Background <p>Dislocation of the mobile bearing insert in unicompartmental knee arthroplasty (UKA) is a rare complication, with anterior dislocation being the most commonly reported pattern. Posterior dislocation is exceedingly uncommon due to the design of the implant and biomechanical constraints. Management typically involves simple bearing exchange or, in severe cases, conversion to total knee arthroplasty (TKA).</p> Case Report <p>We report a rare case of chronic, asymptomatic posterior dislocation of the mobile bearing insert following medial mobile-bearing UKA. The delayed presentation led to progressive medial collateral ligament (MCL) insufficiency and metallosis, ultimately necessitating conversion to a hinged TKA. This case emphasizes the need for high clinical suspicion, especially in patients presenting late with subtle symptoms.</p> Conclusion <p>Posterior dislocation of the polyethylene insert, though rare, can lead to significant complications if left undiagnosed. Early recognition and intervention can prevent progression to extensive soft tissue damage and the need for constrained prostheses.</p>