Fast-track/rapid recovery/Enhanced Recovery After Surgery (ERAS) programs have proven both safe and effective in joint replacement surgery, to the degree where same-day surgery have been tested in selected cases. A transition of the minor partial knee replacement (PKR) procedure, compared with the alternative and more commonly used total knee replacement (TKR) for knee osteoarthritis, into a same-day regime seems reasonable. Especially as PKR is reported to have lower risk of short-term complications than does TKR.

The aim of this study is to assess efficacy, patient satisfaction and safety outcome measures for PKR when using a same-day surgery protocol in a Swedish healthcare context where ERAS programs nowadays are considered common ground.

With no preselection of patients, all PKR cases by one high-volume surgeon will chronologically be scheduled as the first morning case for one consecutive year, and thereby be included in the study. In order for discharge, strict post-surgery criteria will have to be met.

The thesis is that the same-day surgery will be both feasible and safe.

The eligibility for being considered a medial PKR is either antero-medial osteoarthritis or spontaneous osteonecrosis of the knee. The study-protocol has a pragmatic approach to whom of the PKR candidates will be considered for same-day surgery, as no preselection of patients will be done prior to surgery. I.e., everyone considered for a PKR procedure will be informed prior to surgery by a multidisciplinary team (anesthesiologist, nurse, physiotherapist, and surgeon) that if the surgery will be scheduled in the morning, he/she will then be following a same-day surgery protocol and likely be discharged to home later the same day.

All of one high-volume surgeon´s (MT) first morning slots for one consecutive year will be dedicated to patients who will undergo PKR surgery, and thereby also be included in the study. The slots will be filled continuously as the patients are put up for surgery. When all slots have been taken within one week any additional PKR cases will be assigned to surgery later in the day the same week (as will all the THRs and the TKRs during this period) and consequently not taking part in this study. The operation scheduling will be put together on a weekly basis, in a strict chronological order, by a team of nurses without any insight from the surgeon in question.

The criteria for discharge will be based strictly on postoperative parameters categorized into four dimensions - vital parameters, urinary function, bleeding, and mobilization. The vital parameters will be measured and scored by the National Early Warning Score (NEWS), where the threshold for discharge will be conservatively set to zero (NEWS 0). The urinary function algorithm for discharge will, in short, be if no need for catheterization and less than 200 ml of residual volume after spontaneous void. A compression stocking will be removed six hours post-surgery and if no ongoing bleeding discharge can be considered. Last, but not least, a physiotherapist will evaluate whether basic activity of daily living (ADL) can be performed in a safely manner using crutches.

Primary outcome measures for this three-month follow-up will be safety, i.e. number of adverse events and readmissions. Secondary aims will be feasibility such as same-day surgery achievement and extra outpatient visits needed and for what reasons. Patient-satisfaction will also be evaluated after the participants/patients have ben discharged to their own home. A two-arm subgroup analysis will be conducted between those aged <80 yrs and classified as ASA <3 and those aged ≥80 yrs. and/or classified as ASA class ≥3.

Descriptive data will be presented as unadjusted means with standard deviations (SD), medians with range, or as numbers with proportions (%). The between-group differences will be presented as means with 95% confidence intervals (95% CI) and p values using Welch's test, Mann-Whitney U-test and Likelihood Ratio Test (or when violated Fisher's exact test) for numerical, ordinal and categorical variables respectively. Multivariate analyses will be conducted using logistic regression.

Inclusion Criteria:

• All participants/patients that will undergo a partial knee replacement (PKR) procedure within one consecutive year by one high-volume surgeon (i.e. no preselection of patients will be made)

Exclusion Criteria:

• PKR procedures performed by other surgeons
• One-stage bilateral PKR procedures
• PKR procedures not given the first surgery slot of the day