Medial Displacement Calcaneal Osteotomy and FDL- Transfer - With a Human, Allogeneic Cortical Bone Screw

The goal of this observational study is to compare the use of a screw made of human bone (Shark-Screw®, Surgebright-GmbH) with the metal/Bio-Tenodesis screw (Arthrex) in the treatment of the symptomatic flatfoot using the medializing calcaneus osteotomy with flexor digitorum longus transfer (FDL) in adult patients. The advantage of the human bone screw is that after surgery no hardware removal is necessary. The screw is transformed from the body to normal bone. The main questions it aims to answer are: * Can the human bone screw achieve union rates like the metal/Bio-Tenodesis screw? * Is the time to union similar between the different screws? * Is the complication rate similar between the different screws? * Are the activity scores American Orthopaedic Foot and Ankle Society (AOFAS), Foot and Ankle Outcome Score (FAOS) and Foot Function Index (FFI) after surgery similar in the compared patient groups? Participants will have * the surgery * follow-ups at 6 weeks, 6 months, 1 and 2 years. * X-rays are performed at each follow up. * CT-scans are performed after 6 months. * activity scores are collected at the follow up after 6 months, 1 year and 2 years.