Abstract
Recent clinical data suggest improvement in the fixation of tibia trays for total knee arthroplasty when the trays are additive manufactured with highly porous bone ingrowth structures. Currently, press-fit TKA is less common than press-fit THA. This is partly because the loads on the relatively flat, porous, bony apposition area of a tibial tray are more demanding than those same porous materials surrounding a hip stem. Even the most advanced additive manufactured (AM) highly porous structures have bone ingrowth limitations clinically as aseptic loosening still remains more common in press-fit TKA vs. THA implants.
Osseointegration and antibacterial properties have been shown in vitro and in vivo to improve when implants have modified surfaces that have biomimetic nanostructures designed to mimic and interact with biological structures on the nano-scale. Pre-clinical evaluations show that TiO2 nanotubes (TNT), produced by anodization, on Ti6Al4V surfaces positively enhance the rate at which osseointegration occurs and TNT nano-texturization enhances the antibacterial properties of the implant surface.2
In this in vivo sheep study, identical Direct Metal laser Sintered (DMLS) highly porous Ti6Al4V specimens with and without TNT surface treatment are compared to sintered bead specimens with plasma sprayed hydroxyapatite-coated surface treatment. Identical DMLS specimens made from CoCrMo were also implanted in sheep tibia bi-cortically (3 per tibia) and in the cancellous bone of the distal femur and proximal tibia (1 per site). Animals were injected with fluorochrome labels at weeks 1, 2 and 3 after surgery to assess the rate of bone integration. The cortical specimens were mechanically tested and processed for PMMA histology and histomorphometry after 4 or 12 weeks. The cancellous samples were also processed for PMMA histology and histomorphometry. The three types of bone labels were visualized under UV light to examine the rate of new bony integration.
At 4 weeks, a 42% increase in average pull-out shear strength between nanotube treated specimens and non-nanotube treated specimens was shown. A 21% increase in average pull-out shear strength between nanotube treated specimens and hydroxyapatite-coated specimens was shown. At 12 weeks, all specimens had statistically similar pull-out values. Bone labels demonstrated new bone formation into the porous domains on the materials as early as 2 weeks.
A separate in vivo study on 8 rabbits infected with methicillin-resistant Staphylococcus aureus showed bacterial colonization reduction on the surface of the implants treated with TNT. In vitro and in vivo evidence suggests that nanoscale surfaces have an antibacterial effect due to surface energy changes that reduce the ability of bacteria to adhere.
These in vivo studies show that TNT on highly porous AM specimens made from Ti6Al4V enhances new bone integration and also reduce microbial attachment.
