idiopathic transient osteoporosis of the hip
Transient osteoporosis of the hip, also known as (transient) bone marrow edema syndrome of the hip, is a self-limiting clinical entity of unknown cause, although almost certainly a vascular basis and possible overactivity of the sympathetic system exists. There is some controversy as to whether transient osteoporosis of the hip represents a very early, reversible stage of avascular necrosis (AVN) or is secondary to subchondral insufficiency fractures.
Although initially described in pregnancy, transient osteoporosis of the hip tends to affect middle age men (40-55 year olds) with an M:F ratio 3:1. When women are affected, it is usually during the 3 trimester of pregnancy. Generally, only one hip is affected at a time. Recurrence in the same hip is possible.
Typically, patients present with spontaneous onset of hip pain, usually progressive over several weeks. Patients generally do not have risk factors for avascular necrosis and do not go on to form avascular necrosis.
In some instances, patients can go on to develop similar changes in the opposite hip or in other joints; such cases should be referred to as regional migratory osteoporosis.
An insufficiency fracture is a possible complication.
Typically plain radiographs are normal initially but usually become abnormal by 4-8 weeks following the onset of symptoms. Findings include:
- subchondral cortical loss involving femoral head and neck: virtually pathognomonic
- often profound osteopenia of the femoral head and neck region
- joint effusion may be present
- joint space always preserved
MRI demonstrates bone marrow edema pattern involving the femoral head, neck, and even intertrochanteric region:
- T1: decreased signal with loss of normal fatty marrow signal
- NB it is worth having a coronal of both hips so that one can compare distribution of red marrow
- T2: high signal, often heterogeneous, may be striking
Typically, skeletal scintigraphy demonstrates markedly increased homogeneous uptake in the femoral head; a finding which is seen well before osteopenia is seen on plain films.
Treatment and prognosis
Typically transient osteoporosis of the hip resolves spontaneously within 6-8 months after protected weight-bearing and pain control.
General imaging differential considerations include:
- early avascular necrosis
- absence of subchondral low signal on T2 or contrast-enhanced T1 images suggests favorable outcome, i.e. reversible disease.
- dynamic contrast enhanced MRI will show hyperemia ( early enhancement) in transient osteoporosis vs no enhancement in early avascular necrosis.
- stress fracture
- septic arthritis
- reflex sympathetic dystrophy: more commonly involves upper extremities; vasomotor dysfunction and skin changes more often seen; more debilitating