The present study compares the Gender prosthesis against the LPS-flex in a bilateral study, where patients are given two different prostheses: Gender on one side and LPS-flex on the other side. Patients are blinded to which prosthesis is selected for which side. The potential advantages of the Gender prosthesis include: less post-op pain due to better sizing (less overhang), less post-op patellofemoral pain and less compromised extensor mechanism and better patella tracking (due to less overstuffing of the patellofemoral joint).