Transient osteoporosis of the hip

Transient osteoporosis of the hip, also known as (transient) bone marrow oedema 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. It presents a synonym of the subchondral insufficiency fractures of the knee. 

Epidemiology

Although initially described in pregnancy, transient osteoporosis of the hip tends to affect middle-aged men (40-55-year-olds) with an M:F ratio of 3:1. When women are affected, it is usually during the 3^rd trimester of pregnancy. Generally, only one hip is affected at a time. Recurrence in the same hip is possible.

Clinical presentation

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.

Radiographic features

Plain radiograph

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; present in only 20% of cases after 4-8 weeks from symptom onset ²

• often profound osteopenia of the femoral head and neck region

• joint effusion may be present 

• joint space always preserved

MRI 

MRI is the modality of choice for the detection of transient osteoporosis of the hip and demonstrates bone marrow oedema pattern involving the femoral head, neck, and even intertrochanteric region without findings of osteonecrosis ⁵. Subchondral fracture lines can be found in almost half of cases ⁶. Sparing of the medial femoral head by bone marrow oedema can be found in almost 90% of patients and disappears towards the later stages of the disease ⁶.

Signal characteristics

• T1: decreased signal with loss of normal fatty marrow signal

  • it is worth having a coronal of both hips so that one can compare the distribution of red marrow

• T2: high signal, often heterogeneous, may be striking

Nuclear medicine

Typically, bone scintigraphy demonstrates markedly increased homogeneous uptake in the femoral head; a finding which is seen well before osteopenia is seen on plain films. Bone scintigraphy is not the modality of choice for the evaluation of these patients. 

Treatment and prognosis

Typically transient osteoporosis of the hip resolves spontaneously within 6-8 months after protected weight-bearing and pain control. In approximately one-fifth, oedema will recur in the same or another site of the skeleton, in which case the condition is called regional migratory osteoporosis ⁶.

Differential diagnosis

General imaging differential considerations include:

• early avascular necrosis: avascular necrosis at early stages does not present with bone marrow oedema

• late osteonecrosis

  • ​absence of subchondral low signal on T2 or contrast-enhanced T1 images suggests a favourable outcome - it is important to note that once it happens, osteonecrosis is not reversible

  • dynamic contrast-enhanced MRI will show hyperaemia (early enhancement) in transient osteoporosis vs no enhancement in early osteonecrosis

  • no sufficient evidence exists for the use of dynamic contrast-enhanced (DCE) MRI to distinguish between the two conditions

• stress fracture / subchondral stress response ⁵

• septic arthritis

• reflex sympathetic dystrophy: more commonly involves upper extremities; vasomotor dysfunction and skin changes more often seen; more debilitating

• tuberculous arthropathy of the hip