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

Figure 3.1 The axis of articulation is abducted from being perpendicular to the axis of the humerus, by approximately a half of the carrying angle. Thus the carrying angle reduces from maximal in extension close to zero in full flexion (this may be demonstrated by folding a strip of cardboard shaped like an extended upper limb). There is a range of abduction–adduction laxity allowed by the collateral ligaments, which is used during forearm rotation. This laxity reduces in terminal extension, due to tightening of the anterior soft tissues.

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.

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

Figure 3.2 This is a stylized end-view of the radius and ulna during forearm rotation. The radius, at the top, rotates around the axis of rotation, which is near the centre of the wrist. At the same time, the ulna, at the bottom, is carried in an arc of circumduction: it moves in an arc of a circle yet does not rotate (indicated by the line drawn across it). The circumduction motion is a combination of abduction–adduction (Ab-Ad), plus flexion–extension (Fl-Ext) motion components.

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.

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.

Lateral collateral ligament complex

The functional anatomy of the lateral collateral ligament is complex, with part of the insertion of the ligament dissipating into the annular ligament rather than attaching to bone. An accessory ligament lies posteriorly, usually known as the lateral ulnar collateral ligament. This corresponds to the anterior margin of the anconeus, thus linking the lateral epicondyle directly to the supinator ridge of the ulna.

In general, the lateral collateral ligament structures are less important for elbow stability than the medial collateral ligament complex. Morrey and An²⁴