## 4 Discussion
No displacement data were recorded during the cyclic testing, but it was noted by inspection that the threaded anchors had become more displaced from the starting position. Inserting the threaded anchor into a pre-drilled hole may not provide adequate bone purchase around the distal region of the screw, which can lead to screw loosening as the distal threads move relative to the secured shaft within the proximal block (Figure 3). We propose that using a predrilled hole increases the risk of developing a windshield-wiper effect and subsequent anchor loosening. [PERSON] et al. [25] indicated that fixed anchors placed under cyclic loading succumb to a windshield-wiper effect, which increases motion at the bone-anchor interface, leading to loosening. The threadless suture anchor developed for this study was fixed into a predrilled tunnel by press-fitting, generating greater friction at the bone-implant interface and thus improving the fixation strength to prevent excessive interfacial micro-motion. Excessive micro-motion at the bone-implant interface may impair long-term bone ingrowth and ongrowth [26]. Similar 3D printed threadless implants have been successfully used for sacroiliac joint fusion [27; 28; 29], and in-vivo experiments have indicated that incorporating a porous surface can improve biological fixation and implant stability [7]. By referring the previous literatures related to the processing of the selective laser melted (SLM) machine used in this study, our anchor was designed to have porosities of 70% and pore sizes of 600 \(\upmu\)m. [PERSON] et al. [30] evaluated the osteoinduction of SLM titanium implants and found the superior osteoinduction property was observed when the pore sizes were in 500 and 600 \(\upmu\)m. By an in-vivo experiment by [PERSON] et al. [31], it indicated that the Ti6 Al4V device with porosity of 65% and pore size of 600 \(\upmu\)m had more comparable mechanical strength with the bone and significant bone ingrowth than those with pore size of 300 or 900 \(\upmu\)m.  
Overall, the 3D printed suture anchor presented in this study has the potential to improve initial implant stability and long-term fixation, demonstrated by the significantly greater pullout strength and implant stiffness at a 90\({}^{\circ}\) insertion angle. According to our results, the greatest pullout strength for the threadless anchor occurred at a 90\({}^{\circ}\) insertion angle, and therefore we would recommend surgeons to maintain this angle when using threadless anchors. Because our threadless anchor is prototype, we only evaluated the feasibility of pullout and stability performance of our threadless anchor as our pilot study. Further studies including the mechanical and biocompability tests (cytotoxicity test, sensitization test, intracutaneous irritation test, etc.) according to ASTM and ISO standards respectively shall be performed to validate the design in this study. The current study has some limitations to be considered. Synthetic bone blocks are intended to replicate the properties of bone, but cannot truly represent the anisotropic and viscoelastic material properties of human bone. However, synthetic bone does have some notable advantages, including negligible inter-specimen variability, low cost, readily available, and minimal specimen preparation. The use of synthetic bone compliments the aims of this study to compare the pullout properties of different suture anchors [11; 19; 24]. It should also be recognized that this study only used one commercially available suture anchor for comparison against the novel 3D printed threadless anchor. Future studies should consider a wider range of anchor designs. Regarding the test setup, the loading conditions applied in this study did not accurately represent physiological loading, and the models did not account for the mechanical environment at a specific repair site or using different implantation methods.