Abstract
Introduction
Recent advances in nano-surface modification technologies are improving osseointegration response between implant materials and surrounding tissue. Living cells have been shown to sense and respond to cues on the nanoscale which in turn direct stem cell differentiation. One commercially practical surface treatment technique of particular promise is the modification of titanium implant surfaces via electrochemical anodization to form arrays of vertically aligned, laterally spaced titanium oxide (TiO2) nanotubes on areas of implants where enhanced implant–to-bone fixation is desired. Foundational work has demonstrated that the TiO2 nanotube surface architecture significantly accelerates osteoblast cell growth, improves bone-forming functionality, and even directs mesenchymal stem cell fate. The initial in vitro osteoblast cell response to such TiO2 nanotube surface treatments and corresponding in vivo rabbit tissue response are evaluated.
Methods
Arrays of 30, 50, 70, 100nm diameter TiO2 nanotubes formed onto titanium surfaces were compared to grit blasted titanium controls in vitro (Figure 1). SEM micrographs of bovine cartilage chondrocytes (BCCs) on the nanotube surfaces were evaluated after 2 hours, 24 hours, and 5 days of culture. Additionally 20 samples each of various nanotube diameters and the non-nanotube treated titanium controls were evaluated after exposure to human mesenchymal stem cell (hMSC) after 2 hours and 24 hours.
The left tibia and right tibia of four rabbits were implanted with disk shaped titanium implants (5.0 mm dia. × 1.5 mm) with and without TiO2 nanotubes. The front side of each implant faced the rabbit tibia bone and the back side of the implant had screw holes for post-in vivo tensile testing. After 4 weeks, the bones with implants were retrieved for mechanical testing and histology analysis.
Comparative osteogenic behavior on metal oxide nanotube surfaces applied to other implant material surface chemistries including ZrO2, Ta, and Ta2O5 were also evaluated along with TiO2 nanotubes formed on a thin films of titanium on the surface of zirconia and CoCr alloy orthopedic implants.
Results
A striking difference in ECM fibril formation and cell clustering on the nanotube substrates is evident in larger diameter nanotubes compared to non-treated titanium as shown by the arrows in Figure 2.
The average fracture strength was significantly higher for TiO2 nanotube implants (10.8 N) compared to the grit blasted titanium control implants (1.2 N). The histology at week 4 shown in Figure 3 confirms direct bonded growth of new bone onto the nanotubes with a significantly less trapped amorphous tissue at the implant-bone interface compared to the control.
Conclusions
The TiO2 nanotubes significantly enhanced the adhesion and growth of osteoblast cells (in vitro) by 300 to 400% as compared to non-nanostructure surfaces. In vivo implant tests indicate enhanced osseointegration of new bone cells on the TiO2 nanotube implant surface, with a 600% improvement in adhesion strength compared to conventional sand-blasted titanium surfaces.
Discussion
Both in vitro and in vivo analysis indicates that TiO2 nanotubes enhance the speed and proliferation of osseointegration. This surface treatment technique can be applied to non-porous or porous surfaces on implants where optimized bone fixation is desired.
