Porous Foam Metals for Revision Total Hip and Knee Arthroplasty
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X-ray of a patient aged 44 years with a history of a hip dislocation, 6-cm leg length discrepancy, advanced osteoarthritis, and severe pain.
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A new era of total joint surgery is upon us,” says Herrick J. Siegel, MD, UAB orthopaedic oncologist and joint reconstruction surgeon. Ushering in this new era are porous foam metal prostheses, which Siegel says will be crucial tools in orthopaedic surgeons’ armamentarium during the next decade as demand increases for total joint revision surgeries.
At the UAB Joint Center, Siegel and colleagues K. David Moore, MD, and Stuart X. Stevenson, MD, increasingly are incorporating porous metal prosthetic components and augments into hip and knee revision surgeries for patients with significant bone loss. The Food and Drug Administration- approved metal implants, made of either tantalum or titanium, act similarly to cancellous bone and address major issues inherent in hip and knee revision surgeries: bone stock loss, leg length discrepancies, joint instability, acetabular fracture, and infection.
In revision surgeries, grossly deficient bone stock creates difficulties with acetabular fixation and host bone contact. Surgeons must reproduce the hip center, establish correct leg length, and ensure joint stability. The surgery requires an advanced skill set and experience with extensile exposures, component removal, management of bone defects, fitting and fixation of new components, and use of complex instruments.
“At the Joint Center, we can replace almost any bone around the hip, including the femur and most of the pelvis. We can customize components and assemble them during surgery to fit individual patients’ pathology,” Siegel says. “Bone deficiency around the knee can be addressed similarly.”
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Hip reconstruction with a modular femoral stem and trabecular metal acetabular cup
restores leg length and enables patient to
ambulate with a pain-free, normal gait.
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Modular femoral implants with larger femoral heads equalize leg lengths and improve stability and range of motion. Earlier-generation plastics wore down over time, leading to inflammation and bone loss. Using larger femoral heads with the older materials caused component loosening, which limited femoral head size and compromised joint stability.
Trabecular Metal Components
The element tantalum is the main ingredient in trabecular metal components. The interconnecting pores have a high volumetric porosity (70% to 80%), low modulus of elasticity (3 MPa), and high frictional characteristics. Siegel describes trabecular metal’s open-cell structure as a cotton candy-like scaffold for vascularized soft-tissue ingrowth, including ligaments, tendons, and cartilage. The material acts much like ace-tabular bone in cases requiring muscle and tendon reattachment to a prosthesis.
The low modulus of elasticity allows for more physiologic load transfer and relative preservation of bone stock. The metal’s pores can be adjusted to prevent bone resorption. Trabecular metal’s high friction is conducive to biologic fixation, is osteoinductive, and promotes osteoblast adhesion. Studies with tantalum components have shown excellent osseointegration, stability, and improved postoperative functional and clinical outcomes, Siegel says.
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The porous nature of foam metal replicates the structure of cancellous bone.
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Although the high cost of the material does limit its use, “porous tantalum is an intriguing metal for the design and manufacture of cementless components for total hip arthroplasty and is a promising alternative to traditional implant materials,” Siegel says.
For more information contact Dr. Harrick Siegel at 1-800-UAB-MIST or at mist@uabmc.edu.