Effect of a COntralateral corticoSTeroid Injection in total knee arthroplasty (COSTI) on postoperative patient reported outcomes: a randomised trial protocol.

introductionKnee osteoarthritis (OA) is a prevalent and debilitating condition, with nearly 50% of patients experiencing bilateral symptoms. Persistent contralateral knee pain after unilateral total knee arthroplasty (TKA) is a common contributor to suboptimal functional recovery and patient dissatisfaction. An intra-articular corticosteroid injection is a safe and widely used non-operative treatment for OA, but its perioperative utility in the contralateral (non-operative) knee to improve TKA recovery (operative side) remains underexplored. The METHODS AND ANALYSIS: The COSTI trial is a multi-centre, triple-blinded, randomised controlled superiority trial. An embedded pilot of 60 patients will first be used to assess the feasibility of the definitive trial. To preserve the integrity of the definitive trial, the pilot phase will focus on feasibility outcomes rather than treatment efficacy and no formal between-group analyses of clinical outcomes will be conducted prior to the progression decision. Treatment allocation will remain concealed from investigators and the study statistician throughout the pilot phase. The preliminary planned sample of the definitive trial will consist of 154 patients undergoing unilateral TKA for primary OA with symptomatic contralateral knee OA. Patients will be randomised 1:1 to receive either an ultrasound-guided contralateral intra-articular corticosteroid injection (80 mg methylprednisolone+0.5% bupivacaine/10 cc volume) or placebo (band-aid application) following the administration of spinal regional anaesthesia on the day of surgery. The primary outcome is change in Oxford Knee Score (OKS) from baseline to 4, 8 and 12 weeks and 1 year postoperatively. Secondary outcomes include contralateral knee pain (Visual Analogue Scale), EuroQol five-dimensional descriptive system, Timed Up and Go performance, range of motion, complications, readmissions and adverse events. The study is powered to detect a 5-point between-group difference in OKS with 90% power. ETHICS AND DISSEMINATION: This trial has received approval from the Unity Health Toronto Research Ethics Board (Protocol #24-158) and is registered at ClinicalTrials.gov (NCT06809998). All participants will provide written informed consent. Findings will be disseminated through peer-reviewed publications and academic conferences, and pseudonymised data will be made available upon reasonable request. TRIAL REGISTRATION NUMBER: NCT06809998.