Biomechanical Evaluations of 2-level Arthroplasty, Overstuffed, Hybrid, and 2-level Fusion Constructs in the Cervical Spine with a Mobile-core Arthroplasty Device

Introduction: Cervical total disc arthroplasty has become a popular treatment for cervical degenerative disc disease as it preserves motion and is thought to reduce adjacent level disease compared to ACDF. The sensitivity of implant height on the biomechanics in cervical arthroplasty is currently unknown. The aim of this biomechanical study was to determine the effect of various instrumentation constructs on the biomechanics of the cervical spine. The constructs included 1- and 2-level cervical total disc arthroplasty (cTDA), overstuffing, hybrid, and 2-level fusion.

Methods: Eight human cadaveric cervical spine specimens (C2-T1) were subjected to flexion-extension loading up to 2 Nm using a dual gimbal, six degree-of-freedom servo-hydraulic spine simulator. Motion was tracked using an optoelectronic system. Each specimen was tested in the native state and in five different intervention constructs: 1-level cTDA, 2-level cTDA, 2-level cTDA with one device oversized by 1 mm (overstuffing), hybrid, and 2-level fusion with ACDF and posterior fusion. Range of motion (ROM) were compared between the six different interventions at the total cervical spine, index levels, and adjacent levels.

Results: The introduction of a mobile-core cTDA device replicated healthy native range of motion for both 1- and 2-level cervical arthroplasty. The overstuffed, hybrid, and 2-level fusion interventions all caused significant drops in total cervical ROM compared to the intact, 1-level, and 2-level cTDA interventions. Overstuffing one level of a cTDA device resulted in a significant 15% drop in total cervical range of motion and a 53% reduction in ROM at C4-C5 and a 55% reduction in ROM at C5-C6 compared to intact specimens. No significant differences were recorded between the hybrid and overstuffed interventions.

Conclusion : Proper sizing of a cTDA device is critical to ensuring that the device allows for proper motion and reduces risk of inadvertent fusion and adjacent level disease.