2.1
Biomaterials for Total Hip Implants
2.1.1 Introduction
Total hip arthroplasty (THA) is a safe and well‐recognized surgical technique performed in the field of orthopedics. The goal of this surgical procedure is to restore a patient back to their normal day‐to‐day activities and range of motion without any discomfort or pain. As of 2003, the United States had more than 200,000 THA surgeries yearly for its residents. It has been recorded that nearly 2.5 million people have one type of hip replacement implanted or another [1]. And the numbers are speculated to increase to 572,000 by 2030 [2]. The National Joint Registry in England indicated that in the span of 12 years, i.e. from 2003 to 2015, a total of 790,000 THAs have been performed [3]. Even the Korea Medical Service has reported that 60,000 hip replacement surgeries have been carried out between 2010 and 2017, and the increase rate of incidence is alarming [4]. The principal factors that impact the current field of design of artificial hips are mainly mechanical strength and reliability [5–14], biocompatibility [15–18], bioactivity [19–28], and ability to resist wear and corrosion. A few complications associated with failed implants include pain and discomfort, disability, and the need for revision surgery. However, over the years, it has been identified that the fundamental reason for the failure of THA is an inborn immune response generated by the deterioration of bearing materials in THA. The wear debris absorbed by macrophage and polynuclear giants causes cytokinesis release, which results in the inflammation of tissues and activates osteoclast activities. This finally leads to implant loosening. Thus, new biomaterials continue to be studied and developed for THA. This paper seeks to provide the latest update of various development materials for THA applications.
2.1.2 History of THA Development
For over a decade (1955–1965), large ball bearings have been the source material for metal‐on‐metal (MoM) bearings [29]. However, a decline in its use was observed in the 1970s when John Charnley instigated an HA device predicated on metal‐on‐polymer (MoP) made of a tiny metal ball and cemented PE cup [30]. The prolonged rate of survival of these initial implants was quite satisfactory, with a successful performance of about 77–81%, 25 years after early THA [31]. Due to the growing utilization of THA among young active patients, there have been increasing revision rates [32] and concerns about PE particles influencing bone lysis and causing implant loosening [31]. Hence, modern materials continue to be discussed in preventing attrition and bone lysis. French surgeon Pierre Boutin, foreseeing current issues with PE disease, started exploring alumina and ceramic‐on‐ceramic (CoC) hip implants in the 1970s [33]. Since the twentieth century, wide applications of CoC hip devices in THA have paved the way for the development of Ceramic on PE (CoP) implants as better bearing alternatives alongside MoM and CoC. According to history, stainless steel was among the initial set of alloys used in the field of orthopedics [34]. But the issue of corrosion necessitates that it be used temporarily for only short‐term purposes [35]. Ti and alloys, Co–Cr, PE, and ceramics are the principal materials used in THAs.
2.1.3 Metallic Materials
2.1.3.1 Stainless Steel
They are alloys universally containing Cr, Ni, Mo, Mn, and C. Orthopedic implants consisting of fracture‐fixation devices are made from austenitic (316 series) alloys. The resistance of stainless steel to oxidation and the economically viable ease of manufacturing make it an excellent material choice for most hip prostheses. However, the reason it is less exploited nowadays in THA is because of poor biocompatibility, even though it is still available in some locations (more specifically, the United Kingdom).
2.1.3.2 Cobalt–Chromium Alloys
Cobalt–chromium are alloys composed of cobalt and chromium fusion. It was first discovered in the early 1900s by Elwood Haynes. Even though early applications were in the field of dentistry, it has now become a major material used for hip implants. This is due to its improved mechanical properties, and ability to withstand wear and corrosion. Young’s modulus of Co–Cr alloys serves as a cement‐type material for femoral stems even much better than titanium alloys.
2.1.3.3 Titanium Alloys
Titanium is the gold standard for THAs. Concerning femoral stems and acetabular non‐cemented parts of THAs, commercial grade titanium (Ti6‐Al‐4V) and pure titanium (pTi) are the most readily used metallic alloy in bone applications due to their improved mechanical properties, biocompatibility, lower density, and excellent corrosion resistance [36]. However, the poor wear resistance of Ti makes it impossible to be used for the manufacturing of femoral heads. Other titanium alloys void of vanadium like α + β titanium‐6Al‐7Nb with excellent biocompatibility evolved during the last two decades by building in biocompatible elements such as niobium [15–18]. Most recently, researchers are now extending their efforts in building a bulk variety of metallic materials with lower Young’s modulus, thus increasing the significance of β‐Ti alloys.
2.1.3.4 Alloy Surface Modifications
The innate biomaterial limitations stemming from traditional materials such as forged beads, fiber metal, and plasma sprays limit their application in classic implants. For effective osseointegration and necessary bone–implant contact, porous metals and coatings were developed [37] to avoid the risk of implant loosening. In comparison, stainless steel, cobalt–chromium alloys, Ti, and tantalum are seen as the most favorable choice for use in orthopedics. Thus, to achieve better osseointegration [38] and keep the implant right in the bone bed, hydroxyapatite was introduced. Porous metals have been explored greatly for achieving natural fixation and increasing the long‐term performance of orthopedic implants in the body [39]. Newly introduced metallic materials with improved porosity have contributed tremendously to osseointegration and bone healing in orthopedic devices [40].
2.1.4 Exploited Materials for Bearing Surface
2.1.4.1 Polymers
2.1.4.1.1 Ultrahigh Molecular Weight Polyethylene (UHMWPE)
The first bearing material was used for the Charnley hip prosthesis in the twentieth century. A hip arthroplasty with minimal friction comprises a cemented affix, 22–25 mm femoral bearing surface, and all PE cup [41]. Sterilization of traditional PE is done with gamma radiation in an open space environment. This technique allows for molecular conjoining, which produces oxidizable free radicals that can lower degradation rates and PE wear [42]. There are several factors that contribute to PE wear [43]. Examples are the activities of the patient, the width of the femoral head, acetabular cup alignment, and modular cementless cup usage [44, 45]. It is reported that osteolysis mediated by UHMWPE wear remains a major concern in THA [46, 47].
2.1.4.1.2 High Cross‐Linked UHMWPE (XLPE)
The target of modern XLPE is to enhance UHMWPE in both cementless and cemented implants. While reducing PE wear is a major concern, scientists are also putting in efforts to stop the oxidation process and maintain the right mechanical properties in XLPE [48]. Gamma or electron beam radiation are key radioactive technologies for achieving cross‐linking, i.e. breaking molecular bonds in XLPE. There exist three key processes in the manufacture of XLPE cross‐linking, heat treatment, and sterilization. The bulk of the cross‐linking is higher using radioactive rays with a dose range between 50 and 100 kGy to improve abrasion. The essence of the heat treatment is to eliminate the free radicals produced after cross‐linking. Thermal treatment beyond or less than the melting point of the polymer (137°C) is helpful to achieve this goal. In vivo studies highlighting the wear resistance of XLPE, and conventional PE showed that the wear rates of XLPE were significantly lower relative to traditional PE [49, 50]. Biological activity levels and osteolysis of XLPE showed similar results [49–54].
2.1.4.1.3 Antioxidant‐Doped PE
The oxidation process can be stopped without jeopardizing the mechanical behavior of the material through heat application; additional antioxidants like vitamin E are added to stabilize XLPE. This inhibits free radicals from being oxidized after cross‐linking with the intention to improve PE wear [5, 6, 55]. But even though the initial results and testing seem promising, long‐term studies should still be engaged as there are no clinical test results for this newer materials.
2.1.4.1.4 Poly(2‐methacryloyloxyethyl phosphorylcholine) (PMPC)
The effect of tribological contact on XLPE is a key area where progress has been made [7]. Surface treatment of XLPE with a chemically thin layer (100–200 nm) showed a greater wear resistance, and poly(2‐methacryloyloxyethyl phosphorylcholine) (PMPC) created through light copolymerization produces a superb emollient layer that mimics natural joint cartilage [8]. A hip simulator wear research study highlighted that the polymer, poly (2‐methacryloyloxyethyl phosphorylcholine) (PMPC), joined to the XLPE surface drastically lowered the wear to 70 million cycles [56].
2.1.4.2 Ceramics
2.1.4.2.1 Alumina
Since the 1970s, ceramics made of alumina have been exploited as a surface bearing in HAs [57] because of their chemical resilience, high wear resistance, and ability to not cause any adverse host–cell reaction. The wear of alumina ceramics was as small as a few micrometers in a 15‐year period, which is almost two thousand and hundred times lower than regular metal on a polymer couple and metal on metal prostheses [58]. Despite alumina ceramics having a greater abrasion resistance compared to MoP, it is still reported in history to have a high incident rate of fracture [59]. Alumina’s improved manufacturability as a result of the high wear incidence was made possible by reducing the grain size and porousness and adjusting the processes for increased toughness [60]. This has greatly reduced the fracture incidence of alumina ceramics in recent years, making it an excellent option for young active patients [59].
2.1.4.2.2 Zirconia
The earliest introduction of femoral heads made of zirconia in Europe was in 1985 and 1989 for the United States [61]. The transition went from alumina as the main femoral head component to zirconia due to reasons such as alumina’s high rate of incidence of fracture, fracture toughness, and larger bend strength than alumina [62]. However, it is also reported that there is a newly identified possibility for zirconia undergoing monoclinic revamp in vivo, thus greatly increasing the chances of fracture and wear degradation [61, 63]. For example, in 2021, the leading producers of zirconia revoked all their goods and services due to issues associated with the thermal processing of those earlier batches [61]. Ever since this incident, there has been a sudden decline in zirconia usage, especially in those maintained with yttria. However, in recent times, scientists have revealed a new developing trend toward alumina–zirconia composites [64] as their combination makes ceramic bearings competitive.
2.1.4.2.3 Alumina–Zirconia Composites
Despite the well‐established history in terms of the clinical significance of zirconia and alumina in hip replacements, there have also been some limitations associated with their use. Manufacturer’s efforts to combine the two materials: hard alumina and tough zirconia yielded the composite called “zirconia‐toughened alumina (ZTA)”. CeramTec company was the first to commercialize the product in the twenty‐first century. The alumina substrate in the matrix is essential for contributing hardness to the composite, whereas the zirconia aggregate is to ensure higher resistance to crack propagation [62]. ZTA’s ability to slow down the kinetics of hydrothermal aging, thus, withstanding thermal processing makes it very competitive than zirconia.
2.1.4.2.4 Silicon Nitride
A ceramic material consists of silicon and nitrogen but is void of oxygen (Si3N4). For more than 50 years, silicon nitride has been utilized in surface bearings and turbine blades due to its high strength and toughness. In the medical field, silicon nitride has been utilized since 2008 for cervical spacers and spinal fusers, and there have been no catastrophic results so far from 25,000 implanted spinal cages [65, 66]. Most recently, Si3N4 has been considered for artificial hips because of its excellent biocompatibility, high Vickers hardness, fracture toughness, and flexural strength, as well as its modulus of elasticity [67]. Thus, there has been a great improvement in fracture toughness, flexural strength, and resistance to hydrothermal degradation. Si3N4, after testing, showed no adverse effects, thus performing like that of alumina [68], making it more biocompatible in hip joints.
2.1.4.2.5 Hybrid Design of Oxide Ceramic Layer on Metal (Oxinium™)
The first introduction of zirconium and its alloy (Zr‐2.5Nb) for hip arthroplasty was in 2003 [66]. Heat treatment of zirconium alloy in open spaces or environments turns the metallic region to a black‐colored zirconium‐oxide [60, 69, 70]. The zirconium femoral head commercially oxidized as Oxinium is not considered a coating; it is an oxygen diffusion surface hardening process targeted at increasing the resistance of zirconium alloy to wear when subjected to a stress. Reports from hip simulation studies showed Oxinium femoral heads generating wear less than 45%, compared to flat Co–Cr heads. The change was even more vivid after roughing the heads, producing over 61% less wear. A comparison of 50 Co–Cr and 50 Oxinium heads by Lewis et al. indicated an equivalent clinical outcome after two years of follow‐up [71]. Even though OxZr’s head has been clinically utilized for more than eight years, more clinical trials are needed to ascertain its performance.
2.1.4.2.6 Ultra‐Hard Coatings on Metals
Although self‐mated cobalt–chromium couples rubbed on XLPE or PE are often utilized in HA, more than half of failed fabricated hip joints are primarily because of aseptic loosening moderated by osteolysis and prolonged metallic ion allergies [72]. The backup technique is to coat alloys of metal with solid suitable surface layers, e.g. diamond‐like carbon (DLC, 5000 HV) [73] or titanium nitride (TiN, 2100 HV) [74]. This method is to ensure that the actual attributes of high toughness of the base metal are secured when supporting a surface bearing, and inhibit the letting out of harmful metallic by‐products from beneath the alloy. However, there are a few concerns associated with this technique, such as crevice corrosion, local delamination, and third‐body wear [74, 75]. Hence, the alternative in achieving this is to cover the metal head with pTi. In respect to this, 3–100 nm grain size of ultra‐nanocrystalline diamond (UND) was directly applied to titanium and cobalt–chromium alloys by the CVD [76, 77]. The three major characteristic features of UNDs are high abrasion resistance to third‐body abrasion particles, low roughness in the surface, and high hardness [78]. Nonetheless, high‐stress concentrations are still retained in these coatings due to contaminants near grain interfaces, inhibiting better adhesivity to the base metal [79].
2.1.4.2.7 Clinical Aspects of Bearing Surface
A low friction coefficient, malleability, and wear resistance with high surface roughness and hardness are critical for these couple bearings. Moreover, surfaces directly interacting with biological tissues should be biocompatible, bioinert, and noncytotoxic to tissues. Currently, these myriad bearings are still used for clinical applications (Figure 2.1.1).
Figure 2.1.1 Materials utilized in modern Total Hip Arthroplasty (THA) bearings including (a) Metal‐on‐Polyethylene (MoP) bearings, (b) large head Metal‐on‐Metal (MoM) bearings, (c) small head Metal‐on‐Metal (MoM) bearings, (d) Ceramic‐on‐Ceramic (CoC) articulation, and (e) Ceramic‐on‐Polyethylene (CoP) articulation.
(Ref. [125]/American Association for the Advancement of Science ‐ AAAS/CC BY 4.0).
2.1.4.3 MoP Articulation
2.1.4.3.1 Advantages
In 1963, MoP only comprises a small‐round metal ball and a cemented PE cup [80]. Joints made of Co–Cr femoral heads and UHMWPE have been an excellent bearing surface couple for most hip prostheses due to the long‐lasting results over the last few decades. Based on previous literature reports, the wear rate of the XLPE group in comparison with the traditional PE group was significantly reduced (XLPE groups, 0.035 mm/yr.; conventional PE group, 0.118 mm/yr) [81]. Several other bearing articulations have this as their benchmark for wear testing. The long‐term performance of MoP surface bearings in elderly patients have made it more clinically accepted in THA [82].
2.1.4.3.2 Disadvantages
Despite the pros, it is evident that PE liner produces a lot of wear debris due to osteolysis, causing implant loosening and subsequent failure of the implant. Osteolysis occurrence is typical for any wear rates higher than 0.1 mm/yr, and atypical for any wear rates below 0.05 mm/yr [83, 84]. Clinical studies show that the osteolysis rate of MoP is as high as 26% and the aseptic loosening rate is at 3% after a 10‐year follow‐up [85]. In decreasing PE wear, different manufacturers continue to make efforts during the past decade to develop new biomaterials such as PMPC, XLPE, and antioxidant‐doped PE. By this, better performance of newer XLPE groups has been achieved compared to traditional or first‐generation XLPE [86].
2.1.4.3.3 Wear Mechanism
There are sticky features patched on PE surfaces corresponding with a metallic head [87]
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