Cervical spine injuries due to tensile neck loading are among the most fatal and catastrophic. They include basilar skull fractures, craniocervical dislocations, Hangman's fractures and a variety of lower cervical injuries. The increasing use of passive restraints, which decrease compressive injury incidence, may increase the incidence of tensile injuries. Tensile neck injuries to occupants interacting with airbags and noncontact tensile injuries due to head inertia have been reported (Blacksin, 1993; Huelke, 1994). Unfortunately, very little data currently exists on the tensile responses and tolerance of the cervical spine.
Our current research will . . .
- Determine the tolerance of the cervical spine under the action of a pure tensile load applied through the center of gravity of the head.
- Determine local behavior of the spine within the sagittal plane by using instrumentation to determine all loads and displacements within the sagittal plane.
- Determine local tolerance through segmental testing by retesting uninjured sections of the spine.
- Determine through a computational model the effects of surrounding musculature.
The tests use unembalmed human cadaver heads with intact ligamentous cervical spines to T2 with surrounding musculature removed. A test fixture was developed to apply a pure vertical load (based on the global coordinate system) at the center of gravity (cg) of the head. A high speed CCD camera is used to capture the motion of the individual vertebral bodies within the sagittal plane. Following preconditioning the specimen are loaded quasistatically to failure.
The effect of musculature is being examined using a computational model. Currently research is being conducted in our lab to quantitatively describe the musculature of the cervical spine. Using the tolerance data of the ligamentous spine with skeletal muscle constitutive data available in the literature (Myers et al, 1995) and the muscle geometry from the current project, a structural model has been developed. Muscles are incorporated into the model as forces based on PCSA measurements of the cervical muscles, and lines of action are defined along the areal centroid of each individual muscle. Muscles with broad origins and insertions are subdivided, and individual forces represent the subdivisions in a method similar to that used by Van Der Helm et al (1991).
The effects of muscular tone can be examined by adjusting the stress level used to derive muscle forces from the PCSA data. Under the action of a tensile load the model forces at each vertebral level will be compared with the data from the ligamentous spine experiments. This model will be able to quantify the degree of load sharing between the muscles and the ligamentous spine. This model, if combined with local tolerance data, will be used to predict injuries to the cervical spine under tensile loading including muscular influence. The hypothesis that muscles offer a protective mechanism to the cervical spine, particularly the more caudal cervical spine, has been confirmed.