14 The Elbow Joint
14.1 The Confluent Layered Anatomy of the Elbow
The student of elbow anatomy will quickly realize that it is difficult to see all portions of the joint without taking down or dividing many of the more superficial structures. The many vessels and nerves that traverse the joint, particularly medially and anteriorly, have branches to the proximal musculature that prevent deep dissection without division of some of the branches that innervate the proximal musculature. The tendons that appear separate in the middle and distal forearm are inseparable proximally, since many of them converge and take origin close to the flexion/extension axis of the elbow. In addition, there is no easily identifiable plane between superficial and deep structures of the most proximal medial and lateral elbow tendon origins. Tendon fibers underlap or overlap each other to allow for each to arise close to the elbow flexion/extension axis. Also, the tendon fibers of the flexor pronator and extensor supinator origins arise not only from bone but also from the deeper medial and lateral ligament structures.
The following dissections will cover the superficial and deep anatomy of the elbow. To do so, the more superficial layers have been removed to allow for visualization of the deeper ligamentous anatomy.
Nerve and vascular anatomy is covered in chapters 9 and 11, so it will only be mentioned here when particularly relevant to the elbow structures.
14.2 Articular Anatomy
The elbow joint contains three different portions or articular components, all within the same capsular synovial cavity.
Ulnohumeral joint: The trochlea is shaped like a grooved spool positioned between the medial epicondyle and the lateral epicondyle (▶Fig. 14.1a). The trochlea, which articulates with the coronoid and olecranon of the proximal ulna (trochlear notch), provides a stable surface for flexion and extension of the elbow. The medial part of the trochlear groove is wider and deeper anteriorly and medially than on its shorter lateral surface. It is separated from the capitellum by a small ridge of bone.
Radiocapitellar joint: The capitellum articulates with the radial head to form the second portion of the elbow joint (radiocapitellar articulation) that allows for rotation of the radial head and for flexion and extension (▶Fig. 14.1b).
Proximal radioulnar joint: Third, the circumferential surface of the radial head articulates with the sigmoid notch of proximal ulna to comprise the proximal radioulnar joint. The radial head is stabilized to the ulna at the “sigmoid notch” (▶Fig. 14.2a,b) by the strong or stout annular ligament that encircles the radial head by arising from ulna anteriorly and attaching to ulna posteriorly. The annular ligament exists as part of the “lateral collateral ligament complex” and contains also the fibers of origin of the supinator as well as the extensors in the forearm. The radial fossa subtends an arc of 66° of the radial head, or 18% of the radial head circumference. And 215° of the radial head consists of articular surface that articulates through full pronation to full supination. 1
The radial head is slightly oblong in shape and is concentrically located in the sigmoid notch in neutral rotation. It shifts anteriorly with pronation and posteriorly with supination.
14.3 Effect of Elbow Joint Morphology on Elbow Alignment
The morphology of the trochlea and capitellum provides for valgus elbow alignment with elbow extension and varus alignment in flexion. In the coronal plane, the medial trochlea extends more distal than the lateral trochlea and capitellum (▶Fig. 14.1b). In elbow extension, this places the elbow in a valgus carrying angle, which facilitates holding or carrying objects away from the body (▶Fig. 14.1b). When the elbow flexes, the anterior part of the capitellum, which projects more anteriorly than does the trochlea, swings the forearm into slight varus, which facilitates placing the hand close to the head or mouth (▶Fig. 14.1a).
14.4 Structures That Provide for Varus/Valgus Stability
Valgus stability depends on the intact ulnar collateral ligament medially and the buttressing effect of the radial head laterally (▶Fig. 14.1b). In the opposite direction, stability against varus stress requires an intact medial coronoid and lateral collateral ligament complex (▶Fig. 14.1b).
14.5 Structures That Resist Posterolateral Elbow Dislocation
When the intact surfaces of the trochlea and trochlear notch of ulna are compressed together by muscle forces of brachialis and triceps, the joint is remarkably stable. The coronoid process and radial head both function as an anterior buttress constraint against posterior dislocation (▶Fig. 14.1a,c). The lateral collateral ligament complex (▶Fig. 14.11a) prevents opening of the lateral and posterolateral elbow joint. When disrupted, the ulnohumeral joint opens laterally (ulnohumeral supination). As this occurs, the radial head falls posterior to the capitellum and loses its buttressing support with the capitellum (▶Fig. 14.1c).
Because of the projections of the radial head and coronoid process, an adequate fossa is required for both to allow for full elbow flexion. The normal elbow has an abundant anterior coronoid recess and capitellar (radial) recess to facilitate flexion (▶Fig. 14.1b). An abundant triangular olecranon fossa is required to provide or allow for full elbow extension without abutment of the olecranon against the posterior humerus (▶Fig. 14.3a,b). The coronoid fossa anteriorly and olecranon fossa posteriorly are separated by a thin lamina of bone that at times may have a perforation known as the supratrochlear foramen.
Posteriorly, the triceps tendon inserts onto the olecranon process, not on the tip of the process, but in a small subtle transverse sulcus approximately 5 to 10 mm anterior to the posterior-most proximal ulna (▶Fig. 14.3a). In the normal elbow, the anterior capsule in front of the elbow and the posterior capsule (▶Fig. 14.3b) are normally quite soft, compliant, and thin.
▶Fig. 14.3c–e shows the end-on anatomy of the distal humerus, the olecranon, and the proximal ulna and the radial head.
14.5.1 Posteromedial Anatomy
Proximal to the elbow, the intermuscular septum (▶Fig. 14.4a) separates the posterior compartment with triceps from the anterior compartment, which contains the brachialis and biceps muscles. The ulnar nerve lies posterior to the intermuscular septum (▶Fig. 14.4b) and is covered by a fascial layer that extends from the medial triceps to the intermuscular septum. Around 3 to 10 cm proximal to the medial epicondyle, a 2- to 5-cm thickening, termed the arcade of Struthers (▶Fig. 14.4c), may exist which can potentially compress the ulnar nerve. When present, it derives from a thickening fascia of the distal part of the arm, by superficial muscle fibers of the medial head of the triceps muscle arising from the medial intermuscular septum, and by attachments of the internal brachial ligament. 2
As the ulnar nerve approaches the elbow, it gains segmental vascular supply from the inferior ulnar collateral vessels (▶Fig. 14.4b). Just distal to the medial epicondyle, the ulnar nerve passes beneath the ligament of Osborne, a ligamentous structure between olecranon and medial epicondyle, and the fascial aponeurosis between the humeral and ulnar heads of the flexor carpi ulnaris (▶Fig. 14.4b). In elbow extension (as shown in the figure), the ligament is lax, but with elbow flexion it tightens, as does the posterior band of the ulnar collateral ligament just deep to the ulnar nerve. In flexion, the tightening of these two structures compresses the ulnar nerve. At the level of the ligament of Osborne, the ulnar nerve delivers innervation to the proximal ulnar head of the flexor carpi ulnaris and then more distally to the more anterior humeral head of the flexor carpi ulnaris (▶Fig. 14.4c). In this figure, the fascia and the ulnar head of the flexor carpi ulnaris have been reflected to expose the ulnar nerve along its course from the medial epicondyle distally. The nerve at the level of the released ligament of Osborne in this specimen has been compressed and there is precompressive neuromatous enlargement of the nerve just proximally. The first most relevant branch to the deeper humeral head of the flexor carpi ulnaris enters muscle 5- to 7-cm distal to the medial epicondyle. The posterior band of the ulnar collateral ligament serves as the floor of the cubital tunnel deep to the ulnar nerve. The anterior band of the ulnar collateral ligament lays one fingerbreadth anterior to the ulnar nerve under the flexor pronator origin. Access to the anterior band of the ulnar collateral ligament and to the medial coronoid is most easily obtained leaving the ulnar nerve in situ rather than by transposing it anteriorly, where the branches, as shown here, would be in the way of deeper dissection.
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