Thoracic Spine Extension Injuries in Occupants with Pre-Existing Conditions during Rear End Collisions 2019-01-1222

Certain ankylosing spondyloarthropathies such as ankylosing spondylitis (AS) or diffuse idiopathic skeletal hyperostosis (DISH) can significantly alter clinicopathologic spine biomechanics and injury mechanisms in rear end motor vehicle collisions. AS is an inflammatory disease which can lead to structural impairments of the spine secondary to flowing ossification along the spinal column, including ossification across the spinal discs, facet joints, and ligaments, and has been associated with diffuse osteoporosis of the spine. DISH is characterized by excess bone formation along the spinal column, encompassing the annulus and forming the thickest and strongest bridging of the vertebral bodies at the level of the disc space. In both conditions the spine is generally stiffer and more kyphotic than a healthy spine. This paper presents a series of case studies in which a front-seat occupant with ankylosing spondyloarthropathy experienced a moderate- or high-speed rear-end collision and sustained a thoracic spine fracture/dislocation, often with spinal cord injuries. Forward acceleration of the occupant by the seat back in each case resulted in straightening of the kyphotic thoracic spine and consequent extension fractures of the pathologically stiff and brittle thoracic spine. This paper illustrates the increased risk of thoracic fracture for this segment of the population with degenerative spinal conditions such as DISH and AS in rear impacts and discusses this injury risk in the context of arguments for stiffer or more rigid seat backs, which would tend to exacerbate the mechanism of injury in these individuals.