Are Ankle Implants Worth Another Look?

Author(s): 
By Mark H. Feldman, MS, DPM, and John Grady, DPM

Yes, Mark H. Feldman, MS, DPM, cites technical advances in the devices and promising results from studies. Diligent study of normal ankle biomechanics and review of previous implant failures has led to the development of a new generation of total ankle replacement (TAR) implants. The newer implants provide a better means of dissipating the rotational forces at the joint surface by using a meniscus-like bearing between the tibial and talar components, while maintaining the integrity and stability of the joint.1-4 This improvement, coupled with improved cementless fixation, has led to prosthesis designs that allow for more anatomic ankle motion with decreased rates of implant failure.5 A strong understanding of ankle joint biomechanics plays a pivotal role in the design and function of total ankle replacements. Motion and stability of the ankle joint are essential for normal function during gait. The ankle has two degrees of motion and three degrees of stability.6 The two degrees of ankle motion are plantar dorsiflexion and internal-external (axial) rotation.7-11 An inversion-eversion component is also apparent, but Lundberg has described the greater portion of this motion as being associated with the subtalar joint.12 The normal ankle joint is stable and constrained from significant motion in three degrees: anterior-posterior, medial-lateral and inversion-eversion. This is accomplished by intrinsic and extrinsic factors. Both the anterior-posterior and inversion-eversion motions of the ankle are constrained by extrinsic ligaments about the ankle. Medial-lateral movement depends upon the intrinsic support of the ankle mortise itself by the medial and lateral malleolus. Ankle implant devices must take into consideration the above factors of motion and stability in order to provide near-anatomic motion around the joint. Previous devices did not completely allow for the important rotational (axial) forces and failed to totally consider, and design for, the intrinsic and extrinsic anatomic stability of the ankle mortise. Currently, there are 11 ankle joint prostheses in use around the world. These include the Agility Ankle (DePuy), STAR, Beuchel-Pappas (Endotec), Salto, Alpha OSG, AES, Albatros, Hintermann, Ramses and a ceramic design used in Japan. The Eska ankle is implanted from the lateral side after an osteotomy of the fibula and reflecting it posteriorly. The implant that I’ve had the most experience with is the Beuchel-Pappas implant, which has been used by surgeons in Europe for over 15 years. The implant has been undergoing FDA clinical investigational trials in the United States since 1998. A Closer Look At The Beuchel-Pappas (BP) Implant Frederick Beuchel, MD, and Michael Pappas, PhD, developed and implanted their first ankle design in 1974.13 Over the years, they’ve made several improvements to the device. These improvements included: • the addition of a second talar fin to reduce the risk of talar necrosis and add stability; • the deepening of the talar component’s tibial sulcus in order to reduce early problems of bearing subluxation; • an increase of the platform thickness in response to early observation of high stress loads at the tibial component’s platform edges; • the use of titanium alloy and ceramic coating technology, which reduced contact wear; and • the application of an improved porous coating at the bone contact area for better bony ingrowth, which facilitated a more solid fixation surface.14,15 As described above, the current Beuchel-Pappas device is a three-piece mobile implant. The current design of the articulating surface of the tibial component consists of a flat loading plate with a single fixation stem at a seven-degree anterior incline. The articular surface of the talar component has a convex surface with a central trochlear groove. The talar fixation surface has two anchoring stems, which allow for minimal talar resection and decreased risk of talar avascular necrosis. You may also implant a thick talar component in cases of talar AVN or when the talar bone stock has been previously reduced or is absent. In cases where the talus is absent, the thick talar component is seated directly on the calcaneus. The meniscal bearing articulates congruently with both components matching the flat tibial surface and the trochlear talar surface.

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