Effect of robotic arm-assisted bi-unicompartmental knee arthroplasty versus total knee arthroplasty on objectively measured physical activity: secondary outcomes of a randomized controlled trial.

AIMS: Bi-unicompartmental knee arthroplasty (Bi-UKA) is a bone-conserving, cruciate-sparing alternative to total knee arthroplasty (TKA). Patient-reported outcome measures (PROMs) do not differ between procedures. Conflicting data exist on gait preservation, and little is known about physical activity (PA) outcomes. PA is understudied in arthroplasty, despite its links with all-cause mortality. We compared PA volume and intensity in patients randomized to Bi-UKA or TKA. METHODS: We analyzed 38 TKA and 31 Bi-UKA patients from a prospective randomized controlled trial. A triaxial accelerometer was worn on the surgical thigh for seven days, with a three-day minimum wear criterion. Data were collected preoperatively, and at one and two years postoperatively. Outcomes were PA energy expenditure (PAEE), moderate-to-vigorous intensity PA (MVPA), light-intensity PA (LPA), and sedentary time (ST), plus proportional PAEE by intensity. Group differences were assessed with mixed-effects models including baseline PA and biphasic gait as covariates. Spearman's rank correlation between PROMs and PA was performed. RESULTS: Preoperative characteristics were comparable. Activity levels were higher following surgery, but postoperative differences in PAEE, MVPA, LPA, or ST between groups failed to reach statistical significance. Baseline PAEE (β =+ 0.27, p = 0.037) and MVPA (β =+ 0.25, p = 0.024) were associated with improvements in postoperative activity. Postoperative correlations between PROMs and objective PA were weak across groups, and appeared to diminish over time. CONCLUSION: Although free-living PA improved following Bi-UKA and TKA, differences between groups failed to reach statistical significance. Across both procedures, habitual and behavioural factors appear to be stronger determinants of postoperative PA than surgical procedure, as reflected by the limited correlation between PROMs and objective PA. Larger, long-term randomized studies are needed to confirm these findings.