A Closer Look At The Future Of Total Ankle Arthroplasty

Adam R. Johnson, DPM

   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.

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