Adjacent segment motion following multi-level ACDF: a kinematic and clinical study in patients with zero-profile anchored spacer or plate

To investigate the adjacent segment kinematics, including the instantaneous axis of rotation (IAR) and range of motion (ROM), after anterior cervical discectomy and fusion (ACDF), and to compare between ACDF with zero-profile anchored spacer (ACDF-Z) and ACDF with plate (ACDF-P).Eighty-seven patients (ACDF-Z = 63; ACDF-P = 24) were included. Flexion, extension and neutral cervical radiographs were obtained before operation and at 1-year follow-up. C2-C7 ROM, adjacent segment ROMs, and IARs were measured. Clinical evaluation was based on the Visual Analogue Scale, Neck Disability Index, and Japanese Orthopaedic Association score.After ACDF-Z, location of the superior IAR-AP reduced 1.60 mm, which represents 8% of the vertebral body (P 0.001), and location of the inferior IAR-SI reduced 2.19 mm, 17% of the vertebral body (P = 0.02). After ACDF-P, location of the superior IAR-AP increased 0.8 mm, which means 6% of the vertebral body (P = 0.008), location of the inferior IAR-AP increased 3.34 mm, 22% of the vertebral body (P = 0.03), and location of the inferior IAR-SI reduced 3.14 mm, 25% of the vertebral body (P = 0.002). C2-C7 ROM significantly decreased after both ACDF-Z and ACDF-P (P 0.001). Neither ACDF-Z nor ACDF-P significantly affected the adjacent segment ROMs (P 0.05).Both ACDF-Z and ACDF-P significantly impacted cervical kinematics, although both procedures obtained satisfactory clinical results in the treatment of cervical spondylosis. After both ACDF-Z and ACDF-P, C2-C7 ROM decreased significantly, while adjacent segment ROMs were preserved. ACDF-Z and ACDF-P impact the location of adjacent segment IAR-SI in similar way, while impact the location of adjacent segment IAR-AP in diverse ways. These slides can be retrieved under Electronic Supplementary Material.