Ankle Arthritis: Part II. Total Ankle Arthroplasty



Ankle Arthritis: Part II. Total Ankle Arthroplasty


Thos Harnroongroj, MD

Constantine A. Demetracopoulos, MD


Dr. Demetracopoulos or an immediate family member has received royalties from Exactech, Inc.; is a member of a speakers’ bureau or has made paid presentations on behalf of Exactech, Inc., Integra LifeSciences, Paragon 28, Royal Biologics, Stryker, and Wright Medical Technology, Inc.; serves as a paid consultant to or is an employee of Exactech, Inc., Integra LifeSciences, Paragon 28, Royal Biologics, RTI Surgical, Stryker, and Wright Medical Technology, Inc.; and has received research or institutional support from Integra LifeSciences. Neither Dr. Harnroongroj nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.


This chapter is adapted from Smith WB, Berlet GC: Ankle arthritis: Part II. Total ankle arthroplasty, in Chou LB, ed: Orthopaedic Knowledge Update: Foot and Ankle 5. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2014, pp 129-143.





Introduction

Over the past 10 years, an increasing number of patients with end-stage ankle arthritis have been treated with total ankle arthroplasty (TAA). This trend is due to many factors, including innovations in implant design, improvements in surgical instrumentation, a better understanding of surgical techniques for deformity correction, and promising early to midterm results with modern prostheses. The first reported ankle arthroplasty was in 1970, and the authors subsequently published their initial results of 12 patients in 1973.1 The implant consisted of a tibial stem and polyethylene talar-replacing component that necessitated a subtalar fusion. The ball-in-socket design of the first total ankle implant was modeled after hip implant design, and results were poor.

Early studies of first-generation ankle arthroplasty demonstrated promising results, but midterm follow-up revealed evidence of substantial radiographic loosening and high failure rates.2 Many initial ankle prostheses were designed as a simple, constrained hinge. Motion was allowed only in the sagittal plane. These early implants necessitated substantial bone resection, thus placing the components in softer, metaphyseal bone. In addition, most early implants were cemented. These highly constrained designs transmitted significant forces to the bone-implant interface, which led to early loosening, implant subsidence, and failure.2

Subsequent designs were much less constrained, such as the New Jersey low-contact stress prosthesis (DePuy), the Newton prosthesis (Stryker Howmedica Osteonics), and the Smith prosthesis (Dow Corning Wright). These implants were designed to rely on the stability of the ligamentous envelope of the ankle joint. However, this proved insufficient, and these implants also suffered from a high incidence of impingement, and significant early failure.2


The current generation of TAA implants emphasize limited bone resection, improved fixation to the surrounding bone for initial stability and to encourage bony ingrowth, and seek to restore the physiologic constraint and articulation of the ankle joint. In addition, there are advances in revision TAA systems which compensate for bone loss both on the tibial and talar side. Current TAA implants have two common features: first, all have porous coated surface for enhancing bone ingrowth, and second, all are made of titanium alloy with a cobalt-chrome-polyethylene articulation.3

In the United States, there are nine TAA systems currently available: the Scandinavian Total Ankle Replacement ([STAR] Stryker) (Figure 1), the Salto-Talaris and Salto XT Total Ankles, the Cadence Total Ankle (Integra LifeSciences) (Figures 2 and 3), the Infinity, INBONE II, and INVISION implants (Wright Medical Technology) (Figures 4, 5, 6), the Trabecular Metal Total Ankle (Zimmer) (Figure 7), and the Vantage Total Ankle (Exactech) (Figure 8). All fixed-bearing TAAs in the United States are FDA cleared for cemented use, and the STAR is a mobile-bearing implant FDA approved for use without cement.






FIGURE 1 Scandinavian Total Ankle Replacement (STAR), Stryker.






FIGURE 2 Salto-Talaris Total Ankle (Tornier).






FIGURE 3 The Cadence (Integra Life Sciences







FIGURE 4 INBONE II total ankle (Wright Medical).






FIGURE 5 The Infinity (Wright Medical).






FIGURE 6 The INVISION (Wright Medical).






FIGURE 7 The Trabecular Metal Total Ankle (Zimmer).







FIGURE 8 The Vantage (Exactech).


Design Issues and Rationale


Anatomy and Biomechanics

The ankle joint consists of three bony interactions: the tibia, fibula, and talus. The morphology of each bone affects the function and relationship of the ankle joint.

Many patients who present with ankle arthritis will also demonstrate radiographic and clinically significant adjacent joint arthritis in the hindfoot, most commonly the subtalar joint. Also, adjacent joint arthritis is known to significantly progress with ankle arthrodesis. The biomechanical rationale for increased wear in adjacent joints was elucidated in a 2009 study, which showed that motion in the subtalar joint in the sagittal, coronal, and transverse planes changes to move in the opposite direction when compared with healthy ankles.4 The complication of adjacent joint arthritis has fueled the search for ankle fusion alternatives.

The current generation of implants have sought to address the shortcomings of initial implants. Improved materials and fixation techniques were implemented. By minimizing resection, the implants are placed on stronger, subchondral bone. In addition, by improving initial fixation and ingrowth, implants may be successfully placed without cement. Modern implant designs are semiconstrained, which reduces the stress at the bone-implant interface, and has been successful in improving midterm survivorship.

Because the ankle allows for both gliding and rolling motion, implants are designed to accommodate complex three-dimensional ankle motions. Implant use entails a balance of compromises. Fixed-bearing devices offer inherent stability but sacrifice certain planes of motion, particularly rotational. Mobile-bearing devices allow for more rotational motion, thus decreasing stress to the implant, but they also can reduce stability, allow for possible bearing impingement, edge loading of the polyethylene, and create the potential for backside polyethylene wear.

Current ankle prostheses can strike the necessary balance between providing implant stability and decreasing stress transfer to bone. On the tibial side, most implants of the current generation emphasize a similar approach of minimizing the amount of bone resection to allow for stable fixation on the cortical rim of the distal tibia. Additional stability of the tibial component is achieved with either a keel, barrels, pegs, a cage, or a stem.

The complex anatomy of the talus poses challenges to both implant designers and surgeons. Talar anatomy features complex three-dimensional conical wedges. The radii of curvature on each side of the talus (medial, lateral, anterior, and posterior) all differ. The axis of rotation of the talus has been described as having a changing instant center of rotation,5 with the primary axis of rotation correlated with the transmalleolar plane and externally rotated 23°.6 To further confound the mechanism, the ankle also rotates about 5° in the transverse plane.7 Current primary total ankle implants have a “resurfacing” talar component. Most implant systems do so by making an anterior and posterior chamfer after performing a cut on the top of the talus. Two current implants create a curved surface on the talus, which allows for perpendicular loading of the implant against the bone, with the intention of minimizing the effect of shear forces.

Previously, resurfacing the medial and lateral facets was believed to help reduce the incidence of gutter impingement. However, current implants, which do not resect the bone of the medial and lateral talus to spare bone, do not have a higher incidence of gutter impingement postoperatively.

The blood supply of the talus is known to be sensitive to injury and surgical manipulation. Each technique puts different vascular structures at risk. Tennant et al conducted a cadaveric injection study and reported that INBONE subtalar drill hole had a risk of transection of artery of the tarsal canal. While the lateral approach of the Trabecular Metal Total Ankle had a risk of injury to the first perforator of peroneal artery, the STAR had a risk of injury to the deltoid branches.8,9 Ultimately, how our surgical approaches and techniques affect in vivo blood supply to the talus is not known, and it is unclear how these effect influence survivorship of the TAA implants.









Approaches

Currently, the anterior approach to the ankle is the approach for which there is the most relevant literature for evaluation. A newer implant that was released in 2013 (the Trabecular Metal Total Ankle) uses a lateral, transfibular approach to the ankle joint. The posterior approach for TAA is another option. Because the posterior soft tissue of the ankle is often more robust, it seems to be an attractive alternative in the patient who is a risk for poor wound healing anteriorly. A limited number of anecdotal and case reports mention the posterior approach for the current generation of implants.24 Because the current implant systems are not designed for placement of the implant from a posterior approach, this approach has limited appeal at this time.

Feb 27, 2020 | Posted by in ORTHOPEDIC | Comments Off on Ankle Arthritis: Part II. Total Ankle Arthroplasty
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