Morphometric X-ray absorptiometry in the assessment of vertebral fractures in renal transplant patients

Abstract

Bone mineral density (BMD) is widely employed to evaluate the risk of fractures, but more than mineral content is bone quality that accounts for bone strength. In fact, occasionally, subjects with normal or only mildly reduced BMD also experience pathologic fractures. In metabolic bone disease, like renal transplantation, the agreement between fractures and BMD is less predictable. We assessed the prevalence of vertebral fractures by means of a new, DEXA-based method (morphometric absorptiometry, MXA) and compared its concordance with the two mostly employed radiological techniques (Visual semi-quantitative, SQ, and morphometric radiography, MRX) in an asymptomatic population of transplanted patients. Moreover, the association of bone fractures with low BMD values was examined. Fifty-three renal transplant patients underwent spinal radiographs and BMD measurements by DEXA. In order to obtain a morphometric assessment, a lateral DEXA scan was performed. Patients with vertebral fracture were 17/53 (32%) with both SQ and MRX, and 12/53 (23%) with MXA (chi(2) = n.s.). A single fracture was detected in 9/53 patients (17%) with SQ and MRX, and in 4/53 (7.5%) with MXA; multiple fractures were 8/53 (15%) with any technique. With SQ as the standard reference, predictive indexes were excellent with MRX (invariably 100%) and quite good with MXA (sensitivity 70.6%; specificity 100%). Of the total of 689 vertebrae, 49 were fractured with SQ, 54 with MRX and 41 with MXA. Mild deformities were present in 21 (SQ), 26 (MRX) and 13 (MXA) vertebral bodies, respectively, while moderate-severe deformities were 28 with any technique. Again, with SQ as standard reference, predictive indexes were good (MRX: sensitivity 100%, specificity 99.2%; MXA: sensitivity 83.7%, specificity 100%). When we classified patients according to BMD T-score values, SQ and MRX recognized fractures in 4/18 normal (22%), 10/22 osteopenic (45%) and 3/13 osteoporotic (23%). With MXA 3/18 (17%) normal, 6/22 (27%) osteopenic and 3/13 (23%) osteoporotic were fractured. The lower performance of MXA was very likely due to the poor quality of images from the upper thoracic spine of obese subjects. Prevalence of vertebral fractures in renal transplant patients is quite high and randomly associated with reduced BMD. A surveillance of the spine by Rx, implemented with vertebral morphometry, is therefore warranted to recognize the disease. (MXA is a reliable alternative technique, especially in more severely affected individuals.) MXA, although less sensitive than the conventional techniques because of possible technical biases, is very specific, and can be proposed for follow-up purposes in this population of patients.