In this study, two novel designs of Modular Mega prosthesis (MDF 1 and MDF 2) are generated with different geometries and dimensions, with a reference to a normal knee Modular Prosthesis emerging as one of the most promising alternatives to replace the outdated amputation procedures for bone cancer treatment. To obtain a prosthesis with full flexion ability, a better understanding of normal and healthy knees is needed. To replicate knee movements, 3–axis simulations to acquire interference between femoral condyles and the tibial base is applied to both designs and normal knee to better compare the range of movements. Volume interference as an indicator of high - flexion in knee rotation movements is established. The result showed that MDF 2 can give a superior result of flexion-extension rotations compared to normal knees and MDF 1. The range of movement in internal-external and varus-valgus rotations of normal knees is still higher than both MDF 1 and MDF 2, although the results of movement for both novel designs are not too distinctive. The simulations in this study showed that both designs, MDF 1 and MDF 2 have great potential as a solution after limb-salvage resection. © 2024 American Institute of Physics Inc.. All rights reserved.