# Implant Model Generation Method for Mandibular Defect Based on Improved 3D Unet
## 1 Introduction  
Owing to trauma, congenital defects, plastic surgery, or the resection of diseased parts, some areas of the mandible can become damaged, which affects not only the appearance of the patient but also important physiological functions, such as chewing [1]. Oral maxillofacial trauma accounts from 7.4 to 8.7% of medical emergencies, both in developed and developing countries [2]. To restore the anatomical shape and aesthetic effect of the mandible as much as possible, mandibular defect repair requires the consistency of shape based on the original shape and characteristic parameters of the mandible [3]. In related studies of bone defects, a \"critically sized\" defect was considered as one that would not heal spontaneously and which requires surgical treatment. The definition of a \"critically sized\" defect includes a defect length greater than 1-2 cm and a greater than 50% loss of the circumference of the bone [4]. During the surgery, the implant needs to match the defect position as much as possible and conform to the defect surface to restore the morphology and function of the mandible. Currently, there are two commonly used methods for repairing and reconstructing defective mandbles, i.e., autologous bone grafts and personalized 3D implants for implant repair [5]. The advantage of autologous bone grafting is immunocompatibility, accessibility, abundance, and low cost. Currently, the most used donor sites for mandibular reconstruction include the forearm radius, scapula, ilium, and fibula [6]. The vascularized fibula flap transplantation method has advantages that other autologous bone graft repair and reconstruction methods do not have, such as a sufficient bone volume in the bone donorarea, low mobility of the bone donor area, a rich blood supply, and a high survival rate of the graft. [7]. Restoring the geometric shape of the mandible with reconstructed bone is difficult because the shape, size, and volume of autogenous bone are considerably different from those of the mandibular defect, which leads to difficulties in overlaying, fixed denture restoration, and dental implant restoration. In recent years, personalized 3D implants, as a new type of bionic repair method, can not only achieve functional repair but also accurately match the shape of the human mandible, which has become a new development direction for repairing mandibular defects. The process for realizing personalized 3D implants is as follows: First, the patient's mandibular bone information is obtained using computerized tomography (CT) [8]. Then, using the scanned information as a reference, a mandibular implant model with a personalized shape and complex internal structure is designed with the help of computer-aided design technology. Finally, through additive manufacturing (3D printing) technology, mandibular implants that meet the surgical requirements are manufactured. This method can effectively restore the patient's personalized appearance and chewing function [9; 10]. However, designing the desired shape and structure of a mandibular implant is challenging. At present, the usual method of bone defect implant design in surgery is based on the symmetry or bone shape before the defect [11; 12]. However, the pre-defective shape of the mandible is hardly obtainable. Therefore, it is difficult to completely match the geometric model of the implant obtained by these methods with the shape of the actual defect. The 3D modeling and design of implants are always done under the guidance of orthopedic surgeons, which takes a considerable amount of time and effort, and this also limits their applicability. For defects with relatively complex structures, simplification is usually carried out to reduce the difficulty of design and manufacture. However, this can make it difficult for the shape of the implant to precisely match the shape of the defect, leading to design failure [13]. 3D shape completion [14] has been a research hotspot in recent years, aiming to restore the shape and structure of the defect part of the 3D model.