Osteonecrosis

Description

Osteonecrosis , also called AVascular Necrosis (hence the initials “AVN”), refers to bone infarction (that is, tissue death caused by an interruption of the  blood supply) near a joint.  (The generic term “bone infarction” is typically applied only to bone death that is not near a joint.)  

Osteonecrosis  can be painful in and of itself, but it’s main significance is that death of sub-chondral bone (ie, the bone right under the joint surface) can lead to collapse of the joint surface and end stage arthritis. Osteonecrosis is most common in the hip and shoulder.

Clinical Manifestations

Osteonecrosis can be asymptomatic and found incidentally on imaging.

Most patients present because of pain, either from the infarction itself or from arthritis. Patients with osteonecrosis of the femoral head often complain of groin pain that worsens with weight-bearing and motion and less commonly complain of thigh and buttock pain.  Rest pain occurs in about two-thirds of patients, while night pain occurs in about one-third of patients. 

The initial physical exam findings are often non-specific, but after osteonecrosis progresses, joint function deteriorates and the patient will present with the findings of arthrosis: a limp, tenderness around the affected bone, restricted motion, deformity and the like. In the femoral head, osteonecrosis causes particular limitation in internal rotation and abduction.  

Red flags

Severe bone pain, especially in patients with known risk factors for osteonecrosis, should prompt consideration of this diagnosis. It is particularly helpful to detect osteonecrosis before there is joint collapse and possibly protect the joint until the infarcted area can heal.

Epidemiology

In the United States, approximately 10,000 to 20,000 new cases of osteonecrosis are diagnosed per year; osteonecrosis accounts for more than 10% of the total hip replacement surgeries that are performed.  The peak age of prevalence for avn is the fourth or fifth decade.  The male-to-female ratio of idiopathic osteonecrosis is 8:1.  Osteonecrosisis is bilateral in about 55% of cases and the most common site is the hip.  The overall prevalence of osteonecrosis in patients with sickle cell anemia  is about 10 percent.

Pathology and pathophysiology

The lesions in osteonecrosis are those seen in any infarction: there is a geographic (ie, focal) area of necrosis surrounded by a rim of increased debris removal (osteoclastic activity here) and synthesis (osteoblastic reparative function). 

Edema, hemorrhage, fibrilloreticulosis, and hypocellularity may be present in bone marrow lesions.  In AVN, the necrosis always involves the medullary bone first. (The cortex may be spared because it has a collateral blood supply.) Likewise, the overlying articular cartilage receives nutrition from the synovial fluid and remains viable.  The dead bone has empty lacunae and is surrounded by necrotic adipocytes that often rupture and release their fatty acids.  These fatty acids can bind calcium and form insoluble calcium soaps.  

During the healing process, osteoclasts resorb the necrotic trabeculae, while the remaining ones serve as scaffolding for the deposition of new bone (i.e., creeping substitution).  Nevertheless, the pace of creeping substitution may not be fast enough to be effective, and, as a result, the necrotic cancellous bone collapses.    

In some case, the cause of the infarct is clear (ie, in sickle cell anemia, mechanical blockade by rigid erythrocytes is likely). In others, the reason is less clear. Vascular damage, increased intraosseous pressure and mechanical stresses are putative causes. 

The basic steps of osteonecrosis causing arthritis are follows:

• If bone is deprived of its blood supply, ie gets ischemic, it dies;
• If bone dies, it does not remodel. (Recall that bone remodeling is the process by which osteoclasts secrete acid and proteolytic enzymes to digest the bone matrix and osteoblasts synthesize new organic matrix----leading, of course, not only to mineral homeostasis, but the deposition of newer, better bone.)
• If bone does not remodel, micro-damage does not get repaired and the mechanical properties of the bone are marred.
• If enough damage accumulates, the sub-chondral bone can be weakened to the point of collapse.
• If the sub-chondral bone collapses, the joint surface of course becomes irregular and no longer smooth;
• If one side of the joint surface is not smooth it will damage the other surface. 

Differential diagnosis

The “differential diagnosis” can be thought of in a few separate ways. That is, the clinician may wonder "why does this person (with normal xrays) have pain?" or "why does this fairly young person have arthritis?" or "what could be the cause of this lesion I see on x--rays?".

To be sure, osteonecrosis is a possible cause of pain in the setting of normal x--rays, but is likely only in patients with known risk factors (see below). Without risk factors, more prosaic causes such as sprains, strains, arthrosis, bursitis and the like are more plausible. 

Similarly, osteonecrosis is a possible reason that a person may have premature osteoarthritis. Again, the likelihood of this without a risk factor is low.
The differential diagnosis for a bone lesion includes osteonecrosis but osteomyelitis and bone tumors are the primary considerations.

Etiology

The four main risk factors are responsible for most cases of AVN: trauma (specifically hip dislocation and femoral neck fracture for the hip and fractures of the scaphoid and talus);  corticosteroid use; alcohol abuse; or sickle cell disease (or other hemoglobinopathies).

The precise mechanism by which corticosteroids and alcohol lead to AVN is not known. In sickle cell disease, AVN  results from  the “vaso-occlusive” process of sickled cells with low compliance simply getting stuck and behaving like an embolism. A hip dislocation, in particular, is a risk for AVN because the femoral circumflex arteries of the femur ordinarily do no move much relative to the pelvis, and when a dislocation demands that they move, they are less able to comply.  As such, when the hip is dislocated, it is correspondingly more likely that the blood vessels are apt to be damaged (as compared to say the shoulder).

Radiographic and laboratory findings

 No laboratory test findings specifically suggest or confirm the presence of AVN.

Plain radiograph may be normal for months after the onset of symptoms of osteonecrosis, but early findings may include mild density changes followed by sclerosis and cysts as it progresses.  The pathognomonic crescent sign (subchondral radiolucency) presages  subchondral collapse.   
In late stages of AVN, loss of sphericity and collapse of the femoral head and joint-space narrowing and degenerative changes in the acetabulum can be seen. 
 
Bone scanning can show increased bone turnover at the junction of dead and reactive bone, but it is significantly less sensitive than MRI in diagnosing osteonecrosis. 

MRI has been reported to have a sensitivity of 91% and changes can often be seen on MRI early in the course of disease (unlike on plain radiograph and bone scanning).  On T-1 weighted imaging, focal lesions are well-demarcated and inhomogeneous; in early stages of AVN, one may see a single density line that reflects the separation of normal and ischemic bone.  On T-2 weighted imaging,  a second high intensity line that reflects hypervascular granulation tissue (i.e., the pathognomonic double-line sign) can be found.

CT scanning may be used to determine the extent of the disease and calcification, but it is not as sensitive as MRI.  There is a direct relationship between the radiographic findings and the pathophysiology because the lesions seen on imaging reflect the joint surface damage and  bone collapse that ultimately result from ischemia. 

Risk factors and prevention

1. History of trauma, especially a joint dislocation
2. Corticosteroid use or Cushing's disease
3. Alcohol abuse
4. Sickle cell disease/hemoglobinopathies
5. Systemic Lupus Erythematosus
6. Antiphospholipid Antbody Syndrome
7. Metabolic diseases such as hyperlipidemia
8. Renal failure (in renal transplantation, medication may be responsible).
9. HIV
10. Prior radiation therapy
11. Chemotherapy
12. Decompression sickness (diving)
13. Bisphosphonates (AVN of the jaw, in particular, with high dose use in cancer treatment)

The first four are particular worth recalling, as patients with any of them need heightened scrutiny should they present with bone or joint pain and normal plain radiographs.

To reduce the risk of AVN, the minimum effective dose of systemic corticosteroids should be used and, if possible, steroid-sparing agents should be used.   Patients at high risk of AVN (e.g., with prolonged corticosteroid use, hemoglobinopathies, renal transplant) should be educated about AVN and advised to report symptoms as soon as possible to facilitate treatment. 

Treatment options

 Treatment for AVN is directed by whether the bone has collapsed or not.  If it has, reperfusion and healing of the infarcted area will not restore the joint.

Limited weight bearing with crutches may be offered, but there is little evidence that such management is effective at halting progression of disease. 

Uncontrolled studies have suggested that bisphosphonates (ironically, since high dose use can be associated with inducing AVN) may delay collapse of the femoral head and delay the need for surgical intervention, but further studies are required to assess long-term results. 

Although there is no consensus on the best surgical procedure for AVN, core decompression with or without bone graft is appropriate in early stages of AVN in the hip and total hip arthroplasty is appropriate in the late stages (collapse, femoral head deformity, secondary osteoarthritis).  Core decompression may enhance circulation by decreasing intramedullary pressure  and preventing more ischemia. The evidence supporting core decompression comes primarily from case series and retrospective studies.  Such evidence suggests that core decompression is superior to non--operative management.  Decompression is often useful for pain relief and is especially effective in the early stages of disease. 

Bone grafts (especially free vascularized fibular grafts) provide structural support to the subchondral bone and can be a source of mesenchymal stem cells and a vascular supply to the necrotic tissue. 

Focal areas of collapse can be treated with an osteotomy to move the area of necrosis away from the major load-transmitting area of the acetabulum and redistribute the weight-bearing to the articular cartilage, but the results are variable. 

Total joint replacement is recommended for patients with advanced disease.  Total hip arthroplasty provides significant pain relief for several years, though revision rates are often high. A hip fusion is poor poorly received.

Outcomes

 
Outcomes of treatment directly correlate with the stage of disease.  No medical treatment has been demonstrated to be effective in preventing the disease process and half of patients with subchondral collapse of the femoral head develop AVN in the contralateral hip.  Total hip replacement is an ideal medium term treatment, but some studies have showed that revision rates in patients with avascular necrosis are significantly higher than revision rates in patients with other disorders.  For example, one retrospective review compared total hip replacement of hips in patients with osteonecrosis with replacement of hips in patients with osteoarthritis and found that the time of revision was an average of 5 years less in osteonecrosis patients.

The results of hip arthroplasty for avn related to sickle cell anemia  are poor.

Holistic medicine

Patients with arthritis from post-traumatic osteonecrosis may have unrealistic treatment expectations, as they are otherwise young and healthy.

Miscellany

The athlete Bo Jackson developed avascular necrosis of his left hip after a hip injury playing professional football.  He had a hip replacement and returned to play professional baseball with  the Chicago White Sox in 1991. 

Key terms

Osteonecrosis

Subchondral collapse
Bone remodelling

Crescent sign

Skills and competencies

Recognize patients at risk for AVN

Describe imaging findings