A biconcave polyethylene (PE) bearing was developed for mobile-bearing unicompartmental knee arthroplasties (UKA) to reduce PE dislocation.A modification of the BalanSys Unicompartmental knee system with a biconcave PE and convex tibial component was used in 32 prostheses in 28 patients. Clinical outcomes and five cases of PE fracture are reported and extensively analyzed ex vivo.Visual Analogue Score of pain and satisfaction, and Knee Society Scores improved for all patients. The passive range of motion was 130°. No PE bearings dislocated. Five bearings fractured with oxidation, cracking and delamination at the thinnest central region of the PE. The combination of increased stress and decreased poly thickness was associated with increased creep. PE oxidation caused embrittlement and contributed to fractures in the thin waist of the implant.The unforeseen consequence of a novel design of a UKA that resulted in a specific mechanical and tribological mode of failure is reported. We conclude the PE failed due to a biconcave design that increased stress on the implant at its region of risk combined with a decrease of the polyethylene thickness by 1 mm in the central area.