Background context: In cases of low back pain associated with biomechanical lumbar instability, anterior interbody fixation can be used as a surgical treatment, but its affect on facet joint capsule strains is unknown.
Purpose: To determine the effect of a single-level anterolateral interbody fixation, the changes in lumbar facet joint capsule strains at the level of and adjacent to the fixation were evaluated.
Study design/setting: Human cadaveric lumbar spine specimens were tested under displacement control before and after the addition of a single anterior thoracolumbar plate (ATLP) on the L4-L5 motion body.
Methods: Ligamentous lumbar spine specimens (n=7) were potted and actuated before and after fixation of the L4-L5 motion segment with an ATLP in motions of extension, flexion, left and right bending. Joint moments were calculated from the applied load and respective moment arms. Intervertebral angulation was measured using biaxial inclinometers mounted onto adjacent vertebrae. Plane strains of the capsules were measured by optically tracking the displacements of small, infrared reflective markers glued to capsule surfaces. Statistical differences (p<.05) in moment, intervertebral angle and capsular strain were assessed using analysis of variance and comparison of linear regression lines.
Results: Fixation resulted in an increase in moment at the three vertebral levels for all motions. There was also an increase in intervertebral angle at L3-L4 and L5-S1, and a decrease in intervertebral angle at L4-L5 for all motions. Plane strains in the L3-L4 and L5-S1 facet capsules increased as a result of the fixation. L4-L5 facet capsules experienced decreased and increased strains ipsilateral and contralateral, respectively, to the instrumentation.
Conclusion: Restriction of a vertebral motion segment using a single ATLP increased adjacent capsular strains, which if suprathreshold for capsule nociceptors, could play a role in low back pain.