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Painful knee: a dilemma

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Q1: What are the findings on the bone scan and the MRI scans?

The isotope bone scan after injection of the technetium-99 methylene diphosphonate shows significantly increased uptake of the radiopharmaceutical by the right femoral head and the proximal femur (fig 2). Coronal T1 and fat saturated T2, MRI images of both hips were obtained. On T1 weighted image there is reduced signal intensity noted in relation to the head, neck, and proximal right femur (fig 3A). On T2 weighted image, there is patchy increased signal intensity in the same areas. A small joint effusion is also noticeable on the right side on T2 weighted image (fig 3B). The acetabulum appears normal in both films.

Q2: What is the diagnosis?

Physical examination, MRI images, and the pattern of radionuclide uptake are highly suggestive of transient osteoporosis of the right hip. In majority of the cases suffering from this disorder, osteopenic changes are evident on plain film in the proximal femur few weeks from onset. In rare cases, no loss of bone density may be detected throughout the duration of the disease. This variant of transient osteoporosis has been called “transient bone marrow oedema syndrome”1 The differential diagnosis includes avascular necrosis, rheumatoid arthritis, osteomyelitis, septic arthritis, and reflex sympathetic dystrophy. However in view of the clinical details and laboratory investigations these can easily be out ruled. Our patient was treated with NSAIDs and non-weight bearing on the affected hip. She was able to partially weight bear after nine months and her condition fully resolved at 12 months without surgical intervention.

Discussion

Transient osteoporosis was initially described as a clinical syndrome, which affected women in the third trimester of pregnancy. It is characterised by pain and focal loss of radiodensity in the affected hip. Contralateral hip may be involved in 30% of the cases. This is a rare, self limiting disorder where function and bone density of the hip are restored in six to 12 months. Absence of any underlying medical condition or drug therapy separates it from “secondary osteoporosis”.2 Since the earlier reports, it is now known that this condition may affect both males and females irrespective of their race, age, or sex. However only few cases have been documented with patients in their sixth decade of life, with an atypical presentation of referred pain from the hip joint. A symptom often leading to diagnostic confusion. If the examining clinician does not perform a careful and complete physical examination.

The symptoms of transient osteoporosis are of sudden onset of hip pain, inability to weight bear, and restriction of motion. These symptoms are generally associated with proximal femoral osteoporosis on plain radiographs and increased radionuclide uptake on isotope bone scan. Such features are the early manifestations of a number of possible differential diagnoses including avascular necrosis, septic arthritis, rheumatoid arthritis, tuberculosis, pigmented and villonodular synovitis, and neoplasm. However, the diagnosis is not difficult if the clinician is aware of the characteristic features.

There is controversy about the aetiology, pathophysiology, and the outcome of the disease. It has been shown that increased bone metabolism contributes to the accumulation of free water in the bone marrow, leading to raised intramedullary pressure. This in turn may cause venous stasis rather than obstruct arterial blood supply and lead to the marrow changes. Metabolic, viral, and neurological factors have been suggested as possible links to the aetiology.1Transient osteoporosis has also been regarded as a possible variant of reflex sympathetic dystrophy and the early phase of avascular necrosis of the femoral head.

The three main diagnostic tools used in evaluating transient osteoporosis are conventional radiographs, technetium-99m radionuclide scans, and MRI scans. Plain radiographs are the least sensitive of the group. Typical changes of osteopenia with patchy demineralisation in the femoral head do not develop for four to six weeks, until then the radiographs remain normal. However these changes may not occur at all and it has been suggested that this may be a separate radiological entity or a variant of transient osteoporosis called “bone marrow oedema syndrome”.3 This phenomenon was observed in our patient where no osteopenia was detected on standard film throughout the course of the disease, leading to diagnostic confusion for clinicians unaware of this entity. Radionuclide scan shows increased uptake in the femoral head and the proximal femur within days of onset, but it lacks the anatomical detail and is non-specific. However MRI scan is able to demonstrate the changes within the marrow, cortex, and the surrounding soft tissue with surprisingly good clarity. Three phases of transient osteoporosis have been identified on MRI scan.4 The initial “diffuse stage” with patchy hypointensities on the T1 weighted images with hyperintensive marrow oedema and joint effusion on T2 weighted images. This may be seen as early as 48 hours. Focal stage takes uptil three months to develop with hypointensive areas localised to the weight bearing regions of the femoral head. Finally the “residual phase”, which occurs six to 12 months from the onset of symptoms displaying return to normal marrow intensity associated with resolution of symptoms.

Transient osteoporosis only affects epiphyseal and metaphyseal areas without affecting soft tissues. It has been suggested that transient osteoporosis is an early stage of AVN as initial MRI findings are similar in both cases and some documented cases of transient osteoporosis progressed to osteonecrosis.5 However in osteonecrosis the presence of double line sign on T2 weighted images representing an outer rim of lower intensity (zone of fibrosis) and an inner rim of hyperaemia (zone of granulation tissue) of high signal intensity is considered pathognomic. In addition on technetium-99m bone scan an area of avascular necrosis appears as a “cold defect” excluding transient osteoporosis. In comparison patients with septic arthritis, joint effusion, marrow oedema, and bony erosions appear as low signal intensity on T1 images within 24 hours of infection. However the final diagnosis requires culture of the aspirate.6 As the disease is self limiting the treatment remains NSAIDs, protected weight bearing, and an active and passive range of motion exercises. Core decompression of the femoral neck has been used in a small number of patients with reduction in recovery time. This procedure requires further evaluation to be recommended.5

A review of literature suggests that transient osteoporosis is not as rare as initially presumed and may often result in unnecessary expensive, prolonged and often painful diagnostic procedures for a self limiting condition. We feel it is important to include this condition in the differential diagnosis of hip or knee pain. The availability of MRI has not only improved the chances of early detection of this disorder but can also help in out ruling other more serious conditions.

Final diagnosis

Unilateral transient bone marrow oedema syndrome.

References

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