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A functional and stable elbow allows for the complex motions of flexion and extension and pronation–supination necessary for daily life.
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The functional range of motion of the elbow joint has been determined to be 30 to 130 degrees in the flexion extension arc and 50 degrees each of pronation and supination.
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The elbow joint is a trochleoginglymoid joint that has complex motion in flexion–extension and axial rotation difficult to reapproximate with implants or external fixators.
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Stability of the elbow is conferred by bony congruity, ligamentous structures, and dynamic action of muscular forces.
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A posterior utilitarian approach to the elbow is useful for addressing many conditions. Variations exist to deal with specific problems.
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An understanding and appreciation of the complex anatomy of the elbow joint, including the soft tissues and neurovascular structures, is essential to understand and treat pathology about this joint.
The elbow joint functions as a link between the arm and forearm to position the hand in space and allow activities of prehension; it transmits forces and it allows the forearm to act as a lever in lifting and carrying. A functional and stable elbow allows for the complex motions of flexion and extension and pronation–supination necessary for daily life. Stability of the elbow is conferred by bony congruity, ligamentous restraints, and dynamic stabilization by muscular forces. Biomechanical aspects of the elbow are considered in the context of motion, function, and stability. An understanding of the anatomic features contributing to these roles is critical and is outlined in this chapter.
Biomechanics
It has been determined that although the normal arc of motion of the elbow is 0 to 150 or 160 degrees in the flexion extension arc and 75 to 85 degrees each of pronation and supination, a functional range of motion in which most activities of daily living can be accomplished is 30 to 130 degrees in flexion extension and 50 degrees each of pronation and supination. When the elbow is affected by pathologic conditions, the ability to place the hand in space is diminished.
In full extension, 60% of axial loads are transmitted across the radiocapitellar joint while 40% of loads are transmitted across the ulnohumeral joint ( Fig. 3-1 , online). With elbow flexion, the relationship is altered such that loads are equally shared between the ulnohumeral and radiocapitellar articulations.
In the flexion–extension arc the elbow does not follow motion of a simple hinge joint; the obliquity of the trochlear groove and ulnar articulation results in a helical pattern of motion , ( Fig. 3-2 ). The varus–valgus laxity over the arc of flexion–extension measures 3 to 4 degrees. The center of rotation in the sagittal plane lies anterior to the midline of the humerus and is colinear with the anterior cortex of the distal humerus. The axis of rotation runs through the center of the articular surface on both the anteroposterior and lateral planes ( Fig. 3-3 , online). Forearm rotation occurs through an axis oblique to both the longitudinal axis of the radius and the ulna, through an imaginary line between the radial head at the proximal radioulnar joint and the ulnar head at the distal radioulnar joint. As described in the analogy by Kapandji, the distal and proximal radioulnar joints function as the hinges of a door. Disruption of either hinge results in loss of complete motion in pronation or supination. ,
Anatomy
Osteology
Palpable bony landmarks about the elbow include the medial and lateral epicondyles, the radial head, and the olecranon ( Figs. 3-4 and 3-5 ).
The prominent medial and lateral epicondyles serve as the attachment point for the medial collateral ligament (MCL), the flexor pronator group and lateral collateral ligament complex (LCL), and the common extensor tendon origin. The distal humerus articulates with the proximal ulna via the trochlea, a spool-shaped surface. The center of the medullary canal is offset laterally to the center of the trochlea. The olecranon, together with the coronoid process, forms the semilunar or greater sigmoid notch of the ulna ( Fig. 3-6 ). This articulates with the trochlea of the humerus and confers stability and facilitates motion in the anteroposterior plane. The lateral ridge of the trochlea is less prominent than the medial side, resulting in a 6- to 8-degree valgus orientation and creating the valgus carrying angle of the arm. Laterally, the capitellum articulates with the proximal radius. The radius also articulates with the lesser sigmoid notch of the ulna. Together, the hinge motion at the trochlea–proximal ulnar articulation and the rotational motion at the radiocapitellar joint provide the complex motion at the elbow in flexion–extension and forearm rotation.
Anatomically, a transverse “bare area” devoid of cartilage is found at the midpoint between the coronoid and the tip of the olecranon. The unwary surgeon may inadvertently discard structurally significant portions of the olecranon if this is not considered when reconstructing a fracture. The anterior portion of the sigmoid notch is represented by the coronoid, which has increasingly been recognized as an important contributor to stability of the elbow ( Fig. 3-7 , online). Posteriorly, the olecranon tip is the attachment site of the triceps. McKeever and Buck determined in the laboratory that one may excise up to 80% of the olecranon without sacrificing stability if the coronoid and anterior soft tissues are intact. , In addition, An and colleagues noted increasing instability of the elbow with olecranon excision in a linear fashion, with laboratory data suggesting that loss of up to 50% of the olecranon may be associated with no instability. If anterior damage is present, instability results if too much proximal ulna is excised. Significant coronoid loss, such as occurs with untreated coronoid fractures, will lead to instability. , The clinical importance is that severely comminuted olecranon fractures in the absence of anterior injury may be treated with partial excision, particularly in elderly or low-demand patients. However, if significant anterior damage is present, reconstruction is essential ( Fig. 3-8 ).
The articular surface of the radial head is oriented at a 15-degree angle to the neck away from the radial tuberosity ( Fig. 3-9 ). The radial head has been called a secondary stabilizer of the elbow. In the setting of a ligamentously intact elbow, fracture or removal of the radial head renders the joint unstable. However, in the setting of MCL deficiency, the radial head becomes crucial to stability against valgus forces ( Fig. 3-10 , online). The portion of the radial head that articulates with the capitellum is an eccentric dish-shaped structure with a variable offset from the neck, both of which factors have implications for fracture fixation or prosthetic replacement. Likewise, the portion of the radial head that articulates with the proximal ulna has clinically important features. The cartilage of the radial head encompasses an arc of about 280 degrees about the rim of the radius.
Ligamentous Anatomy
Ligamentous structures that contribute to the stability of the elbow joint include the collateral ligaments and the capsule both anteriorly and posteriorly. Dynamic stability is conferred by the actions of the muscles crossing the joint.
Medially, the medial collateral ligament consists of the anterior oblique ligament (AOL), the posterior oblique ligament (POL), and the transverse ligament , ( Fig. 3-11 ). The AOL of the MCL is the most important stabilizer to valgus stresses and should be preserved or reconstructed. The AOL has two bands: an anterior band that is tight from 0 to 60 degrees and a posterior band that is tight from 60 to 120 degrees. The AOL and POL arise from the central portion of the anterior inferior medial epicondyle and insert near the sublime tubercle (AOL) and in a fan-shaped insertion along the semilunar notch (POL). The transverse segment of the MCL appears to have little functional significance.
