New titanium coating improves joint replacements

Tag : Coating Material

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ATLANTA - Research at the Georgia Institute of Technology shows that coatinga titanium implant with a new biologically inspired materialenhances tissue healing, improves bone growth around the implantand strengthens the attachment and integration of the implant tothe bone.

“We designed a coating that specifically communicates withcells and we’re telling the cells to grow bone around theimplant,” said Andrés Garcia, professor and WoodruffFaculty Fellow in Georgia Tech’s Woodruff School ofMechanical Engineering and the Petit Institute for Bioengineeringand Bioscience.

Details of the new coating appear in the July issue of the journalBiomaterials. The research was supported by the National Institutesof Health, the Arthritis Foundation and the Georgia Tech/EmoryNational Science Foundation Engineering Research Center on theEngineering of Living Tissues.

Total knee and hip replacements typically last about 15 years untilthe components wear down or loosen. For many younger patients, thismeans a second surgery to replace the first artificial joint. Withapproximately 40 percent of the 712,000 total hip and kneereplacements in the United States in 2004 performed on youngerpatients 45-64 years old, improving the lifetime of the titaniumjoints and creating a better connection with the bone becomesextremely important.

Current clinical practice includes roughening the surface of thetitanium implant or coating it with a flaky, hard-to-apply ceramicthat bonds directly to bone.

In collaboration with Georgia Tech School of Chemistry andBiochemistry professor David Collard, graduate students Tim Petrieand Jenny Raynor, and research technician Kellie Burns, Garciacoated the 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 topresent our bio-inspired, bioactive protein,” explainedGarcia.

In this case, the polymer presented controlled amounts of anengineered protein that mimics fibronectin, a protein in the bodythat acts as a binding site for cell surface receptors calledintegrins.

It is important to control the integrins binding to the titaniumimplant because integrins provide signals that direct boneformation. Therefore, controlling integrin binding to the titaniumwill result in targeted signals that enhance bone formation aroundthe implant.

To bind integrins to titanium, researchers previously coatedtitanium with a small biological signal containing the sequencearginine-glycine-aspartic acid (RGD) that binds to integrins.However, this region alone binds many different integrin receptorsand with much less affinity than the full fibronectin protein.

“It has been common to mimic only very small sections offibronectin, but when you take a small section and ignore the restof the molecule you lose specificity and activity, and thereforesignaling is impaired,” said Garcia.

For that reason, Garcia engineered a much longer region of the sametype of fibronectin that included the RGD peptide sequence as wellas new sections also known to have sites that participate inintegrin binding.

To evaluate the in vivo performance of the coated titanium in bonehealing, chemists Raynor and Collard coated the surfaces of tinyclinical-grade titanium cylinders with the polymer brushes. Thenengineers Petrie and Garcia modified them with peptide sequences.
Two-millimeter circular defects were drilled into a rat’stibia bone and the cylinders were pressed into the holes. Theresearchers tested three types of coatings: uncoated titanium,titanium coated with the RGD peptide and titanium coated withdifferent densities of the engineered fibronectin fragment.

To investigate the function of these novel surfaces in promotingbone growth, the researchers quantified osseointegration, or thegrowth of bone around the implant and strength of the attachment ofthe implant to the bone.

Analysis of the bone-implant interface four weeks later revealedextensive and contiguous bone matrix and a 70 percent enhancementin the amount of contact between the implant and bone with thetitanium implants coated with the engineered fibronectin fragmentover the uncoated or RGD-coated titanium.

Garcia and Petrie tested the fixation of the implants by measuringthe amount of force required to pull the implants out of the bone.The study showed significantly higher mechanical fixation of theimplants coated with the engineered fibronectin fragment over theimplants with the other coating and uncoated titanium.

In addition to total joint replacements, Garcia is studying how tofill large gaps between bones, which sometimes occur after atraumatic injury or tumor removal.

“We are developing a strategy to present peptides thatencourage the surrounding bone to grow in and fill in around thegap,” said Garcia.

By improving communication with the body’s cells, Garcia cancontrol the integration and healing response of the body to anyimplanted device. Currently, most become encapsulated by a collagensheath, which affects the performance and long-term viability ofthe device. Garcia aims to use these biomaterials to help integratedevices implanted in the body.