## 4 Discussion  
Failure of suture anchors at the bone-anchor interface, such as through pullout, is not uncommon, especially in elderly osteopororotic patients. The fixation ability is adversely affected by poor bone quality and can be aggravated by decoritation, such as migration, loosening, or interference from surrounding tissues. However, 3D printing allows for an open fenestrated core to be incorporated in the implant design, which increases the contact area with bone and also provides an accessible route for bone integration [7, 19]. Currently, the technologies in 3D-printing medical device have been developed to apply in metal and polymer materials [20]; however, the polymer materials have been considered that have some problems such as poor implant strength, damage to soft tissue, and development of inflammatory reactions leading to osteolysis. The literatures also indicated that polyetheretherketone (PEEK) has been shown to be poor osseointegration [21, 22, 23]. Porosity titanium implant have been shown excellent osseointegration property, and therefore we choose the 3D printed titanium technology to create our anchors. In addition, our threadless anchor is made of Ti6 Al4V ELI powder that has been satisfied the requirements of ASTM F3001-14 which is relevant to medical devices and recognized by US FDA. In this study, we compared the primary fixation stability of a novel threadless 3D-printed suture anchor against a commercially available threaded suture anchor. While the pullout strength of threaded suture anchors has been assessed in previous studies [11, 17, 19, 24], research has indicated that the pullout strength weakens considerably with a larger angle between the load direction and long-axis of the anchor [17, 24]. This was also evident from the results of this study whereby inserting the threaded suture anchor at 135\({}^{\circ}\) gave a greater pullout strength than the 90\({}^{\circ}\) insertion angle. However, the opposite was found for the threadless anchor, in that the smaller 90\({}^{\circ}\) insertion angle provided a greater pullout strength than the anchors inserted at 135\({}^{\circ}\). Similar results were reported by [PERSON] et al. [14], who found that a 90\({}^{\circ}\) insertion angle provided the greatest pullout strength for a titanium metal threadless anchor. The static pullout strength of the threadless anchor was also significantly higher than the threaded anchor when both were inserted at 90\({}^{\circ}\). However, when using a 135\({}^{\circ}\) insertion angle, the threaded anchor had a superior pullout strength, although the difference was not significant. The initial pullout resistance is primarily influenced by friction generated by the axial component of the applied force. Most commercially available threaded anchors have a smooth surface and so the resistance capability is related to the thread design. However, the 3D printed anchor with rectangular cross-section generates friction through the surface  
Figure 5: Compared to different insertion angle, cycled force and cycle numbers of survival proportions. roughness and the press-fit force. When the pullout force was aligned normally with the long axis of the anchor (90\({}^{\circ}\) insertion angle), the threadless anchor had a better initial stability than the threaded anchor, with the resistance to pullout mainly arising from the overall contact area and the press-fit force of the rectangular shape to reduce movement or tilting. The maximum displacement (structural stiffness) of the threadless anchor was less than the threaded anchor for the 90\({}^{\circ}\) insertion angle, demonstrating its resistance to tilting and migration. No failures were reported for the threadless anchor constructs with a 90\({}^{\circ}\) insertion angle, but the threaded anchors showed signs of gross loosening after about 21-30 cycles. For the 135\({}^{\circ}\) angle, the two anchor groups showed a similar trend to failure.