An in Vivo, Three-Dimensional (3D), Functional Centers of Rotation of the Healthy Cervical Spine

<black square> OBJECTIVE: This study examined cervical center of rotation (COR) positions in 7 postures using validated cone beam computed tomography (CBCT) combined with 3D -3D registration in healthy volunteers. <black square> METHODS: CBCT scans were performed on 20 healthy volunteers in 7 functional positions, constructing a threedimensional (3D) model. Images were registered to the neutral position using 3D -3D registration, allowing analysis of kinematic differences and rotational axes. COR measurements were obtained for each segment (C2/3 to C6/ 7) in each posture. <black square> RESULTS: The CORs of C2/3 to C6/7 were predominantly posterior (-5.3 +/- 3.8 -0.6 +/- 1.2 mm) and superior (16.5 +/- 6.0 23.6 +/- 3.2 mm) to the intervertebral disc's geometric center (GC) in flexion and extension. However, the C4/5 segment's COR was anterior to the GC (2.0 +/- 9.8 mm) during flexion and close to it in the right-left direction. During leftright twisting, the CORs of C2/3 -C6/7 were posterior (-21.8 +/- 10.5 -0.9 +/- 0.8 mm) and superior (3.1 +/- 7.5 23.2 +/- 3.6 mm) to the GCs in anterior-posterior and superior-inferior directions, without consistent right-left directionality. During left-right bending, each segment's COR was predominantly posterior (-25.2 +/- 13.1 -6.5 +/- 9.9 mm) and superior (0.3 +/- 12.5 12.1 +/- 5.1 mm) to the GC in anterior-posterior and superior-inferior directions, except for the C2/3 segment, located inferiorly (-5.9 +/- 4.1 mm) in left bending. The rightleft COR position varied across segments. <black square> CONCLUSIONS: Our findings reveal segment-specific and posture -dependent COR variations. Notably, the CORs of C3/4, C4/5, and C5/6 consistently align near the intervertebral disc's GC at different postures, supporting their suitability for total disc replacement surgery within the C3/ 4 to C5/6 segments.