Despite improvements in prosthesis technology and use of high-quality materials in recent years, the number of revisions related to implant failures (4.7%) remains high. Several phenomena were reported in literature as reasons for dislocation and fracture of tibial insert in mobile bearing prosthesis: incorrect positioning of components, discrepancy of extension and flexion balancing or ligament weakness. However, in our cases neither of these causes were observed. The authors consider that bio-physiological and biomechanical aspects of total knee arthroplasty and knee joint prosthesis should be thoroughly studied and implemented into the clinical practice. In the described cases, multiple damage of the insert due to flexion instability and “twist-hyperflexion” in overweight patients is perceived as the reason for failures.