Meniscus tear, anterior/posterior collateral ligament tear, patellofemoral pain syndrome, tibial fracture, Osgood-Schlatters disease, popliteal cyst rupture, and osteonecrosis.
Differentiating OCD from osteonecrosis is difficult, but the most significant clue is the age of the patient. Younger patients tend to develop OCD, and older patients tend to develop osteonecrosis.