This study is a nationwide cross-sectional survey on pain and satisfaction after knee arthroplasty (KA). Please find the full protocol uploaded at ClinicalTrials.gov

The goal of this observational study is to study pain and satisfaction in patients operated with knee arthroplasty. The main question it aims to answer are:

• What is the incidence of persistent postsurgical pain in unselected knee arthroplasty patients
• What is the level of satisfaction in unselected knee arthroplasty patients?

Participants will be asked to answer a 22-item questionnaire.

The study is registered at the Capitol region of Denmark's regional research listing (Pactius) with identifier P-2023-4, approved 4 January 2023. The resulting paper will be reported according to the CROSS checklist for standardised reporting of survey studies. Wherever needed, a trained linguist translated questions from English to Danish and vice versa. The Danish and an English version of the questionnaire will be available in the final publication.

• Study design: This study is a nationwide cross-sectional survey.
• Patient involvement: A panel of KA patients helped develop the questionnaire, i.e., testing and selecting the questions.
• Questionnaire: The questionnaire is composed of 22 questions (please see the uploaded protocol), however patients answering "No" to question 3 skips questions 4-17 concerning pain in the operated knee. Only question 3, 4, 20, and 21 are mandatory as to avoid non-respondents. The full questionnaire translated to English can be found in the supplementary material in the protocol. Contact information for the first author (JL) is supplied in the contact letter in case the patients have difficulties with filling in the questionnaire, have questions, or believe they have been selected mistakenly. Similar or identical questions to those in a survey published in 2006 to increase the ability to compare results (i.e., questions 3-5, 8, 10 and 18)
• Sample characteristics: Eligible patients are all adult (18 years or older) patients operated with primary total or medial unicompartmental KA for osteoarthritis between August 1 and November 30, 2023. Legally incompetent citizens, i.e., persons with a legal guardian, will not be asked to participate. For total KA, patients are identified from the Danish National Patient Register through the Danish Health Data Authority (SKS-code DM17 [knee osteoarthritis] + KNGB20, KNGB30 or KNGB40 [primary total knee arthroplasty]). For medial unicompartmental KA, patients are identified from the Danish National Patient Register through the Danish Health Data Authority (SKS-code DM17 [knee osteoarthritis] + KNGB01 or KNGB11 [primary medial unicompartmental knee arthroplasty]).
• Sample size calculation: The sample size calculation is based on the primary outcome in total KA patients. With an estimated 16% incidence of the primary outcome3 and a 70% response rate, 3172 patients are needed to yield a 95% confidence interval of 3 percentage points (14.6-17.6). In our opinion, this level of certainty is appropriate because differences less than this, may be hard to interpret by clinicians and patients. Sampling patients that were operated during 4 months outside holiday season should yield approximately this number of identified patients
• Ethical considerations: The local institutional review board approved the study, and the Danish Health Data Authority will provide contact information for potential respondents. Telephone numbers for non-respondents are found by searching the CPR number in the electronic patient files, but without accessing the patients' health data. According to Danish legislation, approval from the national ethics committee is neither required nor possible to obtain for survey studies.

Inclusion Criteria:

- Patients operated with primary knee arthroplasty for osteoarthritis 12-15 months before survey distribution

Exclusion Criteria:

- Legally incompetent citizens, i.e., persons with a legal guardian, will not be asked to participate.

For total KA, patients are identified from the Danish National Patient Register through the Danish Health Data Authority (SKS-code DM17 [knee osteoarthritis] + KNGB20, KNGB30 or KNGB40 [primary total knee arthroplasty]).

For medial unicompartmental KA, patients are identified from the Danish National Patient Register through the Danish Health Data Authority (SKS-code DM17 [knee osteoarthritis] + KNGB01 or KNGB11 [primary medial unicompartmental knee arthroplasty]).

The full dataset will be stored at the Danish national archive (www.rigsarkivet.dk) if possible