A method for creating long-term stable bone implants used to replace a missing or diseased portion of bone has been devised by scientists based in the USA.

The aim is to embed a biocompatible material, such as a titanium coil, in the surface of a polymer bone implant to provide a porous coating

Problems with current implant designs arise from the difference in mechanical properties between the materials used in the implant system and the bone itself.

The intramedullary implants being used today are generally made from metal, using an alloy of either titanium (Ti) or cobalt chrome (Co-Cr). The joint replacement implants are primarily made with a Co-Cr alloy containing molybdenum (Mo). Long bone replacement implants are most commonly made from Ti, either in its commercially pure state or as an alloy with aluminium and vanadium. The implant must remain stationary so the bone tissue can begin to grow around it.

The implant is stabilized initially with bone cement (polymethylmethacrylate) but long term stabilization of the implant in bone is achieved by having a porous coating on the surface of the implant. The porous coating is either added or moulded onto the surface of the implant.

Ti or hydroxyapatite (HA) are often used to create this porous coating, which must have large enough pores to allow the bone cells to travel through and create a strong interlocking fixation by reconnecting with adjacent bone tissue throughout the mesh. If the bone tissue does not grow fast or is not strong enough, the implant is not completely stabilized and there can be micromotion, say the inventors of these methods.

The use of titanium as the porous coating requires new apparatus and methods for embedding the biocompatible material in the polymer bone implant surface, they say.

They describe three methods of embedding the titanium coil in the implant:

* Wrapping it around the implant and rotating it while heating it with a hot gas stream. Spring-loaded rollers push the coil into the surface of the implant.

* Compressing the coil, placing it onto a needle wire which creates a channel in the softened polymer and feeding the stretched coil into it. The needle wire then pulls the polymer over a portion of the coil as it passes.

* Wrapping the coil around the implant with an optional titanium foil then wrapped around the coil-implant and inductively heated. A ceramic clamp and elastic material can be used to apply pressure.

For further information, contact: Johns Hopkins University School of Medicine, 720 Rutland Avenue, Baltimore, Maryland 21205, USA; tel: +1-410-955-5880; Internet address: http://hopkins.med.jhu.edu; Inventors are: Paul Biermann, Amy Corvelli and Jack Roberts