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

The study is registered at the Capitol region of Denmark's regional research listing (Pactius) with identifier P-2022-933, approved 23 December 2022. 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 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 hip. 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 13)
• Sample characteristics: Eligible patients are all adult (18 years or older) patients operated with primary THA for osteoarthritis between 1 March and May 31, 2022. Legally incompetent citizens, i.e., persons with a legal guardian, will not be asked to participate. Patients will be identified from the Danish National Patient Register through the Danish Health Data Authority (SKS-code DM16 [hip osteoarthritis] + KNFB [Primary Total Hip Arthroplasty]). Baseline data of respondents and non-respondents are gathered from the Danish Hip Arthroplasty Register. To evaluate the generalisability of the study, respondents' baseline data will be compared to the non-respondents baseline data in the paper's Table 1.
• Sample size calculation: With an estimated 12% incidence of the primary outcome (patients with moderate or severe pain at rest and/or while walking) and a 70% response rate, 2577 patients are needed to yield a 95% confidence interval of 3 percentage points (10.5-13.5). 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 3 months outside holiday season should yield approximately this number of identified patients.
• Ethical considerations: This study was approved by the local institutional review board and contact information are provided by the Danish Health Data Authority. 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 (see attached 'exempt from notification' letter from the Danish national ethics committee).

Inclusion Criteria:

• Patients operated with primary THA 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.

Patients are identifed from the Danish National Patient Register through the Danish Health Data Authority (SKS-code DM16 [hip osteoarthritis] + KNFB [Primary Total Hip Arthroplasty]). Baseline data of respondents and non-respondents are gathered from the Danish Hip Arthroplasty Register.

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