Influence of DISH, Ankylosis, Spondylosis and Osteophytes on Serious-to-Fatal Spinal Fractures and Cord Injury in Rear Impacts

Seats have become stronger over the past two decades and remain more upright in rear impacts. While head restraints are higher and more forward providing support for the head and neck, serious-to-fatal injuries to the thoracic and cervical spine have been seen in occupants with spinal disorders, such as DISH (diffuse idiopathic skeletal hyperostosis), ankylosis, spondylosis and/or osteophytes that ossify the joints in the spine. This case study addresses the influence of spinal disorders on fracture-dislocation and spinal cord injury in rear impacts with relatively upright seats. Nineteen field accidents were investigated where serious-to-fatal injuries of the thoracic and cervical spine occurred with the seat remaining upright or slightly reclined. The occupants were lap-shoulder belted, some with belt pretensioning and cinching latch plate. The occupants were older and had pre-existing disorders of the spine, including DISH, ankylosis, spondylosis and/or osteophytes that ossify the spinal joints. The crashes were summarized and the mechanism for injury was analyzed. The 19 cases involved fracture-dislocation and spinal cord injury at areas of the spine where DISH, ankylosis, spondylosis and/or osteophytes ossified the intervertebral soft tissues causing stiff and brittle joints that were vulnerable to fracture-dislocation by straightening of the spine. Published sled tests at 40 km/h (25 mph) with the 50^th Hybrid III showed that peak chest acceleration was 13.5 ± 2.4 g (n=7) and head acceleration was 26.0 ± 12.0 g (n=8). Sled testing at 16 km/h (10 mph) with the BioRID IIg involved T1 x-accelerations of 12.6 ± 2.4 g (n=12) and head x-accelerations of 10.1 ± 0.2 g (n=12). These levels of acceleration are sufficient to fracture the calcified spine of the older occupants without ramping or moving off the support from the seatback and head restraint. A new injury mechanism for spinal fracture-dislocation is described in older occupants with spinal disorders. The occupant remains supported by the relatively upright seatback and high and forward head restraint. The accelerations that bring the occupant up to the delta V are sufficient to fracture-dislocate the calcified spine that tries to straighten in the crash.