## 1 Introduction  
Pelvic implants require complex geometrical shapes to reconstruct unusual areas of bone defects, as well as mechanical strength to endure high compressive loads [1]. For limb salvage procedures, custom pelvic prostheses have been utilized for reconstruction after the resection of bone tumors [1, 2]. Custom pelvic prostheses have been fabricated by machining a solid titanium block, and this requires intensive labor and a long fabrication time [2]. Electron-beam melting (EBM), a type of 3D printing technology, can directly fabricate complex metallic structures with excellent material properties (almost no porosity) [3] and shapes, on the basis of digital models [4]. Thus, the EBM has been utilized to fabricate orthopedic components such as knee, hip, and jaw replacements, and maxillofacial plates [5; 6; 7; 8].  
For custom pelvic implants, fixation design is important, considering that the fixation secures the pelvic implants to the remaining bones, and should bear substantial loads as well [9]. A double column plating or a single column plating combined with lag screws has been commonly utilized to fix pelvic bones. The double column plating is effective to fix the complex fractures of pelvic bones [10]. However, the double column plating requires more screws for penetration during surgery, and this often results in a serious traumatic complication [11; 12] that can lead to the development of osteoarthritis [11]. The single column plating combined with lag screws was regarded as the preferred technique, as it produces minimal exposure and devascularization of the pelvis [13], with comparable stability to the double column plating [13; 14]. Therefore, in pelvic implants, two or three straight column platings combined with lag screws have been conventionally utilized for fixation [15; 16]. These fixation types reportedly incur high traumatic fracture risks under compressive loading conditions [16; 17]. Enhancements of the mechanical stability using spinal rod connectors, which have a bridge component between two straight connectors, have been reported in spine surgery publications [18; 19]. Thus far, this bridge component design has not been utilized for pelvic implant fixation. We designed a horseshoe-shaped plate fixation with a bridge component between two straight plates, in an effort to enhance the mechanical stability of pelvic implants. Several studies have evaluated the strength of a conventional straight-type plate fixation for the posterior wall or transverse acetabular fractures [20; 21; 22; 23]. In pelvic implants, there have been no studies comparing the biomechanical stabilities of the new horseshoe-shaped plate fixation with the conventional straight-type plate fixation consisting of column plates and lag screws.  
The aim of this study is to investigate the biomechanics of the horseshoe-shaped plate fixation in a 3D-printed pelvic implant, using a finite element (FE) simulation. First, computed tomography (CT) scans were acquired from a patient suffering from periacetabular bone tumors. Second, 3D FE implant models, in this case a conventional straight-type plate model and the horseshoe-shaped plate model, were created using the patient's Digital Imaging and Communications in Medicine (DICOM) data. Third, a FE simulation was performed, and the stress distribution between the straight-type plate and the horseshoe-shaped plate models was compared.