Hip osteonecrosis is a condition where there is death of the bone that comprises the hip joint. The injury and subsequent death of bone tissue is due to a disruption in the blood supply to the bone. Therefore it is also known as avascular necrosis of the hip. Since the bone death is not due to an infection, one of the more common causes of bone disease, it is therefore also known as aseptic necrosis of the hip. In the early stages the condition can be managed to some degree with medication but surgery is usually needed and there may also need to be some degree of joint destruction in the later stages.
Up to 20,000 new cases of hip osteonecrosis are diagnosed every year in the United States. While trauma tends to affect only one side, non-traumatic causes affect both hip joints in about half of all cases. Men are 4 times as likely to suffer with hip osteonecrosis as women. In non-traumatic causes, other joints are also affected along with the hip joint.
The hip joint is made up of a hollow rounded cavity in the pelvic bone (acetabulum) and the rounded head of the thigh bone (femur). Therefore the acetabulum and femoral head form a ball and socket joint. Hip osteonecrosis occurs when there is death of the tissue of the femoral head. As the affected bone of the femur collapses. Between the femoral head and acetabulum of the pelvic bone lies the articular cartilage. It reduces friction between the two bones articulating to form the hip joint. Cartilage is firm and flexible and shapes itself to the joint surfaces. Therefore it also collapses when the affected part of the femoral head collapses.
Bone has an extensive blood supply and is not dead tissue as is commonly thought. Blood vessels within the bone ensure that the living tissue has sufficient oxygen and nutrients via the arteries and wastes and carbon dioxide exit via the veins. As with any part of the body, if the blood supply is interrupted the tissue first undergoes injury known as ischemia and then tissue death known as infarct. Once the cells die (necrosis), there is a break down in the structure of the bone and the overlying cartilage is also disrupted.
There are several reasons why the blood supply to the bone may be affected :
- Injury may injure the blood vessels.
- Enlargement of fat tissue around the blood vessels compresses it.
- Obstruction of the blood vessel with an embolism – gas or fat emobli.
- Abnormally shaped blood cells can stick together and block the vessel.
- Increased coagulability of the blood predisposes to clot formation within the vessel.
- Increased pressure within the joint for any reason compresses the blood vessel.
New bone tissue gradually replaces the dead bone. However, during this natural bone repair process the hip joint becomes unstable. Movement is not only painful but articulation is abnormal. Normal use of the joint can increase pressure on the damaged bone and already unstable joint and may worsen the joint damage.
Pain and difficulty walking or standing are the main symptoms of hip osteonecrosis. Both hip joints are usually affected.
Deep hip pain is the first symptom of hip osteonecrosis. The onset can be sudden in acute causes or gradual in more slowly progressing causes. Standing and walking worsens the pain. Typically it is worse at night and is felt as a deep dull ache or a throbbing pain. Hip osteonecrosis pain is felt in the groin and sometimes the buttocks.
Initially movement is hampered due to the pain but as the condition progresses, the joint becomes unstable. Apart from pain with walking and standing, the normal range of motion of the hip joint becomes limited. A person may feel a popping sensation of the hip joint when walking and the normal “smoothness” of the articulation is impaired.
The causes of hip osteonecrosis is similar to osteonecrosis elsewhere in the body, like knee osteoporosis. can be divided into traumatic and non-traumatic (atraumatic) factors. Any injury to the hip joint, especially when there is a fracture or dislocation, can interrupt the blood supply to the femoral head. Tissue death can occur within hours or days and the onset is acute. The causes of atraumatic hip osteonecrosis is more extensive and includes :
- Corticosteroid use (common)
- Alcohol abuse (common)
- Sickle cell anemia
- Caisson disease
- Gaucher disease
- Systemic lupus erythematosus (SLE)
- Crohn’s disease
- Arterial embolism and thrombosis
The third most common atraumatic cause is unknown (idiopathic) after corticosteroid use and alcohol abuse.
Patients with osteonecrosis have hip pain, experience difficulty with walking and standing and have an abnormal way of walking known as Trendelenburg gait. However, further investigations are needed as these symptoms are non-specific for hip osteonecrosis. The main diagnostic investigations to conclusively identify hip osteonecrosis includes :
- Magnetic resonance imaging (MRI)
- Bone scans
- Core biopsy
An MRI is the best method to visualize the derangement in bone structure. Bone scanning is one alternative when an MRI is not easily available. Laboratory tests may be conducted to identify the cause of hip osteonecrosis. The extent of the condition is determined by the Ficat classification (stage 0 to IV) based on the findings of these diagnostic investigations.
The treatment should be directed at the cause of the osteonecrosis as well as the restoring the bone structure and joint stability. Surgery is usually necessary.
Non-surgical measures have limited scope in treating hip osteonecrosis and is only effective in the early stages. These measures include :
- Electrical stimulation
- Extracorporeal shockwave therapy
- Nonsteroidal anti-inflammatory drugs (NSAIDs) for pain relief.
- Physical therapy
- Mobility aids like crutches
There are several surgical procedures that may be utilized in the treatment of osteonecrosis. The choice of procedure depends on several factors including the underlying cause, stage of the disease, severity of bone death and degree to which the hip joint is affected. Some of these procedures include :
- Core decompression to reduce pressure within the bone and stimulate the formation of new blood vessels.
- Bone graft which often follows decompression involves filling the area from where the dead bone tissue is removed.
- Arthrodesis is where the cartilage of the joint is removed and the femoral head is fused to the acetabulum of the pelvic bone.
- Arthroplasty is a procedure where the dead bone tissue is removed (resection), the femoral head resurfaced or replaced with a prosthetic along with the cartilage.