CHAPTER 3 Biomechanics of the Elbow
INTRODUCTION
In our practice, a working knowledge of biomechanics has been extremely important and rewarding. Clinical relevance includes elbow joint design and technique, the rationale and execution of trauma management, and ligament reconstruction. In short, a clear understanding of biomechanics provides a scientific basis for clinical practice.5
KINEMATICS
FLEXION-EXTENSION
Because of the congruity at the ulnohumeral articulation and surrounding soft tissue constraint, elbow joint motion is considered primarily a hinge type. Yet, two separate three-dimensional studies of passive motion at the elbow revealed that the elbow does not function as a simple hinge joint.51,69 The position of the axis of elbow flexion, as measured from the intersection of the instantaneous axis with the sagittal plane, follows an irregular course. A type of helical motion of the flexion axis has been demonstrated.69 This pattern was previously suggested26,50,61 and was attributed to the obliquity of the trochlear groove along which the ulna moves.52 An electromagnetic tracking device that allows a three-dimensional measurement of simulated active elbow joint motion reveals the amount of potential varus-valgus and axial laxity that occurs during elbow flexion to average about 3 to 4 degrees. This has been confirmed with more advanced electromagnetic tracking technology.101
CENTER OF ROTATION
The axis of motion in flexion and extension has been the subject of many investigations.60 Fischer (1909), using Reuleaux’s technique, found the so-called locus of the instant center of rotation to be an area 2 to 3 mm in diameter at the center of the trochlea (Fig. 3-1).34 Subsequent experiments with the same technique described a much larger locus.32 In a three-dimensional study of passive motion of the elbow joint, the observations of Fischer were confirmed by using the biplanar x-ray technique.69 Based on direct experimental study as well as analytic investigation, Youm and associates109 concluded that the axis does not change during flexion-extension. In our study, however, variations of up to 8 degrees in the position of the screw axis from individual to individual have been shown. As seen from below, the axis of rotation is internally rotated 3 to 8 degrees relative to the plane of the epicondyles. In the coronal plane, a line perpendicular to the axis of rotation forms a proximally and laterally opening angle of 4 to 8 degrees with the long axis of the humerus.105 These data, coupled with the clinical information regarding implant loosening, have inspired the development of less constrained but coupled elbow joint replacement designs. It recently has been demonstrated that these designs do function as semiconstrained implants and allow for the normal out-of-plane rotations noted earlier (see Chapter 49).75
From a practical point of view, despite the different findings of various investigators, the deviation of the center of joint rotation is minimal and the reported variation is probably due to limitations in the experimental design. Thus, the ulnohumeral joint could be assumed to move as a uniaxial articulation except at the extremes of flexion and extension. The axis of rotation passes through the center of the arcs formed by the trochlear sulcus and capitellum.56
The center of rotation can be identified from external landmarks. In the sagittal plane, the axis lies anterior to the midline of the humerus92 and lies on a line that is colinear with the anterior cortex of the distal humerus.69 The coronal orientation is defined by the plane of the posterior cortex of the distal humerus.19 This axis emerges from the center of the projected center of the capitellum and from the anteroinferior aspect of the medial epicondyle (see Fig. 3-1).
Similarly, the effect of altering the center of rotation on the kinematics of the forearm has been recently studied. Alterations of as much as 5 mm proximally, distally, anteriorly, or posteriorly have been shown to have only a slight effect on elbow kinematics (Fig. 3-2). This observation has great clinical relevance regarding the design and insertion of prosthetic replacement and articulating external fixation devices.
FOREARM ROTATION
The radiohumeral joint, which forms the lateral half of the elbow joint, has a common transverse axis with the elbow joint, which coincides with the ulnohumeral axis during flexion-extension motion. In addition, the radius rotates around the ulna, allowing for forearm rotation or supination-pronation. In general, the longitudinal axis of the forearm is considered to pass through the convex head of the radius in the proximal radioulnar joint and through the convex articular surface of the ulna at the distal radioulnar joint.34,97 The axis therefore is oblique to the longitudinal axes of both the radius and the ulna (Fig. 3-3), and rotation is independent of elbow position.45
Mori has characterized the axis of forearm rotation as passing through the attachment of the interosseous membrane at the ulna in the distal fourth of the forearm (see Fig. 3-32).62 This may have particular applications with regard to the sensitivity of forearm rotation to angular deformity in this particular portion of the bone. Clinically and experimentally, less than 10% angulation of either the radius or the ulna causes no functionally significant loss of forearm rotation.91
In the past, ulnar rotation was described as being coupled with forearm rotation.106 This observation could not be reproduced in a subsequent study by Youm and associates.108 By using a metal rod introduced transversely into the ulna, extension, lateral rotation, and then flexion of the ulna was described with rotation from pronation to supination. The axial rotational movements of the ulna were also observed by others.14,22,30,43,69,88,108
Ray and associates88 also suggested that varus-valgus movement of the ulna occurs if the forearm rotates on an axis extending from the head of the radius to the index finger. Experiments from our laboratory76 have demonstrated external axial rotation of the ulna with forearm supination. Internal rotation or closure of the lateral ulnohumeral joint occurs with pronation.
Finally, the radius has been shown to migrate 1 to 2 mm proximally with pronation.67 This observation had not been reported previously but has been confirmed by observations at the wrist.82
CARRYING ANGLE
The carrying angle is defined as that formed by the long axis of the humerus and the long axis of the ulna. It averages 10 to 15 degrees in men and is about 5 degrees greater in women.1,18,53,97
However, uncertainty has arisen over the use of the term carrying angle in the dynamic setting. Dempster27 described an oscillatory pattern during elbow flexion, whereas Morrey and Chao69 reported a linear change, with the valgus angle being the greatest at full extension and diminishing during flexion. The confusion arises because three descriptions based on different reference systems have been adopted for the measurement of carrying angle changes.
Definition 1
The carrying angle is the acute angle formed by the long axis of the humerus as the long axis of the ulna projects on the plane containing the humerus (Fig. 3-4A).
Definition 2
The carrying angle is described as the acute angle formed by the long axis of the ulna and the projection of the long axis of the humerus onto the plane of the ulna (see Fig. 3-4B).
Definition 3
From an anatomic point of view, it is not difficult to conclude that the existence of the carrying angle is due to the existence of obliquities, or cubital angles, between the proximal humeral shaft, the trochlea, and the distal ulnar shaft. By assuming that the ulnohumeral joint is a pure hinge joint and that the axis of rotation coincides with the axis of the trochlea, the change in the carrying angle during flexion can be defined as a function of anatomic variations of the obliquity of the articulations according to simple trigonometric calculations.8 If the first or second definition is accepted, the carrying angle changes minimally during flexion. The specific varus/valgus relationship of the forearm to the humerus during flexion therefore depends on the relative angular relationship of the humeral and ulnar articulations (Fig. 3-5).
RESTRICTION OF MOTION
In normal circumstances, elbow flexion ranges from 0 degrees or slightly hyperextended to about 150 degrees in flexion. Forearm rotation averages from about 75 degrees (pronation) to 85 degrees (supination) (see Chapter 2). The cartilage of the trochlea forms an arc of about 320 degrees, whereas the sigmoid notch creates an arc of about 180 degrees. Generally, the arc of the radial head depression is about 40 degrees,97 which articulates with the capitellum, presenting an angle of 180 degrees.
The significance of the 30-degree anterior angulation of the trochlea with the 30-degree posterior orientation of the greater sigmoid notch to flexion and extension and stability of the elbow joint is discussed in detail in Chapter 1 (Fig. 3-6). Impact of the olecranon process on the olecranon fossa and the tension of the anterior ligament and the flexor muscles as well as tautness of the anterior bundle of the medial collateral ligament have been described as serving as a check to extension.40,52 The anterior muscle bulk of the arm and forearm, along with contraction of the triceps, is also reported to prevent active flexion beyond 145 degrees.52 However, the factors limiting passive flexion include the impact of the head of the radius against the radial fossa, the impact of the coronoid process against the coronoid fossa, and tension from the capsule and triceps.
For pronation and supination, Braune and Flugel20 found that passive resistance of the stretched antagonistmuscle restricts the excursion range more than that of the ligamentous structures. Spinner and Kaplan,96 however, have shown that the quadrate ligament does provide some static constraint to forearm rotation. Impingement of tissue restrains pronation, especially by the flexor pollicis longus, which is forced against the deep finger flexors. The entire range of active excursion in an intact arm is about 150 degrees, whereas when the muscles are removed from a cadaver specimen, the range increases to 185 to 190 degrees. With cutting the ligaments, the range increased up to 205 to 210 degrees.
CAPACITY AND CONTACT AREA OF THE ELBOW JOINT
The capacity of the elbow joint recently has been shown to average about 25 mL. The maximum capacity is observed to occur with the elbow at about 80 degrees of flexion.78 This explains the clinical observation that stiff elbows tend to have fixed deformities at about 80 to 90 degrees of flexion.63
Accurate measurement of the contact points of the elbow is extremely difficult, and several techniques have been applied to this highly congruous joint.99 Silicone casting, Fuji Prescale film, and reversible cartilage staining are most commonly used. Each has advantages and disadvantages. The contact area of the articular surface during elbow joint motion has been investigated by Goodfellow and Bullough, using a staining technique.39 They found that the central depression of the radial head articulates with the dome of the capitellum and that the medial triangular facet was always in contact with the ulna. The upper rim of the radial head made no contact at all. At the humeroulnar joint, the articular surfaces were always in contact during some phases of movement. Others have verified these observations.107 The contact areas on the ulna occurred anteriorly and posteriorly and tended to move together and slightly inward from each side from 0 to 90 degrees of flexion and with increasing load.31,74 Using a wax casting technique, in full extension, the contact has been observed to be on the lower medial aspect of the ulna, whereas in other postures, the pressure areas described a strip extending from posterolateral to anteromedial.37 The radiocapitellar joint also revealed contact during flexion without externally applied load. Investigations in our laboratory show that the contact areas of the elbow occur at four facets: two at the coronoid and two at the olecranon (Fig. 3-7). Only a slight increase in total surface area occurred with elbow flexion and with a sevenfold increase in load.99 With a 10-N load, about 9% contact of the articular surfaces occurs, and with 1280 N, the area increased to about 73%.31
FIGURE 3-7 Contact in the sigmoid fossa moves toward the center of the fossa during elbow flexion.
(Redrawn from Walker, P. S.: Human Joints and Their Artificial Replacements. Springfield, IL, Charles C. Thomas, 1977.)
When varus and valgus loads are applied to the forearm, the contact changes medially and laterally. This implies a pivot point about which the radioulnar articulation rotates on the humerus in the anteroposterior (AP) plane in extension with varus and valgus stress. In vivo experiments have demonstrated the varus-valgus pivot point of the elbow to reside in the midpoint of the lateral face of the trochlea (Fig. 3-8).
ELBOW STABILITY
The static soft tissue stabilizers include the collateral ligament complexes and the anterior capsule. Studies from our laboratory regarding the anatomy of the lateral collateral ligament68,77 and others36,79,95 have been discussed previously (see Chapter 2). The lateral collateral ligament and the anterior bundle of the medial collateral ligament originate from points through which the axis of rotation passes. Furthermore, the medial collateral ligament has two discrete components.66,93 The anterior bundle has been shown to be taut in extension; the converse is true for the posterior fibers of the anterior bundle. Because elbow joint motion occurs about a nearly perfect hinge axis through the center of the capitellum and trochlea, the posterior bundle of the medial collateral ligament complex will be taut at different positions of elbow flexion (Fig. 3-9). The lateral collateral ligament and the anterior bundle lying on the axis of rotation will assume a rather uniform tension, regardless of elbow position. Furthermore, the lateral ulnar collateral ligament has inserts on the ulna and, as such, helps to stabilize the lateral ulnohumeral joint (Fig. 3-10).23,66,77,81 In experiments performed in our laboratory, O’Driscoll and associates have demonstrated that the lateral ulnar collateral ligament is essential to control the pivot shift maneuver (see Chapter 4). Further evidence of the contribution of the lateral ligament complex to elbow stability is offered by S∅︀jbjerg and associates.94 These investigators also attributed a major role in varus and valgus stability to the annular ligament. Although our work suggests that the major component in the varus and rotatory stability is the structure termed the lateral ulnar collateral ligament, the parallel findings of these investigators suggest that the lateral complex is, in fact, a major valgus stabilizer of the elbow joint and functions with or without the radial head.80
ARTICULAR AND LIGAMENTOUS INTERACTION
The influence of the ligamentous and articular components on joint stability are usually studied with the use of the materials testing machine by imparting a given and controlled displacement to the elbow.47,65,87 The relative contribution of each stabilizing structure can be demonstrated by sequentially eliminating each element and observing the load recorded by the load cell for the constant displacement imparted, usually 2 to 5 degrees95 (Fig. 3-11).
A simplified summary of the observations from such an experiment is shown in Table 3-1. In extension, the anterior capsule provides about 70% of the soft tissue restraint to distraction, whereas the medial collateral ligament assumes this function at 90 degrees of flexion. Varus stress is checked in extension equally by the joint articulation (55%) and the soft tissue, lateral collateral ligament, and capsule. In flexion, the articulation provides 75% of the varus stability. Valgus stress in extension is equally divided between the medial collateral ligament, the capsule, and the joint surface. With flexion, the capsular contribution is assumed by the medial collateral ligament, which is the primary stabilizer (54%) to valgus stress at this position. Furthermore, for all practical purposes, the anterior portion of the medial collateral ligament provides virtually all of the structure’s functional contribution.
Limitations of this experimental model have resulted in an overestimation of the role of the radial head inresisting valgus load.47,65,90 This has prompted the development of an experimental technique that allows simultaneous and accurate measurement of three-dimensional angular and translational changes under given loading conditions (Fig. 3-12). Using the electromagnetic tracking device, an accurate technique for measuring the function of the articular and capsuloligamentous structures was developed.70 More accurate and relevant data were generated.70 Valgus stability is resisted primarily by the medial collateral ligament. With an intact medial collateral ligament, the radial head does not offer any significant additional valgus constraint. With a released or compromised medial collateral ligament, the radial head does resist valgus stress. This important experiment documents that the radial head is a secondary stabilizer for resisting valgus stress, whereas the medial collateral ligament is the primary stabilizer against valgus force (Fig. 3-13). In a laboratory investigation, the hyperextension trauma produces lesions of the anterior capsule, the avulsion of proximal insertions of both medial and lateral collateral ligaments.103 The degree of extension increased by 17 degrees and induced significant joint laxity in forced valgus internal-external rotation, but not varus.103
It has been recently observed that the valgus and varus laxity of the elbow is dependent on forearm rotation.86
