Does Placement of the Axis of Rotation of the Cervical Spine Affect Mechanics during Flexion and Extension?

摘要

Due to current limitations in in-vitro biome-chanical testing, there is no consensus for the location of the inherent center of rotation (CoR) of the spine. The effects of placement of fixed CoRs of the cervical spine on segmental mechanics were investigated. A custom-designed spine robot able to control the motion of the human joint about any desired anatomical location was used. Six fresh human cadaveric cervical motion segment units (MSUs) were mounted in the spine robot and rotated about three points located along the plane of the disc in the anterior-posterior (A-P) direction: at the mid-point of the disc (CI), half-way between the mid-point and anterior aspect of the disc (A 1), and half-way between the mid-point and posterior aspect of the disc(Pl). Three similarly spaced lower points (A3, C3, P3) were located 5mm below the upper points in the end plate of the subjacent body. The MSUs were rotated in flexion and extension to a 2.5 Nm bending moment end limit or stopped if the axial or shear loads exceeded 225N. A one-way RM ANOVA with SNK (P=0.05) was used to compare the MSU mechanics: tension/compression, A-P shear forces, and sagittal plane rotation. In flexion, significantly more motion occurred at the midpoints than the anterior and posterior points, and in extension significant decreases in range of motion occurred moving from posterior to anterior direction along both planes. Along the disc plane in flexion, shear forces were significantly greater at Al and CI as compared to PI. In extension, axial compressive loads significantly increased as rotations moved from posterior to anterior along both planes. These findings displayed that the cervical MSU is highly sensitive to the location of the axis of rotation in terms of mechanics and range of motion.