The pilot study was designed as a prospective, nonrandomized trial for the treatment of displaced fractures of the medial malleolus with a lean, REE-free, Mg-based biodegradable implant in the adult skeleton. The trial was conducted according to the Good Clinical Practice (ISO 14155:2011) standard and the Declaration of Helsinki.The study was performed at the Department of Orthopaedics and Trauma at the Medical University of Graz and approved by the ethics committee (28 - 071 ex 15/16). A total of 20 subjects were recruited and operated on by three experienced surgeons in the centre between July 2018 and October 2019. Inclusion focused on adults aged between 18 and 65 years presenting with a displaced isolated medial malleolus fracture, a bimalleolar ankle fracture, or a trimalleolar ankle fracture. Fracture displacement was defined as diastasis of the fracture in any direction of 2 mm or more. Exclusion criteria were pathological fractures, underlying diseases (particularly bone diseases, kidney diseases, diabetes mellitus), polytraumatized patients, and pregnant or breastfeeding women.Written informed consent was obtained from each patient before the surgery. All radiological data were stored in a picture archiving and communication system (PACS). Fractures were classified according to the Herscovici system.25

Description of the bioresorbable device used: Ultra-high pure Mg (99.999%) was alloyed with Zn and Ca (0.45 wt% Zn and 0.45 wt% Ca) at 750°C under a protective gas atmosphere. Prepared screws had a length of 40 mm and a diameter of 3.5 mm. Implants were threaded at the distal part for use as traction screws.

Surgery: All patients were operated in a supine position. In case of a bimalleolar or trimalleolar fracture, the fibular and the dorsal tibial fragments were treated before the medial malleolus. Subsequently, two parallel Kirschner wires perpendicular to the medial malleolar fracture line were positioned with fluoroscopic control. A cannulated drill bit with 2.7 mm width was used for preparing the hole. Subsequently, one wire was removed and the ZX00 compression screw was inserted into the hole. Final fixation of the fracture was achieved with the second bioabsorbable Mg screw after removal of the second wire. The insertion torque of the screws was limited at a force of 1.5 newton metre with a torque handle. In all patients, no additional implants other than the Mg screws were used for the fixation of the medial malleolus. Patients with bimalleolar fractures and trimalleolar fractures were fixed with titanium plates and screws. Moreover, patients were immobilized with an under-knee plaster cast four to six weeks postoperatively and encouraged to attempt full weight-bearing as tolerated. Ankle movement exercises were started immediately after removal of the plaster.

Assessments and measurements: For each subject, the study consisted of assessments conducted preoperatively, immediately after operation and postoperative followup visits. During follow-up, complications including vital signs, erythema, swelling, pain, secretions, wound healing disorders, wound infections, or implant infections were recorded. Pain was assessed using visual analogue scale (VAS). Blood analysis and detection of Ca and Mg were recorded. Functional outcomes were evaluated through range of motion (ROM) in dorsal and plantar flexion. Additionally, the American Orthopaedic Foot and Ankle Society (AOFAS) score was performed after three months. Anteroposterior and lateral ankle radiographs (X-rays) were used to evaluate the fracture union and loss of reduction during follow-up. Postoperative radiograph evaluation was performed by two independent radiologists.

The study was registered retrospectively on Clinicaltrials.gov.