Comparison of flexion relaxation phenomenon after single-level and multi-level Anterior Cervical Discectomy and Fusion

Background: This study aims to assess the influence of the number of fused segments on cervical paravertebral muscles by analyzing the changes of the cervical flexion relaxation phenomenon (FRP) after single-level and multi-level anterior cervical discectomy and fusion (ACDF). Methods: A total of 28 patients underwent ACDF were included, including 9 cases of 1-level, 12 of 2-level and 7 of 3 or 4-level. The flexion relaxation experiment of cervical erector spinae (CES) muscles was carried out at preoperative 3 days, postoperative 6 months and 12 months by surface electromyography (SEMG). Patients were examined by the neck Visual Analogue Scale (VAS), the cervical Japanese Orthopaedic Association (JOA) score and Neck Disability Index (NDI). C2-7 range of motion (ROM) at flexion and extension was evaluated by X-ray. Results: The FRP of all patients with 1-level ACDF was normalized (100%). Only 2/12 patients with 2-level ACDF had normalized FRP at the last follow-up, with increased flexion relaxation ratio (FRR) and decreased normalized SEMG activity than those before surgery. However, all patients with 3 or 4-level had decreased postoperative FRR and increased normalized SEMG activity in each phase. A significant correlation was found between the changes of C2-7 ROM and FRR in three groups. Conclusions: 1-level ACDF could normalize the FRP of cervical paravertebral muscles, which was not ideally recovered in patients with 2-level ACDF. However, 3 or 4-level ACDF could aggravate the SEMG activity and worsen the function of cervical paravertebral muscles. Our research strongly supported the passive structures hypothesis.