New surgical bone screw biodegrades in two years

March 1, 2010 11:54 AM PST
by Elizabeth Armstrong Moore

For years, people with broken bones have had to suffer through not only the pain of the break, but also the long process of healing, often with the help of titanium screws. Typically, patients must then undergo more surgery to remove the titanium.

When my mom broke her knee in the '90s, they rigged her with so many screws and bars that her X-rays looked more robot than human. She predicted rain with eerie accuracy.

(Credit: Fraunhofer IFAM) This month, researchers at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) in Bremen, Germany, are unveiling a new type of screw that not only biodegrades within two years but actually encourages bone growth into the implant itself so as not to leave gaping holes where the screws used to be. (This has been one goal of fracture putty as well.)

Current biodegradable screws are made of polylactic acid, but those leave holes once they degrade. IFAM researchers developed a moldable composite made of polylactic acid and hydroxylapatite, a ceramic that Philipp Imgrund of IFAM's biomaterial technology department says is the main constituent of bone material.

"We have modified biomaterials in such a way that they can be formed into robust bioactive and resorbable screws by means of a special injection-molding process," Imgrund said. "This composite possesses a higher proportion of hydroxylapatite and promotes the growth of bone into the implant."

IFAM engineers rely on conventional injection-molding methods, but they developed a granulate from the biomaterials with a net shape that results in a robust screw. The prototype's properties are very similar to a real bone's, with compressive strength of more than 130 newtons per square millimeter (a real bone withstands 130 to 180).

Moreover, the temperature required for compression is about 280 degrees Fahrenheit, much less than the 2,500-degree temperatures required in typical powder injection molds. So there are energy savings to celebrate as well.