While there have been a number of advances with total ankle replacement, these authors review key fundamentals for balancing unstable ankle joints, offering pearls and tips on procedure selection for soft tissue balancing as well as correction of varus and valgus imbalances.
After four generations of total ankle replacements (TARs), numerous modifications in design and alterations in approach, TAR has evolved into a reliable alternative to ankle fusion in the appropriate setting. Modern long-term TAR follow-up shows comparable results with those of ankle arthrodesis and a recent systematic review of intermediate and long-term outcomes comparing TAR with arthrodesis shows similar risks of early complications and long-term failure for both procedures.1-5
Additionally, recent studies of TAR describe implant survival rates ranging from 70 to 95 percent during a seven- to 16-year follow-up.6 These encouraging clinical data are supported by bench analyses that show TAR more closely resembles normal gait in comparison with ankle arthrodesis.7 The principle behind the development of TAR is that anatomic restoration of the ankle joint may not only alleviate pain and improve function, but also delay the onset of adjacent joint arthritis.8-10
Indeed, restoration of ankle alignment and stability is perhaps the most important technical consideration and goal of TAR surgery. The complex configuration of the ankle and its dynamic nature have had significant implications in the design and evolution of total ankle prostheses. Numerous studies and empirical evidence show that a more anatomic approximation of ankle alignment and stability will generally endure greater longevity.7,9,10 It is imperative to give attention and respect to balancing unstable ankle joints when pursuing TAR, regardless of the specific system one utilizes.
Understanding The Biomechanics Of Restoring Ankle Joint Balance
Balance about the ankle joint is represented by both a static and dynamic interaction of anatomic alignment and stability. Alignment and stability of the ankle joint are not mutually exclusive considerations for TAR. They overlap significantly during ankle joint performance. Alignment, a static and dynamic factor, is predominantly dependent upon osseous quantity and quality. Stability, representing a largely dynamic factor of ankle performance and integrity, is dependent upon the osseous geometry and soft tissue quality. Together, ankle alignment and stability and their associated components provide balance about the joint, and are essential for the successful performance and durability of any TAR.
Alignment. Sufficient bone quantity and quality are fundamental to the success of any prosthetic arthroplasty. Following removal of the subchondral plate in the ankle joint, compressive resistance may be 30 to 50 percent lower. Removing the subchondral bone 1 cm proximal to the subchondral plate may provide a 70 to 90 percent reduction. In general, to avoid subsidence, one should perform minimal bone resection to preserve the stronger superficial subchondral bone. Conserving as much bone as possible at the time of the index surgery not only maintains structural support and alignment but also saves bone, which may be useful should revision become necessary. In this way, osseous quality and quantity provide for the realization and maintenance of proper alignment.11
Stability. The stability of the ankle joint depends upon both the joint’s osseous geometry and ligamentous structures.12-15 The congruity of the articular surface of the ankle joint thus creates an inherently stable articulation with loading. The articular surfaces provide the sole restraint of the joint under loaded conditions. During most activities, the soft tissue structures are the main torsional and anteroposterior stabilizers of the ankle while the ankle’s articular surface geometry acts as the major inversion/eversion stabilizer with the collateral ligaments playing a secondary role.13,15–17
Ankle ligaments have a passive stabilizing effect on the ankle joint. Research has shown the deep deltoid ligament is secondarily resistive against lateral and anterior talar excursion whereas the anterior talofibular ligament is the only restraint against anterior talar excursion.12,16–20 The anterior talofibular ligament is the ligament that is most susceptible to injury and subsequent insufficiency, often leading to anterolateral dislocation of the talus out of the mortise and posterior dislocation of the fibula respectively.21
Rotation about a vertical axis occurs at the ankle during walking and this rotatory stability results from tensioning in the collateral ligaments by compression of the medial and lateral talar facets against their corresponding malleoli, and the shape of the articular surfaces.12-15,22,23 Due to the truncated conical shape of the talus with its medially directed apex, the three separated lateral ligaments control the greater movement on the lateral side whereas the deltoid ligament controls the lesser movement on the medial side. This has important consequences for ligament balancing in total ankle replacement as non-anatomic prosthetic design and/or inappropriate implantation may incite medial ligamentous stress with a loss of range of motion or lateral ligament insufficiency with subsequent lateral ankle instability.
If a prosthesis does not provide intrinsic inversion and eversion stability, an unbalanced and unstable ankle will result. Thus, the more the geometry of the articular surfaces changes from its physiological condition, the more the prosthesis depends upon the soft tissues for stability. Some authors have even suggested that the indication for operation should be narrowed to those with < 10° to 15° of varus or valgus.24-27 Regardless, a TAR prosthesis should be as anatomic as possible to mimic physiological joint motion and assist in ensuring proper ligamentous balancing.
What You Should Know About Edge Loading And Soft Tissue Balancing
Unquestionably, proper osseous and soft tissue balancing of the prosthetic ankle are integral components in attaining and maintaining functional alignment and stability of the TAR.28-33
Correcting osseous malalignment and addressing soft tissue imbalances at the ankle and foot during primary or revisional TAR procedure provides this functional balance. Failure to address these features properly may lead to edge loading in which asymmetric forces will directly and deleteriously affect the ultra–high molecular weight polyethylene (UHMWPE) spacer, and indirectly the prosthesis to bone interface, causing uneven and increased wear.34 Generally, the process of edge loading may increase the risk of prosthetic component complication and ultimately failure.28-33
One may assess ankle joint balance intraoperatively with the use of an assortment of instruments including spacer blocks, laminar spreaders and tensioning devices as well as the placement of trial components.34 Symmetrical soft tissue balancing during TAR is an important step in optimizing the mechanical balance of the ankle joint. Soft tissue contractures that result from frontal plane deformities can pose an especially difficult problem and the foot and ankle surgeon should have a standard procedure for managing such circumstances.
Soft tissue balancing, especially in the frontal plane, is crucial for long-term success and patient satisfaction. Correction of frontal plane imbalance, whether it is misalignment or instability, has traditionally involved a sequence of procedures that complement one another. The general theory of soft tissue balancing involves the release of the contracted soft tissue on the concave side and plication on the convex side of the ankle.28-33 Techniques for balancing the varus or valgus ankle during TAR include, but are certainly not limited to, osteophyte resection, soft tissue releases, soft tissue plication, tendon transfers, and osteotomies.
Essential Keys To Correcting Varus Imbalance
Varus imbalance correction during primary and revision TAR involves the release of medial tissues and reinforcement of lateral tissues (see photos 1 and 2). This may include:
• the removal of periarticular osteophytes and debridement of the medial, lateral and posterior gutters;
• release of the deltoid ligament from the medial malleolus and/or the medial talus and/or lengthening osteotomy of the medial malleolus;
• lengthening or recession of the posterior tibial tendon;
• correction of pedal deformities with a dorsiflexory first metatarsal osteotomy or lateralizing calcaneal osteotomy; and
• lateral ankle ligamentous plication and/or tendon transfer to reinforce lateral soft tissue restraint.28-37
As the flexor retinaculum is continuous with the superficial fibers of the deltoid ligament, it too can tether the rearfoot into varus following release of the deep deltoid. In these instances, one can perform a tarsal tunnel release with a flexor retinaculum release in order to provide for complete release of the concave side of a varus deformity or instability.34
In the case of retained lateral ankle instability, one may perform an open modified Broström–Gould lateral ankle stabilization as a primary procedure although researchers have also described an “all-inside” arthroscopic technique.38,39
Additionally, if the remaining lateral ankle ligaments are insufficient, then one may transect the peroneus brevis tendon as proximally as possible and transfer the tendon through a bone tunnel to the anterior aspect of the fibula. The surgeon would subsequently attach the tendon under tension to the talar neck in a modified Evans stabilization procedure.
In some cases, abnormal tension of the peroneus longus is present in longstanding varus deformity with lateral ankle instability, causing plantarflexion of the first metatarsal. In this instance, one may dissect the peroneus longus tendon through a small incision over the cuboid and perform tenodesis to the fifth metatarsal base.
How To Address Valgus Imbalance
Valgus imbalance correction may involve release of lateral tissues and reinforcement of medial tissues (see photos 3 and 4). This may include:
• removal of periarticular osteophytes and debridement of the ankle joint gutters;
• complete release of the lateral ligament complex from the lateral malleolus and/or lengthening osteotomy of the lateral malleolus:
• correction of pedal deformities with medializing calcaneal osteotomy or medial column, isolated or combined midfoot/hindfoot arthrodesis; and
• deltoid ligament plication and/or tendon transfer to reinforce medial soft tissue restraints.29-35,40-42
For patients in whom the remaining deltoid ligamentous structures are insufficient to provide for medial ankle stability, one may perform a reverse modified Evans procedure. This entails transecting the peroneus brevis tendon as proximally as possible, transferring it through a bone tunnel through the talar neck and attaching the tendon under tension to the anterior tibia. This provides a simple and reproducible medial ankle stabilization for the correction of longstanding valgus contractures, especially where medial structures are insufficient following TAR.
As the literature demonstrates, total ankle prostheses continue to evolve in design, technique and outcomes, showing ever increasing longevity. However, the success of TAR depends largely on a constellation of multifaceted factors including surgeon experience, appropriate patient selection and, perhaps most importantly, how closely the prosthetic design and its technical insertion can approximate normal biomechanics of the ankle joint. Several studies have shown that TAR is functionally closer to the normal ankle than ankle fusion.1–5
Still, a careful and comprehensive patient evaluation including clinical and radiographic assessment is of the utmost importance as a reference for proper procedural selection. There is an assortment of methods surgeons must be aware of and adept in to attain and retain structural alignment and stability with TAR procedures. Every surgeon performing TAR should have an awareness of the probable imbalances about the ankle and how to address them expediently in order to provide the most successful outcome possible.
Dr. Atves is a Fellow with the Pennsylvania Intensive Lower Extremity Fellowship at Premier Orthopaedic and Sports Medicine and the Pennsylvania Orthopaedic Center in Malvern, PA.
Dr. Miller is the Podiatric Residency Director at the Tower Health/Phoenixville Hospital in Phoenixville, PA. He is an Adjunct Associate Professor in the Department of Surgery at the Temple University School of Podiatric Medicine. Dr. Miller is the Director of the Pennsylvania Intensive Lower Extremity Fellowship Program at Premier Orthopedics and the Pennsylvania Orthopaedic Center in Malvern, PA.
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