Stephen Morton knows firsthand the trouble a failed bone implant can cause: His father had a hip implant that had to be replaced within two years.
"Both times, he was bedridden for about six months," Morton said. "Now he walks with a kind of persistent limp. Because of the failure of the initial implant, it led to a decrease in the quality of life."
Today, as a doctoral student at the Massachusetts Institute of Technology, Morton is part of a research team that may have found a way to make bone implants less likely to fail — a high-tech adhesive that more securely bonds implants to bone by promoting cell growth between natural and artificial body parts.
In a study published in the June 26 online edition of Science Translational Medicine, the MIT team and its collaborators from several other institutions reported that the implant adhesive — a multilayered coating of ceramic and nanolayers of polymers infused with proteins — worked so well on lab rats that they will soon be ready to test it in humans.
The nanolayers, or super-thin sheets of material, hold therapies such as growth factors that attract and encourage the formation of bone cells, causing them to firmly attach to the titanium implant. The coated implants required significantly more force to pull free than uncoated ones; indeed, the researchers said the resulting bond is so strong that under stress, the bone would fracture first before the interface with the implant.
"If you have bonding that is so strong that you actually break the bone, and you don't get failure at the implant site, that would be very significant," said Guillermo Ameer, a professor of biomedical engineering at Northwestern University, who was not involved in the study. "It's pretty exciting if this is scaled up" to humans.
With the US population aging and becoming more active, failing bone joints are increasingly a common problem and a multibillion-dollar industry. Hip and knee replacement surgeries are two of the most commonly performed surgeries in the United States.
But about 12 percent of all implants worldwide must be replaced within 10 years, said Nisarg Shah, the lead author of the MIT project; other research indicates the cost of replacing failed implants will reach $2 billion a year by 2030. Implants typically fail due to gradual loosening at the attachment site, often because the cement used to bond it has degraded.
Both Shah and Morton are doctoral students of MIT professor Paula Hammond, who specializes in materials design and molecule delivery at very small scales. She's primarily responsible for the original design of the nanolayers on which the new implant coating is based. Hammond was the first African-American woman in MIT's chemical engineering department when hired in 1995. She was elected to the prestigious American Academy of Arts and Sciences in April.
The implant coating she helped design works like a tiny, elegant machine. The top coating consists of repeating layers, each impossibly thin, that contain the bone growth factor BMP-2. The layers gradually break apart over a period of weeks, releasing BMP-2 into the body. The factor then stimulates stem cells in bone marrow to transform themselves into new bone cells.
The bottom part of the coating is made of a ceramic that mimics bone, thereby attracting bone cells to its surface. This side of the coating is attached to the implant, and recently formed bone cells tend to affix to this ceramic and grow outward, adhering like "superglue" to attach the implant to the bone, Shah said.
The conventional approach to adhering implants uses a polymer called bone cement to attach them to bone. This cement can fragment and loosen over time. Moreover, because the body recognizes the cement as a foreign material, it often surrounds it with scar tissue, preventing bone from firmly attaching to the implant.
In contrast, most of the materials in the nanocoating either exist in the body or mimic natural substances such as bone. Most are also already FDA-approved, Hammond said. If larger animal trials and then clinical trials are successful, Hammond estimated the nanocoating could become a product "within a four- to five-year period." Her group expects to launch trials in larger animals over the next few years.
The nanocoating is flexible enough to use on surfaces other than metal implants, Hammond said. By applying it to polymer scaffolds that mimic body parts, "this kind of technology can be adapted for replacing bone," she said.
"This is another area where there is a lot of potential. There's a range of needs that are associated with dentistry, dental implants, and cranial facial reconstruction."
The dental implant industry could be one major benefactor of this technology, said Myron Nevins, a practicing periodontist and associate professor at the Harvard School of Dental Medicine.
"If it attracts more bone cells, which we believe it will, it will make the implants stable and secure more quickly," said Nevins, who was not involved in Hammond's research.
Currently, patients often wait months after their initial procedure until bone grows around the implant and the tooth can be safely affixed to it.
"Patients are usually anxious to have the work completed," Nevins said. With the new technology, "we could consider putting teeth on the implants more rapidly. It's an encouraging beginning, very encouraging."
Myron Spector, director of the orthopedic research laboratory at Brigham and Women's Hospital and a coauthor of the paper, cautioned that the technology could increase the cost of an implant, so it may first work best for patients already at risk of failure, such as the elderly and people with preexisting conditions that hamper bone regeneration, like osteoporosis. Most people will respond well to traditional implant methods and won't need a new approach.
Still, Spector said, the new coating could bring successful implants to more patients.
"The value of the technology is not so much that it should be routinely used," he said. "It's to extend the success rate. It's important to get this into the clinic, move it forward, and get it to the people who need it."