118. Zero profile biomechanics: comparison with anterior fusion and cervical disc arthroplasty

Zero profile interbody devices were developed to accommodate decompression and stabilization adjacent to a fusion that may be necessary for certain patients with adjacent segment degeneration who have undergone a prior anterior cervical fusion. Zero profile devices have been used as interbody implants in lieu of traditional anterior interbody fusion and plating (ACDFP) and cervical disc arthroplasty (CDA) for treating cervical radiculopathy. Our hypothesis is that zero profile implants will provide more favorable biomechanics than traditional ACDFP and may provide less development of adjacent segment stenosis than traditional ACDFP. Finite element modeling. Finite element models of the intact cervical spine C2-T1 and with C5-6 ACDFP or C5-6 Mobi-C (CDA) were simulated. A previously validated three-dimensional osteoligamentous FEM of the cervical column (C2-T1) was used. The model simulated the vertebral body, posterior elements, intervertebral disc, and ligaments. Each vertebral body consisted of a thin cortical shell, a softer cancellous bone, endplate, and posterior elements. The cortical bone was modeled as a linear isotropic material of a 0.5 mm thick shell surrounding the cancellous bone, and a 0.2 mm thick endplate was placed on the superior and inferior surfaces of the disc. The modeled disc comprised the nucleus pulposus, annulus ground substance, and annulus fibrosus. All major ligaments were included. Material properties from the literature were used. The intact spine was modified at the C5-C6 vertebral level to simulate the biomechanics of an ACDF, Mobi-C, or a zero-profile device. The anterior longitudinal ligament was removed at the surgical level in all cases. In the case of Mobi-C and zero-profile devices, a cavity was created at the surgical level for implant placement. For ACDFP, disc properties were altered at the surgical level to simulate trabecular bone and solid arthrodesis. A titanium plate and 4 screws were placed anteriorly at C5-C6 All spines were fixed at the first thoracic vertebra in all degrees of freedom, and the load was applied at the superior endplate of the C2. A follower load of 75 N and a moment of 2 Nm under flexion, extension, and lateral bending were applied to the three groups: intact, ACDFP, and CDA. The range of motion (ROM) was measured at each level and across C3-C7 levels. A hybrid loading protocol using variable moment loading was then applied. The bending moment was varied until the ACDFP and CDA responded with the same total C3-C7 ROM. ROM and facet forces were obtained at the index and adjacent segments under each loading mode, and intradiscal pressures were determined at adjacent levels. Zero profile implants preserved 23% of ROM whereas ACDFP preserves 2% in flexion, preserved almost 50% of ROM in extension and lateral bending compared to ACDFP, allowed adjacent segments to be more flexible than adjacent to ACDFP, and provided less adjacent facet forces as compared to Mobi-C. These matched-pair multi-modal spine studies indicate that the zero-profile device when used as an interbody implant, provides improved ROM and less adjacent level facet forces which would favor less development of adjacent segment stenosis than traditional ACDF and is less mobile than Mobi-C. This abstract does not discuss or include any applicable devices or drugs.