Influence of DISH, Ankylosis, Spondylosis and Osteophytes on Serious-to-Fatal Spinal Fractures and Cord Injuries in Rear Impacts
To be published on April 2, 2019 by SAE International in United States
Objective: 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 study addresses the influence of spinal disorders on fracture-dislocations and spinal cord injury in rear impacts with relatively upright seats. Methods: Twenty-two field accidents have been investigated in-depth where serious-to-fatal injuries of the thoracic and cervical spine have occurred in rear impacts with the seat remaining upright or slightly reclined and having a high and forward head restraint. The occupants are lap-shoulder belted, some with belt pretensioning and cinching latch plates. The occupants are mid-age and older and have pre-existing disorders of the spine, including DISH, ankylosis, spondylosis and/or osteophytes that ossify the spine. The crashes have been summarized and the mechanism for injury analyzed in this study. Results: Spinal disorders often develop in people as they age. DISH is a severe condition with the ossification of adjacent segments of the spine where the person may be unaware that they have it until a radiology examination is performed after the crash. A number of studies have addressed the prevalence of DISH. One study found 27.3% of men and 12.8% of women older than 50 years old had DISH. The occurrence of other disorders is greater. The 22 cases were investigated involving fracture-dislocation and spinal cord injury at areas of the spine where DISH, ankylosis, spondylosis and/or osteophytes ossify the intervertebral soft tissues causing stiff and brittle joints that are vulnerable to fracture-dislocation by straightening of the spine in a rear impact. Sled testing at 40 km/h (25 mph) with the 50th Hybrid III shows that chest acceleration averaged 13.5 ± 2.4 g (n=7) and head acceleration 26.0 ± 12.0 g (n=8). Sled testing at 16 km/h (10 mph) with the BioRID IIg shows that T1 chest x-acceleration averaged 12.6 ± 2.4 g (n=12) and head x-acceleration 10.1 ± 0.2 g (n=12). These levels of acceleration are sufficient to straighten and fracture the calcified spine of the older occupants without ramping or moving off the support from the seatback and head restraint. Significantly higher accelerations are experienced in stiffer seats, such as ABTS. Conclusion: A new injury mechanism is described in older occupants with spinal disorders, such as The occupant remains supported by the relatively upright seatback and high and forward head restraint. The acceleration from the seat that brings the occupant up to the delta V is sufficient to fracture-dislocate the calcified spine that straightens in the crash. Stiffer seats increase the risk for serious injury.