Implications of overstuffing in cervical total disc replacement: biomechanical assessment of index and adjacent level range of motion.

PURPOSE: We assessed the biomechanical effects of oversizing a cervical total disc replacement (cTDR) device using a cadaver model. We hypothesized that an overstuffed cTDR would significantly reduce flexion-extension range of motion (FE ROM) compared to the native spine and a 2-level cTDR, while still providing more 2-level motion compared to hybrid (ACDF adjacent to a cTDR).

METHODS

Eight C2-T1 cadaveric spines were instrumented at C4-C6 by a board-certified neurosurgeon with the following interventions: 2-level cTDR, cTDR with one overstuffed level (+ 1 mm height), and hybrid with ACDF. All cTDR interventions were completed with the Mobi-C Cervical Disc. Flexion-extension range of motion (FE ROM) was measured after each intervention using a spine simulator and optical motion capture. Total FE ROM was extracted for the full spine and each motion segment and normalized to each specimen's native FE ROM to facilitate statistical comparison between interventions.

RESULTS

An overstuffed cTDR reduced index level FE ROM by 50%, which scaled to a 14% reduction in the full cervical spine (C2-T1). 2-level FE ROM across C4-C6 was clinically comparable between overstuffed and hybrid interventions. The cTDR device adjacent to the index level remained consistent in FE ROM through all interventions.

CONCLUSION

Overstuffing a cTDR device by just +1 mm height resulted in substantial and clinically meaningful reductions in motion at the index level and full cervical spine. These significant biomechanical consequences, observed with even minimal oversizing, highlight the need for expanded implant heights, standardized height reporting, and improved alignment of device design with population anatomy.