Study design: Biomechanical evaluation of lumbosacral fixation using a porcine model.
Objectives: The primary objective of the current study was to quantify and compare the changes in lumbosacral range of motion produced by four different methods of surgical stabilization, as well as comparing the relative effects of the four constructs in destructive testing.
Summary of background data: The lumbosacral junction continues to be a difficult region to obtain a successful spinal arthrodesis and is one of the primary regions for construct failure.
Methods: Twenty-four fresh-frozen porcine lumbosacral spines were used in this investigation. Following intact analysis, the specimens were radically destabilized at the lumbosacral junction and randomized into four treatment groups based on reconstruction: 1) L7-S1 pedicle screws alone (n = 6); 2) L7-S1 pedicle screws and interbody cage (titanium mesh) (n = 6); 3) L7-S1 pedicle screws and iliac screws (n = 6); and 4) L7-S1 pedicle/iliac screws and interbody cage (n = 6). Nondestructive, multidirectional flexibility analyses included four loading methods and fatigue component followed by a destructive flexural load to failure. Lumbosacral peak range of motion (millimeters or degrees) and ultimate failure load (Nm) of the four reconstruction techniques were statistically compared using a one-way analysis of variance combined with Fisher's PLSD.
Results: 1) Axial compression: There were no differences in lumbosacral flexibility among the four treatment groups (P > 0.05). 2) Axial rotation: Iliac screw constructs, with or without cages, decreased flexibility at the lumbosacral junction compared with pedicle screws alone (P < 0.05). However, interbody cage reconstructions did not significantly reduce motion. 3) Flexion-extension: Iliac screws with interbody cages reduced segmental motion about the lumbosacral junction, which was significantly different from the remaining treatments (P < 0.05). 4) Lateral bending: The iliac screw constructs afforded significantly less lumbosacral motion compared with bothpedicle screw constructs (with or without cages). With destructive testing, pullout for L7-S1 constructs occurred at the sacrum, whereas failure for the L7-S1/iliac screw, with and without cages, occurred at the proximal adjacent level.
Conclusion: Iliac screw constructs reduced lumbosacral flexibility levels in three of four loading methods (axial rotation, flexion-extension, and lateral bending) compared with pedicle screw reconstructions. The addition of interbody cages decreased lumbosacral motion for the iliac screw treatments under flexion-extension loading and pedicle screw constructs under axial rotation but did not protect the sacral screws in destructive testing as the iliac screws did. Based on evaluation using an porcine model, both iliac screws and interbody cages effectively reduce the multidirectional flexibility properties of the lumbosacral junction; however, iliac screws are more restrictive of motion (at the lumbosacral joint) and protective of the S1 screws.