Feature Article

A “metal foam” that has a similar elasticity to bone could lead to the development of biomedical implants that avoid bone rejection that often results from more rigid implant materials, such as titanium. Researchers at North Carolina State University (Raleigh, NC, USA; www.ncsu.edu) have developed the metal foam, which is lighter than solid aluminium and can be made of 100% steel or a combination of steel and aluminium.

The researchers have reported that, in addition to the high-energy absorption capability and light weight of their novel composite foams, the “modulus of elasticity” of the foam is similar to that of bone. The rough surface of the foam would also foster bone growth into the implant, improving the strength of implant, according to the scientists. Modulus of elasticity, which is measured in gigapascals (GPa), is extremely important for biomedical implants, explains Dr. Afsaneh Rabiei, an associate professor of mechanical and aerospace engineering and an associate faculty member of biomedical engineering at NC State and co-author of the paper.

“When an orthopaedic or dental implant is placed in the body to replace a bone or a part of a bone, it needs to handle the loads in the same way as its surrounding bone,” Rabiei says. “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. This will cause the loosening of the implant and eventually ends in failure. This is known as “‘stress shielding.’” When this happens, the patient will need a revision surgery to replace the implant. Our composite foam can be a perfect match as an implant  to prevent stress shielding,” Rabiei explains.

To give an idea of the difference between the modulus of elasticity of bone and that of traditional implants, bone has a modulus of between 10 and 30 GPa. Titanium has a modulus of approximately 100 GPa. The new composite foam has a modulus that is consistent with bone, and is also relatively light because it is porous.

The rough surface of the metal foam will bond well with the new bone formed around it and let the body build inside its surface porosities,” Rabiei says. “This will increase the mechanical stability and strength of the implant inside the body.”