The effect of grinding conditions on the toxicity of cobalt-chrome-molybdenum particles in vitro

Toxicology of wear particles of cobalt-chromium alloy metal-on-metal hip implants Part II: Importance of physicochemical properties and dose in animal and in vitro studies as a basis for risk assessment

The objective of the Part II analysis was to evaluate animal and in vitro toxicology studies of CoCr particles with respect to their physicochemistry and dose relevance to metal-on-metal (MoM) implant patients as derived from Part I. In the various toxicology studies, physicochemical characteristics were infrequently considered and administered doses were orders of magnitude higher than what occurs in patients. Co was consistently shown to rapidly release from CoCr particles for distribution and elimination from the body. CoCr micron sized particles appear more biopersistent in vivo resulting in inflammatory responses that are not seen with similar mass concentrations of nanoparticles. We conclude, that in an attempt to obtain data for a complete risk assessment, future studies need to focus on physicochemical characteristics of nano and micron sized particles and on doses and dose metrics relevant to those generated in patients or in properly conducted hip simulator studies.

Influence of boron addition to Ti-13Zr-13Nb alloy on MG63 osteoblast cell viability and protein adsorption

Cell proliferation, cell morphology and protein adsorption on near β-type Ti-13Zr-13Nb (TZN) alloy and Ti-13Zr-13Nb-0.5B (TZNB) composite have been investigated and compared to evaluate the effect of boron addition which has been added to the Ti alloy to improve their poor tribological properties by forming in situ TiB precipitates. MG63 cell proliferation on substrates with different chemistry but the same topography was compared. The MTT assay test showed that the cell viability on the TZN alloy was higher than the boron containing TZNB composite after 36 h of incubation and the difference was pronounced after 7 days. However, both the materials showed substantially higher cell attachment than the control (polystyrene). For the same period of incubation in fetal bovine serum (FBS), the amount of protein adsorbed on the surface of boron free TZN samples was higher than that in the case of boron containing TZNB composite. The presence of boron in the TZN alloy influenced protein adsorption and cell response and they are lower in TZNB than in TZN as a result of the associated difference in chemical characteristics.

Cobalt ions influence proliferation and function of human osteoblast-like cells

Cobalt is the major component in many orthopedic implants and the introduction of a second generation of metal on metal bearing prosthesis systems actualizes the toxicity and biocompatibility of this compound. We studied the effect of cobalt ions on primary cultures of human osteoblast-like cells. Cobalt ions dissolved in cell culture medium caused a dose-dependent decrease in proliferation of human osteoblasts measured as (3H)thymidine incorporation. We also found that cobalt ion-enriched medium increased the production of interleukin-6 from the osteoblast-like cells. Furthermore, incubation of osteoblasts with cobalt ion-enriched medium reduced collagen type I and osteocalcin production in a dose-dependent manner when 1,25 dihydroxyvitamin-D3 was added to the culture medium. Cobalt concentrations below 10 microg/mL or 0.17 mmollmL in the cell culture medium had no significant effect on human osteoblast proliferation and function.

Effect of mechanical surface pretreatment on metal ion release

The degree of metal ion dissolution from Ti-6Al-4V alloy hip replacement stems subjected to various mechanical and chemical surface pretreatments was analysed in vitro. High-dissolution rates were observed for nitric acid passivated samples that had been mechanically surface treated to increase the implant surface area. Significantly lower ion release levels were observed for mechanically treated samples which had been aged in de-ionised water. The application of an hydroxyapatite coating decreased the metal ion release from the nitric acid passivated samples (compared to the uncoated sample) and increased the metal ion dissolution from the aged samples. The dissolution behaviour of the samples is explained in terms of the diffusion processes occurring at the stem/solution interface and the morphological and chemical characteristics of the surface treated stems.

Effect of surface roughness and composition on costochondral chondrocytes is dependent on cell maturation state

During endochondral bone formation, as occurs in fracture healing, chondrocytes are one of the first cells to see an implant surface. We tested the hypothesis that chemical composition and surface roughness affect chondrocyte differentiation, matrix synthesis, and local factor production and that the nature of the response is dependent on the state of maturation of the cells. To do this, we harvested rat growth zone and resting zone chondrocytes and examined their response to smooth and rough disk surfaces manufactured from either commercially pure titanium or titanium alloy. Profilometry, scanning electron microscopy, Auger spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the surfaces. Average roughness values were 0.22 microm for smooth titanium surfaces, 0.23 microm for smooth titanium alloy surfaces, 4.24 microm for rough titanium surfaces, and 3.20 microm for rough titanium alloy surfaces. Cells were grown on the different disk surfaces until the cultures had reached confluence on plastic. The effect of the surfaces was determined by assaying cell number and [3H]thymidine incorporation as measures of cell proliferation, cell layer and cell alkaline phosphatase specific activity as markers of differentiation, and collagen production and [35S]sulfate incorporation as indicators of extracellular matrix production. In addition, the synthesis of prostaglandin E2 and transforming growth factor-beta were examined to measure changes in local factor synthesis. In growth zone and resting zone cultures, cell number and [3H]thymidine incorporation were decreased on rough surfaces; however, this effect was greater on commercially pure titanium surfaces. Cell layer and cell alkaline phosphatase specific activity were decreased in resting zone cells grown on rough surfaces. Cell alkaline phosphatase specific activity in growth zone cells was decreased on rough surfaces, whereas cell layer alkaline phosphatase specific activity was increased only in growth zone cells grown on rough commercially pure titanium surfaces. Resting zone cell collagen production was decreased only on rough commercially pure titanium, whereas in growth zone cells, collagen production was increased. Increased prostaglandin E2 release into the media was found for growth zone and resting zone cell cultures on the disks with rough surfaces. The observed effect was greater on rough commercially pure titanium. Production of transforming growth factor-beta by resting zones was similarly affected, whereas an increase in its production by growth zone cells was measured only on rough commercially pure titanium. These results indicate that surface roughness affects chondrocyte proliferation, differentiation, matrix synthesis, and local factor production and that these parameters are also affected by chemical composition. Furthermore, the nature and extent of the cell response is dependent on cell maturation. The overriding variable in response to an implant material, however, appears to be roughness of the surface.

Biologic effects of cobalt chrome in cell and animal models

The literature on animal and cellular models used to study the response to cobalt chrome alloy implants and wear and corrosion products is reviewed. Animal studies show that in solid form cobalt chrome alloy is relatively well tolerated. Injections of large numbers of particles in a single bolus lead to acute inflammation and necrosis, followed by a chronic inflammatory response. Macrophages are the predominant cell type and may persist in the tissues for years. Long term studies have failed to confirm the induction of tumors. In vitro studies confirm the toxic effects of cobalt chrome alloy corrosion products and wear particles, especially cobalt, and show that intracellular corrosion is an important mechanism for early release of cobalt ions. In vitro studies show that cobalt chrome alloy particles induce the release of inflammatory mediators from macrophages before causing cell death. These mediators have significant effects on osteoblastlike cells, as well as inducing bone resorption. Variations in the cell types, implantation site, and characteristics of the particles used in experimental models make interpretation of the results difficult. Standardized methods to control for size, shape, and number of particles for testing are proposed. It is important that in vitro and in vivo findings not be taken in isolation, but be compared with the results of human studies.

Effect of titanium surface roughness on chondrocyte proliferation, matrix production, and differentiation depends on the state of cell maturation

Although it is well accepted that implant success is dependent on various surface properties, little is known about the effect of surface roughness on cell metabolism or differentiation, or whether the effects vary with the maturational state of the cells interacting with the implant. In the current study, we examined the effect of titanium (Ti) surface roughness on chondrocyte proliferation, differentiation, and matrix synthesis using cells derived from known stages of endochondral development. Chondrocytes derived from the resting zone (RCs) and growth zone (GCs) of rat costochondral cartilage were cultured on Ti disks that were prepared as follows: HF-HNO3-treated and washed (PT); PT-treated and electropolished (EP); fine sand-blasted, HCl-H2SO4-etched, and washed (FA); coarse sand-blasted, HCl-H2SO4-etched, and washed (CA); or Ti plasma-sprayed (TPS). Based on surface analysis, the Ti surfaces were ranked from smoothest to roughest: EP, PT, FA, CA, and TPS. Cell proliferation was assessed by cell number and [3H]-thymidine incorporation, and RNA synthesis was assessed by [3H]-uridine incorporation. Differentiation was determined by alkaline phosphatase specific activity (AL-Pase). Matrix production was measured by [3H]-proline incorporation into collagenase-digestible (CDP) and noncollagenase-digestible (NCP) protein and by [35S]-sulfate incorporation into proteoglycan. GCs required two trypsinizations for complete removal from the culture disks; the number of cells released by the first trypsinization was generally decreased with increasing surface roughness while that released by the second trypsinization was increased. In RC cultures, cell number was similarly decreased on the rougher surfaces; only minimal numbers of RCs were released by a second trypsinization. [3H]-thymidine incorporation by RCs decreased with increasing surface roughness while that by GCs was increased. [3H]-Uridine incorporation by both GCs and RCs was greater on rough surfaces. Conversely, ALPase in the cell layer and isolated cells of both cell types was significantly decreased. GC CDP and NCP production was significantly decreased on rough surfaces while CDP production by RC cells was significantly decreased on smooth surfaces. [35S]-sulfate incorporation by RCs and GCs was decreased on all surfaces compared to tissue culture plastic. The results of this study indicate that surface roughness affects chondrocyte proliferation, differentiation, and matrix synthesis, and that this regulation is cell maturation dependent.

Effect of titanium surface roughness on chondrocyte proliferation, matrix production, and differentiation depends on the state of cell maturation

Although it is well accepted that implant success is dependent on various surface properties, little is known about the effect of surface roughness on cell metabolism or differentiation, or whether the effects vary with the maturational state of the cells interacting with the implant. In the current study, we examined the effect of titanium (Ti) surface roughness on chondrocyte proliferation, differentiation, and matrix synthesis using cells derived from known stages of endochondral development. Chondrocytes derived from the resting zone (RCs) and growth zone (GCs) of rat costochondral cartilage were cultured on Ti disks that were prepared as follows: HF-HNO3-treated and washed (PT); PT-treated and electropolished (EP); fine sand-blasted, HCl-H2SO4-etched, and washed (FA); coarse sand-blasted, HCl-H2SO4-etched, and washed (CA); or Ti plasma-sprayed (TPS). Based on surface analysis, the Ti surfaces were ranked from smoothest to roughest: EP, PT, FA, CA, and TPS. Cell proliferation was assessed by cell number and [3H]-thymidine incorporation, and RNA synthesis was assessed by [3H]-uridine incorporation. Differentiation was determined by alkaline phosphatase specific activity (AL-Pase). Matrix production was measured by [3H]-proline incorporation into collagenase-digestible (CDP) and noncollagenase-digestible (NCP) protein and by [35S]-sulfate incorporation into proteoglycan. GCs required two trypsinizations for complete removal from the culture disks; the number of cells released by the first trypsinization was generally decreased with increasing surface roughness while that released by the second trypsinization was increased. In RC cultures, cell number was similarly decreased on the rougher surfaces; only minimal numbers of RCs were released by a second trypsinization. [3H]-thymidine incorporation by RCs decreased with increasing surface roughness while that by GCs was increased. [3H]-Uridine incorporation by both GCs and RCs was greater on rough surfaces. Conversely, ALPase in the cell layer and isolated cells of both cell types was significantly decreased. GC CDP and NCP production was significantly decreased on rough surfaces while CDP production by RC cells was significantly decreased on smooth surfaces. [35S]-sulfate incorporation by RCs and GCs was decreased on all surfaces compared to tissue culture plastic. The results of this study indicate that surface roughness affects chondrocyte proliferation, differentiation, and matrix synthesis, and that this regulation is cell maturation dependent.

Effect of titanium surface roughness on proliferation, differentiation, and protein synthesis of human osteoblast-like cells (MG63)

The effect of surface roughness on osteoblast proliferation, differentiation, and protein synthesis was examined. Human osteoblast-like cells (MG63) were cultured on titanium (Ti) disks that had been prepared by one of five different treatment regimens. All disks were pretreated with hydrofluroic acid-nitric acid and washed (PT). PT disks were also: washed, and then electropolished (EP); fine sandblasted, etched with HCl and H2SO4, and washed (FA); coarse sandblasted, etched with HCl and H2SO4, and washed (CA); or Ti plasma-sprayed (TPS). Standard tissue culture plastic was used as a control. Surface topography and profile were evaluated by brightfield and darkfield microscopy, cold field emission scanning electron microscopy, and laser confocal microscopy, while chemical composition was mapped using energy dispersion X-ray analysis and elemental distribution determined using Auger electron spectroscopy. The effect of surface roughness on the cells was evaluated by measuring cell number, [3H]thymidine incorporation into DNA, alkaline phosphatase specific activity, [3H]uridine incorporation into RNA, [3H]proline incorporation into collagenase digestible protein (CDP) and noncollagenase-digestible protein (NCP), and [35S]sulfate incorporation into proteoglycan. Based on surface analysis, the five different Ti surfaces were ranked in order of smoothest to roughest: EP, PT, FA, CA, and TPS. A TiO2 layer was found on all surfaces that ranged in thickness from 100 A in the smoothest group to 300 A in the roughest. When compared to confluent cultures of cells on plastic, the number of cells was reduced on the TPS surfaces and increased on the EP surfaces, while the number of cells on the other surfaces was equivalent to plastic. [3H]Thymidine incorporation was inversely related to surface roughness. Alkaline phosphatase specific activity in isolated cells was found to decrease with increasing surface roughness, except for those cells cultured on CA. In contrast, enzyme activity in the cell layer was only decreased in cultures grown on FA- and TPS-treated surfaces. A direct correlation between surface roughness and RNA and CDP production was found. Surface roughness had no apparent effect on NCP production. Proteoglycan synthesis by the cells was inhibited on all the surfaces studied, with the largest inhibition observed in the CA and EP groups. These results demonstrate that surface roughness alters osteoblast proliferation, differentiation, and matrix production in vitro. The results also suggest that implant surface roughness may play a role in determining phenotypic expression of cells in vivo.

Underlying mechanisms at the bone-biomaterial interface

In order to understand how biomaterials influence bone formation in vivo, it is necessary to examine cellular response to materials in the context of wound healing. Four interrelated properties of biomaterials (chemical composition, surface energy, surface roughness, and surface topography) affect mesenchymal cells in vitro. Attachment, proliferation, metabolism, matrix synthesis, and differentiation of osteoblast-like cell lines and primary chondrocytes are sensitive to one or more of these properties. The nature of the response depends on cell maturation state. Rarely do differentiated osteoblasts or chondrocytes see a material prior to its modification by biological fluids, immune cells and less differentiated mesenchymal cells in vivo. Studies using the rat marrow ablation model of endosteal wound healing indicate that ability of osteoblasts to synthesize and calcify their extracellular matrix is affected by the local presence of the material. Changes in the morphology and biochemistry of matrix vesicles, extracellular organelles associated with matrix maturation and calcification, seen in normal endosteal healing, are altered by implants. Moreover, the material exerts a systemic effect on endosteal healing as well. This may be due to local effects on growth factor production and secretion into the circulation, as well as to the fact that the implant may serve as a bioreactor.

Cell damage in vitro following direct contact with fine particles of titanium, titanium alloy and cobalt-chrome-molybdenum alloy

Fibroblastic cells in vitro were exposed to powders of titanium, titanium-aluminium-vanadium alloy and cobalt-chrome-molybdenum (Co-Cr-Mo) alloy, either in direct contact with the cells or separated from the cells by a microporous membrane. Fine particles of all the materials reduced cell growth when in direct contact with cells, but only the finest particles of Co-Cr-Mo alloy caused cell damage through the microporous membrane. This provides further evidence that there is a mechanism of cell damage in vitro which depends on a direct interaction between cells and particles and is largely independent of the chemical nature of the particle.

Size and shape of biomaterial wear debris

A literature review of wear debris is presented. Included are debris retrieved at revision of total joint replacement and at autopsy, as well as debris produced in vitro in wear testers and joint simulators or otherwise fabricated for biological experiments. Observations of wear debris in vivo and in vitro are classified in tabular form according to material type, origin, size, shape and color. Polymer particles, most commonly ultra-high molecular weight polyethylene (UHMWPE), exhibit the largest size range and appear as granules, splinters or flakes, while ceramic particles possess the smallest size range and have a granular structure. Metal particles seen in vivo and in vitro, whether from cobalt-chromium alloys or, less frequently, other alloys, form granular or needle-like shapes and generally are smaller than polymer particles but larger than ceramic particles. Particles generated in joint simulators resemble the size and shape of in vivo wear particles from total joint replacement (TJR) retrieved at revision or autopsy. However, particles prepared in vitro, whether in simulators or by other means, do not consistently resemble wear debris particles from TJR.

A method for production and characterization of metal prosthesis wear particles

The wear of joint prostheses generates wear particles that produce an inflammatory response in the surrounding tissues and may contribute to bone resorption resulting in prosthetic loosening. Although the effects of particles produced from prosthetic materials have been studied extensively in vitro and in vivo, little attention has been paid to the standardisation of methods for the generation and characterization of these particles. This paper describes a reproducible method for generation of metal particles by the abrasive shaking of joint replacement components. Particular attention was given to the production of metal particles that closely resembled particles found around solid and loose human prostheses. To achieve this, particle size, size distribution, chemical composition, and shape were characterized. Particles that were 0.5-3.0 microns in diameter were isolated by differential sedimentation, and the distribution of particle sizes was determined with use of a Coulter Multisizer. Chemical composition was measured by atomic absorption spectrophotometry, and transmission electron microscopy was used to characterize particle shape. The techniques were shown to be reproducible, since there was little variation between batches over a lengthy time period. These or similar methods of particle production and characterization should be an essential part of future in vitro and in vivo studies of wear particles.

The bone-titanium interface in vitro

Commercially pure 5-mm-diameter titanium (cpTi) discs received droplet inoculations of cells derived from rat bone marrow and were maintained in supplemented culture medium for 2-3 weeks. The cells and extracellular matrix (ECM) were processed for observation by light (LM), scanning (SEM), and transmission electron (TEM) microscopy. The latter was achieved by freeze-fracturing the solid metal from the resin-embedded tissue using a method which preserved the interface. Surface staining of whole discs revealed cells separated from the metal substratum by areas of ECM which stained positively using von Kossa's method to identify mineralization. At SEM, the ECM comprised dense interwoven collagen fiber networks which were partially obscured by globular masses (GMs). Individual GMs were associated with collagen fibers, especially at fiber intersections. EDAX line scan analysis confirmed the presence of Ca and P in these areas which were assumed to be spheritic foci of calcification since the Ca and P peaks diminished in areas which demonstrated only collagen fibers or the underlying cpTi. TEM examination confirmed the presence of globular mineralization and also revealed the presence of an interfacial zone between the metal substratum and the mineralized ECM elaborated by osteoblasts during the culture period. The interfacial zone comprised two layers, a bonding zone containing few collagen fragments and a ruthenium red positive layer containing more densely packed collagen fibers. We believe that this is the first report of both the formation of bonelike tissue on solid titanium substrata in vitro and demonstration of an interface which bears close morphological similarities to that known to develop in vivo.