Symposium probes clinical issues

In the next 5 to 10 years, orthopaedic surgeons can expect to see a jump in the number of patients who require implant revision surgery, according to Wayne G. Paprosky, MD, FACS, associate professor of orthopaedics at Rush University Medical Center.

Although the number of individuals who receive total joint arthroplasty is skyrocketing, the increase in revisions isn’t tied just to population growth. Dr. Paprosky and other attendees of the 2007 AAOS/NIH Osteolysis and Implant Wear Research Symposium delved into the clinical reasons for joint revision surgery and explored issues such as the diagnosis and treatment of osteolysis, the challenges surgeons face during revision surgeries, and the continuing need for research into these complex subjects.

Osteolysis: an ongoing problem
Many “evolutionary milestones” have occurred in implant materials and design in recent years, according to Dr. Paprosky and two of his colleagues—Amanda Marshall, MD, assistant professor in the department of orthopaedics at the University of Texas Health Science Center at San Antonio; and Michael D. Ries, MD, professor of orthopaedic surgery and chief of arthroplasty at the University of California at San Francisco.

The physicians pointed to advances such as the development of porous in-growth surfaces in the 1980s to facilitate cementless fixation of implant components and thereby eliminate cement as a potential contributor to implant wear and osteolysis.

Despite these evolutionary milestones, said the physicians, periprosthetic lesions have remained a problem, demonstrating that particulate debris from other implant materials, most notably wear debris from materials such as ultra-high molecular weight polyethylene (UHMWPE), contribute to osteolysis (Fig. 1).

Although improved bearings implanted since 2000 have markedly reduced wear and osteolysis, osteolysis is still prevalent due to components implanted before the mid-1990s. Suboptimal polyethylene sterilization techniques have led to oxidative degradation and increased wear in these implants; inadequate locking mechanisms have also caused increased particle wear and subsequent osteolysis.

“More severe bone osteolysis is due to the increase in metal particulates as metal heads wear completely through the polyethylene, resulting in chrome cobalt femoral heads articulating with titanium shells,” said Dr. Paprosky.

“Because the metal shell geometry is destroyed,” he continued, “many of these cups must be removed due to locking mechanism destruction and the inability to cement in new liners. In almost all cases, the porous shells remain well-fixed to the severely damaged acetabulum. If liner exchange is not possible, major reconstruction of the acetabulum may be required after the shell is removed, with a potential of not achieving stable fixation.”

According to Drs. Paprosky, Marshall, and Ries, early results with highly cross-linked UHMWPE, metal-on-metal, and ceramic-on-ceramic bearing surfaces are encouraging and may eventually reduce the revision burden.

Implant wear risk factors
Audrey K. Tsao, MD, consultant orthopaedic surgeon with research interest in osteonecrosis, osteoporotic fractures, and total joint reconstruction, reviewed the factors, such as obesity, that put patients at high risk for developing implant wear-induced osteolysis.

“Although a high body mass index alone is not predictive of increased wear,” said Dr. Tsao, “it elevates the technical difficulty of surgery, thus increasing the potential for greater malalignment. In addition, higher applied loads can lead to accelerated wear patterns.”

Another group of patients—high-demand athletes, such as marathon runners—has not been shown to have increased implant wear during short-term follow-up. According to Dr. Tsao, these patients may face implant wear and osteolysis issues in the future due to the repetitive impact-loading and pivoting activities they perform.

Finally, age plays a large role in long-term implant outcomes. Older patients with osteoporosis are more susceptible to osteolysis, while younger patients place greater demands on artificial joints due to their longer life expectancies.

Surgical accuracy is key
According to John J. Callaghan, MD, professor in the University of Iowa’s orthopaedic surgery and biomedical engineering departments, alternative bearing couples are highly sensitive to surgical technique. Malpositioning of cross-linked polyethylene bearings can lead to implant fractures, said Dr. Callaghan. In ceramic-on-ceramic bearings, squeaking and impingement wear can result; in metal-on-metal bearings, problems can include unexplained increases in wear debris, known as “runaway wear,” and osteolysis.

“For all three types of bearings, proper positioning of the acetabular component to avoid excessive lateral opening and anteversion will prevent some of the complications associated with these alternative bearings,” said Dr. Callaghan.

Dr. Tsao elaborated on how surgical factors can affect wear mechanisms, referring specifically to knee prostheses.

“Surgical techniques—such as thickness of resections, alignment of each component, and ligamentous balance—influence the tracking of the components and the contact stresses experienced by the articulating materials,” she explained. “Minimally invasive surgery, with its significantly diminished exposure, may increase the role of these factors in the long-term wear of the components.”

The influence of surgical technique on osteolysis and implant wear will only continue to intensify in the future, according to symposium co-chair Timothy Wright, PhD, F.M. Kirby chair of orthopaedic biomechanics at the Hospital for Special Surgery and professor of applied biomechanics at Cornell University’s Weill Medical College.

“Orthopaedic surgeons should realize that they’re going to see implants designed more for function and less for combating wear because biomedical engineers have done a good job of combating wear on the materials side,” said Dr. Wright. “More functional joints need to be implanted very accurately. They’re not as forgiving as joints that have large, curved surfaces.”

Diagnosis and surveillance
The diagnosis and surveillance of wear-induced osteolysis require the measurement of the extent of the generated wear debris and the use of imaging methods to evaluate the extent, location, and progression of the periprosthetic osteolysis process, reported Henrik Malchau, MD, PhD, co-director of the Harris Orthopaedic Biomechanics and Biomaterials Laboratory at Massachusetts General Hospital, and Hollis Potter, MD, chief of the division of magnetic resonance imaging and director of research for the department of radiology and imaging at the Hospital for Special Surgery.

Because conventional radiographs underestimate the prevalence and extent of osteolysis in many instances, Drs. Malchau and Potter noted that diagnosis and surveillance should be performed with radiographic edge detection (to provide a more precise and accurate estimate of the boundaries of osteolytic lesions), spiral computed tomography (CT), magnetic resonance imaging (MRI), radiostereometric analysis, and quantitation of wear and osteolysis, including bone and soft-tissue lesions.

Drs. Malchau and Potter noted that helical CT has demonstrated excellent specificity in identifying and quantifying the extent of osteolysis. According to the physicians, MRI can more accurately localize both osseous and soft-tissue particulate disease and can directly detect granuloma and compression on adjacent nerves and vessels.

Treatment
“Treatment interventions for osteolysis can be both nonsurgical and surgical; the choice should be determined by the severity of symptoms and the extent of bone loss identified on 3-dimensional imaging studies,” said Bernard N. Stulberg, MD, director of the Cleveland Center for Joint Reconstruction; and Alejandro González Della Valle, MD, assistant attending orthopaedic surgeon at the Hospital for Special Surgery.

In asymptomatic patients with smaller lesions, Drs. Stulberg and González Della Valle explained, osteolytic lesions can be managed nonsurgically—with frequent clinical and radiographic follow-up, and the possible use of pharmacologic agents. When osteolysis is detected, the patient should be closely monitored. Revision surgery is recommended if the patient becomes symptomatic, substantial bone loss occurs, or there is risk of catastrophic implant failure or periprosthetic fracture.

According to the physicians, the orthopaedic surgeon and the patient undergoing a total hip or knee replacement should commit to postoperative follow-up evaluations at 6 weeks, 3 to 4 months, and 1 year after initial implantation. After the initial year of implantation, regular clinical and radiographic follow-up should be performed every 3 to 5 years for patients without clinical symptoms, osteolysis, or other known problems who have radiographically stable prostheses. Young, active, heavy patients are in particular need of more frequent, regular follow-up. After osteolysis/implant wear is identified, a plan for closer follow-up every 3 to 6 months should be instituted until changes appear stable.

Ultimately, agreed Drs. Stulberg and González Della Valle, wear-induced osteolysis is “a progressive phenomenon; one that is best treated by surgical intervention.” The goals of surgery are to remove all debris, restore bone stock as needed, and provide a mechanically sound reconstruction while removing the source of wear debris particles.

“A contemporary grading system for bone loss should be used to guide implant and grafting choices during revision arthroplasty, and to characterize deficiencies appropriately for clinical and research purposes,” advised Dr. Stulberg.

Surgical options can vary from débridement and change of worn bearing surfaces (with or without bone grafting of osteolytic lesions) to more extensive procedures that replace the entire prosthesis and address structural bone loss.

Research directions
The clinicians at the symposium agreed that, in particular, the use of CT and MRI in diagnosing the extent of osteolysis should be extended to large cohorts to determine the efficacy of these diagnostic tools and to establish guidelines for their clinical use in the evaluation of new hip and knee replacement designs. They also stated that the relative indications and contraindications for choosing one bearing surface over another need to be established, as no guidelines to help the practicing surgeon currently exist, other than those provided by industry.

In addition, Dr. Callaghan, John M. Cuckler, MD; James I. Huddleston, MD; Jorge O. Galante, MD, and other symposium attendees called for prospective, randomized, multicenter studies to compare available alternate bearing surfaces in total joint arthroplasty, especially in young, active patient populations. They also encouraged the development of a national arthroplasty registry to evaluate differences in longevity of the different bearing alternatives in the general population. Further basic science and clinical studies are necessary, they added, to evaluate the early complications noted with all three bearing couples and to provide a mechanistic explanation that will lead to further improvements. In addition, the development of patient education materials that focus on the lack of early symptoms of osteolysis and the risk of delaying treatment until failure occurs should be undertaken.

Jennie McKee is a staff writer for AAOS Now. She can be reached at mckee@aaos.org

Symposium coverage, part 2
This is the second in a series of articles on the AAOS/National Institutes of Health (NIH) Osteolysis and Implant Wear Research Symposium: Biological, Biomedical Engineering, and Surgical Principles, held Nov. 9 – 11, 2007, in Austin, Texas. Co-chaired by Stuart Goodman, MD, PhD, and Timothy Wright, PhD, the invitation-only symposium built upon previous workshops on osteolysis and implant wear held in 1995 and 2000.

This issue of AAOS Now focuses on the clinical issues, such as risk factors for implant wear and osteolysis, surgical techniques, outcomes, diagnosis, and treatment. For coverage of biologic markers, alternative bearing surfaces, and future research, see the January 2008 AAOS Now online at www.aaos.org/now

Grant support for the symposium came from the AAOS, the American Association of Hip and Knee Surgeons, The Knee Society, the Orthopaedic Research and Education Foundation, and the Orthopaedic Research Society. The symposium also received grant funding from the National Institute of Arthritis and Musculoskeletal and Skin Diseases/NIH, through a U-13 conference support grant. Industry supporters included DePuy, DePuy Spine, Smith & Nephew, Stryker Orthopaedics, Synthes, and Zimmer. The symposium findings will be published in their entirety in a special supplement to the Journal of the AAOS later this year.