DIFFERENT OPTIMAL ALIGNMENT BUT EQUIVALENT FUNCTIONAL OUTCOMES IN MEDIAL AND LATERAL ROBOTIC-ASSISTED UNICOMPARTMENTAL KNEE ARTHROPLASTY

INTRODUCTION Medial and lateral unicompartmental knee arthroplasty (UKA) are both reliable treatment options for isolated osteoarthritis. Postoperative lower leg alignment is known to play an important role on short-term functional outcomes, which is an important argument for the use of robotic-assisted surgery. Since several anatomical and kinematic differences exist between both compartments, it seems inaccurate to aim for similar postoperative lower leg alignment in medial and lateral UKA. Purpose of this study was (I) to compare outcomes between both procedures and (II) to assess the role of preoperative and postoperative alignment on short-term outcomes in both procedures. METHODS Patients who underwent robotic-assisted medial or lateral UKA were included if they completed functional outcomes questionnaires preoperatively and postoperatively (Western Ontario and McMaster Universities Arthritis score) and completed an artificial joint awareness questionnaire (Forgotten Joint Score) postoperatively (not used preoperatively). A total of 143 medial UKA and 36 lateral UKA patients were included and mean follow-up was 2.4-years (range: 2.0 – 5.0 year). Postoperative alignment was measured using hip-knee-ankle radiographs with a standardized method. Alignment was categorized in medial and lateral UKA as undercorrection (3° to 7° varus or valgus, respectively), neutral (−1° to 3° varus or valgus, respectively), or overcorrection (3° to 7° valgus or varus, respectively). Outcomes were compared using independent t-tests and Pearson correlation analysis was performed to assess a correlation between alignment and outcomes. RESULTS No preoperative differences in functional outcomes were seen between medial UKA (54.9 ±14.9) and lateral UKA (50.3 ±13.4, p =0.304). Postoperatively, equivalent outcomes were noted between medial and lateral UKA in overall function (89.8 ±11.7 vs. 90.2 ±12.4, respectively, p =0.855) and joint awareness (71.2 ±24.5 vs. 70.9 ±28.2, respectively, p =0.956). Correlation analysis did not show a correlation between preoperative alignment and both functional outcomes and joint awareness for both procedures (all p > 0.4, Figure 1). More undercorrection was correlated with better functional outcomes (−0.355, p = 0.039) and less joint awareness (−0.540, p =0.005) in lateral UKA (Figure 2). In medial UKA, no correlation was noted between postoperative alignment and both functional outcomes ( p =0.104) and joint awareness ( p =0.069, Figure 2). With neutral postoperative alignment, less joint awareness was noted following medial UKA than lateral UKA (72.6 ±22.6 vs. 55.3 ±28.5, p =0.024). With undercorrection, however, significantly less joint awareness (85.3 ±19.5 vs. 68.2 ±26.8, p =0.024) and better functional outcomes (96.0 ±5.4 vs. 88.5 ±11.6, p =0.001) were noted following lateral UKA than medial UKA (Figure 3). CONCLUSION At short-term follow-up, equivalent outcomes were noted between medial and lateral UKA but the optimal postoperative alignment differed between both procedures. Undercorrection (3° to 7° valgus) resulted in most optimal outcomes in lateral UKA, while this was with neutral alignment (−1° to 3° varus) in medial UKA. This study showed that postoperative alignment plays a role on short-term outcomes of UKA and suggests that precise control of postoperative alignment should be pursued, which is possible with computer navigation or robotic-assisted surgery in UKA. For any figures or tables, please contact authors directly (see Info & Metrics tab above).