Underestimation of bone loss of the spine with posterior-anterior dual-energy X-ray absorptiometry in patients with spinal cord injury

Background: Bone mineral density (BMD) of the lumbar spine (L-spine) has been reported to be normal by routine posterior-anterior (PA) bone density imaging in patients with chronic spinal cord injury (SCI). Objective: To determine BMD of the L-spine by PA and lateral (LAT) dual-energy radiographic absorptiometry (DXA) in patients with chronic SCI. Design: Prospective study. Setting: Veterans Affairs Medical Center and a private rehabilitation facility. Methods: Measurements of the PA and LAT L-spine and hip were performed in 15 patients with SCI: 9 with tetraplegia and 6 with paraplegia. The DXA (GE Lunar Advance DXA) images were obtained using standard software. Results are reported as mean ± SD. Results: The mean age was 35 ± 15 years (range = 20-62 years), and the duration of injury was 57 ± 74 months (range = 3-240 months). T- and Z-scores were lower for the LAT L-spine than those for PA L-spine (T-scores L2: -0.7 ± 1.2 vs 0.0 ± 1.4, P < 0.01; L3: -0.9 ± 1.6 vs 0.3 ± 1.3, P < 0.002; L2-L3: -0.8 ± 1.3 vs 0.2 ± 1.3, P < 0.001; Z-scores L2: -0.3 ± 1.1 vs 0.2 ± 1.2, P < 0.05; L3: -0.6 ± 1.3 vs 0.5 ± 1.3, P < 0.01; L2-L3: -0.4 ± 1.1 vs 0.4 ± 1.2, P < 0.005). The T- and Z-scores for the total hip (-1.1 ± 1.0 and - 1.0 ± 1.0, respectively) and L2-L3 LAT L-spine demonstrated remarkable similarity, whereas the L2-L3 PA L-spine scores were not reduced. Bone mineral density of the LAT L-spine, but not the PA L-spine, was significantly reduced with increasing duration of injury. Conclusions: Individuals with SCI may have bone loss of the L-spine that is evident on LAT DXA that may be misdiagnosed by PA DXA, underestimating the potential risk of fracture.