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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A close investigation of the sawbone blocks after the completion of the tests and the removal of the inserted screws revealed that failure occurred due to compression (crush of the pilot hole), while the screw remained intact without undergoing any plastic deformation. This is clearly shown in Fig. 5.

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Fig. 5. Photos of sawbones after completion of a typical test. (a) The 30 mm-long block was displaced vertically parallel to the 60 mm-long block (pure shear); (b) The sawbone failed due to compression (crush of the pilot hole), resulting in nonuniform deformation of the pilot hole. 4. Discussion Certain fracture types are inherently subjected to shear forces due to the skeleton's structural architecture, such as tibial plateau fractures and pelvic ring fractures. However, torsional shear stress may develop in any fracture type due to the motion of the neighboring articulations (Perren, 1979). Consequently, it is essential to investigate the effect of shear loading when interfragmentary compression is utilized as a fixation method. In the case of porous materials, like cancellous bone or its substitutes, the length of engagement has been found to correlate with the holding power of screws. Chapman et al. (1996) and Asnis et al. (1996) proposed the use of a formula that predicts the failure force necessary to strip the internal threads formed in cancellous bone (ultimate shear strength of cancellous bone) to predict the pullout force of cancellous bone screws. According to this formula, the predicted pullout force of a cancellous screw is proportional to the length of thread engagement and other screw geometry factors, such as the major diameter of the screw, the dread depth , and inversely proportional to the pitch . Chapman’s formula has been utilized in various studies in the literature to accurately predict the pullout force of cancellous screws. However, recent research has demonstrated that this formula may not be suitable for screws with specific characteristics, such as pedicle screws (Tsai et al., 2009; Chatzistergos, 2010; Benoit et al., 2023). The effect of thread length on fixation under shear loading has been investigated by Downey et al. (2015), Phen et al. (2020), and Shannon et al. (2021). In their biomechanical studies, Downey et al. and Phen et al. utilized sawbone blocks with 15PCF as substitutes for cancellous bone. The blocks were fixed with either fully threaded screws (lag by technique) or partially threaded screws (lag by design). As a result, axial compression was produced between the two sawbone block fragments. The constructs were then subjected to shear loading, and various mechanical parameters such as the stiffness, yield force, yield displacement, and ultimate force of the constructs were calculated and compared between the two groups. In another study conducted by Shannon et al. (2021) on cadaver pelvis, which simulated sacral zone 2 fractures, the authors compared the biomechanical results of the fixation with two partially threaded screws or two fully threaded screws applied by the lag screw technique (lag-by-design and lag-by-technique corre spondingly). In all these three studies, it was concluded that fully threaded constructs exhibited statistically significant ly greater yield force and ultimate force than the constructs fixed with partially threaded screws. In the present study, partially threaded screws (short- and long-threaded) were used for fixation, which differs from the aforementioned studies in which partially threaded and fully threaded screws were compared. In addition, in this study, a high-density cancellous bone substitute of 20PCF was utilized, which differs from the lower-density bone substitute of 15PCF used in the studies by Downey et al. (2015) and Phen et al. (2020). Despite these differences, the findings of our study are in line with those of previous studies. The group with constructs fixed with the longer threaded screw (Group B) demonstrated statistically significantly greater stiffness, proportionality limit, and yield load compared to that with the partially threaded ones, in high-density cancellous bone substitute. The thread length of screws is believed to have an impact on the level of compression between two fragments or blocks. Research suggests that screws with longer thread lengths tend to produce more axial compression between the two engaged objects (Liu, 2019). This phenomenon can also be attributed to the fact that screws with different thread lengths require different insertion torque during the screw-tightening process. The variation in insertion torque leads to differences in the tensile force generated along the screw axis when a screw is tightened, resulting in varying compression forces between the two engaged objects (greater insertion torque generates higher compression) (Hughes & Jordan, 1972). In our study, constructs fixed with 16 mm threaded screws were tightened to a torque limit of 2 Nm,