IN-VIVO DYNAMIC STIFFNESS OF THE PORCINE LUMBAR SPINE EXPOSED TO CYCLIC LOADING - INFLUENCE OF LOAD AND DEGENERATION

The dynamic axial stiffness of the L2-3 motion segment subjected to vi bratory loading under intact and injured states of the intervertebral disc was studied using an in vivo porcine model. Three groups of anima ls with the following states of the intervertebral discs were studied: intact disc, acutely injured disc, and degenerated disc. A miniaturiz ed servo-hydraulic exciter was used to sinusoidally vibrate the motion segment from 0.05 to 25 Hz under a compressive load with a peak value of either 100 or 200 N. The dynamic axial stiffness of the interverte bral disc was calculated at 1-Hz intervals over the frequency range. T he results showed that the dynamic axial stiffness was frequency depen dent. A positive relationship was found between an increase in mean dy namic stiffness and load magnitude. An increase in mean stiffness with successive exposures at the same load magnitude was observed, despite the allowance of a recovery period between loading. The greatest diff erence was noted between the first and second load sets. No significan t change in stiffness was found due to an acute disc injury, whereas a significant increase in mean stiffness was found for the degenerated dsc group as compared with the intact group. The form of the frequency response curve, however, remained relatively unaltered regardless of the degenerated state of the disc. With heavier loads, repeated loadin g, and/or disc degeneration, the stiffness of the intervertebral disc increases. An increase in stiffness can mean a reduction in the amount of allowable motion within the motion segment or a potentially harmfu l increase in force to obtain the desired motion. This may locally res ult in greater stresses due to an altered ability of the disc to distr ibute loads.