Metal mimics bone
A metal foam that exhibits similar modulus of elasticity to bone is said to have been developed, according to researchers in the USA. The material could be used in a new generation of biomedical implants for load-bearing areas, preventing the rejection that often results from more rigid implant materials such as titanium.
Seventy per cent lighter than the bulk metal they are made of, the passive metal foam can be coated with a biocompatible layer of hydroxyapatite and is made of 100% steel or other elements such as titanium.
Dr Afsaneh Rabiei from the Department of Mechanical and Aerospace Engineering at the North Carolina State University explains, ‘When an orthopaedic 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.
‘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.’
Bone has a modulus of 10-30GPa while titanium has a value of approximately 100GPa. The new composite foam has a modulus that is said to be
consistent with bone, about 10-12GPa, and is also relatively light because it is porous.
Rabiei says 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. This will increase the mechanical stability and strength of the implant inside the body.
‘The roughness depends on the spheres’ size and processing technique as well as the final finishing of the product and the coating. All of these factors can be controlled and tailored to the needs of the patient’, she says.
The foam is said to be ideal for hip, knee and shoulder replacements, where strength is particularly important. ‘A ceramic scaffold cannot provide enough strength for these applications’, Rabiei explains. Ceramics are famous for their low fracture toughness and as a result they will fracture under loading faster than metal casting.’
The elastic behaviour of the foam, fabricated using powder metallurgy techniques, has been characterised by compression experiments, constitutive scaling equations and 2D finite element modelling.
The implants can be manufactured cost-effectively using casting or powder metallurgy and are ready for industry scale-up, according to the researchers. In vivo trials will soon be conducted.Materials World Magazine, 01 Apr 2010
Seventy per cent lighter than the bulk metal they are made of, the passive metal foam can be coated with a biocompatible layer of hydroxyapatite and is made of 100% steel or other elements such as titanium.
Dr Afsaneh Rabiei from the Department of Mechanical and Aerospace Engineering at the North Carolina State University explains, ‘When an orthopaedic 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.
‘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.’
Bone has a modulus of 10-30GPa while titanium has a value of approximately 100GPa. The new composite foam has a modulus that is said to be
consistent with bone, about 10-12GPa, and is also relatively light because it is porous.
Rabiei says 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. This will increase the mechanical stability and strength of the implant inside the body.
‘The roughness depends on the spheres’ size and processing technique as well as the final finishing of the product and the coating. All of these factors can be controlled and tailored to the needs of the patient’, she says.
The foam is said to be ideal for hip, knee and shoulder replacements, where strength is particularly important. ‘A ceramic scaffold cannot provide enough strength for these applications’, Rabiei explains. Ceramics are famous for their low fracture toughness and as a result they will fracture under loading faster than metal casting.’
The elastic behaviour of the foam, fabricated using powder metallurgy techniques, has been characterised by compression experiments, constitutive scaling equations and 2D finite element modelling.
The implants can be manufactured cost-effectively using casting or powder metallurgy and are ready for industry scale-up, according to the researchers. In vivo trials will soon be conducted.Materials World Magazine, 01 Apr 2010
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