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Transient osteoporosis of pregnancy involving the hips has been reported widely. The knee is much less commonly affected and only isolated cases have been reported. We report the case of a woman in the third trimester of pregnancy with bilateral transient osteoporosis of the knees.
A 36 year old woman presented with a one month history of pain in the left knee followed a week later by pain in the right. The right knee deteriorated over one to two weeks until she was unable to weight bear. There was no pain at rest. At the time of presentation she was 30 weeks pregnant with her first child conceived through in vitro fertilisation. There was no past history of joint problems, alcohol excess, corticosteroid use, preceding trauma, acute medical illness, or any constitutional symptoms. She walked with two crutches. Joint examination showed a slightly warm right knee with no clinical evidence of an effusion. Both knees were non-tender to palpation but painful at maximal flexion.
Investigations showed an erythrocyte sedimentation rate of 33 mm/1st h, C reactive protein slightly raised at 10 mg/l (normal <3), and globulins at 36 g/l (25–35) with a normal protein electrophoresis. Full blood count, creatinine, electrolytes, 25-hydroxyvitamin D3, parathyroid hormone, creatine kinase, and liver and thyroid function tests were all normal. Antinuclear antibodies, rheumatoid factor, and HLA-B27 were negative. Plain radiographs of the knees were normal. Magnetic resonance imaging (MRI) showed extensive oedema in the medial and lateral femoral condyles and some oedema in the surrounding soft tissues of the right knee (fig 1). There was an effusion in the suprapatellar bursa and some synovial proliferation. In the left knee there was a moderate amount of oedema in the medial femoral condyle and a trace of oedema in the medial tibial condyle.
A diagnosis of transient osteoporosis of pregnancy was entertained and the patient was treated with simple analgesics and followed up closely throughout the remainder of her pregnancy. She presented to the delivery suite in labour at 39 weeks' gestation and successfully delivered a healthy 4500 g male infant by fast normal vaginal delivery. By three weeks post partum the pain had begun to resolve and she could walk without any aids. At three months post partum there was only residual discomfort in the right knee walking up and down stairs. Repeat MRI of the knees showed dramatic improvement in the bone marrow oedema. In the right knee there was only minor patchy oedema in the distal femur and proximal tibia. In the left knee there was some residual spotty marrow inhomogeneity in the lateral tibial plateau.
Transient osteoporosis is an uncommon condition affecting middle aged men or women in the third trimester of pregnancy. The hip joints are most commonly affected, being reported in 76% of cases.1There are only isolated case reports of the knee being affected in pregnant women,2 3 and none with both knees involved as in this report.
Plain radiography of the affected joint is often normal early in the course of disease, with changes only becoming apparent after four to six weeks.4 Demineralisation of the bone and resultant osteopenia may be severe. Joint space is preserved throughout the course and there is no progression to joint erosion.5 6MRI is the radiological investigation of choice showing transient bone marrow oedema (the term preferred to “transient osteoporosis”) and almost always a joint effusion.5-7 The cause is unknown, but the MRI appearances and limited histological information7 8 suggest an active inflammatory process. Although a variety of treatments have been used,2 9 a conservative approach is favoured during pregnancy.
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