# Mechanical Strength Study of a Cranial Implant Using Computational Tools
## 1 Introduction  
Cranioplasty is a medical-surgical procedure that aims to fill skull defects provoked by a craniectomy or skull fracture, recovering the skull's protective function and also bringing aesthetic satisfaction to patients. Craniomy is a surgical procedure where a part of the skull is removed to reduce pressure on the brain resulting from swelling or bleeding. Then, the surgeon fills the gap in the head using autologous bone or alloplastic material. The human skull is a vital part of the human body, mainly because it has the task of protecting the brain, an essential organ of the human body. It is possible to divide the skull into two parts: the braincase (neurocranium) and the facial skeleton (viscerocranium) [1]. Skullbone is composed of both cortical bone and trabecular bone. Cortical bone is treated as a compact bone that acts as the outer layer of the skeleton, surrounding the trabecular bone and providing a hard covering for the skeleton [2]. On the other hand, trabecular bone is a sponge-like tissue configured in a lattice consistency situated in the core of bones [2]. Additionally, the skull contains cranial sutures, which act as connectors of the skull bones and are composed mainly of collagen [3]. Since the brain is a susceptible organ and needs the protection of all parts that surrounds it against impact loads, a good indicator of the effectiveness of external components, such as a cranial implant, is their capacity to protect the brain against severe injuries. Traumatic brain injury (TBI) can beclassified as a combination of two parameters: deformation processes due to brain motion and indentation processes on the skull (or implant) [4]. Nowadays, evolution in Computer-Aided Design (CAD) and Additive Manufacturing (AM) has led to an increase in patient-specific cranial implant production in craniplasty, which offers multiple advantages including good definition on the implant contour, high precision of curvature, reduced risk of surgical complications, and improved aesthetic results [5]. Four prominent materials can be used in craniplasty: allografts or autografts, metal, polymers, and ceramic [6]. Furthermore, three materials have emerged as the most applied ones: titanium, poly(methyl methacrylate) (PMMA), and poly-ether-ether ketone (PEEK). In fact, PEEK is being widely used these, days since its mechanical properties, mainly the elastic modulus, are identical to bone's, which helps prevent the stress shielding effect [7]. Moreover, it has many advantages such as high thermal stability, toughness, and rigidity; the facility to process, self-lubrication, abrasion resistance, and compatibility with computed tomography (CT) and magnetic resonance imaging (MRI) [7; 8]. For these reasons, besides cranial implants, many applications of PEEK related to prosthodontics have also been reported [9; 10; 11]. As a crucial step for the implant design, computer-aided design for this type of components can follow templates, such as an average skull or a left-right mirrored image (in the case of a one-sided defect) [6]. When incorporating tissue engineering during implant development, CAD techniques should be employed to obtain the scaffold structure during the design process [12; 13; 14; 15]. Otherwise, when creating a PEEK implant with no porosity, there is the necessity to incorporate small holes to help plural tack-up sutures and muscle suspension [16]. The technologies used to fabricate cranial implants by additive manufacture depend on the material family. In the case of synthetic polymers, since working with PMMA is a challenging task, PEEK obtained via SLS has been exploited by every company in this area [6]. One advantage of PEEK is that the design of implants can be modified to create material properties similar to the adjacent skull, making PEEK cranial implants more suitable than titanium ones [6]. The surgeon frequently chooses the fixation methods in craniplasty based on experience.