Instrumentation affects spinal biomechanics and making accurate analysis of post-surgical outcomes using simulation models is crucial in aiding clinical decisions. Geometrical morphological finite element models streamline development and improve inference of surgical instrumentation's biomechanical impact on patient-specific spine segments. This study develops a comprehensive hexahedral morphological lumbosacral finite element model to analyse motion range, disc pressures, and facet contact forces in intact and instrumented spines.

Extensive validation across physiological loading regimes—pure moment, compression, and combined loading—compares model results with six in vitro studies, showing statistically significant agreement. Flexion, extension, and bending moment curves align favorably with experimental data, while axial torque curves match in four out of five lumbar functional units. Facet contact force predictions also correlate with in vitro results. Despite using fewer elements, the developed model maintains accuracy, enhancing computational efficiency compared to contemporary models. Sensitivity analysis on seven anatomical parameters explores their impact on facet contact force, disk pressure, and motion range.

The study then analyses both bilateral and unilateral instrumentations and reveals comparable immediate post-fusion stabilities, suggesting the unilateral approach's viability for specific patients due to its minimally invasive nature and maximum tissue retention.

Overall, the model shows promise in predicting patient-specific biomechanical responses, improving post-surgical stability and fixation.