There are three different types of osteonecrosis of the knee. There is Spontaneous Osteonecrosis of the Knee, Post Arthroscopic Osteonecrosis of the knee, and Secondary Osteonecrosis of the knee. Osteonecrosis is further classified by severity using Ficat Stages of Knee Osteonecrosis. In Stage I, the x-ray appears normal. Stage II, Sclerosis of the condyle is present. In Stage III, the crescent sign is found as well as a subchondral fracture. Stage IV, there is a collapse of subchondral bone.
Spontaneous Osteonecrosis of the Knee typically occurs in females older than 55. Usually one joint and one compartment is affected (medial femoral compartment). No etiology is known. Symptoms typically consist of a sudden onset of severe pain with decreased range of motion as well as swelling in the knee. X-rays will probably appear to be normal. An MRI is helpful, the provider may find a crescent shaped lesion. This condition
can cause arthritis. If severe knee pain is present in a middle aged or elderly female patient, and the x-ray is negative, the provider should order an MRI to rule out osteonecrosis of the knee. Treatment consists of protected weightbearing as well as therapy and NSAIDs. An arthroplasty may be required when conservative treatment fails. A unicompartmental knee arthroplasty will be performed for small lesions. A total knee arthroplasty will be completed for large lesions or collapse.
Post Arthroscopy Osteonecrosis of the Knee most commonly occurs in middle aged 
women after a knee arthroplasty. Secondary Osteonecrosis of the knee is common in women under 55 years of age and has associated risk factors. This condition involves more than one compartment or the metaphysis of the knee. Secondary Osteonecrosis occurs bilaterally in 80% of cases and multifocal lesions may be seen. There is a cause for Secondary Osteonecrosis of the knee, which is why it can be bilateral, multiple, and everywhere. Risk factors include:
- Alcohol use
- Sickle cell disease
- Steroid use
- Trauma
- HIV medications
- Gaucher Disease
These patients should be screened for other joint involvement. The lesion is a subcondylar insufficiency fracture and the patient will have severe pain with weight bearing either standing or sitting. An x-ray may show a wedge-shaped lesion and MRI is the better study. A differential diagnosis is Osteochondral Dissecans, which is located in the lateral aspect of the medial femoral condyle in younger patients. Other differentials include: Occult trauma, bone bruise and overuse, as well as transient osteoporosis which is found more in middle-aged men and usually in the hip rather than the knee. Treatment consists of NSAIDs, a decrease in activity and weightbearing, and physical therapy. A scope surgery may be necessary to remove loose fragments or core decompression for lesions not extending to the joint. An Osteocondylar allograft may be performed for large, painful lesions in younger patients. A total knee replacement may be done for larger lesions, for collapse, or if multiple compartments are involved. Conservative treatment is not as successful with secondary avascular necrosis. Without surgery, secondary AVN will advance to osteoarthritis. Bisphosphonates have no effect on knee osteonecrosis.
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