STUDY DESIGN: This study evaluates the effect of iliac screw fixation versus interbody femoral ring allograft (FRA) on lumbosacral kinematics and sacral screw strain in long segment instrumentations. OBJECTIVE: (1) Quantify kinematic properties of 3 lumbosacral fixation techniques; (2) Evaluate sacral screw strain as instrumented levels extend cephalad; and (3) Determine whether iliac screws or FRA biomechanically protect sacral screws. SUMMARY OF BACKGROUND DATA: High failure rates at the lumbosacral junction have been reported with long posterior instrumentation ending with S1 pedicle screws. Achieving lumbosacral arthrodesis remains a clinical challenge. METHODS: Seven human cadavaric lumbosacral spines were biomechanically evaluated intact and in 3 instrumented conditions: pedicle screw fixation alone (pedicle screw group), pedicle screw fixation supplemented with iliac screws (iliac screw group), and pedicle screw fixation supplemented with FRA (allograft group). Each condition was tested spanning L5-S1, L4-S1, L3-S1, L2-S1, and L1-S1. Testing included pure unconstrained moments (±10 Nm) in axial rotation, flexion/extension, and lateral bending, with quantification of S1 screw strain and lumbosacral range of motion (ROM). RESULTS: Testing revealed decreasing lumbosacral ROM as instrumentation extended cephalad (P < 0.05). In axial rotation, ROM was markedly higher for the allograft group compared to pedicle screw and iliac screw groups with instrumentation to L4 (P < 0.05). In flexion/extension, length of instrumentation in each group correlated with ROM. As length of instrumentation increased, ROM decreased, particularly for the iliac screw group. In lateral bending, ROM decreased in all groups as instrumentation lengthened (P < 0.05). Strain on unprotected sacral screws increased in flexion, extension, and lateral bending as instrumentation extended to L3 (P < 0.05). Iliac screws reduced strain in constructs to L3 and above (P < 0.05). Allograft reduced strain when fixation reached L2, but was not as effective as iliac screws overall. Neither iliac screws nor allograft reduced strain in constructs terminating at L5 or L4. (P > 0.05) CONCLUSION.: For instrumented fusions extending above L3, sacral screws should be protected with supplemental iliac screws or FRA at L5-S1. Of the two, iliac screws appear more effective.