Can Cervical Arthroplasty Impact Alignment? A Comparison of the Synergy Disc with Cervical Fusion

Background context: Synergy disc is a new cervical disc prosthesis that incorporates alignment restoration while providing full intervertebral disc kinematics.Purpose: This follow-up study with 40 Synergy Disc patients with 24-month follow-up compared cervical alignment changes with a retrospective cohort of 30 single level anterior cervical discectomy and fusion (ACDF) patients.Study design/setting: The pilot trial was a multi-center, prospective, consecutive patient enrollment study using the Synergy Disc for the treatment of single and two-level degenerative disc disease of the cervical spine.Patient sample: The procedure was performed on 43 patients (45 implants) with follow-up on 40 patients (42 implants). For the historical cohort ACDF arm, 30 patients with similar follow-up with single level anterior discectomy, fusion and plating were used for segmental lordosis measurements.Outcome measures: For the Synergy Disc group, the kinematic parameters included: range of motion (ROM), shell angle (SA), disc height (DH), sagittal plane translation and center of rotation (COR) in the X and Y direction. Standard assessments of clinical outcomes were also measured (Neck Disability Index, Visual Analog Scale). For the fusion arm, only functional spinal unit (FSU) angle was recorded using a single pre-operative and post-operative standing lateral cervical radiograph.Methods: In the Synergy Disc group, static and dynamic radiological assessments were performed in 43 consecutive patients prior to the placement of the Synergy Disc. Forty patients were studied for the course of the study protocol (3 patients lost to follow-up). For the Synergy Disc group, all kinematic parameters were examined at a minimum of 24 months follow-up. Neck Disability Index and Visual Analog Scale for arm and neck pain were collected and analyzed. For the fusion group, standing lateral radiographs were reviewed.Results: At a mean of 28 months with all patients having a minimum of 24-month follow-up (40 patients, 42 implants), the average SA of the Synergy Disc was maintained at 6 ± 2.7˚ of lordosis. There was significant improvement in all clinical outcome measures. In the fusion group, with a similar follow-up period, there was a 4˚ increase in lordosis of the FSU.Conclusion: The Synergy Disc had an endplate angle of 6 ± 2.7˚ at 2 years following surgery. This was comparable to the lordotic correction provided by an anterior cervical discectomy with interbody fusion and plating.