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We present an in vitro biomechanical comparison of adjacent segment motion at the cranial segment (L3–L4) for an L4–L5 versus an L4–S1 fusion model using cadaveric lumbosacral spines.The purpose is to determine the biomechanical effect on the unfused cranial segment of extending a short lumbar fusion to the sacrum versus stopping at L5.Radiographic evidence of adjacent segment degeneration can occur as a late sequela in patients following lumbar and lumbosacral spinal fusions. It is believed that altered biomechanics adjacent to the fusion construct contribute to these degenerative changes. Little is known regarding changes in cranial adjacent segment mechanics resulting from inclusion of the sacrum compared to ending a fusion at L5.Seven human cadaveric lumbosacral spines were instrumented with pedicle screws at L4, L5, and S1. Rods were placed from L4–L5 and from L4–S1 to simulate the corresponding fusion models. A material testing system was used to apply load-controlled moments to the spines in flexion-extension, lateral bending, and axial rotation. Electromagnetic sensors were used to record 6 df motion across the L3–L4, L4–L5, and L5–S1 motion segments. Angular displacements were recorded and system stiffness was calculated for each spine and construct. A paired sample t test was used to determine significance of recorded differences.Under flexion-extension loading, the angular displacement in the sagittal plane at L3–L4 for the L4–S1 model was 9.0° compared to 7.8° for the L4–L5 model (+15%; P = 0.002). Under lateral bending loading, L3–L4 motion in the coronal plane for the L4–S1 model was 12.8° and was 14.5° for the L4–L5 model (−12%; P = 0.002). In axial rotation testing, L3–L4 torsional motion for the L4–S1 model was equivalent to the L4–L5 model. Overall system stiffness increased for the L4–S1 model compared with the L4–L5 model.In this load-controlled model, extending fusion across L5–S1 did not consistently increase motion at L3–L4. While it may be difficult to translate this finding to a clinical setting, avoiding fusion to the sacrum in a lower lumbar fusion may not provide significant benefit from the standpoint of avoiding adjacent segment disease.