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Can We Achieve the Revision-Free Implant?

Published: March 1, 2011 Shana Leonard MPMN Medtech Pulse

I attended an interesting conference session at the MD&M West trade show last month in which speaker Paul Wooley, research director at the Center of Innovation for Biomaterials in Orthopaedic Research at Wichita State University, asked: Can we achieve the revision-free implant? While the concept might not be ideal for OEMs‘ business, the notion of a long-lasting, durable hip or knee implant is an undeniably exciting prospect for patients. And it might not be a pipe dream, according to Wooley. The catch: Current materials just won’t cut it.

Although hip and knee implants are designed to have an estimated 15-year lifespan, they can experience failure stemming from materials issues in fewer than two years, Wooley notes. He adds that 30% of patients in their 50s and 55% of patients younger than 50 will require revision surgery in fewer than 15 years. Inadequate surgical techniques, infection, and patient noncompliance can play a role in the need for revision surgery. However, bearing surface wear and the resulting debris are widely regarded as the primary culprits for such issues as osteolysis, loosening, breakage or fracture that lead to implant failure.

In light of the metal-on-metal bearing surface backlash, many implant manufacturers have turned to highly crosslinked (HXL) polyethylene as a more wear-resistant option. Yet despite claims of 70 to 90% reductions in volumetric wear that have further bolstered support for the material, HXL polyethylene might not be the wear panacea that engineers perceive it to be, Wooley says. “The particle number has probably not changed that much even though we’re seeing less wear,” he states. “But the question with new materials that arises is: Does a different form of wear constitute a novel problem?” Wooley thinks that it could, but it’s too soon to tell just yet.

“I would contend that current materials are suboptimal,” Wooley surmises. “From a bioengineering standpoint, perhaps there isn’t a great deal we can achieve through changing the basic design—although there are a couple of interesting, novel improvements out there. We need to improve biocompatibility, wear, and this idea that we can use materials to improve osteointegration.”

So, where does the promise lie for that revision-free implant? Wooley proposes that composite materials may be the missing ingredient in these ambitious implants— but they may come from unlikely sources. His group, for example, is exploring the use of commercially available aerospace composites in orthopedic implant design. Carbon foam, which is used in aircraft wings, shows promise for such applications, he says. “This material actually mimics the structure of bone. The issue now becomes the balance of structural strength as we go from a more open-cell material to a closed-cell materials. We have to account for both the bioengineering as well as the biological properties.” Composites allow engineers to create honeycomb structures, which promote bone in-growth, Wooley adds.

In addition to someday possibly enabling the development of revision-free implants, composite materials could prevent increased implant wear if employed in orthopedic surgical tools, according to Wooley.

“Stainless-steel instruments have this bad habit in orthopedic surgery when used incorrectly or carelessly of actually scratching the highly polished surface of the bearings,” he says. “Once you have that scratch, you have a cutting surface, and your wear rate through conventional highly crosslinked poly is going to be hugely magnified.”

Composite materials, in contrast, can be scratch resistant.

It’s no doubt that current hip and knee implants have dramatically improved the quality of life for patients. However, overcoming materials-related implant problems and failures that result in the need for risky revision surgeries is a novel notion. Maybe, as Wooley notes, engineers need to think beyond the usual materials suspects— provided, of course, that the materials are safe and biocompatible. Could composites be the key to next-generation, revision-free implants? Do you think a revision-free implant is possible? Weigh in down in the comments section below. Plus, check out MPMN’s archives for more information on bearing surfaces and facilitating implant acceptance. —Shana Leonard