Acute mechanical symptoms due to excess retained cement in the posterior compartment of the knee joint following unicondylar knee arthroplasty (UKA) are uncommon. Infection, aseptic loosening, polyethylene wear and progressive arthritis are well-documented complications of UKA procedure. We present a patient with acute pain and 'clicking' sensation in the knee joint due to cement extrusion in the posteromedial compartment after UKA. Full functional recovery was achieved after arthroscopic removal of the cement debris. Of retrospectively screened 43 UKA cases, asymptomatic cement extrusion was detected in 8 patients in the posteromedial compartment on direct X-rays. Careful inspection of components is essential to minimize the risk of cement extrusion into the posterior compartment and perioperative fluoroscopy may be helpful during UKA procedure.