Accuracy of ROSA® Partial Knee System in Tibial Alignment During Medial Unicompartmental Knee Arthroplasty: An Observational Study

Background/Objectives: The ROSA® Partial Knee System (Zimmer Biomet, Warsaw, IN, USA) is an imageless robotic platform developed to improve the reproducibility of tibial resection and implant positioning in unicompartmental knee arthroplasty (UKA). Evidence on imageless ROSA-assisted medial UKA remains limited. This preliminary single-center study evaluated the early radiographic accuracy of the system in reproducing planned tibial coronal alignment and tibial slope during robotic-assisted medial UKA, while perioperative and short-term clinical findings were assessed as secondary exploratory observations. Methods: A retrospective analysis of a prospectively maintained database was performed on 23 consecutive patients who underwent robotic-assisted medial UKA using the ROSA® Partial Knee System and Persona® Partial Knee implant between December 2025 and March 2026. Planned and robot-validated values were compared with postoperative radiographic measurements, which were used as the achieved alignment reference. Deviations >2° and >3° from target were defined as outliers. Paired clinical analyses were restricted to patients with data available at both time points. Results: Mean age was 68.4 ± 8.5 years and mean BMI was 28.2 ± 4.1 kg/m2. Preoperative HKA was 5.4 ± 3.4° varus, while final robot-validated and postoperative radiographic HKA values were 2.3 ± 0.7° and 2.6 ± 0.9° varus, respectively (p = 0.08). Robot-validated and postoperative radiographic MPTA values were 2.0 ± 0.5° and 2.1 ± 0.7°, respectively (p = 0.41). Planned, robot-validated, and postoperative radiographic tibial slope values were 4.5 ± 0.6°, 4.6 ± 0.6°, and 4.8 ± 0.8°, respectively (p = 0.27). Mean absolute postoperative deviation from target was 0.42 ± 0.38° for HKA, 0.34 ± 0.29° for MPTA, and 0.46 ± 0.39° for tibial slope. Within 2° of target, accuracy ranged from 91.3% to 95.7%, and all cases were within 3°. Short-term clinical outcomes improved in the available paired subsets (n = 14 for VAS and KSS, n = 12 for ROM, and n = 9 for UCLA activity score). Conclusions: In this preliminary single-center observational series, the ROSA® Partial Knee System showed high early radiographic accuracy in reproducing planned tibial coronal alignment and tibial slope during robotic-assisted medial UKA, with low outlier rates. Short-term clinical findings were favorable in the available paired subsets but should be interpreted as exploratory. Larger comparative studies with longer follow-up are needed to determine the clinical relevance of this technical accuracy.