Injury mechanisms of the cervical intervertebral disc during simulated whiplash

Study Design. A kinematic analysis of cervical intervertebral disc deformation during simulated whiplash using the whole cervical spine with muscle force replication model was performed. Objectives. To quantify anulus fibrosus fiber strain, disc shear strain, and axial disc deformation in the cervical spine during simulated whiplash. Summary of Background Data. Clinical studies have documented acute intervertebral disc injury and accelerated disc degeneration in whiplash patients, although there has been no biomechanical investigation of the disc injury mechanisms. Methods. A bench-top sled was used to simulate whiplash at 3.5, 5, 6.5, and 8 g using six specimens. The 30 and 150 fiber strains, disc shear strains, and axial disc deformations during whiplash were compared with the sagittal physiologic levels. Results. Increases over sagittal physiologic levels (P < 0.05) were first observed during the 3.5 g simulation. Peak fiber strain was greatest in the posterior 150 fibers (running posterosuperiorly), reaching a maximum of 51.4% at C5-C6 during the 8 g simulation. Peak disc shear strain was also greatest at the posterior region of C5-C6, reaching a maximum of 1.0 radian due to posterior translation during the 8 g simulation. Axial deformation at the anterior disc region exceeded physiologic levels at 3.5 g and above, while axial deformation at the posterior region exceeded physiologic limits only at C5-C6 at 6.5 g and 8 g. Conclusions. The cervical intervertebral discs may be at risk for injury during whiplash because of excessive 150° fiber strain, disc shear strain, and anterior axial deformation.