Sensitivity of spinal load estimates to vertebral landmark error in biplanar radiograph-based musculoskeletal models

Patient-specific musculoskeletal models from biplanar radiographs offer a clinically feasible route to estimating spinal loading, but how vertebral landmark error affects force prediction remains unclear. Paired radiograph landmarks reconstruct the three-dimensional spine, making placement error a source of uncertainty in load estimates. We quantified how landmark-placement error affects compression and shear across perturbation magnitude, direction, and spinal region. Using perturbed landmarks and multi-rater annotations, compression maintained excellent ground-truth agreement up to 6.7 mm error. Shear reliability degraded at 2.2 mm, corresponding to 80 N mean absolute error and 26% normalized error. Therefore, these shear estimates require cautious clinical interpretation.