Clinical effect of occipitocervical and subaxial cervical fusion constructs on range of motion: comprehensive guide based on biomechanical cadaveric testing on 1009 motion segments.

Understanding the relative contribution of each cervical motion segment is vital for assessing the effect of fusion constructs on range of motion (ROM). Many spine surgeons are familiar with the work of Panjabi and White, from which these values have historically been cited. However, their data were obtained from a limited number of subjects, and methodological shortcomings have since been identified. In this study, the authors sought to improve understanding of segmental ROM using data from standardized biomechanical tests involving a large number of intact cervical spine specimens. Flexibility data from 1009 cervical spine motion segments from 286 cadaveric spine specimens spanning the occiput (Occ)-T1 were analyzed. Specimens were subjected to standardized pure moment flexibility tests and loaded to 1.5 Nm in 3 anatomical axes: flexion-extension, axial rotation, and lateral bending. Intervertebral ROM was measured optoelectronically. Hypothetical ROM values of various fusion constructs were calculated, assuming complete loss of segmental ROM across treated segments and lack of compensatory changes in ROM for unfused segments. The overall mean ROM values for the entire cervical spine (Occ-T1) in flexion-extension, axial rotation, and lateral bending were 109.8°, 79.3°, and 37.7°, respectively. The greatest segmental contribution to flexion-extension ROM was the Occ-C1 joint (24% of overall ROM) at a mean (SD) of 26.4° (6.4°), which differed significantly from the values of all other levels (p < 0.001). In axial rotation, C1-2 contributed 53% of overall ROM (41.6° [14.7°]) (all p < 0.001). C3-4 accounted for 16% of lateral bending ROM (5.9° [1.9°]). Cervical ROM after hypothetical Occ-C2 fusion was 59% of the ROM of the unfused spine in flexion-extension, 36% in axial rotation, and 76% in lateral bending. Fusion from C2 to T1 maintained 41% of ROM in flexion-extension, 64% in axial rotation, and 24% in lateral bending. Increasing the length of a subaxial fusion construct leads to a steady decrease in the remaining ROM in all 3 planes of movement. This study demonstrates the segmental ROM values of the intact cervical spine and evaluates the calculated effects of cervical instrumentation on regional ROM based on data from the largest reported number of similarly tested cervical motion segments. These findings can help surgeons to plan surgery and counsel patients regarding the clinical effect of cervical fusion on ROM.