Tensile properties of the human muscular and ligamentous cervical spine.

Tensile neck injuries are amongst the most serious cervical injuries. However, because neither reliable human cervical tensile tolerance data nor tensile structural data are currently available, the quantification of tensile injury risk is limited. The purpose of this study is to provide previously unavailable kinetic and tolerance data for the ligamentous cervical spine and determine the effect of neck muscle on tensile load response and tolerance. Using six male human cadaver specimens, isolated ligamentous cervical spine tests (occiput - T1) were conducted to quantify the significant differences in kinetics due to head end condition and anteroposterior eccentricity of the tensile load. The spine was then separated into motion segments for tension failure testing. The upper cervical spine tolerance of 2400 +/- 270 N (occiput-C2) was found to be significantly greater (p < 0.01) than the lower cervical spine tolerance of 1780 +/- 230 N (C4-C5 and C6-C7 segments). Data from these experiments were used to develop and validate a computational model of the ligamentous spine. The model predicted the end condition and eccentricity responses for the tensile force-displacement relationship. Cervical muscular geometry data derived from cadaver dissection and MRI imaging were used to incorporate a muscular response into the model. The cervical musculature under maximal stimulation increased the tolerance of the cervical spine from 1800 N to 4160 N. In addition, the cervical musculature resulted in a shift in the site of injury from the lower cervical spine to the upper cervical spine and offers an explanation for the mechanism of upper cervical spine tension injuries observed clinically. The results from this study predict a range in tensile tolerance from 1.8 - 4.2 kN based on the varying role of the cervical musculature.