Saving Joints

BIOMATERIALS

Bone loss around the site of an implant caused by an inflammatory response to wear-particles is the most important factor limiting the lifetime of artificial joints. Japanese researchers now show that grafting a biocompatible polymer layer onto artificial joints reduces wear and bone loss.

The team, led by Hiroshi Kawaguchi from the department of orthopedic surgery at the University of Tokyo, covalently attaches a biocompatible phospholipid polymer, 2-methacryloyloxyethyl phosphorylcholine (MPC), to the liner surface of an artificial hip made of polyethylene [Nat. Mater., published online Oct. 24, http://dx.doi.org/10.1038/nmat1233].

In mechanical tests of the coated joint in a hip-joint simulator, the MPC layer decreases friction and the production of wear-particles compared to uncoated joints. "The reduction of wear is likely to arise from the hydrated lubricating layer formed by the MPC grafting," Kawaguchi says.

In addition, the MPC coating appears to prevent bone loss: MPC-coated polystyrene particles injected into mice do not induce bone resorption. The researchers use polystyrene because trying to isolate the wear-particles from the hip-joint simulator damages the MPC layer. "Since the structure of MPC polymer is similar to that of biomembranes, it prevents macrophages from recognizing the particles as foreign bodies," Kawaguchi says. The usual inflammatory response that triggers bone loss around artificial joints does not ensue.

"These are promising results," says Timmie Topoleski, a biomaterials scientist at the University of Maryland, Baltimore County, who also studies joint implants. "The ultimate test will be going through the hurdles to apply the new material to orthopedic devices and then study results in patients."

Kawaguchi and his colleagues plan to start clinical trials of MPC-grafted implants by June 2005 and expect results by the end of 2007.