Transient osteoporosis of the hip (TOH) is an uncommon disease that has very characteristic clinical features (1). Disabling hip pain develops without antecedent trauma and disappears spontaneously in several months. Radiologically, osteopenia of the proximal femur is the only one abnormal finding and it also disappears in parallel with symptoms. The osteopenic area shows increased radionuclide uptake in bone scintigram and bone marrow edema pattern on MR images.

Although TOH affects middle-aged men more frequently, the first reported cases were in women during the last trimester of pregnancy. The etiology of TOH remains unknown, but it was recently proposed that subchondral fracture of the femoral head (SFFH) causes TOH (2-4).

Recently, the authors treated a woman with bilateral TOH that occurred sequentially during the peripartum period of her first delivery. Serial computerized tomograms (CTs) of the femoral heads showed evidences of trabecular injury. The patient was informed that data concerning this case would be submitted for publication.

A 28-yr-old woman presented with pain in both hips. The patient, a previously healthy housewife, had delivered her first baby about 3 weeks previously. Pain had first developed in the left hip some 3 months previously, at gestation week 32, and in the right hip 3 weeks previously (immediately after delivery). She had to use crutches because of disabling hip pain on weight bearing. At the time of presentation (postpartum 3 weeks, 3 months after left hip pain onset), the left hip pain was improving, but the right hip pain was worsening. Plain radiographs taken at 1 week postpartum showed diffuse osteopenia in both proximal femora, which was more marked in the left side (Fig. 1). Magnetic resonance (MR) images taken at 2 weeks postpartum showed a diffuse bone marrow edema pattern in the femoral head, neck and intertrochanteric areas of both femurs. No definite MR crescent sign (5) was detected (Fig. 2). The patient was treated conservatively under a diagnosis of TOH. Symptoms improved gradually and disappeared completely at 7 weeks postpartum (4 months after left hip pain onset) in the left hip and at 11 weeks postpartum in the right hip. Bone densities of proximal femora recovered in parallel with symptomatic improvement (Fig. 3). Follow-up MR images taken at 7 weeks postpartum showed markedly reduced bone marrow edema in the left and increased bone marrow edema in the right. (Fig. 4).

Coronal multiplanar reformation (MPR) CT images were taken serially to evaluate the condition of trabecular bone of the femoral heads (Fig. 5). The first CT images taken at 4 weeks postpartum (4 weeks after right hip pain onset, and 12 weeks after left hip pain onset) showed marked decreases in the sclerotic densities of primary compression trabeculae with irregular discontinuation, more so in the right. In addition, a focal area showing irregular thickening of trabeculae was observed in the left femoral head, and in the right femoral head, breakages of subchondral bone with a linear dense sclerotic line crossing vertical trabeculae (suggestive a fracture line) were observed. On the second CT images taken at 11 weeks postpartum (11 weeks after right hip pain onset, 19 weeks after left hip pain onset), sclerotic densities of primary compression trabeculae had recovered markedly in both femoral heads. A focal area showing irregular thickening of trabeculae was also observed in the right femoral head. Third CT images taken at 20 weeks postpartum (20 weeks after right hip pain onset, 28 weeks after left hip pain onset) demonstrated increased trabecular density in both femoral heads, and the focal area of irregular trabecular thickening disappeared in both heads. The fourth set of CT images taken at 1 yr postpartum showed almost complete recovery of primary compression trabeculae in both femoral heads.

The etiology of TOH remains unknown, but several hypotheses have been proposed (1, 6). These include intermittent compression of the obturator nerve, a non-traumatic form of reflex sympathetic dystrophy, viral infection, transient ischemic insult to bone, hormonal imbalance and trauma. Recently, SFFH had been proposed as a possible etiology (2-4, 6, 7).

Noorda et al. (6) and Plenk et al. (8) have described the histological findings of biopsy specimens of the femoral head in TOH. Both observed irregular new bone formation in apposition to trabeculae. Noorda et al. (6) considered that microfracture might play a role in the early pathophysiology in TOH, whereas Plenk et al. (8) described new bone formation as microcallus-like, but considered that it was not associated with microfractures. In our case, irregular trabecular bone thickening was observed in both femoral heads in late stage of TOH, and this disappeared gradually. In view of our CT findings, which suggested subchondral fracture in the right femoral head, we consider that irregular trabecular thickening is due to callus formation around injured trabeculae which is gradually remodeled.

Todd et al. (9) observed lesions of irregular bone accretion around trabeculae of the femoral head in cadavers and patients suffering from femoral neck fractures, osteoarthrosis, and rheumatoid arthritis. They concluded that these lesions were uniting fatigue fractures of isolated trabecula. This suggests that some degree of fatigue type microfracture and repair occur continuously in normal healthy adults, but that when microfracture occurs massively, it might become symptomatic like TOH. A sudden increase in body weight during the third trimester of pregnancy could exacerbate any microfracture process.

The clinical course and image findings of TOH closely resemble those of SFFH (2-4, 6, 7). The only difference between them is the presence of the subchondral fracture line on MR images in SFFH. In cases with SFFH, the presence of a fracture line has been reported to be detectable only on some slices of MR images made with various techniques including T1-weighted, T2-weighted, and gadolinium enhanced (1, 10). It is expected that it is more difficult or impossible to detect subchondral fracture line when fractures are incomplete. In the present case, we were unable to detect any definite evidence of subchondral fracture of the femoral head on MR images. However, on MPR CT images, we observed a linear dense sclerotic line suggestive of a fracture line in the right femoral head, and a definite area of irregular trabecular thickening in both femoral heads, which suggested a subchondral trabecular injury and subsequent repair.

The presented case study confirmed the evidence of subchondral trabecular injury in TOH which might be the milder form of injury than SFFH. The authors recommend that CT be used to evaluate trabeculae of the femoral head of TOH cases diagnosed by MRI.