Medical Implants Made of Foam?

Scientists have developed a new lightweight “metal foam” with elasticity similar to bone that could lead to a new generation of medical implants, likely overcoming a range of problems associated with devices currently in widespread use.

“We can tailor this material’s properties for both orthopedic and dental implants,” said Afsaneh Rabiei, associate professor of mechanical and aerospace engineering and an associate faculty in biomedical engineering at North Carolina State University. “It has the potential to revolutionize the field.”

The new material, which is lighter than solid aluminum because it is porous--and can be made of 100 percent steel, titanium, cobalt-chromium or a combination of various elements--should solve the bone rejection that often results from the use of more rigid implants, such as those made from bulk titanium, Rabiei said.

Furthermore, its “modulus of elasticity”--the measure of how the material can change shape (elongate, contract, etc.) when force is applied--is very similar to that of bone. 

Modulus of elasticity is measured in gigapascals (GPa) where one gigapascal is equivalent to 10,000 times the atmospheric pressure at sea level. Bone has a modulus of elasticity between 10 and 30 GPa, while titanium, the material in conventional implants, has a modulus of approximately 100 GPa.

When an implant is placed in the body, it must handle the loads as much as its surrounding bone does.  If the modulus of elasticity of the implant is too much bigger than the bone, the implant will take over the load bearing and the surrounding bone will start to die, causing the loosening of the implant which can eventually result in its failure--a process known as “stress shielding.” When this happens, the patient often must have the implant replaced.

“Our composite foam can be a perfect implant material to prevent stress shielding,” said Rabiei. “This is because the modulus of our composite foam is matching perfectly with that of bone. That means when the implant is in the body and a load is applied to the bone, as a result of walking, chewing, etc., both the implant and the surrounding bone will take a fair share of the load,” Rabiei said. “It is unlike bulk metal implants, in which the metal takes on the majority of load.”  When this occurs, “the bone starts to become lazy and eventually dies because it is ‘left out,’ and is not active anymore,” she added.

Moreover, the rough surface of the metal foam will bond well with the new bone formed around it, promoting bone growth into the implant.  “This will increase the mechanical stability and strength of the implant inside the body,” she added.

The material has not yet been studied in humans, a process that could take several more years. 

In order to make the material, Rabiei and her team mix prefabricated metallic hollow spheres with the powder of a metal, then baked the mixture in a furnace, a process known as powder metallurgy technique, or PM.

“In some cases, we cast molten metal around the hollow spheres that are placed in a mold, and then we remove the product from the mold,” Rabiei explained. “The selection of processing technique will depend on the properties expected from the product, as well as the type of material that needs to be processed.”

Rabiei said she does not expect implants made from the new material to be more expensive than products currently in use. “The added cost would be minimal, even none,” she said. “Remember that the major cost of this kind of operation for a patient is not related to the implant material, but more related to the medical procedure and implantation. So any minimal additional cost will be less considerable when you look at the big picture.”

She stressed, however, that once the new material is approved for human use, patients who already have an implant should not consider having it replaced unless the current device is loose or otherwise failing.

“Revision surgery is painful and expensive,” she said. “I am not a medical doctor and prefer to leave the judgment to the patients and their doctors about whether they should replace their implants with a new one made out of our composite foams or not. However, if I were a patient, I would most likely keep my old implant until it starts giving me trouble. This is simply because I rather not to take the risk going through another surgery. For new patients, it will be a completely different story, as they will have a choice.”