Background <p>Rehabilitation models following total knee arthroplasty (TKA) vary internationally. The German healthcare system typically involves an initial hospital stay of 8–10 days, followed by a 3-week intensive rehabilitation program, which is delivered in either an inpatient or outpatient setting. Only a few studies have evaluated the outcomes of these two models on functional recovery in TKA patients. Therefore, this study compared the effectiveness of inpatient versus outpatient rehabilitation on clinical and functional outcomes in a German TKA cohort.</p> Methods <p>This multicenter, prospective, non-randomized, active-controlled, investigator- and outcome assessor-blinded clinical trial allocated patients with primary TKA based on their preference to either a 3-week inpatient (<i>n</i> = 26) or outpatient (<i>n</i> = 16) rehabilitation program. Measurements were conducted at two time points: pre-test (one day prior to discharge from acute care hospital; pre-rehabilitation) and post-test (three months following TKA). The primary outcome, step count, was measured using 7-day activPAL™ accelerometery during the first week of inpatient and outpatient rehabilitation, respectively. The clinical outcomes included knee pain, swelling, and range of motion (ROM), whereas the functional outcomes included step count, sit-to-stand transitions, timed up and go (TUG) and stair climbing (SC) performance, joint position sense (JPS), and gait performance. Between-group differences were analysed using analysis of covariance, adjusting for baseline values (where available), age, BMI, sex, hospital, and the total number of interventions during rehabilitation.</p> Results <p>At pre-test, the outpatient group had a significantly higher covariate-adjusted step count compared to the inpatient group (<i>P</i> &lt; 0.001; <i>η</i>_<i>p</i>^<i>2</i> = 0.338; +73.8%). At post-test, the outpatient group demonstrated significantly better JPS, with a lower mean absolute error at both 30° (<i>P</i> &lt; 0.001; <i>η</i>_<i>p</i>^<i>2</i> = 0.366; − 64.6%) and 50° (<i>P</i> = 0.010; <i>η</i>_<i>p</i>^<i>2</i> = 0.180; − 47.1%) of knee flexion. The outpatient group also showed superior SC performance, completing the task 5.3 s faster (<i>P</i> = 0.004; <i>η</i>_<i>p</i>^<i>2</i> = 0.218; − 23.7%). No significant between-group differences were found for knee pain, swelling, ROM, TUG performance, and gait performance (post-hoc power <i>β</i> &lt; 0.50, suggesting a high risk of type II error).</p> Conclusions <p>Results of the study indicate that outpatient rehabilitation promoted higher early physical activity levels (step count) and superior functional outcomes (JPS and SC performance). These preliminary findings must be interpreted with caution due to study limitations, including the non-randomized design and a small sample size. </p> Trial registration <p>ClinicalTrials.gov, NCT02120313. Registered 23 April 2014.</p> Level of evidence <p>Level 3 (OCEBM Levels of Evidence Working Group). "The Oxford Levels of Evidence 2". Oxford Centre for Evidence-Based Medicine. http://www.cebm.net/index.aspx?o=5653</p>