Purpose: This study examines the anatomic proportions of the interspinous space and the spinous processes, considering the optimal placement of an interspinous spacer.
Methods: Between January 2008 and December 2009, 565 patients undergoing computed tomography (CT) scans of the abdomen for various reasons were collected retrospectively for the study. Using the CT scan data, spinous processes of the lumbar spine L1-5 and the interspinous spaces T12-L5 were measured.
Results: The average measured interspinous space was 9.1 ± 2.5 mm. This space became significantly (p < 0.001) smaller from anterior to posterior. Average cortical thickness of all lumbar spinous processes was 2.5 ± 0.5 mm. Cortical thickness decreased significantly (p < 0.001) from anterior to posterior. The cortex of the spinous processes from L2 (2.67 ± 0.45 mm) and L3 (2.66 ± 0.94 mm) was significantly thicker (p < 0.001) than that of the others. The spinous process of L5 had the thinnest (p < 0.001) cortex (2.10 ± 0.41 mm), as well as the smallest (p < 0.001) volume (3.0 ± 1.0 ml) and the shortest (p < 0.001) height (16.6 ± 3.6 mm).
Conclusions: The spinous processes of L2 and L3 are the largest and sturdiest, and that of L5 is the weakest. The L4/5 segment features the smallest interspinous space with the thinnest cortex of all lumbar spinous processes. Because the interspinous space narrows posteriorly and the cortex is thicker anteriorly, it seems that the best anatomic position for a stand alone interspinous spacers is anterior.