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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In addition to these bones, the knee joint also includes cartilage, muscles, tendons, and ligaments. Among the ligaments that connect the knee bones, the Anterior Cruciate Ligament (ACL) prevents the tibia from moving forward and limits the rotation of the knee, i. e., stabilizes the leg in rotational movements. The need to withstand body weight while standing, walking, running, or jumping, renders the knee highly vulnerable to injuries. An ACL injury occurs when the forces that the ligament is subjected to overcome the ligament inherent mechanical resistance. Studies have found that under normal daily activities the ACL is subjected to a maximum load of 450 N, with the ultimate tensile loads that it can withstand being as high as 2 300 N [1], [2]. Contrary to the behavior of the bone tissue, in case of complete or incomplete tearing, cruciate ligaments have no spontaneous healing capacity which decreases drastically the chance of successful healing without surgery [3]. Recently, there has been an increase in ACL reconstruction surgeries [4] and, consequently, many improvements and innovations are being proposed. During ACL reconstruction, it is necessary to provide a good fixation system capable of keeping the graft inside the tibial tunnel without motion until it is fully integrated and fixed within the bone tissue. At the same time, the reconstruction must be stiff and strong enough to sustain the loads that the ACL is usually subjected to. This work is focused on the development of a new bioabsorbable interferential screw for ACL reconstruction, aiming at improved mechanical and biological performance, as well as promoting an early rehabilitation process. Several screw geometries, with different thread geometries, different pitch, different drive geometries, and different hole geometries, were created using a 3D CAD modeling software and analyzed using a finite element analysis (FEA) program to assess which geometry provides the most favorable mechanical performance to the implant. Selected screw geometries were produced using fused filament fabrication (FFF) and insertion tests were performed. 2. Bioabsorbable screws Nowadays, the most used distal fixation device in ACL reconstruction is the interference screw due to its high initial mechanical strength, which provides sufficient fixation strength to make the postoperative rehabilitation process successful, with a rapid osteointegration enabling an earlier rehabilitation [5], [6] . In their primordial development stages, interference screws were made of metallic biomaterials capable of provinding good initial fixation strength due to their high strength and elastic modulus. Insertion of these screws caused graft laceration so metallic screws with blunt threads were developed and also the introduction of cannulated screw designs allowed the use of guide wires to minimize the screw-tunnel divergence during insertion [7], [8]. The increasing knowledge regarding bioabsorbable materials has led to their greater prioritization over traditional metals. Despite the initial distrust in bioabsorbable interference screws, mechanical studies have steadily reported that they can provide equivalent fixation strength and suficient resistance to pullout force [9]. Besides proper mechanical characteristics, bioabsorbable interference screws allow better postoperative images, less laceration of graft during insertion and uncompromised revision surgery when compared to metallic interference screws [7], [10]. One of the main problems associated with these types of screws is the failure during insertion, a phenomenon which is also related with drive shape, drive diameter, drive length and core diameter [11]. Figure 1 displays examples of bioabsorbable interference srews that were compared in the work performed by Costi et al. [10], where authors evaluate the torque resistance of screws with different geometries (length, diameter, and drive geometry).

Fig. 1. Examples of bioabsorbable interference screws. Adapted from [10].