Objective: The purpose of this study was to investigate the effect of posture and loading history on the compressive strength and site of failure in the spine.
Design: An in vitro experiment was performed using a porcine cervical model that provided a homogeneous population of young healthy spines.
Background: The distribution of stresses amongst the many load bearing tissues of the spine is altered throughout the day by posture and the history of loading, but it is not clear how this modulates tissue damage or the risk of injury.
Methods: 48 porcine cervical spines were harvested and dissected into motion segments containing two vertebrae and the intervening disc (C3/4 and C5/6). Compressive loads and rotational torques (flexion/extension) were applied so that the effects of four loading histories (hydrated, neutral dehydration, flexed dehydration, superhydrated) and two failure postures (neutral, flexed) could be examined. Levels of dehydration were based on those reported over the course of a day. Dissection techniques and X-rays were used to document tissue damage. RESULTS. Specimens had a lower yield point (43--63%) and ultimate compressive strength (23--47%) when in a flexed posture than when in a neutral posture. When injured in a neutral posture, superhydrated specimens had a lower strength (22--29%) than dehydrated specimens. Loading history also modulated the site of failure.
Conclusions: The spine may be more prone to injury early in the morning when the discs are at their greatest level of hydration and/or when they are in a fully flexed posture.