ABSTRACT
As nowadays statistics have reported results about the former generation implants and are showing us that wear and the associated propagation of wear debris represented the main issue in obtaining long-term durability of total hip arthroplasty, tremendous efforts have been made and successes achieved in the last decade in improving the quality of biomaterials for joint bearing surfaces. In this book, we have given a critical review starting from the formerly established to the most recently developed biomaterials, including several generations of ceramics, conventional and highly cross-linked polyethylene, and the most advanced evolution of metal implants with oxidized surfaces (cf. Chapters 3, 5, and Section 7.1). The many possible material combinations currently available in designing the sliding couple of an artificial joint provide the surgeons with a large number of choices, to be made according to personal preferences for a given surgical procedure, experience about different individual brands, and differentiation between young and middle-aged patients. Actual trends in new load-bearing biomaterials for hip joints are presently strongly influenced by a widespread quest for increasing head diameters; in order to achieve both improved range of motion and dislocation stability. Nevertheless, large head diameters have been so far a strict prerogative of metallic implants due to structural reliability
reasons. The available design solutions based on large ceramic head diameters are yet seldom found and ceramic resurfacing technology is yet in embryo (cf. Section 7.2). On the other hand, the main advantage of ceramic-on-ceramic bearings remains the fact that this type of articulation provides the lowest wear rates compared with any other material options in hip arthroplasty, even lower than ceramic versus highly cross-linked polyethylene [20]. The main disadvantage resides in a quite low (but, anyway, non-zero) risk of fracture (i.e., 2 in 10.000 or less) either for the ceramic head or for the ceramic liner. The current life expectancy for a successfully implanted hip joint can be approximately estimated to be 15 years when “standard” articulations consisting of metal/UHMWPE or alumina/UHMWPE are implanted. However, with the most recently developed alternatives of low-wear bearings, one could expect (or, at least, assertively hope) that future statistical results will show the achievement of longer lifetimes, owing to a significant decrease in the volume of wear (e.g., in the case of advanced ceramic-on-ceramic heads or ceramic composite heads sliding against UHMWPE liners with engineered structures, as described in Chapters 5 and 6, respectively). The burden to the lymphatic system could be reduced and the life expectancy for an artificial hip joint could be significantly improved as a result. The final goal in lowering wear rate at the joint surface, namely the design of a hip prosthesis that reliably experiences a longer lifetime than the patient who has received the surgery, could be closer to a satisfactory level of achievement than we could predict at the time of writing this book. Nevertheless, deeper scientific understanding and continuous technological monitoring need to be unremittingly pursued, at least for a full establishment of the clinical procedures involved with the newly developed implants.What is next in hip bearings? This is the crucial question that we should ultimately be able to answer at the end of the dissertations given in this book. Summarizing the present situation, one could certainly state that the current generation of hard-on-soft and hard-on-hard hip bearings has become to exhibit extremely low wear when compared with traditional implants made of the cobalt-chromium alloy coupled with conventional polyethylene used in the past. At more than 10 years’ clinical follow-up for hard-on-hard bearings and more than 5 years’ follow-up for highly cross-linked polyethylene materials, we might soon obtain
officially compiled statistics showing a significant reduction of particle-mediated osteolysis compared with the past [765, 1168]. However, further issues (i.e., other than wear) certainly remain unsolved, as we have discussed them in some detail in the previous chapter. For better summarizing, in these conclusive remarks, one could say that ceramic-on-ceramic bearings are quite innovative, but also quite sensitive to cup position and neck impingement and, although in quite rare instances, could fracture. Metal-on-metal bearings are also sensitive to cup position and edge loading, including the possibility of unexpected pain, allergic reaction, soft tissue necrosis, pseudotumor, aseptic lymphocyte dominated vasculitis associated lesion (ALVAL), and metallic ion contamination (metallosis). Metal or ceramic heads on highly cross-linked polyethylene acetabular cups might appear more forgiving than hard-on-hard bearings with regard to the degree of precision required in the surgical technique. However, edge loading arising from joint laxity or component malposition might increase the actual polyethylene wear rate with these couples as well (i.e., although without emission of any detectable noise). Moreover, even in advanced polymer components, high contact stresses due to very thin liners or edge loading might ultimately lead to component fracture [1169-1173]. Several reports have detailed the cause of fracture for highly cross-linked polyethylene liners. Typically in these cases, fracture occurs in a region of thin and/or unsupported polyethylene, in association with superiorly directed edge-loading conditions, in turn associated to an excessive inclination of the implanted acetabular component. The reported cases of polyethylene liner fracture obviously have forced biomaterials scientists and technologists to give a second thought to the long-term benefit of the low wear rate obtained with highly cross-linked polyethylene. Such benefits can be achieved and exploited only if short-term mechanical failures are avoided. The potential for rim loading on thin polyethylene, particularly for patients with a relatively vertical cup orientation, is nowadays carefully assessed before considering the use of a cross-linked liner in combination with a large-size femoral head. A further unsolved issue, which we have mentioned at the end of Section 7.2, is that contemporary hip bearings are supported by femoral stems and acetabular shells made of materials that are rather stiff with respect to adjacent bone. Such a large imbalance in stiffness between synthetic components and bone
potentially leads to retroacetabular bone loss and acetabular stress shielding. We have also reported in the previous chapter that attempts to fix this long-known problem have recently been made by combining revolutionary choices for the bearing biomaterials and advances in component design. While such quite recent developments again confirm that paradigmatic progresses in hip arthroplasty cannot prescind from the development of new biomaterials, such progresses are also expected to occur in line with the long-discussed trends in developing less intrusive and more patient-friendly implants.In the literature review necessary to compile this book, we came across an interesting analysis by Morscher [1174]. This report is indeed, for some aspects, “disconcerting” for any passionate scientist, who, as in the case of the writer, compellingly believes in innovation. In reviewing past failures and successes in total hip replacement, this researcher gives a clever dis-quisition on the reasons why “good ideas may not work” in joint arthroplasty. In the paper by Morscher, the analysis of the way endoprosthetic practices have progressed (and presumably will progress from now on) is presented in a realistic and over-whelming way. The main concept put forward in that analysis is that as a general issue in technological developments, “success is the greatest obstacle to further progress.” From the scrutiny of past experiences, a general reasoning is given, which describes the curve of progress toward a hypothetical perfection, whatever the field of application, as a discontinuous one, definitely asymptotic in any advanced field, except for the occurrence of abrupt jumps in correspondence of discrete tipping points. This trend is qualitatively shown in Fig. 8.1, which also summarizes the discussions on future trends extensively made in the previous Chapter 7. In other words, as further progress is searched for, more efforts (and finances) become needed. According to the analysis of what might actually be feasible in the middle-range development of hip arthroplasty, and with what resources this could become available, one might come across a tangible probability that the medical device market, as presently structured, could even turn back to adopting less advanced technologies in the near future. The first step or initial development of hip arthroplastic surgery, namely what one could define the immediate achievement of short-term aims, has been fully achieved and consisted of liberating the patients from pain,
Figure 8.1 Line of development for artificial hip materials and implants and the prediction of a tipping point in surgical development reached with the establishment of cartilage healing technologies. improving the mobility of the affected joints, and restoring the ability of the patients to stand and to walk. A subsequent progress, presently in a progressive stage in the field of arthroplasty, resides in a systematic recognition and in a responsible con-sideration of the risk factors that could eventually compromise the longevity of an artificial joint. Such second step has necessarily involved and continues to require reiterative developments of improved prosthetic materials (and surgical techniques) as we have described in Chapter 7, thus necessarily demanding enormous economical efforts but also resulting in a substantial decrease in the severity and frequency of surgical complications. Then, what can we expect next? According to Morscher, an innovation might solve one or more problems, but it will also definitely create a number of new problems. If this concept will prove valid, after the many innovations the arthroplasty field has reached in the last decade, we should expect a wave of new problems coming back to us in the very near future (i.e., one might even suspect that the stream of new problems has already started with the metal-on-metal resurfacing recall mentioned in the previous chapter). There is no doubt that the pioneering age of joint replacement has already come to an end, and that technological developments seem now to approach a saturated stage. However, what additional choices do we have but continue to believe in research as the only way to further progress it?
