A system for a unicompartmental knee replacement is described. The system can include a femoral prosthesis component, a tibial prosthesis component and a meniscal bearing. The meniscal bearing can be insertable between the femoral prosthesis component and the tibial prosthesis component. The meniscal bearing can include a first bearing surface configured to bear against the femoral prosthesis component, a second bearing surface opposed to the first bearing surface and configured to bear against the tibial prosthesis component, and a sidewall separating the first bearing surface from the second bearing surface, wherein the sidewall has a retention groove formed in a portion thereof, and wherein the meniscal bearing and the tibial prosthesis component are configured such that the retaining flange enters the retention groove on movement of the meniscal bearing relative to the tibial prosthesis component towards the medial wall.