Years ago, surgeons asked the question: joint replacement has worked for the hip, knee, shoulder, and hand — why not for the ankle? It could spare ankle motion and would certainly be better than a fusion with no motion. But early attempts failed. The ankle is just so much more complex in its biomechanical design than even the shoulder.
So it was back to the drawing board. And now there’s a second-generation of implants that seem to have better results. Second-generation refers to the new and improved designs that have replaced those first implants. The studies available are somewhat limited, but it looks like there are fewer problems and a lower rate of failures with the newer ankle prosthetics.
In this report, surgeons from the Orthopedic Foot and Ankle Division of the New Jersey Medical School bring us up to date on the subject of modern total ankle arthroplasty (replacement). They review current implant design, patient selection, and survival rates. They suggest that based on more recent results, there is a new interest in the use of ankle replacements.
The modern implant design tries to reproduce sliding, gliding, and rotational movements present in the natural ankle. By experimenting with different coatings sprayed on the implant, researchers have found materials that preserve bone and foster improved bone growth around the implant. With better ingrowth, cement is no longer needed to hold the implant in place. That helps eliminate problems caused by the use of cement.
Improved polyethylene (plastic) parts have also improved movement between the parts and reduced overall wear on the implant. That means they are less likely to break or shift causing a partial or complete joint dislocation.
The second-generation prosthetic design can have fixed component parts or they can be mobile-bearing. Fixed means motion is somewhat limited. Translation and rotation are not allowed in all directions. This makes for a more stable joint but with less motion. There is less stress where the implant meets the bone and therefore less chance of implant failure from loosening.
Mobile-bearing means it can move in more than one direction. That allows for greater torque (twisting motions) and rotation. Multiplane motion also means the joint is less stable. Strong ligamentous support around the ankle is needed for this type of implant. Mobile-bearing implants have not been approved for use in the United States yet. Results reported are from studies in Europe and Canada.
The authors describe fixed implant designs currently available in the United States. Food and Drug Administration (FDA)-approved implants include the total ankle systems from DePuy, Wright Medical, Tornier, and Integra. Compared to other parts of the world, the selection of implants in the U.S. is fairly limited right now. With more studies confirming successful results, this trend is expected to change in the near future.
Choosing the right patient is as important as selecting the best implant design. Not everyone is a good candidate for a total ankle replacement. The patients most likely to benefit from this procedure are those with severe osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis. They have tried and failed with conservative management. They are candidates for ankle fusion called arthrodesis but would like to save joint motion.
Most of the patients selected for ankle replacement are older and less active. They have severe pain from their ankle arthritis. Those with rheumatoid arthritis seem to have the best results so far. Using these implants for younger, more active adults remains a topic that is debated.
For sure, anyone with a large amount of bone loss, infection, or severe deformity won’t qualify as a candidate for ankle replacement. Surgeons may carefully screen patients who are obese, have severe ankle instability, engage in heavy-labor, or have poor skin or bone quality. These people may be more likely candidates in the future but right now, they aren’t considered good candidates for this procedure.
Implants are expected to last at least five years. Some studies show fair-to-good survival rates at 10 years. Slow healing and fracture of the ankle bone are the two main problems that develop. There’s evidence to suggest that these problems are less common as the surgeon’s level of experience increases.
Sometimes the implant migrates (moves) or sinks down into the bone (called subsidence). That doesn’t always mean the implant is a failure. Many patients still report great improvement over their pre-operative state of severe pain and loss of function. They can move their ankle through a greater arc of motion. And they can walk with a normal or near-normal gait pattern. Some even participate in sports.
The authors advise surgeons to be aware that ankle replacements have improved but still carry a high degree of risk and potential problems even when carried out by an experienced surgeon. Choosing patients carefully, being familiar with the various implants, and knowing how to do the surgery are essential for a successful outcome. The surgeon must plan carefully for individual patient variations in anatomy, alignment, and joint biomechanics.