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Nanoscale Film Coating Strengthens Joint Implants

Hydroxyapatite nanoparticles (Hammond Lab/MIT)

Hydroxyapatite nanoparticles (Hammond Lab/MIT)

Chemical engineers at Massachusetts Institute of Technology have developed an ultra-thin film that can strengthen the bond of knee or hip implants and promote bone growth. The findings are described in a recent issue of the journal Advanced Materials (paid subscription required).

Knee and hip replacements are becoming more common, with some 773,000 Americans having these procedures annually, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases. While the implants are designed to last many years, about one in six patients who receive a total joint replacement find the implant loosens and has to be replaced early, which can cause dangerous complications for elderly patients.

The paper’s first author Nisarg Shah, a graduate student in the lab of engineering professor and senior author Paula Hammond, developed a new coating for implants that could help them better adhere to the patient’s bone, preventing premature failure. In most cases today, surgeons use bone cement to secure the implant, a polymer that resembles glass when hardened. In some cases, say the authors, this cement ends up cracking and the implant detaches from the bone, causing chronic pain and loss of mobility for the patient.

“Our idea is to prevent failure by coating these implants with materials that can induce native bone that is generated within the body,” says Shah. “That bone grows into the implant and helps fix it in place.” To accomplish this goal, Shah and colleagues devised a new coating made of a film, as thin as 100 nanometers (1 nanometer – 1 billionth of a meter) in some cases, composed of layers of two materials that promote bone growth.

One of the materials, hydroxyapatite (pictured at top), is a natural component of bone, made of calcium and phosphate that attracts mesenchymal stem cells from the bone marrow and provides a surface where new bone can form. The second layer releases a growth factor that stimulates mesenchymal stem cells to transform into bone-producing cells called osteoblasts.

Once the osteoblasts form, they produce new bone to fill in the spaces surrounding the implant, securing the implant to the existing bone and eliminating the need for bone cement. Having healthy tissue in that space creates a stronger bond and reduces the risk of infection around the implant. Bone cement sometimes leaves dead spaces not reached by blood vessels, where bacteria can collect and proliferate unchecked by the body’s immune system.

Previous attempts at using hydroxyapatite and the growth factor for osteoblasts were not successful due to the inability to control application of these compounds in the precise quantities needed to substitute for bone cement. The MIT team says they can control the thickness of the film and 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.

The researchers are testing the process on lab animals and report that the coatings lead to rapid bone formation, locking the implants in place. Shah says the technology can be applied to cases where severely fractured bones need to be set with plates and screws, or even with dental implants. “It is very versatile,” says Shah. “You can apply it to any geometry and have uniform coating all around.”

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