A Biomechanical Evaluation of a Versatile and Novel Anterior Cervical Fusion Device Possessing Modular and Integrated Fixation Capabilities

Abstract

IntroductionA novel ACDF (anterior cervical discectomy and fusion) construct possessing integrated screw and modular plate fixation (MPF) capabilities has been introduced in an effort to provide versatility when selecting a stabilization mechanism. Construct features allow the surgeon to switch between zero-profile (2 screws), half-plate (3 screws), and full-plate (4 screws) system. Inherently, the device can be readily adapted to the patient's anatomical landscape, accommodating adjacent level fixation, as well as diminishing hardware prominences when necessary. Additionally, the MPF technology, which creates a singular rigid body about the index level, affords ideal plate orientation/alignment, eliminates potential for cage migration/subsidence, and ensures physiological compression of the cage/graft. However, the translated effects of such novel features on segmental stability have not yet been characterized in the literature. The objective of this study was to assess the segmental rigidity achieved by the novel ACDF device iterations as compared with traditional ACDF (cage and anterior cervical plate system) and supplementation with posterior cervical constructs with lateral mass screws (LMS). Material and MethodsEighteen human cervical spine specimens (C3-T1) were tested. Osseous integrity was confirmed via DEXA scans and radiographs. Specimens were divided into three groups (n = 6) such that the mean bone quality across each group was consistent. The C3 and T1 vertebral bodies were potted. Each spine was first tested in an intact state. An anterior discectomy (C5/C6) was then performed, followed by sequential iterative construct instrumentation and testing (see Results for sequence). The three group protocol was executed such that each respective group would receive only integrated zero-profile, integrated half-plate, or integrated full-plate fixation (Alta ACDF System – Zimmer-Biomet Spine); diminishing vertebral body compromise due to excessive screw removal/placement. For posterior supplemented constructs, lateral mass screws (Lineum – Zimmer-Biomet Spine) were placed bilaterally at the index level. A 2Nm moment was applied in flexion-extension (FE), lateral-bending (LB), and axial-rotation (AR) using a six degree-of-freedom Bionix® Spine Kinematics System (MTS, Systems, MN). Segmental range-of-motion (ROM) was tracked using Optotrak Certus (NDI, Inc, Canada) motion analysis software. Mean ROM relative to intact conditions (100%) was measured. ResultsROM (% Intact): FE/LB/AR. Integrated Zero Profile ACDF (n = 6): 69 / 48 / 76. Integrated Half-Plate ACDF (n = 6): 42/ 25 / 67. Integrated Full-Plate ACDF (n = 6): 32 / 29 / 40. Traditional ACDF (n = 18): 37 / 39 / 61. Traditional ACDF + LMS (n = 18): 12 / 11 / 22. Integrated Zero Profile ACDF + LMS (n = 6): 14 / 13 / 31. Integrated Half-Plate ACDF + LMS (n = 6): 18 / 10 / 31. Integrated Full-Plate ACDF + LMS (n = 6): 12 / 10 / 16. ConclusionThe full-plate and half-plate constructs both appeared advantageous in comparison to the traditional ACDF construct (MaxAn – Zimmer-Biomet Spine) when used without posterior supplementation, demonstrating an inherent benefit of the MPF technology. Motion reductions with zero-profile fixation were not as robust; however, it can be argued that clinically significant stability was still achieved. Lastly, supplemental fixation with LMS appeared to be a clear leveling factor across all constructs, facilitating significant motion reduction in all directions.