Quest for better TKR bearing surface materials full of promise, some detours

NEW ORLEANS — Finding alternative materials for total knee replacement bearings that wear less over time through resistance to adhesive, abrasive and fatigue wear, and produce less osteolysis, has been a priority for orthopedic researchers and manufacturers, the director of biomechanics at New York’s Hospital for Special Surgery recently said.

What lies ahead is demonstrating the clinical effectiveness of advances made thus far in this area, such as with highly crosslinked polyethylene (PE) and oxidized zirconia, in total knee replacements (TKRs).

“I think the good news is wear is definitely on the decrease, but again, as always, questions remain for the long-term,” Timothy M. Wright, PhD, said.

He reviewed issues affecting the longevity of TKR bearings and discussed alternative solutions being explored during the Knee Society Specialty Day Meeting held during the 2010 Annual Meeting of the American Academy of Orthopaedic Surgeons.

Timothy M. Wright

PE issues

Wright focused on the problem with bearing surface materials currently being used and possible solutions arising from advances in material science related to PE and ceramics.

The new PEs used in orthopedic surgery, which have elevated levels of crosslinking or have undergone thermal treatments, have been associated with clinical success in total hip replacement, but have not fared as well in the knee. The crosslinking processes have adversely affected the PE’s properties of fatigue and fracture, so that when the material is designed into TKR components with posts, pegs or locking mechanisms, it can fail, Wright explained.

A scanning electron photomicrograph shows a damaged region of an oxidized zirconium TKR bearing surface. This type of damage has not been significantly correlated with increased PE wear of the opposing surface, but it indicates these ceramic surfaces should be handled with care. In extreme situations, if they are too damaged they may need to be exchanged during revision surgery. Image: Wright TM

“The good news is that wear simulation studies from a number of laboratories show that crosslinked PE greatly reduces wear both in terms of removal of material from the surface from abrasion and adhesion, but also an apparent elimination of the types of pitting and delamination we often see in degraded PE,” he said.

Wright noted that the diminished mechanical properties of PE with increased crosslinking used in TKR have been a liability. This can cause incipient cracks and other structural problems that signal mechanical insufficiencies.

Oxidized zirconia

“Ceramics certainly have advantages,” including being inert, biocompatible, harder and having a higher elastic modulus (than metallic alloys), which allows them to be polished into a smooth articulating surface, Wright said.

But, they can be weak in tension and are brittle, lacking the ability to deform plastically.

Oxidizing zirconium alloy by exposing it to heat and pressure creates a ceramic bearing surface while maintaining a ductile metallic alloy below the surface. From short-term component retrievals and wear studies done in our laboratory, “We certainly do not see any major or any different wear mechanisms than we have seen with more conventional metal-on-plastic bearings,” Wright said.

Fractures, cracks or roughening of the surface of ceramic TKR components appear to affect it minimally, according to scanning electron microscopy analyses. This may be the material’s “saving grace,” he said. — by Susan M. Rapp

Reference:

• Wright TM. Alternative bearing surfaces for total knee replacement. Presented at the Knee Society Specialty Day Meeting. March 13, 2010. New Orleans.

• Timothy M. Wright, PhD, can be reached at Hospital for Special Surgery, Caspary Research Building, 541 East 71st St., New York, NY 10021, 212-606-1093; e-mail: wrightt@hss.edu. He receives royalties from Mathys Ltd., Exactech Inc., and Zimmer, research/institutional support from Stryker, Synthes and Zimmer, and has stock/stock options with Exactech Inc.

Advances in engineering design and materials science of total hip and knee arthroplasty have improved the clinical success rate over the past 2 decades, although the process has been accompanied by major setbacks. Hard-on-hard bearings continue to be promising, but have not solved the wear problem with convincing finality. Ceramic-on-ceramic bearings have low wear rates in most cases, but the problems of fracture and squeaking still remain. Metal-on-metal bearings in the hip seem to be plagued with metal hypersensitivity and massive tissue destruction, the same problems that occurred in the early 1970s with the McKee-Farrar and Ring prostheses.

Much of the wear problems associated with total knee arthroplasty have been solved by eliminating gamma sterilization and by adding compression molding to the manufacturing process for polyethylene. Although bench testing of highly crosslinked polyethylene looks favorable in the knee, there is little to be gained, especially in older patients, by risking major new changes in the polyethylene. Ceramic-on-polyethylene is probably the next step, and has already been proven effective by the Oxinium material [Smith & Nephew], a ceramic-coated metal femoral component.

— Leo A. Whiteside, MD
Missouri Bone & Joint Center, Saint Louis, Mo.