Delfos Clinica, Rheumatology Unit, Barcelona and
1 Hospital Duran i Reynais, Department of CT and MR Imaging, Barcelona, Spain
SIR, Pain in the low back and sacrum is a common complaint during pregnancy and the post-partum period and is usually due to mechanical lesions of the pelvic soft tissues or ligaments [1, 2]. This fact tends to overshadow less common but important causes, such as infections of the lumbar spine or sacroiliac joints and stress fractures of the pelvis.
We report a case of stress fracture of the sacrum during the post-partum period in a woman without pregnancy-related osteoporosis.
A 40-yr-old woman with no significant past medical history was referred with a 2-month history of severe mechanical pain in her left buttock that had started after vaginal delivery of a 3900 g baby. The patient was primiparous and the pregnancy was uncomplicated except for a maximal gain in body weight of 14 kg. She was a non-smoker and there was no regular alcohol consumption either before or during pregnancy. There was no history of pelvic trauma or family history of osteoporosis. Daily calcium intake was more than 1 g, no prolonged bedrest or immobilization occurred and, except for 3 days of low-molecular-weight heparin (fraxiparine 5000 IU daily) immediately after caesarean delivery, the patient did not receive medication affecting bone metabolism. The patient breast fed for 3 months.
Physical examination showed an exquisitely painful point on the left sacroiliac articulation radiating to the dorsolateral aspect of the thigh. Her body weight at the time of examination was 71 kg and her height was 154 cm. Laboratory data showed normal values of erythrocyte sedimentation rate, C-reactive protein, blood count, renal and liver function tests, serum levels of calcium, phosphorus and alkaline phosphatase, thyroid and parathyroid hormones, serum 25-(OH) vitamin D3 and urinary excretion of calcium. Plain radiographs of the pelvis and lumbar spine showed no osteoarticular abnormalities. Magnetic resonance imaging of the pelvis showed a linear area of low signal intensity on the left sacral wing corresponding to a stress fracture, surrounded by ill-defined bone marrow oedema changes. Dual-energy X-ray absorptiometry examination performed 2 months after delivery showed normal bone mineral density values with T-scores of +0.75 and +0.59 at the lumbar and femoral regions, respectively.
Management consisted of rest and analgesics. After 2 months the pain disappeared completely and the patient could perform normal daily activities without recurrences.
Stress fractures can occur if normal bone is exposed to repeated abnormal stress (fatigue fractures) or if normal stress is placed on bones with compromised elastic resistance (insufficiency fractures). Sacral stress fractures are rare during pregnancy and the post-partum period. A review of the literature revealed only six previously reported cases: three fatigue fractures [35] and one insufficiency fracture [6]; in the other two cases [7, 8] described as stress fractures, no information was given about bone mineral density. However, the prevalence of this complication is probably underestimated because of the lack of specificity of the symptoms. In this sense, although lumbosacral pain is a common complaint during pregnancy and in the early post-partum period, it is frequently dismissed as trivial and inevitable; for this reason, imaging studies are rarely performed to investigate this symptom.
Fatigue fracture is the most likely diagnosis in our patient, probably due to unaccustomed stress related to rapid and excessive weight gain in the last trimester of pregnancy. No risk factors for osteoporosis were identified and bone mineral density performed 2 months after delivery was normal, making the diagnosis of pregnancy-associated osteoporosis improbable. Risk factors for sacral fatigue fractures during pregnancy and the post-partum period include vaginal delivery of a high-birth-weight infant, increased lumbar lordosis, excessive weight gain and rapid vaginal delivery [9].
In summary, although rare, our case advises that sacral stress fracture should be included in the differential diagnosis of buttock pain during pregnancy or the early post-partal period. Magnetic resonance imaging, permitted during pregnancy, may be very useful in the confirmation of the diagnosis in these cases.
Notes
Correspondence to: F. J. Narvaez Garcia. E-mail: 31577edd{at}comb.es
References