J. Chris Coetzee and Steven L. Haddad
DEFINITION
The Agility ankle replacement is a fixed bearing device with a tibial base plate that requires a fusion between the distal tibia and fibula. This unique design feature allows a large surface area for bone ingrowth and also limits the likelihood of subsidence of the tibial component into the cancellous bone of the distal tibia.
The tibial component is a porous-coated titanium implant designed to be positioned in 23 degrees of external rotation.
It has an ultrahigh-molecular-weight polyethylene (UHMWPE) insert available in different thicknesses. The Agility LP uses a front-loading UHMWPE spacer, which makes insertion and revision simple.
The talar component is a dome-shaped cobalt chrome alloy with a porous-coated undersurface. The Agility LP talar base plate covers the entire talar cut surface. The current design has six sizes and is a fixed bearing implant that is partially conforming (FIG 1).
ANATOMY
The ankle joint is complex in that it involves four structures: the lower end and medial malleolus of the tibia and the lateral malleolus of the fibula and the trochlear surface of the talus (FIG 2).
The ankle joint resembles a mortise-and-tenon joint as used in carpentry. The tibia and fibula must be bound together for the mortise to be stable. This is done by the syndesmosis, which consists of the anterior tibiofibular ligament, interosseous ligament, and posterior tibiofibular ligament. Instability of the mortise could lead to degenerative changes of the joint (FIG 3).
The ankle acts mainly as a hinge joint, allowing plantarflexion and dorsiflexion. The ankle is strengthened on the medial side by the triangular deltoid ligament, which radiates from the medial malleolus to the sustentaculum tali of the calcaneus, the medial border of the plantar calcaneonavicular (“spring”) ligament, the tuberosity of the navicular, and the neck of the talus.
The lateral collateral ligament consists of the anterior and posterior tibiofibular ligaments and a calcaneofibular ligament.
All these structures are essential for accurate function and stability of the joint.
FIG 1 • The two-component design of the Agility ankle replacement with a fixed front-loading polyethylene bearing.
PATHOGENESIS
The complexity of the ankle anatomy adds to the difficulty of successful ankle replacements.
Most ankle arthritis is secondary to previous fractures. Intraarticular fractures are common, and especially pilon fractures have a high likelihood of resulting in degenerative changes.
Syndesmosis injuries are notorious for causing ankle arthritis. One millimeter of translation of the talus in the mortis causes a 40% increase in force in the articular cartilage.4
Collateral ankle ligament instabilities are also a major cause of ankle arthritis. Due to the close-packed nature of the ankle joint, any instability results in a significant increase in stress and force in the ankle.
FIG 2 • Part of the complexity of ankle replacements stems from the fact that the ankle joint involves four “separate” entities: (1) the distal tibia, (2) the medial malleolus, (3) the lateral malleolus, and (4) the talus.
FIG 3 • Anterior and lateral views of the ankle show the multiple ligamentous structures involved in keeping the ankle and mortise stable.
The most common is lateral instability. This is accentuated if there is a hindfoot varus deformity.
The foot plays a major role in the pathogenesis of ankle arthritis, and also the outcome of ankle replacement surgery. A stable plantigrade foot is a prerequisite for a successful ankle replacement.
Close attention should be paid to posterior tibial tendon insufficiencies, deltoid attenuation, gastrocnemius contracture, hindfoot varus, and forefoot supination in planning an ankle replacement. Any of these factors should be addressed before or at the time of the ankle replacement.
At present a ligamentous instability of more than 20 degrees varus or valgus is felt to be a contraindication for a total ankle arthroplasty.
NATURAL HISTORY
Degenerative change of the ankle occurs either after a fracture or after ligamentous instability. Only a few cases are truly idiopathic.9
The postfracture group could be divided in two groups. The first comprises patients with severe soft tissue injury, highenergy injury, and multiple operated tibial pilon. These patients usually have a compromised, scarred soft tissue envelope, and the ankle has limited motion. Pain is due to the ankle arthritis but also the soft tissue problems, including scar and damaged lymphatic and venous outflow.
The second group comprises patients with simple malleolar fractures, low-energy pilon with minimal soft tissue compromise. This group behaves more like the ligament instability or idiopathic group in that the soft tissues are friendly and the ankle range of motion is generally very well preserved.
The instability group could have additional issues, including peroneal tendinosis or rupture as well as secondary subtalar arthritis or hindfoot varus.
PATIENT HISTORY AND PHYSICAL FINDINGS
The history is usually very similar within the postfracture group. Depending on the severity and energy of the injury, as well as the accuracy of the reduction of the ankle mortise, the degenerative process will start early or many years after the incident.
Patients in the ligamentous instability group usually present many years after multiple ankle sprains. The most common history is that of multiple ankle sprains while in school or college that were treated suboptimally. There is usually a history of ongoing instability and the need to use an ankle brace while playing sports in later years.
Physical examination should include:
Range of motion of the ankle. Maximum plantarflexion and dorsiflexion are measured. At least 5 degrees of dorsiflexion is required for normal gait. As a general rule preoperative range of motion determines postoperative range of motion.
Gastrocnemius contracture. The examiner should lock the midfoot and then test passive dorsiflexion first with the knee extended and then with the knee flexed. With a gastrocnemius contracture dorsiflexion of the ankle is less with the knee extended. A gastrocnemius lengthening might need to be done.
Tibialis posterior tendon function. Evaluating the foot while the patient is standing and walking will show the triad of deformities: too many toes and loss of medial arch and hindfoot valgus. Grade 1 has no deformity but pain, swelling, and weakness. Grade 2 has weakness and correctable deformity. Grade 3 involves a rigid deformity.5 Tibialis posterior tendon dysfunction is best treated before proceeding with ankle replacement surgery.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Weight-bearing AP, lateral, and oblique radiographs of the ankle are necessary. The lateral radiograph should include the entire foot to evaluate for midfoot and forefoot collapse.
Obtaining weight bearing maximum plantarflexion and dorsiflexion radiographs of the ankle is the only reliable way to measure tibiotalar and midfoot motion.2
Long-leg standing radiographs that include the knee and ankle will help to determine the axis of the leg and any alignment issues not shown on an ankle radiograph alone.
CT scan and MRI could be helpful to determine the presence and size of bone cysts and avascular necrosis of the talus.
DIFFERENTIAL DIAGNOSIS
Posttraumatic degenerative joint disease
Degenerative joint disease secondary to ligamentous instability
Rheumatoid or other seronegative arthritis
Avascular necrosis
Infection
NONOPERATIVE MANAGEMENT
Medications
Nonsteroidal anti-inflammatories might give good medium-term relief.
Corticosteroids could be a valuable tool to delay total ankle replacement.
Injections
Diagnostic
Invaluable; provides a way to determine if most of the pain is coming from the ankle joint
Palliative
Corticosteroids can give good anti-inflammatory and pain control over the short to medium term, but it is seldom, if ever, permanent pain relief.
Footwear modifications
Wide, extra-depth, comfortable shoes with a low heel can help normalize the gait.
Heel wedges can compensate for a leg-length discrepancy or an equinus deformity.
Sole flares provide additional stability to the foot and ankle.
Medial heel flare provides stability for a valgus deformity of the hindfoot.
Lateral flare provides stability for a varus deformity of the hindfoot.
Rocker-bottom sole or a solid ankle cushioned heel (SACH) improves forward progression, reduces impact on the ankle at heel strike, and reduces the amount of plantarflexion required at gait.
Orthotics: in-shoe1,11
Semirigid: vary from simple felt pads to custom-molded inserts
Accommodative inserts are best for rigid deformities. They can also support the medial arch and unload pressure areas. They can control an axial deformity to some degree.
Functional inserts are for flexible deformities. They support the foot and help maintain the axial alignment.
Rigid orthotics
Give better control of axial deformity or misalignment and might help to control instability patterns to some degree
Unload pressure areas but might create new “hot spots”
The UCBL orthotic is a rigid polypropylene insert that aims to correct a flexible hindfoot deformity. It restricts painful hindfoot motion, supports the longitudinal arch, stabilizes the midfoot, and controls the forefoot.
Laced-up ankle brace
Made from various materials (fabric, leather, plastic). It gives reasonable support and correction.
Limits motion to a variable degree (depending on the material)
Helps for swelling
Might simulate a fusion
Ankle–foot orthosis (AFO)
Helps correct and maintain axial malalignment
Mimics an ankle fusion
Provides ankle stability
Might reduce pain but does not completely unload axial forces
SURGICAL MANAGEMENT
It is of paramount importance to have a stable balanced foot before doing an ankle replacement. Any deviation from this tenet increases the likelihood of component malalignment and subsequent prosthetic failure.
A concurrent tibialis posterior tendon dysfunction should be treated before the ankle replacement, especially if there are already secondary changes including hindfoot valgus, loss of the medial arch, or forefoot supination.
Preoperative range of motion determines postoperative range of motion. On average there will be only a 5-degree increase in motion after a replacement.2 Realistic expectations are therefore important.
Preoperative Planning
The appropriate radiographs and other imaging studies should be available.
If there is a significant ligamentous instability, one should plan to do a reconstruction at the time of the ankle replacement.
Concurrent subtalar or talonavicular arthritis poses a challenge. If a diagnostic ankle joint injection relieved most of the pain, one should not fuse these joints. If it was necessary to inject these joints as well to get adequate pain relief, they should probably be fused at the time of the replacement.
Positioning
The patient is placed supine on the table with a sand bag under the ipsilateral hip. It is easier to visualize the ankle if the foot is perpendicular to the bed (FIG 4). The operative extremity is placed on blankets to elevate the leg above the adjacent nonoperative extremity. This will allow easy visualization of the operative extremity on sagittal plane fluoroscopy.
FIG 4 • The patient should be positioned with the foot close to the end of the bed. That makes it easier for the surgeon to visualize the joint without having to lean forward for an extended period. A sand bag is placed under the ipsilateral buttock to turn the foot perpendicular to the bed for equal access to the medial and lateral sides of the joint. The lower calf is supported to allow the ankle to hang free. That posteriorly translates the joint and also relaxes the posterior structures.
TECHNIQUES
EXPOSURE OF THE ANKLE
Use an anterior approach between the extensor hallucis longus tendon and the tibialis anterior. Leave the sheath of the tibialis anterior tendon intact, and perform the dissection lateral to the tendon (TECH FIG 1).
The medial branch of the superficial peroneal nerve is often found in the subcutaneous tissues in the distal half of the wound. It should be identified, protected, and retracted laterally.
The deep neurovascular bundle is found deep to the extensor hallucis longus tendon. The medial malleolar arterial branches are coagulated or divided to free the neurovascular bundle so that it can be retracted laterally.
Incise the ankle capsule longitudinally over the midpoint of the ankle; it may be necessary to excise the central portion of this capsule to gain good exposure. An extensile exposure is required: the entire medial malleolus, syndesmosis, and lateral malleolus should be visible (TECH FIG 2).
TECH FIG 1 • Anterior approach to the ankle between tibialis anterior and extensor hallucis longus.
TECH FIG 2 • Adequate exposure is critical. This shows the medial malleolus, fibula, and syndesmosis. At this point the syndesmosis is already prepared for fusion by removing all the soft tissues and decorticating the apposing surfaces.
Remove the anterior osteophytes on the tibia with an osteotome to expose the extent of the depression in the tibial plafond.
Also remove the osteophytes from the anterior aspect of the talus to allow the cutting block to be adequately placed (TECH FIG 3).
Identify the medial and lateral sides of the talus. It is possible at this stage to assess whether soft tissue procedures are needed to realign the foot. Severe deformity is very difficult to correct, and deformity over 20 degrees may be regarded as a contraindication for ankle replacement with the Agility ankle. The lack of complete congruent contact between the dome of the talar component and the plafond of the tibial tray may encourage tilt of the prosthesis postoperatively.
The syndesmosis is visualized and prepared for fusion using the same incision. Remove all the soft tissues and decorticate the apposing surfaces of the tibia and fibula over the distal 4 cm.
TECH FIG 3 • After adequate exposure of the ankle the anterior osteophytes are removed from the distal tibia and the talar neck. After the removal the entire joint should be visible. The neurovascular bundle is retracted laterally.
APPLICATION OF THE EXTERNAL FIXATION
Apply the distractor with two pins in the foot and two in the tibia, all inserted from the medial side.
The first pin goes into the talar neck. This pin is critical and should be parallel to the talar dome. For example, if the ankle is in valgus, the pin is inserted perpendicular to the axis of the deformity, which would be corrected as distraction is applied.
With the dissection done first the actual placement of the talar pin can be verified under direct vision, which ensures accurate placement (TECH FIG 4).
TECH FIG 4 • The talar pin placement is critical. It should be in the “soft spot” between the medial malleolus proximal, navicular distal, tibialis anterior tendon anterior, and tibialis posterior tendon posterior.
Cancellous pins are used for the talus and calcaneus. Use the distractor guide to place the second pin through the calcaneus. Accurate placement of the first (talar) pin will ensure that the calcaneal pin is posterior and superior to the neurovascular bundle.
This is followed by placing two proximal pins through the distractor guide into the tibia.
Tighten all the distractor joints with the foot at 90 degrees to the tibial axis.
Slowly distract the joint. There is no set distance for distraction, but the goal is to get close to the deltoid endpoint, where the deltoid is under tension. This is usually about 1 cm of distraction.
If no distraction is possible due to scarring and ankylosis, use an osteotome to manually loosen the joint.
Be careful not to overdistract. It could cause malleolar fractures, or “overstuffing” of the joint with limited range of motion.
If distraction tilts the joint in varus or valgus, make adjustments to bring the ankle back to neutral before making the bone cuts.
Use fluoroscopy liberally to ensure that the articular surfaces are parallel, joint space is restored, and the ankle is not in equinus (TECH FIG 5).
TECH FIG 5 • Proper placement of the distractor is critical to ensure correct bone cuts.
ALIGNMENT JIG AND CUTTING BLOCK
Place the yoke of the tibial alignment jig on the leg so that the clamp adjustment bar lies over the anterior crest of the tibia.
Center the proximal end of the ankle clamp alignment jig over the tibial tubercle and hold it in position by wrapping the ankle clamp spring around the leg. The adjustment tube on the yoke post and the extending tibial rod should be parallel to the tibia to give the correct angle for the cutting block at the ankle (TECH FIG 6).
Turn the cutting block clamp fine-tuning screw to its halfway point. That will allow proximal and distal adjustment after securing the alignment guide to the tibia.
Proper rotation of the cutting block is critical. With the jig parallel to the tibia, use the alignment stylus to ensure that the cutting block lines up with the second metatarsal. Then tighten the footpad assembly screw.
TECH FIG 6 • The yoke of the alignment jig should be centered over the tibial crest on an AP view and should be parallel to the tibia on a lateral view.
TECH FIG 7 • The correct-size cutting block will remove equal bone from the tibia and talus, and about one third of the medial and lateral malleolus. At this point the alignment jig is secured to the tibia, but the cutting block can still be moved distal or proximal and lateral or medial. The correct position should be confirmed before securing the cutting block to the tibia.
Insert the correct-size cutting block into the alignment jig slot and roughly center it over the ankle joint. Secure the footpads to the tibia with the stabilizing pins.
Center the cutting block over the ankle. Fluoroscopy is valuable. There is a notch on the lateral and medial wall of the cutting block to show the level of the joint. A lateral radiographic view can also be used to show that equal distances of the tibia and talus will be resected (TECH FIG 7).
First insert the stabilizing pin on the proximal-medial aspect of the cutting block to ensure that the medial-lateral placement of the block will take equal bone from the medial and lateral malleolus (about one third of the width). With adequate placement of the cutting block, insert one or two more stabilizing pins.
BONE RESECTION
Recheck the position of the ankle, especially in the sagittal plane, before making the saw cuts (TECH FIG 8).
Perform the tibial, tibial keel, malleolar, and talar bone cuts using the respective slots in the cutting block. The tibial keel cut should go about half the depth of the tibia.
Remove the cutting block and if necessary complete the corner cuts with a reciprocal saw.
Remove the distal tibial bone, taking care not to rotate the fragments, as it can put excess pressure on the malleoli.
This is followed by the talar fin cut. To allow placement of the burr guide, remove the distraction device to allow full plantarflexion of the ankle. Then place the burr guide centered on the cut surface of the talus, with the alignment jig parallel to the second metatarsal.
Secure the burr guide to the talus with pins and prepare the keel. Remove the burr guide (TECH FIG 9).
TECH FIG 8 • A. The distractor is locked with the ankle in neutral. Note the distraction of the joint. Before securing the cutting block to the tibia, saw blades are used to verify that equal amounts of tibia and talus will be removed. B. The talar cut is complete and the tibia cut is being done.
TECH FIG 9 • A. After removal of the bone from the tibia and malleoli, the burr guide for the talar keel cut is placed and secured. B. Final bone cuts, including the tibial and talar keel cuts. In the posterior-medial corner the tibialis posterior tendon is visible.
TIBIAL AND TALAR COMPONENT INSERTION
Insert the trial components using the alignment handles. It might be helpful to gently spread the syndesmosis with a wide osteotome. The tibial component is inserted first, followed by the talus. A standard front-loading polyethylene component is then inserted into the tibial tray. It could be replaced with a +1 component if needed (TECH FIG 10).
TECH FIG 10 • The trial components are inserted. The insertion handles should diverge about 23 degrees to confirm correct alignment of the two components. If it has not been done before, the distractor should be removed at this point and the ankle stability tested.
Test the range of motion and ankle ligament stability. If there is not at least 5 degrees dorsiflexion consider a gastrocnemius lengthening.
Remove the trial components and thoroughly rinse the ankle to remove all debris. The final components are now inserted. The talar component insertion usually requires the foot to be in maximum plantarflexion (TECH FIG 11).
TECH FIG 11 • Final component placement.
SYNDESMOSIS FUSION
Morselize the bone taken from the bone cuts in the tibia and talus and pack it in the syndesmosis.
Place a threeor four-hole semitubular plate over the lateral aspect of the fibula through the anterior incision.
Insert two screws percutaneously through the plate, fibula, and tibia to compress the syndesmosis. The distal screw should be about 1 cm proximal to the keel of the tibial component (TECH FIG 12).
TECH FIG 12 • The syndesmosis is fused with a small plate on the fibula and two screws into the tibia. Bone taken from the bone cuts is morselized and packed into the syndesmosis.
POSTOPERATIVE CARE
The leg is placed in a short-leg posterior splint or leg walker with the ankle in neutral.
The patient should be non–weight-bearing, or at most toe touch.
The splint is removed at 2 weeks to remove the sutures.
A leg walker is applied, and the patient can start with non–weight-bearing range of motion for 5 minutes three times a day.
The patient continues to be non–weight-bearing for a further 4 weeks.
At 6 weeks radiographs are obtained, and if there are adequate signs of syndesmosis healing the patient can progress to full weight bearing and start physical therapy to increase range of motion, proprioception, and strength.
If soft tissue procedures were done to correct ligamentous imbalance, it is advisable to use a short-leg cast for the first 6 weeks.
OUTCOMES
Alvine's series6 has the longest follow-up (7 to 16 years) on the Agility ankle replacement.
At a mean 9-year follow-up the revision rate was 11% (either a revision or a fusion).
More than 90% of patients reported that they had decreased pain and were satisfied with the outcome of the surgery.
Eighty-nine (76%) of the 117 ankles had some evidence of peri-implant radiolucency.
Syndesmosis nonunion had a negative impact on the clinical and radiologic outcome.
Deland et al7 reported results at 3.5 years of follow-up on 38 patients. The American Orthopaedic Foot and Ankle Society (AOFAS) ankle–hindfoot scores increased from 33.6 preoperatively to 83.3 at final follow-up (P < 0.001).
Postoperative Medical Outcomes Study Short Form-36 (SF36) Physical Component Summary (PCS) and Mental Component Summary (MCS) scores averaged 49.5 and 56.1, respectively.
Migration or subsidence of components was noted in 18 ankles. Overall, 37 of 38 patients were satisfied with the outcome of their surgery and would have the same procedure under similar circumstances.
COMPLICATIONS
Hansen et al10 reported on the complications on 306 consecutive ankle replacements.
28% underwent reoperations; the most common procedures were débridement of heterotopic bone, correction of axial malalignment, and component replacement. The belowthe-knee amputation rate was 3.5%.
Malleolar fractures happen in about 10% of cases. Further perioperative complications include tibial nerve injury, tendon injuries, and wound problems.8
Late complications include syndesmosis nonunions in 6% to 26% of cases.3
Infection
Progressive varus or valgus deformities due to ligamentous imbalance
Osteolysis, bone cysts, and subsidence
REFERENCES
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2. Coetzee JC, Castro MD. Accurate measurement of ankle range of motion after total ankle arthroplasty. Clin Orthop Relat Res 2004;424:27–31.
3. Coetzee JC, Pomeroy GC, Watts JD, Barrow C. The use of autologous concentrated growth factors to promote syndesmosis fusion in the Agility total ankle replacement: a preliminary study. Foot Ankle Int 2005;26:840–846.
4. Gardner MJ, Demetrakopoulos D, Briggs SM, et al. Malreduction of the tibiofibular syndesmosis in ankle fractures. Foot Ankle Int 2006;27:788–792.
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