The Journal of Foot & Ankle Surgery 50 (2011) 641–647

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The Journal of Foot & Ankle Surgery

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Early Clinical Results of the BOX Ankle Replacement Are Satisfactory: A Multicenter Feasibility Study of 158 Ankles

Sandro Giannini, MD 1,2, Matteo Romagnoli, MD 1, John J. O’Connor, PhD 3, Fabio Catani, MD 1,2, Livio Nogarin, MD 4, Bruno Magnan, MD 5, Francesco Malerba, MD 6, Leo Massari, MD 6, Marco Guelfi,MD7, Luigi Milano, MD 8, Antonio Volpe, MD 9, Arturo Rebeccato, MD 10, Alberto Leardini, DPhil 11

1 Orthopaedic Surgeon, Department of Orthopaedic Surgery, Istituto Ortopedico Rizzoli, Bologna, Italy 2 Orthopaedic Surgeon, Movement Analysis Laboratory, Istituto Ortopedico Rizzoli, Bologna, Italy 3 Full Professor, University of Oxford, Oxford, England 4 Orthopaedic Surgeon, Casa di Cura S. Clemente, Mantova, Italy 5 Orthopaedic Surgeon, Department of Orthopaedics, University of Verona, Italy, Italy 6 Orthopaedic Surgeon, Clinica Universitaria S. Anna, Ferrara, Italy 7 Orthopaedic Surgeon, Dipartimento Caviglia Piede, Gruppo Policlinico di Monza, Clinica S. Gaudenzio, Novara, Italy 8 Orthopaedic Surgeon, Clinica Cellini, Torino, Italy 9 Orthopaedic Surgeon, Casa di Cura Abano Terme, Abano Terme, Italy 10 Orthopaedic Surgeon, Ospedale San Bortolo, Vicenza, Italy 11 Senior Researcher, Movement Analysis Laboratory, Istituto Ortopedico Rizzoli, Bologna, Italy article info abstract

Level of Clinical Evidence: 2 A new design for a 3-part ankle replacement was developed in an effort to achieve compatibility with the Keywords: naturally occurring ligaments of the ankle by allowing certain fibers to remain isometric during passive AOFAS hindfoot-ankle score motion. In order to test the design concept clinically, 158 prostheses were implanted in 156 patients within prosthesis a 9-center trial and were followed up for a mean of 17 (range 6 to 48) months. The mean age at the time of range of motion surgery was 60.5 (range 29.7 to 82.5) years. Outcome measures included the American Orthopaedic Foot & revision Ankle Surgery hindfoot-ankle score and range of motion measured on lateral radiographs of the ankle. The surgery talus preoperative American Orthopaedic Foot & Ankle Surgery score of 36.3 rose to 74.6, 78.6, 76.4, and 79.0, tibia respectively, at 12, 24, 36, and 48 months. A significant correlation between meniscal bearing movement on total ankle replacement the tibial component (mean 3.3 mm; range 2 to 11 mm) and range of flexion at the replaced ankle (mean 26.5; range 14 to 53) was observed in radiograms at extreme flexions. Two (1.3%) revisions in the second and third postoperative years necessitated component removal (neither were for implant failure), and 7 (4.4%) further secondary operations were required. The results of this investigation demonstrated that non- anatomic–shaped talar and tibial components, with a fully conforming interposed meniscal bearing, can provide safety and efficacy in the short term, although a longer follow-up period is required to more thor- oughly evaluate this ankle implant. Ó 2011 by the American College of Foot and Ankle Surgeons. All rights reserved.

In the early to mid-1970s, total ankle arthroplasty (TAA) was ankle, but long-term results of the pioneering designs were disap- introduced (1,2) as a possible alternative to arthrodesis for the pointing (3–5). More modern designs have produced better results, treatment of severe erosions of the articular surfaces of the human contributing to a renewed interest in TAA over the past decade (6–17), but TAA still has not been associated with the degree of success Financial Disclosure: The Istituto Ortopedico Rizzoli and 1 of the authors (J.J.O.) observed with total hip and total knee arthroplasty procedures. have a licensing arrangement with the company producing the prosthesis, under Recent publications from Swedish (18), Norwegian (19), and New which they receive royalties. However, the study was performed on a voluntary basis, and the company was not involved in any of the phases (i.e., planning of the experi- Zealand (20) registries revealed a steady annual revision rate of 2% to ment, collection or analyses or interpretation of the data, or writing of the manu- 3%, whereas a similar population in California showed a 4.6% annual script). revision rate (21). Although recent reviews (22–26) recommend Conflict of Interest: Four of the authors (S.G., J.J.O., F.C., and A.L.) are inventors of arthroplasty instead of arthrodesis, they point out that the clinical the patent for the device here analyzed. results of current ankle implant designs are still not fully satisfactory. Address correspondence to: Alberto Leardini, DPhil, Movement Analysis Laboratory, Istituto Ortopedico Rizzoli, Via di Barbiano 1/10, 40136 Bologna, Italy. An inadequate understanding of ankle function and the structures E-mail address: [email protected] (A. Leardini). guiding ankle motion in the natural state, namely the ligaments and

1067-2516/$ - see front matter Ó 2011 by the American College of Foot and Ankle Surgeons. All rights reserved. doi:10.1053/j.jfas.2011.06.003 642 S. Giannini et al. / The Journal of Foot & Ankle Surgery 50 (2011) 641–647 articular surfaces, as well as insufficient restoration of these functions in the prosthetic joint may be responsible for the limited range of postoperative joint mobility that is commonly observed after TAA (9). Recently, a newly designed TAA was developed, in which the shape of the articular surfaces in the sagittal plane were intended to natu- rally interact with the preserved ankle ligaments (27–29). The ankle implant was designed based on clinical experience as well as findings derived from a number of investigations, including studies that analyzed ankle function in cadaver specimens, in virtually unloaded conditions, and in mathematical models (30–34). The results of these investigations showed that the articular surfaces and ligaments of the ankle interact together in a complementary and mutually compatible manner. A key feature of the surface and ligament interaction of the new ankle implant is that it allows fibers within the calcaneofibular and tibiocalcaneal ligaments, namely the central superficial fibers of the deltoid ligament complex, to remain isometric over the range of passive motion while the remaining ligamentous fibers tighten only at the limits of plantarflexion or dorsiflexion. Previous approaches to the design of TAA implants focused almost exclusively on the geometry of the prosthetic components in relation to the morphological features of the intact articular surface of the talus (1,12,35,36). Moreover, mathematical analyses showed that either fixed articular surfaces should have anatomic shapes, or both Fig. 1. The 3 components of the BOX ankle prosthesis, immediately after implantation: tibial component (above), meniscal component (in between), talar component (below). should be nonanatomic (27–29). Currently available 3-part ankle prostheses approximate the natural convexity of the talus, whereas the tibial component of these devices takes on a nonanatomic flat The meniscal bearing is machined from super-pressed sheets of PUR 1020 (Finsbury fi – Orthopaedics Ltd), a low-calcium medical-grade ultra-high molecular-weight poly- con guration (6,14,15,17,37 40). This combination of anatomic and ethylene. The bearing is placed in gas-impermeable film packaging, evacuated and back nonanatomic surfaces cannot be compatible with the retained liga- filled with nitrogen before sterilization by gamma irradiation with a Co60 source to 25- À ments (27,28). 35 kGy, giving a sterility assurance level of 10 6. The bi-concave meniscal bearing fully With the aforementioned information in mind, we hypothesized conforms to the corresponding highly polished tibial and talar surfaces, irrespective of joint position. Fully conforming meniscal bearings minimize polyethylene wear in knee that a ligament-compatible TAA design could achieve satisfactory replacement (43,44) and are likely to do so in ankle replacement. The proximal surface clinical results on par with or better than results achieved with more of the bearing is slightly longer posteriorly so that contact area is maximized, and conventional ankle prostheses. Having established the feasibility of double concavity ensures entrapment of the meniscus. In essence, the difference the operation in a number of cases, the designers invited surgeons between maximum and minimum thicknesses, which is similar to the design of from 8 other hospitals to participate in a prospective, multicenter currently available 3-part implants, remains aligned despite the larger radius of curvature of the talar arc in the sagittal plane. The minimum thickness of the central cohort study, the results of which we present in this report. component (meniscal bearing) varies in 1-mm increments from 5 to 8 mm, and the most appropriate thickness is chosen to adjust ligament tension after implantation of the tibial and talar metallic components. The same components are used for left and right ankles and are currently available in 3 different sizes. It is recommended that the Patients and Methods tibial and talar components be matched within 1 size up or down, and that the meniscal component corresponds with the size of the talar component. To allow radiologic The BOX (Bologna and Oxford Universities) Total Ankle Arthroplasty Design detection, the meniscal bearing contains 3 tantalum spheres (0.8-mm diameter), 2 anterior and 1 posterior, attached to polyethylene pegs. The Bologna and Oxford Universities (BOX) (BOX Ankle, Finsbury Orthopaedics Ltd, Leatheread, UK) (29) is a 3-part TAA implant, with cast cobalt-chrome-molybdenum alloy components fixed to the body of the talus and the distal portion of the tibia, Surgical Technique along with an interposed meniscal bearing (Fig. 1). This implant has previously been discussed in other published reviews (24,26,41,42). The distal tibial component of the For the prosthesis to work properly, the fixed components must be implanted BOX implant has a convex spherical surface that corresponds to a proximal, concave correctly with respect to the preserved ligamentous attachments (29). To satisfy this spherical surface of the meniscal bearing with an equal radius. The proximal surface of condition in practice and, in particular, for the meniscal bearing to slide smoothly on the talar component has a circular, convex sagittal plane arc, with a radius of curvature both metallic components, it is necessary that a constant gap be maintained between that is compatible with the chosen radius of the tibial arc, thereby allowing fibers the articular surfaces of the tibial and the talar components throughout the arc of within the calcaneofibular and tibiocalcaneal ligaments to remain isometric during rotation. This is, in fact, a critical goal of the surgical implantation procedure. To this passive joint motion (27,29). In order to allow the ligaments to remain isometric, end, a longitudinal incision is made, and this is situated either anteromedially or a radius of curvature larger than that of the natural talus was required, and this differed anterolaterally, the latter being preferred, in our opinion, for easier access to the more from the design of most other 3-part ankle implants. It also required that the meniscal critical lateral malleolus. Using a talar cutting block mounted on a tibial alignment jig bearing move forward on the tibial and talar components during dorsiflexion, and (Fig. 2) to guide the saw blade, a horizontal surface, oriented perpendicular to the long backward during plantarflexion. The talar component of the BOX ankle implant, when axis of the shaft of the tibia with the ankle in neutral position, is then made across the viewed in the frontal plane, displays a concave sulcus that limits medial to lateral superior aspect of the body of the talus by removing a section of the talar dome  4mm dislocation of the meniscal component. Furthermore, the sagittal arcs of the metal in thickness. components are slightly longer posteriorly, in order to allow for more plantarflexion Thereafter, the subsequent amount of tibial bone resection is determined by than dorsiflexion. Still further, the talar component is narrower posteriorly so that it considering the necessary minimum overall thickness of the prosthesis, taking into more accurately matches the morphology of the talus. consideration the desired amount of final tension on the preserved ankle ligaments, For cementless component-to-bone fixation, the nonarticulating metal surfaces are which have to be balanced (Fig. 2). With the use of a joint distraction ratchet and 4 covered with small cast-in balls and also coated by plasma spray with a 50-mm-thick different metal tensioners, each corresponding to the thickness of a specific meniscal layer of hydroxyapatite. The tibial component has 2 parallel cylindrical bars running bearing component, the final size of the meniscal bearing is determined, as are the anteroposteriorly (AP) on its proximal flat surface. On its undersurface, the talar anticipated final stability of the TAA and the planned level and orientation of the tibial component has a flat, central, horizontal surface, as well as flat anterior and posterior osteotomy in the transverse plane; all these variables being determined before any chamfers to match the prepared talar dome. In addition, 2 pegs are used, 1 on the sawing of the tibia is performed. The surfaces of the tibia and the malleoli are then anterior chamfer and the other on the central surface. The pegs are oriented posteriorly prepared with a tibial cutting block attached to the alignment jig (Fig. 2), and holes for to facilitate component implantation. the 2 parallel bars of the tibial component are also drilled. S. Giannini et al. / The Journal of Foot & Ankle Surgery 50 (2011) 641–647 643

Postoperative Management

The operated leg was held in a plaster cast without weight-bearing for 2 weeks, followed by active and passive movements and partial weight-bearing with a rigid, below-the-knee boot. Complete weight-bearing with the boot started 1 month after the operation and free weight-bearing was resumed 2 months postoperative.

Surgical, Clinical, and Radiological Assessments

Intraoperatively, the final motion of the meniscal bearing component was assessed in regard to smoothness of the excursion, and alignment of the implant components throughout the range of ankle motion. The American Orthopaedic Foot & Ankle Surgery (AOFAS) hindfoot-ankle score (45,46) was obtained in each of the centers preopera- tively and, when possible, at the following follow-ups: 1, 3, 6, 12, 18, 24, 36, and 48 months. In 31 ankles from 1 center, those patients who agreed to undergo additional radiography, radiographs were also taken with the implanted ankle in maximum plantarflexion and dorsiflexion (Fig. 3), with a minimum of 12 months of follow-up, and the movement of the meniscal bearing relative to the tibia was also measured. Pros- thesis component alignment over time and progressive radiolucency were analyzed radiographically. The radiographs were specifically examined at 5 areas inferior to the talar component and in 5 areas superior to the tibial component, both in the lateral and frontal views. All 156 patients in the study were seen at least once at a minimum of 6 months postoperatively, and 1 (0.63% of ankles, 0.64% of patients) of the patients Fig. 2. Photograph taken in the surgical theater during the tibial bone preparation phase (unilateral TAA) died for reasons unrelated to the operation 12 months after the of the operation. While the selected tibial cutting block is inserted onto the tibial align- operation. Postoperative complications associated with the ankle implant were also ment guide in optimal medio-lateral adjustment with respect to the malleoli, a tibial recorded. tensioner of the appropriate thickness is inserted through the slot on the tibial cutting block, and tension is applied to the joint through the ratchet knob. This would represent the final tension in the replaced joint, providing the meniscal implant has the same Results labeled thickness of the tibial tensioner. Surgical Implantation

In our experience, the most appropriate AP position for the talar component is Overall, the instrumentation worked smoothly and enabled reli- determined by means of a step-by-step procedure using a talar template and a series of able implantation of the 3 components in the target location, even for plastic gap gauges. In every tentative AP position, the gap between the flat tibial surface fi and the top (superior surface) of the talar template is measured in both maximal the initial implantation at each center. After implantation of the nal plantarflexion and dorsiflexion. This procedure is repeated after removal of bone from components and before layered wound closure, the meniscal bearing the front (anterior portion) of the talar dome until these gaps are equal to within 1 mm. was observed to move anteriorly during dorsiflexion and posteriorly The talar dome is then prepared, with the posterior chamfer and the holes for the 2 during plantarflexion over both the tibial and talar components in all pegs, using a talar chamfer guide pinned in place. With the trial talar component implanted, the same step-by-step procedure is repeated for the determination of the operated patients. The motion was smooth and continuous, and most appropriate AP position of the trial spherical tibial component, in case length- complete congruence among the 3 components was maintained in all ening of the holes for the 2 parallel bars is required. Final talar and tibial components joint positions for 155 (98.1%) of the 158 ankles. In 9 (5.7%) of the are implanted and the most appropriate meniscal bearing is inserted between the ankles, slight misalignment in the AP direction of the 2 metal metallic tibial and talar components, after tests with corresponding trials. components was observed, and this was associated with a partial anterior subluxation of the talus that was noted in the preoperative period in these patients. Patients A number of adjunct procedures were performed at the time that The surgeons involved at the different hospitals shared the following general the BOX ankle prosthesis was implanted, including distal first meta- indications: primary or posttraumatic osteoarthritic patients with a relatively low tarsal osteotomy (47) in 11 (7.0%) ankles, reduction of previous functional demand, tentatively in patients > 50 years of age; all patients with rheu- tibiofibular mortise diastasis with a syndesmotic screw in 2 (1.3%) matoid arthritis; all patients rejecting arthrodesis, but compatible with the exclusion ankles, lateral ligament reconstruction in 5 (3.2%) ankles, calcaneal criteria. The general contraindications adopted from the beginning of the trial were morphological defects of the ankle, significant osteoporosis or osteonecrosis affecting osteotomy in 2 (1.3%) ankles, and percutaneous Achilles tendon the talus, previous or current infections of the foot, vascular pathologies or severe lengthening in 109 (69.0%) ankles-to achieve satisfactory dorsiflexion neurological disorders, previous arthrodesis of the homolateral hip or knee or severe after component implantation. deformities of these joints. The following contraindications were resolved before or during total ankle replacement: capsuloligamentous instability, and hindfoot or fore- foot deformities affecting a correct posture. Clinical and Radiological Assessment Between July 2003 and October 2007 (approximately 4 years and 4 months), 192 BOX ankle prostheses were implanted in 190 patients, in 9 hospitals in northern Italy, Follow-up radiographs (Fig. 3) showed no variation in the posi- each with a single surgeon involved. Staff from the inventors’ (S.G., A.L., F.C., and J.J.O.) tions of the implanted components. Radiolucencies > 1mmwere team participated at the very first intervention in each center. The outcomes of 158 observed in 12 (7.6%)-ankles, although not progressive (i.e., observed ankles in 156 patients (82.3% of the 192 ankles, and 82.1% of the 190 ankles) who had undergone BOX TAA at least 6 months before October 2007, thereby assuring a follow- at the first follow-up but maintained stable at the subsequent follow- up duration of at least 6 months, were selected for inclusion in the cohort described in up). Radiographs at maximal dorsiflexion and maximal plantar- this report. The distribution of TAA over the 9 centers was as follows: 75 (47.5%) flexion, as viewed fluoroscopically, confirmed that the meniscal (primary data collecting center), 7 (4.4%), 13 (8.2%), 32 (20.3%), 4 (2.5%), 9 (5.7%), 4 bearing component moved in the direction predicted by the (2.5%), 10 (6.3%), and 4 (2.5%). The mean follow-up duration was 17.7 (range 6 to 48) months. The mean age of the patients was 60.5 (range 29.7 to 82.5) years. The distri- computer-based models (27,28) in all cases. Excursion of the meniscal bution of the cause of the ankle arthrosis, for each ankle replaced, was as follows: 127 bearing on the tibial component, measured radiographically at the (80.4%) posttraumatic osteoarthritis, 18 (11.4%) primary osteoarthritis, 10 (6.3%) rheu- limits of dorsiflexion and plantarflexion (mean 3.3 mm; range 2 to 11 matoid arthritis, 1 (0.63%) ochronotic arthritis, 1 (0.63%) psoriatic arthritis, and 1 mm), correlated with the corresponding range of joint flexion (mean (0.63%) posttraumatic osteoarthritis combined with rheumatoid arthritis. The 2 26.5; range 14 to 53) in the 31 ankles analyzed (Fig. 4). patients who underwent bilateral TAA had posttraumatic osteoarthritis in both ankles, and these patients had the second ankle implanted at 14 and 11 months, respectively, The mean AOFAS scores recorded preoperatively and then at 12, after the first operation. 24, 36, and 48 months’ follow-up were, respectively, 36.3 (range 0 to 644 S. Giannini et al. / The Journal of Foot & Ankle Surgery 50 (2011) 641–647

Fig. 3. Two fluoroscopic radiographs taken in double leg stance at maximum plantarflexion (A) and maximum dorsiflexion (B) from a patient at 24 months’ follow-up duration. The single posterior tantalum bead on the meniscus is pointed out by the white circle.

74), 74.6 (range 12 to 100), 78.6 (range 43 to 99), 76.4 (range 24 to 92), The percentage of poor (total score 50), fair (51 to 75) and good- and 79.0 (range 76 to 82) (Fig. 5). Using Student’s paired sample plus-excellent scores rose from 73, 27, and 0 preoperative, to 3, 44, t tests, changes at every single follow-up with respect to the preop- and 53 at 12 months, to 2, 26, and 72 at 24 months, and to 4, 29, and erative were found to be statistically significant (p < .001), whereas 67 at 36 months (Fig. 5). the changes in scores after 6 months postoperative were not (p > .05). These AOFAS scores were, respectively, 35.7, 73.7, 79.9, 76.2, and 79.0 Complications and Implant Survival where only posttraumatic osteoarthritic patients were considered, and 29.7, 79.0, 78.0, and 81.5 (none at 48 months) where only rheu- Some intraoperative complications occurred, including 1 (0.63%) matoid arthritic patients were considered; the differences between lateral and 2 (1.3%) medial malleolar fractures; 2 were treated with the 2 groups were not statistically significant (p > .05). Kirschner wires and 1 was treated with an interfragmental “Function” domain within the overall AOFAS score (maximum compression screw. A single (0.63%) major surgical technical error value 50 points) improved from 20.6 preoperative to 37.8 at 36 resulted in incorrect talar component positioning; namely, the talar months postoperative, and the “pain relief” domain score (maximum component was positioned too far posteriorly, and this required value 40 points) changed from 10.1 preoperatively to 30.0 at 36 surgical revision by means of reimplantation, which was undertaken months, and this difference was/was not statistically significant. The on postoperative day 3. Complications observed in the postoperative “alignment” domain score (maximum value 10 points) changed from period included 4 (2.5%) problems with wound healing that resolved 5.5 preoperatively to 8.8 at 36 months, and this difference was/was with local care and did not require a return to the operating room, and not statistically significant. Ankle dorsiflexion increased from 0.1 2 (1.3%) malleolar stress fractures, 1 of which was also treated with preoperatively to 7.8,7.7, and 6.1 at 12, 24, and 36 months, subtalar distractional realignment arthrodesis. Three (1.9%) patients respectively, whereas plantarflexion increased from 16.0 preopera- complained of severe pain on the medial side, which proved to be tively to 20.3, 20.4, and 18.5 at 12, 24, and 36 months, respectively. associated with arthrofibrosis and osteophytes at the medial tibiotalar All these 3 improvements were statistically significant (p < .0001). articulation and responded successfully to reoperations for tissue and bone removal (in 1 of these 3 operations, calcaneal osteotomy was also performed). One (0.63%) isolated calcaneal osteotomy and 1 (0.63%) subtalar fusion were performed, for a total of 11 (7%) secondary operations with no component removal. Osteoarthritis of the talo-navicular joint was observed in 2 (1.3%) patients. Pain at 24

Fig. 4. Correlation plot between range of ankle dorsiflexion and plantarflexion and meniscal anteroposterior displacement from measurements on 31 ankles; the significance Fig. 5. Bar plot reporting mean and standard deviation values of the AOFAS hindfoot- of the correlation is reported. The correlation remained statistically significant even after ankle score over the duration of follow-up, also subdivided for Pain, Function, and the point at the extreme right-hand side of the plot, which had considerable leverage on Alignment sub-scores. The figure records the number of ankles from which data were the distribution of the data set, was omitted. obtained at each follow-up time. S. Giannini et al. / The Journal of Foot & Ankle Surgery 50 (2011) 641–647 645 and 36 months was designated as moderate (relief score equal to 20) Based on our experience with the patients in this report, we in 11 (7%) and 4 (2.5%) patients, respectively, and severe (score equal believe that the observations at operation and postoperatively to 0) in 0 and 1 (0.63%) patients. Component removal was necessary in confirmed the principles on which the design was based (27). After 2 (1.3%) cases, in 2 different centers (Table 1). One of these cases appropriate component placement and ligament tensioning, the gap involved lateral impingement due to ankle varus deformity in between the metal components remained the same throughout the a patient with Charcot-Marie-Tooth disease who was treated after 24 flexion arc, indicative of smooth and congruent AP motion of the months with successful arthrodesis with the use of a retrograde meniscal bearing. The fact that, with appropriate positioning of the locking nail. The other patient complained of severe pain of unknown metal components, a constant gap could be maintained between their origin and was treated after 12 months with a triple arthrodesis, surfaces is a demonstration that some ligament fibers remain effec- although this did not lead to resolution of the pain (Table 1). After tively isometric over the range of motion. more than 4 years of follow-up, 152 (96.1%) of the implanted BOX A more limited range of movement was observed in ankles with ankle prostheses remained functional. more limited bearing movement (Fig. 4), as observed in those replacements not fully aligned because of original misalignments Discussion such as talar subluxation. Despite the longevity of some current 3-part ankle implant designs, we are not aware of any published data In total joint replacement design, for the articular surfaces to specifically detailing the meniscal bearing movement. The limited complement the joint ligaments in order to provide natural rolling range of dorsiflexion and plantar flexion achieved by these previous and sliding, the shapes of the prosthetic surfaces should be chosen not designs (49) could perhaps be explained by the absence of bearing arbitrarily, but deduced by a mathematical model of joint mobility movement resulting from a ligament-incompatible combination of (48). We applied this analysis in the design of an ankle prosthesis, anatomic and nonanatomic surfaces. With restoration of the physio- resulting in the use of nonanatomic surfaces for both talar and tibial logic role of the ligaments, natural joint motion could be reproduced components, which, nonetheless, can be compatible with the liga- (27,28), as well as natural muscle lever arm lengths (33). In other ments of the ankle (29). The results of our investigation show that words, because the function of the ligaments in controlling passive compatibility between bearing movement on the tibia and range of joint mobility seems to have been restored with the BOX ankle motion has been achieved. The first implantation took place in July prosthesis, their physiologic contribution to joint stability appears to 2003, after extensive biomechanical studies (30–34). The device in have been restored. Interestingly, gait analysis of a group of these itself combines freedom from restraint in the sagittal plane with patients confirms restoration of function (50). congruity of the components throughout the 3-dimensional motion These early clinical results of the BOX ankle implant have also arc. This should contribute eventually to physiologic stress in the demonstrated safety and efficacy. We think that the present survival ligaments as well as minimizing wear of the meniscal bearing. These rate at 4 years (96.1%) compares very well with multicenter 5-year features should result in longevity and efficacy of the TAA, although rates published by the Swedish (18) (531 cases, survival 78%), this assumption has yet to be borne out in regard to the ankle implant Norwegian (19) (257 cases, 89% survival), and New Zealand (20) (202 evaluated in this investigation. These assertions have to be proved in cases, 86% survival) registries. In addition, the type and number of clinical trials, and this preliminary study seeks to contribute in this complications, range of motion, and radiographic analyses that we direction. observed were also encouraging. The only observation consistent with For the new design to function as claimed, not only must the the previous literature is the degree of remaining pain after TAA, which prosthesis components be designed according to biomechanical at the ankle can be accounted for by many factors and can be attributed principles, they must also be implanted correctly relative to the to ailments at several different joint levels in the involved extremity. ligamentous attachments. A strong force was often necessary to insert There were issues that could have complicated this series. Nine the meniscal bearing between the fixed tibial and talar components. different surgeons were involved, with 7 out of 9 performing less than This did not result in a limited flexion arc, and we believe that it may 15 TAA procedures. It has been shown recently from a large national have guaranteed safe entrapment of the bearing. The results in the registry (18) that the survival rate from the initial 30 TAAs from each cohort described in this report demonstrated that the status of the surgeon are usually worse than the subsequent, and our results ligaments in severely arthritic ankles can still allow implantation of appear to be consistent with those from other centers. We also found ligament-compatible TAA designs. Unlike total knee replacement, it interesting that, in terms of mobility and ligament function, rheu- which regularly sacrifices 1 if not both of the cruciate ligaments, ankle matic patients can perform better (best survivability) than one might ligaments can in fact be easily retained with the use of an anterior expect, and this was also observed in the Swedish study (18). In fact, in incision and access. In addition, the mean thickness of the meniscal our cohort, improvement in the AOFAS score from preoperative to 36 component used in this clinical trial was 5.5 mm, and there was months was 40.6 for the 127 patients with posttraumatic osteoar- a trend from thicker to thinner over time for each surgeon, which thritis, and 51.8 for the 10 rheumatic patients. implied to us that the instrumentation enabled an accurate prediction The meta-analysis by Stengel et al (49) provides a convenient of this parameter and that, with experience, the minimum required basis for comparison of the clinical results. The overall AOFAS score amount of bone could be removed. improvement in the present trial patient population (36.3

Table 1 Survival table of the BOX ankle replacement

Year No. of Prostheses No. of Ankles No. of Ankles No. of Survival Cumulative Lower 95% Upper 95% Entering Withdrawn at Risk Failures Rate Survival Confidence Confidence 1 158 32 142 0 1 1 100.0 100.0 2 126 76 88 1 0.9886 0.9886 96.6 100.0 3 49 27 35.5 1 0.9718 0.9608 90.3 100.0 4 21 19 11.5 0 1 0.9608 90.3 100.0 5 2 2 1 0 1 0.9608 90.3 100.0

For further information, see Murray DW, Carr AJ, Bulstrode C. Survival analysis of joint replacements. J Bone Joint Surg Br 75(5):697–704, 1993. 646 S. Giannini et al. / The Journal of Foot & Ankle Surgery 50 (2011) 641–647 preoperative to 79.0 at 48 months) was similar to that of the meta- References analysis, and the approximately 18-point increase in the “function” score that we observed also compared well with the mean 12.5-point 1. Kempson GE, Freeman MAR, Tuke MA. Engineering considerations in the design of an ankle joint. Biomed Eng 10:166–180, 1975. increase reported in the meta-analysis. Pain relief in the present 2. Freeman MAR. Current state of total joint replacement. Br Med J 27:1301–1304, study, which ranged from 18.8 to 20.5 points over the 12- to 36- 1976. month follow-up visits, was worse than the 28-point improvement 3. Lachiewicz PF. Total ankle arthroplasty: indications, techniques, and results. Orthop Rev 23:315–320, 1994. reported in the meta-analysis. It is also interesting to note that pain is 4. Rush J. Management of the rheumatoid ankle and hindfoot. Curr Orthop 10:174– very subjective, and after consideration of the causes of postoperative 178, 1996. pain identified in our cohort, we appreciate the fact that the patients 5. Conti SF, Wong YS. Complications of total ankle replacement. Clin Orthop – who had the most pain had their symptoms attributed to the subtalar 391:105 114, 2001. 6. Kofoed HA, Sorensen TS. Ankle arthroplasty for rheumatoid arthritis and osteo- and midtarsal joints. There was a mean gain in the range of motion of arthritis: prospective long-term study of cemented replacements. J Bone Joint Surg approximately 10 in our cohort, compared with 7 reported in the Br 80:328–332, 1998. meta-analysis. The most encouraging finding that we observed, 7. Pyevich MT, Saltzman CL, Callaghan JJ, Alvine FG. Total ankle arthroplasty: a unique design: two to twelve-year follow-up. 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