1. The patellae of rabbits can be displaced without opening or otherwise injuring the knee joint. Such patellar displacement is of itself adequate cause for degenerative and hypertrophic changes in cartilage, similar to those of degenerative joint disease.

2. The undue pressure and friction of the displaced patellae resulted in rapid loss of the underlying articular cartilage with subsequent eburnation of the subchondral bone.

3. The absence of normal patellar apposition and, in consequence, of lubrication of the non-articulating surfaces with synovial fluid was responsible for articular-cartilage degeneration and atrophy.

4. Proliferation of the connective tissue at the perichondrial margins of the articular surfaces led to well-defined marginal overgrowth. This marginal overgrowth was sufficiently marked to form a new patellar articulating surface on the medial side of the femur.

5. The repeated stretching of the synovial membrane at its insertion, coincident with full extension, would seem adequate stimulus to cause the very vascular, undifferentiated, perichondrial connective tissue to proliferate sufficiently to result ultimately in marginal lipping. Such marginal proliferation occurs to some degree in all joints with increasing age and long-continued use, but develops much more rapidly in joints with anatomical derangements such as patellar displacement.

6. The clinical and pathological findings from one patient with bilateral displacement of the patella were identical with those of degenerative joint disease.

7. These studies emphasize the importance of early corrective measures in patients with displaced patella if irreparable joint damage is to be prevented.