The influence of diffuse idiopathic skeletal hyperostosis on bone mineral density measurements of the spine.

OBJECTIVES BMD has been described to be increased in patients with DISH. The contribution of the ossified anterior longitudinal ligament (ALL) on vertebral body BMD is currently unknown. We investigated the influence of DISH on BMD measurements using an experimental DXA scanning protocol. METHODS Ten DISH specimens and 10 matched human cadaveric spines were used. After assessment of the localization and orientation of the ossified ALL with CT, BMD was measured using an experimental DXA protocol, exploiting the asymmetry of DISH in the thoracic spine. For controls, identical orientations were used and both groups were compared for differences in BMD. RESULTS Specimens with DISH displayed a significantly higher BMD than their matched controls when the ossified ALL was present in the scanning field. Measurements of the left half of the spine were comparable for DISH specimens and controls (P = 0.446). The right-left difference in anteroposterior view was statistically significant within DISH specimens (P = 0.001), but not in controls (P = 0.825). CONCLUSIONS The variability in measurements in different scanning orientations suggests a substantial contribution of the ossified ALL to the total BMD in DISH specimens, ranging from 23.6 to 39.0%. Vertebral body BMD does not seem to be increased, as demonstrated by comparable BMDs in the unaffected left half of the spine. It is suggested that routine anteroposterior DXA scanning may overestimate the true vertebral body BMD in DISH patients.