Textured titanium medical implants may be better than smooth ones

Jun 14 2004

Medical implants – from catheters that deliver long-term life support to joint replacements – may work better when their surfaces are on the rough side, new research suggests.

Implants often have surfaces that soft tissue, such as skin and connective tissue, cannot attach to, said Andreas von Recum, the study's lead author and a professor of biomedical engineering at Ohio State University. So the body in turn forms a tissue capsule around the implant, sealing it off from the rest of the body.

That seclusion can lead to a variety of serious problems, von Recum said.

For instance, there are more than 300,000 hip and knee replacements each year in the United States. Such implants usually last for 10 to 12 years until they become loose and quite painful and need to be replaced.

"Being encased in connective tissue seriously compromises an implant's function," von Recum said. "And connective tissue can't tolerate constantly moving against a foreign object. This friction – and ensuing inflammation – kills healthy cells and creates a steadily growing capsule of dead tissue."

Adding texture to an implant's surface – the researchers used titanium in this study – increased compatibility with connective tissue cells, called fibroblasts, considerably.

And while the researchers conducted their experiments using titanium, which is commonly used to make implants, all implants can benefit from having a textured surface, von Recum said.

The study appears in a recent issue of the Journal of Biomedical Materials Research Part A. von Recum conducted the study with Rakhi Jain, a former doctoral student at Ohio State. von Recum is also the associate dean for research in veterinary medicine at Ohio State.

The researchers coated disk-shaped polyester wafers with titanium. The disks were round and about the size of a nickel. Some of the disks were covered with grooves only several microns deep. Some bone implants currently used have similar texture, von Recum said, although these grooves tend to be 100 to 200 microns deep and can interlock with hard tissue, such as bone.

"The grooves on our disks were a hundred times smaller than an individual fibroblast," von Recum said, adding that the cells attached to the textured disks by depositing proteins into the grooves.

Fibroblasts from mice were left to grow on both the smooth and textured disks for three days. At the end of that time, the researchers used photomicrography – taking an image through a confocal microscope – to determine the distance between cell membranes and disk surfaces.

The distance between the fibroblasts and the surface of the textured disks was immeasurable, suggesting that these cells had adhered to the surface. Conversely, the researchers could measure the distance – although very small, on the order of nanometers – between cell membranes and the surface of a smooth disk.

"Texturing provided a strong adherence between the surface of a cell and the surface of a disk," von Recum said. "That's exactly what we want in an actual implant."

That the cells responded so readily to physical changes on the disk surfaces surprised the researchers.

"Almost everything in the biological world responds primarily to chemical signals," von Recum said. "It was astonishing to find that these fibroblasts responded strongly to mechanical signals on the disk surface."

Adding texture to implants isn't a new concept; it first gained popularity in the 1960s.

"The problem with those implants was that the scale of roughness was a hundred to a thousand times larger than what we've found to be effective," von Recum said. "The interface went beyond a layer of connective tissue cells – these old implants essentially became embedded in the body as connective tissue and bone grew into them. Their removal was nearly impossible, as it usually resulted in major bone loss in some types of implants.

"The technique we're proposing – and it will be some time before such implants are made and used – doesn't adhere nearly as strongly to the surrounding tissues," he continued.

von Recum realized the problems smooth-surfaced implants presented while working with catheters used for long-term life support.

"Tissue eventually builds up around the area where the smooth plastic of these catheters enter the skin," he said. "The area is at risk for infection and becomes a source of discomfort and possibly mortality – people who depend on a catheter usually can't survive an infection.

"If cells could directly attach to an implant, problems with dead tissue build up, infections and implant replacements would be far less common."