[An experimental study on the etiology of chondromalacia patellae]

Chondromalacia patellae is one of the common causes of patellofemoral disorders in young subjects. It frequently occurs in patients with lateral subluxation of the patella. However, the etiology of this disease is still obscure. The purpose of this study was to elucidate the histological changes of articular cartilage and bone in chondromalacia patellae. Lateral subluxations of the patellae were produced experimentally in forty New Zealand white rabbits. The mean tilting angle of the patellae was 14.2 degrees just after surgery, 10.7 degrees after 6 weeks, and 7.1 degrees after 12 weeks respectively. Contact pressure was lower on the medial facets than on the lateral facets and both facets of the controls. Macroscopic changes on the patellar cartilage did not appear until 12 weeks postoperatively. On the other hand, histological changes such as vascular invasion of the calcified layer, irregularity of tidemark and a decrease in the number of chondrocytes in the deep layer of the cartilage were observed on the medial facet about 4 weeks postoperatively. The degeneration of chondrocytes and matrix gradually progressed from the deep layer to the superficial layer. Twelve weeks after, a depressed area was shown in the medial facet macroscopically. Repairing process was substantiated such as the proliferation of synovial tissue, the cluster formation of chondrocytes and the recurrence of the tidemark about 30 weeks postoperatively. From the results mentioned above, it is suggested that cartilaginous and bony degeneration were caused by a decrease in contact pressure on the articular cartilage. The histological features of the chondromalacia patellae would be eventually developed by degenerative changes and restorative reaction.