Wrist and hand

The articulating surfaces are between the convex distal head of the ulna and the concave ulnar notch of the radius, and are connected by an articular disc.

The fibrous capsule is thicker anteriorly and posteriorly, but the proximal part of the capsule is lax.

Articular disc

The articular disc is fibrocartilaginous (collagen with few elastic fibres in the young) and is triangular, binding the distal ends of the ulna and radius. Its periphery is thicker, its centre sometimes perforated. The disc is attached by a blunt, thick apex to a depression between the ulnar styloid process and distal articular surface, and by its wider thin base to the prominent edge between the ulnar notch and carpal articular surface of the radius. Its margins are united to adjacent carpal ligaments, its surfaces are smooth and concave: the proximal articulates with the ulnar head, the distal is part of the radiocarpal joint, and articulates with the lunate and, when the hand is adducted, the triquetrum. The disc shows age-related degeneration, becoming thinned and ultimately perforated in about half the subjects over the age of 60.

The capsule is lined by synovial membrane which projects proximally between the radius and ulna as a recessus sacciformis in front of the distal part of the interosseous membrane.

Vascular supply and lymphatic drainage

The arterial supply to the distal radio-ulnar joint and disc is mainly derived from the palmar and dorsal branches of the anterior interosseous artery, reinforced by the posterior interosseous and ulnar arteries.

The distal radio-ulnar joint is innervated by branches of the anterior and posterior interosseous nerves.

Movements

Movements at the radio-ulnar joint complex pronate and supinate the hand. In pronation the radius, carrying the hand, turns anteromedially and obliquely across the ulna, its proximal end remains lateral, its distal becomes medial. During this action the interosseous membrane becomes spiralled. In supination the radius returns to a position lateral and parallel to the ulna and the interosseous membrane becomes unspiralled. The hand can be turned through 140–150°: with the elbow extended, this can be increased to nearly 360° by humeral rotation and scapular movements. Power is greater in supination, a fact which has affected the design of nuts, bolts and screws, which are tightened by supination in right-handed subjects. Moreover, supination is an antigravity movement with a pendent upper arm and semiflexed forearm; in seizing objects for examination or manipulation, pronation is merely a preliminary and is aided by gravity.

Forearm rotation occurs between the articulation of the head of the ulna and sigmoid notch distally, and the head of the radius and the radial notch of the ulna proximally. These distal and proximal radio-ulnar joints are pivot-type synovial joints: they act as a pair permitting stable rotary motion (pronation 61–66°, supination 70–77°). During rotation, the radius moves around the ulnar head. The axis for pronation and supination is often represented as a line through the centre of the radial head (proximal) and the ulnar attachment of the articular disc (distal). More correctly this is the axis of movement of the radius relative to the ulna and it does not remain stationary. The radial head rotates in the fibro-osseous ring: its distal lower end and articular disc swing round the ulnar head. During rotation of the radial head its proximal surface spins on the humeral capitulum. As the forearm moves from full pronation into supination the ulna translocates medially by 9–10 mm, such that the axis of rotation shifts but still passes through the ulnar head. In addition the sigmoid notch changes its contact position with the ulnar head, lying dorsal proximal in pronation and volar distal in supination. The distal end of the ulna is not stationary during these movements; it moves a variable amount along a curved course, posterolaterally in pronation, anteromedially in supination. The axis of movement, as defined above, is therefore displaced laterally in pronation, medially in supination. Hence the axis for supination and pronation of the whole forearm and hand passes between the bones at both the superior and distal radio-ulnar joints when ulnar movement is marked, but through the centres of the radial head and ulnar styloid when it is minimal. The axis may be prolonged through any digit, depending on the medial or lateral displacement of the distal end of the ulna. The hand will rotate further than the forearm because of the sliding–rotatory movement which occurs between the carpal bones and the bases of the metacarpals and, to a very minor degree, at the radiocarpal joint.

Accessory movements

Accessory movements include anterior and posterior translation of the radial head on the ulnar radial notch, and of the ulnar head likewise on the radial ulnar notch.

Muscles producing movement

The muscles producing movements at the distal radio-ulnar joint are as follows.

Pronation

Pronator quadratus, aided in rapid movement and against resistance by pronator teres. Gravity also assists.

Supination

Supinator, in slow unresisted movement and extension, assisted by biceps in fast movements in flexion, especially when resisted.

Electromyographic studies have not confirmed activity in brachioradialis during pronation and supination.

RADIOCARPAL (WRIST) JOINT

Fig. 50.15 Joints and ligaments of the left hand. A, Palmar aspect. B, Dorsal aspect.

The radiocarpal joint is a synovial biaxial and ellipsoid joint formed by articulation of the distal end of the radius and the triangular fibrocartilage with the scaphoid, lunate and triquetrum (Fig. 50.15 and Fig. 50.16). In the neutral position of the wrist, only the scaphoid and lunate are in contact with the radius and articular disc: the triquetrum comes into apposition with the disc only in full adduction of the wrist joint. The radial articular surface and distal discal surface form an almost elliptical, concave surface with a transverse long axis. The radial surface is bisected by a low ridge into two concavities. A similar ridge usually appears between the medial radial concavity and the concave distal discal surface. The proximal articular surfaces of the scaphoid, lunate and triquetrum, and their interosseous ligaments, form a smooth convex surface which is received into the proximal concavity.

Fig. 50.16 MRI left wrist. A, Axial view at level of distal radio-ulnar joint. B, Axial view at level of carpal tunnel. C, Coronal MRI through the wrist demonstrating normal bony anatomy and the triangular fibrocartilage.

Part A: 1. Median nerve. 2. Flexor carpi ulnaris. 3. Flexor digitorum profundus. 4. Ulna. 5. Extensor carpi ulnaris. 6. Extensor digiti minimi. 7. Extensor digitorum and extensor indicis. 8. Extensor pollicis longus. 9. Flexor digitorum superficiales. 10. Flexor carpi radialis. 11. Flexor pollicis longus. 12. Radial artery. 13. Abductor pollicis longus. 14. Extensor pollicis brevis. 15. Radius. 16. Extensor carpi radialis longus. 17. Extensor carpi radialis brevis.

Part B: 1. Ulnar artery and nerve. 2. Median nerve. 3. Hypothenar muscles. 4. Hamate. 5. Capitate. 6. Base of 3rd metacarpal. 7. Thenar muscles. 8. Flexor digitorum superficialis. 9. Flexor digitorum profundus. 10. Base of 1st metacarpal. 11. Trapezium. 12. Trapezoid.

Part C: 1. 5th metacarpal. 2. Hamate. 3. Triquetrum. 4. Lunate. 5. Triangular fibrocartilage. 6. Distal ulna. 7. Base of 2nd metacarpal. 8. Trapezoid. 9. Capitate. 10. Scaphoid. 11. Scapholunate ligament. 12. Distal radius.

The fibrous capsule is lined by synovial membrane which is usually separate from that of the distal radio-ulnar and intercarpal joints. A protruding prestyloid recess (recessus sacciformis), anterior to the articular disc, is present and ascends close to the styloid process. The recess is bounded distally by a fibrocartilaginous meniscus, which projects from the ulnar collateral ligament between the tip of the ulnar styloid process and the triquetrum; both are clothed with hyaline articular cartilage. The meniscus may ossify. The capsule is strengthened by palmar radiocarpal and ulnocarpal, dorsal radiocarpal and radial and ulnar collateral ligaments.

The synovial membrane lines the fibrous capsule.

The radiocarpal joint is supplied by branches of the anterior interosseous artery, anterior and posterior carpal branches of the radial and ulnar arteries, palmar and dorsal metacarpal arteries and recurrent rami of the deep palmar arch.

The radiocarpal joint is innervated by the anterior and posterior interosseous nerves with contributions from the median, ulnar and radial nerves.

Muscles producing movement

Movements accompany those of the intercarpal and midcarpal joints and are described on page 871.

CARPAL JOINTS

The intercarpal joints interconnect the carpal bones. They may be summarized as the joints between the proximal and distal rows of carpal bones, and the midcarpal joint, a complex joint between the rows.

Carpal bones are connected by an extensive array of ligaments, not all of which are specifically named. The flexor retinaculum is an accessory intercarpal ligament. Articular surfaces are either sellar, ellipsoid or spheroidal.

Joints of the proximal carpal row

Joints of the proximal carpal row are between the scaphoid, lunate and triquetrum. In addition, the pisiform articulates with the palmar surface of the triquetrum at a small, oval, almost flat, synovial pisotriquetral joint.

A thin capsule surrounds the joint. The synovial cavity is usually separate but may communicate with that of the radiocarpal joint.

Joints of the distal carpal row

Joints of the distal carpal row are between the trapezium, trapezoid, capitate and hamate. There is virtually no movement at these joints.

Midcarpal joint

The midcarpal joint, between the scaphoid, lunate and triquetrum (proximally) and trapezium, trapezoid, capitate and hamate (distally) is a compound articulation that may be divided descriptively into medial and lateral parts. Throughout most of the medial compartment the convexity formed by the head of the capitate and hamate articulates with a reciprocal concavity formed by the scaphoid, lunate and much of the triquetrum. However, most medially the curvatures are reversed, forming a compound sellar joint. In the lateral compartment the trapezium and trapezoid articulate with the scaphoid, forming a second compound articulation, often said to be plane, but which is also sellar.

Carpal synovial membrane

The carpal synovial membrane is most extensive, and lines an irregular articular cavity. Its proximal part is between the distal surfaces of the scaphoid, lunate and triquetrum and the proximal surfaces of the second carpal row. It has proximal prolongations between the scaphoid and lunate, and lunate and triquetrum, and three distal prolongations between the four bones of the second row. The absence of an interosseous ligament means that the prolongation between the trapezium and trapezoid and/or between the trapezoid and capitate is often continuous with corresponding carpometacarpal joints, variably from the second to the fifth, or often the second and third only. In the latter case, the joint between the hamate and fourth and fifth metacarpal bones has a separate synovial membrane and the carpometacarpal interosseous ligament is interposed. Synovial cavities of carpometacarpal joints are prolonged slightly between the metacarpal bases. The synovial joint between the pisiform and triquetrum is usually isolated.

The carpal joints are supplied by the posterior carpal branches of the radial and ulnar arteries and by the anterior interosseous artery.

Precise details about innervation of the carpal joints are lacking. They appear to be innervated by small branches from the deep terminal branch of the ulnar nerve, the median nerve and its anterior interosseous branch, the superficial radial nerve and the posterior interosseous branch of the radial nerve.

Wrist ligaments

Wrist ligaments situated between the fibrous and synovial layers of the wrist joint are termed intracapsular, while those lying superficial to the fibrous layer are extracapsular. Almost all ligaments of the wrist actually lie within the joint capsule and the only exceptions are the flexor and extensor retinaculae and the pisotriquetral ligament. The intracapsular ligaments appear to blend one into another and the edges of the ligaments may not be distinct or discrete.

The ligaments are further classified into extrinsic and intrinsic named ligaments. In addition there are superficial and deep parts to some of the extrinsic ligaments; the latter are identifiable at wrist arthroscopy, the former are not.

It is important to appreciate that the wrist ligaments are named from proximal to distal and from radial to ulnar. Thus a ligament passing between the capitate, scaphoid and radius is called the radioscaphocapitate ligament.

Extrinsic ligaments

Extrinsic ligaments connect the carpus with the forearm bones. The extrinsic ligaments as a group tend to be longer than the intrinsic ligaments. They are approximately one-third as strong but easier to repair following rupture.

Extrinsic palmar carpal ligaments

When the synovial lining of the carpal tunnel is dissected away, two V-shaped ligamentous bands are visible with their apices lying distally (Fig. 50.15A). The limbs of the ‘V’ take origin from the radius and ulna respectively: the apex of one ‘V’ attaches to the distal row and that of the second ‘V’ to the proximal row.

Radioscaphocapitate ligament

The radioscaphocapitate ligament originates from the radial styloid and the palmar lip of the radius. It courses distally and is then described by some authors as having three parts. The first is the most radial and inserts onto the lateral aspect of the waist of the scaphoid (radial collateral ligament). The second continues as part of the distal ‘V’ and inserts onto the distal pole of the scaphoid. The third passes over the proximal pole of the scaphoid towards the midcarpus and blends with the fibres originating from the ulnar side – part of the triangular fibrocartilage complex – to form the arcuate ligament over the palmar aspect of the capitate. A few of the fibres of the radioscaphocapitate ligament attach to the body of the capitate. There is a discrete interval between the inferior margin of this ligament and the palmar horn of the lunate which is known as the space of Poirier.

Long radiolunate ligament

The long radiolunate ligament takes origin adjacent to the radioscaphocapitate ligament on the palmar lip of the radius. It passes over and supports the proximal pole of the scaphoid before inserting into the palmar horn of the lunate. This ligament is discrete from the radioscaphocapitate ligament and the visible separation is known as the interligamentous sulcus (continuous with the space of Poirier).

Radioscapholunate (ligament of Testut)

Histological studies have shown the radioscapholunate ligament is not a true ligament because it contains neurovascular structures which supply the scapholunate interosseous membrane and is covered by a thick synovial lining. However it a visible landmark inside the wrist joint when undertaking wrist arthroscopy.

Short radiolunate ligament

The short radiolunate ligament is part of the proximal ‘V’. It arises from the palmar lip of the lunate fossa of the radius and passes directly to the palmar horn of the lunate. To the ulnar side its fibres blend with those of the palmar triangular fibrocartilage complex as these also pass to their insertion on the lunate. This ligament contributes to the stability of the lunate.

Ulnolunate ligament

The ulnolunate ligament originates from the palmar aspect of the ulna adjacent to the short radiolunate ligament and inserts onto the palmar horn of the lunate. Part of this fibre complex arches radially and blends with part of the radioscaphocapitate complex, forming the arcuate ligament.

Ulnotriquetral (ulnar collateral) ligament

The ulnotriquetral ligament arises from the palmar aspect of the ulna and inserts into the medial aspect of the triquetrum. It continues distally to a further attachment to the medial aspect of the hamate. It is generally thought that the ulnolunate and ulnotriquetral ligaments also take some origin from the marginal ligament of the triangular fibrocartilage complex.

Extrinsic dorsal carpal ligaments

The dorsal wrist ligaments are comparatively thin. They are reinforced by the floor and septa of the fibrous tunnels for the six dorsal compartments. The extrinsic dorsal carpal ligaments and the intrinsic dorsal intercarpal ligaments have a ‘Z-shaped’ configuration (Fig. 50.15B). The pattern and shape of these ligaments is utilized in one surgical approach to the dorsum of the wrist joint where incisions are oriented parallel with the ligaments: this reduces scarring and restriction of subsequent motion caused by the arthrotomy.

Dorsal radiolunotriquetral ligament

The dorsal radiolunotriquetral ligament is a true intracapsular ligament. It is the only extrinsic ligament on the dorsum of the carpus and has superficial and deep components. The superficial part connects the radius and triquetrum, and the deep part connects the radius, lunate and triquetrum. The two components are inseparable. The wide superficial component arises from the dorsal margin of the distal radius and courses ulnarly to insert on the dorsal edge of the triquetrum. The deep component takes a narrower origin from the ulnar aspect of the distal dorsal radius and passes in an ulnar direction to attach to part of the lunotriquetral articulation and the intrinsic lunotriquetral ligament.

Intrinsic ligaments

Intrinsic ligaments of the wrist are attached to carpal bones. They are stronger and shorter than extrinsic ligaments and are connected with the extrinsic ligament complexes by interdigitating fibres. Rupture of one or more intrinsic ligaments frequently leads to a clinical instability of the carpus. The intrinsic ligaments are subdivided into ligaments which connect the carpal bones of the proximal and distal rows respectively and ligaments which connect the rows by crossing over the midcarpal joint.

Proximal row interosseous ligaments

The scapholunate and lunotriquetral ligaments are clinically and biomechanically important structures. In the sagittal plane they have an approximately horseshoe-shape configuration with palmar, midcarpal and dorsal components. The scapholunate ligament contains short transverse fibres connecting the dorsal aspect of the respective bones and more obliquely oriented fibres connecting the palmar aspect. The functional significance of this arrangement is that the tighter dorsal component of the ligament acts as a hinge facilitating flexion and extension of the scaphoid: these movements are important to carpal mechanics. The ligament continues on its midcarpal section as an interosseous membrane. The lunotriquetral ligament has similar dorsal, midcarpal interosseous and palmar components, but the fibres of the dorsal and palmar components are similarly rather than differentially oriented, which precludes the same pattern of preferential movement as occurs between the lunate and scaphoid. The interosseous membranes of the scapholunate and lunotriquetral ligaments separate the midcarpal from the radiocarpal joint spaces. Dye injected into one of these joint spaces which leaks to the other denotes a tear of one of these ligaments.

Distal row interosseous ligaments

The distal row interosseous ligaments are powerful ligaments between the capitate, hamate, trapezium and trapezoid, with an important stabilizing function for the distal carpal row. They have superficial and deep components, and unlike the ligaments of the proximal row, are seldom torn.

Palmar midcarpal ligaments

Anterolaterally lies the fan-shaped palmar scaphocapitate – trapezoid ligament which originates from the scaphoid tuberosity and is thought to be an important stabilizer of the scaphoid. This ligament may be subdivided into two parts known as the scaphotrapeziotrapezoidal ligament (having the attachments as its name suggests) and a more ulnar component known as the scaphocapitate ligament. The triquetrohamate and triquetrocapitate ligaments lie towards the ulnar side of the carpus.

Dorsal midcarpal ligaments

The dorsal intercarpal ligament assists in stabilization of the proximal carpal row. It arises from the trapezoid and distal pole of scaphoid, and passes across the dorsal horn of the lunate to be attached to the triquetrum. The ligament forms the floor of the fourth and fifth extensor compartments. On the radial side is the lateral scaphotrapeziotrapezoidal ligament which acts as a stabilizer of the scaphoid and trapezium.

Triangular fibrocartilage complex (TFCC) and distal radio-ulnar ligaments

The triangular fibrocartilage complex (TFCC) is a ligamentous and cartilaginous structure which suspends the distal radius and ulnar carpus from the distal ulna. The TFCC stabilizes the ulnocarpal and radio-ulnar joints, transmits and distributes load from the carpus to the ulna, and facilitates complex movements at the wrist (Fig. 50.17). By definition, it is made up of the cartilaginous disc, the meniscus homologue (an embryological remnant of the ‘ulnar’ wrist that is only occasionally present), volar and dorsal distal radio-ulnar ligaments, ulnar collateral ligament, floor of extensor carpi ulnaris subsheath, ulnolunate and ulnotriquetral ligaments. The triangular fibrocartilage proper (TFC) is a biconcave body composed of chondroid fibrocartilage. It extends across the dome of the ulnar head and varies between 2 and 5 mm in thickness.

Fig. 50.17 Representation of the manner in which the radius moves around the ulna during forearm rotation. The curvature of the shaft of the radius is important in facilitating this movement. The axis of forearm rotation passes through the head of the ulna and varies slightly in position through the range of rotation. The degree of ulnar translocation of the ulna during pronation (shown by the difference in length between the two double arrowheads) can be seen with reference to the distance the ulnar styloid has moved from the plumb line (vertical black line shown)

From its distal aspect, the TFCC resembles a hammock supporting the ulnar carpus with the disc proper as the base of the hammock. From the proximal side, the TFCC appears in the shape of a fan extending from the fovea of the ulna along either side of the sigmoid notch. This fan-shaped structure is divided into dorsal, central and palmar portions where the central portion is the triangular fibrocartilage and the peripheral margins are thick lamellar collagen, structurally adapted to tensile loading and known as the distal (palmar and dorsal) radio-ulnar ligaments. In keeping with other extrinsic ligaments of the wrist joint proper there are thought to be superficial and deep components of the distal radio-ulnar ligaments which act as a functional couple stabilizing the rotation of the ulnar head on the sigmoid notch of the radius.

Stability of the distal radio-ulnar joint during forearm rotation is conferred by the TFCC complex, dorsal and volar distal radio-ulnar ligaments, interosseous membrane and subsheath of the extensor carpi ulnaris tendon (floor of sixth dorsal extensor compartment).

COORDINATED MOVEMENTS AND LOAD-BEARING AT THE WRIST JOINT

Wrist movements

The movements at the radiocarpal and intercarpal joints are considered together since they are both involved in all movements as well as being acted upon by the same muscles. Active movements are flexion (85°), extension (85°), adduction or ulnar deviation (45°), abduction or radial deviation (15°) and circumduction (all movements are approximate).

The range of flexion is greater at the radiocarpal joint, while in extension there is more movement at the midcarpal joint (Fig. 50.18). Hence the proximal surfaces extend further posteriorly on the lunate and scaphoid bones. These movements are limited chiefly by antagonistic muscles, and therefore the range of flexion is perceptibly diminished when the fingers are flexed, due to increased tension in the extensors. Only when the joints are forced to the limits of flexion or extension are the dorsal or palmar ligaments fully stretched (but see below).

Fig. 50.18 A, Radiograph of the hand and wrist in full flexion: lateral aspect. Compare with B, and note the relative positions of the capitate and lunate, and the lunate and radius. B, Radiograph of the hand and wrist: lateral aspect. The long axes of the third metacarpal, capitate and lunate are, approximately, in line with the long axis of the radius. Note the relative positions of the capitate and lunate, and the lunate and radius. C, Radiograph of the hand and wrist in full extension: lateral aspect. Compare with B and note the alterations in the relative positions of the capitate and lunate, and the lunate and the radius. 1. Capitate. 2. Lunate. 3. Tubercle of the trapezium. 4. Tubercle of the scaphoid.

Adduction of the hand is considerably greater than abduction, perhaps due to the more proximal site of the ulnar styloid process. Most adduction occurs at the radiocarpal joint. The lunate articulates with both the radius and articular disc when the hand is in the midposition, but in adduction it articulates solely with the radius, and the triquetrum now comes into contact with the articular disc (Fig. 50.19A). Much of the proximal articular surface of the scaphoid becomes subcapsular beneath the radial collateral ligament and forms a smooth, convex, palpable prominence in the floor of the ‘anatomical snuff-box’.

Fig. 50.19 A, Radiograph of the hand in full adduction (ulnar deviation), dorsopalmar projection. The arrows point to the scaphoid on the radial side and to the pisiform on the ulnar side. Note that the shadow of the pisiform bone overlaps the shadow of the tip of the styloid process of the ulna. Compare with B and observe that the movements occur at both the radiocarpal and intercarpal joints. B, Radiograph of the same hand in full abduction (radial deviation). The arrows point to the hamate and pisiform. Compare with A and note that: (1) the scaphoid and lunate have passed medially (ulnarly) so that the latter articulates to a large extent with the articular disc of the distal radio-ulnar joint; (2) the pisiform is now widely separated from the styloid process of the ulna; (3) the scaphoid, having rotated round a transverse axis, is much foreshortened; (4) the apex of the hamate has been thrust away from the lunate by the rotation of the capitate around an anteroposterior axis; (5) a gap has opened up between the distal portions of the hamate and triquetrum; and (6) the long axes of the capitate and lunate are now almost in the same straight line.

Abduction from the neutral position occurs at the midcarpal joint, the proximal carpal row not moving. Radiographs of abducted hands show that the capitate rotates round an anteroposterior axis so that its head passes medially and the hamate conforms to this: the distance between the lunate and the apex of the hamate is increased (Fig. 50.19B). The scaphoid rotates around a transverse axis, and its proximal articular surface moves away from the capsule to articulate solely with the radius. Movements are limited by antagonistic muscles and, at extremes, by the carpal collateral ligaments.

Circumduction of the hand is not rotatory, but involves successive flexion, adduction, extension and abduction or vice versa.

Integrated model of wrist movement (carpal kinematics)

The proximal row (scaphoid, lunate and triquetrum) is an intercalated segment: no tendons insert onto the bones of the row. It is inherently unstable and controlled by specific retaining and gliding ligaments. Its relative position is determined by the spatial configurations of the radius, triangular fibrocartilage complex (TFCC) and ulna on one side, and the rigid distal carpal row on the other. The proximal carpal row is subject to two opposing moments: the scaphoid straddles the proximal and distal rows and tends to rotate the proximal row into flexion under axial load and radial deviation. At the same time there is a force tending to extend the proximal row which is initiated by the distal row and transmitted via the midcarpal ligaments to the triquetrum (Fig. 50.20). Stability of the midcarpal joint is thus ensured during both movement and loading.

Fig. 50.20 The balance of rotatory moments occurring during axial loading are explained in this series of sagittal sections of the wrist. A, The scaphoid tends to move into flexion with axial loading and is simultaneously constrained palmarly by the scaphotrapeziotrapezoidal ligaments and dorsally by the scapholunate ligament. B, The lunate tends to follow the scaphoid restrained only by the dorsal radiolunate ligament. C, The triquetrum tends to flex along with the two other bones of the proximal carpal row and is constrained palmarly by the triquetrohamate and triquetrocapitate ligaments and dorsally by the radiotriquetral ligament.

The distal carpal row (trapezium, trapezoid, capitate and hamate) can be regarded as one rigid structure tightly bound together. The scaphoid bridges the proximal and distal carpal rows and provides a functional couple between the two.

The carpus was originally thought to move simply as proximal and distal rows (row or rigid body theory). According to this view, during the composite movement of wrist flexion and extension, approximately two-thirds of movement occurs at the radiocarpal joint and one-third at the midcarpal joint. The carpus was later judged to move in lateral central and medial columns more than it did in rows, and the radius–lunate–capitate was described as a three-bar linkage system (column theory). This theory was modified to incorporate the specific stabilizing role of the scaphoid as it bridges the proximal and distal rows. A further theory proposed that the bones were linked by their ligaments in a ring configuration, so that any breakage of the key links leads to instability (ring theory). Most recently the ‘four unit’ theory suggests that the distal carpal row moves as a single unit, and the scaphoid, lunate and triquetrum move in complex but characteristic relationships which are dependent on the given movement (Fig. 50.21). Clinical observation provides some support for each of these theories.

Fig. 50.21 Different theories of carpal mechanics. A, Row or rigid body theory; B, central column theory of Navarro; C, ring theory of Taleisnik; D, four-unit concept of Viegas.

Wrist loading

Axial loading refers to load or force applied along a line parallel to the long bones of the arm and therefore corresponds to power grip manoeuvres, e.g. clenching the fist. In the normal activities of daily living, loading is usually multiplanar with a combination of vectors of force, e.g. grasping an object and then lifting it against gravity with the elbow flexed would engender transverse and axial loading with respect to the long axis of the forearm.

Force transmission

Radiocarpal joint

The articulation of the carpus with the radius (radiocarpal) and ulna (ulnocarpal) can be described in terms of specific fossae, i.e. the scaphoid and lunate fossae of the radius and the facet formed by the distal aspect of the TFCC. The scaphoid fossa contributes 43%, the lunate fossa 46%, and the TFCC 11%, of the total area of this articulating surface. Under physiological conditions, the contact area between this surface and the proximal carpal row is of the order of 20% of the whole. It varies consistently with position: greater contact is seen in forearm supination, radial deviation and dorsiflexion of the wrist, and lesser contact in forearm pronation, ulnar deviation and palmar flexion of the wrist. Further increase in contact area is seen with axial loading. Maximum contact areas are recorded at 40%. Force across the joint also varies with the position of the forearm and the degree of wrist flexion. For a given load in wrist neutral position, 50% of force passes across the scaphoid fossa, 35% through the lunate fossa, and 15% across the TFCC. With a power grip there is slight ulnar deviation, and the proportion of force passing across the scaphoid and lunate fossae reverses.

Midcarpal joint

With the wrist in neutral position, 50–60% of a given load is transmitted from the distal row through the capitate to the scaphoid and lunate. Up to 30% of the load is transmitted via the scaphotrapeziotrapezoid joint, and up to 20% via the hamate-triquetral joint.

Distal radio-ulnar joint (DRUJ)

The DRUJ is part of a more complex articulation that includes the proximal radio-ulnar joint and the interosseous membrane. Static axial loading of the wrist increases the contact areas of the joint surfaces of the distal radio-ulnar joint, DRUJ. Force transmitted across the DRUJ also increases with axial loading and varies with position of forearm rotation. It is maximal at 60° supination, when the ulnar head lies most directly under the ulnar-sided carpus, and least in full pronation. It has been shown that with the wrist in neutral position, 70–80% of axial load passes down the radius and 20–30% down the ulna. The proportion of force passing down the ulna varies with increasing load, and changes with position of the hand and carpus relative to the forearm bones. The interosseous membrane between the radius and ulna plays a role in distributing load in the forearm. Distal radio-ulnar joint loading also depends on relative radial and ulnar lengths, i.e. ulnar negative, neutral or positive variance.

Specific actions

Power grip

There is a predictable pathway of force transmission during axial loading of the hand such as occurs in power grip. During this action the wrist is positioned in mid-dorsiflexion and the object is tightly gripped. Force is transmitted from the hand and wrist via the radiocarpal joint and TFCC to both bones of the forearm, through the elbow joint to the humerus, and thence via the glenohumeral joint to the pectoral girdle. Dynamic muscle action accounts for transmission of a large part of the force, and a smaller proportion is transmitted through the osseoligamentous structures. The distribution of force between the radius and ulna varies dynamically. It depends on the absolute value of load, degree of forearm rotation, and position of the carpus relative to the forearm.

Hammer action

The action exemplified by hammering, where the wrist alternates between the dorsoradial and the palmar-ulnar position, is an important movement. The reciprocal movement is more constrained and awkward to perform. The former movement is important in a wide variety of activities of daily living. In primates this movement is needed (along with forearm rotation at the distal radio-ulnar joint) for brachiation.

It is believed that load distributes dynamically as the position of the carpus moves relative to the forearm and there are consequent changes in the relative position of the carpal bones.

Wrist instability

During evolution the upper limb has developed to facilitate prehension, i.e. the placing of the hand in three-dimensional space. However, mobility of the hand on the forearm has evolved at the expense of stability. A wrist is said to be kinetically unstable when it cannot bear physiological loads without giving way or causing injury. A wrist is kinematically unstable when it exhibits sudden changes in carpal alignment, i.e. during a specific movement there is a ‘clunk’ as one or more of the carpal bones moves abnormally with respect to the others. Stability of the various joints such that the bones maintain normal anatomical relations with respect to each other throughout the normal range of motion is essential for physiological load-bearing.

Some degree of carpal instability may occur as part of inherent hypermobility in affected individuals, but usually it occurs only after rupture or attenuation of intrinsic and extrinsic carpal ligaments and may be detected either clinically or evidenced on radiographs as malalignment of the carpal bones (Fig. 50.22).

Fig. 50.22 Scapholunate dissociation shown in posteroanterior power grip view (A) and lateral view (B). Dissociation between the scaphoid and lunate as shown here leads to collapse of the scaphoid into flexion and the lunate into extension (dorsal intercalated segment instability, or DISI) thus removing the inbuilt tension across the proximal row. This causes the gap (so called Terry Thomas sign) between the scaphoid and lunate in the PA radiograph (marked with asterisk) and increased scapholunate angle above the upper limit of normal of 60 degrees, as shown in B.

CARPOMETACARPAL JOINTS

The first carpometacarpal joint, CMCJ, or basal joint of the thumb, is a modified saddle joint that permits opposition of the thumb and confers the ability to hold and manipulate objects: the actions of ‘pinch’ grip, ‘tripod’ pinch and ‘chuck’ grip are specifically facilitated. Significant forces pass across and compress this joint compared to the remaining CMCJs. The second to fifth CMCJs exhibit an increasing range of movement progressing from the radial to the ulnar side. Thus there is little mobility at the base of the index ray, but considerable mobility at the base of the small finger ray, which facilitates ‘cupping’ of the palm of the hand. Compressive forces between the metacarpals and the distal carpal row are estimated to be upwards of ten times the forces at the tips of the fingers during ‘pinch’ or ‘chuck’ grip.

Carpometacarpal joint of the thumb

The carpometacarpal joint of the thumb is a sellar (saddle) joint between the first metacarpal base and trapezium. It enjoys wide mobility due to its extensive articular surfaces and their topology.

The first metacarpal and trapezium are connected by lateral, anterior and posterior ligaments and a fibrous capsule. The broad lateral ligament runs from the lateral surface of the trapezium to the radial side of the metacarpal base. The palmar and dorsal ligaments are oblique bands which converge to the ulnar side of the metacarpal base from the palmar and dorsal surfaces of the trapezium respectively.

The synovial membrane lines the joint capsule and is separate from it.

The carpometacarpal joint of the thumb receives its blood supply from the radial artery and its first dorsal metacarpal branch.

The carpometacarpal joint of the thumb is innervated by articular twigs from the posterior interosseous nerve and the superficial branch of the radial nerve.

Joint movements

Except at initiation, flexion is accompanied by medial rotation; conversely, medial rotation involves flexion. Linkage of movements is due largely to the shape of the articular surfaces (which impose some conjunct rotation), and to the obliquity of the dorsal ligament (which, when taut, anchors the ulnar side of the metacarpal base while its radial side continues to move). Contraction of flexor pollicis brevis, assisted by opponens pollicis, thus produces medial rotation with flexion; combined with abduction this brings the thumb pulp into contact with the pulps of the slightly flexed fingers, a movement termed opposition. (The flexed fingers have varying degrees of lateral metacarpophalangeal rotation, which is minimal in the index, and maximal in the minimus.) Conversely, full extension of the thumb metacarpal entails slight lateral rotation, attributable to the sellar form of the joint and to the action of the palmar ligament (which is similar to that of the dorsal ligament in flexion). (See also p. 888.)

The muscles producing movements at the carpometacarpal joint of the thumb are as follows.

Flexion

Flexor pollicis brevis and opponens pollicis, aided by flexor pollicis longus when the other joints of the thumb are flexed. Flexion entails medial rotation.

Extension

Abductor pollicis longus and extensors pollicis brevis and longus. In full extension extensor pollicis longus, owing to its oblique pull and the disposition of the palmar ligament, rotates the thumb laterally and draws it dorsally, i.e. slightly adducts the thumb.

Abduction

Abductors pollicis brevis and longus. When abduction is maximal the digit and metacarpal are not in line, and the thumb is abducted at both metacarpophalangeal and carpometacarpal joints.

Adduction

Adductor pollicis alone.

Opposition

Opponens pollicis and flexor brevis pollicis simultaneously flex and medially rotate the abducted thumb. Interpulpal pressure, or that generated by digital grasping, is increased by adductor pollicis and flexor pollicis longus.

Circumduction

Extensors, abductors, flexors and adductors acting consecutively in this, or reverse, order.

Second to fifth carpometacarpal joints

The second to fifth carpometacarpal joints are synovial ellipsoid joints between the carpus and second to fifth metacarpals. Although widely classed as plane, they have curved articular surfaces which are often of complex sellar shape. The bones are united by articular capsules, and dorsal, palmar and interosseous ligaments.

The dorsal ligaments are the strongest, and connect the dorsal surfaces of the carpal and metacarpal bones. The second metacarpal has two ligaments, from the trapezium and trapezoid; the third has two, from the trapezoid and capitate; the fourth has two, from the capitate and hamate; the fifth has a single band from the hamate, which is continuous with a similar palmar ligament, forming an incomplete capsule.

The palmar ligaments are similar, except that the third metacarpal has three: a lateral from the trapezium, superficial to the tendon sheath of flexor carpi radialis, an intermediate from the capitate, and a medial from the hamate.

The interosseous ligaments consist of two short, thick, fibrous bands. They are limited to one part of the carpometacarpal articulation and connect contiguous distal margins of the capitate and hamate with adjacent surfaces of the third and fourth metacarpal bones: they may be united proximally.

Synovial membranes

The synovial membranes are often continuous with those of the intercarpal joints. Occasionally, the joint between the hamate and fourth and fifth metacarpal bones has a separate synovial cavity, bounded laterally by the medial interosseous ligament and its extensions to the palmar and dorsal parts of the capsule.

The second to fifth carpometacarpal joints are supplied by the posterior carpal branches of the radial and ulnar arteries, by twigs from the anterior interosseous artery, and from the palmar digital arteries.

The second to fifth carpometacarpal joints are innervated by the deep terminal branches of the ulnar nerve, the anterior interosseous branch of the median nerve, and the short branches of the superficial radial and dorsal ulnar nerves.

Slight gliding movements are effected by the long flexor and extensor muscles of the digits.

INTERMETACARPAL JOINTS

The second to fifth metacarpal bases articulate reciprocally by small cartilage-covered facets connected by dorsal, palmar and interosseous ligaments.

The intermetacarpal joints have fibrous capsules.

The dorsal and palmar ligaments pass transversely from bone to bone. The interosseous ligaments connect contiguous surfaces just distal to their articular facets.

Synovial membranes

The synovial membranes are continuous with those of the carpometacarpal articulations.

Movements at the carpometacarpal and intermetacarpal joints

Movements at the carpometacarpal and intermetacarpal articulations are limited to slight gliding, sufficient to permit some flexion–extension and adjunct rotation: ranges vary in different joints. They are partly accessory movements occurring when the palm is ‘cupped’, as in grasping an object. Active movements also occur and are familiar, e.g. to the pianist or violinist. The fifth metacarpal is most movable and the second and third are the least mobile. These variations are easily demonstrated by opposing each digit to the thumb over the palmar centre. About two-thirds of the movements are pollicial, as described above, but during opposition to minimus, the latter is flexed, abducted and laterally rotated, accounting for the remaining third of the movement. These actions occur at both the carpometacarpal and metacarpophalangeal joints. The close-packed position probably coincides with carpal extension, as in gripping. Further accessory movements are spiral twisting of the whole metacarpus on the carpus.

Movements at carpometacarpal and intermetacarpal joints are effected by the flexors and extensors of the second to fifth digits.

METACARPOPHALANGEAL JOINTS

The metacarpophalangeal joints are usually considered ellipsoid. However, the metacarpal heads are adapted to shallow concavities on the phalangeal bases: they are not regularly convex but partially divided on their palmar aspects and thus almost bicondylar (Fig. 50.23).

Fig. 50.23 Metacarpophalangeal and digital joints of the left third finger: medial aspect.

The metacarpophalangeal joints all have fibrous capsules.

Each metacarpophalangeal joint has a palmar and two collateral ligaments.

Palmar ligaments

The palmar (volar plates) ligaments are unusual. They are thick, dense and fibrocartilaginous, and sited between, and connected to, the collateral ligaments. They are attached loosely to the metacarpals but firmly to the phalangeal bases. Their palmar aspects are blended with the deep transverse palmar ligaments and are grooved for the flexor tendons, whose fibrous sheaths connect with the sides of the grooves. Their deep surfaces increase articular areas for the metacarpal heads.

Deep transverse metacarpal ligaments

The deep transverse metacarpal ligaments are three short, wide, flat bands which connect the palmar ligaments of the second to fifth metacarpophalangeal joints. They are related anteriorly to the lumbricals and digital vessels and nerves, and posteriorly to the interossei. Bands from the digital slips of the central palmar aponeurosis join their palmar surfaces. On both sides of the third and fourth metacarpophalangeal joints, but only the ulnar side of the second and radial side of the fifth, transverse bands of the dorsal digital expansions join the deep transverse metacarpal ligaments. The lumbricals and the phalangeal attachments of the dorsal interossei lie anterior to this band, and the remaining attachments of dorsal interossei and palmar interossei are posterior to it (see Fig. 50.30).

Collateral ligaments

The collateral ligaments are strong, round cords which flank the joints. Each is attached to the posterior tubercle and adjacent pit on the side of its metacarpal head, and each passes distoanteriorly to the side of the anterior aspect of its phalangeal base (Fig. 50.23).

The metacarpophalangeal joints are lined by a synovial membrane.

The metacarpophalangeal joints receive their blood supply from the dorsal and palmar metacarpal arteries, the arteria princeps pollicis, and the arteria radialis indicis.

The metacarpophalangeal joints are innervated by twigs from the palmar digital branches of the median nerve, the deep terminal branch of the ulnar nerve, and the posterior interosseous nerve.

Joint movements

Flexion, extension, adduction, abduction, circumduction and limited rotation all take place at the metacarpophalangeal joints. Rotation cannot occur in isolation, but may accompany flexion–extension. This may be initiated voluntarily in the free hand (e.g. each finger flexing and rotating to place its tip near the palmar centre); the range of rotation is frequently increased as a result of the resistance of a grasped object. Flexion is almost 90°, whereas extension is only a few degrees: both movements are limited mostly by antagonistic muscles. Flexion is often terminated by the resistance offered by a grasped object. The metacarpophalangeal joint of the thumb has a flexion–extension range of approximately 60°, which is almost entirely flexion. Slight lateral rotation accompanies digital flexion of digits 3–5. Flexion of the index finger may be accompanied by minimal lateral rotation or no rotation: a small degree of medial rotation is frequently observed. Other movements are adduction–abduction (maximal range 25°), which invariably accompanies the corresponding carpometacarpal movements and increases their combined range, and slight conjunct rotation, but greater adjunct rotation, which accompanies flexion–extension. Of the second to fifth metacarpophalangeal joints, the second is most mobile in adduction–abduction (approximately 30°), followed by the fifth, fourth, and third.

Accessory movements

Accessory movements are further rotation (most marked in the thumb), anteroposterior and lateral translation of a phalanx or metacarpal, and distraction.

The muscles producing movements at the metacarpophalangeal joints are as follows.

Flexion

Flexores digitorum superficialis and profundus, assisted by the lumbricals and interossei and, in the minimus, flexor digiti minimi brevis. In the thumb, flexores pollicis longus and brevis and the first palmar interosseous.

Extension

Extensor digitorum, assisted in the second and fifth digits by extensor indicis and extensor digiti minimi respectively. In the thumb, extensors pollicis longus and brevis.

Adduction

In extended fingers, palmar interossei; the long flexors are predominant during flexion. In the thumb, limited metacarpophalangeal adduction is possible and may be attributable to adductor pollicis and the first palmar interosseous.

Abduction

In extended fingers, dorsal interossei assisted by the long extensors (except in the middle finger), and abductor digiti minimi in the little finger. In the thumb, abductor pollicis brevis (which also contributes to opposition). When the fingers are flexed at the IP joints, active abduction is impossible: if the long digital flexors are inactive, passive abduction is free. Inability to abduct actively in this position may be due to shortening of the dorsal interossei and abductor digiti minimi by flexion. However the altered line of pull of the interossei relative to the axis of movement is probably the determining factor: in digital extension the axis of lateral movements is anteroposterior, whereas in flexion it is proximodistal, and the line of pull of the interossei is then nearly parallel to the axis.

Gamekeeper’s thumb

The ulnar collateral ligament of the metacarpophalangeal joint of the thumb can be avulsed from its distal attachment to the base of the proximal phalanx by a combination of forced extension and radial deviation of the joint. The injury was reported to occur in gamekeepers while breaking the necks of game, but can occur following simple falls onto the outstretched hand, especially in skiers who have the straps of ski poles wrapped around their thumbs.

The ulnar collateral ligament is normally covered by the aponeurotic insertion of the adductor pollicis tendon. Following avulsion, the proximal end can become superficially displaced around the proximal free edge of the aponeurotic insertion of the adductor. This soft tissue interposition prevents the ends of the avulsed ligament from healing. If left untreated, a firm palpable inflammatory swelling occurs, known as the ‘Stener lesion’. Surgical repair requires division of the aponeurotic insertion of adductor pollicis, followed by bony reattachment of the avulsed ligament and repair of the aponeurosis.

INTERPHALANGEAL JOINTS

The interphalangeal joints are uniaxial hinge joints (Fig. 50.23).

Each interphalangeal joint has a fibrous capsule.

Each interphalangeal joint has a palmar ligament (also known as the volar plate) and two collateral ligaments. The long extensor tendons take the place of the dorsal capsular ligaments. Extensions from the extensor expansion, each collateral ligament and the palmar ligament, all pass into the joint cavity and provide a significant increase to the articular surface area of the phalangeal base: their deformable nature improves joint congruence.

Palmar ligament (volar plate)

The volar plate constitutes the floor of the interphalangeal joint. In the proximal interphalangeal joint, the distal end of the volar plate is thickened laterally where it is firmly attached to the base of the middle phalanx at the position of the true collateral ligament attachment. Centrally, it is more delicate and blends with the volar periosteum of the middle phalanx. Proximally, the volar plate is also very delicate in its central portion, but it is thickened laterally to form the so-called check rein ligaments which attach to the periosteum of the proximal phalanx just within the walls of the A2 pulley component of the fibrous flexor sheath. Nutrient branches of the digital arteries pass underneath these check ligaments to reach the vinculae (Fig. 50.24). A detailed account of the anatomy of the volar plate can be found in Bowers et al (1980).

Fig. 50.24 Anatomy of the proximal interphalangeal joint volar plate. The blood vessel supplying the flexor tendons via the vincula brevum is shown, but the mesentery forming the vinculum has been omitted.

Collateral ligaments

The collateral ligaments pass from the lateral aspect of the head of one phalanx to the volar aspect of the base of the adjacent phalanx. An accessory component to the collateral ligament arises in continuity with the main ligament and passes in a volar direction to attach to the volar plate.

Each interphalangeal joint has a synovial lining.

The interphalangeal joints are supplied by branches from the palmar digital arteries.

The interphalangeal joints are innervated by the palmar digital branches of the median nerve (to the thumb, index, middle and ring fingers), and the ulnar nerve (to the ring and little fingers).

Factors maintaining stability

Stability is conferred by the articular contours of the joint surfaces and the collateral ligaments. The flexor and extensor tendons and retinacular ligaments provide secondary stabilization. Stability against hyperextension of the proximal interphalangeal joint is enhanced by the three-dimensional box arrangement that is produced by the collateral ligament–volar plate complex.

Joint movements

Movements (active) at the interphalangeal joints are flexion and extension, and are greater in range at the proximal joints. Flexion is considerable, whereas extension is limited by tension of the digital flexors and terminated by tension in the palmar ligaments and nonarticular compression. Full extension is the close-packed position, which is assumed whenever the fingers are used as props to transmit body weight or powerful thrust. In contrast, the fully clenched fist may also be used for this purpose. Flexion and extension are accompanied by slight conjunct rotation: during flexion this turns the digital pulps slightly laterally, i.e. to face the opposed thumb, and an opposite rotation occurs during extension.

Accessory movements

Accessory movements are limited rotation, abduction, adduction and anteroposterior translation. They permit the fingers to adapt to the shapes of gripped objects and help to protect against stresses and strains.

The muscles producing movements at the interphalangeal joints are as follows.

Flexion

At proximal interphalangeal joints, flexores digitorum superficialis and profundus; at distal interphalangeal joints, flexor digitorum profundus; at the thumb interphalangeal joint, flexor pollicis longus.

Extension

Extensores digitorum, digiti minimi and pollicis longus, in association with abductor pollicis longus and extensor pollicis brevis. Extension occurs simultaneously in both joints in digits 2–5.

Simultaneous flexion at the metacarpophalangeal joints and extension at the interphalangeal joints of a digit are essential for the fine movements of writing, drawing, threading a needle, etc. The lumbricals and interossei have long been accepted as not only primary agents in flexing the metacarpophalangeal joints but also in extending the interphalangeal joints via their attachments to the dorsal digital expansions.

Mallet finger

Mallet finger is a traumatic rupture, avulsion fracture or laceration of the terminal slip of the extensor tendon to the terminal phalanx of the finger. It can result from forced flexion of the distal phalanx of the fully extended finger and is a well-known injury in cricketers. The patient is unable actively to extend the terminal phalanx although the distal interphalangeal joint can be extended passively.

Swan neck deformity

Swan neck deformity involves hyperextension of the proximal interphalangeal joint together with distal interphalangeal joint flexion. It is caused by a relative overactivity of the extensors acting at the proximal interphalangeal joint compared with the flexors. It arises as a result of intrinsic muscle spasm, long standing mallet finger or dysfunction of flexor digitorum superficialis and can only occur if there is also some laxity of the volar plate.

Boutonnière deformity

Boutonnière deformity is a flexion deformity of the proximal interphalangeal joint following either division or laxity in the central slip of the extensor tendon, which normally inserts into the base of the middle phalanx. An essential feaure is hyperextension at the distal interphalangeal joint. It usually occurs as a result of either trauma or rheumatoid arthritis which cause the lateral bands of the extensor tendon to migrate in a volar direction and the head of the proximal phalanx to migrate dorsally. Initially the deformity is passively correctable. With time, the soft tissues around the joint contract and a fixed deformity results.

MUSCLE

EXTRINSIC LONG FLEXORS AND EXTENSORS

The extrinsic long flexors and extensors are described on page 845.

Flexor retinaculum

The flexor retinaculum is a strong, fibrous band (Fig. 50.25). It crosses the front of the carpus and converts its anterior concavity into the carpal tunnel which transmits the flexor tendons of the digits and the median nerve. The retinaculum is short and broad, measuring 2.5–3 cm both transversely and proximodistally. It is attached medially to the pisiform and the hook of the hamate. Laterally, it splits into superficial and deep laminae. The superficial lamina is attached to the tubercles of the scaphoid and trapezium. The deep lamina is attached to the medial lip of the groove on the trapezium. Together with this groove, the two laminae form a tunnel, lined by a synovial sheath, which contains the tendon of flexor carpi radialis. The retinaculum is crossed superficially by the ulnar vessels and nerve – immediately radial to the pisiform – and by the palmar cutaneous branches of the median and ulnar nerves. A slender band of fascia, the superficial part of the flexor retinaculum, bridges over the ulnar neurovascular bundle and attaches to the radial side of the pisiform, forming a tunnel (Guyon’s canal) which is an occasional site of ulnar nerve entrapment. The tendons of palmaris longus and flexor carpi ulnaris are partly attached to the anterior surface of the retinaculum. Distally some of the intrinsic muscles of the thumb and little finger are attached to the retinaculum.

Fig. 50.25 Transverse section through the left wrist at the level of the carpus, showing the muscle tendons and their synovial sheaths.

Long flexor tendon apparatus

Flexor tendon sheaths

The fibrous sheaths of the flexor tendons are specialized parts of the palmar fascial continuum. Each finger has an osseoaponeurotic tunnel which extends from midpalm to the distal phalanx. The thumb has a tunnel for flexor pollicis longus which extends from the metacarpal to the distal phalanx. The proximal border is to some extent a matter of definition, because the transverse fibres of the palmar aponeurosis may be considered to be a part of the pulley system. The sheath consists of arcuate fibres which arch anteriorly over bone, tendons (where the sheath is required to be stiff), and the centres of joints (where a bucket-handle of arcuate fibres is a mechanically favourable arrangement). In contrast, where the sheath is required to fold to permit joint flexion, it consists of cruciate fibres. These fibrous sheaths are lined by a thin synovial membrane which provides a sealed lubrication system containing synovial fluid. The synovial membrane extends from the distal phalanx to midpalm in the case of the index, middle and ring fingers, and further proximally in the case of the little finger (Fig. 50.26). The sheaths around the thumb and little finger are continuous with the flexor sheaths in front of the wrist. The parietal synovial membrane is reflected onto the surface of the flexor tendon, forming a visceral synovium.

Fig. 50.26 Synovial sheaths of the tendons and flexor tendon sheaths on the flexor aspect of the left wrist and hand. Where they are exposed, the synovial sheaths are shown in blue. A, Lateral view of the anular and cruciate pulleys of the flexor tendon sheath. B, Palmar aspect showing the synovial sheaths and details of the anular and cruciate pulleys in the middle finger. Note the extension of the digital arteries and nerves to the end of the middle finger have been omitted to allow clarity in showing the pulley system. C, Variation in the arrangement to the synovial sheaths to the digits.

A standard nomenclature for the anular and cruciform pulleys that make up the sheath has been adopted by the American Society for Surgery of the Hand: the letters A and C respectively are used (Doyle & Blythe 1975).

The usual pattern is as follows (Fig. 50.26). The A1 pulley is situated anterior to the palmar cartilaginous plate of the metacarpophalangeal joint and may extend over the proximal part of the proximal phalanx. The A2 overlies the middle third of the proximal phalanx. It is the strongest pulley and arises from well-defined longitudinal ridges on the palmar aspect of the phalanx. Its distal edge is well developed. A pouch or recess of synovium extends superficial to the free edge of the pulley fibres so that the free edge forms a lip protruding into the synovial space. A3 is a narrow pulley lying palmar to the proximal interphalangeal joint. A4 overlies the middle third of the middle phalanx, and A5 overlies the distal interphalangeal joint. The cruciate fibres are numbered in a slightly different manner. C0 is palmar to the metacarpophalangeal joint. There are two cruciate zones, C1 and C2, at the proximal interphalangeal joint and they lie just proximal and distal respectively to A3. At the distal interphalangeal joint there is one pronounced cruciate system, C3, which lies between A4 and A5. Variations occur frequently. During flexion, the cruciate fibres become orientated more transversely in the digits and the edges of adjacent anular pulleys approximate so that they form, in full flexion, a continuous tunnel of transversely orientated fibres. Surgically the most important pulleys which prevent bowstringing of the flexor tendons are the A2 and A4 pulleys.

The pulley arrangement in the thumb is different from that in the other digits. There are three constant pulleys – two anular and one oblique (Fig. 50.27). The A1 pulley is located at the metacarpophalangeal joint. The oblique pulley is located over the mid-portion of the proximal phalanx, and its fibres pass from the ulnar aspect proximally to the radial aspect dorsally. The A2 pulley is thinner than the A1 pulley and is situated just proximal to the interphalangeal joint. The oblique pulley is the most important pulley in the thumb for maintaining the action of flexor pollicis longus.

Fig. 50.27 Flexor sheath of the left thumb showing the anular and oblique pulleys.

(From Doyle JR, Blythe WF 1977 Anatomy of the flexor tendon sheath and pulleys of the thumb. J Hand Surg 2: 149–51. With permission from the American Society for Surgery of the Hand.)

Synovial sheaths of the carpal flexor tendons

Two synovial sheaths envelop the flexor tendons as they traverse the carpal tunnel, one for the flexores digitorum superficialis and profundus, the other for flexor pollicis longus (Fig. 50.25). These sheaths typically extend into the forearm for 2.5 cm proximal to the flexor retinaculum, and occasionally communicate with each other deep to it. The sheath of the flexores digitorum tendons reaches about halfway along the metacarpal bones, where it ends in blind diverticula around the tendons to the index, middle and ring fingers (Fig. 50.26). The sheath is prolonged around the tendons to the little finger and is usually continuous with their digital synovial sheath. A transverse section through the carpus shows that the tendons are invaginated into the sheath from the lateral side (Fig. 50.25). The parietal layer lines the flexor retinaculum and the floor of the carpal tunnel. It is reflected laterally as the visceral layer over the tendons of flexor digitorum superficialis ventrally and flexor digitorum profundus dorsally. Medially a recess formed by the visceral layer of the sheath insinuates between the two groups of tendons and passes laterally for a variable distance. The sheath of flexor pollicis longus, which is usually separate, is continued along the thumb as far as the insertion of the tendon.

Vincula

The phenomenon of tendon gliding within a fibrous sheath requires a very specialized arrangement of the vascular supply. Folds of synovial membrane containing a loose plexus of fascial fibres carry blood vessels to the tendons at certain defined points (Fig. 50.28). These folds, vincula tendinum, are of two kinds. Vincula brevia, of which there are two in each finger, are attached to the deep surfaces of the tendons near to their insertions. There is thus one vinculum brevium attaching flexor digitorum profundus to the region of the distal interphalangeal joint, and a more proximal vinculum deep to flexor digitorum superficialis at the proximal interphalangeal joint. Vincula longa are filiform: usually two are attached to each superficial tendon, one to each deep tendon.

Fig. 50.28 Lateral part of the left hand showing the tendons and vincula tendinum of the index finger and the muscles in the first intermetacarpal space.

Trigger finger

Trigger finger is a stenosing tenovaginitis which affects the fibrous flexor sheaths of the fingers or thumb within the palm. The affected sheath thickens and entraps the contained tendons, which become constricted at the site of entrapment and bulge distal to it, to produce a distinct nodule in the palm of the hand. The finger now snaps as the tendon nodule passes through the constriction on flexing the finger. The corresponding extensor muscle is insufficiently powerful to extend the affected finger. The patient does this passively, accompanied by a painful snap. Treatment frequently requires surgical division of the A1 pulley of the flexor sheath to relieve the stricture.

Extensor retinaculum

The extensor retinaculum is a strong, fibrous band which extends obliquely across the back of the wrist (Fig. 50.29). It is attached laterally to the anterior border of the radius, medially to the triquetral and pisiform bones, and, in passing across the wrist, to the ridges on the dorsal aspect of the distal end of the radius. It prevents bowstringing of the tendons across the wrist joint.

Fig. 50.29 The synovial sheaths of the tendons on the extensor aspect of the left wrist.

(From Sobotta 2006.)

Synovial sheaths of the carpal extensor tendons

Six tunnels deep to the extensor retinaculum transmit the extensor tendons, each contains a synovial sheath. The tendons of abductor pollicis longus and extensor pollicis brevis lie in a tunnel on the lateral side of the styloid process of the radius. Occasionally there may be a separate synovial sheath for each, or the tendon of the abductor may be double. The tendons of extensores carpi radiales longus and brevis lie behind the styloid process. The tendon of extensor pollicis longus lies on the medial side of the dorsal tubercle of the radius and the tendons of extensor digitorum and extensor indicis lie in a tunnel on the medial side of the tubercle. The tendon of extensor digiti minimi lies opposite the interval between the radius and ulna, and the tendon of extensor carpi ulnaris lies between the head and the styloid process of the ulna.

The tendon sheaths of abductor pollicis longus, extensores pollicis brevis and longus, extensores carpi radiales and extensor carpi ulnaris stop immediately proximal to the bases of the metacarpal bones; those of extensor digitorum, extensor indicis and extensor digiti minimi are sometimes prolonged a little more distally along the metacarpus.

De Quervain’s tenovaginitis

De Quervain’s tenovaginitis is a stenosing tenovaginitis of unknown aetiology which occurs in the first dorsal compartment at the level of the radial styloid. It involves the common extensor sheath containing the tendons of abductor pollicis longus and extensor pollicis brevis. There is palpable thickening of the tendon sheath with painful limitation of extension of the thumb. Treatment frequently requires division of the thickened sheath, care being taken to avoid the adjacent superficial radial nerve. Division of the sheath produces no functional impairment.

Extensor tendon apparatus

The extensor digitorum tendons emerge through the fourth dorsal compartment onto the dorsum of the hand, where they are joined together distally by a varying pattern of oblique interconnections, the juncturae tendinae. These typically pass in a distal direction from middle finger to index finger and from ring finger to middle and little fingers. Proximal lacerations to the middle finger extensor tendon may result in only partial loss of extension because of these tendinous interconnections.

At the level of the metacarpophalangeal joint, each extensor tendon is held in a central position over the dorsum of the joint by a flat fibrous extensor expansion (Fig. 50.30). The expansion extends onto the dorsum of the proximal phalanx of each digit. It forms a movable hood, which moves distally when the metacarpophalangeal joint is flexed, and proximally when it is extended (in which position it is most closely applied to the joint).

Each extensor tendon blends with the extensor expansion along its central axis, and is separated from the metacarpophalangeal joint by a small bursa. The expansion is triangular in shape, with its base proximal. It receives the conjoined tendons of the intrinsic muscles. The expansion is almost translucent between its margins and the extensor digitorum tendon. Transverse fibres (the sagittal bands) pass to the volar plate and transverse metacarpal ligaments. They separate the phalangeal attachment of the dorsal interosseus from the rest of the muscle, and the palmar interosseus from the lumbrical muscle. Injuries to the sagittal bands can lead to subluxation of the extensor tendon.

The margins of the extensor expansions are thickened on the radial side by the tendons of lumbrical and interosseous muscles and on the ulnar side by the tendon of an interosseous alone or, in the case of the fifth digit, by abductor digiti minimi.

The interossei tendons join the extensor expansion at the level of the proximal portion of the proximal phalanx, while the lumbrical tendons join the extensor mechanism further distally at the mid-portion of the proximal phalanx. Their line of pull is proximal to the axis of rotation at the metacarpophalangeal joint, but dorsal to the axis of rotation at the proximal interphalangeal joint. The extensor mechanism trifurcates into a central slip and two lateral bands just proximal to the proximal interphalangeal joint. The central slip receives a contribution from the lumbrical and interosseous tendons via the lateral bands. Similarly, some fibres from the central region pass to each lateral band, producing a criss-cross arrangement of fibres. The central slip attaches to the base of the middle phalanx, while the lateral bands continue distally and eventually fuse together and insert into the distal phalanx. The tension in the central slip and the lateral bands varies as the finger moves between flexion and extension and plays a crucial role in coordinating synchronous activity between the proximal and distal interphalangeal joints.

The transverse and oblique retinacular ligaments of Landsmeer connect the fibrous flexor sheath to the extensor apparatus. The transverse retinacular ligament passes from the A3 pulley of the fibrous flexor sheath at the level of the proximal interphalangeal joint to the lateral border of the lateral extensor band. The oblique retinacular ligament lies deep to the transverse retinacular ligament. It originates from the lateral aspect of the proximal phalanx and flexor sheath (A2 pulley) and passes volar to the axis of rotation of the proximal interphalangeal joint but in a dorsal and distal direction to insert into the terminal extensor tendon.

INTRINSIC MUSCLES OF THE HAND

The intrinsic muscles of the hand are organized into three groups plus a superficial muscle. Flexor pollicis brevis, abductor pollicis brevis, opponens pollicis and adductor pollicis all act on the thumb and are known collectively as the thenar muscles. Abductor digiti minimi, flexor digiti minimi brevis and opponens digiti minimi all act on the little finger and are known collectively as hypothenar muscles. Interossei and lumbricals act on the fingers. Palmaris brevis is a superficial muscle that lies beneath the ulnar palmar skin.

Flexor pollicis brevis

Fig. 50.31 Muscles of the left hand. Deep layer after cutting the flexor retinaculum and partial removal of several superficial muscles: palmar aspect.

Flexor pollicis brevis lies medial to abductor pollicis brevis (Fig. 50.31). It has superficial and deep parts. The superficial head arises from the distal border of the flexor retinaculum and the distal part of the tubercle of the trapezium, and passes along the radial side of the tendon of flexor pollicis longus. It is attached, by a tendon that contains a sesamoid bone, to the radial side of the base of the proximal phalanx of the thumb. The deep part arises from the trapezoid and capitate bones and from the palmar ligaments of the distal row of carpal bones, and passes deep to the tendon of flexor pollicis longus. It unites with the superficial head on the sesamoid bone and base of the first phalanx.

(From Sobotta 2006.)

The superficial head is frequently blended with opponens pollicis. The deep head varies considerably in size and may even be absent.

Flexor pollicis brevis lies superficial in the thenar eminence and is distal to abductor pollicis brevis. It is crossed by the motor branch of the median nerve.

Flexor pollicis brevis is supplied by the superficial palmar branch of the radial artery, and branches from the princeps pollicis artery and radialis indicis artery.

The superficial head of flexor pollicis brevis is usually innervated by the lateral terminal branch (motor branch) of the median nerve. The deep head is usually innervated by the deep branch of the ulnar nerve, C8 and T1. Some variation is common.

Flexor pollicis brevis flexes the metacarpophalangeal joint.

Flexor pollicis brevis is palpated in the thenar eminence while the subject flexes the metacarpophalangeal joint of the thumb, keeping the interphalangeal joint fully extended.

Abductor pollicis brevis

Abductor pollicis brevis is a thin, subcutaneous muscle in the proximolateral part of the thenar eminence (Fig. 50.31). It arises mainly from the flexor retinaculum, but a few fibres spring from the tubercles of the scaphoid bone and trapezium, and from the tendon of abductor pollicis longus. Its medial fibres are attached by a thin, flat tendon to the radial side of the base of the proximal phalanx of the thumb, and its lateral fibres join the dorsal digital expansion of the thumb. The muscle may receive accessory slips from the long and short extensors of the thumb, opponens pollicis, or the styloid process of the radius.

Abductor pollicis brevis lies proximomedial to flexor pollicis brevis in the superficial part of the thenar eminence.

Abductor pollicis brevis is supplied by the superficial palmar branch of the radial artery, and often by an independent branch directly from the radial artery lying on the radial aspect of thumb.

Abductor pollicis brevis is innervated by the lateral terminal branch (motor branch) of the median nerve, C8 and T1. Note that this is the only muscle of the thenar eminence which is constantly supplied by the median nerve.

Abductor pollicis brevis draws the thumb ventrally in a plane at right angles to the palm of the hand (abduction).

The hand is placed with its dorsum on the table. The subject’s thumb is placed so that the nail is in a plane at right angles to the palm of the hand. The subject is instructed to touch with his thumb an object held vertically above the thumb. This is a highly specific test for a median nerve lesion.

Opponens pollicis

Opponens pollicis lies deep to abductor pollicis brevis (Fig. 50.31). It arises from the tubercle of the trapezium and the flexor retinaculum, and is attached to the whole length of the lateral border, and the adjoining lateral half of the palmar surface of the metacarpal bone of the thumb.

Opponens pollicis lies deep between the other two muscles of the thenar eminence and is only revealed when they are retracted.

Opponens pollicis is supplied by the superficial palmar branch of the radial artery, and by branches from the first palmar metacarpal artery (when present), princeps pollicis artery, radialis indicis artery and the deep palmar arch.

Opponens pollicis is innervated by the lateral terminal branch of the median nerve, C8 and T1, and commonly by a branch of the deep terminal branch of the ulnar nerve.

Opponens pollicis flexes the metacarpal bone of the thumb.

The subject tries, against resistance, to touch the tip of the little finger with the thumb. The key observation is whether or not the thumb can be strongly adducted without flexing the IP joint.

Adductor pollicis

Adductor pollicis arises by oblique and transverse heads (Fig. 50.31). The oblique head is attached to the capitate bone, the bases of the second and third metacarpal bones, the palmar ligaments of the carpus and the sheath of the tendon of flexor carpi radialis. Most of the fibres converge into a tendon (which contains a sesamoid bone), which unites with the tendon of the transverse head, and is attached to the ulnar side of the base of the proximal phalanx of the thumb. The deepest fibres may pass into the medial side of the dorsal digital expansion of the thumb. On the lateral side of the oblique head a considerable fasciculus passes deep to the tendon of flexor pollicis longus to join flexor pollicis brevis; this has been described as the ‘deep head’ of flexor pollicis brevis. The transverse head is the deepest of the pollicial muscles, it is triangular, and arises from the distal two-thirds of the palmar surface of the third metacarpal. The fibres converge to be attached, with the oblique head and the first palmar interosseous, to the base of the proximal phalanx of the thumb. The two parts of the adductor vary in relative size and degree of connection.

The deep palmar arch and the deep branch of the ulnar nerve pass between the two heads of the muscle. Anteriorly adductor pollicis is crossed by the flexor tendons of the index finger and their sheath, and the first lumbrical, and is overlapped by flexor pollicis brevis. Posteriorly it abuts against the first dorsal interosseous muscle: together these muscles form the mass of the first web space of the hand.

Adductor pollicis is supplied by the arteria princeps pollicis and the arteria radialis indicis (which are sometimes combined as the first palmar metacarpal artery) and by branches from the deep palmar arch.

Adductor pollicis is innervated by the deep branch of the ulnar nerve, C8 and T1.

Adductor pollicis approximates the thumb to the palm of the hand. It acts to greatest advantage when the abducted, rotated and flexed thumb is opposed to the fingers in gripping.

With the thumb lying along the palmar aspect of the index finger, and with its nail in a plane at right angles to the palm, the subject is asked to retain a strip of paper between the thumb and palm against resistance.

Abductor digiti minimi

The muscle may have two or three slips and may be fused with flexor digiti minimi brevis. An additional slip may arise from flexor retinaculum, antebrachial fascia, or tendons of palmaris longus or flexor carpi ulnaris. It may be partly attached to the fifth metacarpal by a slip from the pisiform.

Abductor digiti minimi arises from the pisiform bone, the tendon of flexor carpi ulnaris and the pisohamate ligament (Fig. 50.31). It ends in a flat tendon which divides into two slips: one is attached to the ulnar side of the base of the proximal phalanx of the little finger, and the other to the ulnar border of the dorsal digital expansion of extensor digiti minimi.

Abductor digiti minimi lies along the ulnar border of flexor digiti minimi brevis and overlies opponens digiti minimi.

Abductor digiti minimi is supplied by the deep palmar branch of the ulnar artery, branches from the ulnar end of the superficial palmar arch and the palmar digital artery for the medial border of the little finger.

Abductor digiti minimi is innervated by the deep branch of the ulnar nerve, C8 and T1.

Abductor digiti minimi abducts the little finger away from the fourth, e.g. in the habitual spreading of the digits when they are extended. Abduction is also possible when digits 2–4 are tightly adducted in flexion or extension.

Abductor digiti minimi is tested by abducting the little finger against resistance.

Flexor digiti minimi brevis

Flexor digiti minimi brevis arises from the convex surface of the hook of the hamate and the palmar surface of the flexor retinaculum. It inserts into the ulnar side of the base of the proximal phalanx of the little finger with abductor digiti minimi.

It may be absent, or fused with the abductor, and it may attach to the distal end of the fifth metacarpal by a muscular slip.

Flexor digiti minimi brevis lies lateral to the abductor. Its origin is separated from that of the abductor by the deep branches of the ulnar artery and nerve.

Flexor digiti minimi brevis is supplied by the deep palmar branch of the ulnar artery, branches from the ulnar end of the superficial palmar arch, and the palmar digital artery for the medial border of the little finger.

Flexor digiti minimi brevis is innervated by the deep branch of the ulnar nerve, C8 and T1.

Flexor digiti minimi brevis produces flexion of the little finger at its metacarpophalangeal joint, together with some lateral rotation.

The subject flexes the metacarpophalangeal joint of the little finger while the examiner maintains the interphalangeal joints in extension.

Opponens digiti minimi

Opponens digiti minimi is a triangular muscle, lying under cover of the flexor and abductor. It arises from the convexity of the hook of the hamate, and the contiguous portion of the flexor retinaculum, and inserts along the whole length of the ulnar margin of the fifth metacarpal bone, and the adjacent palmar surface.

The muscle is often divided into two lamellae by the deep branches of the ulnar artery and nerve. It blends to a variable degree with its neighbours.

Opponens digiti minimi lies on the ulnar margin and adjacent palmar surface of the fifth metacarpal, overlapped by, and often partly fused with, the other two muscles of the hypothenar eminence.

Opponens digiti minimi is supplied by the deep palmar branch of the ulnar artery, and branches from the medial end of the deep palmar arch.

Opponens digiti minimi is innervated by the deep branch of the ulnar nerve, C8 and T1.

Opponens digiti minimi flexes the fifth metacarpal bone, drawing it forwards and rotating it laterally at the carpometacarpal joint: this deepens the hollow of the palm. These actions, together with flexion and some lateral rotation at the metacarpophalangeal and interphalangeal joints, bring the digit into opposition with the thumb.

The subject is asked to place the tip of the little finger, against resistance, onto the tip of the opposed thumb while the examiner maintains the interphalangeal joints of the little finger in extension.

Palmaris brevis

Palmaris brevis is a thin, quadrilateral muscle, lying beneath the skin of the ulnar side of the palm (see Fig. 50.4). It arises from the flexor retinaculum and the medial border of the central part of the palmar aponeurosis, and is attached to the dermis on the ulnar border of the hand.

Palmaris brevis is superficial to the ulnar artery and the superficial terminal branch of the ulnar nerve.

Palmaris brevis is supplied by branches from the ulnar end of the superficial palmar arch.

Palmaris brevis is innervated by the superficial branch of the ulnar nerve, C8 and T1.

Palmaris brevis wrinkles the skin on the ulnar side of the palm of the hand and deepens the hollow of the palm by accentuating the hypothenar eminence. In this way it may contribute to the security of the palmar grip.

Against resistance, the subject touches the tip of the opposed thumb with the tip of the little finger; the skin on the ulnar side of the palm is seen to pucker.

Interossei

The interossei occupy the intervals between the metacarpal bones, and are divided into a palmar and a dorsal set.

Palmar interossei

Fig. 50.32 Palmar interossei of the left hand: palmar aspect. Note that there is no first palmar interosseous in this specimen.

Palmar interossei are smaller than dorsal interossei and lie on the palmar surfaces of the metacarpal bones rather than between them (Fig. 50.32). With the exception of the first, each of the three arises from the entire length of the metacarpal bone of one finger, and passes to the appropriate (adductor) side of the dorsal digital expansion.

(From Sobotta 2006.)

The middle finger has no palmar interosseus. The remaining digits have palmar interossei on their aspects which face the middle finger. The first arises from the ulnar side of the palmar surface of the base of the first metacarpal bone, and is inserted into a sesamoid bone on the ulnar side of the proximal phalanx and from there passes to the phalanx and usually also into the dorsal digital expansion. It lies in front of the lateral head of the first dorsal interosseous, and is overlapped anteriorly by the oblique head of adductor pollicis (Fig. 50.31, Fig. 50.32). It is often very rudimentary because the thumb has its own powerful adductor. The second arises from the ulnar side of the second metacarpal bone, and is inserted into the same side of the digital expansion of the index finger. The third arises from the radial side of the fourth metacarpal bone, and is inserted together with the third lumbrical. The fourth arises from the radial side of the fifth metacarpal bone, and is attached with the fourth lumbrical and also to the base of the proximal phalanx. The attachment of these muscles to the dorsal digital expansions (Fig. 50.30) stabilizes the extensor tendons on the convex heads of the metacarpal bones during flexion and extension at the metacarpophalangeal joints.

The interossei show little variation in their arrangement. They are occasionally reduplicated.

The first palmar interosseous lies anterior to the lateral head of the first dorsal interosseous and is overlapped anteriorly by the oblique head of adductor pollicis, which also crosses anterior to the second palmar interosseous. The third and fourth are overlapped by the long flexor tendons of the ring and little finger respectively within their flexor sheaths.

Palmar interossei are supplied by branches from the deep palmar arch, princeps pollicis artery, radialis indicis artery, palmar metacarpal arteries, proximal and distal perforating arteries and common and proper digital (palmar) arteries.

All the interossei are innervated by the deep branch of the ulnar nerve, C8 and T1.

Palmar interossei adduct the fingers to an imaginary longitudinal axis through the centre of the middle finger. Interossei have a considerable cross-sectional area and therefore contribute strongly to metacarpophalangeal joint flexion and interphalangeal extension. When paralysed, as in ulnar nerve paralysis, the grip strength of the hand is severely impaired and the arc of finger motion is abnormal, with a tendency for the fingers to claw. Each interosseous has a considerable ability to rotate the digit at the metacarpophalangeal joint. Generally this is not obvious, because interossei act in pairs, but it may occur where one interosseus is deficient as a result of injury or congenital deformity.

Adduction of the index, ring and little fingers against resistance. The subject grips a piece of paper between the index and middle fingers with the hand placed flat on the table. The examiner can assess the strength of adduction on trying to pull the paper away. This is a reliable test of ulnar nerve integrity.

Dorsal interossei

Fig. 50.33 Dorsal interossei of the left hand: palmar aspect.

Dorsal interossei consist of four bipennate muscles, each arising from the adjacent sides of two metacarpal bones, but more extensively from the metacarpal bone of the finger into which the muscle passes (Fig. 50.33). They insert on the bases of the proximal phalanges and separately into the dorsal digital expansions. Between the double origin of each of these muscles there is a narrow triangular interval. The radial artery passes through the first of these intervals, and a perforating branch from the deep palmar arch passes through each of the others. The first and largest muscle is sometimes named abductor indicis. It is attached to the radial side of the proximal phalanx of the index finger and to the capsule of the adjoining metacarpophalangeal joint. The second and third are attached to the radial and ulnar sides of the middle finger, respectively. Whereas the second generally reaches the digital expansion and the proximal phalanx, the third usually extends only to the digital expansion (Fig. 50.30). The fourth may be wholly attached to the digital expansion, but it often sends an additional slip to the proximal phalanx.

(From Sobotta 2006.)

The first dorsal interosseous abuts anteriorly with adductor pollicis. The other dorsal interossei occupy the spaces between the second and the fifth metacarpal bones.

Dorsal interossei are supplied by the dorsal metacarpal arteries (1st–4th), palmar metacarpal arteries (2nd–4th), radial artery (1st); princeps pollicis artery, radialis indicis artery, three perforating branches from the deep palmar arch (proximal perforating arteries), and three distal perforating branches. The tendons are supplied by branches from the common and proper palmar digital arteries and the dorsal digital arteries.

All the interossei are innervated by the deep branch of the ulnar nerve, C8 and T1.

Dorsal interossei abduct the fingers from an imaginary longitudinal axis through the centre of the middle finger. See also Action of palmar interossei.

Abduction of the index, middle and ring fingers against resistance. Most conveniently, the first dorsal interosseous is tested with the subject’s fingers and palm flat upon the table. The subject tries to abduct the index finger against the examiner’s resistance. The muscle belly can be felt and seen. This provides a reliable test for the integrity of the ulnar nerve.

Lumbricals

Fig. 50.34 The lumbricals of the left hand.

The lumbricals are four small fasciculi which arise from the tendons of flexor digitorum profundus (Fig. 50.34). The first and second arise from the radial sides and palmar surfaces of the tendons of the index and middle fingers respectively. The third arises from the adjacent sides of the tendons of the middle and ring fingers, and the fourth from the adjoining sides of the tendons of the ring and little fingers. Each passes to the radial side of the corresponding finger, and is attached to the lateral margin of the dorsal digital expansion of extensor digitorum which covers the dorsal surface of the finger.

(From Sobotta 2006.)

Variations in the attachments of the lumbricals are common. Any of them may be unipennate or bipennate. When they are bipennate, the two heads arise from adjoining tendons of flexor digitorum profundus, and, in the case of the first lumbrical, from the tendon of flexor pollicis longus. Accessory lumbrical slips may be attached to an adjacent tendon of flexor digitorum superficialis.

The first and second lumbricals are supplied by the first and second dorsal metacarpal and dorsal digital arteries. The third and fourth lumbricals are supplied by the second and third common palmar digital arteries, and the third and fourth dorsal digital arteries and their anastomoses with the palmar digital arteries.

The first and second lumbricals are innervated by the median nerve, C8 and T1, and the third and fourth lumbricals by the deep terminal branch of the ulnar nerve, C8 and T1. The third lumbrical frequently receives a supply from the median nerve. The first and second lumbricals are occasionally innervated by the deep terminal branch of the ulnar nerve.

Lumbricals arise from flexor tendons and insert into the extensor apparatus. Since both attachments are mobile, they have the potential for producing movement at either. The action on the extensor apparatus is easier to understand and consists of extension of both interphalangeal joints in a coordinated manner. The mode of action at the metacarpophalangeal joints is disputed, but if there is a flexor action it is very weak. The effect on the flexor digitorum profundus attachment is to pull the tendon distally. The combined action on both origin and insertion is therefore to alter the posture of the finger to allow more interphalangeal extension. Pinching the index finger against the thumb without a lumbrical would result in a nail-to-nail contact: the addition of the lumbrical increases the interphalangeal joint extension, and results in pulp-to-pulp pinch. Lumbricals contain many muscle spindles and have a long fibre length: it is therefore reasonable to assume that they play a role in proprioception.