Biomechanics of the Elbow

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Chapter 3 Biomechanics of the Elbow

Range of motion and kinematics related to the joint geometry

The complex motion of the elbow joint is divided into two components that act about different axes, enabling flexion–extension and pronation–supination. However, the literature includes papers that have sought to provide a deeper understanding of the subtleties, such as variation of the carrying angle, which might lead to more physiological designs of joint replacements, even if the potential benefits are as yet unproven.

Flexion–extension: range of motion

The accepted nomenclature for describing elbow flexion–extension range of motion is that of the American Academy of Orthopaedic Surgeons.1 They defined the fully extended position, with a straight arm, as 0°, and the fully flexed position to be approximately 146°. Some earlier papers had used the ‘elbow angle’, which is the angle between the limb segments, but that convention has fallen from use among orthopaedic surgeons, although it remains common among veterinary surgeons. A review of the literature shows that many papers have merely estimated the range of normal motion, but those where it has actually been measured have reported a mean range from 0° to 142° of active elbow flexion.2 The distribution of range of motion was investigated by West,3 who found that of 517 elbows that were measured 25% hyperextended by 5° or more, and more than 90% had maximum active flexion of between 140° and 150°.

There are several factors which can affect the range of elbow flexion–extension. Glanville and Kreezer4 examined the difference between active and passive elbow flexion, and found that passive movement allowed the range to increase from 141° to 146°. Active motion is limited by apposition of the soft tissues between the humerus and forearm and, as such, motion with the flexor muscles relaxed would be expected to be greater.

Several studies have looked for side-to-side differences in range of motion, and also examined the differences between dominant and non-dominant arms. These studies have found no significant differences in motion and therefore loss of motion can be calculated by comparison with the uninjured limb.

Other authors have noted that the build of the subject affects the range of motion, with slim subjects typically having approximately 10° more flexion than muscular or fat individuals.

There has also been agreement that the range of motion reduces with increasing age, with children hyperextending by a mean of 5°. After the fourth decade there is a loss of several degrees of flexion, with several more degrees of extension being lost after the sixth decade.

The final factor that affects the range of flexion–extension is the sex of the subject. Studies have shown that females have 5–8° more hyperextension than males.2

Flexion–extension and the carrying angle kinematics

Elbow motion results largely from the articular geometry, particularly the shape of the humeral trochlea and capitellum. These have been approximated as a spool and hemisphere placed coaxially across the distal humerus. In the past, some authors have produced diagrams which showed relative axial motion of the radius alongside the ulna, as the elbow flexed and extended, suggesting that this resulted from the capitellum being displaced from the trochlear axis, thus acting like a cam. However, there has been clear radiographic evidence of the coaxiality of the articular surfaces, as long as the X-ray beam is aligned accurately.5 Provided this is the case, the articular features are seen as a series of concentric circles and arcs of circles representing the waist of the trochlea, the capitellum and the medial lip of the trochlea. These structures are situated anterior to the shaft of the humerus, with the medial epicondyle hanging back posteriorly and proximal to the axis. The lateral view also shows the coronoid and olecranon fossae, which approach each other from anteriorly and posteriorly to produce a thin membrane of bone immediately proximal to the waist of the trochlea. This membrane may actually be perforated in elbows that hyperextend, and this supratrochlear foramen has an ethnic basis.6 The presence of these fossae increases the range of elbow flexion–extension before it is limited by bony impingement from the coronoid and olecranon processes. There is also a matching shallow anterior fossa that accommodates the rim of the radial head in terminal elbow flexion. However, in life elbow flexion is more likely to be limited by apposition of the bulky soft tissues between the shafts of the humerus and forearm bones. This tendency is reduced by the anterior offset of the distal humeral articulations and also the anterior position of the trochlear notch of the ulna, in relation to its shaft. These features combine to widen the gap between the bone shafts in deep elbow flexion. As much of the soft tissue is muscle, it follows that the range of passive elbow flexion is greater than that of active flexion.

The early work that described the axes of the trochlea and capitellum as being eccentric when viewed laterally suffered from lack of appreciation of the orientation of the axis about which flexion occurs. The key to this is the carrying angle and how that angle varies with elbow flexion. The carrying angle has been defined in many specific ways, but all of these seek to describe the lateral deviation of the extended forearm from the sagittal plane, that is, aligned along the anatomical axis of the shaft of the humerus. Specific definitions have depended on how the axis of the forearm has been defined, either in terms of a line passing from the centre of the capitellum to the centre of the ulnar head distally, an axis based on bony anatomical features, or else as a line which passes along the centre of the forearm, irrespective of the structures within it.7 It is widely recognized that the carrying angle is maximal when the elbow is extended, and reduces until the forearm overlays the upper arm in maximum elbow flexion. Provided that the humero-ulnar joint acts as a uniaxial hinge this variation can be obtained by inclining the flexion axis so that it bisects the carrying angle. While the coaxial circles seen on an accurate lateral radiograph5 suggest this to be true, the literature has found it contentious until the recent advent of more modern 3-D kinematic measurement methods. In particular, it is possible to define the motion of one limb segment in relation to another in terms of the ‘instantaneous screw axis’, which allows a series of axes to be defined as the motion progresses incrementally. It has been shown8 that these instant axes are close to being parallel and intersect through a small zone at the centre of the trochlea. For this reason we can assume for all practical purposes that the elbow acts like a simple hinge under defined loads. The orientation of the hinge axis is not perpendicular to the sagittal plane of the humerus, but is abducted from that orientation by one-half of the carrying angle (Fig. 3.1).9

The literature shows that the carrying angle is larger in females than in males, typically being 14° and 11°, respectively.2,7,10 It has also been shown that the carrying angle relates to build, with carrying angle increasing with heaviness of body type.10 Although Paraskevas et al10 found that the carrying angle was larger in the dominant limb than in the non-dominant limb, that difference was small, and most authors have found that they are effectively symmetrical. That observation is useful in children, because the carrying angle develops up to puberty and so there is no standard value. Smith11 found that 9% of children aged less than 11 years had a zero carrying angle and a further 36% had less than 5°. The biomechanical and clinical importance of the carrying angle relates to both its effect on the appearance of the whole of the upper limb and its effect on the distribution of forces across the distal humerus. Inaccurate reduction of supracondylar humeral fractures will result in deformity and alteration of the carrying angle, while heavily loaded activities will result in significant force transmission across the elbow, which may affect the overall stability of the joint.

Summary Box 3.1 Elbow flexion and carrying angle kinematics

Elbow flexion Uniaxial hinge motion
Flexion axis Abducted by a half of the carrying angle
Carrying angle Maximal in extension. Approximately 14° in women and 11° in men
Carrying angle Increases throughout childhood and differs between subjects. To assess accurately examine for abnormal alignment by side-to-side comparison

Pronation–supination motion

The second principal component of elbow motion is rotation of the forearm. The early literature assumed that this motion took place about a fixed axis, with the ulna not moving in relation to the humerus. This was thought to be due to the constraint of the congruent humero-ulnar joint, with the coronoid and olecranon processes gripping the waist of the trochlea. However, it has since been appreciated that the joint is not so closely constrained, with the carrying angle typically varying by ±5°, due to sloppiness of the fit of the articular surfaces and some slackness of the collateral ligaments.9 The classic paper that showed this variation was by Ray et al,12 who took double-exposure radiographs of their own elbows and forearms in pronation and supination. They immobilized the humerus using transcutaneous pins into the epicondyles, and applied traction to the fingers using a Chinese finger basket. They showed that if the forearm rotation axis was aligned through the ulna, due to traction on the little finger, it resulted in the ulna remaining stationary, with the forearm swinging around it like the handle on a bucket. However, if the axis was aligned through the centre of the wrist, with traction on the long finger, then the centreline of the forearm did not move during forearm rotation and the ulna did move in relation to the fixed humerus. Thus, pronation from neutral rotation entailed ulnar abduction, and supination entailed adduction, with the range of ulnar ab–adduction motion corresponding to the ligamentous laxity noted above. It has been suggested that the ulnar abduction during pronation is a reason for the anconeus muscle. As the forearm rotates away from neutral rotation, so the ulna also flexes a little, as it moves alongside the radius. It was shown long ago13 that this was a circumduction motion, in which the distal ulna moves in a part-circular path, yet does not itself rotate (Fig. 3.2). This has been investigated again using modern technology, with the same findings.14

At the elbow, forearm rotation is accommodated by the radial head rotating in the ulnar notch on the lateral side of the coronoid. This notch is less than 180° in extent and therefore the radial head is vulnerable to subluxation under the anterior force vector applied to the radial tuberosity by the biceps tendon. This vulnerability is controlled by the tension in the surrounding annular ligament, which envelops the radial head and attaches securely to the ridges at the anterior and posterior edges of the ulnar notch. At the wrist, the axis of rotation passes through the centre of the head of the ulna, so the axis of rotation slants across from the capitellum proximally to the ulna distally, when viewed in the coronal plane. In the sagittal plane, the axis of forearm rotation is relatively anterior proximally, again being centred in the capitellum, and is anterior to the interosseous border of the ulna. This has great significance for the function of the interosseous membrane which, when the forearm is in neutral rotation, will be tightest when the radius is furthest away from the interosseous border of the ulna,. It will then slacken when the forearm rotates away from that posture.15 In particular, the interosseous membrane is slack when the forearm is in pronation, which is the posture taken up during falls onto the outstretched hand, when load transmission along the forearm is important. This will be discussed below.

Range of forearm rotation motion

Forearm rotation motion is usually measured from a neutral position with the upper arm alongside the trunk and the elbow flexed 90°. By having the elbow flexed, humeral rotation may be eliminated or controlled. The neutral position is when the extended thumb is parallel to the humerus. Pronation is usually approximately 80°, while supination is approximately 90°.

Various methods have been used to measure forearm rotation. These comprise devices held in the hand, such as a rotational pendulum or bubble goniometer, or those which seek to eliminate accessory motion at the wrist and hand (a very common way in which patients try to ‘cheat’), by using a device located around the wrist. Patrick15 found that hand grip motion was 27° greater than that measured simultaneously at the wrist, which is true forearm rotation. Also, when the radius was vertically above the ulna at the wrist, the hand grip was supinated 11°. An analysis of the then published literature2 found that the mean motion reported from hand-gripping devices was 77/106° pronation/supination, while wrist cuff devices led to means of 76/80°. A comparative study by Darcus and Salter16 found that wrist cuff goniometers resulted in much more repeatable readings for forearm rotation, and for this reason they are recommended for research studies. An updated version of this method uses an electromagnetic sensor from a motion analysis system attached to the wrist via a strap.

The literature clearly shows that forearm rotation varies between normal subjects and, therefore, when being measured after injury it should always be compared to the contralateral normal limb. This recommendation is supported by a number of studies that have found no significant difference between the dominant and non-dominant arms. Salter and Darcus17 found that females had a mean of 8° more active forearm rotation than males, while Glanville and Kreezer4 found that there was a mean of 31° more forearm rotation when the hand was held and rotated passively, presumably because that allowed muscles to relax, which might otherwise block motion between the radius and ulna.

Muscle actions causing forearm rotation

Both pronation and supination are each driven by two principal muscles,18 and in each case one is relatively long, while the other consists of a compact bulk of fibres that cross transversely from the ulna to the radius. Travill and Basmajian19,20 found that unresisted rotation was driven by the short supinator and pronator quadratus muscles, with the longer muscles becoming active as the motion was resisted.

Forceful supination is driven primarily by the biceps, the tendon of which wraps around the proximal radius inserting into the radial tuberosity. The biceps is augmented by the supinator muscle, the fibres of which are oriented anterodistally from the lateral supinator ridge to the radius. The supinator has a more extensive ulnar origin than radial attachment, with superficial fibres that spiral around the neck of the radius, attaching both proximal and distal to the biceps tuberosity. Although not usually seen during surgery, the supinator also has a deep layer of fibres which are oriented transversely to the axis of the forearm, and which wrap directly around the proximal neck of the radius from the supinator ridge. This fibre group is well oriented to resist the tendency of the biceps to cause anterior subluxation of the head of the radius when the forearm is actively flexed. Forceful supination demands coordinated action from many muscles, because of the subluxing and flexing effects of biceps contraction. Thus at least the triceps will also act to stabilize the elbow, but also probably almost all of the other forearm muscles, as they cause the hand to grasp whatever object is being rotated and also stabilize the wrist. In particular, the brachioradialis may act to return the forearm to neutral rotation from either pronation or supination.21

Pronation is driven by the combined actions of pronator teres and pronator quadratus. The pronator teres, which has both humeral and ulnar origins, also tends to flex the elbow when it contracts, in a similar manner to the action of the biceps during supination. The pronator quadratus fills the space between the anterior aspect of the distal shafts of the radius and the ulna, with its fibres oriented transversely. As it is close to the axis of rotation, it is not as effective as the pronator teres in producing pronation. Its line of action, however, means that it helps to maintain coaptation of the distal radio-ulnar joint, in synergy with the triangular fibrocartilage. The strength of forearm rotation will be described below.

Summary Box 3.2 Forearm rotation

Forearm rotation Must be assessed by comparison with the contralateral limb due to variability between subjects
Forearm rotation – supination Achieved by biceps and supinator
Forearm rotation – pronation Achieved by pronator teres and pronator quadratus

Passive soft tissue stabilizers of the elbow

Most daily activities which cause the elbow to be heavily loaded entail active contraction of groups of muscles which span across the elbow. These muscle tensions act mostly parallel to either the forearm or the humerus, and act to compress the radial head or ulnar trochlear notch against the capitellum and trochlea of the humerus. Thus the muscles act to stabilize the elbow. If the external load applied – usually via the hand and forearm – overcomes the active stabilizing actions of the muscles, then the passive ligamentous and capsular restraints come into play in order to prevent subluxation or instability.

Medial collateral ligament complex

The principal structures crossing the joint are the anterior and posterior bands of the medial collateral ligament (MCL). The humeral attachment is spread around the base of the medial epicondyle, across its distal and posterior–distal aspects, thus spanning across the site of the flexion–extension axis, which emerges from the centre of the circular medial end-face of the trochlea, at the distal edge of the epicondyle. This means that the most anterior fibres of the anterior band of the MCL are close to being isometric, and therefore tight throughout the range of elbow flexion–extension. Moving posteriorly across the width of the MCL complex, there is an increasing tendency for the ligament fibres to slacken as the elbow is extended.22 Thus the posterior band of the MCL is only tightened completely when it becomes wrapped around the distal aspect of the epicondyle beyond 90° of elbow flexion.23 Ciccotti et al22 found that the progressive tightening of the MCL fibres as the elbow flexed correlated with a reducing range of valgus laxity. Morrey and An24 found that the MCL resisted approximately one-third of the valgus moment applied to the elbow when it was extended. In addition, they noted that because the anterior joint capsule was tight in this position it also took one-third of the load, with the articular surfaces taking the remainder. As the elbow flexed, so the MCL took a larger proportion of the load, reaching 54% at 90° flexion. Thus they felt that the MCL was the primary restraint to valgus laxity. This was consistent with the findings of Sojbjerg et al,25 who reported that cutting the anterior band of the MCL led to 14° of valgus laxity at 70° of elbow flexion. At this point the posterior band became tight and if this was also cut the elbow became unstable, with up to 31° of valgus deformity.