THE EFFECT OF LOADING RATE ON ANNULUS FIBROSUS STRENGTH FOLLOWING ENDPLATE FRACTURE

Low back pain is the most prevalent cause of chronic pain for North Americans, and its correlation with endplate damage, and deficient mechanical properties in the annulus fibrosus (AF) makes this injury particularly concerning. To date, biomechanical alterations in the AF following endplate fracture have not been well described. Owing to the AF’s collagenous composition, and the intrinsic biomechanics of intervertebral discs (IVDs) during compression, it was hypothesized that the mechanical properties of AF specimens would show rate-dependent alterations in their tensile and adhesive strengths following endplate fracture. The purpose of the present study was to quantify the mechanical properties of the AF from IVDs that had sustained endplate fracture via different rates of applied compression. Porcine cervical spines acquired from a common source were stored at -20oC until the night before testing at which they were left to thaw at room temperature. Specimens were dissected into functional spinal units (vertebra-disc-vertebra) at the levels C3/4 and C5/6. Segments were preconditioned with 300N of compressive force for 15 minutes using a uniaxial electromechanical material testing system. Following preconditioning, functional spinal units were compressed until fracture under displacement control at one of three loading rates: 15mm/s (fast), 1.5mm/s (medium), or 0.15mm/s (slow). Following fracture, two samples dissected from the AF were mechanically tested from each disc: 1) Bilayer samples were tested in tension at 1%/s to 50% strain; 2) multilayer samples were tested using a 180° peel test configuration at 0.5mm/s. Mechanical properties were compared with a 2-way analysis of variance and Tukey-Kramer post-hoc analyses. Results indicated significant differences in the compressive strength and stiffness of fast and medium-compressed functional spinal units compared to those compressed at the slowest rate of velocity. However, at the AF tissue-level, the present experiment did not detect alterations in mechanical properties stemming from different rates of endplate-fracture velocity. Previous work has shown that rapid internal intervertebral disc pressurization resulted in endplate fracture, along with decreased adhesion strength in these AF specimens compared to non-pressurized/non-fractured controls. While at the whole-disc level, the present experiment reports quantifiable differences in the strength and stiffness of functional spinal units, it is likely that velocities of compression were much too slow to incur the previously documented mechanism of decreased adhesion strength. Future research should seek to investigate a compressive model of endplate injury at high rates of velocity commensurate with falls from height or motor vehicle accidents