Background: Adult acquired flatfoot deformity (AAFD) is a deformity of the foot which leads to pain and an altered gait pattern. There are several different surgical interventions available for treatment of AAFD. One of the most common treatments for mobile AAFD is medial displacement calcaneal osteotomy (MDCO) with or without adjuvant soft tissue procedures. However, the medial displacement of the calcaneus only aims to correct the deformity in one plane despite AAFD being a deformity of three planes with hypereversion of the calcaneus, anterior translation of the talus over the calcaneus and tilting/drop of the calcaneal head. Because of these multiple forces, the healing of MDCO in the intended position can sometimes be hard to achieve and medial soft tissue is still stressed leading to recurring deformity. The investigators believe that adjuvant insertion of a sinus tarsi implant (STI) could work as an internal splint, protecting soft tissue procedures on the medial side, reversing anterior translation of the talus, opposing calcaneus eversion, and lifting talar head. This additional procedure will create a better correction of the deformity with enhanced appearance of the foot and better long-term functional results.

Methods: This is a multi-center randomized controlled trial designed to assess the efficacy of STI as an adjuvant procedure to MDCO in patients with mobile AAFD (Stage II). Patients aged 16 to 75 years, referred to one of the orthopedic centers involved in this study, will be invited to participate if they fulfil the trials eligibility criteria. In total, 130 patients who provide informed consent will be randomized to either MDCO with STI or MDCO without STI (65 patients in each group). The included patients will be clinically and radiographically examined. They will also fill out a form before surgery and 4-5 months, 1 year and 2 years postoperatively. The form will include Self-Reported Foot and Ankle Score (SEFAS) and Euro-QoL 5 Dimensions (EQ-5D). The trials primary outcome will be change in Meary's angle. Secondary outcomes include additional radiographic changes, change in SEFAS score, EQ-5D index, pain according to visual analog scale (VAS), satisfaction assessment, clinical outcome measures, the length of postoperative sick-leave and rate of complications. Discussion: This is the first randomized controlled trial assessing the efficacy of a sinus tarsi implant as an adjuvant procedure to MDCO.