MAKING THE CASE
Osteonecrosis of the femoral head is a progressive disease that affects patients between their 30s and 50s. The average age of presentation is 33, and the estimated incidence is 20,000 cases/year in the U.S. In fact, the condition accounts for as many as 5-18% of total hip replacements.

Osteonecrosis, formally known as avascular necrosis, is now the preferred terminology to describe an "avascular" process that leads to necrotic or "dead" bone. The process actually isn't avascular except for the very early stage - it becomes hyperemic as bone turnover progresses. While a list of potential risk factors and conditions has been identified (Table 1), to date, neither the etiology nor the natural history of the disease has been definitively determined.

Symptoms
Patients are often asymptomatic during the early course of the disease. The first sign of trouble is usually groin pain on ambulation or a deep pain in the groin. You may also find limited range of motion and pain with internal rotation of the hip. When young patients present with atraumatic groin pain and a history of one or more of the mentioned risk factors, you should investigate the person without delay for the onset of osteonecrosis of the hip. Progression can occur from a viable round head to advanced collapse within 4 months -- so urgent referral is needed.

Initial workup
Start with plain x-rays including anteroposterior and frog-leg lateral views. Radiograph changes in the femoral head are associated with the various stages of the disease (Table 2) and they include cysts (Figure 1), sclerosis or a crescent sign (best visualized on a frog-leg lateral view [Figure 2]). A crescent sign represents a subcondral fracture line and will progress to cystic changes and collapse. For earlier stages of the disease, MRI has become the imaging modality of choice due to its high sensitivity and specificity of 99% (Figure 3). Pertinent changes on the MRI include a serpiginious black line on T1 that represents a bone reformation layer at the necrotic margin.

Differential diagnosis
The only true differential diagnosis in this age group with both radiographic and MRI changes is a condition called idiopathic transient osteoporosis, which occurs most often in middle-aged men and pregnant women. Symptoms include pain and limp with local muscle wasting. The condition has different findings on MRI (no serpi­ginious line), which helps to distinguish it from osteonecrosis. Treatment is conservative, i.e. protected weight bearing with crutches, as full weight bearing will result in hip fracture.

Treatment
Conservative therapy, such as crutch ambulation or bed rest - though ineffective in terms of final treatment - should be instituted for symptomatic patients until an orthopedic consultation can be obtained to prevent collapse in the interim. All individuals with osteonecrosis warrant urgent orthopedic referral and surgical treatment because without therapy, a symptomatic hip will uniformly progress to collapse of the femoral head.

Although MRI isn't an absolute necessity for diagnosis, many surgeons prefer to have this study done in order to plan surgery. It also helps with staging and can influence eventual treatment protocols. As well, a baseline MRI is useful if the patient is at a very early stage of the disease and is going to be followed without surgery. There's no place in the treatment algorithm for electrical or ultrasound stimulation or even medical therapy at this time. Patients need to be given crutches until they're seen by a specialist.

Surgical options
Core decompression
This approach is widely used for early pre-collapse stages of osteonecrosis. It involves making a single or multiple drill tracks from the side of the femur into the area of necrosis. Sometimes it's performed in combination with insertion of a nonvascularized allograft bone. The goal is to decompress the femoral head and thereby reduce the intraosseous pressure. This should restore normal vascular flow and alleviate the pain in the hip. Early stages treated in this way may cease to progress, but after stage 2 the procedure only relieves pain and doesn't prevent collapse.

Free vascularized fibular graft
A free fibular graft is used to prevent collapse of the femoral head and enhance vascular supply to that region. It involves harvesting the fibula from the same limb with its peroneal artery and veins and inserting it from the lateral femur up into the femoral neck to within 3-5 mm of the subcondral bone. The ascending branch of the lateral femoral circumflex artery and vein are anastomosed to the peroneal vessels of the fibula. In about 80% of patients with stage 3-4 disease, the hip has a 10-year lifespan with this approach. The procedure is mostly indicated for individuals under the age of 50 with little or no collapse of the femoral head.

Femoral head resurfacing - partial hip replacement
This is a viable option for young patients with pre- or post-collapsed lesions but without acetabular involvement. Its main indication is to delay the eventual necessity for a total hip replacement. In this process, the collapsed femoral head is reshaped, thereby removing the damaged cartilage. A titanium-alloy shell is then cemented onto the reshaped femoral neck to replace the femoral head. The success rate is about 80-92% over 5 years and 61-74% over 10 years.

Total hip replacement
This is the only treatment that's known to provide enduring pain relief with excellent functional outcome. It's indicated after collapse of the femoral head with acetabular involvement and secondary degenerative arthritis. Typically, a total hip replacement will last > 10 years in 94-98% of cases. In younger patients, however, one must balance this excellent clinical result against the fact that a large amount of host bone will be taken, which narrows future treatment options.

No treatment method has proven to be complete, and the ultimate goal from an orthopedic surgeon's perspective is to relieve pain, to preserve the femoral head, and to delay the need for a total hip replacement for as long as possible.