Total knee and hip replacement implants have been used with success for years, but ankle implants have had a long history of failure. In the late ’70s and early ’80s the ankle implants wore out, popped out or collapsed into the bone. Ankle implant surgery disappeared, for the most part, for almost two decades, with surgeons recommending an ankle fusion for those patients with severe pain and disability due to ankle arthritis. In the earlier part of this decade, ankle implants have started to make a resurgence with the new emerging technology.
Why the failures? Early ankle replacement devices were fixed to the bone with cement. The fixation was lost with the excess shearing forces causing the bone support to fail. Compared with the hip and knee joints, the ankle joint has a much smaller surface with much greater weight to support and greater shearing forces.
An ankle fusion, the current standard of care for surgical treatment of chronic ankle pain due to arthritis, is limiting because of the lack of motion at the ankle after the procedure. The newer ankle implant devices have higher success rates than the older models. There are many two component implants on the market and currently being used, including the Inbone Total Ankle Replacement (Wright Medical Technology), Agility Total Ankle System (DePuy), the Eclipse Total Ankle Implant (Kinetikos Medical, Inc.) and the Salto-Talaris Total Ankle Prosthesis (Tornier).
Mobile bearing, three component ankle replacement systems have been under review and in clinical trials. Entering the market, but still under review, are the Buechel-Pappas Ultra Total Ankle Replacement (Endotec) and Hintegra Total Ankle Replacement (Integra). The S.T.A.R. (Small Bone Innovations, Inc) total ankle replacement system was approved by the FDA in May, 2009, for ankle joint replacement due to osteoarthritis, post-traumatic arthritis or rheumatoid arthritis. The advantage of the mobile bearing system is the third component, made of polyethylene, a medical grade plastic. This insert allows for translation and rotation and decreases excess shearing and breakdown of the polyethylene, common in the two-component systems. Excess shear and stress on the interface between the metal component and the bone can lead to implant failure.
Small Bone Innovations, Inc. (SBi) notes that the S.T.A.R. ® Ankle has been in development for 30 years and the most recent design has been implanted in over 15,200 patients around the world. There have been thirty-five peer-reviewed outcomes papers published on the S.T.A.R. ® Ankle replacement device.
In August of 2000, a U.S. IDE Trial of S.T.A.R.® was initiated to evaluate and safety and effectiveness of the device. The 2 year, prospective, multi-center controlled study evaluated 670 patients and compared the S.T.A.R. implant to joint fusion surgery. The results showed superior efficacy and safety with the S.T.A.R. ® Ankle compared to ankle fusion.
The Scandinavian Total Ankle Replacement (S.T.A.R.) system is a 3- component, mobile bearing device, approved for use without cement. The first component is designed to fit into the tibia, the bottom portion of the shin bone. The second component is designed to fit on the talus, also known as the ankle bone. The third component is called the UHMWPE Sliding Core, the mobile bearing made of medical grade plastic. The Sliding Core is designed to move between the two metal components, to simulate normal ankle joint motion.
Indications for ankle joint replacement systems include severe pain and loss of mobility and function due to post traumatic arthritis, ankle osteoarthritis and rheumatoid arthritis.