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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3.1. Design of bioabsorbable screws The design features of the screw will most likely influence its function. The tightening torque required to insert it into the bone tunnel is one of the most critical stages of a screw’s life . Notably, both thread shape and pitch have a significant influence on the contact area between bone and screw, which directly relates to the stress concentration locations and magnitude, during insertion. For this study, 52 models of interference screws with different geometric characteristics were designed. Screws were created with two different drive geometries, hexagonal and Torx plus, and the pitch was also varied between 4 and 5 mm. Furthermore, 4 different thread geometries were considered, differing in thread inclination (proximal angle and distal angle) and thread height (H). Figure 2(A) shows some of the most common geometric parameters to consider for interference screw design. Additionally, two different shapes of perforations were used: In some screws vertical elliptical holes were created; in others screws elliptical perforations, following the screw thread, were produced. These were selected to create an alternative to the circular holes that the bioabsorbable interference screws commercially available have, aiming to facilitate the flow of the bone cement. Finally, among the screws with a pitch of 4 mm, screws were perforated either with 21 holes or 12 holes, and in the group of screws with a pitch of 5 mm, screws were drilled either with 17 holes or 10 holes. Figure 2(B) shows a representative example of screw models with variable pitch and hole geometry.

(A) (B) Fig. 2. Schematic representation of studied models. (A) Geometric elements of the screw thread identifying: Proximal angle (PA), Distal angle (DA), Thread depth (H), and Thread Width (T), among others, adapted from [27]. (B) Developed screw models with different pitch and perforation geometry: (I) pitch: 4mm with vertical and elliptical holes; (II) pitch: 5mm and elliptical and inclined holes.

3.2. Finite element analysis A preselection of screws to be produced with FFF was performed with FEA using the Siemens NX software version 1904. Notable material properties for PLLA and Cancellous bone were defined as follows: • PLLA - Elastic Modulus (E=3600 MPa); Shear Modulus (G=1384 MPa); Density (ρ=1 .25g/cm 3 ). • Bone - Elastic Modulus (E=1000 MPa); Shear Modulus (G=385 MPa); Density (ρ=0 .64g/cm 3 ). Among the analyses performed, one aimed to evaluate the drive and screw geometries performance during tightening and another the pullout resistance of the thread. Drive geometry greatly influences the strength of the screw during its insertion into the bone tunnel. In this FEA, the application of torque of magnitude of 2 Nm on a screwdriver was simulated. The fixation system is considered the weakest region in the first weeks after ACL reconstruction, so the pullout resistance of the interference screw is also important to ensure that the graft remains correctly placed and without damage. In this study, the evaluation of the