Clinical findings:
(1) usually an elderly patient, typically > 65 years
(2) sudden onset of knee pain, typically along the medial side
(3) no history of an inciting cause such as trauma or surgery
(4) recurrent swelling of the knee after full weight bearing, due to the presence of an effusion
(5) tenderness over the affected femoral condyle
(6) restricted range of motion in the affected knee
(7) imaging changes consistent with osteonecrosis (may initially only be detected on bone scan or MRI, while advanced stages are detectable on routine X-rays of the knee)
MRI changes may take several weeks to develop, so MRI should be delayed for at least 6 weeks after the onset of symptoms.
Differential diagnosis:
(1) degenerative meniscal tear: Some patients may undergo arthroscopy without benefit or with worsening of symptoms.
(2) osteoarthritis
