Knee pain has multiple causes, the most common being osteoarthritis, particularly in the older population. However, there are other conditions besides osteoarthritis which can cause pain, such as meniscal cartilage tears and ligament injuries of the knee, or issues that affect blood circulation in the surrounding bone area, leading to a condition called osteonecrosis.
One must note that bone is a living tissue - much like any other part of the body - and requires blood and oxygen to survive. In osteonecrosis, the blood supply to an area of bone is interrupted and results in death of that segment of bone. The term “osteonecrosis” is a Latin term for “bone death” and the condition is also referred to as avascular necrosis (or AVN).
When an area of bone loses its blood supply as a result of osteonecrosis, the body tries to replace it with living bone in a process sometimes referred to as "creeping substitution." However, in this progression the softening and absorption of necrotic bone outpaces new bone formation. During this replacement process there is a temporary weakening - and the possibility of collapse - of this segment of bone.
When osteonecrosis involves a weight bearing surface near a joint such as the knee, the weakened surface may break or collapse under normal loading. This surface fracture, called a subchondral fracture, may cause sudden, acute pain in the joint.
The knee joint is made up of three bone surfaces:
Figure 1: Anatomy of the Knee
Osteonecrosis of the knee is most commonly seen in the femoral condyle, usually on the inner side of the knee (the medial femoral condyle). On occasion, osteonecrosis can occur on the outside of the knee (the lateral femoral condyle) or on top of the shin bone (the tibial joint surface), known as the tibial plateau.
Although osteonecrosis has several causes, in the overwhelming majority of patients the exact cause of the osteonecrosis is not known; this is referred to as idiopathic AVN. Although the exact mechanism is not fully understood, idiopathic AVN is associated with certain disease conditions as described in this section.
One theory is that fat globules form inside the microvessels of the bone, resulting in blockage of the vessels and diminished circulation. In addition, some patients have a specific activity or trauma associated with their pain, and this can be a result of a bone contusion (bone bruise) or stress fracture. It has been noted that if the pressure within the bone is measured in an area of osteonecrosis, there is usually marked increase in pressure along with very fatty bone marrow.
Women are more commonly affected, typically three times that of males, and it is more common in those 60+ years of age.
The conditions that are associated with osteonecrosis of the knee are:
In thalassemia and sickle cell anemia, the avascular necrosis is a result of a change of shape of the blood cells, which cause them to clump and block off the small, microvessels in the bone.
Steroid-induced osteonecrosis is usually the result of prolonged high-dose steroid therapy as sometimes necessary in the treatment of lupus and other diseases, or more rarely, in patients who receive a large single dose. Steroid-induced osteonecrosis may affect multiple joints such as the hip, knee, and shoulder, and can be seen in younger patient groups.
Another common association with osteonecrosis is that of high alcohol intake. Alcoholics are at higher risk for developing osteonecrosis, again occurring in the hip, knee, and elsewhere.
Osteonecrosis can also be seen in patients with asthma who receive steroid treatment,.
Typically, osteonecrosis in the knee results in sudden onset of pain. It may be triggered by a specific seemingly routine activity or minor injury. Also, patients who have known mild-to-moderate osteoarthritis who suddenly get worse may be experiencing a local area of osteonecrosis that suddenly worsens their condition.
Osteonecrosis is often associated with increased pain with activity and at night. It may also cause swelling of the knee and sensitivity to touch and pressure, and can result in limited motion due to pain and swelling.
Osteonecrosis in the early stages can be difficult to diagnose because often it is not apparent on plain radiographs (x-rays). More sophisticated imaging, such as a bone scan or MRI scan, may be necessary to diagnose early stages of the disease.
Typically, in early stage disease (also known as stage I) the symptoms may be quite intense and, because routine x-rays are normal, a positive bone scan or MRI is needed to make the diagnosis.
The initial treatment is usually non-surgical, focusing on pain relief, protected weight bearing, and treating the underlying metabolic cause of the disease if it exists.
Patients with early osteonecrosis of the knee (stage I) may be treated with crutches with protected weight bearing to prevent further collapse of the weak joint surface. Knee braces, designed to relieve pressure on the involved joint surface, are sometimes beneficial. Medical treatment may consist of treatment with bisphosphonates (an antiresorptive medication such as Fosamax) to try and prevent excessive resorption and weakening of the bone, and/or fat metabolism-altering drugs known as statins. These medications theoretically affect fat metabolism, which can cause the disease, in addition to treating the bony issues as well.
In stage II disease there are boney changes in the area of osteonecrosis that may be visible on plain x-ray. It can take anywhere from several weeks to months to progress to this stage. The x-rays will typically show collapse of the bone just underneath the cartilage, known as subchondral collapse. An MRI or bone scan can be used to confirm the disease if it is not well visualized on x-ray. Occasionally, a CAT scan can be used to further delineate the area of necrosis.
Patients who have reached this stage are more likely to develop progressive osteoarthritis of the knee and may need surgical intervention.
Surgery of stage I & II disease is controversial. There have been some studies indicating that drilling of the osteonecrotic area may stimulate revascularization, a new blood supply, to facilitate regeneration of new bone. Cartilage grafting is also considered in stage II disease.
Osteonecrosis is classified stage III when the joint surface has collapsed and become depressed or flattened. Routine x-rays usually show this collapse and irregularity of the joint surface. The associated damage to the overlying articular cartilage is not visible on routine x-ray but can be seen on MRI scan. Operative treatment such as drilling of the lesion, local bone grafting or placement of a cartilage graft may be considered in younger patients. In older individuals who have progressed to advanced osteoarthritis, joint replacement surgery may eventually be necessary.
Stage IV disease is when osteonecrosis has progressed to severe damage, osteoarthritis, of the joint. The surface articular cartilage has been destroyed and marked osteoarthritic changes are seen on the x-ray. These patients continue to have symptoms and are treated like typical osteoarthritic patients, which includes symptomatic treatment until such time that knee replacement is necessary.
The eventual need for surgical intervention in osteonecrosis of the knee is based on several factors, including the area where the osteonecrosis occurs and the extent of the damage to the joint. Small lesions may not go on to extensive collapse and joint damage. Osteonecrosis lesions that are not in the weight bearing area may cause limited symptoms which resolve when the lesion heals. Patients who develop osteonecrosis in the weight bearing part of the knee joint with a large area of involvement are more likely to eventually require surgery.
When conservative measures fail to relieve symptoms, including activity modification, protected weight bearing using a cane or crutches, the use of braces, and appropriate medications, surgical options are considered.
For younger patients, typically under the age of 50 and depending on the area and extent of involvement, various surgical procedures may be indicated. Among these are arthroscopic removal of damaged cartilage and/or drilling (to reduce pressure in the bone and reestablish blood supply), and realignment procedures and osteotomies to shift load bearing away from the damaged surface of the knee. There are also surgical procedures to replace or help regenerate involved bone and cartilage. For the older age population, full or partial knee replacement is the usual surgical treatment.
Treatment options depend on the extent and location of the osteonecrotic area, patient age, and level of activity. It is important to consult an orthopedic surgeon who is experienced in treating this condition, including all of the surgical options that may result in the best possible outcome.