## 2 Materials and Methods  
This study aimed to replicate implantation in a healthy human humerus. To achieve this, synthetic sawbones (Sawbones, Pacific Research Laboratories, Vashon, WA, USA) were used which had a similar density (0.24 g/cm\({}^{3}\), 15 PCF, solid polyurethane foam) to the greater tuberosity [15; 16]. The sawbones were shaped into sample blocks of size 60 mm \(\times\) 40 mm \(\times\) 40 mm (W \(\times\) D \(\times\) H). Two types of suture anchors were assessed in this study; (i) threaded titanium anchor, outer diameter 3.5 mm and 10 mm in length (TwinFix(tm) Ti 3.5, Smith & Nephew, Andover, MA, USA), and (ii) a custom-made, 3D printed titanium alloy suture anchor of size 2.5 mm \(\times\) 2.5 mm \(\times\) 10 mm (W \(\times\) D \(\times\) H) with rectangular cross-section and surface fenestration (Figure 1). The anchors were modeled in Creo 2.0 (PTC, Boston, MA, USA) and these 3D CAD models were used to print the anchors using EOS Titanium Ti64 ELI powder on an EOS M290 Selective Laser Melting Metal Additive Manufacturing Machine (EOS GMBH, Krailling, Germany). The following machine parameters recommended by the equipment manufacturer were used: laser power of 280 W, scan speed of 1200 mm/s, resolution of 0.02 mm, and layer thickness of 30 \(\upmu\)m.  
The sawbone samples were held in a custom jig and a pilot hole of diameter 2.5 mm and depth 10 mm drilled in each. Each anchor was inserted at angles of 90\({}^{\circ}\) and 135\({}^{\circ}\) from the surface of the sawbone to the same depth (Figure 2). The aim of this study was to compare the pull-out properties of different anchors, and so braided polyethylene sutures were used for all tests to negate the potential influence of different suture materials or designs. The sutures were hand-tied around the pulling jig with a single surgeon's knot and four square knots [17]. A universal testing machine (eXpert 3910, ADMET, UK) equipped with a custom-made pulling jig was used to perform the pullout tests (Figure 3). Each anchor was preloaded with a 10 N force applied at 1 mm/s at an angle of 180\({}^{\circ}\) (parallel) to the surface to ensure sufficient contact between the implant and bone [15,18]. This preloading stage was immediately followed by the pull-test in the static model with the load being applied at a rate of 1 mm/s. The fixation strength of each anchor was defined by the maximum load recorded prior to failure by any means. Each test setup was performed 6 times with separate anchors and separate sawbone samples. Six constructs were tested for each insertion angle. The stiffness of each anchor was determined through linear regression modelling of the force-displacement curve for each construct ranging from 20-80% of the ultimate load [17]. A one-tailed t test model with a significance level (a) of 0.05 was used.  
Cyclic testing was performed in accordance with the protocol devised by [PERSON] et al. [17]. Each specimen was cycled from 0 to 50 N for 10 cycles, from 0 to 70 N for 10 cycles, and from 0 to 90 N for 10 cycles, up to a maximum of 30 cycles at a rate of 1 Hz. The number of cycles required to induce complete anchor pullout was recorded.  
Figure 1: (**a**) Commercially-available threaded suture anchor (TwinFixTM Ti 3.5, Smith & Nephew, Andover, MA, USA); (**b**) custom-made threadess suture anchor with rectangular cross-section; (**c**) design drawing of the custom-made threadess suture anchor.  
Figure 3: Experimental setup. The sawbone was clamped in place and the braided polyethylene lines were tied to a custom-made pulling jig.  
Figure 2: Loading was applied at 180\({}^{\circ}\) (parallel) to the bone surface. (**A**) Anchor inserted at 90\({}^{\circ}\) from the bone surface. (**B**) Anchor inserted at 135\({}^{\circ}\) from the bone surface. (**C**) Contact surface between threaded suture anchor and pre-drilled hole. (**D**) Contact surface between custom-made suture anchor and pre-drilled hole.