Alloy With "Memory" Helps Bones Heal Faster And More Reliably

Researchers compared Nitinol - a nickel and titanium alloy - to stainless steel in an experiment designed to see which metal worked best on a simulated broken bone. Where the stainless steel wire lost tension during the simulation, the Nitinol kept a continuous pull. Loss of tension could cause a bone to heal improperly.

"Metals like Nitinol remember their original shape," said Alan Litsky, an associate professor of orthopaedics and biomedical engineering at Ohio State University. "This shape 'memory' can be used to help compress broken bones, which could mean a quicker healing time, and a better fusion of the bone."

Litsky will present his findings May 20 at the World Biomaterials Congress in Kamuela, Hawaii.

One common way surgeons treat certain fractures is to tie wire around the affected bone in order to pull the fractured pieces together. The problem is that stainless steel - the most common metal used in these wires - tends to loosen over time as the metal slowly migrates through the bone. This reduces the amount of pressure applied to the fracture and can result in poor healing.

Nitinol, if it is stretched before being wrapped around a fractured bone, resists loosening because it remembers its original shape. This "memory" allows the wire to continue compressing the bone even after it starts to be absorbed by the bone.

Nitinol's shape can be changed below a certain temperature; it then pops back into its original shape when heated. To fix a fracture, Nitinol is cooled, stretched and wrapped around the broken bone. As it warms to body temperature, the wire "remembers" its original length. It tightens around the bone, and begins to compress the fracture to a greater extent than could be achieved by simply tightening the wire.

In this study, the researchers used cylindrical tubes to represent bones. The tubes were connected to a hydraulic system that could change the position of the tubes to simulate what would happen during a six-week fracture healing period. In separate experiments, the researchers wrapped the cylinders with stainless steel wire and with Nitinol to compare their effectiveness at holding the "bones" together with proper compression.

The Nitinol wire was stretched to 108 percent of its length before it was wrapped around the cylinders. It was then heated. When the researchers slowly moved the cylinders together to simulate what would happen as the wire migrated into the bone, the wire tightened. Nitinol was still able to maintain nearly the same level of compression between the cylinders.

The researchers performed the same experiment with stainless steel wire. As the cylinders began moving toward each other, the stainless steel "immediately lost its ability to compress the fracture," Litsky said. As bone heals, stainless steel tends to loosen, losing its ability to place pressure on the bone.

"Stainless steel will hold things where they were, while Nitinol will push the pieces of a broken bone together," Litsky said. "If you use a stainless steel wire and something loosens, the compression on the fracture is lost."

When using Nitinol to repair a broken bone, a surgeon would flush the fracture site with water slightly cooler than body temperature, then wrap pre-stretched Nitinol wire around the site of the break. After the wire warmed, it would attempt to regain its original shape, and then continuously push the bone back together.

"The key with Nitinol wire is its ability to achieve and maintain a high level of compression on the bone while it heals," Litsky said.

Litsky conducted the research in the Orthopaedic BioMaterials Laboratory at Ohio State with Todd Ritzman, a student in Ohio State's College of Medicine and Public Health; Brian Sears of Grant Hospital in Columbus; and Ajay Seth, an orthopaedic resident at Ohio State.