Functional morphology of forelimb joints in the woolly monkey Lagothrix lagothricha

This gross anatomical study of embalmed forelimb joints of the South American woolly monkey Lagothrix lagothricha focuses on the problem of determining in osteoligamentous preparations how the disposition of the capsular apparatus and the geometry of the articular surfaces govern the amount and types of movement permitted at a joint, and then correlates these findings with the use of the forelimb in the positional capabilities of captive wooly monkeys observed by this author. Data collection was by dissection, quantitative range of motion studies on osteoligamentous preparations, and by qualitative manipulations of these preparations and of disarticulated bones. Supplemental evidence was obtained from radiographs and from an estimate of angular values of articular surfaces. Presented for the shoulder, elbow, radioulnar, and hand joint complexes are the functional anatomy of the capsular apparatus and articular surfaces, the quantitative range of motion data, and proposed mechanisms of movement that combine the functional morphology with the observed use of the forelimb in activities by the animal. The structurofunctional framework of MacConaill is the basis for the joint analysis. The evidence suggests that the functional anatomy of these joints correlates well with the possible positional capabilities of Lagothrix. The morphology of the capsular apparatus and joint surfaces reflect both the arboreal quadrupedalism and forelimb suspension of the woolly monkey. The positions of maximum congruency and close-pack approximate the forelimb's weight-bearing stance during palmigrade arboreal quadrupedalism. During forelimb suspension, the taut and twisted capsular apparatus of close-packed joints maintains the integrity of forelimb links in tension. This is the first known report of the internal band of the collateral ligament of the proximal interphalangeal joints of the second through fifth digits being incorporated into the terminal tendon of the extensor assembly. This author believes this is a mechanism not only for maintaining the integrity of these joints during forelimb suspensory activities when the body is supported by the middle phalanges, but also for helping to coordinate the flexion-extension actions of the phalanges for smooth grasping and release maneuvers in an arboreal environment. When sufficient comparative data have been collected, such correlations of the functional morphology of joints with positional behavior may be used to postulate positional capabilities of fossil primates.