Highly porous metals: New class of materials leads to novel orthopedic tools and solutions

From abandoned US government plans for an orbiting spaceplane to a Mayo Clinic Department of Orthopedic Surgery bone ingrowth laboratory, highly porous metals (HPMs) command attention in select scientific circles. Why all the interest in HPMs, such as porous tantalum, over the past 20 years?

Electron micrography image shows elemental tantalum metal closely replicates properties of cancellous bone

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Elemental tantalum metal provides a high coefficient of friction against cancellous bone

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Mayo Clinic orthopedic surgeon David G. Lewallen, M.D., cites at least 6 compelling reasons. All relate to HPMs' cellular architecture and ability to form strong, rapid biological fixation of implants that improves spanning of large defects. From there, many possibilities emerge—such as one day creating smart bioactive implants for cancer patients that structurally replace bone after tumor resections, while also delivering chemotherapy to reduce chances of local recurrence.

"This is a real possibility moving forward," Dr. Lewallen says. "And while the technology is not quite like what we see in movies yet, there is tremendous potential for new and robust orthopedic solutions from HPM research."

Adds his Mayo colleague, orthopedic surgeon and researcher Arlen D. Hanssen, M.D.: "The work with HPMs is great news for patients, because we do have a need for larger and more complex implants. Our research is very much driven by the underlying patient need."

Some of their recent findings, published in the August 2010 issue of Journal of Arthroplasty, suggest that in acetabular revision, highly porous tantalum acetabular components provide superior mechanical stability over the traditional cementless implant when tested in hemipelvis specimens with superolateral defects.

Six ways to apply HPMs to orthopedic problems

Dr. Lewallen began researching potential orthopedic applications of HPMs in the early 1990s because of his interest in porous ingrowth implants. The data immediately caught his attention because the high volume of revision work performed at Mayo creates a need for reliable fixation and for filling large bone defects around hip and knee joints.

"My first look at the data showed impressive evidence in animals of rapid bone ingrowth around HPM joints," Dr. Lewallen recalls. "Growth became more and more rapid, in fact, and I was intrigued. It suggested so many possibilities." Among them:

1. Improving durable fixation of cementless implants through vigorous and strong bone ingrowth.
2. Using blocks or shaped segments of HPM to build up defects related to total joint replacements or bone loss from periarticular tumor resection.
3. Creating bioimplants loaded with growth factors that can modify the tissue environment to encourage tissue ingrowth or otherwise mediate therapeutic effects.
4. Exploiting the spongy nature of HPM by filling it with antibiotics or chemotherapy medicines to deliver therapeutic agents to the implant interface with bone.
5. Achieving ligament or tendon ingrowth to a prosthesis. The intricate architecture of HPM is like a geodesic dome, Dr. Lewallen explains, which not only supports robust bone attachment but also allows for tendon and ligament ingrowth to the implant at near-normal bone attachment strength in animal studies.
6. Promoting cartilage growth. In the laboratory, Mayo researchers have grown cartilage derived from periosteal grafts on the top of a rough scaffolding, suggesting the future possibility of combination tissue and prosthetic implants for clinical use.

Next steps

The versatility and biocompatibility of this new material continue to impress Dr. Lewallen. One especially appealing possibility to him is to create clinical devices that allow for tendon attachment to the implant. Another is to cue cartilage growth for the resurfacing of portions of damaged joints.

"Down the line, the prospect exists for development of composite implants of human tissue and artificial materials for the restoration of damaged joints and bone defects, which is a tremendously exciting prospect," Dr. Lewallen says. "That possibility could open all sorts of therapeutic doors now closed to us."