PSI - Issue 42

Guilherme Opinião et al. / Procedia Structural Integrity 42 (2022) 1266–1273 Guilherme Opinião / Structural Integrity Procedia 00 (2022) 000 – 000

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Another study by Weiler et al. [12] analyzed six bioabsorbable interference screws with different geometries and materials and compared them with a titanium interference screw. This study also confirms that the drive geometry influences the torque resistance of the screws and their failure modes. The use of interference screws made of bioabsorbable materials has brought some problems related to the degradation process of these materials inside the human body. Among the most common polymers that have been used to produce bioabsorbable interference screws, PLLA (Poly-L-Lactic Acid), PGA, and their copolymers are the most common [7]. When in contact with body fluids and human tissue, some polymers initiate the degradation process releasing acidic products that can cause inflammatory reactions, cyst formation, and tunnel widening, which can result in graft laxity and failure [13], [14]. To deal with these issues, bio-composites have been developed enabling, among other, an improved replacement ability of the screw by bone. To produce these biocomposites, bioceramics like hydroxyapatite (HA) or calcium phosphates have been used to prevent the release of acidic products and their related problems [15]. In 2008, Hunt and Callaghan [16] demonstrated that a composite bioabsorbable screw made of PLLA/HA significantly increased new bone formation and decreased inflammatory reactions when compared with a PLLA screw. Johnston et al. [17] carried out a 5-year follow-up of 65 patients undergoing ACL reconstruction by biocomposite PLLA/HA interference screws and the results of computed tomography analysis revealed no tunnel widening, cysts, or inflammatory changes. 2.1. Additively manufactured bioabsorbable screws Additive Manufacturing (AM) technologies allow the production of highly complex geometric systems in a layer by-layer manner, being able to use a wide variety of materials such as metal, polymers, ceramics, and even living cells [18]. Regarding medical-related devices, the most relevant technologies are Fusion Filament Fabrication (FFF), Inkjet Printing (IJP), Selective Laser Sintering (SLS), Electron Beam Melting (EBM) and Selective Laser Melting (SLM), being FFF, IJP, and SLS among the most used [19], [20]. In FFF an extrusion head system is attached to a carriage, where a thermoplastic material is heated to a semi-molten state and extruded in a raster configuration. FFF can create porous structures where the porosity can be manipulated, and the material chosen, to ensure that an implant has the necessary biomechanical properties such as promoting bone cell growth [19]. Fused filament fabricated screws can be designed to have predictable degradation phenomena, according to each patient ’ s needs [21], [22]. Furthermore, screws can be composed of intricate cellular structures or have tailored structural cores, which could optimize the amount of material used, its degradation process, and their mechanical resistance. By controlling printing parameters such as infill density, print speed, layer height, and build direction, it is possible to control design parameters such as the porosity, and biological properties of the structure [19], [23]. PLA (Poly-Lactic Acid) is the most used raw material in FFF, partially due to its biodegradability and environmentally friendly properties [24]. Using FFF, Dhandapani et al. produced PLA cortical screws concluding that osteointegration is promoted using a porous structure [25]. A study performed by Tappa et al. evaluated the influence of infill density and printing orientation on properties such as hardness, stiffness, yield stress, wearability, and degradation time [26]. Thus, with AM it is possible to combine different printing conditions, porosity, pore size, and biomaterials to produce a medical implant with mechanical and biological properties that will allow the implant to fulfill its function, such as degrading predictability over time. 3. Design and development Various interference screw models currently available adopt different geometries. Since several standards must be met, screws with different lengths, diameters, and thread geometry can be found. Concerning the current study interference screw design, authors focused on implants with 25 mm length and 8 mm in diameter. The 3D modelling of the screws was performed using Solidworks® 2020 software.