Background and purpose:​ Radiotherapy is known to produce long-term skeletal complications. We aim to evaluate the biomechanical effect on femoral neck bone from hypothetical eccentric loads on pre- and post-radiotherapy CT-images for patients treated for rectal cancer.

Material and methods:​ Pre- and post-radiotherapy CT-images of rectal cancer from 10 patients were selected randomly. The cortical and trabecular bone was segmented by hand. The biomechanical simulations of 4 eccentric loads and one load aligned with the femoral neck axis were completed using finite element analysis (FEA) in both pre- and post-patient models.

Results:​ A comparative statistical study was completed of pre- and post-radiotherapy patient models of stress and displacement factors. Significant differences were found in eccentric loads in both factors. Natural load has a significant difference in stress, but no differences were found for displacements. The absolute difference in eccentric load applied to the anterior area location on the same patient implies from 5.3% to 40.5% of the stress yield values reported in previous studies.

Conclusions:​ Loads applied to the anterior area of the femoral head must be considered in fracture simulations because the percentage of yield stress of pre- and post-irradiated bones shows a significant biomechanical change.