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New coating for hip implants could prevent premature failure

19 April 2012

Artificial hip or knee prostheses are designed to last many years, but for a percentage of patients who receive a total joint replacement, the implant eventually loosens and has to be replaced. To help minimise the need for such interventions, a team of chemical engineers at the Massachusetts Institute of Technology (MIT) has developed a new coating for implants that could help them better adhere to the patient’s bone, preventing premature failure.

Hydroxyapatite nanoparticles are incorporated into multilayer coatings for faster bone tissue growth. Image reproduced, courtesy of the Hammond Lab

The new coating consists of a very thin film, ranging from 100 nanometers to one micron, composed of layers of materials that help promote rapid bone growth. One of the materials, hydroxyapatite, is a natural component of bone, made of calcium and phosphate. This material attracts mesenchymal stem cells from the bone marrow and provides an interface for the formation of new bone. The other layer releases a growth factor that stimulates mesenchymal stem cells to transform into bone-producing cells called osteoblasts.
 

“This would allow the implant to last much longer, to its natural lifetime, with lower risk of failure or infection,” says Paula Hammond, the David H. Koch Professor in Engineering at MIT and senior author of a paper on the work appearing in the journal Advanced Materials. The coating could also be used to help heal fractures and to improve dental implants, according to Paula Hammond and lead author Nisarg Shah, a graduate student in Hammond’s lab.

Once the osteoblasts form, they start producing new bone to fill in the spaces surrounding the implant, securing it to the existing bone and eliminating the need for bone cement. Having healthy tissue in that space creates a stronger bond and greatly reduces the risk of bacterial infection around the implant.

It takes at least two or three weeks for the bone to fill in and completely stabilize the implant, but a patient would still be able to walk and do physical therapy during this time, according to the researchers.

While there have been previous efforts to coat orthopedic implants with hydroxyapatite, the films end up being quite thick and unstable, and tend to break away from the implant, Shah says. Other researchers have experimented with injecting the growth factor or depositing it directly on the implant, but most of it ends up draining away from the implant site, leaving too little behind to have any effect.

The MIT team can control the thickness of its film and the amount of growth factor released by using a method called layer-by-layer assembly, in which the desired components are laid down one layer at a time until the desired thickness and drug composition are achieved.


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