Introduction. Cementless total knee arthroplasty (TKA) has several advantages compared to the cemented approach, including elimination of bone cement, a quicker and easier surgical technique, and potentially a stronger long-term fixation. However, to ensure the successful long-term biological fixation between the porous implant and the bone, initial press-fit stability is of great importance. Undesired motion at the bone-implant interface may inhibit osseointegration and cause failure of biological fixation. Initial stability of a cementless femoral implant is affected by implant geometry, bone press-fit dimension, and characteristics of the porous coating. The purpose of this study was to compare the initial fixation stability of two types of porous femoral implants by quantifying the pull-out force using a paired cadaveric study design. Methods. The two types of cementless TKA femoral implants evaluated in this study had identical implant geometry but different porous coatings (Figure 1). The first type had a conventional spherical-bead coating (Type A), while the second type had an innovative irregularly-shaped-powder coating (Type B). The porous coating thickness was equivalent for both types of implants, thus the dimensional press-fit with bone was also equivalent. Three pairs of cadaveric femurs were prepared using standard TKA surgical technique, with each pair of the femurs receiving one of each porous implant type. An Instron 3366 load frame (Norwood, MA, USA) was used to pull the femoral implant out from the distal femur bone (Figure 2). The testing fixture was designed to allow free rotation between the implant and the actuator. The pullout was performed under a displacement control scheme (5 mm/min). Peak pull-out force was recorded and compared between the two implant groups. Results. Mean pull-out force for the Type B porous femoral implants (512 ± 246 N) was greater than that of the Type A porous femoral implants (310 ± 185 N), although the difference was not statistically significant (p>0.05) (Figure 3). Discussion. This paired cadaveric study showed that the innovative Type B porous coating provides equivalent and potentially greater pull-out force than the conventional Type A porous coating. Lack of statistical significance could be attributed to the limited sample size. Although pull-out testing is not a physiological loading scenario for TKA implant, it provides a relevant assessment of the implant-bone press-fit stability. With all other factors the same, the greater pull-out force observed in the Type B implants is likely related to the higher roughness and friction of the new porous coating. Previous experiments have shown that the Type B porous coating has significantly greater friction against Sawbones surface (coefficient of friction 0.89) compared to Type A porous coating (coefficient of friction 0.50), which was consistent with the findings in this study. Greater initial fixation stability is more favorable in cementless TKA as it reduces the risk of interface motion and better facilitates long-term biological fixation

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

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

The clinical outcome and radiographic analysis of 82 patients undergoing total hip arthroplasty using a titanium acetabular component coated with a new proprietary Titanium Porous Coating inserted without cement are reported. All total hip replacements were performed by a single surgeon and utilized a porous coated, cementless femoral component. Pre clinical testing was carried out in an animal model to evaluate the new porous coating. THR was performed using a cementless acetabular component of the same geometrical design inserted without cement. The component is coated with a new proprietary Titanium Porous Coating wherein the non-spherical bead itself is also porous. This creates a “lava rock” type of structure and gives variability in the pore sizes that aids in the in-growth and apposition of bone (fig 5). The inter-bead pore size: the pore size between each non-spherical bead = 200–525 μm while the Intra-bead pore size: the pore size within each non-spherical bead = 25–65 μm. The resulting surface is extremely rough and provides a robust initial “bite” or “stick” to the bone. Clinical results were evaluated using the Harris Hip score and were recorded prospectively preoperatively and at 6 weeks, 6 months, and 1 year postoperatively. Radiographs were evaluated for component migration, subsidence, and cortical and cancellous biologic response as well as zonal analysis of radiolucent lines, using the Muller THR template. Pre-clinical animal testing of the new porous coating was carried out in 50 sheep using a metacarpal intramedulary implant (similar to a hip stem) designed to function as a Percutaneous Osseointegrated Prosthesis (POP) for amputees and evaluated Apposition Bone Index (ABI) (fig 1), Mineral Apposition Rate (MAR) (fig 2),% Bone In-growth (fig 3), and Axial Pull-out Force (fig 4). Sheep were sacrificed at time points of 0, 3, 6, 9, and 12 months to measure and evaluate the above parameters. Human clinical and radiographic follow up averaged 10.5 months (range 2–18 months). There were 39 females and 43 males. Average age was 59 years. The clinical results were excellent with respect to both pain and function at mid term follow up. Patient satisfaction was high. Radiographic analysis showed no migration or change in the angle of inclination at latest follow up. Femoral component subsidence was detected in 2 cases and averaged 1.8 mm. No polyethylene wear was detected. No hips dislocated. No hips underwent additional surgery. Pre-clinical test data demonstrated excellent mechanical and biological attributes. Average tensile strength of the coating surpassed the FDA minimum requirement by 3X. Animal testing in the sheep showed no evidence of stem loosening or need for revision after 12 months, and corroborated well with clinical results. Correlation between the pre-clinical testing and the human experience was exceptional. Application of a new titanium porous coating utilizing a proprietary dual pore size structure to the surface of the acetabular component provides an extremely rough surface and robust initial fixation during cementless THR. Excellent early clinical and radiographic results are demonstrated. The addition of this new type of porous coating to other arthroplasty components may confer additional clinical advantages