Can we achieve the revision-free implant? Paul Wooley, research director at the Center of Innovation for Biomaterials in Orthopaedic Research at Wichita State University, posed this intriguing question at a conference session I attended recently. 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. 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 materials-related failure in fewer than two years, Wooley notes. 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 a 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. "The particle number has probably not changed that much even though we're seeing less wear," Wooley 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.
"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. 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--but they may come from unlikely sources. His group, for example, is exploring the use of aerospace composites in orthopedic implant design. Carbon foam, which is used in aircraft wings, shows promise for such applications because it 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 material. We have to account for both the bioengineering as well as the biological properties," Wooley says. Composites allow engineers to create honeycomb structures, which can promote bone in-growth.
Current orthopedic implants have dramatically improved patients' quality of life. However, overcoming materials-related implant problems that result in the need for risky revision surgeries is imperative in future implant design. Maybe, as Wooley notes, engineers need to think beyond the usual materials suspects--provided, of course, that the materials are safe and biocompatible.