Abstract
Background:
With the increasing use of total ankle prostheses, kinematic analysis of these implants is important to our understanding of their specific biomechanics. Fluoroscopic analysis as used in this study has distinct advantages over previous experimental approaches and allows kinematic determination in vivo of dynamic weightbearing motions.
Methods:
Ten patients with unilateral Agility (Depuy, A Johnson & Johnson Company, Warsaw, IN) total ankle replacements were tested using video fluoroscopy in weightbearing dynamic gait conditions. Their prosthetic ankle and normal ankle kinematics were then analyzed by computer with two-dimensional and three-dimensional model-fitting techniques.
Results:
All of the total ankle prostheses in this study demonstrated less than 3.5 mm of posterior-to-anterior translation from heel strike to toe-off. In comparison, more variability was seen in posterior-to-anterior motion of the normal contralateral ankles, with three ankles translating more than 6 mm. When inversion and eversion and internal and external rotation of the ankle were measured, wide variations were seen among patients.
Conclusions:
While the relative incongruence theoretically decreases the shear stresses transmitted to the bone-prosthesis interface, it does allow more inversion-eversion and rotational freedom that can lead to edge loading and higher contact stresses. Despite good medium-term results having been published for this prosthesis, polyethylene wear and osteolysis have been observed. Further studies looking at the effect of the inversion and eversion and rotational freedom on polyethylene wear and prosthesis survival will assist in our understanding of the factors leading to successful outcome of total ankle prostheses.
Keywords
INTRODUCTION
Total ankle arthroplasty (TAA) is an area of growing interest. Initially used in the 1970s, TAA was largely abandoned by the late 1980s. Although initial results were promising, the long-term results demonstrated an unacceptable failure rate. 1,4,14,19,20,21,23,34,47 Arthrodesis continued to be the principal means to treat ankle arthritis; however, documented problems associated with a fused talocrural 2,3,7,27,30 –32,45 joint combined with success of hip and knee arthroplasty provided motivation to develop second generation ankle prostheses by the mid-1990s. The advancement in prosthetic design and operative techniques were promising but, based on past experiences, skepticism remained as early studies on the second generation prostheses reported success. 13,35,46 Currently, a number of long-term studies have demonstrated acceptable results, 4,22,25,24 which has revived interest in TAA.
As TAA becomes a more common treatment for ankle arthritis, kinematic studies are of increasing importance to help gain an understanding of the effect the implanted prosthesis has on lower leg biomechanics. Information gained from these analyses can be useful to surgeons in selecting a prosthetic device and may help improve operative techniques and prosthetic design. Studies on ankle kinematics include cadaver studies, gait analysis with external markers, and quasi-static stereophotogrammetric studies. 5,6,16,17,33,36,37 Cadaver and quasi-static stereophotogrammetric studies may not simulate in vivo conditions accurately, while gait analysis with external markers may have error associated with motion occurring between the skin and the osseous landmarks. Fluoroscopic analysis, as used in this study, has distinct advantages over these other experimental approaches and allows kinematic determination in vivo of dynamic weightbearing motions. Fluoroscopic analysis has previously provided valuable information in the study of total knee arthroplasty. 8,10,9,11,12,18,40,39,42,43,41,38,44 A previous study using in vivo fluoroscopic weightbearing methodology measured rotational and translational motions at the ankle after TAA using the Buechel-Pappas prosthesis (Endotec, South Orange, NJ). 26 The purpose of this study was to move forward from the weightbearing study by Komistek et al. 26 and measure motions at the ankle during the stance-phase of gait in patients with total ankle arthroplasty.
MATERIALS AND METHODS
Ankle kinematics were determined for 10 patients having a nonimplanted and an implanted ankle (Agility Ankle, DePuy, A Johnson & Johnson Company, Warsaw, IN). The patients, therefore, served as their own controls. All patients had clinically and radiographically normal ankles on the nonimplanted side. The average age of our patients was 62 years (range 52–72 years). They were tested at an average of 2.8 years after their ankle arthroplasty and all patients were at least 1 year from our patients had ankle arthroplasty for treatment of post-traumatic arthritis. All 10 implanted ankles were judged to be clinically successful without demonstrable pain or ligament instability. All had successful syndesmotic fusion as determined radiographically and clinically.
Each ankle was analyzed using video fluoroscopy under in vivo, weightbearing, dynamic gait conditions. During ambulation, each of the patients was analyzed from heel-strike to toe-off. The video was then downloaded to a workstation computer for analysis. The nonimplanted ankles were analyzed in two-dimensions (2D) using digitization, and the implanted ankles were analyzed using a three-dimensional (3D) model-fitting technique that obtains 3D component positions from 2D fluoroscopic images. 18
Nonimplanted Ankles 2D Digitization
Initially, four discrete points on the tibia (A, B, C, D) and two points on the talus (E, F) were located and defined (Figure 1). Three lines were then drawn, bisecting points A and B, C and D, and E and F. The lines AB, CD, and EF were constructed, and the midpoints of each of the lines were found; G was denoted as the midpoint of line AB, H of line CD, and I of line EF. A line was then drawn from point G to point H, through the distal tibia. A second line was then constructed perpendicular to line EF, through point I in the proximal talar direction. The contact point between the tibia and the talus was then found and denoted as J. The distance from line GH to the contact point J was measured relative to the tibia, and the distance from the perpendicular line through point I to the contact point J was measured relative to the talus. If the contact point was anterior to line GH it was denoted as positive and posterior was denoted as negative. Also, relative to the talus, if the contact point was anterior to the perpendicular line through point I, it was denoted as positive and posterior was denoted as negative. Each patient was analyzed at heel-strike, at 33%, and 66% of stance-phase and toe-off.
3D Model-Fitting
As in the 2D analysis, the fluoroscopic images were downloaded to a computer workstation. Images representing heel-strike, 33%, and 66% of stance-phase and toe-off were analyzed. Three-dimensional computer assisted device models of the tibial and talar components were obtained from DePuy (Johnson & Johnson Company, Warsaw, IN). Using the software package ModelFit, developed at the Rocky Mountain Musculoskeletal Research Laboratory (Denver, Colorado), the 3D components were then fit onto the 2D fluoroscopic image 9,10,18 An error analysis was conducted using the ModelFit software package. The translational accuracy was less than 0.5 mm and the rotational accuracy was less than 0.5 degrees. The precision of the system was less than 0.1 mm in translation and less than 0.1 degrees in rotation. 18
Reference points as used for digitization of video fluoroscopy footage.
Once the computer had correctly fit the 3D CAD models onto the 2D fluoroscopic image, the tibial and talar components were grouped together in the reference frame of the talar component and rotated to a pure sagittal view. The contact point of the talar component, relative to the tibial component was measured from the midpoint of the talus in the sagittal plane. If the contact point between the two components was anterior to the midpoint of the talus in the sagittal plane, it was denoted as positive and a posterior position was denoted as negative. For analysis, if desired, the two components could be viewed from any direction, and measurements could be made of any of the 6 degrees-of-freedom.
RESULTS
Anteroposterior Contact Positions
On average, the anteroposterior contact positions did not translate significantly for the nonimplanted ankles. At heel-strike, the contact position was posterior to the midline in the sagittal plane, and at toe-off the contact position moved in the anterior direction (Figure 2). From heel-strike to toe-off, the average change in anteroposterior contact position was only 2.5 mm. Seven of the 10 patients experienced less than 2.0 mm of movement, while three experienced more than 6.0 mm of movement.
The maximal amount of movement was 10.6 mm, from a posterior position at heel-strike to an anterior contact position at toe-off (Figure 3).
Overall, the implanted ankles experienced even less anteroposterior motion than the nonimplanted ankles. On average, at heel-strike, the contact position was 0.4 mm anterior of the midline in the sagittal plane. At toe-off, the contact position moved anterior to 0.7 mm. Therefore, from heel-strike to toe-off, the average amount of anteroposterior movement was only 0.3 mm (Figure 4). From heel-strike to toe-off, all 10 patients experienced less than 3.5 mm of anteroposterior motion, and 8 of the 10 experienced less than 2.0 mm of movement.
Ankle Rotation
On average, the implanted ankles experienced minimal inversion and eversion and internal and external rotation, but they experienced greater variability during these rotations compared to the anteroposterior measurements. On average, from heel-strike to toe-off the patients experienced 1.3 degrees of ankle inversion and 2.8 degrees of ankle external rotation (Figures 5 and 6).
Average contact positions in the nonimplanted ankles during the stance phase of gait.
Maximal motion in a nonimplanted ankle during the stance phase of gait.
Average contact positions of the implanted ankles during the stance phase of gait.
During ankle inversion and eversion, the maximal amount of rotation was 14.1 degrees of ankle eversion. Two patients also experienced 11.3 degrees of ankle inversion. Therefore, although the average amount of ankle inversion and eversion was minimal, subject-to-subject measurements deviated significantly from the average. Only one patient experienced less than 1.0-degree of ankle inversion and eversion.
Average amount of implanted ankle inversion and eversion.
Average amount of implanted ankle medial and lateral rotation.
Similarly, the amount of ankle internal and external rotation during the stance-phase of gait also was quite different for each patient. The maximal amount of motion was 10.4 degrees of ankle internal rotation. Six of 10 patients experienced greater than 6.0 degrees of ankle internal or external rotation. Eight experienced ankle internal rotation and two external rotation. During both ankle inversion and eversion and internal and external rotation the 3D overlay procedure revealed these large rotations occurring from heel-strike to toe-off.
DISCUSSION
As the popularity of TAA gains momentum, an understanding of the biomechanics of each of the prostheses will become increasingly important to improve prosthesis design and ligament-balancing technique. In this fluoroscopic kinematic study, during the stance phase of gait with the Agility Total Ankle System the following findings were noted: 1) small magnitude of anterior and posterior movement of the contact point in the native ankle, 2) decreased anterior to posterior motion of the contact point in the Agility ankle compared to the native ankle, and 3) wide variability in inversion and eversion and internal and external rotation of the Agility ankle among patients.
One goal of TAA should be to establish a balance between constraint and freedom while mimicking movements of the normal ankle articulation. 13 Normal ankle motion includes 3D movement of the talocrural joint with a talar axis that continuously changes. It is well established that the native talus has differing medial and lateral talar dome radii, thereby allowing the talus to internally rotate relative to the tibia as the ankle plantarflexes. As movement is transferred between the foot and tibia, this rotation and translation of the talus is important to help dissipate forces and coordinate movement between the lower leg and the foot. 15,28,29
The Agility is a two-component prosthesis made up of a wide metallic tibial component and a narrower talar component. Variability in the movement of the talus may be caused by the deliberate mismatch in size between the larger tibial component and the smaller talar component. While the relative incongruence theoretically decreases the shear stresses transmitted to the bone-prosthesis interface, it does allow more inversion and eversion and rotational freedom, which can lead to edge loading and higher contact stresses. This variability in inversion and eversion and rotational freedom for the Agility prosthesis was confirmed by our kinematic gait study. The results are similar to those described by Komistek et al. 26 in an in vivo weightbearing kinematic study done using the Buechel-Pappas Total Ankle (Endotec, South Orange, NJ).
The observations in this study also are similar to those of Michelson et al. 33 In their study using a different unconstrained fixed bearing prosthesis (Irvine Ankle Arthroplasty, Howmedica, Rutherford, NJ), the kinematics were observed in vitro with conditions simulating partial weightbearing. They did, however, observe “toggling” of the tibia over the talar dome in the axial and coronal planes as the ankle moved sagittally. They stated that this reflected the incongruent nature of the prosthesis compared to the highly congruent native ankle. Our study confirmed that these same kinematic characteristics are seen in a weightbearing in vivo model using the Agility prosthesis.
Anterior and posterior translation of the contact point and variability of rotational and inversion and eversion movements are kinematic characteristics that may provide valuable information. In a recently published 7- to 16-year followup study of the Agility prosthesis, 15% of patients demonstrated expansile lysis, 22 which was attributed to wear-particle inflammatory reaction, similar to osteolysis seen around total hip implants. Increased translational movements, both anterior to posterior and rotational, of the talus within the mortise, as well as, edge loading may be causes of wear-particle inflammatory reaction. Based on this study the degree of anterior to posterior translation of the talus is relatively small and probably contributes minimally to polyethylene wear. It is a more likely hypothesis that generation of polyethylene particles is caused by edge loading and shear forces related to the increased inversion and eversion and rotational movement.
Despite the variability of inversion and eversion and rotational movements seen among patients in this study, the Agility prosthesis has already demonstrated excellent medium term results. 22 Further studies looking at the effect of the inversion and eversion and rotational freedom on polyethylene wear and prosthesis survival will assist in our understanding of the factors that lead to success or failure of total ankle prostheses.
Our study did not allow assessment of inversion and eversion and rotational freedom of the native ankle. This could be further investigated in another study, but based on the highly congruent nature of the native ankle, we would expect to see less freedom of motion in these planes when compared to this prosthetic ankle. The small number of patients and the lack of preoperative measurements also are methodological concerns with this pilot study but could certainly be included in future work. Despite these shortcomings, the observations of this study and the questions this study raises are noteworthy to surgeons who are interested in the improvement of ankle arthroplasty technology.