Due to development of orthopedic surgery, prolongation of the average population age, increasing quest or desire for living actively in older age without pain, and other factors like, e.g., obesity, the prevalence of patients with implanted joint replacements is increasing. Prolongation of the lifetime of joint implants is an important goal for each individual patient and his/her personal burden, but also is for society as a whole as it is related to large direct medical costs for treatments and indirect economic costs due to patient inability to work. The number of total knee and hip replacements in the United States by 2030 is expected to increase by 673% and 174%, respectively.1 The prevalence in a population of 100,000 in the age group above 65 years reaches about 15,000 for men and 25,000 for women.2 Roughly translating this prevalence to the number of people aged ≥65 years in the European Union, it can be predicted that about 17 million people in the targeted population are diagnosed with osteoarthritis. At the same time, the number of patients younger than 65 years with long life expectancies will also increase. Both groups require proper strategies and treatments. Age-related public expenditures (pensions, health-care, and long-term care) are projected to increase by 4.1% to around 29% of the GDP between 2010 and 2060.3
Joint replacement, or joint implant, replaces the diseased joint, restores its function, and enables the patient to live actively without pain, aiming for a 20 year lifetime. Most components of implants are made of metal—stainless steel, cobalt chromium, and titanium alloys for permanent joint implants, and magnesium alloys for temporary implants. During functioning in vitro under simulated physiological solution, or in vivo in the relatively aggressive environment of the human body, metals are subjected to various corrosion, tribocorrosion, and wear processes. These phenomena are generally termed as biocorrosion. Understanding the mechanism of biocorrosion and metal degradation in human body—identification of the metal corrosion and wear products, and their effects on human body—is of paramount importance for the safety and long-term well-being of the patients. Translation of in vitro mechanisms to in vivo clinical situation is not straightforward: only the strong combination of knowledge regarding both material properties as well as clinical conditions enables the understanding of the long-term operative mechanisms. One of the prerequisites to reach this goal is a fruitful collaboration between scientists in the laboratories and orthopedic surgeons in the operating theaters.
This special issue of CORROSION aims to emphasize the importance of studies of different aspects of materials used as orthopedic implants. It can be divided into two parts: (i) six in vitro studies and (ii) six retrieval and model clinical studies related to orthopedic implants.
The first three in vitro articles are concerned with the basic mechanism of passivation and corrosion of different metals and alloys in various simulated physiological solutions. Metikoš-Huković, et al., studied the role of alloying elements of Co-28Cr-6Mo on its corrosion properties by electrochemical techniques in Hanks inorganic simulated physiological solution. Mott-Schottky tests were used to determine the semiconducting parameters of the passive films formed on the alloy taking into account frequency dispersion. The beneficial effect of alloying Co-based alloy with Mo was explained by annihilation of mobile cation vacancies with immobile highly charged Mo ions, thus leading to an increased corrosion resistance. Virtanen and Wagener presented the importance of choice of simulated physiological solution, especially in the case of magnesium metal. Magnesium forms calcium phosphate layers in both solutions investigated—simulated body fluid and the cell culture medium DMEM (Dulbecco’s Modified Eagle’s Medium)—but the layer formed in DMEM was more homogeneous. The formation of dense layer was attributed to weak buffering capacity of DMEM and decreased Mg corrosion at higher pH. Hedberg, et al., investigated the interaction of albumin and fibrinogen with stainless steels through sequential exposure in phosphate buffered saline. Proteins increased the release of metals; in mixed protein solutions, adsorbed albumin was replaced by larger protein fibrinogen, indicating that Vroman effect was operative. The importance of precipitation of metal-rich protein aggregates was emphasized.
The next three articles are concerned with the material modifications in order to increase osseointegration ability or to resist bacterial attack. Mišković-Stanković, et al., electrochemically synthesized silver doped poly(vinyl alcohol)/chitosan/graphene hydrogels aiming to produce nanocomposites for various applications in biomedicine. Chitosan is a natural polymer often used for controlled release drug carriers, while graphene is excellent reinforcement material for functionalization within polymer matrix. Silver nanoparticles contributed to antibacterial activity. Ceré, et al., modified the surface of magnesium alloys AZ31 and AZ91 by nitrate-free sol-gel bioactive 58S glass synthesized from SiO2, CaO, and P2O5. Coating retarded the degradation of Mg alloys in Hanks simulated physiological solution and demonstrated increased bioactivity compared to bare alloys by forming apatite-like compounds. Hirimoto and Doi investigated self-healing behavior of sodium polyacrylate (SPA)-hydroxyapatite (HAp) coatings on biodegradable Mg AZ31 alloy. The combination of SAP and HAp promoted in situ healing of coating under the cracks, as proved by static immersion test, and enhanced self-ability of the HAp coating under tensile deformation.
The review presented by Gilbert presents the transition from in vitro research to real biological environment of human body. This comprehensive review describes the corrosion and tribocorrosion mechanisms of orthopedic alloys in vivo and presents examples of corrosion damage on retrieved medical devices. The role of inflammatory species and biological processes on metal corrosion are discussed. Milošev reviewed the physical-chemical properties of main orthopedic alloys and presented examples of micro- and nanosized metal wear and corrosion products released into periprosthetic tissue during corrosion and tribocorrosion processes. Possible adverse effects of metal products are discussed.
The four following articles are all concerned with model clinical, or retrieval studies of orthopedic implants. Cao, et al., used tribocorrosion model based on the combination of mechanical and corrosion concepts to rationalize experimental observation and clinical outcomes for hip implants with metal-on-metal Co-28Cr-6Mo bearing combination. This model emphasizes the importance of patient specific parameters such as body weight, gait speed, and synovial fluid viscosity on the degree of tribocorrosion. The need to better understand the in vivo corrosion conditions, in particular the role of electrode potential, is emphasized. Baleani, et al., presented in vitro comparative study of fretting-corrosion resistance of Ti-6Al-4V and Co-28Cr-6Mo in a taper joint subjected to high bending moment. Adoption of Co alloy neck reduces the risk of mechanical failure but it also increases the risk of adverse soft tissue reaction to corrosion products. Campbell, et al., described an interesting case of retrieved distal femoral endoprosthesis used to treat osteosarcoma in a child. The modular design allowed expansion during the years but despite multiple connections, solid corrosion products were found only during the last period, leading to removal of surface features and focal corrosion. Modular design of hip prostheses has been recently related to numerous clinical problems of corrosion and fractures. Higgs, et al., investigated the possibility that in vivo taper corrosion affects the strength of the head-stem connection of the hip implants. The analysis of 109 femoral stems that were retrieved from revision surgery and cadaver donors shows that the severe taper corrosion results in slightly higher disassembly force between the components. Therefore, the hypothesis that the corrosion weakens the taper junction is not supported.
I would like to thank Prof. John R. Scully, Technical Editor in Chief, for kind invitation to serve as a guest editor for this special issue. The valuable help and advice by Sammy Miles, Managing Editor in Chief, and Marlene Walters, Journal Production Manager, are greatly appreciated. Invited authors presented the best of their work and thus created strong contributions to the presentation of this important field of research. Reviewers also carried out an important task in the creation of this issue that improved the quality of all manuscripts. It was my pleasure and honor to serve as a Guest Editor of the Biocorrosion special issue in CORROSION. I hope that it will be interesting for readers not yet familiar with the biocorrosion topic as the starting point for further study, as well as for researchers already devoted to this topic who may find more deepened insights into matters of interest.
