This spiral trochlear configuration allows the forearm to move in a sagittal plane while maintaining the stability of ulnohumeral contact through the cam effect of the ulnar condyle during humeral rotation. ![]() The articular surface on the proximal ulna is oblique in orientation, and the distal half retains an articulation with the ulnar condyle. The trochleariform distal humeral articular surface in modern mammals largely came about by widening the intercondylar groove and the development of a ridge within it (see Fig. The oblique orientation of the humeroulnar joint resembled a spiral configuration, which helped to keep forearm movement in a sagittal plane as the humerus underwent a compound motion involving adduction, elevation, and rotation during propulsion. The ulnar notch had articular surfaces for both the ulnar and the radial condyles, each matching the configuration of the corresponding humeral surface. The two condyles were separated by an intercondylar groove. However, the radial condyle was more protuberant than the ulnar, and the ulnar condyle was more linear and obliquely oriented (see Fig. This ridge articulated with the groove between the radial and ulnar condyles displaying some features in common with the “tongue and groove” (trochleariform) type of humeroulnar articulation characteristic of many modern mammals.Įarly mammals from the Triassic (210 to 160 mya) and Jurassic (160 to 130 mya) periods also had radial and ulnar condyles. The lateral flange on the ulna for articulation with the radius was separated from this surface by a low ridge. The proximal ulnar articular surface was an elongate spoon shape for articulation with the humeroulnar condyle. The distal humeral articular surface consisted of radial and ulnar condyles separated by a shallow groove (see Fig. 137:281, 1973.)Ĭynodonts, the more immediate ancestors of mammals from the Permo-Triassic period (235 to 160 mya), had their limbs underneath their bodies rather than at the sides. Jr.: The functional anatomy and evolution of the mammalian humeroulnar articulation. Subsequent evolutionary stages show accommodations to increasing mobility. The form of the pelycosaur elbow was designed to maximize stability. The distal ends of the humeri are shown on the left, and the corresponding radius and ulna are on the right. ![]() It appears, therefore, that stability rather than mobility was the major functional characteristic of the elbow of these late Paleozoic reptiles.įIGURE 1-1 The major evolutionary stages in the development of the elbow joint from pelycosaurs to advanced mammals. ![]() The ulnohumeral joint, with its dual articular surfaces, was well suited to resist the valgus/varus stress produced by humeral rotation, and the proximal end of the radius was flat and triangular, precluding pronosupination. Elbow flexion and extension probably were useful only in side-to-side motions. Forward motion was brought about by rotation of the humerus around its long axis, which propelled the body forward relative to the fixed forefoot. The humerus was held more or less horizontal, the elbow flexed to 90 degrees, and the forearm was sagittally oriented. Reconstruction of the forelimb of these reptiles suggests that they walked with limbs splayed out to the side. 11 The proximal articular surface of the ulna was similarly divided into two surfaces separated by a low ridge. The articulation with the ulna was formed by two distinct surfaces: a slightly concave ventral surface and a more flat dorsal surface (Fig. The distal humerus of pelycosaurs, the late Paleozoic (255 to 235 mya) reptiles that probably gave rise to more advanced mammal-like reptiles, possessed a bulbous capitellum laterally and medially.
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