Cervical Spine Under High G-force: How Does ACDF Affect the Fighter Pilot in Ejection Scenario?

Introduction: The cervical spine of a military pilot is subject to high G-forces during practice, battle, and specifically in an ejection scenario. An ejection scenario exposes the pilot to a high Gz pulse that could injure the spine. Biomechanical studies looking at high G-forces are lacking. With a high prevalence of cervical spine injury among pilots, ACDF and arthroplasty are surgical options. The present study quantified the internal anterior and posterior column loading with ACDF at index and adjacent level during high-g forces that the pilots may encounter routinely.

Methods: An intact global human body consortium finite element model was seated upright with horizontal Frankfort plane and was placed in a five-point restraint with a 1.5 kg helmet. -Gz pulse of maximum amplitude of 6G was applied to the seat. Finite element model was validated against human cadaver results. ACDF was simulated at C5-6. Range of motion, disc pressure, and facet loads were measured and compared between intact and fused cervical spines.

Results: Range of motion and facet loads at the surgical level (C5-6) decreased by two-third and one-third respectively. At C4-5, disc pressure, motion, and facet load increased by less than 10%. These increased at C6-7 by up to 38%.

Conclusion : This is the first biomechanical study to investigate the biomechanical effects of ACDF on a pilot in a high G-force scenario (ejection). ACDF at C5-6 is a clinical scenario that is encountered for these patients who are regularly exposed to high G-force during training and combat. Patients with C5-C6 may transfer the intrinsic loads due to ACDF to the C6-C7, i.e., predispose to adjacent segment degeneration, a finding that appears to fit with military studies that show that C5-C6 and C6-C7 are the two most common levels for ACDF/CDA . Further studies would be needed with alternative techniques such as standalone spacer and arthroplasty. These findings are applicable to ground troops encountering IED, commercial pilots, and civilian patient ejection scenarios in high-speed motor vehicle collisions such as rollovers.