Post-traumatic differential osteoporosis of the patella.

The differential density developing after patella fractures between the proximal and distal poles has been attributed to avascular necrosis of the proximal fragment. Six cases are presented here where differential densities between the proximal and distal poles developed after trauma to the patella or knee. Based on the natural history of the condition, the rapidity of onset of the differential densities, osteoporosis of the whole patella later and complete resolution of the changes finally, we postulate that this phenomenon is due to a differential rate of osteoporosis of the proximal and distal parts of the patella because of the differences in their blood supply and not because of avascular necrosis of the proximal pole.