EFFECT OF SACRAL AND ILIAC INSTRUMENTATION ON STRAINS IN THE PELVIS -A BIOMECHANICAL STUDY

Study Design. This study is a biomechanical study on human cadaver pel ves using strain gauges to measure strains at different locations in r esponse to different load states. Objectives. To examine the changes i n strains, at different sites on the pelvis, and after instrumentation of the lumbosacral joint and the lumbosacroiliac joint. Summary of Ba ckground Data. Pelvic girdle pain after long instrumented fusions to t he sacrum has been described and studied along with sacroiliac and pub ic instability. Fractures of the pelvic rami or the iliac wing have be en reported in patients after surgical fusions that incorporated the s acrum and sacroiliac joint. Methods. Pelves with lumbar spine attached had three axis rosette strain gauges cemented to specific sites on th e pelves (ileum and rami). Specimens were tested in different configur ations involving lumbosacral and sacroiliac instrumentation. Iliac bon e harvest was also tested. Strain values were computed and compared fo r each configuration. Results. The ''Galveston'' and ''sacral'' instru mentation configurations caused significant stress shielding. in pelve s under all compression loads applied. Axial rotations on the instrume nted specimens caused higher strains at all sites. Harvesting of bone from the ilium did not alter the strains seen at the different sites. Conclusions. This study suggests significant stress shielding provided by sacro and sacroiliac instrumentation systems, supporting the possi bility of development of device-related osteopenia that may predispose patients to late, insufficiency-type fractures as their activity incr eases. The rigid instrumentation of the lumbosacral spine was found to increase the stress during torsion, which may lead to failure during activity.