Total ankle replacement (TAR)
Indications and Contraindications for TAR
Total ankle replacement (TAR) is an alternative to ankle arthrodesis for the treatment of end-stage ankle osteroarthritis (OA) in select patients with advanced painful arthropathy of all three main etiologies: primary, post-traumatic, and secondary. One of the main advantages of TAR compared with ankle arthrodesis is preservation of functional range of motion (ROM) which is sacrificed in ankle fusion. Improved ROM allows patients to better perform activities of daily living and possibly regain athletic activities.
The ideal candidate for TAR is an older (middle- to old-aged), reasonably mobile patient with no significant co-morbidities, a normal or low body mass index, adequate bone stock, a well-aligned and stable hindfoot, good soft tissues conditions, and no neurovascular impairment of the lower extremity.
Another indication for TAR is bilateral end-stage ankle OA. Bilateral ankle fusion may not be the most appropriate treatment option given its significant detrimental effects on gait and functional status of patients.1 This is particularly important to consider in patients with secondary OA due to hemochromatosis or hemophilia.2,3 Furthermore, in patients with previously performed subtalar, triple, and/or midfoot fusion the tibiotalar fusion would completely “stiffen” the hindfoot, while the TAR may preserve functional motion. It has been shown that the clinical outcome of TAR when combined with hindfoot fusion is comparable to that of ankle replacement alone.4
The relative contraindications for TAR are severe osteoporosis, history of osteomyelitis, diffuse osteonecrosis, or significant bone defect on the tibial and/or talar site. Previous long-term therapy with steroids or immunosuppressive substances may also reduce bone quality, resulting in compromised osteointegration of prosthesis components. Further relative contraindications for TAR include heavy physical work, medium level of sports activities (eg, tennis, jogging, and downhill ski), high body mass index, diabetes, and smoking.
Significant preoperative varus or valgus deformity (>10°) has also been seen as a contraindication for TAR. Doets et al found that preoperative deformity in the frontal plane is difficult to correct, causing instability and subluxation of the bearing, which may result in the prosthesis failure.5 Wood and Deakin observed in their study including 200 STAR implants that preoperative varus or valgus deformity >15° may cause edge loading of the mobile bearing.6 Therefore, they stated that this may be a relative contraindication for TAR.7
However, the preoperative hindfoot deformity should not be an absolute contraindication, as long as additional realignment procedures (supramalleolar and/or calcaneal osteotomies, ligament reconstruction, subtalar fusion) may correct the deformity.8-11 Karantana et al did not observe any differences in functional outcome and prosthesis component survivorship between patients with and without preoperative deformities as long the deformity was addressed at the time of prosthesis implantation.12,13 Daniels et al demonstrated that correction of moderate to severe varus deformities is possible and results in good functional outcome and stability of prosthesis components.14 Kim et al reported that the clinical outcome of TAR performed in ankles with preoperative varus deformity >10° is comparable with that of neutrally aligned ankles.15 However, the simultaneous surgical procedures addressing the preoperative deformity are necessary to achieve good results.15
The absolute contraindications for TAR include:
- Neuroarthropathy (Charcot foot)
- Non-manageable hindfoot malalignment
- Massive joint laxity (eg, patients with Marfan disease)
- Highly compromised periarticular soft tissues (eg, in patients with posttraumatic OA who underwent several previous surgeries)
- Severe sensomotoric dysfunction of foot/ankle
- Active soft-tissue or bony infection
Additionally, TAR should not be considered as the first-choice therapy in patients with a high level of functional demand (eg, contact sports).
While many authors suggest that previous ankle infection is an absolute contraindication for TAR,16-20 we do not confirm this idea. In a series of 17 consecutive patients who underwent HINTEGRA TAR, we achieved good functional results and did not observe any recurrence of infection (unpublished data). Eichinger et al and Espinosa and Klammer also recommend TAR in patients with ankle OA due to previous infection.21,22
In several studies, avascular necrosis of the talus has been identified as an absolute contraindication for TAR.17,23-29 However, it should be considered that some prosthesis designs offer the possibility to use the revision talar component. Also custom made components may be used replacing the whole body of talus.30
First-Generation Total Ankle Replacements
Since the 1970s, ankle arthrodesis has been recognized as having limitations regarding complication rate and functional outcome.31-33 In most articles addressing TAR history,22,31,32,34-38 the study by Lord and Marotte is described as the first clinical study with TAR patients.39
However, Muir et al described in 2002 outcome results in a 71-year-old male who underwent talar dome resurfacing with a custom Vitallium implant for post-traumatic OA in 1962.40 The clinical examination at 40-year follow up showed mild hindfoot malalignment with slightly decreased ROM (25° plantar flexion), AOFAS score of 85, no pain, and no activity limitation.40
In their study, Lord and Marotte used an inverted hip stem, which was implanted into the tibia.39 After the talus had been completely removed, they implanted a cemented acetabular cup in the calcaneus. This procedure was performed in 25 consecutive patients and only seven patients reported satisfaction postoperatively.41 Twelve of the 25 arthroplasties failed, and therefore the authors did not recommend the further use of this prosthesis design.41 At the time, the authors recognized the complexity of ankle biomechanics and concluded that a simple hinge prosthesis system with plantar flexion and dorsiflexion would be insufficient to mimic the normal ankle joint.41 Overall, the majority of first-generation prostheses were eventually withdrawn from the market because of high failure rates with subsidence, continued patient pain, or progressive alignment deformities.
The research published by these original investigators led to the development of the first-generation of TARs. These included:
- St. Georg-Buchholz ankle prosthesis, a semi-constrained prosthesis type introduced in 1973
- Imperial College of London Hospital prosthesis, a two-component, constrained implant with a polyethylene tibial component 42,43
- Irvine Ankle TAR (Howmedica prosthesis), one of the first early ankle prostheses where a special talus anatomy was regarded, with the prosthesis designers performed anatomical measurements of 32 tali to establish the morphology of talus 44
- Richard Smith TAR, a non-constrained so-called “ball-and-socket” (spherocentric) prosthesis that was introduced in 1975 45
- Conaxial Beck-Steffee ankle prosthesis, a very constrained prosthesis type 46
- Newton ankle implant, a non-constrained cemented prosthesis including the high density polyethylene tibial and Vitallium talar components
- Bath-Wessex TAR, an unconstrained, two-component total ankle design.
- Mayo TAR, a highly congruent two-component design including a polyethylene tibial component with cement fixation 47,48
- Oregon ankle prosthesis, a single-axis, two-component TAR design
- Thompson-Richard prosthesis, a two-component semi-constrained cemented implant that included a polyethylene tibial component with a concave articular surface and a lip on each side 1,49
- New Jersey or Cylindrical TAR, developed by a bioengineer and an orthopaedic surgeon 50
Second-Generation of Total Ankle Replacements
Based on research demonstrating the high complication and failure rates and lack of patient satisfaction with the first-generation of TARs, a second-generation was developed, including:
- The Agility ankle prosthesis (Depuy), the first of a new generation of ankle prostheses, has been used since 1984.51 The Agility has been approved by the FDA and is currently the most widely used ankle prosthesis in the United States, and with more than 20 years of implantations, it has the longest follow up of any fixed-bearing TAR.52 The Agility ankle prosthesis is a semi-constrained, two-component prosthesis consisting of a titanium tibial and cobalt-chromium talar component. For improved osseous integration, both components have a sintered titanium bead surface. As this prosthesis is a two-component system, a modular polyethylene insert is locked into the tibial component. In 2007, the Agility LP Total Ankle System was introduced with some modification of its design.52 All improvements were designed after careful analysis of published data to improve the outcome and avoid mid- and long-term complications. The new prosthesis features include: a redesigned broad-based talar component (to avoid subsidence of the tibial component, especially in patients with nonunion of tibiofibular syndesmosis), the ability to mix and match component sizes to match native anatomy, and a front-loading polyethylene (easier surgical technique for exchange of insert).52
- The Buechel-Pappas ankle prosthesis is the first reported three-component prosthesis with a mobile bearing. It is the evolution of the first-generation New Jersey ankle prosthesis.38,50 In the first Buechel-Pappas design (Mark I), the anterior-posterior constraint between the tibial and mobile bearing components was removed. This shallow sulcus design allowed more ROM without compromising the intrinsic sagittal stability of the ankle replacement. Postoperative complications included delayed wound healing, reflex sympathetic dystrophy, deep infection, mobile bearing subluxation, talar component subsidence, severe bearing wear, malleolar fracture, and osteolysis. Analysis of complications from using this prosthesis led to modifications resulting in the Mark II Buechel-Pappas prosthesis. This new design (also known as the deep sulcus design) included two fins, a thicker meniscal component, and deeper sulcus with a gap in the plastic.
- The Scandinavian Total Ankle Replacement (STAR) was developed as a two-component, anatomic, unconstrained resurfacing ankle prosthesis with congruent parts covering the medial and lateral facet joints.53 Since 1986, the tibial part of the STAR prosthesis has included a polyethylene component.53 This modification was performed to minimize rotational stress at the implant-bone interface. The current design of the STAR prosthesis is a congruent, cylindrical, three-component prosthesis. Initial osseous integration of the prosthesis is secured by a single fin on the talar side and by two cylindrical fins on the tibial side. Both metallic components have hydroxyapatite coated surfaces. The STAR prosthesis, one of the most popular TARs used in Europe, has one of the longest histories in ankle replacement surgery, with several modifications made during its clinical use.54
Results of First- and Second-Generation TARs
The majority of first-generation TAR designs were two-component prostheses that used cement fixation on both the talar and tibial sides. The reason for cement fixation was simple: in the 1970s cementless fixation had not been widely used in knee and hip prostheses and cement fixation led to acceptable early component stability. However, an extremely high complication rate was observed with increased incidence of loosening, wide osteolysis, subsidence, and mechanical failure of prosthesis components.
Cement fixation required a larger bone resection. Therefore, bone quality at the cement-bone interface was not optimal, as the main load transfer occurred on the weaker methaphyseal bone. Most TARs included a polyethylene concave tibial and a metal convex component for the talus, usually made using cobalt chrome alloy. Both types of prostheses – constrained and unconstrained – were available at that time. One design feature of most first-generation TAR designs was a tibial component that was significantly larger than the talar. The idea was to allow physiological dorsi-/plantar flexion ROM as well as axial rotation. However, due to low intrinsic stability, significant increases in shear forces occurred. This was especially true in patients with chronic ligamental instability which resulted in prosthesis loosening early on.
Most clinical studies addressing outcomes in patients who underwent first-generation TAR were case reports or studies including between 20 to 40 patients.55 Another critical factor was the short follow-up periods reported (mostly 5 years or less). Patient satisfaction with first generation TAR was reciprocally proportional to the length of follow-up and varied between 19% and 81%.55
Generally, the clinical results of first-generation TAR were highly discouraging. The alarmingly high prosthesis component failure rate along with other complications like wound healing and unacceptable functional results were the reasons that foot and ankle surgeons were advised to use ankle fusion as the primary treatment option for ankle OA.29,48,56-60 Failure analysis of first generation TARs showed that only significant improvements in prosthetic design, change of fixation (elimination of use of cement), and improved anatomical access would change arthroplasty outcomes.61
An analysis of the main failure reasons of the first-generation TAR designs was crucial for the development of the second-generation ankle prostheses. More conservative and sparing bone cuts and the elimination of bone cement have significantly improved problems with component loosening. New biologic interfaces with special porous coatings for bony ingrowth and/or adding of hydroxyapatite were investigated as another possible method to ensure the primary prosthesis fixation.62,63 To reduce subsidence, second-generation ankle prostheses were designed to increase the surface area of the metallic components. Increased surface area sought to decrease the average local contact pressure and pressure peaks during gait.
The three main second generation TAR designs – Agility, Buechel-Pappas, and STAR prostheses – have been implanted with encouraging mid- and long-term results.64 Positive clinical results, high patient satisfaction, and acceptable survivorship of prosthesis components presented at national meetings and published in orthopaedic literature led to rethinking that ankle fusion may not be the only one reasonable treatment option for patients with severe ankle OA. The continued critical review of second-generation implant failures and biomechanical studies provided important data that led to the development of modern TAR designs.
Modern TAR Designs (Third-Generation TARs)
- The Salto Total Ankle was developed between 1994 and 1996 by Michel Bonnin.65 This TAR represents the third-generation of cementless meniscal-bearing designs. The tibial component has a flat surface toward the mobile bearing, allowing its free translation and rotation. The 3-mm medial rim is designed to avoid insert impingement against the medial malleolus. For osseous integration, the component has a keel and a fixation peg. The specific shape of the talar component mimics the natural talar geometry with the anterior width being wider than the posterior, and the lateral flange having a larger curvature radius than the medial. The mobile bearing is manufactured from ultra-high-molecular-weight polyethylene (UHMWPE) and has full congruency with the talar component in flexion and extension. All components are available in three sizes.65
- The HINTEGRA TAR is an unconstrained, three-component system that provides inversion-eversion stability and was designed in 2000 by Beat Hintermann, Greta Dereymaeker, Ramon Viladot, and Patrice Diebold. The mobile bearing provides axial rotation and normal flexion-extension mobility.66-68 The HINTEGRA TAR includes two metallic components and an ultrahigh-density polyethylene mobile bearing. The non-articulating surfaces have a porous coating with 20% porosity and are covered by titanium fluid and hydroxyapatite. The tibial component has a flat, 4-mm thick loading plate with pyramidal peaks against the tibia. Additional stability may be achieved by fixation with two screws. The talar component is conically shaped with a smaller radius medially than laterally, mimicking the normal anatomy of talus. It has 2.5-mm high rims on each side that ensure stable positioning and guide the anteroposterior translation of the mobile bearing. The anterior shield of this component increases primary bone support, especially in cases with weaker bone, and may prevent the adherence of scar tissue and avoid restriction of ROM in cases with arthrofibrosis.67
- The Mobility Ankle System was developed by Pascal Rippstein, Peter Wood, and Chris Coetzee. This is a three-component Buechel-Pappas type prosthesis with a short, conical tibial stem. The talar component of the Mobility implant resurfaces the superior dome of the talus, while the medial and lateral aspects of the talus remain untreated (unlike the Buechel-Pappas prosthesis). The talar component has a central, longitudinal sulcus and two fins, enhancing its intrinsic stability. The non-articulating surfaces are porous coated with a titanium spray.
- The Ramses TAR was developed in 1987 and first implanted in 1989 by a French design group.69,70 The Ramses TAR is a three-component, semi-constrained prosthesis with the high-density mobile bearing.70 Initially, a cemented fixation of prosthesis was used between 1980 and 2000.
- Since its introduction in 1975, the TNK total ankle replacement has undergone many modifications to address the material of the components (stainless steel, polyethylene, alumina ceramic), coating (without/with hydroxyapatite), and fixation (cement/cementless fixation). Currently, this is the only TAR design containing alumina ceramic components. While the studies by the designer reported favorable results using the third-generation TNK prosthesis,71,72 independent studies addressing TAR results in patients with rheumatoid OA show less promising results.73,74
- The Ankle Evolutive System (AES) TAR is a further development of the Buechel-Pappas-type prosthesis. This design has a modular stem and allows hemi-replacement of the medial tibiotalar and talofibular joints.75 This prosthesis has been widely used in England and France,76,77 and has also been introduced in Norway.78 Recent studies of the AES TAR reported a high rate of osteolytic lesions.79-82 It is still unclear whether the osteolysis process is a result of failure of the hydroxyapatite coating of the metal components or failure of the mobile bearing. As a result of independently published results showing high osteolysis rate, the AES prosthesis has been withdrawn from the market.83
- The BOX TAR was developed in the late 1990s by Leardini et al. This prosthesis is a three-component implant with metal components fixed to the proximal talus and the distal tibia and interposed UHMWPE meniscal bearing. The biomechanical development of this prosthesis type has been well documented in the literature by its designers.84-86
- The ESKA ankle is a two-component prosthesis designed for cementless implantation between 1985 and 1989.87,88 The following features were included to improve biomechanics of the replaced ankle: cementless implantation and porous-structured implant surface for faster osteointegration, shear force reduction by shape design of both metallic components, and easy replacement of the polyethylene without disturbing prosthesis anchoring.87,88 Because of the ridge-like shaping and its transverse anchoring peg in both metallic components, a lateral or in special cases, medial malleolar approach has to be used for implantation.87
- Other modern TAR designs include:
- German Ankle System, a three-component prosthesis allowing rotation around each of the three possible movement axes
- Alphanorm Total Ankle Replacement, a non-constrained Buechel-Pappas type design with a 90° tibial stem without inclination
- TARIC Total Ankle Replacement, which has a titanium coating and is optionally available with an additional hydroxyapatite coating
- INBONE TAR, a fixed-bearing, two-component total ankle system with a modular stem system for both metallic components
The TAR Learning Curve
It has been shown that there is a steep learning curve associated with performing TAR. The following intraoperative complications have been commonly reported:
- Medial and/or lateral malleolar fractures
- Laceration to the tendons (posterior tibial tendon, flexor digitorum/hallucis longus)
- Nerve injures (deep/superficial peroneal nerve) 89-92
The influence of surgeon experience on complication rates in patient receiving the Agility prosthesis was examined by Saltzman et al.91 The perioperative records of the first 10 cases of nine surgeons with different training levels were recorded. The authors did not identify any specific training method that significantly decreased complication rates.91
Myerson and Mocked performed a retrospective radiographic and chart review of 50 arthroplasties performed by the same surgeon using the Agility prosthesis.90 Patients were divided into two groups due to surgeon’s experience, each including 25 patients. The number of minor wound complications decreased from six in the first group to two in the second group. Also, the number of intraoperative fractures was different in favor of the second group (five vs. two fractures). The nerve or tendon lacerations (n=4) all occurred in the first group. Regarding the overall decreased complication rate in the second group of 25 patients, the authors stated that there was a notable learning curve in TAR performance.90
A similar study with 50 patients who underwent Agility TAR was performed by Schuberth et al.93 In this study, patients were also divided into two 25-patient groups. There was a significantly decreased rate of the following complications with increased surgeon experience: medial and lateral fractures, major revisions, and malpositioning of prosthesis components.93 Schutte and Louwerens reported their initial results obtained in 49 patients who received a STAR prosthesis.92 The following intraoperative complications were detected: six fractures of the medial and two of the lateral malleolus, three fractures of the distal tibia, one injury of the peroneal nerve, and two malpositions of the tibial and two of talar components. Based on these numbers, the authors concluded that TAR should be performed only by an experienced orthopaedic foot and ankle surgeon.92
The aforementioned studies addressed intraoperative complications in patients receiving second-generation TARs.89-92 Similar studies have been conducted for patient groups receiving third-generation TARs.89,94 Lee et al addressed the perioperative complications in the 25 initial patients to receive the HINTEGRA prosthesis and compared these results with those from a subsequent 25 cases.89 In the first group, perioperative complications occurred in 60% of all cases, while in the second group, only five complications (20%) were observed. All major complications (deep infection and aseptic loosening) occurred in the first group. The rate of minor complications (fractures, minor wound problem, nerve/tendon injuries, and heterotopic ossifications) significantly decreased in the second group. However, the authors were unable to show a decrease in the number of malpositioning of prosthesis components as a result of increased surgeon experience.89 The same working group compared the perioperative complication of the HINTEGRA total ankle system with the MOBILITY total ankle system.94 The authors did not find any differences in perioperative complications between the two total ankle systems, but medial malleolar fractures did occur more frequently when using the MOBILITY prosthesis.94
Recently, Reuver et al addressed short-term results of TARs performed in low-volume arthroplasty centers.95 In total, 64 TARs were performed using Salto implants between 2003 and 2007 at four low-volume centers. Fifty-five patients (59 ankles) were reviewed at a mean follow up of 36 months. Seven ankles had to undergo revision surgery – two revision arthroplasties and five fusions – because of loosening, and two cases of deep infection resulting in a survivorship of 86% at final follow up. Significant pain relief and functional improvement were observed in this review, as assessed using VAS and AOFAS score. The authors felt confident that results of TAR performed in low-volume centers are comparable to most high-volume centers. However, the survival of implanted components was significantly lower, especially regarding the relatively short follow up.95
In summary, despite the encouraging results reported by studies using modern third-generation TAR, we believe, that TAR should be limited to foot and ankle orthopaedic surgeons with special training and adequate experience in arthroplasty techniques.
See Special Situations for Total Ankle Replacement for more on the use of TAR in painful ankle arthrodesis and simultaneous bilateral TAR.
Modern TAR Designs: Promising Results
Stengel et al performed a systematic review and meta-analysis to address the efficacy of TAR with meniscal-bearing implants.96 The following inclusion criteria were defined for this study: a minimum sample size of 20 subjects, at least 1 year of follow up, and a clinically relevant study endpoint. In total, 18 studies with 1,086 patients were included in the review. Most patients experienced significant functional improvement (average 45.2 points using standardized 100-point ankle and hindfoot scores) and a mild increase of ROM (mean increase = 6.3%, 95% CI, 2.2% - 10.5%). Weighted complication rates ranged from 1.6% (deep infection) to 14.7% (impingement). Secondary surgeries were necessary in 12.5% of all patients. Ankle fusions were required in 6.3% of patients due to implant failure, resulting in 1- and 5-year survivorship of 96.9% (95% CI, 94.9% - 98.8%) and 90.6% (95% CI, 84.1% - 97.1%), respectively. The data of this meta-analysis showed that TARs using current three-component designs provide an acceptable benefit-risk ratio. However, the results should be interpreted with caution due to non-optimal methodological quality, sample sizes, and short follow ups.96
SooHoo et al compared reoperation rates following ankle fusion and TAR using California’s hospital discharge database.97 A total of 4,705 ankle fusions and 480 TARs were included in the review during the 10-year study period (1995 through 2004). It was shown that patients who underwent TAR had an increased risk of periprosthetic infection. The rates of major revision surgery after TAR were 9% at 1 year and 23% at 5 years compared with 5% and 11% following ankle fusion. However, TAR was shown to have advantages in terms of functional results.97
In another study from 2004, SooHoo and Kominski performed cost analyses of TAR compared to ankle fusion.98 The authors performed a thorough literature review to identify possible outcomes and their probabilities following ankle fusion versus TAR. They found that TAR generated total expected lifetime treatment costs of $16,568, which is $6,990 more than costs following ankle fusion. Furthermore, TAR had an incremental cost-effectiveness ratio in the reference case of $18,419 for each quality-adjusted life year gained.98
In 2007, Haddad et al performed a systematic literature review addressing the intermediate and long-term outcomes of interest in TAR and ankle fusion.99 In total, 10 studies including 852 TAR patients and 39 studies including 1,262 with ankle fusion were analyzed. The authors showed that 38% of patients had excellent postoperative results, 30.5% had good results, 5.5% had fair results, and 24% had poor results. The corresponding values in patients who underwent ankle fusion were 31%, 37%, 13%, and 13%, respectively. The 5- and 10-year survivorship for TAR was 78% (95% CI, 69.0% - 87.6%) and 77% (95% CI, 63.3% - 90.8%), respectively. The revision rate in patients with TAR and ankle fusion was 7% (95% CI, 3.5% - 10.9%) and 9% (95% CI, 5.5% - 11.6%), respectively. The main reasons for revision surgery were component loosening and subsidence in the TAR group and non-union in the fusion group. The results of this study suggest that the intermediate outcome is comparable in both procedures.99
In 2010, Gougoulias et al performed a systematic review of the literature to address the outcome of TAR implants currently in use.100 In total, 13 Level IV peer-reviewed studies were included, reporting the outcome of 1,105 TARs:
- 234 Agility
- 344 STAR
- 153 Buechel-Pappas
- 152 HINTEGRA
- 98 Salto
- 70 TNK
- 54 Mobility implants
Postoperatively, a remarkable portion of patients still had residual pain (range, 27% - 60%). Also, superficial wound complications and deep infection were often reported, with rates up to 14.7% and 4.6%, respectively. Overall failure rate at 5 years had a wide range, 0% and 32%. In general, most patients experienced significant functional improvement as assessed by the AOFAS score. However, the postoperative improvement of ROM was relatively small (0° - 14°). Therefore, the patients should be informed that significantly improved ROM is not one of the postoperatively expected benefits of TAR.100
Recently, Slobogean et al compared preference-based quality of life in patients with end-stage ankle OA treated with TAR or ankle fusion.101 The quality of life of 107 subjects was assessed using health state values derived from SF-36 (SF-6D transformation). The mean baseline SF-6D health state value in the TAR group was 0.67 (95% CI, 0.64 – 0.69) and in the ankle fusion group 0.66 (95% CI, 0.63 – 0.68). At 1 year follow up, both groups had significant and comparable improvements with 0.73 for the TAR group (95% CI, 0.71 – 0.76) and 0.73 for the ankle fusion group (95% CI, 0.70 – 0.76).101
To date, there has been no clear evidence in the literature that three-component TAR designs are superior compared with two-component designs.102,103 Also, the high-quality comparative studies addressing postoperative outcomes in patients who underwent TAR vs. ankle fusion are rare. There has been only one prospective, controlled, comparative surgical trial performed including both patient cohorts.104 Future high-quality prospective, randomized, controlled studies would significantly help to establish the clinical practice guidelines needed for surgeons to make a correct decision about treatment choice.104-108
TAR is increasingly gaining acceptance as a valuable option for treating patients with end-stage ankle OA. Current reports of this procedure show consistently good to excellent mid-term results with significant pain relief, good functional outcomes, and high patient satisfaction.100,109,110 The high failure rate of the first-generation ankle prostheses has been thoroughly analyzed and the TAR designs have been significantly improved. Current fixation techniques without cement have become the gold standard using “biological surfaces” (eg, introduction of hydroxyapatite in the 1990s) for better osseous integration of metallic prosthesis components.110,111
One of the main principles of TAR surgical technique is preserving adequate bone stock.111 It has been widely recognized, with regard to bone resection, that less is more. An extensive bone resection may drastically limit the revision surgery in case of failure of the TAR, especially on the talar side. Also, the bone resection for implantation of prosthesis components should consider the anatomical inner structure of bone and especially trabecular microarchitecture for optimal load transfer.111 The optimal load transfer is very important because this would avoid pathologically increased pressure peaks, which may cause loosening and subsidence of components. Therefore, the load transfer on a tibial component that incorporates circumferential bony support may be superior to that of a stemmed component in the long term.110 Furthermore, the natural articular geometry of the ankle should be considered during the design of any ankle prostheses.
Modern implants try to retain the radius of the curvature of the talus, resulting in improved and a more natural ROM.111 The modern TAR is not only a resurfacing procedure of the osteoarthritic ankle but also a restoration of normal biomechanics of the entire hindfoot.110 If necessary, additional surgeries should be performed to achieve the appropriate ligamental and osseous balancing of the hindfoot.8-10,110 Failure to correct hindfoot alignment or undercorrection of hindfoot deformity can cause a significant increase of translation forces and movements during gait. Especially in patients with remaining valgus misalignment, this may lead to prosthesis failure because valgus misalignment is tolerated more poorly than varus.110
In conclusion, TAR with current devices, equipment, and techniques has improved considerably over the past several decades to show that the ankle fusion is no longer the “gold standard” treatment for all patients with severe end-stage ankle OA. Future biomechanical and clinical studies addressing the outcomes and biomechanical properties of TAR should be continued with the aim of improving current TAR designs.
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