It is easy to make a metal hip or jaw implant, but hard to get it to match the flexibility of natural bone. That mismatch causes premature wear and failure.

Now Afsaneh Rabiei, associate professor of mechanical, aerospace, and biomedical engineering at North Carolina State University, has found a way to make metal mimic the elastic modulus of natural bone. Not only is Rabiei's porous steel structure more flexible than other metal implants, but it is also lighter and its properties easier to control.

Flexibility and its flip side, stiffness, have always posed problems for implants, especially those used in artificial hips and in dental and jaw implants. Natural bone is only 10 to 30 percent as stiff as titanium, a material used in many implants.

The mismatch in stiffness leads to "stress shielding." This is a condition where the titanium implant carries more of the load than surrounding natural bone. Unfortunately, natural bone needs pressure applied to it in order to thrive. As a result, stiff metal implants reduce the health of the bone surrounding the implant.

Consider how this plays out in an artificial hip. First, stress shielding weakens the bone around the implant. The implant is also much heavier than bone. With each step, all the weight of that stiff titanium implant pounds away at the deteriorating natural bone. Eventually, the implant loosens and the patient needs new surgery.

Rabiei's solves the problem with porous metal composite. By varying the amount of porosity, she can match the foam’s stiffness with the modulus of natural bone, eliminating stress shielding. Composite porous metals are also good energy absorbers, so they cushion the shock of each step. The composite's pores also provide places where natural bone can grow and anchor the implant in place.

Porosity also reduces mass. "I have a hip replacement on my desk, and even though it is made of titanium, it is about three times heavier than a healthy bone with the same size," Rabiei said. "My mother-in-law had one of these implants, and after a while, it pushed into her bone and her physician had to remove it and replace it with another. Our composite weighs up to 70 percent less. If we had used our material, that might not have happened."

Rabiei’s composite porous metal achieves its balance of weight and strength from hollow spheres distributed uniformly within a metal. The spheres act much like straw in ancient bricks or rebar in concrete to impart additional strength to the composite. She makes the material by mixing hollow metal spheres with metal powders and sintering in a furnace, or by casting molten metal around the hollow spheres in a mold.

Others have tried to make porous composites, but the pores weakened them and they fell apart after only moderate use. Rabiei’s formulation, which uses steel spheres, is much more durable.

"We can engineer the stiffness and durability of the implant by controlling size, wall thickness, and percentage of spheres we add to the matrix," she said. She also has a wide range of material options. Rabiei says she can make the composites from steel spheres in a steel matrix, steel spheres in an aluminum matrix, or titanium spheres in titanium matrix. So far, Rabiei’s group has produced steel and aluminum composites and extended the technique to such other metals as titanium and cobalt-chromium.

Rabiei made her first sample seven years ago. "It took time to get it right," she said. She has patented the technology and is currently lining up applications for funding the animal experiments.

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