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Study Design.

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.

Objectives.

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.

Summary of Background Data.

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.

Methods.

Seven human cadaveric lumbosacral spines were instrumented with pedicle screws at L4, L5, and …