Radiographic appearance of the abnormally abducted thumb in a child with diastrophic dysplasia. Discussion Prevalence Diastrophic dysplasia is a rare osteochondrodysplasia. It has been observed in most white populations but is remarkably more prevalent in Finns. In 1990 there were 160 patients known to be affected in Finland, while it is estimated that less than 300 patients have been described in the rest of the world 3. The genetic locus for diastrophic dysplasia has been mapped to the distal part of the long arm of chromosome 5, distal to the gene for adenomatous polyposis coli 3 .
Evidence suggests that diastrophic dysplasia is genetically homogeneous, at least in Finland.
Although the genetic locus has been mapped, the underlying biochemical defect in cartilage has been more difficult to identify. There are broad collagen fibrils surrounding degenerating chondrocytes in the cartilage of patients with diastrophic dysplasia. Stanescu et al suggested that this might be due to a structural defect in type II collagen 5 . However, Murray et al 6 found normal type II collagen in their patients with diastrophic dysplasia and believe the previously reported spacing crystallite pattern was due to an artifact of alignment of the crystallites. In addition, they reported that the broad collagen fibrils thought to be characteristic of diastrophic dysplasia cartilage can be seen in normal subjects when the cartilage structure is disrupted by extraction with guanidine and, thus, may be artifactual.