The minimally invasive implantation of unicompartmental knee arthroplasty (UKA) leads to excellent functional results, but due to the reduced intraoperative visibility the removal of excessive cement may be difficult. In a retrospective study we assessed radiologically the incidence of loose and excess bone cement in 120 UKAs and correlated it to the thickness of the tibial cement layer. In 25 cases loose or attached excess cement was seen. Two of these patients with loose cement bodies required revision surgery. An additional 2 patients not operated at our institution required revision because of pain and loss of motion. The average thickness of the tibial cement layer was 3.1 (1.7–5.0) mm in all the patients. But it was significantly higher in the group with excess cement bodies [3.3 (2.3–5.0) mm] compared to the group without excess cement [3.0 (1.7–4.1) mm] (P < 0.05). Symptomatic free cement bodies need to be removed immediately, if necessary arthroscopically, in order to avoid damage to the implants. To avoid this problem in minimally invasive UKA, intraoperative fluoroscopy, a dental mirror or a nerve hook seem to be useful tools to identify and remove loose or excess cement.