A Closer Look At The Future Of Total Ankle Arthroplasty
While total joint replacement has been successful in the shoulder, the hip and the knee, we have not seen similar success with total ankle replacement in the past.
Initial reports on total ankle replacements were promising in 1979.1 However, long-term follow-up studies painted a different picture as many failures and poor survivorship of the implants led many authors to abandon the procedure in favor of arthrodesis as it had more predictable results and fewer complications.2-4
Yet there has been a recent resurgence of interest in the total ankle arthroplasty with the introduction of new implants on the market and more devices pending approval. These currently available implants include the Agility Total Ankle System (DePuy), the Eclipse Total Ankle Implant (Kinetikos Medical, Inc.), the Inbone Total Ankle Replacement (Wright Medical Technology) and the Salto-Talaris Total Ankle Prosthesis (Tornier).
Early total ankle replacement devices were fixed to the bone with polymethylmethacrylate cement. This fixation was often lost as bone support failed. Comparative research has shown the superior nature of cementless ceramic prostheses in comparison to the cemented metal predecessor.5 Whether it was due to stress shielding, a local reaction to the polymethylmethacrylate or a lack of in-growth into a cemented implant, surgeons have ceased to use bone cement with a prosthesis with the advent of the newly designed total ankle systems.
To help ensure good seating and fixation of cementless implants, surgeons have utilized multiple modalities. Surgeons often cover implant osseous interfaces in metal beads or fibers to help create space for ingrowth of bone. One can spray osteoconductive coatings such as hydroxyapatite onto the implant osseous interfaces as well to help encourage osseous formation at the local site.6 Furthermore, the surgeon can apply chemical coatings of calcium phosphate to the interface to help improve bonding of the osseous structures to the implant.7
Assessing The Evolution Of The Agility Total Ankle System
The Agility Total Ankle System is a two-component prosthesis made up of a narrow talar component that is seated within a wider tibial component. The two components do not seat tightly as the undersized talar component was produced deliberately to help decrease shear stress with the occurrence of normal rotation with ankle motion.
Fluoroscopic kinematic evaluation of the implant showed that rotation freedom did exist. However, researchers have raised questions as to the effects this has on polyethylene wear.8 When the Agility Total Ankle System was first introduced, it occasionally failed secondary to subsidence of the implant into the tibia. Since that time, the installation of the Agility Total Ankle System has been modified to include fusion of the tibia and fibula distally, creating a synostosis and a barrier to tibial subsidence.
While this modification was successful in helping to alleviate the initial complication, this technique does prevent normal biomechanical rotation of the tibia and fibula.
When installing the Agility Total Ankle System, one must first attach an external frame to the tibia and the foot by placing pins into these bones to create a reliable and rigid construct. This system also works to distract the ankle joint and helps ensure good contact of the component system so unnecessary motion does not occur with ambulation. This frame also helps ensure reliable, consistent results with implantation of the device.
Can The Inbone Total Ankle Replacement Have An Impact?
The Inbone Total Ankle Replacement is a multiple module system with two components creating the joint of the system. This system utilizes a modular tibial stem system that one assembles in situ to minimize dissection. This device does not require windowing of the anterior tibial cortex for implantation.
This modular system has been touted as customizable and reportedly provides better support and stress sharing through the tibia to prevent subsidence without compromising the syndesmosis. Furthermore, the new system also does not use an oversized tibial component as does the Agility. Accordingly, it does not require disruption of the fibular head and only minimal medial malleolus resection. The tibial component has multiple sizes of replaceable polyethylene inserts, ranging from 8 mm to 11 mm, to provide for long life of the implant.
In addition to the modular tibial stem, three talar component stems exist to complement the talar component that is shaped to mimic normal anatomy. The talar stems, like the tibial stem, assist in preventing shifting and subsidence of the component. The three talar component stems include a short talar stem, a long talar stem and a subtalar stem (which is not FDA approved at this time). The subtalar stem crosses the subtalar joint to assist when one is performing a concurrent subtalar joint fusion.
A recent design change is the use of a frame, which does not require fixation to the tibia with pins. With this frame, one would use fluoroscopy for intramedullary guidance through the plantar calcaneus in order to ream the component stems and allow for a minimal anterior tibial approach for component insertion.
What About The Merits Of The Salto-Talaris Total Ankle Prosthesis?
The Salto-Talaris Total Ankle Prosthesis is based on the mobile bearing Salto Ankle. However, it is not considered a mobile bearing device. The Salto-Talaris Total Ankle Prosthesis device works to mimic the anatomy in design and recreate the normal flexion and extension translation and rotation that occurs in the natural ankle.9
Normal biomechanics of the ankle cause an inversion/ adduction of the foot with plantarflexion secondary to anteriorly directed rotation of the lateral aspect to the talus. Many of the new implants work to allow for this motion with a polyethylene insert, called a mobile bearing, which is not permanently fixed to the component stem. However, the Salto-Talaris Total Ankle Prosthesis utilizes two distinct radii of curvature to recreate this motion with a fixed bearing design.
Researchers have shown that the rate of polyethylene wear is based on the thickness of the insert and recommendations for the knee are between 4 and 6 mm.10,11 The knee has a larger weightbearing surface than that of the ankle so logic would confer that an ankle polyethylene insert should be even thicker. To accommodate this, the Salto-Talaris Total Ankle Prosthesis has multiple sizes of polyethylene inserts ranging from 8 mm to 11 mm, which are thick enough to hold up to the force of five and a half times the patient’s body weight placed on the ankle joint with ambulation.12 These inserts are also replaceable if excessive wear occurs.
Essential Insights On Emerging Ankle Replacement Devices
Finally, mobile bearing ankle replacement systems soon to enter the market include the Buechel-Pappas Ultra Total Ankle Replacement (Endotec), the Hintegra Total Ankle Replacement (Integra) and the Scandinavian Total Ankle Replacement (STAR) (Small Bone Innovations). These devices are currently being assessed by the Food and Drug Administration (FDA) for approval in the United States. (Editor’s note: As this issue went to press, the FDA issued an approvable letter for the STAR device.)
These systems rely on three separate components for the ankle joint rather than two components. The tibial and talar components remain, but rather than these components having contact, there is a third polyethylene insert (called the mobile bearing) between these components to allow for freedom of translation and rotation.
Normal biomechanics of the ankle cause shear forces on the classic two-component systems. This shear leads to accelerated breakdown of the polyethylene inserts and high amounts of stress at the osseous component interface, which can ultimately result in failure of the device.
With the inclusion of a fully replaceable mobile bearing in the design of these implants, the prevailing thinking is there will be less stress passing through the prosthesis and its osseous interface. In turn, this will hopefully lead to a longer life for the device. Furthermore, allowing for translation and rotation should allow for more natural ambulation for the patient.
Total ankle implants are becoming more widely used by podiatric surgeons in the U.S. and abroad. With a better understanding of biomechanics and learning from previous failures and successes in total joint replacement implants in other areas of the body, perhaps the total ankle implant may soon become the procedure of choice for chronic ankle conditions as opposed to arthrodesis.
Dr. Johnson is a first-year resident at the Hennepin County Medical Center in Minneapolis.
Dr. Burks is a Fellow of the American College of Foot and Ankle Surgeons, and is board-certified in foot and ankle surgery. He is in private practice in Little Rock, Ark.
For further reading, see “A New Solution For The Arthritic Ankle” in the December 2005 issue, “Inside Insights On Ankle Replacement Surgery” in the March 2008 issue, “Total Ankle Arthroplasty: Do The Risks Decrease With Experience?” in the July 2006 issue and “Are Ankle Implants Worth Another Look?” in the April 2003 issue of Podiatry Today.
To check out the archives or get information on reprints, visit the Web site at www.podiatrytoday.com.
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