The Next Big Thing: Opportunities and Innovations in Total Ankle ArthroplastyThe Next Big Thing: Opportunities and Innovations in Total Ankle Arthroplasty

The goal in total ankle arthroplasty (TAA), consistent with other joint prostheses procedures, is to reduce pain while preserving a natural range of motion. Until now, the ankle market has been slow to develop and has remained well below its potential. However, as a result of recent product improvements, it is on the verge of exploding. Availability of advanced technologies such as three-part mobile bearing uncemented TAA devices, penetrating untapped markets such as the United States and Asia, rapid growth of an aging population, and an accelerating incidence of ankle arthritis are the key drivers of the TAA market.

Due to the stresses placed on the complex ankle joint, development of an effective and durable replacement has not been easy. Early attempts at ankle replacement resulted in high failure rates. Revision surgery was difficult because it required removal of the implant and subsequent fusion. These revisions generally left the patient with a shortened ankle, a result of tissue removal during the initial arthroplasty surgery. These poor results led surgeons to abandon ankle arthroplasty and stick with fusion, which has been regarded as the gold standard.

In the past 15 years, there has been a renewed interest in ankle replacement surgery. Newer designs have eliminated some (but not all) of the difficulties experienced with earlier products and have featured less-invasive surgical techniques. The success of ankle arthroplasty in Europe, along with promising medium- to long-term clinical data and a growing number of trained surgeons, have made it a viable alternative to ankle fusion. In addition, many skilled specialist surgeons, along with the emergence of advanced devices and implants, have supported this transformation. Historically, physicians have evolved their thinking and treatment patterns first by focusing on removing the tissue that is the source of pain. The next step is to repair the tissue. Trauma products (i.e., plates and screws) are used to repair broken or damaged bones and facilitate the healing process. Fusion, a procedure in which bones are induced to join together, is classified as repair, but it does not restore full anatomical function. The third phase of the treatment evolution is to replace the damaged tissue with a total joint implant.

Development of Total Ankle Replacements

Total ankle replacements date back to the 1970s. There have been three main device designs—constrained, semiconstrained, and nonconstrained—each of which represent a generation of products. Currently, nonconstrained devices are replacing semiconstrained implants. Nonconstrained, noncemented three-part ankle prostheses are widely accepted in Europe.
The earliest constrained designs treated the ankle as a pure hinge joint. These designs included the Thompson Parkridge Richard (TPR) ankle prosthesis and the Oregon ankle prosthesis. Neither of these ankle prostheses is used today. Constrained designs are very stable, so there is little chance that they will subluxate or impinge on adjacent anatomies. However, treating the ankle solely as a hinge ignores its simultaneous multiple axes of motion and compromises joint mechanics and loading. The resultant stresses cause rapid loosening of the implant’s components. Over time, failure rates approached 90% with the constrained designs.

Second-generation devices were generally two-piece semiconstrained designs. These devices more closely mimicked the anatomy of the ankle by replicating the talus and the mortise. This modification in the design allowed for some lateral motion and minimal rotation. In the United States, DePuy’s Agility ankle had no competition in this category until 2005. The implant is composed of two pieces; the bearing piece fits inside the tibial component and constrains the talus, making it a two-part design.

Known among some surgeons as a sloppy hinge, it requires significant bone removal, and the tibia and fibula must be fused together. Therefore, this implant has a naturally high complication rate due to the requirement for tibia-fibula fusion, among other factors.

Mobile-bearing, nonconstrained designs, which were initially developed in the 1970s, are the most-advanced generation of ankle prostheses. Examples of such devices are Integra LifeScience’s Hintegra (sold outside the United States) and the S.T.A.R. (Scandinavian Total Ankle Replacement System), Small Bone Innovations Inc.’s total ankle replacement system. S.T.A.R. is the only three-part mobile-bearing, uncemented, unconstrained total ankle joint replacement (available worldwide). Nonconstrained devices have a tibial component with a flat surface, a bearing component with a flat top and concave bottom, and a talar component that replicates the talar dome. These designs permit lateral movement at the tibial-bearing interface and flexion and extension at the talar-bearing interface, and perhaps most importantly, rotation of the ankle joint all simultaneously. Fixation is generally achieved by press-fit, and the prostheses are designed to integrate with surrounding bone. Support and stability are provided by intact soft and connective tissues.

The TAA Growth Opportunity

A parallel can be drawn between ankle arthritis and TAA and the evolution of treatment for hip arthritis. The latter has a potential U.S. patient population upwards of 2 million, while the former has a potential patient population of about 400,000 patients today. Both hip and ankle arthritis patients suffer from pain and physical disability as well as decreased range of motion. When hip arthroplasty was shown to decrease pain, improve functionality, and provide patients with a better quality of life, it supplanted fusion as the favored method of treatment for sufferers of hip arthritis. Where medically indicated, hip arthritis patients receive total joint replacements on a regular basis. Patients suffering from ankle arthritis do not enjoy such relief, because fusion, which results in loss of motion, prolonged recovery, and higher rates of complication, is still the preferred method of treating that ailment. TAA can minimize or eliminate negative side effects and should ultimately replace fusion as the gold standard.

The S.T.A.R. ankle is the first three-part ankle prosthesis to receive FDA approval and the first TAA of any kind to complete a randomized, concurrently controlled, multicenter, U.S. investigational device exemption (IDE) study against ankle fusion. Study enrollment began in 1990 and patients have been followed for several years. The IDE study proved S.T.A.R. was statistically superior in efficacy and comparable in safety to ankle fusion.

Demand for effective arthroplasty in the ankle anatomy is in the replacement phase of the small bone and joint sector. The replacement phase is typically characterized by rapid growth as surgeons become comfortable with the procedure. With FDA approval of the S.T.A.R. device, foot and ankle surgeons can do more than fuse or simply medicate to stop pain. They can accept motion preservation as a cofunctioning solution with relieving pain. The S.T.A.R. ankle prosthesis may ultimately be viewed as a more conservative therapy than fusion.

Worldwide TAA Market

Worldwide TAA market revenues are estimated to reach $90 million this year (see Figure 1). Over the next five years, this number will likely increase as innovative ankle implants penetrate the United States, Europe, Asia and the Pacific Rim, the Middle East, and South America. The statistical performance of the S.T.A.R. device shows that TAA enhances treatment efficacy in correctly selected patients. With comprehensive surgeon training, state-of-the-art precision surgical instruments, and long-term clinical performance data, the ankle implant market may rival that of hips and knees one day. Forecast sales are being estimated at a compound annual growth rate (CAGR) of 26%. If this occurs, worldwide TAA revenues could reach $400 million by 2015. Global revenue estimates for 2011 suggest that of the $90 million expected to be generated by the TAA market, approximately $30 million (33%) will be from the S.T.A.R. ankle prosthesis; approximately $18 million (20%) will be from the Salto-Talaris ankle prosthesis (marketed by Tornier Inc.); $11 million (12%) will be from the Inbone ankle prosthesis (marketed by Wright Medical Technology Inc.); and $10 million (11%) will be from the Agility ankle. Several other ankle prostheses will account for the remaining $21 million (24%) in global revenue.

Market Trends. The following are some trends that may occur in the worldwide TAA market:

?    Three-part, mobile-bearing, uncemented ankle implants will be the single largest contributor to the future expansion and growth of the ankle market.
?    Two-part ankle implants, such as the Salto-Talaris, Inbone, and Agility, are expected to lose significant market share over the next five years.
?    Tornier, DePuy, and Wright Medical may consider commencing IDE programs in 2014.
?    Wright Medical will introduce a next-generation version of the two-part semiconstrained Inbone ankle prosthesis this year or in 2012.
?    Also this year, Zimmer Holdings Inc. and Integra LifeSciences may bring in their versions of two-part semiconstrained ankle prostheses with 510(k) marketing clearances to the U.S. market. FDA may require some U.S. patient data.
?    Three-part, mobile-bearing, uncemented technologies with FDA approval such as the S.T.A.R. ankle are likely to rapidly gain market share.

Reimbursement will have a critical effect on the development of the U.S. ankle market. Thus far, Medicare and Medicaid have generally covered two-part ankle implants. Most of the major private insurance carriers have been reluctant to cover any procedure except fusion. Without formal clinical trials, insurers say they don’t have sufficient evidence that these two-part devices provide clinical benefits. However, it is easier for insurance companies to cover FDA-approved products that have demonstrated both safety and efficacy in clinical trials. Since FDA approval of the S.T.A.R. device, proving statistical superiority in efficacy and comparable in safety, there has been a significant shift in attitude to a point where three-part ankle implants can be viewed as routine procedures.

After seeing the success and market potential of a three-part TAA nonfusion ankle technology, it is expected that Tornier, DePuy, and Wright Medical will respond with competitive offerings within the next few years. In addition, Zimmer is expected to seek a 510(k) clearance for its two-part semiconstrained ankle product from FDA sometime this year or in 2012. However, the Zimmer ankle prosthesis uses tantalum metal and a lateral approach. It is expected that surgeon dissatisfaction with both tantalum metal in this device and the lateral approach could lead to the discontinuation of the company’s two-part semiconstrained ankle prostheses strategy.

Conclusion

There are significant growth opportunities in the TAA market. It will follow the hip and knee arthroplasty market paradigms due to the upsurge in interest and demand following the introduction of FDA-approved three-part mobile bearing uncemented ankle devices.

Ankle companies with game-changing technologies such as TAAs could experience dramatic growth in revenues over the next decade. The current gold standard for treating an arthritic ankle—fusion (arthrodesis)—is likely to decline by 50% over the next five years and be succeeded by a new gold standard, namely TAA. Other nonfusion ankle procedures will be converted to TAA procedures at annual rates ranging from 2.5% to 15% over the next five years.

All data sourced from Viscogliosi Bros. (New York City).

Anthony G. Viscogliosi is founder, chairman, and CEO of Small Bone Innovations Inc. (New York City) and principal at Viscogliosi Bros. LLC.