By Nabil A. Ebraheim MD & Dalton Blood MD
Osteonecrosis or avascular necrosis of the hip is
characterized by a disruption in blood flow to the femoral head. Osteonecrosis is
bilateral in about 80% of patients. Even when asymptomatic, the patient’s
contralateral hip should be closely examined. Early diagnosis and treatment may
improve the chances for success of a head preserving surgical procedure, such
as core decompression or bone grafting. In late stages of osteonecrosis, the
femoral head collapses and will need to be replaced. AP and frog leg lateral
x-rays should be the first diagnostic test when evaluating for osteonecrosis of
the hip. The frog leg lateral will show the fracture. MRI is the study of
choice in patients with persistent hip pain, negative radiographs, and
suspected femoral head osteonecrosis. T1 MRI will show a well-defined single
band-like area of low signal intensity corresponding to the ischemic marrow usually
located within the superior anterior portion of the femoral head. This finding
is called the crescent sign. The crescent sign represents the reactive
interface between the necrotic and reparative zone. The single line density
demarcates the normal from the ischemic bone. T2 MRI images demonstrate the double
line sign: a subcortical lesion with a high signal intensity inner border with
a low signal intensity peripheral rim. The high signal intensity represents hyper
vascular granulation tissue while the low signal intensity corresponds to the ischemic
zone. While the presence of bone marrow edema on the MRI predicts worsening
pain and future disease progression, the lesion size is the most important
factor in disease prognosis. The best prognosis occurs in a small lesion with
sclerotic margins. Multifocal osteonecrosis is a disease involving three or more
sites such as the hip, the knee, the shoulder and the ankle, and it occurs in
about 3% of patients. A patient that presents with osteonecrosis at a site
other than the hip should undergo MRI of the hip to rule out the asymptomatic
lesion in the femoral head.
Transient osteoporosis of the femoral head is not an
osteonecrosis of the femoral head. Transient osteoporosis usually affects
pregnant women or men during the 5th decade of life. The symptoms
are usually more severe than the x-ray findings. On x-ray, osteopenia may be
present. On T2 MRI, bone marrow edema signal changes will involve the femoral
head, extend into the neck, and may include the trochanteric area. In transient
osteoporosis, the double density finding, seen on MRI in patients with osteonecrosis,
is absent. Transient osteoporosis is neither a tumor or an osteonecrosis, and
surgery is not required.
