The Influence of Simulated Low Speed Vehicle Impacts and Posture on Passive Intervertebral Mechanics.

Study Design: An in vitro biomechanics investigation exposing porcine functional spinal units (FSUs) to sudden impact loading although in a flexed, neutral, or extended posture.

Objective: To investigate the combined effect of impact severity and postural deviation on intervertebral joint mechanics.

Summary Of Background Data: To date, no in vitro studies have been conducted to explore lumbar tissue injury potential and altered mechanical properties from exposure to impact forces. Typically, after a motor vehicle collision, the cause of a reported acute onset of low back pain is difficult to identify, with potential soft tissue strain injury sites including the intervertebral disc, facet joint and highly innervated facet joint capsule ligament.

Methods: Seventy-two porcine functional spinal units (36 C34, 36 C56), consisting of 2 adjacent vertebrae, ligaments, and the intervening intervertebral disc were included in the study. Each specimen was randomized to 1 of 3 experimental posture conditions (neutral, flexed, or extended) and assigned to 1 of 3 impact severities representing motor vehicle accident accelerations (4 g, 8 g, and 11 g). Before impact (pre) and after impact (post) flexion-extension and anterior-posterior shear neutral zone testing was completed.

Results: A significant two-way interaction was observed between pre-post and impact severity for flexion-extension neutral zone length and stiffness and anterior-posterior shear neutral zone length and stiffness. This was a result of increasedneutral zone range and decreased neutral zone stiffness pre-post for the highest impact severity (11 g), regardless of posture.

Conclusion: Functional spinal units exposed to the highest severity impact (11 g) had significant neutral zone changes, with increases in joint laxity in flexion-extension and anterior-posterior shear and decreased stiffness, suggesting that soft tissue injury may have occurred. Despite observed main effects of impact severity, no influence of posture was observed.Level of Evidence: N/A.