Three weeks postpartum, bilateral knee and ankle pain developed despite full range of motion. Radiographs taken 6 weeks postpartum showed improvement of the hip osteopenia, but periarticular osteopenia developed around the knees and ankles (Figs 3, 4). A bone scan revealed intense tracer uptake in all six major joints in the lower limbs (Fig 5).
Over the ensuing months, the patient experienced gradual resolution of her condition in all affected joints. By 10 weeks, the patient could walk unaided, at 18 weeks her gait was mildly antalgic, and by 12 months she was symptom-free and her radiographs were normal (Fig 6).
The etiology of transient osteoporosis is unclear. Bone loss commonly is associated with estrogen-deficiency states,2,29 yet there is a surfeit of estrogen in pregnancy. The left hip is involved more commonly than the right, with bilateral involvement occurring in 25% to 30% of patients. The only recognized risk factor is pregnancy. Transient osteoporosis of the hip often is overlooked or underdiagnosed.12
The affected regions are characterized by bone marrow edema and increased bone turnover. The precipitating factors and pathogenesis remain elusive. Factors implicated have included: genetic predisposition,1 compression of the obturator nerve,9 Sudeck's atrophy,1,16 bone medullary hypertension and small vessel ischemia,28 and chemical or hormonal factors related to pregnancy.8 A mild loss of bone density occurs during normal pregnancy and lactation. Some suggest that pregnancy probably unmasks rather than causes low bone mass in patients with preexisting low bone mass.15,29,32
An unknown stimulus is thought to activate a large number of osteoclasts in the femoral head.27 Osteoid then is laid down, mineralized, and remodeled.27 Significant bone loss occurs between resorption and formation, which leads to decreased radiographic density.27 This interval is characterized by weakened bone. The bone is vulnerable to microfractures and is considered the cause of pain on weightbearing. If severe, this may lead to stress fracture of the femoral neck.
Clinical presentation is usually with acute pain in the affected joints. There is a characteristic discrepancy between substantial clinical disability and minimal physical findings.17 The clinical course and radiologic findings in middle-aged men are similar to those described for young pregnant patients.
Lequesne and Mauger17 described three phases: (1) increasing pain and disability, but normal radiographs; (2) maximal symptoms with radiologic osteopenia; and (3) regression of the disease and radiographic changes. This condition is characterized by a predictable benign course. The mean duration is 6 to 8 months, but may last 1 year,17 with complete clinical recovery. Radiographs, bone scans, MRI, and the benign self-limiting course will clarify the diagnosis of transient osteoporosis.
Radiologically evident periarticular osteoporosis usually appears within 3 to 8 weeks after the onset of pain.6,26 The joint space always is preserved, and subchondral collapse or cavitation is not seen.6,23,27,34
Bone scans often are positive during the first few days after the onset of symptoms.28 Classically, a target lesion is seen with maximal uptake at the center of the femoral head with decreasing activity peripherally.14
Magnetic resonance imaging characteristically shows low signal in the femoral head on T1-weighted images and high signal on T2-weighted images.36 These changes suggest bone marrow edema.20 The acetabulum also may be involved. Although there invariably is a joint effusion, there is no joint erosion.
A study involving bone densitometry13 showed femoral neck bone density 3 to 5 months after the onset of symptoms to be 20% less than in aged-matched controls. This returned to normal by 2 years.
Biochemical, hematologic, bacteriologic, and serologic tests usually are normal.35 Serum calcium, alkaline phosphatase, ESR, and urinary hydroxyproline excretion may be increased, but the latter two normally are elevated during the third trimester of pregnancy.5
Differential diagnoses include regional migratory osteoporosis, inflammatory arthritis, septic arthritis, avascular necrosis, stress fracture of the femoral neck, pigmented villonodular synovitis, synovial chondromatosis, primary or metastatic carcinoma, and multiple myeloma.21,23
Regional migratory osteoporosis is a condition with considerable overlap with transient osteoporosis. Both are self-limiting disorders, tending to abate in a year, with no sequelae. Regional migratory osteoporosis first was described by Duncan et al in 1969.11 It predominantly affects middle-aged men, with the ankle, foot, and knee most commonly affected.31 It is less common for the hip to be involved.31 Transient osteoporosis has a predisposition for the hips, with unilateral10,22 and bilateral30 knee involvement during pregnancy being rare and only reported in isolated cases. The radiographs, bone scan, and MRI findings are similar for transient osteoporosis and regional migratory osteoporosis. The clinical and pathologic features have a significant resemblance with transient osteoporosis. However, regional migratory osteoporosis frequently recurs, usually in adjacent joints.18 Simultaneous involvement of several regions and overlap of attacks are uncommon in regional migratory osteoporosis.18 The clinical overlap between transient osteoporosis and regional migratory osteoporosis is reflected by possible hip involvement in regional migratory osteoporosis and migratory joint involvement in transient osteoporosis. Difficulty in distinguishing transient osteoporosis and regional migratory osteoporosis suggests that there is likely to be a common etiology.33 This case highlights such an overlap. Our patient likely had an atypical case of transient osteoporosis with multifocal involvement. However, regional migratory osteoporosis during pregnancy (which is unusual) also may be plausible.
Various treatments have been attempted with little benefit. The currently accepted practice is usually supportive.4-6,27,28,34 This entails judicious use of analgesics, NSAIDs, protected weightbearing, and a graduated physiotherapy regime. The aim is to reduce microfractures and prevent pathologic stress fractures. Traction has been used for treatment in men, but not pregnant women. No objective evidence is available to support this over simple analgesia and protected weightbearing.
Elective cesarean sections have been done in patients in whom bilateral severe hip involvement has precluded normal vaginal delivery.2,3 Funk et al13 reported a rapid increase in bone mineral density after the cessation of lactation. They recommended against breast feeding in patients with transient osteoporosis.13
There have been reports that oral and intravenous bisphosphonates may be beneficial, and may speed up recovery.22,24 Calcitonin,10,25 prednisolone,19 and the bone-sparing steroid deflazacort7 also have been used. All of these pharmacologic studies have small sample sizes, with no case controls.4 In some studies, the time until complete recovery have not differed greatly from the natural course of the disease.4 The role of pharmacologic agents in the management of a self-limiting condition, particularly in lactating women remains unclear.
Women in the third trimester of pregnancy and middle-aged men most commonly are affected by transient osteoporosis. Although it usually presents as hip disease, this case shows that a multifocal presentation resembling regional migratory osteoporosis, described predominantly in middle-aged men, is also a possibility. In the pregnant patient, the same optimistic prognosis for transient osteoporosis seems appropriate for this multifocal-type disease.
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