On-Growth and In-Growth Osseointegration Enhancement in PM Porous Ti-Scaffolds by Two Different Bioactivation Strategies: Alkali Thermochemical Treatment and RGD Peptide Coating

Abstract

A lack of primary stability and osteointegration in metallic implants may result in implant loosening and failure. Adding porosity to metallic implants reduces the stress shielding effect and improves implant performance, allowing the surrounding bone tissue to grow into the scaffold. However, a bioactive surface is needed to stimulate implant osteointegration and improve mechanical stability. In this study, porous titanium implants were produced via powder sintering to create different porous diameters and open interconnectivity. Two strategies were used to generate a bioactive surface on the metallic foams: (1) an inorganic alkali thermochemical treatment, (2) grafting a cell adhesive tripeptide (RGD). RGD peptides exhibit an affinity for integrins expressed by osteoblasts, and have been reported to improve osteoblast adhesion, whereas the thermochemical treatment is known to improve titanium implant osseointegration upon implantation. Bioactivated scaffolds and control samples were implanted into the tibiae of rabbits to analyze the effect of these two strategies in vivo regarding bone tissue regeneration through interconnected porosity. Histomorphometric evaluation was performed at 4 and 12 weeks after implantation. Bone-to-implant contact (BIC) and bone in-growth and on-growth were evaluated in different regions of interest (ROIs) inside and outside the implant. The results of this study show that after a long-term postoperative period, the RGD-coated samples presented higher quantification values of quantified newly formed bone tissue in the implant's outer area. However, the total analyzed bone in-growth was observed to be slightly greater in the scaffolds treated with alkali thermochemical treatment. These results suggest that both strategies contribute to enhancing porous metallic implant stability and osteointegration, and a combination of both strategies might be worth pursuing.

Keywords: RGD peptide; bone in-growth; bone on-growth; histomorphometric evaluation; in vivo implantation; osseointegration; thermochemical treatment; titanium foams.

Figure 1

Mediolateral (a) and craniocaudal (b) postoperative X-ray images showing both insertion point location and implant alignment.

Figure 2

New bone formation in titanium porous foam at 4 and 12 weeks after implantation. There were no statistically significant differences (p > 0.05) depending on the type of samples for all analyzed parameters.

Figure 3

BSE-SEM results 4 weeks (left) and 12 weeks (right) after porous titanium implant insertion in transversal view: (a,b) CG; (c,d) TCG; (e,f) PAG.

Figure 4

Total new bone formation in titanium porous foam 4 and 12 weeks after implantation.

Figure 5

BSE-SEM micrographs 4 weeks after implantation in transversal section view at different magnifications: (a,b) CG; (c,d) TCG; (e,f) PAG.

Figure 6

BSE-SEM micrographs 12 weeks after implantation in transversal section view at different magnifications: (a,b) CG; (c,d) TCG; (e,f) PAG.

Figure 7

BSE-SEM results 4 weeks (left) and 12 weeks (right) after porous titanium implant insertion in longitudinal section view: (a,b) CG; (c,d) TCG; (e,f) PAG.

Figure 8

Screen photographic sequence of the surgical method: (a) Medial approach to the proximal aspect of the right tibia, (b) Creating a monocortical bone defect with a 2.5 mm drill bit, (c) Enlarging the monocortical bone defect with a 3.5 mm drill bit, (d) Placing the titanium implant in the monocortical bone defect, (e) Checking the adequate placement and fixation of the titanium implant, (f) Surgical wound sutured with a continuous pattern.

Figure 9

Stitched grayscale image obtained via SEM analysis detailing the different materials identified.

Figure 10

Graphical scheme for assessment of BIC and ROI values.