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Idiopathic transient osteoporosis of the hip

Idiopathic transient osteoporosis of the hip (ITOH) is usually a self limited clinical entity of unknown cause, although a vascular basis almost certainly exists. There is some controversy as to whether ITOH represents a very early, reversible stage of avascular necrosis (AVN).

Epidemiology

Although initially described in pregnancy, ITOH affects middle age men (40-55 year olds) with M:F ratio 3:1. When women are affected; it is usually during 3rd trimester of pregnancy. Generally, only one hip is affected at a time. Recurrence in the same hip can occur.

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 film

Typically plain films 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 2
  • often profound osteopenia of the femoral head and neck region
  • a joint effusion may be present 
  • joint space is always preserved
MRI 

MRI demonstrates bone marrow oedema 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, and may be striking
Bone scan

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 symptomatic support.

Differential diagnosis

General imaging differential considerations include

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