The functional and phylogenetic significance of the carpometacarpal joint of the thumb in anthropoid primates.
Author: Rafferty, Katherine L
Awarding University: New York University, USA
Level : MSc
Subject Terms: Anatomy and physiology/Primates/Thumb/ ;
The ability of catarrhine primates to oppose the thumb has been associated with the unique acquisition of a saddle-shaped first carpometacarpal joint that permits rotation of the first metacarpal. However, it has also been suggested that the saddle-shaped carpometacarpal joint of the thumb is a primitive primate feature and therefore has little functional or phylogenetic significance. To resolve this problem, the bony morphology of the first carpometacarpal joint was surveyed in extant anthropoid primates and in the Miocene primate Proconsul africanus. Cross-sections were made through casts of the trapezium and first metacarpal in the dorsoventral and mediolateral plane in order to study the &;figuration and relationship of the joint surfaces. Platyrrhines exhibit a diversity of morphological patterns of the carpometacarpal joint of the thumb. This joint is consistently concavo-convex (or saddleshaped) in the more generalized platyrrhines, suggesting a saddle-shaped joint is a primitive anthropoid feature. Cross-sections reveal that the joint surfaces that are highly congruent in the dorsoventral and mediolateral planes. This high degree of congruency prevents axial rotation of the first metacarpal necessary for opposition.In cercopithecids, the joint surfaces are reciprocally concavo-convex and show little morphological variability. The base of the first metacarpal has a smaller radius of curvature than the facet of the trapezium in the mediolateral plane. As a result of this joint incongruity,the first metacarpal is highly mobile and unstable in the mid-position and thus can undergo axial (adjunct) rotation, allowing opposition of the thumb. Differences in the prehensile abilities among cercopithecid genera are reflected in the relative lengths of the thumb and index and in soft anatomy, rather than in joint morphology.The primitive anthropoid concavo-convex joint surfaces and the primitive catarrhine mediolateral incongruity are retained in hominoids, except for hylobatids which have a specialized ball-and-socket first carpometacarpal joint. The joint surface of the trapezium of Pan and Gorilla exhibits features that have been associated with indirect (conjunct) rotation of the first metacarpal in humans. In humans and perhaps in African apes as well, the first metacarpal will undergo rotation as a bi-product of the movements of abduction followed by flexion by virtue of the slightly curved path created by differential convexity of the trapezium. The first carpometacarpal joint of Proconsul afn'canus exhibits the mediolaterally incongruent joint surfaces unique to catarrhines among anthropoid primates. Overall, the trapezium of Proconsul africanus is cercopithecid-like, but, like the rest of the carpus, there are hominoid features as well.