summary Osteonecrosis of the knee represents a condition caused by reduced blood flow to bone secondary to a variety of risk factors such as sickle cell disease, steroid use, alcoholism, autoimmune disorders, and hypercoagulable states. Diagnosis is radiographic for advanced disease but requires MRI for detection of early or subclinical disease. Treatment is generally observation with management of the underlying systemic condition. Operative management is indicated for advanced disease with presence of severe subchondral collapse. Epidemiology Demographic women:men 3:1 more common in women <55 years with risk factors reported to be found after knee arthroscopy in middle-aged women Anatomic location typically involves more than one compartment of the knee or even the metaphysis 80% are bilateral multifocal lesions are not uncommon Risk factors alcoholism dysbaric disorders (decompression sickness, "the bends") marrow-replacing diseases (e.g. Gaucher's disease) sickle cell disease hypercoagulable states steroids (either endogenous or exogenous) SLE inflammatory bowel disease transplant patient virus (CMV, hepatitis, HIV, rubella, rubeola, varicella) protease inhibitors (type of HIV medication) trauma Etiology Pathophysiology spontaneous osteonecrosis appears to represent a subchondral insufficiency fracture another hypothesis is an association with a meniscal root tear Physical Exam Symptoms pain with weightbearing, especially sitting to standing Imaging Radiographs first line imaging studies AP knee lateral knee merchant view knee findings wedge-shaped lesion on imaging MRI most useful study findings highest sensitivity and specificity T1: dark T2: bright (marrow edema) Differential Osteochondritis dissecans (OCD) more commonly found at lateral aspect of medial femoral condyle of 15 to 20-year-old males Transient osteoporosis more common in young to middle-aged men multiple joint involvement found in 40% of patients (transient migratory osteoporosis) Occult fractures and bone bruises associated with trauma, weak bones, or overuse Spontaneous osteonecrosis of the knee (SONK) Treatment Nonoperative NSAIDs, limited weightbearing, quadriceps strengthening, activity modification indications first-line of treatment outcomes favorable, but less so than nonoperative management for SONK Operative diagnostic arthroscopy indications remove small, unstable fragments from the joint core decompression indications extra-articular lesions osteochondral allograft indications large symptomatic lesions in younger patients that failed nonoperative management total knee arthroplasty (TKA) indications large area of involvement collapse osteonecrosis in multiple compartments Prognosis Self-limiting condition