Assessment of radiolucent lines in patients with lateral unicompartmental knee arthroplasty and the relationship between these lines and the outcome

Purpose To assess the radiolucent lines (RLLs) around both tibial and femoral components in patients following lateral unicompartmental knee arthroplasty (UKA). Methods We performed a retrospective review of the records of a consecutive series of patients who had undergone lateral UKA. The RLLs were assessed with standard anteroposterior and lateral radiographs post-operatively. The patient-reported outcome measures included the Hospital for Special Surgery (HSS) score and Oxford knee score (OKS). The femoral component position (FCP) and femoral-tibial angle (FTA) were also recorded. Results A total of 198 UKAs that had appropriate radiographs and outcome scores were reviewed with a median follow-up of 33 (range, 12–71) months. The results suggested that 69 cases (34.8%) had RLLs on the standard radiographs. The incidence rates of femoral and tibial physiological RLLs were 11.6% (23/198) and 26% (52/198), respectively, of which 3% (6/198) concerned both components. All RLLs were considered “physiologic lines” that developed within one year after surgery. There were no significant differences among the types of RLLs in any of the outcome measures. No differences in FCP ( P = .359) or FTA ( P = .111) at the last follow-up were seen. Conclusions It was found that one-third of UKAs had RLLs on radiographs following lateral UKA. All RLLs developed within one year after surgery. As a clinical consequence, the development of RLLs does not affect the short-term outcomes after lateral UKA.