Quantifying Endplate Deflection in Response to Cyclic Load Exposures Using a Porcine Cervical Spine Model

Abstract

The vertebral endplate is a thin layer of cartilage and bone that separates the intervertebral disc from adjacent vertebral bodies and facilitates the transmission of compressive force through the spine. Despite this essential function, it remains the weakest component of the vertebra-disc unit and is highly susceptible to mechanical failure. Endplate failure typically arises from localized tensile strains that manifest as deflection, defined as the out-of-plane displacement of the surface under load. While prior work has demonstrated inferior endplates of intervertebral joints exhibiting greater deflection and higher incidence of failure than their superior counterpart, current techniques for quantifying endplate deflection face notable limitations. Early studies using metallic markers or displacement transducers required drilling channels into the vertebral body, potentially exaggerating deformation by weakening subchondral bone support. Imaging-based approaches, particularly micro-CT, offer high spatial resolution but are limited to static or stepwise loading due to temporal constraints. These static conditions do not capture the cyclic loading patterns experienced by the spine during daily activity, where repeated deformation can cause fatigue-induced microdamage and eventual failure. Additionally, static loading promotes excess fluid loss from the nucleus pulposus, altering endplate deflections in ways that do not reflect physiological motion. Consequently, existing measurement techniques may misrepresent true endplate behavior and are unable to evaluate changes in deflection as a function of cyclic load exposure. This study addresses these limitations by developing a unique method to assess endplate deflection during cyclic loading without requiring prolonged stepwise protocols or causing damage to the vertebral bone. By comparing superior and inferior endplates across different load magnitudes and cyclic durations, this work aims to clarify the mechanisms underlying endplate vulnerability and further validate the porcine cervical spine as an experimental model for human lumbar spine deflection. Eighteen porcine cervical spine functional units (C3C4, C4C5, and C5C6; n = 6 per level) were dissected to yield 36 individual vertebrae. High-resolution laser profilometry was then used to capture the topography of the caudal endplates of C3, C4, and C5 and the cranial endplates of C4, C5, and C6. Custom indenters, designed as negative molds of the nucleus-occupying endplate region, were created from the resulting surface scans and fabricated via 3D printing. Specimens were then oriented such that the tested endplate was in a neutral position and subjected to a normalized haversine waveform, ranging from 0.3 kN to 30% of the predicted ultimate compressive strength using a servohydraulic materials testing system. The cycle-dependent changes in endplate deflection were measured at 0, 1000, 3000, and 5000 total cycles. At each time point, endplate deflection measurements were captured via the indenter’s displacement while specimens were exposed to a brief static force of 0.3 kN, 1 kN, and 3 kN, totaling 12 measurements per vertebra. Three separate linear mixed effects models were used to evaluate the impact of loading magnitude, loading cycles, endplate level and the proportion of the nucleus occupying endplate area on superior and inferior endplate deflection within each joint. A fourth linear mixed effects model was used to evaluate the impact of loading magnitude, loading cycles, and joint level on the magnitude of the differences between superior and inferior endplate deflection. Utilizing this novel methodology, this study was the first to quantify endplate deflection under cyclic loading conditions, observing greater deflection of the inferior endplate across all spinal levels, except at baseline (0.3 kN, 0 cycles). This method also enabled comparison of deflection rates between endplates, with the C4C5 and C5C6 inferior endplates showing a significantly greater rate of deflection during the first 1000 cycles. Among joints, C4C5 exhibited the largest difference in superior and inferior endplate deflection compared to C3C4 and C5C6. Endplate deflection was not influenced by the proportion of the nucleus occupying endplate area at any spinal level. Lastly, as the first study to examine endplate deflection in porcine cervical vertebrae, the observation of greater inferior endplate deflection being consistent with human cadaveric studies further supports the validity of this model. Overall, this study demonstrates the utility of a novel methodology for measuring and comparing superior and inferior endplate deflection under cyclic loading.