Although the role of muscles was once thought to be minimal in axial compression, it is now thought that their stabilizing effect may influence the buckling behavior of the inherently unstable articulated column that is the cervical spine. This has been previously investigated experimentally using a very simplified model of neck musculature. The objective of this study was to create and establish the merit of a model of head-first impact with an advanced muscle force replication (AMFR) system using a Hybrid III anthropometric test device head, cadaveric cervical spine, and surrogate spinal cord.
An osteoligamentous cervical spine (occiput-T2, age 60 yrs) was set in dental stone at T1/2 and attached to a Hybrid III head at the occiput using custom adapter plates. The T1/2 potting cup was fixed to a six axis lower neck load cell and mounted to the carriage of a drop tower. The AMFR system modeled four bilateral muscles and three follower loads using fishing line tied to the vertebrae or to the Hybrid III head mounting plate. A radiopaque biofidelic surrogate spinal cord was inserted into the spinal canal. The specimen was dropped from a height of 60 cm onto a padded impact platform overtop a uni-axial load cell. The impact was captured at 1000 frames per second with two high speed video cameras and a high speed x-ray system. Injuries were diagnosed by a fellowship-trained spine surgeon (JS) using x-rays, CT scans, and through dissection.
Lordosis was removed from the specimen and it was aligned with zero eccentricity (anteroposterior distance between the occipital condyles and the T1 vertebral body) using the AMFR system. The peak impact platform and lower neck axial loads were 7930 N and 3830 N, respectively. Injuries included a burst fracture of C1 and a disc and bilateral facet capsule rupture at C5/6. The C1 fracture produced a concurrent peak spinal cord compression of 20%.
This pilot study showed that compression injuries in head-first impacts can be reproduced with a cadaver spine, Hybrid III head, and simulated muscle forces. This model advances in vitro testing as it permits measurement of spinal cord compression and simulation of neck muscle forces with tied attachments on the vertebrae allowing this to be achieved without damaging the structural integrity of the spinal column.