New MIT Coating Could Help Reduce Orthopedic Implant Failure

Everyone gets older, but different medical conditions affect people at different ages. For the huge cohort of baby boomers, orthopedic maladies associated with such parts of the body as hips and knees pose a particular challenge. But at the medical device design and manufacturing end of the spectrum, such orthopedic conditions represent a challenge and an opportunity to develop new implant technologies, new materials, new sensors, and--not least--new coatings.

As the 2010 recall of DePuy's metal-on-metal acetabular head system amply demonstrates, hip implants are susceptible to high failure rates. In fact, in approximately 17% of patients that receive total joint replacements, the implant eventually loosens, requiring explantation and reimplantation. Addressing this issue, chemical engineers at the Massachusetts Institute of Technology (MIT; Cambridge) have developed a new coating that could help adhere the implant to the bone, preventing premature failure. By inducing the body's cells to produce bone that fixes the implant in place, the coating could also be used to help heal fractures.

Orthopedic implants are typically secured using bone cement, a polymer that resembles glass when it hardens. However, this cement can crack, causing the implant to detach from the bone and producing chronic pain and loss of mobility. Consisting of a very thin film ranging in thickness from 100 nm to 1 µm, the MIT researchers' coating is 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.

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.

"When bone cement is used, dead space is created between the existing bone and implant stem, where there are no blood vessels," explains Nisarg Shah, a graduate student involved in the MIT research. "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."

Previous efforts to coat orthopedic implants with hydroxyapatite have been less than satisfactory because these films have been thick and unstable, causing them to separate from the implant, Shah notes. While other researchers have experimented with injecting the growth factor or depositing it directly onto the implant, most of it drains away from the implant site, rendering it largely ineffective. In contrast, 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.