Total ankle replacement is a common procedure with predictable results and is becoming increasingly more frequent for the treatment of ankle osteoarthritis. Registry data from different countries have shown an increase in the use of TAR for ankle osteoarthritis following traumatic event, rheumatoid arthritis and other conditions that cause a progressive loss of integrity of the tibio-tarsal joint [9].
In 2012 the Food and Drug Administration approved a new type of prothesis: the Zimmer Trabecular Metal Total Ankle (Zimmer Inc.,Warsaw, IN). This implant was designed to allow the implant trough lateral approach. The main objective was to introduce a prosthesis that could decrease surgical complication linked to delayed wound healing and to aid the surgeon who was more familiar with the lateral approach used for ankle arthrodesis [10]. Furthermore, this model is equipped with trabecular metal material, which was developed to simulate the normal architecture of the cancellous bone. Bobyn et al. have shown that the porous structure of this material permits bone ingrowth in most of the available surface, thus increasing the stability [11]. Even though the prosthesis in the U.S. is implanted with cement fixation, it is commonly used off-label without cement and fixation relies on bony ingrowth at the bone-implant interface.
The risk of wound healing issues is present in 2-40% of cases of TAR performed throw anterior approach [12, 13]. Early TAR procedures had problems with wound healing but improvements in techniques, materials and postoperative care have reduced the incidence of infections. The percentage of incidence throw lateral approach should be theoretically lower due to the incision lying between two angiosomes and therefore causing a minor vascular damage of the skin.
The other main advantage of a lateral approach is the possibility to have a direct visualization of the rotational center of the joint, and therefore allowing a more accurate bone resection and reconstruction of the ankle joint anatomy and angles.
Some authors report the possibility of a major stability of the components due to the bone perpendicular orientation of the tibial and talar trabeculae that may improve the force transfer from implant and decrease the shear forces at the bone-implant interface [3]. Those data however, don’t have a statistical proof at the moment.
The complications of a lateral approach are associated to the fibular osteotomy with subsequent risk of delayed union, nonunion, change in fibular length, vascular and neural damage and painful hardware. Tennant et al reported the risk of lesion to the perforator peroneal artery during this approach in a cadaveric study [14]. Thus, the risk of blood vessels injury should be taking into account.
In case of a revision a new osteotomy might be necessary if the entire prosthesis must be substituted, with an increased risk of complications. If the damage is isolated to the polyethylene it might be possible to do the exchange through anteromedial arthrotomy or lateral approach.
A study by Usuelli et al reported the possibility of a lower infection rate in total ankle replacement trough lateral approach rather than anterior approach. The incidence was reported to be of 4.9% vs 2.9% for superficial infection and 3.7% vs 1.4% for deep infections, for the anterior and lateral approach respectively. Those differences were not found to be statistically significant [15].
The survival rate of the Zimmer TM TAR has been reported by few studies with insufficient follow-up to be representative of the effectiveness of this prosthesis.
Barg et al. reported a 93% survival at 36 months with a decrease in VAS pain score from 7.9 ± 1.3 to 0.8 ± 1.2 after surgery [16]. Maccario et al. reported a 100% survival rate at 24 month and a decrease in VAS from 7.42 to 1.42 [17]. Tan reported 20 cases of TAR through lateral approach with no failures during the first 12 months, good intra-operative alignment including 8 patients (42.1%) with a mean varus of 10.5± 4.2° and 4 (21.1%) with a mean valgus of 15.5± 8.6° preoperatively. In both groups, the angles decreased to approximately 2° postoperatively [18]. In a recent paper, Usuelli et al. reported a 98.9 % survival rate at 24 months with a rate of major complications requiring surgery of 11.2 %. We found in our patients a survival rate of 100% with a decrease in VAS from 7.81 pre-operatively to 2.29 post-operatively and major complications rate of 23.3 % [19].
In this study, four patients were younger than 40 years old. Younger patients show higher activity level which can theoretical influence polyethylene wear. Therefore, an important concern in these patients is an expected higher revision rate. In literature, there are no studies reporting high long-term survival (> 20 years) of TARs, and we therefore consider younger age a contraindication for TAR not through an absolute age limit, but by taking into consideration the need of higher survivorship expectancy of the implants. When considering a younger patient for TAR, the possibility of future revisions through a revisional arthroplasty or salvage fusion, should be discussed.
A major concern with the lateral approach TAR is associated with fibular osteotomy and ligament sectioning [20]. DeVries et al. reported in a previous study of 16 patients with Zimmer TM TAR, good overall alignment even with valgus or varus hindfoot deformity [10]. However, 4 patients (25%) experienced complications related to the fibular osteotomy (non-union, delayed union, infection). In this study, fibular nonunion was present in two patients and revisional surgery was necessary in one. We described also, the first post-operative case of ankle dislocation after TAR. Despite the challenging treatment, which included reduction and temporary fixation, good mid-term outcome was achieved at the last follow-up. Considering these results, the lateral approach provides a good visualization of the joint but the risk of fibular nonunion and instability should be kept in mind when considering this approach.
The major complication rate in this study (23.3%) was similar to previous studies with different TAR systems. Hofmann et al. reported a major complication rate of 21% with the most common cause for reoperation of gutter impingement [21]. Nunley et. al reported 17% of patients requiring additional procedures after STAR system [22]. In a previous paper about Intramedullary-Fixation Total Ankle Arthroplasty (INBONE I/II), the reoperation rate due to postoperative complications in the short-term follow-up was 24% [23].
Radiolucent lines are a cause for concern after TAR. Barg et al. reported a high rate of radiolucent lines in the tibial and talar-implant interface of 34.5% and 12.7% respectively [24]. Interestingly, we observed radiolucent lines in 10% of the patients. Longer follow-up studies are warranted to understand the real nature of this radiological signs.
The radiographic angles used to assess component’s migration were stable at the last follow-up and comparable to the results found by Haytmanek et al [25]. The authors reported a mean value of 87.5 for the α angle, 88.1 for the β angle and 17.4 for γ angle.
The clinical results of this group of patients are similar to the results for pain outcomes reported in the meta-analysis by Zaidi et al., in which the mean pooled summary VAS scores decreased from 7.4 preoperatively to 1.6 postoperatively [26]. In a previous paper by Usuelli et al. the AOFAS hindfoot score of 67 patients increased from 32.8 to 85.0 [27].
In a previous study, Hsu assessed the clinical outcomes with use of AOFAS ankle-hindfoot score after both INBONE I and II total ankle arthroplasties [23]. The mean overall improvements in the AOFAS score was 43.2 points with most of the patients very satisfied with the final result. Nunley reported a mean change in the AOFAS summary score of 52 points, after the STAR arthroplasty [22]. We report similar results with an increase from 40.95 to 86.38 with a mean increase of 45.22 (range from 26 to 57).
We acknowledge the limitations of this study due to the low number of patients, its retrospective nature and limited follow-up time.