169. Biomechanical analysis of motion following lateral sacroiliac joint screw fixation with or without lumbosacral fixation

BACKGROUND CONTEXT Hypermobility or aberrant mechanics of the sacroiliac joint (SIJ) can be a source of pain. Lumbosacral fixation at L5–S1 may also be a source of postfusion low back pain. Prior biomechanical research has shown that fusion at the lumbar spine level can increase motion and stresses at the sacroiliac joint. However, no biomechanical studies have evaluated the effects of sacroiliac joint fixation on sacroiliac joint range of motion (ROM) in the setting of various ligamentous injuries or adjacent level lumbosacral screw and rod fixation. PURPOSE To assess the ROM of the SIJ after simulated adjacent lumbosacral fixation, with or without SIJ fixation, with lateral sacroiliac screws. STUDY DESIGN/SETTING In vitro biomechanical study. METHODS In this in vitro biomechanical study, 7 cadaveric specimens (L3–P) were tested on a six-degrees-of-freedom machine under load control at 1.5°/s to a maximum moment of ±8.5 Nm. The ligaments at the left posterior sacroiliac joints were destroyed to maximize ROM of the joint. The influence of lumbosacral fixation on SIJ motion, with or without SIJ fixation, was studied. RESULTS During flexion-extension in the setting of posterior SIJ injury and L5–S1 fixation, SIJ ROM increased to 195% of intact. After SIJ fixation with lateral sacroiliac screws, the average ROM was reduced to 144% of intact SIJ motion. SIJ screws thus partially stabilized the joint and reduced motion. The use of 6 bilateral SIJ screws with L5–S1 screw and rod fixation in lateral bending and axial rotation yielded the greatest reduction in SIJ ROM. Without lumbosacral fixation, baseline SIJ motion was reduced, and SIJ screws alone, using either 2, 3, or 6 screws, reduced motion at or below the level of an intact joint. CONCLUSIONS SIJ ligament injury with existing lumbosacral fixation doubled SIJ ROM. After SIJ fixation with lateral sacroiliac screws, ROM decreased. Screw configuration played a minor role; however, 6 SIJ screws generally had the greatest motion reduction. FDA DEVICE/DRUG STATUS SI-LOK screws (Approved for this indication).