ACDF: Anterior Cervical Discectomy and Fusion Assignment

Assignment Task

Introduction

Cervical spine injuries resulting in radicular pain is currently treated using anterior cervical discectomy and fusion (ACDF) or arthroplasty; however, they both involve permanent implantation of the graft or the artificial disc. Posterior cervical foraminotomy (PCF) partially removes the facet to widen the foramen for the damaged or affected nerve root. While the former two procedures have been studied clinically and biomechanically, PCF has received less attention. The objective of this study is to compare the external and internal biomechanics of PCF with ACDF with the underlying working hypothesis that the minimally invasive alternative may be superior for certain patients or occupations.

Methods

A previously validated three-dimensional finite element model of the human cervical spinal column (C2-T1) was used to simulate minimally invasive left-sided PCF: 50, 75, and 100?cet resections (termed graded PCFs); and ACDF with anterior plating and screws. All surgical simulations were performed at the C5-C6 level, commonly injured level in civilian and military populations. They were performed by the neurosurgeon-engineer authors. The finite element model included all subaxial and upper thoracic vertebrae, intervertebral discs, facet joints, and all anterior and posterior spinal ligaments. Each vertebral body consisted of cortical shell, cancellous bone, and superior and inferior endplates. Intervertebral discs were modeled as composite of nucleus pulposus, ground substance and annulus fibers. They had anteroposterior asymmetry with posteriorly displaced nucleus, paralleling the human anatomy. Material properties were obtained from literature. Physiological pure moment of 2 Nm loading under flexion, extension, and lateral bending, and in vivo follower force of 75 N was applied to the intact spine. A hybrid loading protocol that matched the range of motion (ROM) of the intact spine was used for the ACDF and PCF models. The range of motion (ROM), disc pressure, and facet forces at the index at the index (C5-C6) and superior and inferior adjacent levels (C4-C5 and C6-C7) for all cases were obtained and were normalized with respect to the intact spine, under flexion, extension, and lateral bending.

Results

ROM: ACDF decreased ROM at the index and increased motion at the adjacent levels, while all graded PCF responses had an opposite trend: increased motion at the index and decreased motion at adjacent levels. The magnitude of changes depended on the level of resection, spinal level, and loading mode. Motion was greater under left lateral bending. Disc pressure: The pattern of changes in pressures depended on the loading mode and type of surgery although lesser grade resection had smaller changes than the ACDF and 75% and 100% resections. Higher pressures occurred in extension and left lateral bending. Facet Load: Higher facet loads occurred at the superior level in both PCFs and ACDF, while the magnitudes varied with grading. For the intact and four surgical spines, individual results (three modes of loading, three metrics, index and two adjacent levels) will be given in the full-length paper.

Discussion

Using a validated finite model, the present study compared ACDF and graded PCFs with respect to the intact spine using ROM, a clinically relevant metric, and intrinsic parameters of facet loading, representing posterior column load-sharing, and disc pressures, representing anterior column load-sharing within the spinal column. Such metrics can only be studied using finite element models, as cadaver tests are not feasible. Increasing degree of facet resection resulted in greater lateral instability. PCF increased disc pressures and facet loads that mayplay in future disc and facet degeneration or injury. Because the degree of facet resection has important biomechanical consequences, it should be considered with PCF, especially on military populations such as pilots who use head supported mass and have greater demands like the check six posture. This study demonstrates the importance of anatomy-sparing minimally invasive surgical techniques and enabling surgical technology such as intraoperative navigation. A more warfighter- and operation-specific precise surgical planning and execution may be necessary to maximize the chances of return-to-duty after an increasingly prevalent musculoskeletal injury.

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