Avascular Necrosis (AVN) of the Knee

Avascular necrosis (AVN) is the bone death that occurs as a result of disruption of the blood supply to a part of the bone. Avascular necrosis (osteonecrosis) commonly affects the hip but may also occasionally affect the knee joint.

The management of knee joint osteonecrosis is usually surgical and many patients with advanced-stage surgery may need knee replacement surgery.

The bones in our body are living tissues that require a constant supply of blood that brings oxygen and nutrition for their structure and function. Any disruption of the blood supply leads to bone death (infarct) in the affected area.

The body tries to repair the damage by the formation of new blood vessels that creep in to help bone cells remove dead bone and substitute the dead bone with scar tissue or new bone. The process however is completed in avascular necrosis as in AVN there is usually an increase in pressure of the bone marrow.

The increased pressure inside the bone is detrimental to the formation of new blood vessels and the subsequent repair. Further, in the case of osteonecrosis of the bones of a joint such as knee AVN, the bone death near the joint surface may lead to a collapse of the joint.

The knee joint is the largest weight-bearing joint in the human body. The knee joint is formed by the lower part of the thigh bone (femur), the upper part of the shinbone (tibia), and the kneecap (patella). The lower part of the femur has knobby ends known as condyles that divide the knee joint in the inner and the outer compartment.

The ends of the bones that form the knee joint are covered with a glistening tough tissue known as articular cartilage. The bone just beneath the articular cartilage is known as the subchondral bone. The AVN of the knee commonly occurs in the inner knobby part of the femur known as the medial femoral condyle. AVN may also occur in the lateral condyle of the shin bone (the tibial plateau).

Causes and risks of Knee AVN

The majority of the cases of AVN knee are idiopathic, i.e no single cause has been found that leads to avascular necrosis. Spontaneous osteonecrosis of the knee (SONK) is self-limiting avascular necrosis of the inner side of the knee joint. However, there are risk factors that have been identified to play a role in the development of avascular necrosis.

Long-term consumption and abuse of alcohol have been associated with the risk of avascular necrosis. The consumption of alcohol may lead to an increase in free fatty acids in the bloodstream that may block the blood supply of the bone.

Long-term use of corticosteroids has also been associated with avascular necrosis of the knee joint. The steroid therapy may have been used in various diseases such as lupus nephritis, asthma, rheumatoid arthritis, etc. Abuse of corticosteroids in athletes and bodybuilders has also been associated with knee AVN. Steroid-induced osteonecrosis occurs as the steroids may increase the marrow pressure secondary to edema.

The AVN of the knee may also secondary to any injury to the knee that may occur as a result of a fall/accident or any prior surgery around the knee joint. Certain diseases such as thalassemia may cause avascular necrosis as the deformed blood cells may clog the blood supply.

Certain rare diseases such as Gaucher’s disease may lead to avascular necrosis of the knee joint due to abnormal deposition of tissues in the bone marrow. The knee AVN may also occur in patients with a history of a kidney transplant, and at times in patients with HIV infection.

Symptoms

The symptoms of knee AVN may begin suddenly in the form of sharp pain or may begin insidiously. The pain is frequently located in the inner side of the knee joint. The patients may at times report a trivial fall/accident leading to the start of pain or worsening of previous pain. The fall/trauma may lead to the collapse of the subchondral bone weakened by avascular necrosis.

The pain in the AVN of the knee may be associated with swelling and restricted range of motion of the knee joint. The pain may initially be associated with weight-bearing activity but may also occur at rest. The pain worsens over time and may cause the patient to walk with a limp.

Stages and Diagnosis

The initial stages of the knee avascular necrosis may be asymptomatic or may complain of mild pain and the examination of the knee joint may be normal. Radiological studies such as a plain X-ray may be normal in stage 1. There may be subtle findings suggestive of AVN knee in an MRI and a bone scan.

Stage 2 involves the collapse of the subchondral bone and the patient may complain of severe pain in the knee. The finding of subchondral collapse is bisphosphonates usually apparent on an X-ray but may be confirmed on an MRI.

The subsequent stage 3 involves the collapse of the joint surface and osteoarthritic changes in the joint. Stage 4 involves severe osteoarthritis of the knee joint with possible restriction of the range of motion. The findings of stages 3 and 4 are clearly visible on an X-ray. The time between the progression of various stages may vary from weeks to months or years.

Management

The management of the knee AVN depends upon the stage of the disease and the part of the knee involved. AVN of the non-weight bearing part of the knee joint may usually resolve on its own.

Stage 1 management includes protected weight-bearing with the use of crutches, cane, or knee splints. Medications such as bisphonates (anti-resorption) and statins may be used to reduce the progression of avascular necrosis. In the early stages, protected weight-bearing helps to prevent the collapse of the bone. NSAIDs medication may be used to manage the knee pain associated with knee AVN.

In stage 2 of the disease, besides conservative, joint-saving surgical treatment may be used in the form of core decompression, bone and cartilage transplant, autologous chondrocyte implantation, and osteotomy may be used.

In core decompression, multiple small holes are drilled in the area of the AVN to reduce the marrow pressure and increase blood flow. A reduction in marrow pressure helps the new blood vessels to form and start the repair process. Bone graft along with cartilage may also be used to treat the area of bone death after thorough debridement.

Bone cutting surgeries such as osteotomy helps by offloading the joint surface that may stop the progression to joint collapse. Unfortunately, the majority of the cases of knee AVN progress to stages 3 and 4. The treatment option in advanced stages is a total knee replacement surgery that provides excellent relief from the symptoms and improves function.