A prospective, concurrently controlled, multi-center pivotal study to evaluate the safety and effectiveness of the M6-C™ Artificial Cervical Disc compared to anterior cervical discectomy and fusion (ACDF) was performed in the United States under IDE #G050254. To fulfill the concurrent control design of the study, there were clinical sites that evaluated the safety and effectiveness of the M6-C™ Artificial Cervical Disc and there were other clinical sites that evaluated ACDF. Subjects in the study were treated between May 2014 and June 2016 and remain in follow-up. The M6-C™ Artificial Cervical Disc received FDA approval to market on February 6, 2019. As a condition of approval, the FDA requested that the sponsor perform a post market approval study (PAS). The subjects enrolled and remaining in the M6-C IDE pivotal study are the prospective patient population for the PAS.