Variation in bone mineral density of the sacrum in young adults and its significance for sacral fixation

Study Design. Bone mineral density variations throughout the sacrum were me asured and correlated with sacral screw insertion torque. Objective. To quantify bone mineral density variations within the S1 body a nd ala of young human specimens, especially along the pathways of sacral sc rews, and to examine the relation between sacral screw fixation and bone mi neral density. Summary of Background Data. Vertebral bone quality is an essential factor i n anterior or posterior screw fixation of the spine. Several studies have b een conducted regarding bone mineral density variations in the cervical and thoracolumbar spine. However, such variations in bone mineral density in t he sacrum have not been well documented. Methods. The bone mineral density of 13 sacral specimens from young male ca davers (mean age, 31 years) was measured using highly accurate quantitative computed tomography. Variations in bone mineral density were measured in f ive transverse layers and seven vertical columns within the S1 body, and in four transverse layers and six vertical columns within the ala. The sacral screw insertion torque was measured (unicortical and bicortical), and the correlation with bone mineral density was calculated. Results. The mean bone mineral density of the S1 body was 381.9 +/- 59 mg/c m(3), which was 31.9% higher than that of the sacral ala (mean, 296.9 +/- 8 6 mg/cm(3)) (P < 0.05). Bone mineral density of the superior sacral endplat e was higher than that of any other transverse layer. Columns near the late ral posterior and lateral anterior of the S1 body had the highest bone mine ral density. In the ala, bone mineral density values of the internal column s (pedicle) were the highest. Screw insertion torque for bicortical purchas e along the S1 pedicle correlated well with the bone mineral density of the S1 body (r = 0.67, P < 0.05). Conclusion. This study quantified the volumetric bone mineral density varia tions within the S1 body and ala, and a significant linear correlation betw een the screw insertion torque and bone mineral density was found. Optimal sacral screw insertion pathways were also outlined based on bone mineral de nsity values.