Bone density of the first sacral vertebra in relation to sacral screw placement: a computed tomography study.

A visual three-dimensional image of the first sacral vertebra was constructed using computer software to predict the sites of strong density for better screw purchase of upper sacrum. Forty dry sacrum specimens were scanned in the prone position. An axial section, 10 mm below the S1 end plate, was selected for determining density at the region of interest. All images were stored on an optic disc and studied using the NIH Image 1.61 program. Plot analysis assessed the bone density in different regions. Also, three-dimensional pictures of the different screw paths and the related bone density in the upper sacrum were analyzed. Bone density in the anterolateral part of S1 was 115.1 +/- 10.4 pixel. Bone density for males (-99.7 +/- 11.3) was greater than for females (-131.4 +/- 9.6). Bone density in the anterolateral alar region was -108 +/- 10.6. The bone density for males (-95.6 +/- 9.8) and females (121.4 +/- 11.7) was more than the body region. Bone density in the middle anterior cortex of the ala was 759.8 +/- 11.6. Bone density for males (878.2 +/- 10.7) was greater than for females (637.6 +/- 11.9). Using surface plot, the midanterior cortex of the ala had high cortical density compared with other areas. The midanterior cortex of the sacral ala had the highest bone density. Sacral screw purchase in the midanterior cortex provides better mechanical fixation.