Hoffa's fat pad impingement syndrome can be caused by a wide variety of acute or chronic
injuries, most often seen in athletic individuals. In the initial stages hemorrhage and edema may be visualized on MR, and as the disease progresses Hoffa's fat pad may hypertrophy, leading to further impingement and injury. Rarely, this may lead to ossification in the region of impingement, as seen in this case. When this happens it is often referred to as an extraskeletal ossifying chondroma. 

Interestingly, Hoffa disease is often associated with disorders that cause altered patellar tracking (patella alta, trochelar dysplasia, etc) which are not seen in this case. It is because of this that the disease is most often observed within the lateral aspect of Hoffa's fat, but in this case it is medial which is uncommon, possibly related to an old acute trauma.