Improved Osseointegration of a TiNbSn Alloy with a Low Young's Modulus Treated with Anodic Oxidation

Abstract

Ti6Al4V alloy orthopedic implants are widely used as Ti6Al4V alloy is a biocompatible material and resistant to corrosion. However, Ti6Al4V alloy has higher Young's modulus compared with human bone. The difference of elastic modulus between bone and titanium alloy may evoke clinical problems because of stress shielding. To resolve this, we previously developed a TiNbSn alloy offering low Young's modulus and improved biocompatibility. In the present study, the effects of sulfuric acid anodic oxidation on the osseointegration of TiNbSn alloy were assessed. The apatite formation was evaluated with Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy analyses. The biocompatibility of TiNbSN alloy was evaluated in experimental animal models using pull-out tests and quantitative histological analyses. The results of the surface analyses indicated that sulfuric anodic oxidation induced abundant superficial apatite formation of the TiNbSn alloy disks and rods, with a 5.1-µm-thick oxide layer and submicron-sized pores. In vivo, treated rods showed increased mature lamellar bone formation and higher failure loads compared with untreated rods. Overall, our findings indicate that anodic oxidation with sulfuric acid may help to improve the biocompatibility of TiNbSn alloys for osseointegration.

Figure 1

Scanning electron microscopy (SEM) images of apatite formation on anodic oxidation treated CP–Ti and TiNbSn alloy disks. Representative SEM micrograph images of anodic oxidation (AO)-treated pure titanium (CP–Ti; control) disks and TiNbSn alloy disks. (A) AO-treated CP–Ti and (E) AO-treated TiNbSn. Representative images of SEM micrographs of CP–Ti and TiNbSn disks after AO treatment, AO + annealing treatment, and AO + hot water treatment, followed by 7-day incubation in HBSS. (B) AO-treated CP–Ti; (F) AO-treated TiNbSn; (C) AO + annealed CP–Ti; (G) AO + annealed TiNbSn; (D) AO + hot water-treated CP–Ti; and (H) AO + hot water-treated TiNbSn.

Figure 2

X-ray diffraction (XRD) analyses of apatite formation. XRD analyses of anodic oxidation (AO)-treated pure titanium (CP–Ti; control) and TiNbSn disks subsequent 7-day incubation in HBSS. Crystalline apatite formation were observed in both AO-treated CP–Ti and TiNbSn alloy disks.

Figure 3

X-ray photoelectron spectroscopy (XPS) analysis of the surfaces of the TiNbSn alloys. O 1 s XPS profiles of treated TiNbSn disks. (A) Anodic oxidation (AO)-treated TiNbSn disk; (B) AO + hot water-treated TiNbSn disk; and (C) AO + annealed TiNbSn disk. No apparent differences were observed in the fraction of hydroxyl groups of the treated surfaces.

Figure 4

Scanning electron microscopy (SEM) images of the superficial apatite formation of TiNbSn alloy rods. Representative SEM micrograph images of anodic oxidation (AO)-treated (A), AO + annealed (B), AO + hot water-treated rods (C) subsequent 7-day incubation in HBSS. All groups showed abundant apatite formation.

Figure 5

Assessment of bone bonding strength with pull-out tests. At 3 and 6 weeks after rod implantation, the failure loads of treated and untreated TiNbSn rods were shown. The results of pull-out tests of AO-treated, AO + annealed and AO + hot water-treated rods showed higher failure loads compared with the untreated rods both at 3 and 6 weeks. Results are expressed as the median and interquartile range (n = 8 rabbits per group).

Figure 6

Histological images of new bone formation around the TiNbSn rods. Representative histological images of untreated and AO-treated TiNbSn alloy rods implanted in rabbit femurs. (A–D) panels show lower magnification images. (E–H) panels show higher magnification images visualized using a polarization microscope of the rectangular areas indicated in (A–D) panels. Mature lamellar bone (arrow heads) was observed in the AO-treated TiNbSn group. (A,E) Images of distal femur of an AO-treated rod; (B,F) Images of distal femur of an untreated rod; (C,G) Images of proximal femur of an AO-treated rod; and (D,H) Images of proximal femur of an untreated rod.

Figure 7

Quantitative histomorphometric analyses of new bone formation. (A–G) panels indicated the results of distal femur and (H–N) panels indicated the results of proximal femur. The lamellar bone formation in the proximal area is higher in the AO-treated TiNbSn alloy rods compared with that of the untreated rods. Results are expressed as the median and interquartile range (n = 3 rabbits per group). BV/TV, bone volume/tissue volume; Tb.Th, trabecular thickness; OV/TV, osteoid volume/tissue volume; Ob.S/BS, osteoblast surface/bone surface; Oc.S/BS; osteoclast surface/bone surface; MAR, mineral apposition rate.

Figure 8

Transmission electron microscopy (TEM) image of titanium oxide layer on the boundary surface of the rod and bone in AO-treated TiNbSn alloy. TEM image of titanium oxide layer on the boundary surface of the rod and bone, and mapping of elements of Ti, Nb, O, Ti, Ca, and P in the same region. Submicron-sized pores were observed and are indicated by arrows. Mapping revealed that both Ca and P segregated and inserted into the pores.