Proximal radioulnar joints

The bony structures of the elbow are the distal end of the humerus and the proximal ends of the radius and ulna. The elbow joint complex allows two degrees of freedom in motion flexion/extension and pronation/supination. The elbow joint complex is three separate synovial articulations. The humeral-ulnar joint is the articulation between the trochlea of the distal radius and the trochlear fossa of the proximal ulna. The humero-radial joint is formed by the articulation between the capitulum of the distal humerus and the head of the radius. The proximal radioulnar joint is formed by the head of the radius and the radial notch of the proximal ulna. 

Three mechanisms have been proposed for Monteggia injuries hyperextension of the joint, direct blows and hyperpronation. Most falls onto the outstretched limb occur with the arm pronated and in this position, the ligamentous restraints around the proximal radioulnar joint are most lax, predisposing to dislocation. 

Besides flexion and extension, the forearm can rotate around its longitudinal axis. Rotation involves the proximal radioulnar joint of the elbow and the distal radioulnar joint, which lies above the wrist. These motions are observed with the forearm flexed to 90 degrees. 

FIGURE 49.27 The nonlinear relation between the contact area and the load at the proximal wrist joint. The contact area was normalized as a percentage of the available joint surface. The load of 11, 23, 46, and 92 lbs was applied at the position of neutral pronation/supination, neutral radioulnar deviation, and neutral flexion/extension. (From Viegas S.F., Patterson R.M., Peterson P.D., Roefs J., Tencer A., and Choi S. 1989. /. Hand Surg. 14A 458. With... 

The complexity of joint motion at the wrist makes it difficult to calculate the instant center of motion. However, the trajectories of the hand during radioulnar deviation and fiexion/extension, when they occur in a fixed plane, are circular, and the rotation in each plane takes place about a fixed axis. These axes are located within the head of the capitate and are not altered by the position of the hand in the plane of rotation [Youm et al, 1978]. During radioulnar deviation, the instant center of rotation lies at a point in the capitate situated distal to the proximal end of this bone by a distance equivalent to approximately one-quarter of its total length (Figure 49.28). During fiexion/extension, the instant center is close to the... 

FIGURE 49.30 (a) Contact area of the MCP joint in five joint positions, (b) End on view of the contact area on each of the proximal phalanx bases. The radioulnar width of the contact area becomes narrow in the neutral position and expands in both the hyperextended and fully flexed positions. (From An K.N. and Cooney W.P. 1991. In B.F. Morrey (Ed.), Joint Replacement Arthroplasty, pp. 137-146, New York, Churchill Livingstone. By permission of Mayo Foundation.)... 

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