Residual sagittal motion after lumbar fusion: a finite element analysis with implications on radiographic flexion-extension criteria.

STUDY
DESIGN: Finite element analysis of a lumbar fusion model.
OBJECTIVES: To quantify residual sagittal angular motion following various types and levels of completeness of lumbar fusion in order to understand better the validity of current recommendations for interpreting flexion-extension radiographs to assess fusion. SUMMARY OF BACKGROUND DATA: Recommended threshold criteria for solid fusion using flexion-extension radiographs have varied from 0 degrees to 5 degrees of angular motion between vertebrae. Notwithstanding this wide variation and lack of uniform consensus, the validity of these criteria has not been previously biomechanically assessed to the authors' knowledge. To investigate this issue, the authors sought to test various types of simulated healed, noninstrumented lumbar fusions using finite element modeling to determine the amount of residual angular motion under physiologic stresses.
METHODS: A validated 3-dimensional, nonlinear finite element model of an intact adult human L3-L4 motion segment was developed. Four fusion types were simulated using this model, including anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), intertransverse process fusion, and interspinous process fusion. Variations of completeness of fusion were also represented. For ALIF and PLIF, this included tests of solid bridging bone within the posterior or anterior 75%, 50%, or 25% disc space. In addition, PLIF was also tested with either a unilateral or bilateral facetectomy to simulate commonly used surgical techniques. Variations of intertransverse process fusion included unilateral or bilateral bridging bone with or without medial fusion to the pars interarticularis. Only 1 scenario of a healed, solid interspinous process fusion was tested. The intact model and all fusion models were stressed with 10.6-Nm flexion and extension moments. The angular deflections were recorded in degrees.
RESULTS: A wide range of sagittal angular motion was recorded. For ALIF, this ranged from 0.8 degrees (complete, 100% fusion) to 3.3 degrees (solid fusion of the posterior 25% disc space). For PLIF, the numbers were more varied, ranging from 0.7 degrees (complete, 100% fusion) to 6.9 degrees (solid fusion of posterior 25% disc space with bilateral facetectomy). For intertransverse process fusion, the least motion was with a solid bilateral fusion, with medial healing to the pars (2.0 degrees); the greatest motion was found with a solid unilateral fusion without medial healing (6.0 degrees). Interspinous process fusion allowed only 1.9 degrees of motion.
CONCLUSIONS: The amount of residual flexion-extension motion with simulated lumbar fusions (presumably allowed by the bone's inherent elasticity) under physiologically comparable moments varies with fusion type and, more substantially, with varying amounts of completeness. The current study documents a range of sagittal angular motion after several types of simulated lumbar fusion that appear to have considerable overlap with previously purported radiographic criteria for solid fusion using flexion-extension radiographs. However, it also suggests the possibility that some scenarios of solid, yet incomplete, fusion may allow motion that is substantially greater than 5 degrees, which is beyond the most liberal of previously published threshold criteria.