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BACKGROUND CONTEXT Previous studies have identified geometric mismatch between the endplate and interbody as a main risk factor for subsidence. A proper selection of the cage width and placement can potentially reduce the risk of subsidence. Existing studies using surrogate vertebral bodies to evaluate the subsidence performance of interbody cages often omit the morphology of the endplate, which can vary markedly across patients. PURPOSE To comprehensively evaluate the effects of cage width and placement on the subsidence performance using anatomically representative test blocks that cover a different range of endplate concavity depths. STUDY DESIGN/SETTING Surrogate models were fabricated using 15 pcf polyurethane foam representative of the anatomy of a L5 vertebral body. Specifically, an elliptical cylinder was used as the base geometry to capture the overall anatomy of superior half of L5 vertebral body. The major and minor axes were taken from averaged population data (52mm, 36mm). Next, deviations of the shape in the radial direction was included to represent the anterior part, vertebral foramen, left and right pedicles. Lastly, an endplate concavity was created by removing from the endplate surface a concentric ellipsoid with varying depths (1, 2, 3 and 4mm) to simulate different endplate morphology. Lateral cages of two different widths (18mm or 22mm; Modulus XLIF), placed anteriorly or centrally, were assessed for construct stiffness, an indicator of the device's propensity to subside, in static compression per ASTM F2267 standards. PATIENT SAMPLE Sixteen combinations (4 depths, 2 widths, 2 placements) were tested. Five samples were tested in each combination. OUTCOME MEASURES Stiffness measured per ASTM F2267 testing standards was used as the indicator of subsidence performance, with a greater stiffness representing a better resistance to subsidence. METHODS Multiple linear regression was used to examine the role of each variable in subsidence performance. RESULTS Endplate concavity depth and cage width were identified as significant factors to subsidence performance. For each 1mm increase in the endplate concavity depth (more geometric mismatch), a decrease of 634.4 N/mm was observed (p CONCLUSIONS Endplate morphology plays a critical role in the evaluation of subsidence performance of interbody cages. Therefore, using anatomically representative endplate models is recommended. Using a wider cage can mitigate the risk of subsidence, particularly with more concave endplates. FDA DEVICE/DRUG STATUS Modulus XLIF (Approved for this indication). |
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