Improvements in design and technology bring TAR to the fore
For decades, foot surgeons and manufacturers have attempted to develop an implantable device for the ankle that would exhibit the degree of function, safety, and durability achieved by prostheses for hips and knees. With recent advances in device design and with Europe leading the way, total ankle replacement (TAR) is fast becoming the treatment of choice for many patients with ankle arthritis and other disorders.
At the 2010 annual meeting of the American Orthopaedic Foot & Ankle Society (AOFAS), two surgeons—one from Switzerland and one from the United States—surveyed the state of the art of ankle replacement and provided their “takes” on its future.
Lessons from Europe
“Total ankle replacement outside the United States has undergone an overwhelming evolution of new implants and techniques in the last 20 years,” said Beat Hintermann, MD, of Liestal, Switzerland.
“Adding a second interface in three-component ankle prostheses evolved from new expertise in appropriate balancing of the ankle and hindfoot,” he said. “This has not only expanded the indication for TAR, but also has helped establish the new rationale that replacing the destroyed joint surfaces might be a part of overall hindfoot reconstruction. The prosthesis provides free motion in all planes; the components can adapt to the position as given by osseous and ligament structures. This means that the prosthesis has applications in complex hindfoot reconstruction when the original biomechanics of the ankle joint have been destroyed by scarring, injuries, and other deformities. Also, we are moving toward preserving as much bone as possible and minimizing shear forces at bone-implant interfaces.”
Dr. Hintermann provided the following five “take-home points:”
1. The use of three-component ankle prostheses with cementless fixation has become the gold standard.
These appliances constitute 95 percent of the market outside of the United States, and at least 15 models are available. So far, Dr. Hintermann said, the devices have not shown evidence of decrease in stability over time.
2. Current three-component ankle prostheses have shown satisfactory mid- to long-term results.
According to Dr. Hintermann, 5-year survivorship is 98 percent; at 10 years, it is 80 percent to 95 percent.
3. Appropriate ligament balancing is the key for successful replacement of the ankle joint.
“Restored biomechanics depends on the replaced surfaces mimicking anatomy and the collateral ligaments tensioned physiologically,” Dr. Hintermann said. “The closer we can restore talar motion to its original center, the better we can achieve the physiologic ligament load. The better the ligaments are balanced, the less compensatory motion that will be needed along the second interface.”
4. Osseous balancing of the hindfoot is essential for the long-term success of total ankle replacement.
“Osseous balance is the key for achieving physiologic load transfer between bone and implant and for avoiding the pathologic overload of soft-tissue structures such as ligaments and muscle-tendon units,” Dr. Hintermann said. He added that uncorrected or undercorrected misalignment of the hindfoot causes a significant increase of translational forces and movements at the second interface. Valgus misalignment is tolerated more poorly than varus.
5. Load transfer on a tibial component that incorporates circumferential bony support may be superior to that of a stemmed component over the long term.
According to Dr. Hintermann, the main load transfer between the tibial metaphysis and the tibial component occurs through the circumferential cortex. “Evidence shows that anatomically shaped and flat tibial components respond best to this load transfer,” he said. “Other evidence indicates that stems may not account for or may alter load transfer.”
In the States
“I think that 2010 will be a tipping point for ankle replacement,” said Charles L. Saltzman, MD, of the University of Utah. Although fewer TAR implants are approved for use in the United States, current devices, equipment, and techniques have improved enough that “we can say now with pretty good certainty that total ankle arthroplasty is here to stay.”
Five factors—fixation, replacement of articular surfaces, bone resection, articular geometry, and surgical equipment—have accounted for this transformation.
1. Improved fixation leads to higher success rates.
In the 1970s and 1980s, implants relied on cement and failed because of loosening and massive bone loss (Fig. 1). With the introduction of hydroxyapatite in the 1990s, debonding failures were encountered as the coatings wore off. Over the past decade, beaded metal surfaces, either with or without hydroxyapatite coating, have come into use, contributing to improved rates of success.
“It’s possible that in the future we’re going to go to porous metals,” Dr. Saltzman said. “Theoretically that will improve fixation and load transfer, especially in cancellous bone.”
2. Articular surfaces may not need resurfacing.
Dr. Saltzman also noted a shift in resurfacing. “The tendency now is to do less resurfacing and to focus on the superior and inferior joint,” he said.
3. Saving the bone is key.
With regard to bone resection, “less is more,” Dr. Saltzman said. “When bone is removed, it gives us fewer options for the future, particularly on the talar side.” An arthroplasty involving a flat cut can cause problems on the talar side, and he noted that some current prostheses employ a cut more like the natural curve of the talus.
This leads to natural load sharing. “There is less bone resection and it is more natural and oriented in a direction so that the trabeculi can resist the forces across the ankle,” said Dr. Saltzman, who thinks that talar failures in the past have been partially due to bone cuts that leave the talar bone microarchitecture poorly oriented to the implant.
4. Appreciate the articular geometry.
“Articular geometry is underappreciated,” he continued. Designs shifted from the highly unconstrained implants of the 1970s to a highly constrained implant (see Fig. 2) to the current single axis ankle, which uses two planes of motion and multiple degrees of freedom.
Modern implants, he said, “try to respect the radius to curvature and should improve range of motion. The articular geometry enables inversion, eversion, and natural rotation.”
5. Better engineering means better surgical equipment.
Improvements in the equipment used in implant surgery have also been significant and have helped reduce variation in surgical technique, said Dr. Saltzman. The result is more precise guidance that may include provisional positioning of the foot and ankle and fluoroscopic guidance.
A persistent problem in the United States is the Food and Drug Administration (FDA) requirement that every implant must “be packaged as being fitted with cement,” Dr. Saltzman said. He hopes the FDA will soon recognize the shift to porous metals and away from cement.
Looking to the future, Dr. Saltz-man said that many factors remain unknown, including wear and breakage rates of two- versus three-part prostheses, the minimum size and optimal form of polyethylene, how to stop talar subsidence, and how to decrease the learning curve.
The larger question—“Who should have fusion and who should have a replacement?”—is also unanswered.
Disclosure information: Dr. Hintermann—Integra; Dr. Saltzman—Elsevier, Tornier, TotalChart, Twin Star Medical, United Cerebral Palsy Research and Education Fund, Zimmer.
Terry Stanton is the senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org