Although artificial hip or knee prosthesis are designed to last several years, approximately 17% of patients who receive a total joint replacement need early replacement surgery – a procedure which can cause serious complications for elderly patients.

In order to help minimize the need for these operations, a team of chemical engineers at MIT have developed a new coating for implants that could help them better adhere to the patient’s bone, preventing premature failure.

The study is published in the journal Advanced Materials.

Paula Hammond, the David H. Koch Professor in Engineering at MIT and senior author of the study, explained:

“This would allow the implant to last much longer, to its natural lifetime, with lower risk of failure or infection.”

According to Hammond and lead author Nisarg Shah, a graduate student in Hammond’s lab, the coating could also be used to improve dental implants and help heal fractures.

At present, artificial hips and knees are secured using bone cement, a polymer that resembles glass when hardened. However, in some patients, this cement cracks causing the implant to detach from the bone. As a result the patient experiences loss of mobility and chronic pain.

Shah said:

“Typically, in such a case, the implant is removed and replaced, which causes tremendous secondary tissue loss in the patient that wouldn’t have happened if the implant hadn’t failed.

Our idea is to prevent failure by coating these implants with materials that can induce a native bone that is generated within the body. That bone grows into the implant and helps fix it in place.”

The new coating consists of a very thin film that is between 100 nanometers to one micron thick. It is made up from layers of materials that help to encourage a rapid growth of bone, such as hydroxyapatite, a natural component of bone that is made from calcium and phosphate. This material attracts mesenchymal stem cells from the bone marrow and provides an interface for the formation of new bone, whilst the other layer releases a growth factor, which encourages mesenchymal stem cells to transform into bone-producing cells called osteoblasts.

After forming, the osteoblasts generate new bone to fill in the spaces surrounding the implant, securing it to the existing bone. According to the researchers, having healthy tissue in this space creates a stronger bond and considerably lowers the risk of bacterial infection around the implant.

Shah explained:

“When bone cement is used, dead space is created between the existing bone and implant stem, where there are no blood vessels. If bacteria colonize this space they would keep proliferating, as the immune system is unable to reach and destroy them. Such a coating would be helpful in preventing that from occurring.”

The team highlights that although it takes a minimum of two to three weeks in order for the bone to fill in and secure the implant, patients would be able to walk and do physical therapy during this period.

Although prior efforts to coat orthopedic implants with hydroxyapatite have been made, the films tend to detach from the implant, as they are quite thick and unstable. In addition, other studies have tried injecting growth factor or depositing it directly on the implant, however, there is usually not enough to have effect as the majority of it drains away from the implant site.

By using a technique called layer-by-layer assembly, the researchers are able to control how thick the film is, as well as the amount of growth factor released. The method involves laying down each desired component, one at a time, until the desired drug composition and thickness are achieved.

Shah said:

“This is a significant advantage because other systems so far have really not been able to control the amount of growth factor that you need. A lot of devices typically must use quantities that may be orders of magnitude more than you need, which can lead to unwanted side effects.”

At present, the team is testing this coating in animal studies and have found that the coatings result in rapid bone formation, securing the implants in place.

According to Shah, this coating could also be used for fixation plates and screws used to set bone fractures. “It is very versatile. You can apply it to any geometry and have uniform coating all around.”

In addition, the coating could be used in dental implants. Usually, implanting an artificial tooth involves embedding a threaded screw in to the jaw, however, before the new crown can be attached to the screw, the patient must wait several months in order for the screw to stabilize by integrating with the surrounding bone tissue. According to the researchers, this could be reduced to a one-step process in which the patient receives the entire implant using a version of these coatings.

The study received funding from the National Institutes of Health’s National Institute on Aging and conducted at the David H. Koch Institute for Integrative Cancer Research with support from the Institute for Soldier Nanotechnologies at MIT.

Written By Grace Rattue