Histomorphometric study of iliac bones in cervical myelopathy with ossification of the posterior longitudinal ligament

To investigate whether there was any abnormal systemic bone metabolism in patients with ossification of the posterior longitudinal ligament (OPLL), we measured various histomorphometric indices of the iliac trabecular bones in 19 patients with OPLL (14 men, 5 women). For each index, the Z-score for each patient was calculated, using the SD and mean value for non-OPLL control group (n = 159) who underwent orthopaedic surgeries and had no systemic disease, age-matched for each decade. A Z-score of 1.0 meant that the observed data deviated 1 SD from the normal average, and a distribution from -2 to +2 was considered normal. All the averaged Z-scores were within +/-1.0 and there were no significant differences between the OPLL and the control groups. There was also no difference in Z-score among three types of OPLL (segmental, continuous, and mixed). From these results, we concluded that there was no common abnormal bone metabolism affecting the bone histomorphometry of the iliac bone in OPLL patients. However, in two patients, the Z-score for bone volume (BV/TV) was more than 2.0, with increased osteoid volume (OV/TV) and increased trabecular thickness, suggesting that there was some abnormal bone metabolism in these two patients with OPLL.