THE ILIAC BUTTRESS - A COMPUTED TOMOGRAPHIC STUDY OF SACRAL ANATOMY

Fifty randomly selected computed tomographic (CT) scans of the lumbosacral spine (25 males and 25 females) were studied to determine: 1) if the lateral sacral masses could safely accept a 7 mm diameter rod (i.e., intrasacral rod insertion) and 2) what percentage of patients, both males and females, demonstrated coverage of the posterolateral sacrum by the ilia (i.e., iliac buttressing). In all patients the lateral masses (i.e., the lateral intrasacral mass measurements) appeared wide enough on CT to allow for safe insertion of a 7 mm diameter rod, or other similar size implant, down to at least the level of S2. The smallest distance measured for the width between the posteromedial margin of the sacroiliac joint and the lateral cortex of the S1 neuroforamen (i.e., the lateral intrasacral mass measurement) at its location approximately midway (anteroposterior) through the sacrum on CT cuts was 17 mm (mean 28 mm). This would appear to give adequate room for a 7 mm diameter rod to be inserted at this level in the lateral sacrum (i.e., intrasacral rod insertion). Forty-six patients (24 males, 96%; and 22 females, 88%) appeared to have sufficient CT coverage of the sacrum to conceptually provide for so called ''sacroiliac buttressing'' of rods, if rods or other implants were to be inserted distally into the lateral masses. After a review of the sacral anatomy by CT it appears that: 1) insertion of rods into the lateral sacral masses (i.e. intrasacral rod insertions), or intrasacral fixation with other similar size implants, would be possible and apparently safe; and 2) the ilia along with the sacroiliac interosseous ligaments sufficiently surround the back and sides of the posterolateral sacrum in most patients (92%), at least by CT assessment, to conceptually offer an indirect ''buttress'' for implants so inserted. Theoretically, this could biomechanically help resist the flexural loads across the lumbosacral level and possibly provide a method for improved sacral fixation with spinal instrumentation in certain patients.