The morphological characteristics of the antebrachiocarpal joint of the cheetah (Acinonyx jubatus)

Abstract

A morphological study of the structures of the antebrachiocarpal (AC) joint of the cheetah was carried out by dissection of eight forelimbs obtained from four adult cheetahs culled from the Kruger National Park, Republic of South Africa. The aim was to evaluate the deviations of this joint from the normal feline pattern and to consider their possible relationship to the cheetah's adaptation to speed. Although published data on the AC joint of the other felids show general resemblance to that of the cheetah, there are nevertheless slight, but significant variations and modifications which tend to suggest adaptation to speed. The shafts of the radius and ulna of the cheetah are relatively straight and slender, with poorly developed distal ends. The ulnar notch is reduced to a very shallow concavity while the corresponding ulnar facet is a barely noticeable convexity, separated from the distal ulnar articular facet by an ill-defined groove. The movement of the distal radio-ulnar joint is highly restricted by the presence of a fibro-cartilaginous structure and a strong interrosseous membrane, limiting pronation and supination normally achieved by the rotation of the radius around the ulna. The extensor grooves at the distal extremity of the radius are deep and narrow and are guarded by prominent ridges. A thick extensor retinaculum anchors the strong extensor tendons in these grooves. The distal articular surface of the radius is concave in all directions except at the point where it moves into its styloid process. At this point it is convex in the dorsopalmar direction, with a surface that is rather deep and narrow. The proximal row of carpal bones presents a strongly convex surface, which is more pronounced in the dorsopalmar direction with the greatest convexity on the lateral aspect. Medially, there is a ridge·Iike concavity across the base of the tubercle, which rocks on the flexor surface of the radius, limiting excessive flexion as well as restricting lateral deviation of the AC Joint.