Geometry alone? Evaluating the predictive capability of morphed intervertebral disc FEM models with a single material...

Individual intervertebral disc (IVD) characteristics contribute to spinal mechanics and pain, but individualized biomechanical modeling is insufficiently examined, especially regarding the validity of using a single material model across different degeneration grades and disc geometries. Based on MRI images, we generated and simulated 241 patient-specific lumbar finite element method (FEM) IVD models using a single material model calibrated to a healthy L4-L5 disc. This study is twofold: In one respect, it establishes and evaluates the automated morphing algorithm femReg, which is used to generate the models. In another respect, it evaluates the capacity of the geometry-only individualized IVD models to represent the range of motion (ROM) of variously degenerated discs by simulating them in four load cases under 1, 2.5, 5 and 7.5 Nm load. To do so, we grouped numerical models by height loss and compared absolute and normalized ROM results with in vitro data across different degeneration grades. 241 high-quality hexahedral IVD models (mean aspect ratio: 1.65, mean Hausdorff distance: 0.037 mm) were created and simulated within an average runtime of 6 minutes per IVD. Healthy models (n = 149) reproduced experimental ROM with root mean square errors (RMSEs) of 0.96^∘-2.05^∘ across bending load cases. The closest agreement was observed in lateral bending (RMSE = 0.68^∘), where facet joint contributions are minimal. In contrast, degenerated models exhibited reduced height-normalized ROM compared with in vitro data (RMSE = 0.05-0.24^∘/mm), indicating an effectively stiffer response and suggesting that degeneration-specific material parameters, such as fiber loosening or reduced hydration, should be incorporated to better capture pathological behavior.