The Elastic Stable Intramedullary Nailing (ESIN) technique is widely used for treating paediatric femoral shaft fractures. The biomechanical effectiveness of the implant depends on several geometric and surgical factors. However, the lack of quantitative insight into geometric influence on implant performance hinders treatment optimization. This study aimed to develop a parametric model to evaluate, using a multibody approach, the influence of the main geometric factors on the overall implant stability. Two titanium nails (Stryker and Synthes) were characterized through four-point bending test, conducted according to ASTM F1264-24, to obtain force-displacement curves and elastic modulus values. A flexible nail model was then created in Adams View, subjected to the same bending test and tuned to match the experimental results.