Effects of different sites or areas of preoperative lateral compartment bone marrow edema on the midterm clinical outcomes of medial unicompartmental knee arthroplasty

The single imaging feature of bone marrow edema (BME) in the lateral compartment has not been included as an absolute contraindication for medial unicompartmental knee arthroplasty (MUKA). At present, few studies have confirmed whether lateral compartment BME affects the prognosis of MUKA patients. Therefore, it is necessary to explore whether preoperative lateral compartment BME at different sites or areas assessed by magnetic resonance imaging (MRI) affects the midterm clinical results and the imaging alignment of the prosthesis after MUKA to provide surgeons with surgical decision-making and patient selection recommendations. This retrospective study included 202 patients who underwent MUKA between August 2021 and September 2023 and whose preoperative MRI data were available. The patients were divided into a femur group, a tibia group and a femur and tibia group according to their location. One-way analysis of variance was used to analyse the overall difference, and repeated measures analysis of variance was used to analyse the group effect, time effect and interaction effect of the outcome indicators. Patients were also divided into Grade I, Grade II and Grade III according to the overall edema area, and statistical comparisons are exploratory and underpowered because of the small number of events. Clinical outcomes, including range of motion (ROM), visual analogue scale (VAS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), 36-item Short Form Health Survey (SF-36), and Forgotten Joint Score-12 (FJS-12), were assessed at 6 months, 1 year, and 2 years post-operatively. Radiographic parameters at the final follow-up included the sagittal tibial component angle (sTCA), sTCA deviation, mechanical lateral distal femoral angle deviation (mLDFA deviation), and posterior condylar offset ratio (PCOR). Patients with bi-site lateral compartment BME had higher VAS score and lower FJS-12 score and ROM during the early follow-up period (6 months and 1 year, P < 0.05). There were differences in VAS and FJS-12 in patients with Grade III at 1 year and 2 years. At the last follow-up of the midterm period, there were significant differences in the VAS and FJS-12 scores between the femur and tibia groups and the other three groups ( P < 0.05). The time effect and group-time effect of ROM were significant ( P < 0.05), and the group effect, time effect and group-time effect of VAS and FJS-12 scores were significant ( P < 0.05). No significant differences in the WOMAC score or SF-36 score were observed among the groups throughout the follow-up period ( P > 0.05). Radiographic assessment revealed comparable sTCA, sTCA deviation, mLDFA deviation, and PCOR values among the groups, with no increase in postoperative complications or prosthesis-related failures. Lateral compartment BME was associated with delayed recovery after MUKA, particularly with combined femoral and tibial involvement. Patients with single-site or moderate lateral compartment BME achieved satisfactory midterm outcomes. Chictr.org.cn identifier: ChiCTR2500111421. Registered 31 October 2025. Retrospectively registered.