PSI - Issue 68

P. Langourani-Kosteletou et al. / Procedia Structural Integrity 68 (2025) 112–118 P. Langourani-Kosteletou et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction A bone screw acts as a simple machine that is designed with specific features to achieve maximum efficiency when inserted into the bone (Hughes & Jordan, 1972; Weinstein, 2011). The stability of internal fixation depends on the coupling between the bone and the threads of the screw (DeCoster et al., 1990). Analyzing the interaction between bone and implant is challenging as bone is a material with unique anisotropic and heterogeneous mechanical properties (Wu et al., 2018). Cancellous partially threaded solid-core 6.5 screws are commonly employed as lag screws for meta physeal fixation, often through a plate for intra-articular fractures of long bones such as tibial plateau and intercondylar femoral fractures. The procedure involves drilling of a pilot hole into the fracture's proximal and distal ends. The smooth shaft of the partially threaded cancellous screw serves as the gliding hole, while the threaded part engages the distal bone fragment. As the head of the screw is buttressed against the near cortex during tightening, interfragmentary compression is produced (Taha, 2017). Interfragmentary compression offers significant resistance to shear forces when utilized as a fixation method (Perren, 1979). The lag screw fixation technique is a surgical technique used to achieve absolute stability between two bone-engaged fragments. In this case, bone healing is achieved through direct (primary) bone healing, not callus formation (Perren, 1979; Gueorguiev-Ruegg & Stoddart, 2017). The thread length of bone screws is a crucial design parameter since it is directly correlated with the length of the thread engagement in the bone. In the case of porous materials, like cancellous bone or its substitutes, the length of engagement has been found to be correlated with the holding power of screws (Chapman et al., 1996). However, the influence of the thread length in a fixation under shear loading after axial compression has been utilized as a fixation method has not been adequately studied. The primary objective of the present study is to investigate the effect of the thread length on the mechanical behavior of the fixation by means of biomechanical experiments. In this context, compression was applied between two sawbone blocks fixed with a single partially threaded 6.5 cancellous solid-core screws of two different thread lengths (lag by design). Subsequently, a shear load was applied to the constructs, and their mechanical response was assessed. The experimental laboratory data were later analyzed and correlated with the clinical implications of the findings. 2. Materials and Methods Sawbone blocks, composed of solid-rigid polyurethane foam, were employed as a substitute for cancellous bone. The American Society for Testing and Materials (ASTM International Fi839-08, 2021) has confirmed solid-poly urethane foam as a standard material for testing orthopedic devices and instruments. The polyurethane foam selected for this study exhibits the following material properties: Density of 20 PCF (0.32 g/cm³), which is representative of high-density cancellous bone, compressive strength of 5.4 MPa, and elastic modulus of 37 MPa (as per ASTM D1621). In addition, single 6.5 solid-core, partially threaded cancellous screws with a length of 75 mm were used for the fixation. The screws were made of titanium alloy. Eighteen constructs (sawbone blocks and a screw) were used in the present study. As a first step, eighteen pieces of sawbone blocks, each having length equal to 90 mm and cross-sectional area equal to 40 mm x 40 mm, were precisely cut from the original block, and a 3.2 mm drill bit was employed to create the pilot hole of the same length of the screw by the manufacturer's standards. Special attention was paid during drilling in order for the axis of the hole to be normal to the cross-section of the construct (in other words, to be parallel to the length of the construct). After drilling, each sawbone block was precisely cut into two blocks, i.e., one of 30 mm length (block 1) and one of 60 mm length (block 2), which were fixed by a surgeon using one of the aforementioned screws. Two different types of 6.5 solid-core cancellous screws, with varying thread lengths of 16 mm and 32 mm, were employed in the fixation process. The screws were inserted from block 1 (the small one) to ensure that the threaded part of both screw types effectively surpassed the fracture line. It should be mentioned that since the hole was parallel to the length of the construct, the axis of the screws also remained perpendicular to the cross-section of the block. The insertion of the screw was made using the standard hexagonal screwdriver shaft, attached to a torque-limiting screwdriver handle. A torque limit of 2 Nm was implemented for the insertion of 16 mm threaded screws (Group A), while a torque limit of 2.5 Nm was implemented for the insertion of 32 mm threaded screws (Group B). This technique produced axial compression between the two fixed blocks (lag by design). The torque limit was set to ensure that all the screws of the same group were tightened with the same torque. A distinct click from the drive indicated that each