Adjacent Segment Motion After a Simulated Lumbar Fusion in Different Sagittal Alignments: A Biomechanical Analysis

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Abstract

Study Design.

An in vitro biomechanical study of adjacent segment motion (at L3–L4 and L5–S1) after a simulated lumbar interbody fusion of L4–L5 in different sagittal alignments was carried out.

Objectives.

To test the hypothesis that an L4–L5 fixation in different sagittal alignments causes increased angular motion at the adjacent levels (L3–L4 and L5–S1) in comparison with the intact spine.

Summary of Background Data.

Clinical experience has suggested that lumbar fusion in a nonanatomic sagittal alignment can increase degeneration of the adjacent levels. It has been hypothesized that this is the result of increased motion at these levels; however, to the authors’ knowledge no mechanical studies have demonstrated this.

Methods.

Eight fresh human cadaveric lumbar spines (L3–S1) were biomechanically tested. Total angular motion at L3–L4 and L5–S1 under flexion-extension load conditions (7-Nm flexion and 7-Nm extension) was measured. Each specimen was tested intact, and then again after each of three different sagittal fixation angles (at L4–L5): (1) in situ (21°lordosis), (2) hyperlordotic (31°lordosis), and (3) hypolordotic (7°lordosis). The simulated anterior/posterior fusion was performed at L4–L5 with pedicle screws posteriorly, vertebral body screws anteriorly, and an interbody dowel.

Results.

The averaged values for flexion-extension motion at L3–L4 were as follows: intact specimen 2.0°, in situ fixation 4.0°, hyperlordotic fixation 1.7°, hypolordotic fixation 6.5°. The averaged values for flexion-extension motions at L5–S1 were as follows: intact specimen 2.3°, in situ fixation 2.6°, hyperlordotic fixation 3.6°, hypolordotic fixation 2.9°.

Conclusion.

Hypolordotic alignment at L4–L5 caused the greatest amount of flexion-extension motion at L3–L4, and the differences were statistically significant in comparison with intact specimen, in situ fixation, and hyperlordotic fixation. Hyperlordotic alignment at L4–L5 caused the greatest amount of flexion-extension motion at L5–S1, and the difference was statistically significant in comparison with intact specimen but not in situ fixation or hypolordotic fixation.

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