General Session: Biomechanics
Presented by: J. Goodsitt - View Audio/Video Presentation (Members Only)
J. Goodsitt(1), S. Khayatzadeh(1), L. Voronov(1,2), R. Havey(1,2), A. Patwardhan(1,2)
(1) Edward Hines Jr. VA Hospital, Research, Hines, IL, United States
(2) Loyola University Chicago, Orthopaedic Surgery and Rehabilitation, Maywood, IL, United States
Introduction: The ability to rotate the head 45°-60° to each side is essential for safely performing activities such as community walking, running, and driving. Previous kinematic data on the rotational range of motion of middle and lower cervical segments were derived using experiments on short cervical spine segments or single functional spinal units. We investigated the motion experienced by the cervical spine (occiput-T1) in situ during a head-neck axial rotation task. We hypothesized that subaxial segments will experience axial rotation and lateral bending motions of variable magnitudes depending on their sagittal alignment as part of the neck posture.
Methods: We simulated the side-to-side head-neck rotation task on seven human cadaveric cervical spines (Occiput-T1; 36.7±10.0 years) (Fig. 1A). Each specimen was set up in its neutral posture in an apparatus, which allowed adjustment of the specimen's T1-tilt angle and anterior head offset relative to the T1 vertebra (C0-T1 SVA). The occiput was constrained such that its sagittal tilt remained fixed and consistent with horizontal gaze. The occiput could translate vertically and rotate about a vertical axis passing through the midpoint between the left and right external auditory meatus. The occiput was axially rotated from left-to-right through an arc of ±45° to ±60°, but not exceeding a torque of ±1.0 Nm. The three-dimensional motions of the occiput and each mobile vertebra were measured using optoelectronic motion sensors. Segmental motion was expressed as axial rotation (AR), lateral bending (LB), and flexion-extension (FE), in local coordinate systems located on the superior endplate of the inferior vertebra of each segment.
Results: Nearly 63%±6.1% of the axial rotation of the head occurred in the atlanto-axial (C1-C2) joint, with the remainder coming from the subaxial segments. As the occiput was axially rotated to the right, subaxial segments underwent right LB and right AR about the local axes of each segment (Fig. 1B). The C3-C4 and C4-C5 segments experienced larger magnitudes of AR and LB motions as compared to the other segments (p< 0.05) (Fig. 1C).
Conclusions: During the simulated head-neck rotation task, the largest AR motion occurred in the atlanto-axial (C1-C2) joint, followed by the mid-cervical segments (C3-C4-C5) which experienced more AR and LB motion as compared to the other subaxial segments. Therefore, the functional limitations that might result from cervical fusions will depend on the number and locations of fused levels. The proportion of AR and LB experienced in situ depends on neck posture, which is defined by the T1 tilt and C0-T1 SVA. Further studies of the effect of neck posture on in situ segmental motions will elucidate the nature of stresses in cervical discs of the mid- and lower cervical spine in the context of cervical sagittal imbalance.