This retrospective observational study was designed to investigate the association between radiographic Ahlba¨ck osteoarthritis (OA) grade and postoperative joint perception in a cohort of patients undergoing medial robotic arm-assisted unicompartmental knee arthroplasty (RA-UKA), using the Forgotten Joint Status and Patient Acceptable Symptom State (PASS) as outcomes.

Between January 2014 and May 2019, 660 patients (719 knees) underwent medial RA-UKA at two centres. Ahlbäck OA grade was measured on preoperative knee radiographs. Post-operatively, patients were administered the Forgotten-Joint-Score-12 (FJS-12) and 5-Level-Likert-Scale to assess patients’ satisfaction. Correlations were described between FJS-12, satisfaction and Ahlbäck OA grade by means of logistic regression models.

A total of 547 patients (602 knees) were assessed at a mean follow-up of 3.5 years (SD 1.4). A total of 293 cases were graded as Ahlbäck 1 (Group A), 309 knees were graded as Ahlbäck>1 (Group B). Statistically significant difference was detected in mean FJS-12 (p<0.001), but not in the postoperative satisfaction level (p=0.06) between the two groups. Patients in Group B had a significantly higher probability of attaining a “forgotten knee” after the operation, compared to Group A (p<0.05). Cases in Group A had a significantly lower probability of achieving the PASS (p<0.01).

Patients with higher grades of OA (Ahlbäck>1), were more likely to attain a “forgotten knee”, while patients with less severe OA (Ahlbäck 1) were less likely to achieve the PASS after RA-UKA. Although patients with less severe OA reported fairly good outcome, cases in which the results will be poorer are currently difficult to predict.