As discussed previously, the elbow is not just a simple hinge joint but instead possesses two degrees of freedom or motion: flexion-extension and pronation-supination. However, with the primary range of motion occurring in the flexion-extension direction, there are reciprocally convex and concave articular surfaces along the distal humerus, radius, and ulna; a capsule, loose on the sides toward which movement takes place; strong collateral ligaments; and a grouping of muscle masses at the borders where they are not in the direction of movement.
The articular surfaces are the spool-shaped trochlea and the rounded capitellum of the humerus proximally and the trochlear notch of the ulna and the cupped upper surface of the humeral head of the radius distally. The capitellum of the humerus is directed forward and downward, with the articular surfaces most completely in contact when the elbow is flexed to a 90-degree angle. Contact is weak between the humerus and the radius, and both the stability of the joint and its limitation of motion to flexion and extension are due to the ridged and grooved relationship of the humerus and the ulna. The articular capsule is weak in front and behind but strengthened at the sides by the ulnar and radial collateral ligaments. In front, it is attached on the humerus from the medial to the lateral epicondyles along the superior borders of the coronoid and radial fossae. Distally, it is attached to the anterior border of the coronoid process of the ulna and to the annular ligament of the radius; it is continuous on either side with the collateral ligaments. The posterior portion of the capsule is membranous. Its attachments are the margins of the olecranon and the edges of the olecranon fossa, the lateral epicondyle, the annular ligament, and the posterior border of the radial notch of the ulna.
The collateral ligaments are strong, triangular thickenings of the articular capsule, attached by their apices to the medial and lateral epicondyles of the humerus. Their broader distal attachments are to the forearm bones and the annular ligament of the radius. These ligaments place strict limitations on side-to-side displacements of the joint.
The ulnar collateral ligament has thickened borders, with the anterior band reaching the medial edge of the coronoid process and the posterior band attaching to the corresponding edge of the olecranon. The thinner intermediate portion ends below, in transverse fibers stretched between the coronoid process and the olecranon.
The radial collateral ligament, a narrower, less distinct thickening, is stretched between the underside of the lateral epicondyle above and the annular ligament and the margins of the radial notch of the ulna below.
The synovial membrane of the elbow joint lines the capsule and is reflected onto the borders of the radial and coronoid fossae of the humerus in front and the olecranon fossa behind. Below, it continues into the proximal radioulnar articulation.
The hinge action at the elbow joint is not exactly in the line of the long axis of the humerus. In extension, the forearm deviates from a straight line with the arm, forming the carrying angle of the forearm, which is obliterated when the hand is pronated. As discussed previously, the carrying angle has been reported to range from 11 to 14 degrees of valgus in men and from 13 to 16 degrees of valgus in women. Because of a slight spiral orientation of the ridge of the trochlear notch and of the groove of the trochlea, flexion does not bring the forearm bones medial to the humerus. The habitual ease with which the hand is carried to the mouth in elbow flexion is due to the slight medial rotation of the humerus and the semipronated position of the hand.
Proximal Radioulnar Articulation
The head of the radius rotates in a ring formed by the radial notch of the ulna and the annular ligament of the radius. The annular ligament of the radius is a strong, curved band attaching to the anterior and posterior margins of the radial notch of the ulna. It serves as a restraining ligament, which prevents withdrawal of the head of the radius from its socket. The annular ligament receives the radial collateral ligament and blends with the capsule of the elbow joint. Below, a lax band, called the quadrate ligament, passes from the lower border of the radial notch of the ulna to the adjacent medial surface of the neck of the radius.
The synovial membrane of this joint is continuous with that of the elbow joint. A reflection of the membrane below the annular ligament forms a loose sac around the neck of the radius, which accommodates to the rotation of the head of the radius.