Measurement and calibration of the nucleus position and its cross-sectional area ratio to increase the accuracy of finite element analysis

Abstract

Background: As a widely used biomechanical research method, finite element analysis (FEA) is an important tool for investigating the pathogenesis of disc degenerative diseases and optimizing spine surgical methods. However, the definitions of the relative nucleus position and its cross-sectional area ratio do not conform to a uniform standard, thus affecting the accuracy (ACC) of the FEA. Hence, this study aimed to determine a precise definition of the relative nucleus position and its cross-sectional area ratio to increase the ACC of the following FEA studies. Methods: The lumbar relative nucleus position and its cross-sectional area ratio were measured from magnetic resonance imaging data and then calibrated and validated via FEA. Imaging data from patients without disc degeneration were used. The L4-L5 nucleus and disc cross-sectional areas and the distances between the edges of the annulus and nucleus were measured; the ratios between these values were calculated as P1 and P2, respectively. The FEA model was constructed using these measured values, and the relative nucleus position was calibrated by estimating the differences in the range of motion (ROM) between the model, wherein the ligaments, facet joints and nucleus were suppressed, and that of an in vitro study. Then, the ACC was re-estimated in the model with all non-bony structures by comparing the ROM, the intradiscal pressure (IDP), the facet contact force (FCF) and the disc compression (DC) under different sizes and directions of moments magnitudes to validate the measured and calibrated indicators. Results: The interobserver homogeneity was acceptable, and the measured P1 and P2 values were 1.22 and 38%, respectively. Furthermore, an ACC of up to 99% was attained for the model under flexion–extension conditions when the calibrated P1 value (1.62) was used, with a model validation of greater than 90% attained under al most all of the loading conditions considering the different indicators and moment magnitude s. Conclusion: The measured and calibrated relative nucleus position and its cross-sectional area ratio increase the ACC of the FEA model and can therefore be used in subsequent studies.