The development of artificial joints is now considered quite mature, and the main treatment for osteoarthritis. However, in recent unicompartmental knee arthroplasty (UKA) clinical follow-ups, complications due to wear of polyethylene (PE) tibial components still exist. Therefore, this study focused on the possibility of avoiding and minimizing damage to the PE tibial component. Currently, the most common problem in the application of UKA is the malresection of the tibial plateau, often resulting in malpositioning of the tibial implant. This positioning problem may be the main reason for advanced wear and dislocation of a PE tibial component. In this study, finite element analysis (FEA) was used to study the stress change of malpositioned PE tibial components in order to better understand the damaging mechanism on PE tibial components. It was found that anatomically designed unicompartmental knee prostheses (UKP) allowed more positioning error in varus tilt than symmetrically designed ones. And both should avoid any positional error greater than 10° valgus tilt. Otherwise, increased wear of PE tibial components would result in shortened lifetime of the artificial joint.