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New metal biomaterials address cavitary, segmental defects and implant wear
Tantalum, titanium materials have interconnected pores, trabecular bone-like structure.
In the decade since porous metals were first used clinically, they have demonstrated good isoelastic characteristics in orthopedic applications, suggesting they may prove viable in several other implant designs as well.
“[The material] provides reliable and successful fixation and can be used in standard designs, but you can also have a number of unique design concepts and constructs in more bone-friendly designs,” Arlen D. Hanssen, MD, said.
Three brands of porous metals have a track record in prosthetic components sold in the United States. They are Trabecular Metal (Zimmer), made of tantalum, and two products made of titanium or titanium alloy: Regenerex Metal (Biomet) and Tritanium Dimensionalized Metal (Stryker).
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In late January a new product in this category, Biofoam Titanium (Wright Medical), was introduced for use in cementless knee tibial bases. “I believe there is quicker in-growth into the Biofoam base (than in-growth into a hydroxyapatite base),” R. Scott Corpe, MD, said in a press release. The product’s rollout will gradually expand to more markets throughout 2008, company officials said.
Hanssen expects all these products will be used for an increasing number of hip and knee components, but also foresees other similar products emerging.
“It’s not just osteointegration. Porous metals are also highly effective for successful ingrowth of soft tissue, which includes ligaments, tendons and/or cartilage,” he said during the 24th Annual Current Concepts in Joint Replacement Winter Meeting.
At the meeting, Hanssen discussed some of the key advantages of using these porous metals.
It is an “expanding family of porous structures or scaffolds that have the innate characteristic of a 3-dimensional (3-D) interconnected pore structure that’s comparable to trabecular bone,” he said.
The interconnected pores these materials have promote bone growth and provide 3-D fixation. Because their surface is rough due to nanometer-size surface irregularities, these metals are also osteoinductive.
“This roughness promotes osteoblast adhesion and improved bone growth,” Hanssen explained.
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Image: Wright Medical |
Fosters osteoconductivity
The special shape and small size of the pores allows bone to grow through them, making the material osteoconductive as well.
Another advantage is this relationship produces an implant with less stiffness so these materials are more isoelastic with bone, according to Hanssen.
“This concept of isoelasticity, I think, is going to gain more and more favor in prosthetic design because of the need to preserve bone stock, particularly in young patients,” he said.
Isoelasticity relies on the fact the pores are similar in size and structure to those found in cancellous bone.
“The modulus of elasticity is between cancellous and cortical bone, but you can adjust this depending on how you create these [implants], so that you can make it really truly isoelastic and prevent bone resorption,” Hanssen said.
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Images: Hanssen AD |
Low stress shielding
Stress shielding is reduced dramatically with these porous metal implants. In a prospective, randomized trial, Hanssen and David G. Lewallen, MD, studied patients who received either rigid titanium cups or ones made of porous metal. After quantitated computerized tomography analysis of five zones in the cups’ supra-acetabular region, they found no stress shielding in the isoelastic porous metal cups through 7.5 years follow-up. The titanium cups revealed stress shielding and bone loss that exceeded 30% in several of the zones they studied.
Much of porous metal’s potential is as of yet untapped, he said. Some novel structures they are well suited for include hip augments, implants for hip osteonecrosis, and porous metaphyseal cones for severe bone loss in the knee.
“We believe this concept now validated can be put into a variety of other designs,” Hanssen said.
Hardened surface
A transformed surface bearing technology, Oxinium (Smith & Nephew) is a biocompatible zirconium alloy that has been used in more than 200,000 hip and knee implants. A proprietary in situ oxidation hardening process changes just 5 µm of its surface from metal to a ceramic state.
“We have essentially a ceramic implant, but it has the toughness of metal because the ceramic is only at the surface,” said Shilesh C. Jani, Smith & Nephew principal engineer, biomaterials. Oxinium’s hardness is twice that of cobalt chrome with half its friction coefficient. Jani said that unlike monolithic ceramics, Oxinium is not brittle. Lab results showed it produced 45% to 85% less polyethylene wear. This month Smith & Nephew introduces a highly crosslinked polyethylene tibial insert to articulate with Oxinium components expected to provide and maintain better short- and long-term wear rates.
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For more information:
- Arlen D. Hanssen, MD, can be reached in the Dept. of Orthopedic Surgery, Mayo Clinic, 200 First St., SW, Rochester, MN 55905; 507-284-2884; e-mail: hanssen.arlen@mayo.edu. He holds intellectual property rights with Stryker Corp. and Zimmer, Inc.
- R. Scott Corpe, MD, is associate professor of orthopaedic surgery and director of the Joint Replacement Center. He can reached at Medical College of Georgia, Augusta, GA 30912-4030; 706-721-2318. He has indicated he is a paid consultant to Wright Medical.
- Shilesh C. Jani can be reached at Smith & Nephew, 1450 Brooks Road, Memphis, TN 38116; 901-396-2121.
Reference:
- Hanssen AD. Porous metals: An emerging orthopaedic biomaterial. #80. Presented at the 24th Annual Current Concepts in Joint Replacement Winter Meeting. Dec. 12-15, 2007. Orlando, Fla.