Alison Traynor

TiO2-coated CoCrMo: Improving the osteogenic differentiation and adhesion of mesenchymal stem cells in vitro

The current gold standard material for orthopedic applications is titanium (Ti), however, other materials such as cobalt-chromium-molybdenum (CoCrMo) are often preferred due to their wear resistance and mechanical strength. This study investigates if the bioactivity of CoCrMo can be enhanced by coating the surface with titanium oxide (TiO2 ) by atmospheric pressure chemical vapor deposition (CVD), thereby replicating the surface oxide layer found on Ti. CoCrMo, TiO2-coated CoCrMo (CCMT) and Ti substrates were used for this study. Cellular f-actin distribution was shown to be noticeably different between cells on CCMT and CoCrMo after 24 h in osteogenic culture, with cells on CCMT exhibiting greater spread with developed protrusions. Osteogenic differentiation was shown to be enhanced on CCMT compared to CoCrMo, with increased calcium ion content per cell (p < 0.05), greater hydroxyapatite nodule formation (p < 0.05) and reduced type I collagen deposition per cell (p < 0.05). The expression of the focal adhesion protein vinculin was shown to be marginally greater on CCMT compared to CoCrMo, whereas AFM results indicated that CCMT required more force to remove a single cell from the substrate surface compared to CoCrMo (p < 0.0001). These data suggest that CVD TiO2 coatings may have the potential to increase the biocompatibility of CoCrMo implantable devices.

The increase in cobalt release in metal- on-polyethylene hip bearings in tests with third body abrasives

Hypersensitivity reactions in patients receiving metal-on-metal hip replacements have been attributed to corrosion products as observed by elevated cobalt and chromium ions in the blood. Although the majority of cases are reported in metal-on-metal, incidences of these reactions have been reported in the metal-on-polyethylene patient population. To date, no in vitro study has considered cobalt release for this bearing combination. This study considered four 28 mm and seven 52 mm diameter metal-on-polyethylene bearings tested following ISO standard hip simulator conditions as well as under established abrasive conditions. These tests showed measurable cobalt in all bearings under standard conditions. Cobalt release, as well as polyethylene wear, increased with diameter, increasing from 52 to 255 ppb. The introduction of bone cement particles into the articulation doubled polyethylene wear and cobalt release while alumina particles produced significant damage on the heads demonstrated by cobalt levels of 70,700 ppb and an increased polyethylene wear from a mean value of 9–160 mm3/mc. Cobalt release was indicative of head damage and correlated with polyethylene wear at the next gravimetric interval. The removal of third body particles resulted in continued elevated cobalt levels in the 52 mm diameter bearings tested with alumina compared to standard conditions but the bearings tested with bone cement particles returned to standard levels. The polyethylene wear in the bone cement tested bearings also recovered to standard levels, although the alumina tested bearings continued to wear at a higher rate of 475 mm3/mc. Cobalt release was shown to occur in metal-on-polyethylene bearings indicating damage to the metal head resulting in increased polyethylene wear. While large diameter metal-on-polyethylene bearings may provide an increased range of motion and a reduced dislocation risk, increased levels of cobalt are likely to be released and this needs to be fully considered before being widely adopted.

Mesenchymal stem cell response to topographically modified CoCrMo

Abstract Surface roughness on implant materials has been shown to be highly influential on the behavior of osteogenic cells. Four surface topographies were engineered on cobalt chromium molybdenum (CoCrMo) in order to examine this influence on human mesenchymal stem cells (MSC). These treatments were smooth polished (SMO), acid etched (AE) using HCl 7.4% and H2SO4 76% followed by HNO3 30%, sand blasted, and acid etched using either 50 μm Al2O3 (SLA50) or 250 μm Al2O3 grit (SLA250). Characterization of the surfaces included energy dispersive X‐ray analysis (EDX), contact angle, and surface roughness analysis. Human MSCs were cultured onto the four CoCrMo substrates and markers of cell attachment, retention, proliferation, cytotoxicity, and osteogenic differentiation were studied. Residual aluminum was observed on both SLA surfaces although this appeared to be more widely spread on SLA50, whilst SLA250 was shown to have the roughest topography with an R a value greater than 1 μm. All substrates were shown to be largely non‐cytotoxic although both SLA surfaces were shown to reduce cell attachment, whilst SLA50 also delayed cell proliferation. In contrast, SLA250 stimulated a good rate of proliferation resulting in the largest cell population by day 21. In addition, SLA250 stimulated enhanced cell retention, calcium deposition, and hydroxyapatite formation compared to SMO (p < 0.05). The enhanced response stimulated by SLA250 surface modification may prove advantageous for increasing the bioactivity of implants formed of CoCrMo.

Mesenchymal stem cell response to UV-photofunctionalized TiO2 coated CoCrMo

Ultraviolet (UV) photofunctionalization has been shown to be highly effective at improving the osteoconductivity of titanium and TiO2 coated materials. We aimed to assess whether the bioactivity of TiO2 coated cobalt chromium molybdenum (CoCrMo) could be enhanced by UV photofunctionalization of the surface TiO2 layer. Using atmospheric pressure chemical vapour deposition (APCVD) a thin layer of anatase TiO2 was deposited onto smooth CoCrMo discs (referred to as CCMT). Human mesenchymal stem cells (MSCs) were cultured onto CCMT substrates which had been treated with UV light for 24 hours and identical substrates which had not undergone UV treatment. UV treated CCMT promoted a superior cell response in the form of enhancing markers of cell adhesion. This included stimulating the development of larger cells with increased levels of the adhesion protein vinculin and cytoskeletal protein f-actin (p < 0.05). In addition, MSCs were shown to have superior retention to UV treated CCMT after 3 and 24 hours (p < 0.05). Other cellular processes including proliferation, attachment, migration and differentiation were not affected by UV photofunctionalization. Despite this, the enhancement in cellular adhesion alone should result in an improvement in MSC retention to implant surfaces following surgery, and as a consequence, increase MSC resistance to dislodgement from external forces such as blood flow and micro motion.

ECiMa™ for Low Wear, Optimal Mechanical Properties and Oxidation Resistance of Hip Bearings

Peri-prosthetic osteolysis, secondary to wear, is a limitation of conventional UHMWPE. Highly cross-linked polyethylene (HXLPE) materials were introduced to overcome these limitations; however, there are still concerns with regard to material property degradation and oxidation. ECiMa is a cold-irradiated, mechanically annealed, vitamin-E-blended HXLPE developed to maintain mechanical properties, minimise wear and improve long-term oxidation resistance. This chapter reports on the in vitro testing that has been completed for ECiMa. The material has demonstrated a reduced wear rate during in vitro hip simulation when compared to conventional UHMWPE, and improved mechanical properties and long-term oxidation resistance in comparison to HXLPE. This makes ECiMa a promising next-generation bearing material.