Published 3/1/2008
Jennie McKee

Engineering the future of implants

Biomedical engineers explore implant advances

“A principal strategy for reducing the osteolysis problem is to reduce its root cause: wear debris,” said Thomas D. Brown, PhD, director of the Orthopaedic Biomechanics Laboratory at the University of Iowa. “Mechanical implant design is a key consideration in that regard. Mechanical design issues are closely coupled with bearing surface material combinations, the menu of which has become ever more diverse.”

During the 2007 AAOS/National Institutes of Health (NIH) Osteolysis and Implant Wear Research Symposium: Biological, Biomedical Engineering, and Surgical Principles, Dr. Brown and fellow attendees discussed how this “more diverse menu” is affecting the design and engineering of implants. New bearing surfaces, improved sterilization techniques, and other advances in design are helping to create implants that generate less wear debris and are more functional than ever before.

The new generation of implants
In recent years, metal-on-conventional polyethylene (MOCP) implants have been largely supplanted by alternative bearings made of metal-on-highly crosslinked polyethylene (MOXP), ceramic-on-polyethylene (COP), ceramic-on-ceramic (COC), and newer generation metal-on-metal (MOM) materials.

According to Dr. Brown, these newer bearing surface combinations “far out-perform MOCP in terms of volumetric wear rate, although various ancillary issues, such as absolute particle numbers, relative particle osteolytic potency, and metal ion release currently confound direct comparison.”

(Fig. 1 shows some of the modes of debris generation currently being studied.)

Roy Crowninshield, PhD, professor of orthopaedic surgery at Rush Medical College, presented data consistent with earlier predictions that new types of polyethylene and gamma sterilization without oxygen lead to clinically improved bearing performance. He and his team assessed the changes in surface wear/damage and socket wear/creep volume of 46 components (implant duration, 12 to 96 months) used in a single implant system. The implants were made from three different polyethylene types, representative of the sequentially available bearing materials that have been dominant during the past several decades of hip arthroplasty.

“Acetabular components made from highly crosslinked polyethylene had a 50 percent lower total damage score than components made from conventional polyethylene that was gamma-sterilized either in air or in nitrogen,” said Dr. Crowninshield. “The measured wear/creep volume change was 80 percent lower for the highly crosslinked components compared to those gamma sterilized in air and 90 percent lower than those gamma sterilized in nitrogen.

“Although further studies are needed to assess the longer term performance of these implant material types,” he continued, “we believe that these observations, considered with clinical reports of improved wear, support the continued use of these newer materials where prolonged implant longevity is desired.”

Other factors that reduce wear include storing components in an oxygen-free environment, as well as using higher levels of radiation exposure to intentionally crosslink the material. Annealing or melting the ultra-high molecular weight polyethylene (UHMWPE) after irradiation addresses free radicals that can lead to in vivo oxidation.

All of these advances have opened up the design space.

“Implant wear, which used to be a problem that engineers designed around, is now a much smaller constraint,” said 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.

“We can design knee joints to be more anatomic without having to be as concerned about wear,” he continued. “Similarly, hip implants can be designed with bigger femoral heads because UHMWPE wears better.”

Bigger femoral heads, explained Dr. Wright, help reduce the chance of dislocation, a common complication of implant surgery.

Beyond wear reduction
Biomedical engineers and implant designers endeavor not only to reduce wear, but to create more functional joints.

“An important design development has been the availability and implementation of material damage models for both abrasive and fatigue-based damage,” said Donald L. Bartel, PhD, the Willis H. Carter professor in engineering emeritus at Cornell University and senior scientist in the department of biomechanics of the Hospital for Special Surgery. These make it possible to treat maximum stresses and strains as design constraints instead of design objectives so that other objectives, such as better joint function, can be pursued.

Surface clearance is a key consideration in the design of more functional prostheses.

“Appropriate surface clearance is an absolutely crucial parameter for modern hard-on-hard bearings (MOM, COC), whose successful function relies upon fluid lubrication,” said Dr. Brown.

Appropriate clearances for fluid-film lubrication can be determined from engineering analysis. In a typical total hip arthroplasty, maintaining lubrication requires highly polished surfaces and tiny deviations in spherical concentricity between the bearing surfaces. Unfortunately, this idealized behavior can be severely compromised if components are malaligned or component geometrical distortion exists.

Preclinical evaluation and testing
Symposium attendees agreed that new implant designs must receive careful, preclinical evaluation and testing for wear. Comparing wear among designs is difficult, however, because preclinical data are generally limited and based on different testing methods.

Wear is typically measured under conditions that simulate level walking, rather than “heavy-duty” activities. Nonetheless, general conclusions have emerged on design influences. For example, in fixed-bearing total knee arthroplasties, lower conformity between bearing surfaces allows larger, more variable tibiofemoral displacements, but higher contact stresses can lead to elevated wear.

Hip wear tests have also simulated microseparation of the femoral head from the socket. Retrieval analyses suggest that microseparation occurs frequently with hard-on-hard bearings and may be a cause of squeaking.

“Hip wear simulation has been very successful in expanding our understanding of tribology of prostheses and in ranking bearing materials,” said Darryl D’Lima, MD, PhD, director of the Orthopaedic Research Laboratories at the Shiley Center for Orthopaedic Research and Education at Scripps Clinic. “Knee wear simulation is more challenging because of the increased degrees of freedom and reduced articular conformity. In addition, retrievals are the primary source of validation due to the lack of readily available in vivo markers of knee wear.”

Future research directions
According to Dr. Crowninshield and Orhun K. Muratoglu, PhD, co-director of the Harris Orthopedic Biomechanics and Biomaterials Laboratory at Massachusetts General Hospital and associate professor of orthopaedic surgery (biomaterials) at Harvard Medical School, clinical research must document the long-term outcomes of various forms of polyethylene in joint replacement.

“Reliable, accurate methods for measuring clinical wear (such as radiostereometric analysis), together with implant retrieval analysis and imaging assessment of osteolysis, should be used to document performance,” said Dr. Muratoglu.

Symposium attendees agreed that future research should focus on increasing the strength and fatigue resistance of polyethylene without compromising wear and oxidative resistance. These improved materials may provide an opportunity to produce implant systems that are more forgiving of variations in surgical placement and orientation of implant components and allow for the design of more functional implants.

Drs. Crowninshield and Muratoglu also recommended future research into the effects of antioxidants on polyethylene.

“Vitamin E improves the oxidative resistance of irradiated polyethylene; however, the mechanism of action is unknown,” said Dr. Crowninshield. “The use of other antioxidants may have synergistic effects on the wear and mechanical properties of irradiated polyethylene.”

“Clinical studies should be conducted to evaluate the effectiveness of second generation highly crosslinked UHMWPEs that were recently introduced for clinical use,” said Dr. Muratoglu. “Analysis of surgically retrieved implants to evaluate the performance of these newer materials should continue.”

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

Symposium coverage, part 3
This is the final article in a series on the AAOS/National Institutes of Health (NIH) Osteolysis and Implant Wear Research Symposium: Biological, Biomedical Engineering, and Surgical Principles, co-chaired by Stuart Goodman, MD, PhD, and Timothy Wright, PhD, and held Nov. 9–11, 2007, in Austin, Texas.

This issue of AAOS Now focuses on the biomedical engineering concepts discussed at the symposium, from advances in implant material and sterilization techniques to the continuing need for preclinical evaluation and testing of new implant designs. Biologic markers, alternative bearing surfaces, and future research were featured in the January 2008 AAOS Now; the February 2008 issue focused on clinical topics such as risk factors for implant wear and osteolysis, surgical technique, outcomes, diagnosis, and treatment. All issues can be accessed online at www.aaosnow.org