One of the main reasons that orthopaedic implants fail is poor adhesion to the surrounding bone. One of the most commonly used materials today in hip and knee replacements is a cement based on the self-curing polymer PMMA. However, this material does not adhere to bone very well because there is a substantial elastic modulus mismatch at the polymer-bone interface. PMMA is not very strong either and fragments easily. All these factors mean that an implant cemented with PMMA can fail after just a few years and the patient must undergo surgery again to fix the problem.

A team led by Paula Hammond has now come up with a real alternative made of repeating nanolayers that help implants develop a strong interface with host bone. The basic principle on which the coating works is electrostatic interaction – opposite charges attract, explains team member Nisarg Shah. "The implant materials we used were PEEK, a plastic, and medical-grade titanium. It is possible to build up a negative charge on the surface of these materials by bombarding them with oxygen plasma gas. We first dip the implant in a solution of a material that has a net positive charge before immersing it into the negatively-charged polymer solution," he said.

Double role coating

The coating has two roles, he continues. The first is to activate osteoblasts (or bone-forming cells) present in bone marrow. Second, it provides a surface on which new bone can deposit. "The first role is carried out by a bone-forming protein called BMP-2 that releases slowly from a water-based degradation of the polymer coating," he told "The second role is carried out by the so-called osteoconductive base layers (that do not readily degrade in water) containing hydroxyapatite, a calcium phosphate found in bone. These layers stay around for a much longer period (of several months) and act as a sort of ‘superglue’ that helps forms the strong bond between the implant and host tissue."

Shah adds that is it important that the coating plays these two roles.

Preventing premature loosening

The MIT team has already tested its coating on implants in rat tibias with both plastic and metal titanium implants and found that the nanolayer-coated implants are much more difficult to pull out compared to uncoated or partially-coated implants. Radiography images reveal that the coating and bone surfaces are indeed tightly bonded together.

The new coating could help prevent bone implants from prematurely loosening and could find use in both dental and joint prostheses, say the researchers.

"Patients undergoing teeth replacement will be able to receive a coated dental implant in a one-step procedure," explains Shah. "Such an implant would integrate within days following the implant and allow the patient to resume a regular diet. Currently, this procedure involves multiple clinical visits for up to a year before the entire process is complete."

Revision arthroplasty and knee implants

He adds that a nanocoated implant might also be very welcome in procedures such as revision arthroplasty in which the implant has already failed and needs to be replaced. "Such an implant could also be used in a primary joint replacement surgery in osteoporotic patients, where there is a need to augment the lack of local bone. In-growth knee implants, which have largely been abandoned in conventional surgery (because of inadequate bone fixation), might be coated to enhance the bone-forming ability of the implant."

The team says that it is now busy thinking about how its technology might actually be used in the hospital. "There is much work to do before we can test this technology on real patients," said Shah, "but our next immediate step would be to test the nanocoated implants in large animals (such as goats or sheep)."

The researchers are mainly focusing on nanocoating dental implants for the moment though, since their technology could have a real impact in the short term here.

The present work is reported in Science Translational Medicine.