Nanocoating designed to keep hip implants where they belong
By Ben Coxworth
April 24, 2012
Probably the simplest way to describe an artificial hip would be to say that it’s a ball attached to a stem. The stem is often fastened to the open end of the femur using a glass-like polymer known as bone cement, while the ball takes the place of the original hip bone’s ball joint, rotating within a corresponding implant in the socket of the pelvis. Although problems can occur at that ball-and-socket interface, they can also result when the bone cement cracks, causing the stem to detach from the femur. Scientists at MIT, however, have developed a new type of nanoscale film coating, designed to keep that from happening.
According to MIT, about 17 percent of patients who receive artificial hips will eventually require a total replacement of the implant due to loosening of the stem. That loosening typically causes patients to experience considerable discomfort, and a loss of mobility. When the existing implant is removed and a new one installed, tissue loss occurs, along with the various risks associated with surgery on the elderly – the most common recipients of artificial hips.
The MIT film is intended as a substitute for bone cement. Applied at the stem/bone interface, the film ranges in width from 100 nanometers to one micron, and consists of layers of materials that encourage bone to grow from the femur into the implant. One of those materials is a natural component of bone, known as hydroxyapatite. It is composed of calcium and phosphate, and attracts stem cells from the adjacent bone marrow. Also incorporated into the film is a growth factor that causes those attracted stem cells to transform into osteoblasts, which are bone-producing cells.
Once the osteoblasts gets to work, the spaces between the implant and the existing bone are filled in with new bone. Although it takes at least two to three weeks for the implant to become thoroughly attached, patients should still be able to walk and perform physical therapy in the meantime. Not only should the new bone provide a more secure attachment than bone cement, but it should also make infections much less likely – when bone cement is used, bacteria can collect within the gaps that remain between the existing bone and the implant.
Previous attempts have been made at coating implants with hydroxyapatite film, and at introducing growth factor, but they reportedly proved unsatisfactory. The films were thick, unstable and thus broke away from the implants, while it was difficult to keep the growth factor from draining away from the implant site. The MIT scientists, however, are able to precisely control the thickness of the film, and the rate at which it dispenses the growth factor.
So far, the film has been used in animal studies, where it has shown promising results. It is believed that it could be used not only for artificial hips, but also for other metal-in-bone applications such as dental implants, fixation plates, and screws used to set bone fractures.
A paper on the research was recently published in the journal Advanced Materials.