Consistent hydration of intervertebral discs during in vitro testing

Spinal specimens are commonly sprayed with saline solution during mechanical testing to ensure adequate hydration. However, potting of vertebrae inhibits physiological fluid exchange through the porous endplates during loading. This un-physiological flow regime may influence mechanical properties of intervertebral discs and therefore of the whole spinal unit. The objective of this study was to evaluate a new method of spinal specimen hydration through the vertebral body during in vitro testing in order to improve consistency of mechanical behaviour. Ovine lumbar anterior column units were prepared for testing. Half of the specimens were sprayed with Ringer's solution and wrapped in plastic foil. The remainder received an additional saline infusion into the centre of each vertebral body. Three consecutive compression steps were applied by a hydraulic testing machine. Average forces within four sectors of the relaxation curves were compared. Applying saline infusion improves the consistency of consecutive relaxation curves. Differences between consecutive relaxation curves were less than those for standard hydration. The forces at the beginning of the relaxation curve were also lower for the infusion method. The standard deviation between specimens of each group was lower for saline infusion of vertebrae. Hence, the new method leads to more consistent in vitro testing conditions.