# Cranial Implant Design Applying Shape-Based Interpolation Method via Open-Source Software
## 1 Introduction  
The three-dimensional (3D) reconstruction of the craniofacial region is a challenge if involving a very large defect, and it requires the use of sophisticated proprietary image processing and expensive computer-aided design (CAD) software. Previously, the surgical procedure for managing large defects of the skull was complicated as it must be performed manually based on two-dimensional (2D) imaging, namely the shaping, modelling, and placement of the implant, which is made of bone grafts, bone cement, or titanium meshes. Using this conventional method resulted in long and complex operations with poor aesthetic results. The manual process is very labour-intensive and expensive [1]. With the advance in computer and additive manufacturing (AM) technology, an implant that exactly fits the defect can be manufactured pre-operatively from the radiographic data obtained from CT scans. Studies reported the advantages of using several different computer-aided designs and computer-aided manufacturing (CAD/CAM) platforms [2], which resulted in perfectly fitted implants, less surgery time, and better aesthetic results [3]. Functional and aesthetically placed patient-specific cranial implants are extremely important for patients with largecranial defects. Therefore, pre-operative fabrication of the implants is recommended [4] to ensure minimal adjustment during surgery, which would then translate to lower surgical cost and time, as the implants would fit nicely into the defect. Rapid developments in medical imaging and advances in CAD/CAM improved the quality of implants, resulted in improved aesthetic outcomes, and minimised operation time, blood loss, and risk of infection [5; 6]. Patient-specific implants can be produced in any size with an accurate fit using this technology [7]. The creation of the cranial implant with optimal size, shape, and mechanical properties prior to the surgical procedure reduces the operation time and complexity [8]. The main advantage of using CAD/CAM is a better outcome and aesthetic of the implant; therefore, it can be successfully used in the repair of a defect [9]. Using CAD software enables the users to automatically check if the design is within specification. It also enables users to view designs at an earlier stage in the design process. However, CAD software often consumes large amounts of computer processing power. This requires high-quality computer hardware that can be costly, on top of the price of the CAD software [10]. The cost of hardware and software is a significant disadvantage of the CAD/CAM and a major barrier to adopting this technology, particularly for institutions with a limited budget. Another disadvantage of the CAD/CAM technology is the complexity of the software. As the CAD software advances, it becomes more flexible and adaptable and could have much more functions. However, this comes at the cost of making the software more complex. This complexity makes it more difficult for first-time users to master the software. Combined with the cost of training personnel in CAD/CAM technologies, this complexity represents another disadvantage of CAD/CAM. Another technique is the mirror reconstruction method, by replicating the missing bone segment using mirrored healthy skull on the other side where the skull was assumed to be symmetrical along its mid-sagittal plane. The \"Boolean\" operation, which refers to subtracting the mirrored bone from the combined healthy and defected portions of the skull, was then used to cut and obtain the cranial implant that fits perfectly into the defect. The advantage of the mirror image reconstruction method is that it generates the customised cranial implant with a thickness exactly to the thickness of the skull bone, which was about 2.5 mm [11]. [PERSON] et al. [12] used commercial AnalyzeTM software to perform the mirror-imaging function to create cranial implants by using the intact contralateral anatomy as a template for patients with traumatic injuries secondary to MVA.