Cervical spine postural changes related to the degenerative process will lead to the spondylotic spine. In the normal lordotic posture of the cervical spine, the axial force is applied along the instantaneous axis of rotation (IAR) with no deviation from this neutral point of rotation (a). However, with early disc height loss, the lordotic posture is reduced and the axial force is now offset from the instantaneous axis of rotation (d) causing a moment arm at this point of rotation. If an axial force is placed at a particular distance from the center of rotation or the IAR, a bending moment is applied about this point (b), and it will take less force to induce injury to the apical spinal segment. The larger a moment arm, the greater the bending moment, which will cause further progression of the kyphosis (c). (Taken from Benzel EC: Biomechanics of Spine Stabilization. Rolling Meadows, American Association of Neurological Surgeons Publications, 2001 .)
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