Cervical Total Disc Replacement and Anterior Cervical Discectomy and Fusion Responses Under Aircraft Ejection...

Military aviators experience earlier spine degeneration and neck pain than civilians. Concerns exist about the performance of cervical total disc replacement (CTDR) in tactical military flight, including during aircrew ejection. Aviators in ejection-seat aircraft may obtain a medial waiver and return to flight after anterior cervical discectomy and fusion (ACDF); however, ACDF limits index/surgical level motion, increases motion at adjacent segments, accelerates adjacent spinal degeneration, and increases the risk for future cervical spine surgery. CTDR reduces the adverse effects of ACDF, yet CTDR biomechanics are understood based only on quasi-static loading. This study evaluated responses of ACDF and CTDR under simulated hyperdynamic (military aircrew ejection) vertical loading.

Ejection-type vertical (Gz) loading was applied to the human body finite element model. ACDF and CTDR procedures were simulated at the C5-C6 level. The CTDR was modeled using a Mobi-C (ZimVie) design, while the ACDF represented the conventional plating construct used commonly in the United States. Segmental motion and facet force at the index and adjacent segments, and intradiscal pressure at the adjacent segments were obtained and normalized with respect to the intact/native spine. Range-of-motion and load parameters were compared with known failure criteria.

Compared to the native spine, the index level segmental motion decreased by 30% for CTDR and 81% for ACDF. Superior level segmental motions decreased for CTDR (6%) and increased for ACDF (23%), while the motions at the inferior adjacent level increased for both CTDR and ACDF (+11% and +18%). Intradiscal pressure at the superior level increased by <5% for both interventions, as well as at the inferior level (17% for CTDR and 35% for ACDF). Facet forces at the index level increased by 33% and 3% (CTDR vs. ACDF) yet were minimally changed at adjacent levels (<10%, ACDF and CTDR). No condition (native, ACDF, CTDR) resulted in spine failure when referenced to known failure criteria.

This preliminary study investigated the biomechanical responses of post-ACDF and CTDR spines under dynamic vertical loading, applicable to military aviators in ejection-seat aircraft. CTDR preserved motion at the index level and decreased motion transmission to adjacent spinal levels, while increasing the index-level posterior column load sharing. Spine failure criteria were not exceeded in any simulated condition. These findings are important components for the continued evaluation of CTDR safety and performance in the tactical aviator domain.