An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads

Hard Cr2O3 coatings on SS316L substrates prepared by reactive magnetron sputtering technique: a potential candidate for orthopedic implants

316L stainless steel (SS) implants suffer from tribological and biocompatibility problems which limit their service lifetime. In order to improve the surface properties of 316L SS for orthopedic implant applications, hard chromium oxide coatings were applied on 316L SS substrates using a reactive magnetron sputtering technique. The morphological, structural, and phase compositional analyses were conducted on the deposited coatings by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The Rockwell-C indentation tests were performed on the coated substrates to qualitatively evaluate the adhesion of coatings on the steel substrates. The surface characteristics of coatings were measured by using an optical profilometer. The mechanical properties of coatings were reported by measuring the Hardness and Young's modulus. The corrosion resistance of coated and uncoated SS substrates was compared using potentiodynamic polarization tests. An inductively coupled plasma optical emission spectrometry (ICP-OES) was employed to analyze the biocompatibility of the samples by measuring the amount of toxic Cr ions released after the immersion test. The results show that the coatings are adherent and composed of a single Cr₂O₃ phase with a hardness of 25 to 29 GPa. The corrosion resistance of the SS has been improved by applying a chromium oxide coating. The coated SS samples have also demonstrated better wear resistance and lower friction coefficient compared to bare SS samples under a reciprocating sliding condition in saline solution. The biocompatibility of the SS has been enhanced by the Cr₂O₃ coating as much less Cr ions were released after immersion tests. These results indicate that the hard Cr₂O₃ coatings can be considered as a candidate for extending the lifetime of SS implants.

Simultaneous Characterization of Implant Wear and Tribocorrosion Debris within Its Corresponding Tissue Response Using Infrared Chemical Imaging

Biotribology is one of the key branches in the field of artificial joint development. Wear and corrosion are among fundamental processes which cause material loss in a joint biotribological system; the characteristics of wear and corrosion debris are central to determining the in vivo bioreactivity. Much effort has been made elucidating the debris-induced tissue responses. However, due to the complexity of the biological environment of the artificial joint, as well as a lack of effective imaging tools, there is still very little understanding of the size, composition, and concentration of the particles needed to trigger adverse local tissue reactions, including periprosthetic osteolysis. Fourier transform infrared spectroscopic imaging (FTIR-I) provides fast biochemical composition analysis in the direct context of underlying physiological conditions with micron-level spatial resolution, and minimal additional sample preparation in conjunction with the standard histopathological analysis workflow. In this study, we have demonstrated that FTIR-I can be utilized to accurately identify fine polyethylene debris accumulation in macrophages that is not achievable using conventional or polarized light microscope with histological staining. Further, a major tribocorrosion product, chromium phosphate, can be characterized within its histological milieu, while simultaneously identifying the involved immune cell such as macrophages and lymphocytes. In addition, we have shown the different spectral features of particle-laden macrophages through image clustering analysis. The presence of particle composition variance inside macrophages could shed light on debris evolution after detachment from the implant surface. The success of applying FTIR-I in the characterization of prosthetic debris within their biological context may very well open a new avenue of research in the orthopedics community.

The Effect of Coating Density on Functional Properties of SiNx Coated Implants

Ceramic coatings may be applied onto metallic components of joint replacements for improved wear and corrosion resistance as well as enhanced biocompatibility, especially for metal-sensitive patients. Silicon nitride (SiN_x) coatings have recently been developed for this purpose. To achieve a high coating density, necessary to secure a long-term performance, is however challenging, especially for sputter deposited SiN_x coatings, since these coatings are insulating. This study investigates the time-dependent performance of sputter-deposited SiN_x based coatings for joint applications. SiN_x coatings with a thickness in the range of 4.3–6.0 µm were deposited by reactive high power impulse magnetron sputtering onto flat discs as well as hip heads made of CoCrMo. SiN_x compositional analysis by X-ray photoelectron spectroscopy showed N/Si ratios between 0.8 and 1.0. Immersion of the flat disks in fetal bovine serum solution over time as well as short-term wear tests against ultra-high molecular weight polyethylene (UHMWPE) discs showed that a high coating density is required to inhibit tribocorrosion. Coatings that performed best in terms of chemical stability were deposited using a higher target power and process heating.

Chromium oxide coatings with the potential for eliminating the risk of chromium ion release in orthopaedic implants

Chromium oxide coatings prepared by radiofrequency reactive magnetron sputtering on stainless steel substrates were exposed to Ringer's physiological solution and tested for their electrochemical corrosion stability using an open circuit potential measurement, potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky analysis. The coatings were found to be predominantly Cr₂O₃, based on the observation of the dominance of [Formula: see text] and E_g symmetric modes in our Raman spectroscopic investigation and the E_u vibrational modes in our Fourier transform infrared spectroscopic measurements on the coatings. We investigated for the presence of chromium ions in Ringer's solution after all of the above electrochemical tests using atomic absorption spectroscopy, without finding a trace of chromium ions at the ppm level for coatings tested under open circuit and at the lower potentials implants are likely to experience in the human body. The coatings were further exposed to Ringer's solution for one month and tested for adhesion strength changes, and we found that they retained substantial adhesion to the substrates. We expect this finding to be significant for future orthopaedic implants where chromium ion release is still a major challenge.

Chrome nitride coating reduces wear of small, spherical CrCoMo metal-on-metal articulations in a joint simulator

Metal-on-metal articulations have fallen out of favour in larger joint replacements, but are still used in smaller joints. Coating the articulation has been suggested as one way of reducing wear. We compared a standard 6 mm CrCoMo articulation designed for the carpometacarpal joint of the thumb with a chromium nitride-coated version after 512,000 cycles in a joint simulator. A total of 6 articulations in each group were tested with a unidirectional load of 5 kg in Ringer's solution. We found a statistically significant reduction in weight loss, amount of metallic wear produced and volumetric wear for the chromium nitride-coated articulation. Our findings support the use of chromium nitride coating in order to minimize the amount of metallic wear produced.

Clinical outcomes of ceramicized ball heads in total hip replacement bearings: a literature review

BACKGROUND

Metallic ball heads for total hip replacement (THR) bearings with ceramicized surfaces were introduced in orthopedics during the second half of the 1980s, with the aim of decreasing the wear of polyethylene cups.

METHODS

An analysis was made of the literature regarding outcomes for metallic ball heads with ceramicized surfaces now in clinical use (TiN, TiNbN, ZrN, monoclinic ZrO2), as well as carbon coatings (pyrolytic carbon, diamond-like carbon) and silicon nitride as coatings in ball heads for THR bearings.

RESULTS

Notwithstanding the diffusion of ceramicized ball heads in THRs, there are few reports about their clinical outcomes in hip arthroplasty. In addition, several clinical studies and some registry data are putting under scrutiny the clinical advantages of ceramicized ball heads over cobalt chrome (CoCr) alloy and ceramic ball heads.

CONCLUSIONS

The wear of THR bearings with ceramicized ball heads looks like it depends more on the behavior of the polyethylene cups than on the treatment of the ball head surface. The risk of coating damage and of its consequences has to be taken into account in selecting this type of bearing.

A novel method for isolation and recovery of ceramic nanoparticles and metal wear debris from serum lubricants at ultra-low wear rates

Ceramics have been used to deliver significant improvements in the wear properties of orthopaedic bearing materials, which has made it challenging to isolate wear debris from simulator lubricants. Ceramics such as silicon nitride, as well as ceramic-like surface coatings on metal substrates have been explored as potential alternatives to conventional implant materials. Current isolation methods were designed for isolating conventional metal, UHMWPE and ceramic wear debris. In this paper, we describe a methodology for isolation and recovery of ceramic or ceramic-like coating particles and metal wear particles from serum lubricants under ultra-low and low wear performance. Enzymatic digestion was used to digest the serum proteins and sodium polytungstate was used as a novel density gradient medium to isolate particles from proteins and other contaminants by ultracentrifugation. This method demonstrated over 80% recovery of particles and did not alter the size or morphology of ceramic and metal particles during the isolation process.

STATEMENT OF SIGNIFICANCE

Improvements in resistance to wear and mechanical damage of the articulating surfaces have a large influence on longevity and reliability of joint replacement devices. Modern ceramics have demonstrated ultra-low wear rates for hard-on-hard total hip replacements. Generation of very low concentrations of wear debris in simulator lubricants has made it challenging to isolate the particles for characterisation and further analysis. We have introduced a novel method to isolate ceramic and metal particles from serum-based lubricants using enzymatic digestion and novel sodium polytungstate gradients. This is the first study to demonstrate the recovery of ceramic and metal particles from serum lubricants at lowest detectable in vitro wear rates reported in literature.

Lessons learnt from metal-on-metal hip arthroplasties will lead to safer innovation for all medical devices

Metal-on-metal bearings were re-popularised in the late 1990s with the introduction of modern hip resurfacing. Large diameter (LD) metal-on-metal (MoM) hips became more prevalent and have been the least successful group of hip implants ever used. They were rapidly adopted from 2004 until the British Hip Society stopped their use in 2012. Well functioning MoM hip results (including the BHR and Metasul) are hidden in the mire of poor results from the group of all MoM bearings.We have reviewed what happened and we make 3 observations. Firstly, collaboration between surgeons and then between surgeons and other disciplines, first identified and then solved the clinical management problems. Secondly, the problems with MoM hips occurred because hip simulation was inadequate at predicting performance in patients. They gave no indications of the biological effects of wear in the human environment. Lastly, retrieval of failed implants was essential to understanding why failure occurred.These lessons must never be forgotten and must form the basis by which new or altered implants are introduced and how they should be monitored. This will enable safer innovation for patients, surgeons and manufacturers. The problems with MoM hips will not have been in vain.

Lessons from retrievals: Retrievals help understand the reason for revision of coated hip arthroplasties

Coatings have been applied to all surfaces of hip implants with the majority performing well in the laboratory, but there are few reports of their performance in humans. The rationale for coating the metal-on-metal bearing surfaces includes a reduction in metal ion release and risk of adverse reaction to metal debris; yet there are no reports of retrieved coated metal-on-metal hip implants despite the concern that they may delaminate. The aim of this study was to better understand the performance of coated hip implants in humans through findings of three coated metal-on-metal hip resurfacings, retrieved after failure for unexplained pain. Analysis of these implants included quantification of the amount and mechanism of coating loss which was correlated with clinical, imaging and blood data. In all cases, there were large areas of complete coating loss in which the metal substrate was exposed and found to be rougher than the coated areas. The coating loss gave rise to third body abrasive wear of the coating and the exposed metal, the latter of which led to high blood levels of cobalt and chromium. Coating of the bearing surfaces of metal-on-metal hip resurfacings, therefore, do not prevent metal ion release when implanted into humans. This is an example of a need for increased retrieval analysis of newly introduced implants and expansion of laboratory testing regulations to better reflect the clinical environment.

Titanium-Nitride Coating of Orthopaedic Implants: A Review of the Literature

Surfaces of medical implants can be enhanced with the favorable properties of titanium-nitride (TiN). In a review of English medical literature, the effects of TiN-coating on orthopaedic implant material in preclinical studies were identified and the influence of these effects on the clinical outcome of TiN-coated orthopaedic implants was explored. The TiN-coating has a positive effect on the biocompatibility and tribological properties of implant surfaces; however, there are several reports of third body wear due to delamination, increased ultrahigh molecular weight polyethylene wear, and cohesive failure of the TiN-coating. This might be due to the coating process. The TiN-coating process should be optimized and standardized for titanium alloy articulating surfaces. The clinical benefit of TiN-coating of CoCrMo knee implant surfaces should be further investigated.

Articulating Biomaterials

This chapter examines the importance of surface characteristics such as microstructure, composition, crystallographic texture, and surface free energy in achieving desired biocompatibility and tribological properties thereby improving in vivo life of artificial articulating implants. Current implants often fail prematurely due to inadequate mechanical, tribological, biocompatibility, and osseointegration properties, apart from issues related to design and surgical procedures. For long-term in vivo stability, artificial implants intended for articulating joint replacement must exhibit long-term stable articulation surface without stimulating undesirable in vivo effects. Since the implant's surface plays a vital and decisive role in their response to biological environment, and vice versa, surface modification of implants assumes a significant importance. Therefore, overview on important surface modification techniques, their capabilities, properties of modified surfaces/implants are presented in the chapter. The clinical performance of surface modified implants and new surfaces for potential next-generation articulating implant applications are discussed at the end.

Mechanical and tribological behavior of silicon nitride and silicon carbon nitride coatings for total joint replacements

Total joint replacements currently have relatively high success rates at 10-15 years; however, increasing ageing and an active population places higher demands on the longevity of the implants. A wear resistant configuration with wear particles that resorb in vivo can potentially increase the lifetime of an implant. In this study, silicon nitride (SixNy) and silicon carbon nitride (SixCyNz) coatings were produced for this purpose using reactive high power impulse magnetron sputtering (HiPIMS). The coatings are intended for hard bearing surfaces on implants. Hardness and elastic modulus of the coatings were evaluated by nanoindentation, cohesive, and adhesive properties were assessed by micro-scratching and the tribological performance was investigated in a ball-on-disc setup run in a serum solution. The majority of the SixNy coatings showed a hardness close to that of sintered silicon nitride (~18 GPa), and an elastic modulus close to that of cobalt chromium (~200 GPa). Furthermore, all except one of the SixNy coatings offered a wear resistance similar to that of bulk silicon nitride and significantly higher than that of cobalt chromium. In contrast, the SixCyNz coatings did not show as high level of wear resistance.

Fabrication and evaluation of SixNy coatings for total joint replacements

Wear particles from the bearing surfaces of joint implants are one of the main limiting factors for total implant longevity. Si(3)N(4) is a potential wear resistant alternative for total joint replacements. In this study, Si(x)N(y)-coatings were deposited on cobalt chromium-discs and Si-wafers by a physical vapour deposition process. The tribological properties, as well as surface appearance, chemical composition, phase composition, structure and hardness of these coatings were analysed. The coatings were found to be amorphous or nanocrystalline, with a hardness and coefficient of friction against Si(3)N(4) similar to that found for bulk Si(3)N(4). The low wear rate of the coatings indicates that they have a potential as bearing surfaces of joint replacements. The adhesion to the substrates remains to be improved.

What's next? Alternative materials for articulation in total joint replacement

The use of an artificial joint is always related to a certain amount of wear. Its biological effects, e.g., the osteolysis potential, are a function of the bulk material as well as its debris. Following comprehensive experiences with polyethylene (PE) wear, material science is tracking two ways to minimize the risk of a particle-induced aseptic implant loosening: (i) reduction of the PE debris by a low-wearing articulation partner; and (ii) replacement of the PE by other materials. Therefore, new ceramics (e.g., ZTA, Si(3)N(4)), as well as coatings (e.g., TiN, "diamond-like" carbon) and modifications of a bulk metal (e.g., oxidizes zirconium) or cushion bearings (polyurethane, hydrogels), are currently available for total joint replacements or have been used for pre-clinical testing. This review gives a brief overview and evaluates the potential of those that have recently been published in literature.

The John Charnley Award: an accurate and extremely sensitive method to separate, display, and characterize wear debris: part 2: metal and ceramic particles

BACKGROUND

Metal-on-metal and ceramic-on-ceramic bearings were introduced as alternatives to conventional polyethylene in hip arthroplasties to reduce wear. Characterization of wear particles has been particularly challenging due to the low amount and small size of wear particles. Current methods of analysis of such particles have shortcomings, including particle loss, clumping, and inaccurate morphologic and chemical characterization.

QUESTIONS/PURPOSES

We describe a method to recover and characterize metal and ceramic particles that (1) improves particle purification, separation, and display; (2) allows for precise particle shape characterization; (3) allows accurate chemical identification; and (4) minimizes particle loss.

METHODS

After enzymatic digestion, a single pass of ultracentrifugation cleaned and deposited particles onto silicon wafers or grids for imaging analysis. During centrifugation, particles were passed through multiple layers of denaturants and a metal-selective high-density layer that minimized protein and nucleic acid contamination. The protocol prevented aggregation, providing well-dispersed particles for chemical and morphologic analysis. We evaluated the efficacy and accuracy of this protocol by recovering gold nanobeads and metal and ceramic particles from joint simulator wear tests.

RESULTS

The new protocol recovered particles ranging in size from nanometers to micrometers and enabled accurate morphologic and chemical characterization of individual particles.

CONCLUSION

Both polyethylene and metal wear debris can be simultaneously analyzed from the same sample by combining a silicon wafer display protocol for polyethylene and the metal and ceramics silicon wafer display protocol.

CLINICAL RELEVANCE

Accurate analysis of wear debris is essential in understanding the processes that produce debris and a key step in development of more durable and biocompatible implants.

Complications after metal-on-metal hip resurfacing arthroplasty

This article determines the incidence and cause of the complications commonly associated with metal-on-metal hip resurfacing implants and the proposed methods to prevent these complications. The literature available in PubMed was reviewed. Complication rates after hip resurfacing are low, and the procedure has shown both safety and efficacy in the hands of surgeons trained in specialized centers. Proper surgical technique can further reduce the incidence of femoral neck fracture, component loosening, and abnormal wear of the prosthesis. A more systematic detection of adverse local tissue reactions is needed to provide accurate assessments of their prevalence.

Metal-on-metal total hip arthroplasty

The effects of elevated levels of metal ions in patients who have undergone metal-on-metal total hip arthroplasty are not fully understood. The effects of femoral head size on serum metal-ion levels have been the subject of conflicting reports, and further investigation is needed to evaluate the impact of acetabular and femoral component alignment. The conduct of clinical trials of metal-on-metal total hip arthroplasties has been inadequate as few investigators have used a randomized controlled design to compare metal-on-metal bearings with other bearing surfaces. Additional clinical research needs to include appropriate validated patient-reported outcome measures, activity monitoring, and health economics.

A meta-analysis of design- and manufacturing-related parameters influencing the wear behavior of metal-on-metal hip joint replacements

This article aims to clarify the influence of design- and manufacturing-related parameters on wear of metal-on-metal (MoM) joint bearings. A database search for publications on wear simulator studies of MoM bearings was performed. The results of published studies were normalized; groups with individual parameters were defined and analyzed statistically. Fifty-six investigations studying a total of 200 implants were included in the analysis. Clearance, head size, carbon content, and manufacturing method were analyzed as parameters influencing MoM wear. This meta-analysis revealed a strong influence of clearance on running-in wear for implants of 36-mm diameter and an increase in steady-state wear of heat treated components.

Biological response to micron- and nanometer-sized particles known as potential wear products from artificial hip joints: Part I: Selection and characterization of model particles

The aim of this work was to select and characterize model particles, which correspond to real wear products from artificial hip joints, and to investigate the dispersing behavior of these powders. Commercially available nano and microparticles of corundum, graphite, and chromium oxide were selected or alternatively self-produced by milling. These powders were characterized regarding density, specific surface area, crystalline phases, particle size distributions and shape. Volume-based particle size distributions Q(3)(d) were measured after dispersing in water, water with dispersant, Ringers solution, and cell culture solution (Dulbecco's Modified Eagle's Medium (DMEM)) by laser diffraction and ultrasonic spectroscopy. Nanopowders formed agglomerates in the micrometer range in cell culture solutions. The micropowders showed only a marginal agglomeration. The median diameters of the dispersed nanopowders were even bigger than those of micropowders. Calculations of the number-based size distribution Q(0)(d) showed that in spite of the agglomeration the predominant number of the nano and microparticles is in the sub micrometer range, with only one exception, the micrographite powder.

Biocompatibility of diamond-like nanocomposite thin films

Diamond-like nanocomposite (DLN) films consist of network structure of amorphous carbon and quartz like silicon. In the present work, DLN films have been synthesized on pyrex glass and subsequently, their biocompatibility have been investigated through primary and secondary cell adhesion, cytotoxicity, protein adsorption and murine peritoneal macrophage activation experiments. Variable degree of cell and protein response have been found based on variable film synthesis parameters but in overall, required biocompatibility has been established for all types of film-coating.

A finite element method comparison of wear in two metal-on-metal total hip prostheses

The contact mechanics of two metal-on-metal (MOM) total hip prostheses was studied by means of the finite element method (FEM). The purpose of the work was to compare two total hip replacements (Durom™ and Metasul™) with regard to the amount of wear debris released. Wear on the bearing surfaces was evaluated following Reye hypotheses from the pressure distribution, computed by means of three-dimensional FEM models; an approximate analytical model based on Hertz contact theory has also been developed and discussed. The results show that in the dry friction condition the Durom™ joint releases almost twice as much wear volume as produced by the Metasul™ joint. Therefore, while Durom™ implants can improve hip stability by increasing the prosthetic impingement-free range of motion (PIF-ROM), Metasul™ prostheses can be a valuable solution whenever wear represents a critical choice factor.

Tribology of alternative bearings

The tribological performance and biological activity of the wear debris produced has been compared for highly cross-linked polyethylene, ceramic-on-ceramic, metal-on-metal, and modified metal bearings in a series of in vitro studies from a single laboratory. The functional lifetime demand of young and active patients is 10-fold greater than the estimated functional lifetime of traditional polyethylene. There is considerable interest in using larger diameter heads in these high demand patients. Highly cross-linked polyethylene show a four-fold reduction in functional biological activity. Ceramic-on-ceramic bearings have the lowest wear rates and least reactive wear debris. The functional biological activity is 20-fold lower than with highly cross-linked polyethylene. Hence, ceramic-on-ceramic bearings address the tribological lifetime demand of highly active patients. Metal-on-metal bearings have substantially lower wear rates than highly cross-linked polyethylene and wear decreases with head diameter. Bedding in wear is also lower with reduced radial clearance. Differential hardness ceramic-on-metal bearings and the application of ceramic-like coatings reduce metal wear and ion levels.

The tribology of metal-on-metal total hip replacements

Total hip surgery is an effective way of alleviating the pain and discomfort caused by diseased or damaged joints. However, in the majority of cases, these joints have a finite life. The main reason for failure is osteolysis (bone resorption). It is well documented that an important cause of osteolysis, and therefore the subsequent loosening and failure of conventional metal- or ceramic-on-ultra-high molecular weight polyethylene joints, is the body's immunological response to the polyethylene wear particles. To avoid this, interest has been renewed in metal-on-metal joints.

The intention of this paper is to review the studies that have taken place within different laboratories to determine the tribological performance of new-generation metal-on-metal total hip replacements. These types of joint offer a potential solution to enhance the longevity of prosthetic hip systems; however, problems may arise owing to the effects of metal ion release, which are, as yet, not fully understood.

Prediction of scratch resistance of cobalt chromium alloy bearing surface, articulating against ultra-high molecular weight polyethylene, due to third-body wear particles

The entrapment of abrasive particles within the articulation between a cobalt chromium alloy (CoCrMo) femoral component and an ultra-high molecular weight polyethylene (UHMWPE) cup of artificial hip joints or tibial inserts of artificial knee joints usually scratches the metallic bearing surface and consequently increases the surface roughness. This has been recognized as one of the main causes of excessive polyethylene wear, leading to osteolysis and loosening of the prosthetic components. The purpose of this study was to use the finite element method to investigate the resistance of the cobalt chromium alloy bearing surface to plastic deformation, as a first approximation to causing scratches, due to various entrapped debris such as bone, CoCrMo and ZrO2 (contained in radiopaque polymethyl methacrylate cement). A simple axisymmetric micro contact mechanics model was developed, where a spherical third-body wear particle was indented between the two bearing surfaces, modelled as two solid cylinders of a given diameter, under the contact pressure determined from macro-models representing either hip or knee implants. The deformation of both the wear particle and the bearing surfaces was modelled and was treated as elastic-plastic. The indented peak-to-valley height on the CoCrMo bearing surface from the finite element model was found to be in good agreement with that reported in a previous study when the third-body wear particle was assumed to be rigid. Under the physiological contact pressure experienced in both hip and knee implants, ZrO2 wear particles were found to be fully embedded within the UHMWPE bearing surface, and the maximum von Mises stresses within the CoCrMo bearing surface reached the corresponding yield strength. Consequently, the CoCrMo bearing surface was deformed plastically and the corresponding peak-to-valley height (surface roughness) was found to increase with both the hardness and the size of the wear particle. Even in the case of CoCrMo wear particles, with similar mechanical properties to those of the CoCrMo bearing surface, a significant plastic deformation of the bearing surface was also noted; this highlighted the importance of considering the deformation of the wear particles. These findings support the hypotheses made by clinical studies on the contribution of entrapped debris to increased surface roughness of CoCrMo femoral bearing surfaces.

Potential of coatings in total hip replacement

In total hip replacements, the bulk properties of materials, such as proper elasticity and hardness, are important. However, the material interacts with the body mainly at the surfaces. Wear and corrosion are initiated at the surfaces also. Therefore, the control of surface properties using different kinds of treatments or coatings may improve total hip replacements considerably. The most studied surface treatments include ion implantation and methods to control surface topography, such as grit or sand blasting or plasma treatments. Among the large variety of coatings, hydroxyapatite, titanium oxide and nitride, zirconium oxide, pyrolytic carbon, and diamondlike carbon coatings have shown the most promising results. These coatings mainly are used to enhance bone growth; to minimize friction, wear, and corrosion; and to improve biocompatibility of total joint prostheses. The potential of novel coatings to solve some present problems in joint prostheses is discussed based on the structure and properties of different kind of coatings. It can be concluded that currently, coating methods exist to improve the tribologic performance and longevity of the total hip replacements. However, coatings must fulfill two essential requirements: no delamination in biochemical and biomechanical environments and sufficient protection of substrate from corrosion.

Comparative wear under different conditions of surface-engineered metal-on-metal bearings for total hip arthroplasty

Metal-on-metal hip arthroplasties have demonstrated low wear rates. However, the ion release and toxicity of the metal wear particles remains a concern. Modifying the surface of metal bearings with thick chromium nitride (CrN) coatings has the potential to further reduce wear and ion release, and improve the biocompatibility of wear particles produced. The aim of this study was to investigate the application of surface engineering technology to modify metal-on-metal (MOM) bearings to reduce wear and improve the functional biocompatibility of metal-on-metal prostheses. CrN-on-CrN bearings had lower wear rates in comparison to metal-on-metal bearings, particularly under adverse loading conditions. CrN-on-CrN bearings produced similar nanometer-sized particles to metal-on-metal bearings, however, CrN wear particles were less cytotoxic when cocultured with macrophage and fibroblast cells.