Background <p>Osteoarthritis (OA) is a joint disease with a high worldwide prevalence, often leading to replacement surgery. However, despite the high success rate of replacement surgery, some patients fail to experience the intended benefits. Given the importance of patient involvement, this trial aims to assess the effect of shared decision-making (SDM) on decisional quality for patients with severe OA. Specifically, it investigates whether an in-consult patient decision aid (PtDA) enhances decisional quality, involvement, and health outcomes for patients with severe hip or knee OA.</p> Methods <p>This protocol outlines a pragmatic two-armed multicentre cluster-randomised controlled trial (C-RCT), chosen for its ability to assess the combined effect of shared decision-making (SDM) and a newly developed in-consult patient decision aid (PtDA) in routine clinical practice. The PtDA is a tool designed to help patients make informed decisions during the consultation by providing personalised treatment options, benefits, and risks. The C-RCT design is particularly well-suited to this context, as it effectively minimises contamination and bias across treatment groups. A total of 19 orthopaedic surgeons will be randomised 1:1, stratified by centre, to either continue standard consultations without the PtDA or implement SDM using the PtDA. The trial will recruit 615 patients with severe hip or knee OA. Outcomes will be measured at one week, three months, and 12&#xa0;months post treatment start. The primary outcome, decisional quality, will be assessed using the Hip/Knee OA Decision Quality Instrument, focusing on patients being well-informed and receiving their preferred treatment. The secondary outcomes include patient involvement in the decision-making process and consultation durations. The tertiary outcomes include patients’ satisfaction, regret and health-related outcomes. The primary outcome will be analysed using a multilevel mixed-effects logistic regression with surgeons as random effects reporting odds ratios with 95% confidence intervals.</p> Discussion <p>This study aims to contribute insights into the use of SDM facilitated by a PtDA and its impact on improving decisional quality for future patients with hip or knee OA.</p> Trial registration <p><a href="http://www.ClinicalTrials.gov">www.ClinicalTrials.gov</a> (NCT05972525), Data of registration: 06.08.2023.</p>