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Objectives and questions: We assessed the inherent biases, pre-procedural planning accuracy using 2D to 3D X-Atlas^®, and final knee axis outcomes of the ROSA^® Knee System (Zimmer Biomet, Warsaw, IN).

Material and methods: 55 patients who underwent robotic-assisted knee replacement using ROSA^® Knee System (Zimmer Biomet, Warsaw, IN) at a single centre were included. Pre-procedural measurements performed by ROSA were compared to those performed by senior consultants. Component sizes predicted by ROSA^® were compared to those implanted. Final axis measurement was taken during the procedure.

Results: Femur components were exactly matched in (83.64%) cases, accurately matched in further 8 (14.55%), and inaccurate for only 1 (1.82%). Tibial component sizes were exactly matched by the planning for 39 (70.91%), accurately for 12 (21.82%), inaccurately for 4 (7.27%). ANOVA did not show statistically significant differences between the predicted and implanted femur (p = 0.96) nor tibia components (p = 0.27). We show that ROSA^® pre-procedural planning has a statistically significant bias (p=0.001) of 0.83 degrees into varus when assessing knee axis in the coronal plane when compared to senior consultant measurements. The average of final coronal knee axis was 0.37 degrees in varus (SD = 2.49).

Discussion and conclusions: ROSA^® accurately predicts implanted component sizes. Despite small and statistically significant varus bias in initial knee axis assessment, the system results lay within the +/- 3 degrees of neutral knee axis, widely accepted knee replacement standard.