Effect of Sagittal Malalignment on Cervical Spine Biomechanics

Background: Forward head posture (FHP) relative to the shoulders, assessed radiographically using the horizontal offset distance between the C2 and C7 vertebral bodies (C2-C7 SVA), is a measure of global cervical imbalance. This may result from kyphotic alignment of cervical segments, muscle imbalance, as well as malalignment of thoracolumbar spine. Chronic FHP frequently leads to neck pain, headaches, and poor quality of life. Aims: The aims of this thesis were to investigate t five biomechanical aspects of cervical sagittal malalignment: (1) Measuring postural consequence of FHP (2) Measuring the effects of FHP on neuroforaminal and (3) canal spaces in the cervical spine. 4) Estimating muscle length changes associated with increasing FHP severity. (5) To assess whether the presence of cervical sagittal imbalance is an independent risk factor for increasing the mechanical burden on discs adjacent to cervical multilevel fusions. Method: Thirteen fresh-frozen cadaveric cervical spine specimens (Occiput-T1, age: 54±15 yrs old) were tested to assess the postural changes that accommodate different sagittal malalignment postures. The 3D postural data, which was collected by Optotrak motion measurement system, was combined with the specimen-specific anatomy of the spine obtained from Computed Tomography (CT) (vertebral geometry, foraminal area, canal space and muscle attachment points) to measure anatomical changes associated with FHP. The test was repeated after instrumentation across C4-C6 to simulate in situ 2-level fusion. Results and Conclusions: If cervical sagittal malalignment involves only increased SVA and does not include upper thoracic kyphosis, then cervical neuroforaminal and spinal canal space stenosis would not be anticipated as the resultant consequences. However, segmental loading increases in intervertebral discs of the lower cervical spine and severe muscle shortening occurs in suboccipital extensors. If cervical sagittal malalignment involves only kyphosis of upper thoracic spine and does not involve increased SVA, then cervical neuroforaminal and spinal canal stenosis may be anticipated. If cervical sagittal malalignment involves both, kyphotic upper thoracic and increased SVA, then depending on the severity of the kyphosis and FHP, cervical neuroforaminal and spinal canal stenosis may or may not be anticipated.