A Closer Look At The Future Of Total Ankle Arthroplasty
- Volume 22 - Issue 5 - May 2009
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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.