Surface treatment and coating plays a key role in manufacturing artificial joints. Hip and knee replacements typically last about 15 years until the components wear down or loosen. For many younger patients, this means a second surgery to replace the first artificial joint. With approximately 40 percent of the 712,000 total hip and knee replacements in the United States in 2004 performed on younger patients 45 to 64 years old, improving the lifetime of titanium joints and creating a better connection with the bone becomes extremely important.

Medical engineers at the Georgia Institute of Technology (Atlanta) have coated a titanium implant with a new biologically inspired material that enhances tissue healing, improves bone growth around the implant, and strengthens the attachment and integration of the implant to the bone.

“We designed a coating that specifically communicates with cells and we’re telling the cells to grow bone around the implant,” says Andrés Garcia, a bioengineering professor at Georgia Tech’s Woodruff School of Mechanical Engineering and the Petit Institute for Bioengineering and Bioscience.

Current clinical practice includes roughening the surface of the titanium implant or coating it with a flaky, hard-to-apply ceramic that bonds directly to bone. Garcia and his colleagues coated the sample titanium with a thin, dense polymer.

“Our coating consists of a high density of polymer strands, akin to the bristles on a toothbrush, that we can then modify to present our bio-inspired, bioactive protein,” explained Garcia. In this case, the polymer presented controlled amounts of an engineered protein that mimics fibronectin, a protein in the body that acts as a binding site for cell surface receptors called integrins.

According to Garcia, it’s important to control the integrins binding to the titanium implant, because integrins provide signals that direct bone formation. He says controlling integrin binding to the titanium will result in targeted signals that enhance bone formation around the implant.

To bind integrins to titanium, researchers previously coated titanium with a small biological signal containing the sequence arginine-glycine-aspartic acid (RGD) that binds to integrins. “However, this region alone binds many different integrin receptors and with much less affinity than the full fibronectin protein,” Garcia points out. “It has been common to mimic only very small sections of fibronectin. “But, when you take a small section and ignore the rest of the molecule, you lose specificity and activity, and therefore signaling is impaired.”

For that reason, Garcia engineered a much longer region of the same type of fibronectin that included the RGD peptide sequence, as well as new sections also known to have sites that participate in integrin binding.

To evaluate the in vivo performance of the coated titanium in bone healing, chemistry professor David Collard and his colleagues coated the surfaces of tiny clinical-grade titanium cylinders with the polymer brushes. Then, engineers modified them with peptide sequences.

Two-millimeter circular defects were drilled into a rat’s tibia bone and the cylinders were pressed into the holes. The researchers tested three types of coatings: uncoated titanium, titanium coated with the RGD peptide, and titanium coated with different densities of the engineered fibronectin fragment.

To investigate the function of these novel surfaces in promoting bone growth, the researchers quantified osseointegration, or the growth of bone around the implant and strength of the attachment of the implant to the bone. “Analysis of the bone-implant interface four weeks later revealed extensive and contiguous bone matrix, and a 70 percent enhancement in the amount of contact between the implant and bone with the titanium implants coated with the engineered fibronectin fragment over the uncoated or RGD-coated titanium,” says Garcia.

The engineers tested the fixation of the implants by measuring the amount of force required to pull the implants out of the bone. There was significantly higher mechanical fixation of the implants coated with the engineered fibronectin fragment vs. the implants with the other coating and uncoated titanium.