Optimization of the endplate interface for a surrogate intervertebral disc model for wear and fatigue testing of nucleus pulposus replacement devices

Priyanka Preyas Shah

doi:10.17918/etd-3105

Nucleus pulposus replacement devices (NPRDs) have emerged as an alternative to the current standard of care for low back pain. These devices aim to correct spinal load distribution, to restore range-of-motion, and to ultimately relieve pain. Although NPRDs may offer benefits compared to the current standard of care, novel device design features and clinical concerns must be investigated to ensure safety and effectiveness. Therefore, there is a need for a pre-clinical test to characterize device performance. A physical, surrogate annulus fibrosus (AF) model for multidirectional wear and fatigue testing of NPRDs has been designed and validated. However, this model is comprised of the AF only. Within the intervertebral disc, the nucleus is bound by both the AF and endplates. The endplates facilitate uniform load distribution while regulating nutrition and fluid transport across the disc. The current model is designed to include representative endplate interfaces. Twenty-six materials including rubbers, foams, and medical-grade biomaterials were evaluated to identify materials that may represent functional endplate properties. Candidate materials were characterized under unconfined compression. A stepwise stress-relaxation test was performed up to 20% strain, where each step incremented 5% strain at 0.001 strain/sec. The aggregate modulus (HA) was determined as the ratio of equilibrium stress to equilibrium strain and interpreted to identify a representative surrogate endplate material. Eight surrogate intervertebral disc models (SIVDMs) were included in this validation study, where six were utilized for wear and fatigue characterization and two for load soak control. Each model included three injection molded parts: a surrogate NPRD (VytaFlex10, Smooth-On); a surrogate AF (QM264, Quantum Silicones), and superior and inferior surrogate endplates (RenCast6401-1, Huntsman). The SIVDM was validated on a 6DOF spine wear simulator (MTS, EdenPrairie, MN) in accordance with motion and loading profiles defined in ISO 18192-1 up to 2.5Mcycles of wear testing. Each model was subjected to compressive loading at 1.0mm/s to characterize stiffness under denucleated and nucleated states at 0.0 and 2.5Mcycles. Models were submersed in a 37±2o PBS bath and imaged using fluoroscopy. Surrogate NPRDs were characterized for gravimetric and dimensional changes at 0.5Mcycle intervals. Surrogate endplates were characterized for dimensional changes using micrometers and surface roughness changes using white light interferometry at 0.0 and 2.5Mcycles. Representative surrogate NPRDs (n=2) and endplates (n=2) were imaged with optical microscopy and microCT. The surrogate NPRDs exhibited evidence of adhesive/abrasive wear and demonstrated a linear wear rate, 180mg/Mcycles (r2=0.95, p<0.05). The nucleated model was determined to be approximately 30% stiffer than the denucleated model. Weartested endplates maintained 12.35±0.3mm height, and there was no significant change in articulating surface roughness (p>0.05). Surrogate endplates performed superiorly, with respect to wear. Results show no significant changes in height (p>0.99) or surface roughness (p>0.05) after wear testing. Articulating surfaces did not appear worn, and no signs of internal cracking were observed on corresponding microCT images. These results suggest that wear along surrogate NPRDs evaluated in this study are not artifact of interfacing surrogate endplate wear. The endplate material chosen for the SIVDM was validated by comparison of HA to range of moduli reported for cartilaginous endplates in validated finite element models of the human intervertebral disc. A physical SIVDM has been designed and validated for multidirectional wear and fatigue testing of NPRDs. The optimized model has been designed to impart clinically relevant loading patterns and constraint to sample NPRDs. This pre-clinical test was used to characterize wear and fatigue of surrogate devices in terms of mass loss, height loss, contribution to SIVDM stiffness, and physical features of fatigue.

Optimization of the endplate interface for a surrogate intervertebral disc model for wear and fatigue testing of nucleus pulposus replacement devices

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Optimization of the endplate interface for a surrogate intervertebral disc model for wear and fatigue testing of nucleus pulposus replacement devices

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