Investigators focus on wear in total joints
By Timothy M. Wright, PhD
Wear-related failures of total joint arthroplasties remain a serious clinical problem. The release of wear particles from implant components elicits a biological reaction that can culminate in osteolysis and implant loosening. The problem is complex in that implant wear can be affected by patient risk factors, surgical technique and implant design and materials.
Recent advances, however, have markedly improved understanding about the generation and consequences of wear debris and have lead to new means for addressing the problem of wear-related osteolysis. These advances were the focus of a workshop, "Wear 2000," organized through the AAOS Research Committee and sponsored by the AAOS and the National Institutes of Health (NIH). Participants represented the clinical and research communities, orthopaedic implant manufacturers, and the NIH, National Institute for Standards and Technology and the Food and Drug Administration.
Clinically, the size of the wear problem remains difficult to measure. Recent studies have shown that patient activity can vary widely and may be the single most important risk factor affecting wear. Adding to the wear problem is the lack of clinical and radiographic features in daily clinical practice to alert the orthopaedic surgeon to early failure.
When failure does occur, problems, such as bone loss, faced at revision surgery remain unresolved. The surgery is technically demanding and problems related to bone loss remain a challenge. And the introduction of more wear resistant materials will not eliminate the problem given the large number of total joint replacements that have already been performed during the past two decades.
Biologically, the local reaction to wear debris results from the activation of phagocytosis by macrophages, accompanied by local production of cytokines that in turn cause a paracrine activation of periprosthetic osteoclasts. The next level of research needs to ascertain the precise receptors and signaling pathways that incite changes in gene expression. Clinical assessment of patients suggests that host responses to wear particles are heterogeneous.
Identification of the specific factors responsible for these differential responses is of great clinical importance, and technologies are now available for identifying genetic factors that determine the pattern of host responses. Such studies will not only provide an understanding of the pathogenic mechanisms regulating biological response to wear particles, but also could lead to improved therapeutic approaches. The strategy for interventional therapies may ultimately reside with interceding in the activation of the osteoclast.
Perhaps nowhere have more promising advances in combating the problem of implant wear occurred than in the engineering and biomaterials areas. Much remains to be learned about the exact mechanisms by which implant surfaces wear. Recent findings point to important avenues for further research. For example, residual stresses in polyethylene knee component surfaces are now known to be an important factor contributing to the problem of crack propagation and subsequent pitting. The strong correlation between wear rates measured in laboratory simulators and the energies to failure measured in a biaxial punch test suggests that the materials ability to resist such complex stress states is a controlling factor in its wear resistance.
The lack of proven engineering theories for wear notwithstanding, new forms of ultra high molecular weight polyethylene and other alternative bearing materials have been introduced, based primarily on their ability to wear at much lower rates than more conventional polyethylene. Elevated cross-linked polyethylenes have been developed in which energy (e.g., gamma radiation) is used to form an increased number of cross-links between the polyethylene molecular chains. Degradative free radicals that form during the cross-linking are removed through thermal treatments of the material.
The resulting materials have significantly better wear resistance over conventional polyethylene when compared in acetabular components in a hip joint simulator. Cautionary notes were raised by some workshop attendees, however, that the cross-linking of polyethylene does decrease its toughness.
Similarly, metal-on-metal bearing surfaces have been reintroduced into the commercial marketplace with the recent FDA approval of a cobalt alloy-on-cobalt alloy total hip replacement. Other bearing materials such as alumina and zirconia ceramics are also available and research and development is underway on alumina-zirconia composites aimed at boosting the toughness of alumina.
Postmarket surveillance of all these materials, including careful radiographic studies, is required to establish whether the improvements in wear resistance seen in the laboratory carry over to the clinical situation. Current follow-ups are too short to make any definitive conclusions.
The wear workshop is an update of a workshop held in 1995. Participants began to construct the proceedings prior to the workshop by electronically submitting written answers to clinical, biological and bioengineering questions around which the workshop was organized. A web-based tool was then used to make it possible for electronic review of the preliminary answers by other attendees prior to the meeting.
The approach helped focus workshop discussions and expedited subsequent refinement of the answers in breakout sessions held throughout the three-day event.
The AAOS is working to produce the proceedings online for members and other professional groups. As future advances in combating the wear problem occur, timely peer-reviewed revisions will keep the proceedings current.
Timothy M. Wright, PhD, senior scientist, Hospital for Special Surgery and professor of Biomechanics in Orthopaedic Surgery, Weill Medical College of Cornell University; and Stuart B. Goodman, MD, PhD, chairman of orthopaedics, Stanford University Medical Center, Stanford, Calif., were cochairmen of the workshop.