The Challenging Total Ankle Arthroplasty: Management of Associated Foot Pathology

CHAPTER PREVIEW

CHAPTER SYNOPSIS:

Ankle arthritis often occurs with varus or valgus deformity in the foot. Many of these patients will have degeneration of both ankle and hindfoot joints. Total ankle arthroplasty offers theoretically improved functional outcome over pantalar fusion for these patients, provided the arthroplasty can be durable.

IMPORTANT POINTS:

1

The foot and ankle must be neutrally aligned before weight-bearing after total ankle replacement.
2

The heel should be in slight valgus with appropriate tension in the deltoid ligament to achieve good balance.
3

Static deformities in bones and joints, as well as dynamic imbalances in muscles, must be addressed.

CLINICAL/SURGICAL PEARLS:

1

The valgus foot needs restoration of alignment in the hindfoot and in the medial column.
2

The varus foot often has plantarflexion of the medial column.
3

Medial or lateral sliding calcaneal osteotomies can be used to shift the weight-bearing axis of the hindfoot.
4

When foot deformities require extensive reconstruction, it may be wise to stage foot reconstruction and ankle arthroplasty a few months apart.

CLINICAL/SURGICAL PITFALLS:

1

An incompetent deltoid ligament is probably a contraindication to ankle arthroplasty.
2

A medial external fixator/distractor has a tendency to increase valgus in the ankle.
3

A tilting talar component will increase contact pressures in the polyethylene, with polyfracture possible.

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HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

Restoration of alignment is always a key principle in foot and ankle reconstruction.

Many cases of ankle arthritis are the result of misalignment in the foot. Surgical treatment of the arthritis, whether it is osteotomy, arthrodesis, or arthroplasty, requires restoration of alignment to achieve long-term success. Many of the failures seen following “first-generation” ankle arthroplasties may have been due to persistent foot imbalance.

The normal tibiotalar joint is highly constrained, so tilting of the talus in the mortise is only seen as a late finding of advanced deformity/imbalance. Minor misalignment is well tolerated by the ankle for long periods. Modern “second-generation” ankle arthroplasties are all relatively less constrained; slight imbalance in the foundation of the ankle (in other words, the foot) will predictably (and sometimes unpredictably) lead to tilting of the talus in the mortise. Such tilting produces high-contact pressures on the polyethylene, with the potential for early failure. Tilting of the foot also can cause painful osseous impingement medially or laterally. Thus, the goal of ankle arthroplasty should be to maintain neutral alignment (no tilting) of the ankle components through precise balancing of the dynamic and static structures around the ankle, especially in the foot.

The limb must be reasonably well aligned above the ankle implant. This is particularly relevant in the tibia, where posttraumatic deformities must be neutralized. Tibial deformity is discussed in Chapter 12 .

Below the ankle, the foot can be modeled as a tripod, with the heel, the medial column, and the lateral column providing the three legs of support. In the ideal total ankle arthroplasty, the axis of weight-bearing in the coronal plane should pass just medial to the calcaneal tuberosity, thus imparting a slight valgus force on the talus. A properly tensioned deltoid ligament then maintains alignment in the ankle. If the deltoid is overly stressed by a heel that is in too much valgus, then valgus tilting of the talus will follow. By the same thought, an incompetent deltoid ligament is probably a contraindication to ankle arthroplasty. At this point there is still no reliable and reproducible method of reconstructing the deltoid complex as part of a total ankle replacement.

If the heel lies medial to the weight-bearing axis, varus tilting of the talus will predictably follow, since the lateral ligaments do not seem capable of resisting any varus moment.

The medial column of the foot (including the talonavicular, naviculocuneiform, and metatatarsocuneiform joints) must be sufficiently stable to support the ankle. Instability in any of these joints, as often seen in the rheumatoid foot or in the advanced flat foot, will allow valgus collapse. Excessive plantarflexion of the medial column, as might be found with an overactive peroneus longus or a plantarflexed medial column, will force the ankle into varus (forefoot-driven hindfoot varus).

INDICATIONS

Total ankle arthroplasty is a surgical alternative to ankle arthrodesis in the treatment of arthritis. Some have proposed that less than 10 or 15 degrees of coronal plane malalignment is necessary for successful arthroplasty. Any more deformity requires fusion. Rather than rely on any number or absolute degree of deformity, it is best to consider ankle arthroplasty in any situation in which proper static and dynamic balance can be achieved. In other words, arthroplasty is contraindicated in any patient where proper alignment cannot be achieved in the foot below and in the leg above the ankle.

As mentioned in the previous section, a competent deltoid ligament appears to be necessary. There currently are no proved successful deltoid ligament reconstructions, although methods have been proposed and experimental procedures have been described in vitro. Functional plantarflexors and dorsiflexors of the ankle are also necessary for good ankle function; the rare case of arthritis with loss of Achilles function is best treated with a fusion.

At the time of arthroplasty, there must be a stable foot below the ankle. The medial column should be sufficiently stable but not overly plantarflexed. The heel should be in neutral to slight valgus. The Achilles must be sufficiently mobile to allow about 5 degrees dorsiflexion. Many of these factors can be corrected intraoperatively.

CLASSIFICATION

Foot deformity can be broadly categorized into varus or valgus.

Valgus

The basic feature of the valgus foot is loss of medial column support. This could be from hindfoot eversion, perhaps better referred to as dorsilateral peritalar subluxation. This is often seen with posterior tibial tendon disorders. Collapse of the naviculocuneiform or metatarsocuneiform joints, as might be seen as the late sequelae of midfoot trauma, can also result in valgus deformity. Inflammatory arthritis can lead to all of these deformities. Chronic valgus foot alignment leads to lateral tibial plafond overload, with degenerative arthritis following. Severe deformity will eventually lead to valgus tilting of the talus in the mortise.

Muscular imbalance may be at the center of valgus deformity. A contracted gastrocnemius can break down the medial column or could also be the result of longstanding deformity. Posterior tibial tendon failure is often part of the pathology. Spasm or contracture of the peroneus brevis, possibly in a neuromuscular disease, can also result in valgus alignment. Although rare, a disrupted deltoid ligament can lead to a different kind of valgus foot. Deltoid incompetence, whether acquired or iatrogenic, is probably a contraindication to total ankle arthroplasty.

Varus

Although neuromuscular disease can result in impressive cavovarus deformities, nonneuromuscular cavus (the “subtle” cavus foot) is quite a bit more common. Mild varus of the heel may coexist with mild plantarflexion of the medial column. This tends to overload the medial tibial plafond. However, the varus alignment also predisposes to recurrent ankle sprains, and this may cause additional degeneration over time. In late stages, chronic disruption of the anterior talofibular ligament will allow the talus to extrude anteriorly.

The gastrocnemius may be tight in the subtle cavus foot. More obvious, though, is the peroneal imbalance. An overactive peroneus longus plantarflexes the medial column, forcing the hindfoot into varus (forefoot-driven hindfoot varus), while relative weakness of the peroneus brevis leads to further imbalance. Patients with neuromuscular disease may have contracture of the posterior tibial tendon.

SURGICAL TECHNIQUE

Surgery begins with thorough preoperative planning. The deformity must be fully assessed, looking at the leg from the front, back, and side. Muscle contracture and balance should be assessed, as well as osseous alignment when sitting and especially when standing. Weight-bearing radiographs of the ankle and foot should be obtained routinely, and the tibia or upper leg should be imaged when suspicion for deformity exists there.

Once all deformities are identified, a plan for correction can be made. The basic rule is that the foot and ankle must be neutrally balanced before weight-bearing. It is possible that all imbalances can be corrected at one surgery, but often the amount of work involved is extensive, and consideration should be given to staging the procedures. For example, an advanced valgus foot may need a triple arthrodesis to realign the hindfoot. Dorsal stripping of the talus for replacement combined with medial and lateral stripping for hindfoot realignment and fusion would leave no blood supply in the talus. This would impair healing of the fusion and jeopardize osseous integration of the talar component. In such cases, it is wise to stage the hindfoot reconstruction first and then perform ankle replacement 3 to 6 months later. In some cases, the combination of realignment surgery (perhaps a tibial osteotomy) and ankle replacement in 1 day is too much for the limb. Severe swelling and possibly compartment syndrome will occur postoperatively, and soft tissue healing will be impaired. Wound healing complications are the most distressing problems seen after ankle arthroplasty.

When considering multiple procedures at one sitting, the surgical team may actually be the limiting factor. The procedures must be completed in a reasonable amount of time. Even when the surgeon feels comfortable with the amount of surgery planned, a skilled assistant and operating team are required to complete the work safely and efficiently. Whenever there is any question, it is better to stage surgeries than to risk a serious complication.

The Mild Valgus Foot With Ankle Arthritis—One-Stage Surgery

As the arch collapses in an adult flat foot, the medial column (talonavicular, naviculocuneiform, and first tarsometatarsal) joints collapse. As the arch is lost, the ankle tilts into valgus. Realignment begins in the medial column. Every flat foot is different, so some will collapse primarily through the tarsometatarsal joint, while others may collapse elsewhere. Only the joints that have collapsed must be realigned, and this will vary from case to case. In the mild flat foot, the anterior ankle incision can be extended over the top of the medial column to expose the deformed joint(s). The tarsometatarsal joint can be plantarflexed by placing a small block of bone (from the resected distal tibia) into the dorsum of the joint, and then stabilized with a 2.7-mm tubular plate. For the naviculocuneiform joint, the joint should be denuded of cartilage, reduced, and then stabilized with multiple lag screws. This is usually done through a separate medial utility incision, although a dorsal approach can be considered if the soft tissues are not stripped from the bones.

During replacement of the valgus ankle, the surgeon should not use the medial external fixator to apply distraction, since that will increase valgus tilting. Rather, the ankle should be opened and distracted from within, using broad osteotomes on the lateral side to regain tension in the deltoid, and then the medial fixator can be tightened to hold the position. The tibial and talar components should be slightly lateralized, or at least not placed too medially, so that the weight-bearing axis falls more medially across the ankle. This will decrease the valgus moment. Although it may sometimes be desirable to create an overall varus moment in the severe flat foot, it is possible to tilt a valgus ankle into varus. The most reliable technique is to restore the medial column of the foot and to regain appropriate tension in the deltoid.

If the hindfoot remains in valgus once the ankle and forefoot are realigned (so that the heel is laterally translated relative to the leg), then a calcaneal osteotomy should be performed. Through a lateral incision just posterior to the peroneal tendons and sural nerve, either a Tuke or an oscillating saw is used to cut through the tuber, being careful not to plunge medially. The heel is slid medially about a centimeter, and the osteotomy is secured with two 6.5-mm partially threaded screws. By translating the heel medially, the weight-bearing axis is translated as well, decreasing any valgus moment about the ankle.

One must be careful not to translate the heel too far and create a varus moment. If the components are lateralized, and the heel is excessively medialized, in theory the ankle could tilt into varus. In reality, this is unusual, and it is more common to undercorrect, not overcorrect, the deformity.

Soft tissue balancing is essential in reconstructing the valgus foot. Most of these feet will start with a gastrocnemius contracture, but by the time the deformity is advanced, the entire triceps surae is tight. Achilles tendon lengthening can be done percutaneously with a two- or three-incision technique at the time of ankle replacement. Once the new ankle is in place, if the foot cannot be dorsiflexed at least a few degrees past neutral, a lengthening of either the gastrocnemius or, more commonly, the Achilles tendon is needed.

A short leg splint with the ankle neutral is worn for 2 to 3 weeks. The leg is then put into a removable CAM boot, and range-of-motion exercises are begun. Some surgeons may prefer a cast for a total of 2 months. Radiographs are checked 6 to 8 weeks after surgery. With adequate healing of the osseus procedures, weight-bearing is slowly advanced. Patients are usually walking well by 3 months.

The Advanced Flat Foot With Valgus Ankle Arthritis

First Stage

With long-standing hindfoot valgus from posterior tibial tendon failure or rheumatoid arthritis, misalignment through the ankle will lead to erosion of the lateral plafond and talus. When caught early, hindfoot correction, possibly with distal tibial osteotomy, can preserve the ankle. In late stages, hindfoot realignment and fusion may be combined with total ankle arthroplasty as an alternative to pantalar fusion ( Fig. 13-1 ).