Background <p>Mobile-bearing stability in Oxford medial unicompartmental knee arthroplasty (UKA) depends on maintaining a consistent femorotibial component relationship throughout knee motion. This study evaluated whether a modified extramedullary femoral referencing technique, designed to account for extension-phase alignment, was associated with reduced motion-related variation in femorotibial component position and more stable mobile-bearing positional behavior than the previously described extramedullary technique.</p> Methods <p>This single-center retrospective cohort study included 60 patients who underwent primary medial mobile-bearing Oxford UKA between January 2023 and January 2024. Thirty consecutive patients treated with the modified technique (modified group, MG) were compared with 30 individually matched controls treated with the previous extramedullary technique (control group, CG). Standardized standing and 90° flexion anteroposterior radiographs were used to assess the normalized horizontal distance between the femoral component and the tibial lateral wall, bearing horizontal translation, and bearing rotation. Clinical outcomes included range of motion (ROM), Hospital for Special Surgery (HSS) knee score, and postoperative complications.</p> Results <p>Baseline demographic and preoperative clinical characteristics did not differ significantly between groups. Follow-up duration was similar (27.4 ± 3.3 vs. 26.5 ± 2.9 months, <i>p</i> = 0.267). Postoperative ROM and HSS were also comparable, with postoperative HSS scores of 90.6 ± 5.8 in the MG and 89.2 ± 5.2 in the CG (<i>p</i> = 0.319). The MG showed significantly lower motion-related variation in femorotibial component position than the CG (ΔDistance: -0.006 ± 0.040 vs. 0.024 ± 0.054, <i>p</i> = 0.018). Within-group analysis showed no significant extension-versus-flexion change in normalized femorotibial distance in the MG, whereas a significant change was observed in the CG (<i>p</i> = 0.021). Compared with the CG, the MG had lower extension bearing rotation (-0.001 ± 0.054 vs. 0.037 ± 0.077, <i>p</i> = 0.033) and smaller bearing horizontal translation (0.039 ± 0.047 vs. 0.071 ± 0.052, <i>p</i> = 0.018). One bearing dislocation occurred in the CG and none occurred in the MG.</p> Conclusions <p>The modified extramedullary femoral referencing technique was associated with less motion-related variation in femorotibial component position and more stable radiographically derived mobile-bearing positional behavior. These differences were not accompanied by superior short-term ROM or HSS.</p> Clinical trial number <p>Not applicable.</p>