Biomechanical Testing of Charité Disc Replacements versus Fusion Using Hybrid Testing Protocol and Follower Load

by Teng, Edward

Statement of Purpose: To quantify adjacent level effects following Charité disc replacement compared to fusion, using the follower load and the new standard hybrid testing method in flexion, extension, and torsion.

Summary of Background Data: As opposed to fusion, artificial discs not only preserve motion, but may diminish adjacent level effects and thus slow adjacent disc degeneration. This study aims to use hybrid test protocol and follower load to perform in vitro multidirectional testing and quantify adjacent- and other- level (AOL) effects following one- and two-level disc implants compared to fusion.

Methods: Five intact human cadaveric lumbar specimens underwent multidirectional testing in flexion, extension, and bilateral torsion loading with ±10Nm unconstrained pure moments under 400N follower load. Intact specimen (T12-S1) total ranges of motion were first determined for each loading direction and then used as input for the 5 subsequent hybrid tests: 1) one-level Charité disc implant at L5-S1; 2) one-level simulated 3600 fusion at L5-S1; 3) two-level Charité disc implants at levels L4-L5 and L5-S1; 4) combined one-level Charité disc implant at L4-L5 and one-level simulated 3600 fusion at L5-S1; and 5) two-level simulated 3600 fusion at L4-S1. Using repeated measures single factor ANOVA and Bonferroni statistical tests (p < 0.05), intersegmental motion redistribution was compared between intact and testing conditions.

Results: One- and two- level Charité disc preserved physiological motion, whereas one-level L5-S1 fusion increased intersegmental motions at proximal levels. Two-level fusion at L4-L5-S1 produced markedly increased motions at proximal levels, but converting the L4-L5 fusion to a Charité produced results equivalent to a one-level L5-S1 fusion. In axial torsion, both one- and two-level disc implants preserved motions at all levels. Although the one-level fusion did not produce significant changes, the two-level fusion did result in significant increases in motion at proximal levels.

Conclusions: The fusion simulations did not only affect motion re-distribution at adjacent-levels, but also at other-levels cephalic to the fusion. A prosthetic disc placed above a fusion could not buffer the proximal levels from increased motion from the fusion.