TKA implant designers should further study mechanical testing, literature results

Two surgeons recently debated which future efforts would yield longer-lasting TKA implants.

Robert T. Trousdale

Jan Victor

ORLANDO, Fla. — With informed patients asking their physicians about the expected longevity of total knee arthroplasty prostheses, orthopedic surgeons are faced with new questions and opportunities, according to presentations at the Current Concepts in Joint Replacement 2011 Winter Meeting.

Robert T. Trousdale, MD, of the Mayo Clinic in Rochester, Mich., debated Jan Victor, MD, PhD, of Ghent University Hospital, Belgium during a crossfire session on issues they believe are pertinent to total knee arthroplasty (TKA) implant longevity.

Literature predicts durability

Trousdale said that addressing advertising claims is an opportunity and a first step toward better management of patient expectations about TKA implants.

“I tell [patients] for each year they use the knee, they have about a 1% chance of failure, and that chance begins at day 1,” he said. “It is not realistic to tell our patients about 30-year results when there are no clinical data available at 30 years.”

The second step, according to Trousdale, is to use available historical literature to predict which factors most affect implant wear and longevity. While these data might not yet reflect any new technological innovations, the literature is still the best way to gauge how new implants will survive in the future years, he said. As an example, he cited a study he and his colleagues conducted, led by James A. Rand, MD, which examined 11,606 knees operated on at the Mayo Clinic between 1978 and 2000. These data were reliable, Trousdale noted, because every patient had at least a 10-year follow-up.

This preoperative radiograph shows a 52-year-old male 12 years after a closing wedge upper tibial osteotomy.

This postoperative radiograph shows a patient 16 years after TKA. The patient has had a durable result at intermediate follow-up, though there is little data to support durable results at 30-year follow-up. Images: Trousdale RT

Improved survivorship

In Rand’s study, there was approximately a 1% reduction in survivorship at up to 20 years in that study, Trousdale said.

He noted that the women studied had better results and patients with inflammatory arthritis fared better than the patients with non-inflammatory arthritis. Posterior cruciate ligament (PCL)-retaining and non-stabilizing PCL-sacrificing prostheses proved better than PCL-stabilizing implants. Cemented TKA was also more effective than TKA hybrid or cementless fixation, and all-polyethylene metal-backed designs were superior to modular designs, he said.

“Because of this data, the vast majority of my patients presently — this is all ages, young and old — get a conventional, cemented total knee replacement and I use a monoblock, all-polyethylene tibial component in the majority,” Trousdale said.

Severe synovitis intraoperatively due to polyethylene wear is shown. Image: Victor J

Simulator testing favored

In his presentation, Victor proposed that strict testing using wear simulators, rather than relying on historical literature, is the ideal approach that supports continued TKA innovation.

“As doctors working with engineers, we should try to make more use of virtual tools and mechanical testing in order to predict future things that will happen,” Victor said. “We all know of the things that went wrong in orthopedics over the past 20 years. A lot of these things could maybe have been foreseen if the mechanical testing had been a bit better.”

Machines that determine polyethylene wear through repetitive loading and varying kinematic patterns can help replicate whatever problems patients may face after several postoperative years, Victor said, adding that the downside of relying on them to help predict future prosthesis wear and survivorship is that no test standards are currently established for kinematic patterns, applied loads or numbers of cycles. In addition, wear simulators do not account for sudden trauma or long-term exposure to oxygen, both of which could adversely affect implants, he said.

Ultimately, statistical results cannot be applied to individual people, Victor explained, saying he tells patients they can reasonably expect 15-year TKA survivorship from the information available today. Although new prosthesis developments and efforts to promote them are exciting, Victor said he has remained cautious due to the small amount of information available on any new products.

“The native knee is a 340 million-year evolutionary experiment. By contrast, the TKA is a man-made 40-year experiment,” Victor said. “In my view, a lack of evidence of efficacy can be acceptable and we can call that hope. However, a lack of safety analysis is unacceptable and that is when hope turns into hype.” – by Jeff Craven

References:

• Rand JA, Trousdale RT, Ilstrup DM, Harmsen WS. Factors affecting the durability of primary total knee prostheses. J Bone Joint Surg Am. 2003;85:259-265.
• Trousdale RT, Victor J. The 30 year knee: A triumph of hype over reason. Papers #79 and #80. Presented at the Current Concepts in Joint Replacement 2011 Winter Meeting. Dec. 7-10. Orlando, Fla.

• Robert T. Trousdale, MD, can be reached at Mayo Clinic, 200 First St. SW E14B, Rochester, MN 55905; 507-284-3663; email: trousdale.robert@mayo.edu.
• Jan Victor, MD, PhD, can be reached at Ghent University Hospital, Chairman of Orthopaedics, De Pintelaan 185, 9000 Ghent, Belgium; 32-9-3322264; email: jan.victor@ugent.be.
• Disclosures: Trousdale receives royalties from DePuy, MAKO Surgical Corp. and Ortho Development. Victor has intellectual property rights with Smith & Nephew.