Biomechanical testing is essential for evaluating osteosyntheses, particularly in assessing stability, stiffness, and fragment motion. However, traditional flat-fracture models created via osteotomy fail to replicate the complex morphology of real-world fractures, potentially reducing the applicability of results. This study introduces patient-specific distal femur fracture models to investigate the impact of fracture morphology on the biomechanical performance of osteosyntheses. Realistic fracture models were generated using 3D printing and molding, based on CT-derived geometry, alongside traditional osteotomy models. Four groups were tested: osteotomized and realistic fracture models, with and without gaps. All constructs were treated with distal femur locking plates and subjected to axial and torsional loading. Dynamic testing simulated physiological conditions and tracked interfragmentary motions with a 3D optical motion system. Realistic fracture models exhibited higher torsional stiffness and reduced interfragmentary motion compared to osteotomized models, particularly in closed fracture gaps. Axial stiffness increased significantly upon fracture gap closure in all gap groups, transitioning from exclusively plate-bearing to construct-bearing configurations. The irregular geometry of realistic fractures provided enhanced interlocking, improving stability under both axial and torsional loads. Patient-specific fracture models better replicate the mechanical behaviour of clinical distal femur fractures, demonstrating advantages over osteotomized fracture models. The inclusion of realistic fracture geometries in biomechanical testing improves the transfer of biomechanical results into a clinical setting and offers valuable insights for optimizing designs and improving clinical outcomes.