INTERPRETATION OF LUMBAR SPINE DENSITOMETRY IN WOMEN WITH FRACTURES

Identification of postmenopausal women at risk of developing osteoporo tic fractures is a major clinical problem. In this study the use of pr ojected planar lumbar bone density values for individual fracture risk assessment was questioned. Osteodensitometry (DXA) results from 415 n ormal women, 62 women with previous vertebral compressions, and 76 wom en with previous low-energy fractures were analyzed, together with the ir body size and lumbar vertebral body size variables. The following w ere found: (1) Lumbar vertebral projected bone mineral areal density ( BMD) and bone mineral content (BMC) of normal women correlated with bo dy size variables (p<0.001). (2) Lumbar vertebral body size variables also correlated with body size variables (p<0.001). Logistic regressio n analysis of measured and derived physical variables from women witho ut and with vertebral compression fractures (n=477) showed: (3) The be st compression fracture discriminator, significantly better than BMD, was BMC divided by (H(max)/165 cm)1.5 x (D/4.35 cm)1.5, where H(max) i s the body height (cm) at the menopause, and D the mean lumbar vertebr al diameter of the three mid-lumbar vertebral bodies (cm). This parame ter was termed BMC(corr.) ROC analysis showed: (4) At a BMC(corr.) tru e positive ratio of 80% the corresponding uncorrected BMC or BMD true positive ratio was only 60%. The corresponding false positive ratio wa s 6%. Lumbar osteodensitometry could not be used to identify women wit h a history of peripheral low-energy fractures. (5) BMC(corr.) did not , unlike BMC and BMD, correlate with body size and vertebral size vari ables. (6) Likewise, an observed correlation between BMC and lean body mass in a subpopulation of 116 normal women was abolished when BMC(co rr.) replaced BMC. We suggest that vertebral compression fracture risk limits based on BMC, corrected for individual differences in body siz e and vertebral body size, replace the commonly used BMD fracture risk limits. The discriminatory ability of BMC(corr.) for low-energy fract ures needs to be tested in a different population.