The aim of this feasibility trial is to investigate the feasibility of a trial intervention for a population of patients after total knee replacement for osteoarthritis. This study follows a hybrid type 1 design where the primary focus is on the feasibility of the intervention, and the secondary focus is on gaining a better understanding of context and acceptability.

The main questions it aims to answer are:

• Recruitment (Process/Resources): Are patients willing to engage, and stay, in the trial (estimated by inclusion-rate, participant retention, etc.) and what reasons do patients give for not wishing to enroll or later dropping out of the trial? (inquired face-to-face, during enrolment).
• Harms (Scientific): Does the non-exercise intervention appear "safe" (i.e. not harmful) for the patients? (estimated by for instance: adverse events, reasons for dropouts and sense of security).
• Trial procedure feasibility (Management/Scientific): How well does recruitment and trial procedures work at trial sites? (estimated through feedback from site personnel).
• Participant (patient) experienced acceptability of their assigned intervention: An interview-based follow-up using the Theoretical Framework of Acceptability.

Participants will be randomized to one of two interventions, at discharge from the hospital, following knee replacement, which are being tested for feasibility:

1. "Usual-care" - referral to municipal (free-of-charge) physiotherapy (commonly 6-8 weeks of therapeutic exercise).
2. "Return to everyday life" - no referral to municipal physiotherapy.

Participants in both groups are encouraged to follow WHO guidelines of physical activity, to the degree that their post-surgical symptoms allow (within their orthopedic surgeon's recommendations/limitations). All participants are given a "symptom guide", meaning a folder containing information on what to expect, and when and what to react to, during their recovery after the knee-replacement.

For clarity it should be mentioned, that the quantitative and qualitative data-outputs will be reported separately for improved clarity (the study is not a mixed methods design).

Feasibility-criteria (Handling of feasibility outcomes: primary and secondary):

As the feasibility-trial investigates a combination of randomization acceptance, trial procedures, and intervention organization, a primary outcome measure is not specified. The outcome measures planned in the large scale effectiveness trial will be applied, but the outcomes will not be used to assess efficacy, as the trial is not powered for these outcomes. They will be used to indicate efficiency of trial procedures and safety, and to gain information on patients' willingness to enroll and remain in the trial.

Feasibility outcomes will be assessed when all 24 feasibility participants have completed their 12-week follow-up outcome-assessment. An overall assessment of large-scale trial feasibility will be made in the main investigator group, by comparing feasibility research questions with feasibility criteria/outcomes. As there are many components within the feasibility criteria, a summary of findings will be produced. This is used to inform a discussion (main investigator group) to find consensus regarding a final decision on whether a large-scale trial is considered feasible (see Criterion interpretation below).

The feasibility components are organized as related to process, resources, management and scientific, as proposed by Thabane (Thabane et al., 2010). The recruitment (process and resources) is considered the primary feasibility outcome and thusly has specified feasibility criteria.

Criterion interpretation:

It should be noted that feasibility criteria are set to inform a basis for discussion of the trial feasibility in the main investigator group, and the final interpretation is based on the discussion (mentioned above) of which criteria succeeded, which failed, and to which degree. Thusly, a single criterion exactly exceeding or staying just below the criteria set is still subject to discussion and may not singularly mean that the large-scale trial is not feasible (i.e., having only 84% of participants complete the primary outcome does not automatically cancel the large-scale trial, nor does a 85% primary outcome completion rate make the criteria exempt from a discussion regarding feasibility). The trial group discussion aims to result in one of the following interpretations (with elaboration where needed):

1) main study not feasible - stop main study, 2) feasible with modification - continue main study with modifications, 3) feasible without modification - no modifications but close monitoring, 4) feasible as is - continue without modifications.

Inclusion Criteria:

• Over 18 years of age
• Residing in one of fourteen collaboration municipalities
• Referral to primary total knee arthroplasty for osteoarthritis

Exclusion Criteria:

• Insufficient danish skills (read/write/speak) to understand questionnaires and outcomes
• Inability to provide informed consent
• Severe surgical sequelae (i.e. requiring revision surgery, joint mobilization under anaestesia, or joint infection)

There is no plan to share IPD.

Investigator and Outcomes Assessor are blinded to participant group allocation. Further, the personnel administering the intervention after discharge from the surgical ward (Care Provider) is masked.