Pegs are often used in cementless total knee replacement (TKR) to improve fixation strength. Studies have demonstrated that interference fit, surface properties, bone mineral density (BMD) and viscoelasticity affect the performance of press-fit designs. These parameters also affect the insertion force and the bone damage occurring during insertion. We aimed to quantify the effect of the aforementioned parameters on the short-term fixation strength of cementless pegs. 6 mm holes were drilled in twenty-four human femora. BMD was measured using calibrated CT-scans, and randomly assigned to samples. Pegs were produced to investigate the effect of interference fit (diameters 6.5 and 7.6 mm), surface treatment (smooth and rough- porous-coating [friction coefficient: 1.4]) and bone relaxation (relaxation time 0 and 30 min) and interactions were studied using a DOE method. Two additional rough surfaced peg designs (diameters 6.2 and 7.3 mm) were included to scrutinize interference. Further, a peg based on the LCS Porocoat® (DePuy Synthes Joint Reconstruction, Leeds, UK) was added as a clinical baseline. In total seven designs were used (n = 10 for all groups). Pegs were inserted and extracted using an MTS machine (Figure 1), while recording force and displacement. Bone damage was defined as the difference between the cross-sectional hole area prior to and after the test. BMD and interference fit were significant factors for insertion force. BMD had a significant positive correlation with pull-out force and subsequent analyses were therefore normalised for BMD. . Pull-out force increased significantly with interference for both surface coatings at time 0 (p < 0.05). However, after 30 minutes the effect remained significant for rough pegs only (p < 0.05-Figure 2A). Pull-out force reduced significantly with roughness for both peg diameters at time 0 (p < 0.001). However, after 30 minutes the effect remained significant for small pegs only (p < 0.05-Figure 2A). The time dependant interaction was only significant for smooth pegs in both diameters (p < 0.05-Figure 2A). Additionally, the pull-out force increased with diameter in a non-linear manner for the rough pegs (Figure 2B). The two surface treatments were not significantly different to the clinical comparator. Interference fit was the only significant factor for bone damage. BMD was significant for insertion and pull-out forces, reinforcing the need to account for this factor in biomechanical studies and clinical practice. This study also highlights the importance of time in studying bone interactions, with surface treatment and interference showing different interaction effects with relaxation time. Although smooth pegs initially have a higher pull-out force, this effect reduces over time whereas the pullout force for rough pegs is maintained. Smooth pegs also show time sensitivity in relation to interference and the benefit of increased interference reduces over time, whereas it is maintained in rough pegs. This may be explained by different damage (compressive and abrasive) mechanisms associated with different surface treatments. In conclusion, BMD and interference fit are significant factors for initial fixation. Bone relaxation plays an important role as it reduces the initial differences between groups. Therefore, these findings should be strongly considered in the design development of cementless TKR

Most glenoid implants rely on centrally located large fixation features to avoid perforation of the glenoid vault in its peripheral regions [1]. Upon revision of such components there may not be enough bone left for the reinsertion of an anatomical prosthesis, resulting in a large cavity that resembles a sink hole. Multiple press-fit small pegs would allow for less bone resection and strong anchoring in the stiffer and denser peripheral subchondral bone [2], whilst producing a more uniform stress distribution and increased shear resistance per unit volume [3] and avoiding the complications from the use of bone cement. This study assessed the best combination of anchoring strength, assessed as the ratio between push in and pull out forces (Pin/Pout), and spring-back, measured as the elastic displacement immediately after insertion, for five different small press-fitted peg configurations (Figure 1, left) manufactured out of UHMWPE cylinders (5 mm diameter and length).

16 specimens for each configuration were tested in two types of Sawbones solid bone substitute: hard (40 PCF, 0.64 g/cm3, worst-case scenario of Pin) and soft (15 PCF, 0.24 g/cm3, worst-case scenario of spring-back and Pout). Two different interference-fits, Ø, were studied by drilling holes with 4.7 mm and 4.5 mm diameter (Ø 0.3 and Ø 0.5, respectively). A maximum Pin per peg of 50 N was defined, in order to avoid fracture of the glenoid bone during insertion of multiple pegs. The peg specimens were mounted into the single-axis screw-driven Instron through a threaded fixture. A schematic of the experimental set up is made available (Figure 1, centre). The peg was pushed in vertically for a maximum of 5 mm at a 1 mm/s rate, under displacement control, recording Pin. The spring-back effect was assessed by switching to load control and reducing the load to zero. The peg was then pulled out at a rate of 1 mm/s, recording Pout. The test profile is depicted in Figure 1 (right).

Average Pout/Pin, spring back (in mm) and force-displacement curves for all 80 specimens tested are shown in Figure 2. These were split into groups according to the type of bone substitute and interference-fit, with the right column showing the average values for the Pin. High repeatability among samples of the same configuration tested is noted. Configurations #1, #3 and #5 all exceed the maximum Pin per peg for at least one type of bone. Configuration #2 has the lowest Pin of all (best thread aspect ratio), followed by configuration #4 (thinner threads). The peg configurations #4 and #2 had the highest Pin/Pout. The peg configurations with lowest spring-back after insertion were configuration #2 and #4. Interference fit of Ø 0.3 mm was shown to reduce Pin below maximum limit of 50 N without great influence in spring-back.

Implant loosening is one of the primary mechanisms of failure for hip, knee, ankle and shoulder arthroplasty. Many established implant fixation surfaces exist to achieve implant stability and fixation. More recently, additive manufacturing technology has offered exciting new possibilities for implant design such as large, open, porous structures that could encourage bony ingrowth into the implant and improve long-term implant fixation. Indeed, many implant manufacturers are exploiting this technology for their latest hip or knee arthroplasty implants. The purpose of this research is to investigate if the design freedoms offered by additive manufacturing could also be used to improve initial implant stability – a precursor to successful long-term fixation. This would enable fixation equivalent to current technology, but with lower profile fixation features, thus being less invasive, bone conserving and easier to revise. 250 cylindrical specimens with different fixation features were built in Ti6Al4V alloy using a Renishaw AM250 additive manufacturing machine, along with 14 specimens with a surface roughness similar to a conventional titanium fixation surface. Pegs were then pushed into interference fit holes in a synthetic bone material using a dual-axis materials testing machine equipped with a load/torque-cell (figure 1). Specimens were then either pulled-out of the bone, or rotated about their cylindrical axis before being pulled out to quantify their ability to influence initial implant stability. It was found that additively manufactured fixation features could favourably influence push-in/pull-out stability in one of two-ways: firstly the fixation features could be used to increase the amount pull-out force required to remove the peg from the bone. It was found that the optimum fixation feature for maximising pull-out load required a pull-out load of 320 N which was 6× greater than the least optimum design (54 N) and nearly 3× the maximum achieved with the conventional surface (120 N). Secondly, fixation features could also be used to decrease the amount of force required to insert the implant into bone whilst improving fixation (figure 2). Indeed, for some designs the ratio of push-in to pull-out was as high as 2.5, which is a dramatic improvement on current fixation surface technology, which typically achieved a ratio between 0.3–0.6 depending on the level of interference fit. It was also found that the additively manufactured fixation features could influence the level of rotational stability with the optimum design resisting 3× more rotational torque compared to the least optimum design. It is concluded that additive manufacturing technology could be used to improve initial implant stability either by increasing the anchoring force in bone, or by reducing the force required to insert an implant whilst maintaining a fixed level of fixation. This defines a new set of rules for implant fixation using smaller low profile features, which are required for minimally invasive device design