- Imaging of human volunteers showed that cervical spine discs can exhibit large changes in deformation during full-range flexion-extension motion
- The amount of change in deformation between maximum flexion and maximum extension depended on the location in the disc that was studied
- Higher-level discs exhibited more deformation than C6/7 did
Abnormal loading is widely believed to cause cervical spine disc diseases, but surprisingly few studies have investigated the biomechanics of cervical discs during physiologic neck motion. By imaging human volunteers, Massachusetts General Hospital researchers found that the cervical spine experiences a large range of deformation between full flexion and full extension, and that various discs respond to neck motion differently. The study was published in the Journal of Biomechanical Engineering.
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The study led by Thomas D. Cha, MD, MBA, assistant chief of the Orthopaedic Spine Center, and colleagues examined 10 asymptomatic adults (six men and four women, average age 40 years). A spine surgeon confirmed that the subjects did not have a spinal disorder. MRI results ruled out anatomic abnormalities and early disc degeneration.
The researchers used MR images to construct 3D models of the vertebrae from C3 to C7. They then seated each subject within the field of view of two fluoroscopes and asked them to move their neck from full flexion to full extension, and then flex back to the original position.
With the pair of fluoroscopic images, the researchers constructed a virtual dual fluoroscopic system. They introduced the 3D models of the cervical vertebrae into that system and rotated until their contours matched those captured on the fluoroscopic images. Disc deformation was determined as the change of space between the upper and lower endplates of each intervertebral segment. Five points on each disc were analyzed to examine the distribution of deformation.
The results showed a large range of deformation between full flexion and full extension. In general, the anterior point of the discs experienced larger changes in distraction/compression deformation and shear deformation than the center and posterior regions did. Some points on the disc, especially the anterior locations, experienced changes of distraction/compression and shear disc deformation of more than 70%.
The level of the disk also affected the response to neck motion. For example, the anterior points of the C3/4, C4/5, and C5/6 discs exhibited a change in deformation of more than 60% between full flexion and full extension, while the anterior point of C6/7 showed much less deformation.
For now, these findings will be most useful to other researchers. The authors believe, though, that their results could contribute to improvements in fusion and disc replacement surgeries that aim to prevent adjacent-segment degeneration.
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