Strength of the Cervical Spine in Compression and Bending

In Brief Study Design. Cadaveric motion segment experiment. Objectives. To compare the strength in bending and compression of the human cervical spine and to investigate which structures resist bending the most. Summary of Background Data. The strength of the cervical spine when subjected to physiologically reasonable complex loading is unknown, as is the role of individual structures in resisting bending. Methods. A total of 22 human cervical motion segments, 64 to 89 years of age, were subjected to complex loading in bending and compression. Resistance to flexion and to extension was measured in consecutive tests. Sagittal-plane movements were recorded at 50 Hz using an optical two-dimensional “MacReflex” system. Experiments were repeated 1) after surgical removal of the spinous process, 2) after removal of both apophyseal joints, and 3) after the disc-vertebral body unit had been compressed to failure. Results were analyzed using t tests, analysis of variance, and linear regression. Results were compared with published data for the lumbar spine. Results. The elastic limit in flexion was reached at 8.5° (SD, 1.7°) with a bending moment of 6.7 Nm (SD, 1.7 Nm). In extension, values were 9.5° (SD, 1.6°) and 8.4 Nm (3.5 Nm), respectively. Spinous processes (and associated ligaments) provided 48% (SD, 17%) of the resistance to flexion. Apophyseal joints provided 47% (SD, 16%) of the resistance to extension. In compression, the disc-vertebral body units reached the elastic limit at 1.23 kN (SD, 0.46 Nm) and their ultimate compressive strength was 2.40 kN (SD, 0.96 kN). Strength was greater in male specimens, depended on spinal level and tended to decrease with age. Conclusions. The cervical spine has approximately 20% of the bending strength of the lumbar spine but 45% of its compressive strength. This suggests that the neck is relatively vulnerable in bending. Cadaveric cervical motion segments were subjected to physiologically reasonable complex loading in compression and bending, right up to the elastic limit. They were found to have approximately 20% of the bending strength of lumbar motion segments, but 45% of their compressive strength. Most resistance to flexion and extension came from the spinous processes and apophyseal joints, respectively.