# Mechanical Strength Study of a Cranial Implant Using Computational Tools
## 1 Introduction
There are many fixation devices, with the most employed ones being titanium mini-plates, titanium tangential screws, and titanium clamps. The chosen fixation method, as well as the number of fixation points, are variables that play a massive role in avoiding the displacement of the implant. Despite leading to higher strain in the interface between bone and implant, tangential screws offer a more rigid fixation and, since they are directed tangentially into the bone, this method is entirely impalpable [5]. Additionally, there is a lower probability of implant displacement occurring when this is the employed method, as shown in related studies [17]. Different strategies, software, and methodologies have been employed to validate the developed models of cranial implants. These are the most critical steps of the mechanical resistance study, and all the assumptions made have significant impacts on the approximation made by the finite element analysis (FEA). [PERSON] et al. [5] used Abaqus to run an FE analysis on three different models with similar strategies adopted: a PEEK cranial implant fixed with tangential screws, and two ceramic-titanium (CeTi) cranial implants fixed with tangential and axial screws. The complete head model was not used. Instead, to reduce the computational cost, only the part of the skull surrounding the defect was taken into account. All material properties were considered to be linear elastic and the contact between the different parts was assumed to occur only at the screw holes, with complete fixation assumed. Two static loads were applied to the model: one being an impact load due to a fall off a bicycle, applied as a pressure perpendicular to a circular area, and the other one being an internal pressure. Results showed that PEEK offers insufficient brain protection due to its high flexibility and local peak stresses at the bone-screw interface. [PERSON] et al. [12] executed FE analysis using the software ANSYS to study the biomechanical behavior of titanium mesh cranial implants with different thicknesses, pore structures, and surface characteristics, with all material properties treated as linear elastic, isotropic, and homogeneous. The skull was fixed in the bottom, a static load of 50 N was placed in the center of the implant, and the stress of the screws was neglected. Finally, results of the analysis showed that the stress distribution and peak von Mises stress of the implants decreases at the thickness of 1 mm, and implants with a circular pore pattern result in a lower von Mises stress on the bone defect area when compared with triangular and square pore patterns. To compare the mechanical impact response of PEEK and hydroxyapatite (HA), a study was conducted by [PERSON] et al. [15] performing explicit analysis using the software Abaqus (Dassault Systemes, Velizy-Villacoublay, France). A complex finite element head model (FEHM) was used containing scalp, skull, cerebral falk, CSF, and brain tissues, and constitutive models for different parts of the human head were employed based on a selection from the literature. In contrast, for PEEK, a constitutive model previously developed by the authors was adopted. In the numerical model, the contact was modeled with a penalty contact algorithm with a friction coefficient of 0.4. Since the model does not include screws, a parallel analysis was developed to capture the stress concentration introduced by the screws, which was posteriorly applied. A set of numerical simulations were carried out studying the mechanical response of the FEHM for an impact velocity varying from 1 m/s to 7 m/s, including three different scenarios: an FEHM including a PEEK implant, an FEHM including a HA implant, and an FEHM including no defect. The results were then analyzed in three categories: acceleration-time predictions, critical impact velocity predictions, and TBI prediction. Results proved that a PEEK implant could be a much better choice in a demanding mechanical lifestyle, even though HA has better bio-integration properties.