Degenerative changes following spinal fixation in a small animal model

Objective: The objective of this study was to evaluate changes of the lumbar vertebral column following fixation. Design: Using an established small animal (rat) model of spinal fixation (hypomobility), 3 contiguous lumbar segments (L4, L5, L6) were fixed with a specially engineered vertebral fixation device. Spinal segments of control rats were compared with those of animals with 1, 4, or 8 weeks of fixation. Subgroups of these fixation animals subsequently had the fixation device removed for 1, 2, 4, 8, or 12 weeks to evaluate the effects of attempting to reestablish normal forces to the vertebral segments following hypomobility. Setting: This Institutional Animal Care and Use Committee (IACUC) approved study was conducted in a university animal facility. Animals: Eighty-seven animals (23 controls animals and 64 fixation animals) were used in this study. Main Outcome Measures: Outcome measures were degenerative changes of the vertebral bodies (VBs) and intervertebral disks (IVDs), zygapophysial (Z) joint osteophyte fort-nation, and Z joint articular surface degeneration (ASD). Changes found in vertebral segments that were fixed (hypomobile) were compared with changes in adjacent nonfixed vertebral segments, and changes among fixation animals were compared with nonfixed controls. Main Results: Very few degenerative changes were identified on the VBs and IVDs. Z joint changes were significant, both for osteophyte formation (analysis of variance [ANOVA], P < .0001) and ASD (ANOVA, P < .0001). Fixed segments had more degenerative changes than nonfixed segments for all Z joint parameters (ANOVA, P < .0001). Osteophyte formation and ASD were directly dependent on duration of fixation. Conclusions: These findings indicate that fixation (hypomobility) results in time-dependent degenerative changes of the Z joints.