Current options include implant design, materials, surgical technique

Primary total knee arthroplasty (TKA) is quite successful in relieving pain and improving mobility; the National Institutes of Health says that about 85 percent of patients report being satisfied with the results. But members of the Knee Society and the American Association of Hip and Knee Surgeons, meeting in joint session during the 2010 Specialty Day, believe that improvements in surgical technique, as well as advances in implant design and materials, may be able to improve outcomes even more.

Implant designs
“In younger patients—those younger than age 55—the use of optimally designed components and the most wear-resistant plastic are warranted,” said John J. Callaghan, MD, “because of their high demands.”

In the past, said Dr. Callaghan, patients with rheumatoid arthritis accounted for most TKAs performed in younger patients. Implant designs and materials that were suitable for this low-demand population, however, may not be adequate for today’s population of patients with primary osteoarthritis who require knee replacements.

“We now know that the polyethylene used in the late 1980s and early 1990s, when most components were sterilized with gamma irradiation in air, was inadequate, as were many of the polyethylene-to-metal tray locking mechanisms,” said Dr. Callaghan. These designs had a relatively high degree of micromotion between the metal tray and the polyethylene, resulting in increased wear.

Newer designs have better locking mechanisms, and improved polyethylene quality should result in more durable components. “These new designs should be closely followed and evaluated for long-term durability in this age group,” he concluded.

Frederick F. Buechel Sr., MD, agreed that design improvements are leading to better outcomes. “Using ethylene oxide to sterilize the polyethylene bearings and increasing the bearing surface congruity can improve the long-term wear of rotating platform knee designs,” he said.

He cited a prospective study of 257 patients who had a total of 310 cementless, rotating platform TKAs. After 18 years, the survivorship rate was 99.3 percent (using revision for wear or component loosening as the endpoint). To prevent rotary subluxation/dislocation (“spin-out”), a rotational stop-pin was used, which effectively eliminated this complication.

Implants designed for specific populations are another innovation, noted Giles R. Scuderi, MD. For example, studies have found differences between the sexes in the femoral aspect ratio and quadriceps angle, which might be addressed by redesigned knee prostheses. New designs might also be able to address morphotypic variations.

“Endomorphic patients with short stature and wide morphotype have wider knees compared to ectomorphic patients with long and narrow morphotypes, who have narrower femurs,” noted Dr. Scuderi. “More anatomically shaped implant designs may reduce the problems of oversizing the knee, which results in limited motion, or undersizing the knee, which leads to instability.”

Material improvements
Improvements in the materials used to make components may help reduce the impact of wear and osteolysis, said Timothy M. Wright, PhD, and Harry E. Rubash, MD.

Dr. Wright drew a comparison with alternative bearing surfaces in hip replacement, noting that improvements in polyethylene wear resistance have shown dramatic results during the past decade. “Similar improvements in wear resistance are shown in knee simulator studies of highly cross-linked polyethylenes,” he noted, although similar clinical results have not yet been demonstrated.

Hard-on-hard bearings have little advantage in knee replacement, he noted, because large contact stresses and reduced fluid lubrication makes them subject to high wear. But some materials being developed, such as oxidized zirconia, have shown decreased wear in simulators and acceptable short-term clinical outcomes.

Dr. Rubash used examples from his lab to show the dramatic decrease in articular and backside wear as implants moved from conventional polyethylene to the first generation of crosslinked polyethylenes.

“So why go to a second generation when the first generation seems to be performing well?” he asked. “Because, as Dr. Wright pointed out, irradiation and melting result in a decrease in the strength of the material.”

Newer polyethylenes have improved fatigue resistance and material properties. They attempt to address the issue of oxidation by reducing the free radicals, using vitamin E and sequential irradiation. As a result, these materials could be used in different types of TKA systems.

“We know the mechanical environment is different in the knee,” concluded Dr. Rubash. Clinical studies are underway, and developing a reliable and accurate method for quantifying in vivo wear will be critically important as we look at new bearing surfaces.”

Technique matters
Although the hardware and software used to perform computer-assisted surgery (CAS) was introduced less than a decade ago, this new technology has the potential to improve the technical aspects of TKA, said John M. Siliski, MD. Early studies are already showing that CAS results in improved technical accuracy, particularly in axial alignment, but more time is needed to determine whether these improvements will affect long-term functioning and implant survival.

“Questions of cost, surgical time, learning curve, and benefits are still being addressed,” said Dr. Siliski, who noted that these issues have also been impediments to adoption of CAS.

“Doing a computer-assisted TKA takes about 10 minutes longer than doing a conventional TKA, but the promise of improved outcomes may offset that,” he noted. And when the cost of the equipment is amortized, the impact is substantially reduced—to less than that of a unit of transfused blood.

“Digitally based technologies, including CAS, robotic-assisted surgery, and custom cutting blocks, offer significant promise for improving the technical aspects of TKA,” he concluded.

Rehabilitation
According to Clifford W. Colwell Jr., MD, studies on the forces generated during various recreational activities are helping to provide a more scientific rationale for advice on what activities patients should avoid after TKA.

The forces generated on the knee during various recreational activities vary with the activity and level of difficulty. According to the results of an in vivo study using a tibial tray instrumented to measure these forces, skiers may not have to give up the slopes, even if they’ve had a total knee replacement.

Walking on skis to get to the ski lift, for example, generated peak forces of about two times body weight, actually lower than the peak forces generated during walking (about two and a half times body weight). Snowplowing and cruising on a gentle slope generated forces comparable to stationary biking. Stopping while on skis, skating, and stair climbing all generated similar force levels. Skiing on steeper slopes (greater than a 6-degree slope) generated forces as high as those seen during jogging.

“Such quantitative results can assist the surgeon in advising the patient about postoperative exercise,” he said.

Disclosure information: Dr. Buechel—no information available; Dr. Callaghan—DePuy, Wolters Kluwer Health–Lippincott Williams & Wilkins; Dr. Colwell—Stryker, Medical Compression Systems, Journal of Arthroplasty, Journal of Bone and Joint Surgery–American, Clinical Orthopaedics and Related Research; Dr. Rubash —Zimmer; Dr. Scuderi—Zimmer; Dr. Siliski—no information available; Dr. Wright—Mathys Ltd., Exactech, Inc., Synthes, Stryker, Smith & Nephew, Journal of Orthopaedic Research, Wolters Kluwer Health–Lippincott Williams & Wilkins, Knee Society.

Mary Ann Porucznik is managing editor of AAOS Now. She can be reached at porucznik@aaos.org