Of the 700,000 knee replacement surgeries the Centers for Disease Control & Prevention says are conducted each year in the U.S., 12% of them have to be redone within 10 years. The same is true for hip replacements, according to a 2011 study by a group of European orthopedic surgeons.
Part of the reason for the do-overs is bad glue. The go-to method currently used by doctors to stick a titanium or plastic implant to the bone in a person’s joint is to apply polymethyl methacrylate (PMMA) cement, says Paula T. Hammond, a chemical engineer at Massachusetts Institute of Technology. This polymeric glue erodes over time, she says, loosening the implant and shedding bits that can cause joint inflammation.
To create a stronger, longer-lasting interface between bone and an implant, Hammond and her colleagues developed a two-part multilayer implant coating that sticks well and stimulates bone growth (Sci. Transl. Med. 2013, DOI: 10.1126/scitranslmed.3005576).
On the implant surface, the researchers deposit alternating base layers of polyacrylic acid and a mixture of hydroxyapatite, a mineral found in natural bone, and chitosan, a polysaccharide derived from crustacean shells. They then add outer layers of a water-degradable amino ester polymer and a mixture of polyacrylic acid and bone morphogenetic protein-2 (BMP-2), which triggers bone growth.
When surgically inserted into the shinbones of rats, a coated implant controllably released BMP-2 slowly, over a 30-day period, which stimulated steady tissue growth between the implant and the rat shinbone. In tests of the mechanical strength of the implants, the researchers found they couldn’t easily remove them—the new coating is two to three times as strong as PMMA cement.
Given the number of people needing joint replacements, this new coating could have “considerable humanitarian impact,” says Nicholas A. Kotov, a chemical engineer at the University of Michigan who studies nanocomposites and implantable devices.
Hammond and her team have patented the multilayer coating and are seeking funds to test it in larger animals.