The elbow and forearm complex consists of three bones and four joints (Figure 6-1). The humero-ulnar and humeroradial joints form the elbow. The motions of flexion and extension of the elbow provide a means to adjust the overall functional length of the upper limb. This mechanism is used for many important activities, such as feeding, reaching, throwing, and personal hygiene.

FIGURE 6-1. The articulations of the elbow and forearm complex.

The radius and ulna articulate with each other within the forearm at the proximal and distal radio-ulnar joints. This pair of articulations allows the palm of the hand to be turned up (supinated) or down (pronated), without requiring motion of the shoulder. Supination and pronation can be performed in conjunction with, or independent from, elbow flexion and extension. The interaction between the elbow and forearm joints greatly increases the range of effective hand placement.

Four Articulations within the Elbow and Forearm Complex

1. Humero-ulnar joint

2. Humeroradial joint

3. Proximal radio-ulnar joint

4. Distal radio-ulnar joint

OSTEOLOGY

Mid-to-Distal Humerus

The anterior and posterior surfaces of the mid-to-distal humerus provide proximal attachments for the brachialis and the medial head of the triceps brachii (Figures 6-2 and 6-3). The distal end of the shaft of the humerus terminates medially as the trochlea and the medial epicondyle, and laterally as the capitulum and lateral epicondyle. The trochlea resembles a rounded, empty spool of thread. The medial and lateral borders of the trochlea flare slightly to form medial and lateral lips. The medial lip is prominent and extends farther distally than the adjacent lateral lip. Midway between the medial and lateral lips is the trochlear groove, which, when one looks from posterior to anterior, spirals slightly toward the medial direction (Figure 6-4). The coronoid fossa is located just proximal to the anterior aspect of the trochlea (see Figure 6-2).

FIGURE 6-2. The anterior aspect of the right humerus. The muscles’ proximal attachments are shown in red. The dashed lines show the capsular attachments of the elbow joint.

FIGURE 6-3. The posterior aspect of the right humerus. The muscles’ proximal attachments are shown in red. The dashed lines show the capsular attachments around the elbow joint.

FIGURE 6-4. The distal end of the right humerus, inferior view.

Directly lateral to the trochlea is the rounded capitulum. The capitulum forms nearly one half of a sphere. A small radial fossa is located proximal to the anterior surface of the capitulum.

The medial epicondyle of the humerus projects medially from the trochlea (see Figures 6-2 and 6-4). This prominent and easily palpable structure serves as the proximal attachment of the medial collateral ligament of the elbow as well as most forearm pronator and wrist flexor muscles.

The lateral epicondyle of the humerus, less prominent than the medial epicondyle, serves as the proximal attachment for the lateral collateral ligament complex of the elbow as well as most forearm supinator and wrist extensor muscles. Immediately proximal to both epicondyles are the medial and lateral supracondylar ridges, which are relatively superficial and easily palpated.

On the posterior aspect of the humerus, just proximal to the trochlea, is the very deep and broad olecranon fossa. Only a thin sheet of bone or membrane separates the olecranon fossa from the coronoid fossa.

Ulna

The ulna has a very thick proximal end with distinct processes (Figures 6-5 and 6-6). The olecranon process forms the large, blunt, proximal tip of the ulna, making up the “point” of the elbow (Figure 6-7). The roughened posterior surface of the olecranon process accepts the insertion of the triceps brachii. The coronoid process projects sharply from the anterior body of the proximal ulna.

FIGURE 6-5. The anterior aspect of the right radius and ulna. The muscles’ proximal attachments are shown in red and distal attachments in gray. The dashed lines show the capsular attachments around the elbow and wrist and the proximal and distal radio-ulnar joints. The radial head is depicted from above to show the concavity of the fovea.

FIGURE 6-6. The posterior aspect of the right radius and ulna. The muscles’ proximal attachments are shown in red and distal attachments in gray. The dashed lines show the capsular attachments around the elbow and wrist and the proximal and distal radio-ulnar joints.

FIGURE 6-7. A lateral (radial) view of the right proximal ulna, with the radius removed. Note the jawlike shape of the trochlear notch.

The trochlear notch of the ulna is the large jawlike process located between the anterior tips of the olecranon and coronoid processes. This concave notch articulates firmly with the reciprocally shaped trochlea of the humerus, forming the humero-ulnar joint. A thin raised longitudinal crest divides the trochlear notch down its midline.

The radial notch of the ulna is an articular depression just lateral to the inferior aspect of the trochlear notch (see Figures 6-5 and 6-7). Extending distally and slightly dorsally from the radial notch is the supinator crest, marking the attachments for part of the lateral collateral ligament complex and the supinator muscle. The tuberosity of the ulna is a roughened impression just distal to the coronoid process, formed by the attachment of the brachialis muscle (see Figure 6-5).

The ulnar head is located at the distal end of the ulna (Figure 6-8). Most of the rounded ulnar head is covered with articular cartilage. The pointed styloid process (from the Greek root stylos, pillar) projects distally from the posterior-medial region of the extreme distal ulna.

FIGURE 6-8. The distal end of the right radius and ulna with carpal bones removed. The forearm is in full supination. Note the prominent ulnar head and nearby styloid process of the ulna.

Radius

In the fully supinated position, the radius lies parallel and lateral to the ulna (see Figures 6-5 and 6-6). The proximal end of the radius is small and therefore constitutes a relatively small structural component of the elbow. Its distal end, however, is enlarged, forming a major part of the wrist joint.

Osteologic Features of the Radius

• Head

• Neck

• Fovea

• Radial (bicipital) tuberosity

• Ulnar notch

• Styloid process

The radial head is a disclike structure located at the extreme proximal end of the radius. Articular cartilage covers about 280 degrees of the rim of the radial head.⁴⁴ The rim of the radial head contacts the radial notch of the ulna, forming the proximal radio-ulnar joint. Immediately inferior to the radial head is the constricted radial neck (see Figure 6-5).

The superior surface of the radial head consists of a shallow, cup-shaped depression known as the fovea. This cartilage-lined concavity articulates with the capitulum of the humerus, forming the humeroradial joint. The biceps brachii muscle attaches to the radius at the radial (bicipital) tuberosity, a roughened region located at the anterior-medial edge of the proximal radius.

The distal end of the radius articulates with carpal bones to form the radiocarpal joint at the wrist (see Figure 6-8). The ulnar notch of the distal radius accepts the ulnar head at the distal radio-ulnar joint. The prominent styloid process projects from the lateral surface of the distal radius, extending approximately 1 cm farther distal than the ulnar styloid process.

ARTHROLOGY

Joints of the Elbow

GENERAL FEATURES OF THE HUMERO-ULNAR AND HUMERORADIAL JOINTS

The elbow consists of the humero-ulnar and humeroradial articulations. The tight fit between the trochlea and trochlear notch at the humero-ulnar joint provides most of the elbow’s structural stability.

Early anatomists classified the elbow as a ginglymus or hinged joint owing to its predominant uniplanar motion of flexion and extension. The term modified hinge joint is actually more appropriate because the ulna experiences a slight amount of axial rotation (i.e., rotation around its own longitudinal axis) and side-to-side motion as it flexes and extends.⁴⁸ Bioengineers must account for these relatively small “extra-sagittal” accessory motions in the design of elbow joint prostheses. Without attention to this detail, the prosthetic implants are more likely to loosen prematurely.^13,29

Normal “Valgus Angle” of the Elbow: Elbow flexion and extension occur around a relatively stationary medial-lateral axis of rotation, passing through the vicinity of the lateral epicondyle (Figure 6-9, A).⁷¹ From medial to lateral, the axis courses slightly superiorly owing in part to the distal prolongation of the medial lip of the trochlea. This asymmetry in the trochlea causes the ulna to deviate laterally relative to the humerus. The natural frontal plane angle made by the extended elbow is referred to as normal cubitus valgus. (The term “carrying angle” is often used, reflecting the fact that the valgus angle tends to keep carried objects away from the side of the thigh during walking.) Paraskevas and co-workers reported an average cubitus valgus angle in healthy persons of about 13 degrees, with a standard deviation close to 6 degrees.⁵⁸ On average, women had a greater valgus angulation than men by about 2 degrees. Two studies using a large sample of normal subjects have shown that, regardless of gender, valgus angle is greater on the dominant arm.^58,85

FIGURE 6-9. A, The elbow’s axis of rotation (green line) extends slightly obliquely in a medial-lateral direction through the capitulum and the trochlea. Normal cubitus valgus of the elbow is shown with an angle of about 15 degrees from the longitudinal axis of the humerus. B, Excessive cubitus valgus deformity is shown with the forearm deviated laterally 30 degrees. C, Cubitus varus deformity is depicted with the forearm deviated medially 5 degrees.

Occasionally the extended elbow may show an excessive cubitus valgus that exceeds about 20 or 25 degrees (see Figure 6-9, B). In contrast, the forearm may less commonly show a cubitus varus (or “gunstock”) deformity, where the forearm is deviated toward the midline (see Figure 6-9, C). The terms valgus and varus are derived from the Latin turned outward (abducted) and turned inward (adducted), respectively.

A marked varus or valgus deformity may result from trauma, such as a severe fracture through the “growth plate” of the distal humerus in children. Excessive cubitus valgus may overstretch and damage the ulnar nerve as it crosses medial to the elbow.¹¹

PERIARTICULAR CONNECTIVE TISSUE

The articular capsule of the elbow encloses the humero-ulnar joint, the humeroradial joint, and the proximal radio-ulnar joint (Figure 6-10). The articular capsule surrounding these joints is thin and reinforced anteriorly by oblique bands of fibrous tissue. A synovial membrane lines the internal surface of the capsule (Figure 6-11).

FIGURE 6-10. An anterior view of the right elbow showing the capsule and collateral ligaments.

FIGURE 6-11. Anterior view of the right elbow disarticulated to expose the humero-ulnar and humeroradial joints. The margin of the proximal radio-ulnar joint is shown within the elbow’s capsule. Note the small area on the trochlear notch lacking articular cartilage. The synovial membrane lining the internal side of the capsule is shown in blue.

The articular capsule of the elbow is strengthened by collateral ligaments. These ligaments provide an important source of stability to the elbow joint. Motions that increase the tension in the ligaments are listed in Table 6-1. The medial collateral ligament consists of anterior, posterior, and transverse fiber bundles (Figure 6-12). The anterior fibers are the strongest and stiffest fibers of the medial collateral ligament.⁶³ As such, these fibers provide the most significant resistance against a valgus (abduction) force to the elbow. The anterior fibers arise from the anterior part of the medial epicondyle and insert on the medial part of the coronoid process of the ulna.¹⁸ Because the anterior fibers span both sides of the axis of rotation, at least some fibers are taut throughout sagittal plane movement. The anterior fibers therefore provide articular stability throughout the entire range of motion.¹⁰

TABLE 6-1.

Motions That Increase Tension in the Collateral Ligaments of the Elbow

^*Primary valgus or varus stabilizers.

FIGURE 6-12. The components of the medial collateral ligament of the right elbow.

The posterior fibers of the medial collateral ligament are less defined than the anterior fibers and are essentially thickenings of the posterior-medial capsule. As depicted in Figure 6-12, the posterior fibers attach on the posterior part of the medial epicondyle and insert on the medial margin of the olecranon process. The posterior fibers resist a valgus force, as well as become taut in the extremes of elbow flexion.⁶³ A third and poorly developed set of transverse fibers cross from the olecranon to the coronoid process of the ulna. Because these fibers originate and insert on the same bone, they do not provide significant articular stability.

In addition to the medial collateral ligaments, the proximal fibers of the wrist flexor and pronator group of muscles also resist excessive valgus-producing strain at the elbow, most notably by the flexor carpi ulnaris. For this reason, these muscles are referred to as dynamic medial stabilizers of the elbow.³⁸

The medial collateral ligament is susceptible to injury when the fully extended elbow is violently forced into excessive valgus, often from a fall onto an outstretched arm and hand (Figure 6-13).¹² The ligamentous injury may be associated with a fracture within the humeroradial joint or anywhere along the length of the radius—the forearm bone that accepts 80% of the compression force applied through the wrist. A severe valgus-producing force may also injure the ulnar nerve or proximal attachments of the pronator–wrist flexor muscles. The anterior capsule may also be injured if the joint is excessively hyperextended. The medial collateral ligament is also susceptible to injury from repetitive, valgus-producing forces to the elbow in non–weight-bearing activities, such as pitching a baseball and spiking a volleyball.^65,83

FIGURE 6-13. Attempts at catching oneself from a fall may create a severe valgus-producing force to the elbow, causing rupture of the medial collateral ligament and a potentially damaging compression force within the humeroradial joint.

The lateral collateral ligament complex of the elbow is more variable in form than the medial collateral ligament (Figure 6-14). The ligamentous complex originates on the lateral epicondyle and immediately splits into two fiber bundles. One fiber bundle, traditionally known as the radial collateral ligament, fans out to blend with the annular ligament. A second fiber bundle, called the lateral (ulnar) collateral ligament, attaches distally to the supinator crest of the ulna. These fibers become taut during full flexion.⁶³ By attaching to the ulna, the lateral (ulnar) collateral ligament and the anterior fibers of the medial collateral ligament function as collateral “guy wires” to the elbow, providing medial-lateral stability to the ulna during sagittal plane motion.

FIGURE 6-14. The components of the lateral collateral ligament complex of the right elbow.

The lateral collateral ligament complex and the posterior-lateral aspect of the capsule are primary stabilizers against a varus-producing force.⁵⁶ Often after a single traumatic sporting event, rupture of this ligament system can cause not only increased varus (“adduction”) of the elbow, but also posterior-lateral rotary instability. This instability is expressed as excessive external rotation of the forearm with subsequent subluxation of both the humero-ulnar and humeroradial joints.^19,54

The ligaments around the elbow are endowed with mechanoreceptors consisting of Golgi organs, Ruffini terminals, Pacinian corpuscles, and free nerve endings.⁵⁹ These receptors may supply important information to the nervous system for augmenting proprioception and detecting safe limits of passive tension in the structures around the elbow.

As all joints do, the elbow joint has an intracapsular air pressure. This pressure, which is determined by the ratio of the volume of air to the volume of space, is lowest at about 80 degrees of flexion.²⁴ This joint position is often considered the “position of comfort” for persons with joint inflammation and swelling. Maintaining a swollen elbow in a flexed position may improve comfort but may predispose the person to an elbow flexion contracture (from the Latin root contractura, to draw together).

KINEMATICS

Functional Considerations of Flexion and Extension: Elbow flexion performs several important physiologic functions, such as pulling, lifting, feeding, and grooming.⁷⁷ The inability to actively bring the hand to the mouth for feeding, for example, significantly limits the level of functional independence. Persons with a spinal cord injury above the C⁵ nerve root may experience this profound functional impairment because of paralysis of elbow flexor muscles.

Elbow extension occurs with activities such as throwing, pushing, and reaching. Loss of complete extension because of an elbow flexion contracture is often caused by marked stiffness in the elbow flexor muscles. The muscles become abnormally stiff after long periods of immobilization in a flexed and shortened position. Long-term flexion may be the result of casting for a fractured bone or of posttraumatic heterotopic ossification, osteophyte formation, elbow joint inflammation and effusion, muscle spasticity, paralysis of the triceps muscle, or scarring of the skin over the anterior elbow. In addition to the tightness in the flexor muscles, increased stiffness may occur in the anterior capsule and some anterior fibers of the medial collateral ligament.

SPECIAL FOCUS 6-1 Elbow Flexion Contracture and Loss of Forward Reach

A flexion contracture is a tightening of muscular or nonmuscular tissues that restricts normal passive extension. One of the most disabling consequences of an elbow flexion contracture is reduced reaching capacity. The loss of forward reach varies with the degree of elbow flexion contracture. As shown in Figure 6-15, a fully extendable elbow (i.e., with a 0-degree contracture) demonstrates a 0-degree loss in area of forward reach. The area of forward reach diminishes only slightly (less than 6%) with a flexion contracture of less than 30 degrees. A flexion contracture that exceeds 30 degrees, however, results in a much greater loss of forward reach. As noted in the graph, a flexion contracture of 90 degrees reduces total reach by almost 50%. Minimizing a flexion contracture to less than 30 degrees is therefore an important functional goal for patients. Therapeutics typically used to reduce an elbow flexion contracture include reducing inflammation and swelling, positioning the joint in more extension (through splinting, continuous passive-motion machines, or frequent encouragement), stretching structures located anterior to the joint’s medial-lateral axis of rotation, manually mobilizing the joint, and strengthening muscles that produce elbow extension. If these relative conservative treatments are ineffective, then a surgical release may be indicated.⁷⁹ The most effective intervention for elbow flexion contracture, however, is prevention.

FIGURE 6-15. A graph showing the percent loss in area of forward reach of the arm, from the shoulder to finger, as a function of the severity of an elbow flexion contracture. Note the sharp increase in the reduction in reach as the flexion contracture exceeds 30 degrees. The figures across the bottom of the graph depict the progressive loss of reach, indicated by the increased semicircular area, as the flexion contracture becomes more severe.

The maximal range of passive motion generally available to the elbow is from 5 degrees beyond neutral (0 degree) extension through 145 degrees of flexion (Figure 6-16). Research indicates, however, that several common activities of daily living use a more limited “functional arc” of motion, usually between 30 and 130 degrees of flexion.⁴⁷ Unlike in lower extremity joints, such as the knee, the loss of the extremes of motion at the elbow usually results in only minimal functional impairment.

FIGURE 6-16. Range of motion at the elbow. A, A healthy person showing an average range of elbow motion from 5 degrees beyond neutral extension through 145 degrees of flexion. The 100-degree “functional arc” from 30 to 130 degrees of flexion (in red) is based on the data in the histogram. B, The histogram shows the range of motion at the elbow typically needed to perform the following activities of daily living: opening a door, pouring from a pitcher, rising from a chair, holding a newspaper, cutting with a knife, bringing a fork to the mouth, bringing a glass to the mouth, and holding a telephone. (Modified from Morrey BF, Askew LJ, Chao EY: A biomechanical study of normal functional elbow motion, J Bone Joint Surg Am 63:872, 1981.)

Arthrokinematics at the Humero-Ulnar Joint: The humero-ulnar joint is the articulation between the concave trochlear notch of the ulna and the convex trochlea of the humerus (Figure 6-17). Hyaline cartilage covers about 300 degrees of articular surface on the trochlea, compared with only 180 degrees on the trochlear notch. The natural congruency and shape of this joint limits motion primarily within the sagittal plane.

FIGURE 6-17. A sagittal section through the humero-ulnar joint showing the well-fitting joint surfaces between the trochlear notch and trochlea. The synovial membrane lining the internal side of the capsule is shown in blue.

In order for the humero-ulnar joint to be fully extended, sufficient extensibility is required in the dermis anterior to the elbow, flexor muscles, anterior capsule, and anterior fibers of the medial collateral ligament (Figure 6-18, A). Full extension also requires that the prominent tip of the olecranon process become wedged into the olecranon fossa. Excessive ectopic (from the Greek root ecto, outside, + topos, place) bone formation around the olecranon fossa can therefore limit full extension. Normally, once in extension, the healthy humero-ulnar joint is stabilized primarily by articular congruency and also by the increased tension in the stretched connective tissues.

FIGURE 6-18. A sagittal section through the humero-ulnar joint. A, The joint is resting in full extension. B, The joint is passively flexed through full flexion. Note that in full flexion the coronoid process of the ulna fits into the coronoid fossa of the humerus. The medial-lateral axis of rotation is shown through the center of the trochlea. The stretched (taut) structures are shown as thin elongated arrows, and slackened structures are shown as wavy arrows. AC, anterior capsule; MCL-Anterior, some anterior fibers of the medial collateral ligament; MCL-Posterior, posterior fibers of the medial collateral ligament; PC, posterior capsule.

During flexion at the humero-ulnar joint, the concave surface of the trochlear notch rolls and slides on the convex trochlea (see Figure 6-18, B). Full elbow flexion requires elongation of the posterior capsule, extensor muscles, ulnar nerve,^68,73 and certain portions of the collateral ligaments, especially the posterior fibers of the medial collateral ligament. Stretching of the ulnar nerve from prolonged or repetitive elbow flexion activities can lead to neuropathy. A common surgical treatment for this condition is to transfer the ulnar nerve anterior to the medial epicondyle, thereby reducing the tension in the nerve during flexion.⁴³

In severe elbow injuries the trochlear notch of the ulna may dislocate posterior to the trochlea of the humerus. This dislocation is frequently caused from a fall onto an outstretched arm and hand and therefore may also be associated with a fracture of the radius.

Arthrokinematics at the Humeroradial Joint: The humeroradial joint is an articulation between the cuplike fovea of the radial head and the reciprocally shaped rounded capitulum. The arthrokinematics of flexion and extension consist of the fovea of the radius rolling and sliding across the convexity of the capitulum (Figure 6-19). During active flexion, the radial fovea is pulled firmly against the capitulum by contracting muscles.⁴⁶

FIGURE 6-19. A sagittal section through the humeroradial joint during passive flexion. Note the medial-lateral axis of rotation in the center of the capitulum. The stretched (taut) structures are shown as thin elongated arrows, and slackened structures are shown as wavy arrows. Note the elongation of the lateral (ulnar) collateral ligament during flexion.

Compared with the humero-ulnar joint, the humeroradial joint provides minimal sagittal plane stability to the elbow. The humeroradial joint does, however, provide about 50% of the resistance against a valgus-producing force to the elbow.⁴⁹

Structure and Function of the Interosseous Membrane

The radius and ulna are bound together by the interosseous membrane of the forearm (Figure 6-20). Although several accessory fibers have been described, the more prominent central bands are directed distal-medially from the radius, intersecting the shaft of the ulna at about 20 degrees.⁶⁹ The central bands are nearly twice the thickness of other fibers and have an ultimate tensile strength similar to that of the patellar tendon of the knee.⁶¹ A few separate sparse and poorly defined bands flow perpendicular to the central bands of the interosseous membrane. One of these bands, the oblique cord, runs from the lateral side of the tuberosity of the ulna to just distal to the radial tuberosity. Another unnamed band is located at the extreme distal end of the interosseous membrane.

FIGURE 6-20. An anterior view of the right forearm, highlighting the components of the interosseous membrane.

The primary functions of the interosseous membrane are to bind the radius to the ulna, serve as a stable attachment site for several extrinsic muscles of the hand, and provide a mechanism for transmitting force proximally through the upper limb. As illustrated in Figure 6-21, about 80% of the compression force that crosses the wrist is directed through the radiocarpal joint. (This fact accounts, in part, for the relatively high likelihood of fracturing the radius from a fall on an outstretched hand.) The remaining 20% of the force crosses the medial side of the wrist, through the soft tissues located within the “ulnocarpal space.”⁵⁷ Because of the fiber direction of the central bands of the interosseous membrane, part of the proximal directed force through the radius is transferred across the membrane to the ulna.⁶⁰ This mechanism allows a significant portion of the compression force that naturally acts on the radius to cross the elbow via the humero-ulnar joint.⁴⁶ In this way, both the humero-ulnar and humeroradial joints more equally “share” the compression forces that cross the elbow, thereby reducing each individual joint’s long-term wear and tear.

FIGURE 6-21. A compression force through the hand is transmitted primarily through the wrist (1) at the radiocarpal joint and to the radius (2). This force pulls the interosseous membrane taut (shown by two black arrows), thereby transferring a significant part of the compression force to the ulna (3) and across the elbow at the humero-ulnar joint (4). The compression forces that cross the elbow are finally directed toward the shoulder (5). The stretched (taut) structures are shown as thin elongated arrows.

Most elbow flexors, and essentially all primary supinator and pronator muscles, have their distal attachment on the radius. As a consequence, contraction of these muscles pulls the radius proximally against the capitulum of the humerus, especially when the elbow is near full extension. Biomechanical analysis indicates that the resulting compression force at the humeroradial joint reaches three to four times body weight during maximal-effort activities.² Based on the mechanism described in Figure 6-21, the interosseous membrane helps shunt some of the muscular-produced compression forces from the radius to the ulna. In this way the interosseous membrane helps protect the humeroradial joint from large myogenic compression forces. Tears within the interosseous membrane can cause a measurable proximal migration of the radius due to activation of the regional muscles, leading to increased loading and possible degeneration at the humeroradial joint.^32,55 In cases where the head of the radius has been surgically removed because of trauma, the proximal migration is typically pronounced.²⁶ Over time, this proximal “drift” of the radius can cause bony asymmetry in the wrist or distal radio-ulnar joint, causing significant pain and loss of function.¹⁶

The predominant fiber direction of the interosseous membrane is not aligned to resist distally applied forces on the radius. For example, holding a heavy suitcase with the elbow extended causes a distracting force almost entirely through the radius (Figure 6-22). The distal pull on the radius slackens, rather than tenses, most of the interosseous membrane, thereby placing larger demands on other tissues, such as the oblique cord and annular ligament, to accept the weight of the load. Contraction of the brachioradialis or other muscles normally involved with grasp can assist with holding the radius and load firmly against the capitulum of the humerus. A deep aching in the forearm in persons who carry heavy loads for extended periods may be from fatigue in these muscles. Supporting loads through the forearm at shoulder level, for example, like a waiter carrying a tray of food, directs the weight proximally through the radius, so that the interosseous membrane can assist with dispersing the load more evenly through the forearm.

FIGURE 6-22. Holding a load, such as a suitcase, places a distal-directed distracting force predominantly through the radius. This distraction slackens most of the interosseous membrane (shown by wavy arrows over the membrane). Other structures, such as the oblique cord, the annular ligament, and the brachioradialis, must assist with the support of the load. The stretched (taut) structures are shown as thin elongated arrows, and the slackened structures are shown as wavy arrows.

Joints of the Forearm

GENERAL FEATURES OF THE PROXIMAL AND DISTAL RADIO-ULNAR JOINTS

The radius and ulna are bound together by the interosseous membrane and the proximal and distal radio-ulnar joints. This set of joints, situated at either end of the forearm, allows the forearm to rotate into pronation and supination. Forearm supination places the palm up, or supine, and pronation places the palm down, or prone. This forearm rotation occurs around an axis of rotation that extends from the radial head through the ulnar head—an axis that intersects and connects both radio-ulnar joints (Figure 6-23).³¹ Pronation and supination provide a mechanism that allows independent “rotation” of the hand without an obligatory rotation of the ulna or humerus.

FIGURE 6-23. Anterior view of the right forearm. A, In full supination the radius and ulna are parallel. B, Moving into full pronation, the radius crosses over the ulna. The axis of rotation (dashed line) extends obliquely across the forearm from the radial head to the ulnar head. The radius and carpal bones (shown in brown) form the distal segment of the forearm complex. The humerus and ulna (shown in yellow) form the proximal segment of the forearm complex. Note that the thumb stays with the radius during pronation.

The kinematics of forearm rotation are more complicated than those implied by the simple “palm-up and palm-down” terminology. The palm does indeed rotate, but only because the hand and wrist connect firmly to the radius and not to the ulna. The space between the distal ulna and the medial side of the carpus allows the carpal bones to rotate freely, along with the radius, without interference from the distal ulna.

In the anatomic position the forearm is fully supinated when the ulna and radius lie parallel to each other (see Figure 6-23, A). During pronation the distal segment of the forearm complex (i.e., the radius and hand) rotates and crosses over an essentially fixed ulna (see Figure 6-23, B). The ulna, through its firm attachment to the humerus at the humero-ulnar joint, remains nearly stationary during an isolated pronation and supination movement. A stable humero-ulnar joint provides an essential rigid link that the radius, wrist, and hand can pivot on. Movement of the humero-ulnar joint during pronation and supination has been described, but only as a very slight counter-rotation of the ulna relative to the radius.³⁵ It certainly is possible for the ulna to rotate freely during pronation and supination, but only if the humerus is also freely rotating at the glenohumeral joint.

JOINT STRUCTURE AND PERIARTICULAR CONNECTIVE TISSUE

Proximal Radio-Ulnar Joint: The proximal radio-ulnar joint, the humero-ulnar joint, and the humeroradial joint all share one articular capsule. Within this capsule the radial head is held against the proximal ulna by a fibro-osseous ring. This ring is formed by the radial notch of the ulna and the annular ligament (Figure 6-24, A). About 75% of the ring is formed by the annular ligament and 25% by the radial notch of the ulna.

FIGURE 6-24. The right proximal radio-ulnar joint as viewed from above. A, The radius is held against the radial notch of the ulna by the annular ligament. B, The radius is removed, exposing the internal surface of the concave component of the proximal radio-ulnar joint. Note the cartilage lining the entire fibro-osseous ring. The quadrate ligament is cut near its attachment to the neck of the radius.

The annular (from the Latin annulus, ring) ligament is a thick circular band of connective tissue attaching to the ulna on either side of the radial notch (see Figure 6-24, B). The ligament fits snugly around the radial head, holding the proximal radius against the ulna. The internal circumference of the annular ligament is lined with cartilage to reduce the friction against the radial head during pronation and supination. The external surface of the ligament receives attachments from the elbow capsule, the radial collateral ligament, and the supinator muscle.⁸ The quadrate ligament is a thin, fibrous ligament that arises just below the radial notch of the ulna and attaches to the medial surface of the neck of the radius (see Figure 6-24, B). This function of the poorly defined ligament is not clear, although it may support the capsule of the proximal radio-ulnar joint throughout forearm rotation.⁷⁰

SPECIAL FOCUS 6-2 Dislocations of the Proximal Radio-Ulnar Joint: the “Pulled-Elbow” Syndrome

A strenuous pull on the forearm through the hand can cause the radial head to slip through the distal end of the annular ligament. Young children are particularly susceptible to this “pulled-elbow” syndrome because of ligamentous laxity, a nonossified radial head, relative reduced strength and slowed reflexes, and the increased likelihood of others forcefully pulling on their arms—such as a parent, or even a pet dog (Figure 6-25). One of the best ways to prevent this dislocation is to explain to parents how a sharp pull on the child’s hand can cause such a dislocation.

FIGURE 6-25. An example of a cause of “pulled-elbow syndrome” in a child. (Redrawn from Letts RM: Dislocations of the child’s elbow. In Morrey BF, ed: The elbow and its disorders, ed 3, Philadelphia, 2000, Saunders. By permission of the Mayo Foundation for Medical Education and Research.)

Distal Radio-Ulnar Joint: The distal radio-ulnar joint consists of the convex head of the ulna resting on the shallow concavity formed by the ulnar notch on the radius and the proximal surface of an articular disc (Figure 6-26). This important joint firmly connects the distal ends of the radius and ulna. The shallow and often irregularly shaped ulnar notch of the radius affords only marginal osseous containment to the joint. The stability of the distal radio-ulnar joint is furnished through activation of muscles,²⁸ plus an elaborate set of local connective tissues.

FIGURE 6-26. An anterior view of the right distal radio-ulnar joint. A, The ulnar head has been pulled away from the concavity formed by the proximal surface of the articular disc and the ulnar notch of the radius. B, The distal forearm has been tilted slightly to expose part of the distal surface of the articular disc and its connections with the palmar capsular ligament of the distal radio-ulnar joint. The scaphoid and lunate facets on the distal radius show impressions made by these carpal bones at the radiocarpal joint of the wrist.

The articular disc at the distal radio-ulnar joint is also known as the triangular fibrocartilage, indicating its shape and predominant tissue type. As depicted in Figure 6-26, A, the lateral side of the disc attaches along the entire rim of the ulnar notch of the radius. The main body of the disc fans out horizontally into a triangular shape, with its apex attaching medially into the depression on the ulnar head and adjacent styloid process. The anterior and posterior edges of the disc are continuous with the palmar (anterior) and dorsal (posterior) radio-ulnar joint capsular ligaments (see Figure 6-26). The proximal surface of the disc, along with the attached capsular ligaments, holds the head of the ulna snugly against the ulnar notch of the radius during pronation and supination.^51,81 Experimentally cutting the capsular ligaments in fresh cadaver specimens causes marked increases in multidirectional translations of the distal radius in all positions of supination and pronation.⁸⁰

Introduction to the Triangular Fibrocartilage Complex: The articular disc is part of a larger set of connective tissue known as the triangular fibrocartilage complex—typically abbreviated TFCC.25,34,67 The TFCC occupies most of the “ulnocarpal space” between the head of the ulna and the ulnar side of the wrist. Several adjacent connective tissues are typically included with this complex, such as the capsular ligaments of the distal radio-ulnar joint and ulnar collateral ligament (see Figure 6-26, B). The TFCC is the primary stabilizer of the distal radio-ulnar joint.⁸⁰

Other structures that provide stability to the distal radio-ulnar joint are the pronator quadratus, the tendon of the extensor carpi ulnaris, and the more distal fibers of the interosseous membrane.22,82 Tears or disruptions of the TFCC, especially the disc,³⁹

Buy Membership for Rheumatology Category to continue reading. Learn more here