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© 1998 The Journal of Bone and Joint Surgery, Inc. Total Ankle Arthroplasty: a Unique Design. Two to Twelve-Year Follow-up* , IOWA CITY, IOWA , SIOUX FALLS, SOUTH DAKOTAInvestigation performed at the Department of Orthopaedic Surgery, University of Iowa, Iowa City, and Orthopaedic Associates, Sioux Falls
We evaluated the intermediate-term results of a novel total ankle arthroplasty that includes insertion of the components without cement and arthrodesis of the tibiofibular syndesmosis as part of the operative procedure. One hundred consecutive Agility ankle replacements were performed in ninety-five patients between 1984 and 1993. At the time of follow-up, eighty-three patients (eighty-six ankles) were alive and twelve patients (fourteen ankles) had died. Five (6 per cent) of the eighty-six ankles in the living patients had been revised. Including the components that had been revised for loosening, twenty-one (twelve tibial and nine talar) components had migrated. Delayed union of the syndesmosis (twenty-eight ankles) and non-union of the syndesmosis (nine ankles) were associated with the development of lysis around the tibial component. Non-union of the syndesmosis was also associated with migration of the tibial component and circumferential radiolucency around that component. In addition to the patients who died, one patient had a resection of the implant with subsequent arthrodesis. The remaining eighty-two patients (eighty-five ankles) were the basis for the clinical evaluation in the study. The average age at the time of the procedure was sixty-three years (range, twenty-seven to eighty-one years). At the time of the most recent follow-up (range, 2.8 to 12.3 years; average, 4.8 years), forty-seven (55 per cent) of the remaining eighty-five ankles were not painful and twenty-four (28 per cent) were only mildly painful. The range of motion of the fifty-six ankles that were examined at the time of follow-up averaged 36 degrees (range, 10 to 64 degrees), and the results for seventy-nine (93 per cent) of the eighty-five ankles were satisfactory to the patients.
The results of total ankle arthroplasty have not been comparable with those of replacements of other major joints of the lower extremity. Most early reports of the results of ankle arthroplasty after a limited follow-up were optimistic12-14,27,37,39,43-46,52-54; however, with longer follow-up it was found that most implants had failed to provide long-lasting pain relief or improved function4,17,18,22,23,26,38,50,53. In 1982, Newton38 concluded that the results of total ankle arthroplasty in patients who have severe rheumatoid arthritis are so poor that the procedure is contraindicated. Bolton-Maggs et al.4 agreed with Newton and further concluded that arthrodesis should be considered the treatment of choice for a painful, stiff, osteoarthrotic, or arthritic ankle, regardless of the underlying pathological process. Those authors did not believe that total ankle arthroplasty was warranted because of the generally poor long-term results and the high rates of complications associated with the procedure. Despite the disappointing results of most clinical trials of ankle implants, difficulties with alternative forms of operative treatment of osteoarthrosis or arthritis of the ankle have stimulated the continued search for a viable ankle replacement6,25. The principal options that are available for the treatment of debilitating end-stage osteoarthrosis or arthritis of the ankle are arthrodesis and arthroplasty. Although numerous reports have described the benefits of ankle arthrodesis1,2,7-9,15,21,29,31,33-36,40,42,48,49, that operation has been associated with a number of potential problems. First, it may be difficult to obtain an ideal position of the ankle, especially when there is loss of periarticular bone5,28. In addition, pseudarthrosis may be a problem20. Frey et al.14 reported a rate of pseudarthrosis of 41 per cent (thirty-two of seventy-eight ankles), and Stauffer43 stated that rates as high as 50 per cent had been reported, although he did not provide specific numbers. More recent techniques with compression screw fixation and arthroscopic approaches appear to have decreased the rate of pseudarthrosis9,15,49. Attempted arthrodeses have been associated with rates of infection ranging from 0 per cent (of thirty-four ankles)15 to five of eighteen ankles32. In addition, rates of amputation, performed to treat ankles that are unsalvageable because of operative complications, have been as high as 13 per cent (six of forty-eight ankles)11. Patients who have had an arthrodesis often cannot climb stairs or walk on uneven surfaces well, and running can be difficult. The patients' level of satisfaction has also been variable; some patients have persistent pain or a chronic limp1,9,14,31,41,43. Good clinical results have ranged from 65 per cent (fifteen of twenty-three ankles)43 to all of seven patients49. Waters et al.51 found that otherwise healthy patients who had had a successful ankle arthrodesis had a 16 per cent decrease in gait velocity, a 3 per cent increase in oxygen consumption, and an overall gait efficiency of 90 per cent. Mazur et al.35 found that otherwise healthy patients all functioned well in activities of daily living after a successful ankle arthrodesis. Without shoes, however, gait velocity was slowed and the length of the stride was shorter. Gait analysis showed that the loss of ankle motion was compensated for by increased motion of the small joints of the ipsilateral foot and by altered motion of the contralateral ankle, and many patients had degenerative changes in the subtalar and mid-tarsal joints. Moreover, tibiotalar arthrodesis has been shown to increase mechanical demands on the remaining joints of the lower extremity, especially the intertarsal joints, Chopart joints, and the knee joint1,3,5,11,19,22,28,32,35-38,41,43-45. Lidor et al.30 recently reported a series of tibial stress fractures that occurred after ankle arthrodesis, probably as a result of the altered mechanics created by the arthrodesis. In the studies by Waters et al.51 and Mazur et al.35, the patients were relatively young and healthy and had isolated ankle osteoarthrosis with no other systemic disease. Elderly patients with polyarticular arthritis, who already have diffuse underlying joint disease, do not easily tolerate the increased demands that are placed on other joints as a result of ankle arthrodesis10,22,27,45. Furthermore, arthrodesis typically necessitates a long period of convalescence and use of walking aids, which can be difficult for patients who have limited strength, cardiopulmonary difficulties, or arthritis involving the upper extremities. For all of these reasons, the search for a successful ankle prosthesis has continued. According to Bauer et al.3, the long-term results associated with both constrained and non-constrained designs have been disappointing, particularly when methylmethacrylate has been used for fixation3,4,16,23,38,41,50. The few reports on implants that were inserted without cement suggested that fixation without cement is associated with better results than fixation with cement6,21,41,47. The high rates of loosening of constrained implants are believed to be due to larger forces at the bone-prosthesis interface compared with those seen with non-constrained designs. The non-constrained designs, however, have been plagued with problems such as instability and impingement3. The senior one of us (F. G. A.) recognized the shortcomings of early designs of ankle prostheses and developed a novel implant that simulates the anatomical alignment of the talar dome and incorporates arthrodesis of the tibiofibular syndesmosis to increase stability of the components. He hypothesized that these changes would improve the long-term results of total ankle arthroplasty. The purpose of this paper is to present a review of the results, by independent examiners (M. T. P., C. L. S., and J. J. C.), of the first 100 ankle arthroplasties performed with this new implant.
The Implant
This implant is unique because of its semiconstrained two-component design, because it has a relatively large surface area for bone ingrowth, and because it takes advantage of arthrodesis of the tibiofibular syndesmosis to allow transfer of weight to the fibula. The tibial component has a porous coating on its entire superior surface as well as along the medial and lateral sides that are in contact with the medial and lateral malleoli. The talar component has a porous coating on the inferior surface that is exposed to the talar bone. The talar component is slightly wider anteriorly than posteriorly, which theoretically makes it more stable in dorsiflexion than in plantar flexion. Initially, the tibial and talar components were both titanium, but beginning in 1989, and with the twenty-first treated ankle, a cobalt-chromium talar component was used because two talar components had loosened within three years after the initial procedure. Also starting with the twenty-first ankle, the tibial component was made thicker because two tibial components had broken (in the seventh ankle, two and one-half years after the procedure, and in the eighth ankle, three and one-half years after the procedure). The minimum thickness of the polyethylene liner varies (from 3.73 to 4.70 millimeters) depending on the size of the implant.
Operative Technique Under thigh-tourniquet control, the ankle joint is approached anteriorly, between the tibialis anterior and extensor hallucis longus tendons. The anterior aspect of the capsule is incised longitudinally, and the distal part of the tibia is exposed subperiosteally both medially and laterally. The ankle joint is gently distracted five to ten millimeters, and valgus or varus malalignment is corrected with the external fixator. A separate anterolateral approach to the syndesmosis is then performed. The anterior tibiofibular ligament is reflected, the tibiofibular syndesmosis is distracted, and its soft tissues are removed. The alignment jig is secured to the tibia with a unicortical half-pin, and the ankle is slightly distracted in the neutral position. The correct size of tibial jig is chosen to remove as little bone as possible while ensuring that all cartilaginous surfaces, including the cartilage from the lateral malleolus, are removed. Usually five or six millimeters of bone is removed from the distal aspect of the tibia and an additional five to six millimeters is removed from the talar dome. Once the components are in place, graft consisting of bone obtained from the resected articular surfaces is placed at the site of the syndesmosis, after which the syndesmosis is stabilized with lag screws. Only one screw was used initially, but, starting with the forty-first treated ankle, two screws were used. The fixator is then removed, and the skin is closed. After dressings are applied, a posterior splint is placed with the foot in neutral position. The patient remains non-weight-bearing for six weeks but is encouraged to remove the splint and to move the ankle several times a day. During the first six weeks, the splint is worn, on the average, for more than twenty-three hours each day.
Patient Demographics Two patients (two ankles) who had died less than two years after the procedure were excluded from the study. The remaining ninety-eight ankles were evaluated radiographically. The radiographs were made preoperatively, early postoperatively, at six months, at two years, and at the time of the most recent follow-up (average, 4.8 years postoperatively). A total of twelve patients (fourteen ankles), including the two who were followed for less than two years, died from reasons unrelated to the ankle arthroplasty. In addition, one patient had a resection of the implant with subsequent arthrodesis. The remaining eighty-five ankles (eighty-two patients) were the basis for the clinical evaluation in this study. Of the eighty-two patients, forty-three (52 per cent) were male and thirty-nine (48 per cent) were female. The average age at the time of the operation was sixty-three years (range, twenty-seven to eighty-one years), and the patients were followed for an average of 4.8 years (range, 2.8 to 12.3 years). The preoperative diagnoses for the eighty-five ankles were posttraumatic osteoarthrosis (forty-four ankles; 52 per cent), rheumatoid arthritis (nineteen ankles; 22 per cent), primary osteoarthrosis (nineteen ankles; 22 per cent), septic arthritis (two ankles; 2 per cent), and psoriatic arthritis (one ankle; 1 per cent). Two of the three patients with bilateral disease had posttraumatic osteoarthrosis, and the third had severe polyarticular rheumatoid arthritis.
Clinical Evaluation The questionnaire related to the level of pain, function (as compared with preoperative function) in activities of daily living as well as in specific activities (sports and climbing stairs, for example), and satisfaction with the procedure. The patients were asked to rate the pain on a scale of 0 to 10 points. No pain was given 0 points; mild pain, 1, 2, or 3 points; moderate pain, 4, 5, or 6 points; and severe pain, 7 points or more. The clinical examination that was performed on the fifty-four patients (fifty-six ankles) involved a careful assessment of the neurovascular status, the alignment of the ankle with the patient standing, and the range of motion of the ankle. When the patient had had a unilateral arthroplasty, the alignment of the ankle was compared with that on the uninvolved side; when the patient had had a bilateral arthroplasty, the alignment was compared with the average value for the uninvolved ankles of the patients who had had a unilateral arthroplasty. The range of motion was determined with use of a goniometer along the lateral border of the leg and foot and was compared, when the patient had had a unilateral arthroplasty, with that on the uninvolved side. An ankle-hindfoot score, according to the system of the American Orthopaedic Foot and Ankle Society24, was then calculated from the data derived from the examination and the questionnaire for the fifty-six ankles that had had a physical examination by one of us, independent of the surgeon who had performed the operation. The ankle-hindfoot score is determined with use of a 100-point rating system based on symptoms, level of activity, need for walking aids, deformity, and motion.
Radiographic Evaluation
Migration of a component was defined as a shift of a minimum of 5 degrees in the measured angle indicating the position of the component. The 5-degree criterion was based on reliability testing and the rotational effects of radiographic positioning. We assessed the accuracy of the measurements by having the same observer (M. T. P.) repeat the measurements on sixty sets of radiographs with no markings on two separate occasions. We found a high level of intra-rater reliability with relatively small errors of the mean. For angle A, the standard error of the mean was 0.7 degree, the range was 0 to 2 degrees, and the 95 per cent bound was 2 degrees. For angle B, the standard error of the mean was 1.2 degrees, the range was 0 to 4 degrees, and the 95 per cent bound was 3 degrees. For angle D, the standard error of the mean was 0.7 degree, the range was 0 to 2 degrees, and the 95 per cent bound was 2 degrees. Although the radiographs were made in the same radiographic suite by the same technologists, rotation of the limb was not absolutely controlled between serial examinations. In view of this and the modest errors of the mean of the repeat angular measurements, we defined a 5-degree change in an angular measurement as radiographically relevant. On each radiograph, we determined whether the syndesmosis had fused, had had a delayed union (defined as a union that took longer than six months), or had never fused (by the time of the most recent follow-up). Radiolucent lines at the interface between the prosthesis and the bone were also evaluated. On the anteroposterior and mortise radiographs, the tibial component was divided into six zones (Fig. 4-A), and the presence or absence of space between the component and the bone was assessed. Lucency was defined as a radiolucent line of two millimeters in width or less, and ballooning lysis was defined as a radiolucent area of greater than two millimeters. Any change over time was noted. Lucency and lysis were also assessed on the lateral radiograph, but only three zones were identified (Fig. 4-B). The lateral radiograph was also used to evaluate other joints of the foot (in particular, the talonavicular, calcaneocuboid, and subtalar joints) for the progression of osteoarthrosis.
Statistical Methods The McNemar test was used to evaluate any change in the presence of circumferential lucency or ballooning lysis over time, and the Student t test was used to assess whether age or weight was associated with the presence of circumferential lucency or ballooning lysis. To examine the association between pain and the age and weight of the patient at the time of the operation, the range of motion of the ankle, and the initial postoperative position of the component as seen radiographically, one-way analysis of variance followed by the Tukey test for multiple comparison of measures was used. This analysis involved a comparison of the average ages, weights, ranges of motion, and initial postoperative positions of the components associated with no pain, mild pain, moderate pain, and severe pain.
Complications
Clinical Results The patients who had posttraumatic osteoarthrosis reported significantly more pain at the time of follow-up than did either those who had primary osteoarthrosis or those who had rheumatoid arthritis (p < 0.05). Seventeen (40 per cent) of the forty-two patients who had posttraumatic osteoarthrosis were pain-free, whereas fifteen of the eighteen who had rheumatoid arthritis and thirteen of the nineteen who had primary osteoarthrosis were pain-free. Conversely, nine (21 per cent) of the forty-two patients who had posttraumatic osteoarthrosis, two of the eighteen who had rheumatoid arthritis, and one of the nineteen who had primary osteoarthrosis had moderate pain. Both patients who had septic arthritis also reported moderate pain. Of the patients who had had a bilateral arthroplasty, the one with rheumatoid arthritis had no pain in either ankle, one with posttraumatic osteoarthrosis had no pain in either ankle, and the other patient with posttraumatic osteoarthrosis had mild pain bilaterally. With the numbers available for study, we did not find an association between the age or weight of the patient at the time of the operation and postoperative pain. Few patients needed walking aids postoperatively, although fifty-six (68 per cent) of the eighty-two patients reported on the questionnaire that they felt more comfortable walking in shoes, especially ones with a small, elevated heel. Four patients used a cane on a regular basis, and three wore a brace because of a valgus deformity. One of the patients who had a deformity considered the ankle to be painless, and the other two rated the pain as moderate. One of the patients who reported moderate pain despite the use of an ankle-foot orthosis had had six pain-free years after the ankle replacement. As stated, fifty-six ankles were examined at the time of follow-up. The patients who had had a unilateral ankle arthroplasty usually had less motion on the side of the arthroplasty. The average range of motion in the sagittal plane was 36 degrees (range, 10 to 64 degrees). Twenty-eight (50 per cent) of the fifty-six ankles had a plantar-flexion contracture. The contractures averaged 7 degrees (range, 1 to 20 degrees). Six (11 per cent) of the ankles dorsiflexed to neutral, and twenty-two (39 per cent) were able to dorsiflex beyond neutral (average, 4.6 degrees [range, 1 to 10 degrees] beyond neutral). With the numbers available, we were unable to detect an association between pain and a postoperative plantar-flexion contracture. However, thirty-seven (45 per cent) of the eighty-two patients stated that they were not able to climb stairs as well as they would like; ten (27 per cent) of the thirty-seven blamed the ankle, whereas twenty-seven (73 per cent) blamed other problems (age, other joints, and so on). An ankle-hindfoot score was calculated, with use of the system of the American Orthopaedic Foot and Ankle Society24, for all patients who had had a complete follow-up examination. The average overall score was 85 points (range, 40 to 100 points).
Radiographic Results Of the ninety-eight ankles (ninety-three patients) available for radiographic evaluation, sixty-one (62 per cent) had a successful fusion of the syndesmosis (Fig. 5), twenty-eight (29 per cent) had a delayed union (more than six months were required for radiographic union), and nine (9 per cent)—including the ankle that had removal of the implant and tibiotalar arthrodesis—had a non-union.
Of the eighty-five ankles in the patients who had completed the pain questionnaire, fifty-four (64 per cent) had a solid union. Of these fifty-four ankles, thirty were not painful, fourteen were mildly painful, and ten were moderately painful. Of the twenty-three ankles (27 per cent) with a delayed union, eleven were not painful, eight were mildly painful, and four were moderately painful. Of the eight ankles (9 per cent) with a non-union of the syndesmosis, six were not painful, two were mildly painful, and none were moderately or severely painful. There was at least a 5-degree change in the radiographic position of nineteen components (seven talar and twelve tibial). None of the seven migrations of the talar components were associated with a delayed union or a non-union of the syndesmosis, whereas eight of the twelve migrations of the tibial components were associated with a delayed union or a non-union. Migration of the tibial component was found in five of the nine ankles that had a non-union of the syndesmosis, three (11 per cent) of the twenty-eight ankles that had a delayed union, and four (7 per cent) of the sixty-one ankles that had a successful fusion. The risk ratio for migration of the tibial component for ankles with a non-union compared with those with a solid fusion was 8.5 (95 per cent confidence interval, 2.8 to 25.8; p < 0.001). Non-union was also clearly associated with ballooning lysis at the interface between the bone and the tibial component (Fig. 6). By six months after the procedure seven of the nine ankles with a non-union had radiographic evidence of ballooning lysis, and by two years all of the ankles with a non-union had evidence of ballooning lysis. At the time of the most recent follow-up, ballooning lysis was seen in eighteen (64 per cent) of the twenty-eight ankles with a delayed union and in seventeen (28 per cent) of the sixty-one with a solid union. The prevalence of ballooning lysis in the ankles with a non-union was significantly higher than that in the ankles with a delayed or solid union (p < 0.0001). Ballooning lysis was found only in zones 5 and 6. Similarly, the prevalence of ballooning lysis in the ankles with a delayed union was significantly higher than that in the ankles with a solid union (p < 0.05).
Circumferential lucency around the tibial component was seen on the mortise radiographs of twenty-five (26 per cent) of the ninety-eight ankles at the time of follow-up. The lucent lines were rarely progressive after two years (Figs. 7-A and 7-B). Circumferential lucency around the tibial component on the lateral radiograph was associated with non-union of the syndesmosis (p < 0.01). A circumferential line was found in four of the nine ankles with a non-union, two (7 per cent) of the twenty-eight with a delayed union, and one (2 per cent) of the sixty-one with a solid union.
The position of the tibial component had an effect on the outcome. Tibial components that had been placed in more than 4 degrees of valgus, as seen on the mortise radiograph, were associated with a significantly higher rate of pain (p < 0.05). No appreciable progression of degenerative changes over time was observed in the subtalar, talonavicular, or calcaneocuboid joint. We could detect no relationship between age, weight, or preoperative diagnosis and the development of lysis, lucency, or migration of the component.
The early clinical results with the Agility ankle implant are encouraging, although the radiographic findings remain a cause for concern. At the time of follow-up, however, the radiographic findings were not found to be related to the clinical findings. Non-union and delayed union of the syndesmosis were associated with higher rates of ballooning lysis at the interface between the bone and the tibial component. In the present series, fusion of the syndesmosis was tantamount to stability of the tibial component. Despite the somewhat worrisome radiographic appearance of the bone-implant interface, we found no association between pain and the rate of union of the syndesmosis. Eighty (98 per cent) of the eighty-two patients reported that they had obtained some pain relief and seventy-eight (95 per cent) stated that they would have the operation again. In this study, we defined lucency and lysis according to the width of the radiolucent line around the component. The time-course and appearance of these radiographic lines suggest that, in most patients, they were not due to the same biological phenomenon that occurs after other arthroplasties as a result of particles of debris. Lucent lines around these ankle implants were always apparent within two years after the procedure and were typically non-progressive. Similarly, after a few years most areas of lysis (defined as a radiolucent line that was wider than two millimeters) were non-progressive and probably indicated a lack of fixation of the implant. The ballooning lysis almost always occurred along the lateral margin of the implant, between the fibula and the tibial component. The development of lysis in this area was more frequent in the ankles that had a delayed union or a non-union of the syndesmosis. This finding indicates that these radiographic findings may, in most patients, represent osseous resorption in an area of high interfacial shear stresses between the implant and the residual lateral malleolus. At the time of the study, there had been five revisions after the first 100 ankle arthroplasties. Two of the first, titanium talar components loosened and were revised. Loosening of the talar component has not been a problem since cobalt-chromium talar components began to be used early in the series. Another talar component was revised in the first three months because it had been placed in an improper position at the time of insertion. Two tibial base-plates fractured, but only one was revised; the other one was not causing symptoms. No tibial base-plate has fractured since 1989, when the base-plate was made thicker. Finally, one patient had removal of both components and arthrodesis of the ankle secondary to persistent pain. This patient had a non-union of the syndesmosis and no bone ingrowth. Several features of the implant's design are unique and may be responsible for the relative clinical success found in this follow-up study. First, the semiconstrained, two-component design allows slight medial-lateral translation as well as axial rotation of the talar component within the tibial component with motion. Constraint is therefore somewhere in between that of the more restrictive constrained designs and that of the less restrictive non-constrained (usually three-component) designs3,6,25. The reported high rates of loosening of constrained components may have been secondary to increased forces at the bone-prosthesis interface, while non-constrained designs have been plagued with problems such as altered kinematics, instability, and impingement3. Moreover, the Agility implant relies on bone ingrowth, rather than methylmethacrylate, for fixation. Most of the older total ankle implants were inserted with cement. The results from more recent reports of implants inserted without cement have demonstrated overall better results6,25,41,47. Because of its unique design, the Agility ankle implant has a relatively large surface area for bone growth into the tibial component and it allows stress transfer by incorporating fibular weight-bearing through an arthrodesis of the syndesmosis. The early experience with the Agility ankle implant is encouraging; the results appear to be better than those associated with most other ankle prostheses. In the present series, most patients were satisfied with the outcome of the operation and were functioning well with the implant. Additional clinical and radiographic follow-up is needed to determine the long-term efficacy of this design and to clarify whether delayed union or non-union of the syndesmosis is associated with an increased rate of clinically important problems.
*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other non-profit organization with which one or more of the authors is associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was DePuy, Incorporated, Warsaw, Indiana.
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