Biodegradable/bioabsorbable Interference Screws

Kunststoffe-international.com, April 13, 2010

Researchers have developed a new material based on PLA and hydroxylapatite for production of interference screws. The screws degrade biologically without leaving any holes in bones.

A torn ligament in the knee frequently requires an operation to restore stability to the joint. In the course of this operation, the physician replaces the ligament with a piece of tendon from the leg and secures it to the bone with a so-called interference screw. Until now, such screws have been fabricated from titanium - with the drawback that a second operation is required to remove it. Sometimes, biodegradable screws made from polylactic acid (PLA) are used. However holes occasionally remain in the bone after the screw has degraded.

Researchers at the Fraunhofer Institute Manufacturing Technology and Applied Materials Research (IFAM) in Bremen have now developed interference screws that promote growth of bone in the implant and, depending on the formulation, degrade themselves within 24 months. The compressive strength of the material exceeds 130 N/mm². For comparison, real bone has a compressive strength between 130 and 180 N/mm².

The screws consist of an injection moldable formulation of PLA and hydroxylapatite. This mineral accounts for approximately 40% of the material in bone. In the medical field, it is used as a bone replacement or bioactive coating on titanium implants to improve bone growth. The newly developed composite contains a high percentage of hydroxylapatite and can be processed with excellent results in pellet form by means of injection molding. Complex geometries can be produced without the need for post-molding finishing operations. Moreover, the injection molding process has a beneficial side effect. Normally, the powder injection molded part must be densified after molding at very high temperatures of up to 1400°C. In contrast, the new composite material can be processed at 140°C.

Contact
Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung
Wiener Straße 12
28359 Bremen
Tel.: +49 421 2246-0
Fax: +49 421 2246-300
www.ifam.fraunhofer.de

Author
Dr.-Ing. Harald Sambale
sambale@kunststoffe.de