ÐYSCHONDROSTEOSIS, ) a genetically / inherited chondrodystrophy with an autosomal dominant character, occurs more frequerltly than appreciated. It is usually identified by x-ravs of wrist and forearms taken in adolescence. ObservationsCase 1 This 19-year-old girl liars clinically evident &dquo;fork-shaft&dquo; deformity of the distal extremity of the ulna. X-rays of the forearm show evidence of this disease (Fig. 1). There is curving of the radius with posterior and lateral convexity of the shaft which is shortened and the interosseous space is increased. There is obliquity of the lower radial epiphysis, in a longitudinal direction. The carpal arch is ovoid with vertical semilunar bone. The radial head is flattened. Review of x-rays of this patient taken four years earlier show the same forearm changes, less pronounced. The lateral films show curvature of the shaft of ulna with posterior convexity. The lower end of ulna is dislocated backwards and is not part of the wrist articulation.At 19 years of age this patient is only 1.52 meters in height. A family survey shows identical deformities in her sister, father, grandmother, and great grandmother. Her brother is normal. Her sister has bilateral tibio-peroneal exostoses (~ i~. 2), identical to changes described by Kaplan.Case 2 This 1 a-year-old boy was studied because of short stature. He has the characteristic deformities of the forearms with pronounced radial curvature and hypoplasia of the radial head (Fig. 3). The marked hypertrophy of the border of radius is referred to as the &dquo;fin-bone&dquo; appearance. DiscussionWith regard to the pathogenesis of these bone deformities, the onset appears to be at the level of the metaphyseal plate. The primitive defect in the wrist is the lack of development of the medial half of the lower end of radius. Instead of the usual sigmoid curve (Fig. 5), there is obliquity of the articular surface and enlarged semilunar fusing with the medial border of the lower end of the radius. The lower articular end of the radius is displaced anteriorly; this may account for the shape of the wrist with defects of articulation in the wrist joint and limitation of movement. Due to contraction of the pronator muscle and the flexor digitorum superficialis the proximal bones of the wrist are pulled into contact with the lower end of FIG. 1. A l~J-year-old girl with dyschondrosteosis. Curving of the radius, carpal arch ovoid, flattening of radial head.

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