Clinical, biological, design and material considerations of implant wear in total joint replacement
Loose hybrid THR with "cement disease"
By Stuart Goodman, MD, PhD and Timothy Wright, PhD, members, Committee on Biomedical Engineering Committee
Total joint replacement (TJR) is a successful operation in alleviating pain and improving function for increasing numbers of patients with end-stage arthritis and other degenerative diseases. As our population ages and lifespans are extended, and as TJR is being performed in younger more active patients, revision surgeries associated with prosthetic wear are becoming more common. In 1998, more than 29,000 revision hip surgeries were performed, which represents 17% of all total hip replacements that year, and a 7% increase in the number of revision hip surgeries compared to 1994. For total knee replacements, the trends are similar. In 1998, the number of revision knee replacements was 21,364, which is an increase of about 5000 cases compared to 1993. Revision operations are more complex than primary joint replacement, more costly to the patient and society, have higher complication rates and less favorable outcomes. Consequently, the clinician should know the basic clinical, biological, design and material principles relevant to prosthetic wear in order to prevent, diagnose and treat patients with TJRs.
About a year ago, experts in biomechanics and biomaterials, biological science, and medicine and surgery gathered for a 3-day symposium on the subject of implant wear of joint replacements. The workshop was sponsored by the National Institutes of Health, the National Institute of Standards and Technology, the American Academy of Orthopaedic Surgeons, the Orthopaedic Research Society, the Orthopaedic Research Education Foundation, the American Association of Hip and Knee Surgeons, the Knee Society, and two industrial sponsors: Biomet and Stryker Howmedica Osteonics. The symposium was centered on a series of questions, posed to the participants in advance, on issues relevant to implant wear. The results of the symposium are summarized in an inexpensive soft cover book entitled "Implant Wear in Total Joint Replacement", edited by Dr. Tim Wright and Dr. Stuart Goodman and published by the American Academy of Orthopaedic Surgeons. The content of the book will soon be available on the Internet and represents a synthesis of current knowledge and research on the subject of implant wear.
Clinical Aspects of Wear
Implant wear is ubiquitous; all total joint replacements undergo wear of the components at both articulating and non-articulating locations. Implant wear is a silent disease until the late stages, at which time symptoms may be due to the biological sequelae of excessive particulate debris (e.g., chronic inflammation, periprosthetic osteolysis, loss of fixation) and abnormal biomechanics of the involved joint. Clinical and radiographic follow-up are important to avoid catastrophic implant failure. The patient most at risk is the young, heavy, very active male, though wear rates are dependent on numerous factors that encompass the patient, the surgeon and the implant. Rather than a simple function of time in situ, wear of a polyethylene bearing surface is a function of the amount and type of use and the conditions under which the bearing operates.
Surgical technique certainly has a direct impact on prosthetic wear. Suboptimal alignment of hip prostheses (e.g., excessive vertical positioning of the acetabular component) increases wear, especially near the periphery of the component. Anatomic restoration of the hip center of rotation and offset and avoidance of impingement are associated with decreased wear. Optimal surgical technique involves stable fixation to minimize interfacial motion and avoidance of residual particles that could potentially contribute to third body wear. For knee replacements, malalignment, failure to balance the soft tissues, excessive elevation of the joint line, and patellofemoral subluxation are all associated with excessive wear of the bearing surfaces.
Though prosthetic wear is often a silent disease, clinical manifestations of excessive wear may include local and systemic signs and symptoms, including recurrent unexplained joint effusions with inflammation, pain, difficulty bearing weight, abnormal joint sounds, and lymphadenopathy. Catastrophic events such as dislocation, loosening or fracture of the surrounding bone or implant may occur. The toxicologic importance of systemically distributed wear debris remains unknown. Serial physical and radiographic examinations, often including special oblique x-ray views, are the keys to diagnosis. More sophisticated computerized methods to measure wear are currently employed in vivo as research tools in cases of hip replacement, but documentation of wear in knee replacements has proved to be more challenging.
When revision surgery is indicated, the principles of surgical management include: thorough debridement of the involved joint; correction of malalignment and other deficiencies in surgical technique, prosthetic design and materials; replenishing lost bone stock and providing a stable, functional joint replacement. Algorithms for surgical management have been developed and are discussed in the symposium book. The use of cancellous and strut allografts, special revision prostheses and fracture fixation devices have dramatically improved the outcome of these complex procedures. Longer, more detailed functional outcome studies would facilitate the assessment of specific surgical techniques. Adjuvant methods to accelerate osteogenesis or substitute for lost bone stock are currently being assessed, and would greatly assist surgical reconstruction.
Biological Aspects of Wear
Histologic examination of periprosthetic tissues from failed implants has shown a characteristic resemblance to granulomatous inflammation. Wear particles stimulate an adverse biological response consisting of a foreign body and chronic inflammatory reaction that is dependent on the characteristics of the particles, the particle load and the host. The most deleterious particles are in the submicron range and undergo phagocytosis by macrophages and other cells, resulting in cellular activation. In vivo and in vitro studies have demonstrated that particle-induced macrophage activation plays a key role in periprosthetic osteolysis. Particle-cell interactions initiate a cascade of events including the synthesis and release of pro-inflammatory mediators (e.g., prostanoids, cytokines, metalloproteinases, and chemokines) that facilitate osteolysis. Furthermore, particles depress osteoblastic function, tipping the bone-remodeling balance towards bone resorption rather than bone formation. In particular, polyethylene and metallic particles, and opacification agents, such as barium sulfate, in bone cements are implicated in this adverse response. The size, shape, surface area, surface chemistry, concentration and material characteristics of wear particles all affect the biologic response. The presence of endotoxin on the particle surface appears to dramatically exacerbate these adverse events.
Osteoclasts are formed by fusion of bone marrow-derived monocytes/macrophages that circulate in the blood and migrate to the local tissues. Wear particles enhance the differentiation of monocytes/macrophages into multinucleated foreign body cells and osteoclasts, capable of localized bone resorption. These processes are modulated by numerous hormonal, autocrine and paracrine pro- and anti-inflammatory mediators that determine the resultant biological response.
Intermittent loading of the implant can result in pressure waves that distribute particles, cells and inflammatory mediators along the prosthetic interface and to remote sites, such as the regional lymph nodes, liver, and spleen. The host response to implant materials and particulate debris is heterogeneous. Toxic and carcinogenic reactions generally do not occur with the materials currently used for total joint replacement. Whether sensitization and hypersensitivity reactions to specific materials used in TJR may play a role in prosthetic loosening and osteolysis is controversial. It is important to note that serum proteins coat particulate materials in the body; the protein-particle complex may theoretically provoke an adverse immunological reaction in some individuals. Novel technologies are now becoming available to help understand the genetically-determined host response to different physical and chemical properties of implant materials. Although some byproducts of wear or wear-associated biological processes can be found in the blood, urine, synovial fluid and other tissues, no true biological markers currently exist to assess the extent and progression of implant wear.
Currently, experimental biological approaches to mitigate the sequelae of wear have included the systemic use of bisphosphonates, non-steroidal and other anti-inflammatories and metalloproteinase inhibitors, and local delivery of growth factors, anti-inflammatories, and other small molecules that counteract the inflammatory cascade.
Design and Material Aspects of Wear
Engineering strategies have also emerged to combat the wear problem, and involve improved designs and more wear resistant materials. Improved designs aimed at rational solutions to the competing objectives of functional constraint and wear. In total hip replacements, for example, larger head sizes are helpful in combating dislocation, but can cause increased wear since sliding distances between the articular surfaces increase. Introducing elevated cross-linked polyethylenes with improved abrasive and adhesive wear resistance may allow for larger head sizes, though component thickness may still be a concern. Similarly, in total knee replacements, fixed bearing designs have been modified in an attempt to decrease contact stress (i.e., by increasing contact area) while maintaining appropriate constraints for flexion-extension, internal-external rotations, and varus-valgus angulations. Mobile bearing designs have the same aims, but introduce additional articulations that remain a concern.
Alternative bearing combinations, such as ceramic on ceramic and metal on metal couplings, and improved versions of existing materials, such as elevated cross-linked polyethylenes, hold the promise for drastically reducing wear in TJRs. Total joint products are now commercially available based on extensive preclinical in vitro testing and, for alternative bearings, on improvements over earlier, less successful versions of these same bearing couples. A continuing problem in introducing new technologies into the battle against wear is the lack of validated in vitro tests to establish both safety and efficacy prior to introducing them into the clinical community. Joint simulators, the basis for laboratory testing of all new bearing material combinations, can be influenced by a significant number of variables, including lubrication, kinematics, constraint, and applied load. More research is needed, therefore, to develop reliable tools for preclinically establishing the value of new designs and materials aimed at reducing wear. Without them, only long-term clinical observations will suffice to determine the value of these improvements.