Background: Mobile-bearing total ankle replacement (TAR) enables motion at the tibial implant-polyethylene insert interface. This motion could lead to coronal translation of the talus relative to the tibia and may affect radiographic outcome. We aimed to assess the translation of the talus before and after mobile-bearing TAR to determine whether translation of the talus after TAR is associated with coronal plane alignment of the lower limb and hindfoot as well as to investigate the complications associated with talar translation.
Methods: In this retrospective cohort study, we enrolled 153 patients (159 ankles) with a minimum follow-up of 3 years who underwent mobile-bearing TAR. The location of the talus in the coronal plane was quantified with use of talar center migration (TCM) on anteroposterior radiographs both preoperatively and at postoperative intervals, and the relationship between them was investigated. Radiographic parameters in the coronal plane—including mechanical axis deviation (MAD), lateral distal tibial angle (LDTA), hindfoot alignment angle, and hindfoot moment arm—were measured. The relationship between TCM and radiographic parameters in the coronal plane was assessed in each group. The complications associated with talar translation were examined during the same period.
Results: During the 36-month follow-up period, the postoperative TCM showed a strong relationship with the preoperative TCM. Moreover, MAD, LDTA, and hindfoot alignment were significantly related to talar translation (p < 0.01). Complications included medial malleolar impingement in 5 cases (including delayed medial malleolar fracture due to medial impingement in 2 cases), insert dislocation in 1 case, and edge-loading in 2 cases; all of the cases with complications demonstrated implant overhang with talar translation.
Conclusions: Talar translation in the coronal plane after mobile-bearing TAR correlates with the preoperative talar translation. Talar translation arises from deformities of MAD, LDTA, and hindfoot alignment, and it may be accompanied by various complications, as observed on coronal radiography. Therefore, additional realignment procedures for coronal malalignment should be considered to prevent talar translation after mobile-bearing TAR.
Level of Evidence: Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.
Mobile-bearing total ankle replacement (TAR) allows for horizontal motion at the tibial implant-polyethylene insert interface1; therefore, translation of the talus is possible in both the coronal and sagittal planes2-4. Restoration of normal sagittal plane alignment of the implant impacts the longevity of TARs2-4, and a recent study indicated that the position of the talus, relative to the tibial axis in the sagittal plane, improves with accurate tibial and talar cutting5. However, to our knowledge, no reports have described the medial or lateral translation of the talus in the coronal plane. The medial or lateral translation of the talus may also influence the TAR results, as malposition of the talus in the coronal plane could lead to implant overhang or medial malleolar impingement6-10.
Translation of the talus in the coronal plane can be influenced by the slope of the tibial implant in the coronal plane2,4,11,12, as medially sliding momentum develops in cases of varus slope of the tibial implant. The slope of the tibial implant is influenced by the tibial cutting angle and the degree of mechanical axis deviation (MAD) from the tibial axis. Any deformity that increases MAD increases either the varus or valgus slope of the tibial implant; for instance, genu varum at any portion of the tibia increases the varus slope and genu valgum increases the valgus slope.
Hindfoot alignment influences the angular deformity of the talus in the coronal plane; accordingly, varus talar tilt develops in cases of hindfoot varus and valgus talar tilt develops in cases of hindfoot valgus. Hindfoot alignment may also influence the horizontal translation of the talus as angular momentum is divided into horizontal and perpendicular momentum in the coronal plane. Horizontal momentum acts as a shear force between the component and the insert, whereas perpendicular momentum acts as a compression force on the mobile insert.
Although tibial cutting changes the slope of the tibial plafond, MAD and hindfoot alignment do not change during TAR. Hence, the talar position in the coronal plane may remain unchanged after TAR. We hypothesized that the postoperative and preoperative positions of the talus in the coronal plane are related and that the coronal plane position of the talus is influenced by MAD and hindfoot alignment. Accordingly, for a mobile-bearing TAR, we aimed to assess whether there was a correlation between the preoperative and postoperative positions of the talus in the coronal plane, investigate whether MAD and hindfoot alignment influence the coronal plane position of the talus, investigate the degree of impact of MAD and hindfoot alignment on the direction of talar translation, and evaluate whether coronal plane malposition of the talus causes any clinically important complications after TAR.
Materials and Methods
In this retrospective study, we enrolled 159 ankles of 153 patients who underwent mobile-bearing TAR at Inje University Seoul Paik Hospital between January 2004 and December 2012. The mean patient age at the time of the procedure was 62.4 years (range, 40 to 83 years), and the minimum follow-up period was 3 years (range, 3 to 8.6 years). The subjects in the patient group were recruited from among 342 cases of TARs that were performed by a single surgeon using the HINTEGRA prosthesis (Newdeal) during the same period. The inclusion and exclusion criteria of this study are summarized in Figure 1. Radiographs of 73 lower limbs of 73 skeletally mature individuals in a control group were used to determine the normal range of the talar position in the coronal plane; these individuals had no history of ankle surgery and had no gross abnormality of lower-limb alignment or any apparent disorders (Table I). This study was approved by the institutional review board of our hospital.
The following radiographs were obtained for assessment: weight-bearing anteroposterior radiographs of the ankle and full-length anteroposterior radiographs of the lower extremity and in the hindfoot alignment view. The MAD, lateral distal tibial angle (LDTA), hindfoot alignment angle, and hindfoot moment arm were measured. MAD was defined as the angle between the mechanical axis and the tibial axis13, and it was measured with use of full-length anteroposterior radiographs of the lower extremity (Fig. 2-A).
The coronal position of the tibial implant was assessed with use of the LDTA (Fig. 2-B). The LDTA was defined as the angle between the tibial axis and the articular surface line of the tibial component on standing ankle anteroposterior radiographs14. The talar tilt angle was defined as the angle between the articular surface lines of the tibial and talar components. The tibiotalar angle was defined as the angle between the tibial axis and the articular surface line of the talar component.
Hindfoot alignment was assessed with use of the hindfoot alignment angle and hindfoot moment arm (Fig. 2-C). The hindfoot alignment angle was defined as the angle between the tibial axis and the calcaneal axis. The hindfoot moment arm was defined as the distance between the most inferior aspect of the calcaneus and the tibial axis15.
Radiographic evaluations were performed as follows. The weight-bearing anteroposterior radiographs of the ankle were made preoperatively, 6 months postoperatively, and annually thereafter, whereas full-length anteroposterior radiographs of the lower extremity and hindfoot alignment view were made preoperatively and 6 months postoperatively.
Each step of the investigation is described sequentially. The subsequent steps were performed with use of the findings from the previous steps. Therefore, it was important to include some of the results in this section in order to describe these subsequent steps.
The position of the talus in the coronal plane was assessed with use of talar center migration (TCM) on weight-bearing anteroposterior radiographs of the ankle (Fig. 3)16. The ankles were divided into 3 groups on the basis of 2 standard deviations (SDs) of the mean of the TCM in the control group (0.4 ± 0.87 mm). The neutral talar group (65 ankles) included ankles in which the center of the talus was located at or within 2 SDs (2.1 to –1.4 mm), the medial talar translation group (62 ankles) included ankles in which the TCM was located more than 2 SDs medially (>2.1 mm), and the lateral talar translation group (32 ankles) included ankles in which the TCM was located more than 2 SDs laterally (<1.4 mm). The mean change in TCM over the follow-up period was assessed within each group. Furthermore, the mean value of the TCM in each group was compared among the groups at each time period (Fig. 4).
At 6 months postoperatively, the patients could stand with full weight-bearing and the TCM did not differ among the groups; therefore, we assumed that the difference in radiographic parameters at 6 months postoperatively could adequately reflect the differences in TCM at 3 years postoperatively. To assess the cause of the change in TCM over the 3-year follow-up period, the radiographic parameters recorded at 6 months postoperatively were correlated with the TCM at 3 years postoperatively.
Although the influence of each radiographic parameter on the change in talar position in the coronal plane can be determined from previous results, we presumed that the direction and degree of influence exerted by each radiographic parameter would differ. We divided the ankles into 3 groups by using the TCM values at 3 years postoperatively relative to 2 SDs of the TCM of the control group (medial talar translation group at 3 years postoperatively: 44 ankles; neutral talar group at 3 years postoperatively: 75 ankles; and lateral talar translation group at 3 years postoperatively: 40 ankles) to understand the cause of the difference in direction and degree of coronal plane translation of the talus from 6 months postoperatively to 3 years postoperatively. There was a clear correlation of the TCM between the preoperative period and the 3-year postoperative period, and the direction of talar translation changed slightly compared with that during the preoperative period. Moreover, the relationship between talar translation and radiographic parameters in the coronal plane was assessed with use of multinomial logistic regression.
The radiographic parameters were evaluated at the preoperative and postoperative periods. The complications detected on ankle standing radiographs that were related to talar translation were investigated after TAR. In particular, the presence of implant overhang, malleolar impingement, insert dislocation accompanied by talar translation, and edge-loading due to talar tilt were investigated.
An a priori power analysis was performed to determine the appropriate sample size. The control group showed a power of 0.8 for detecting an effect size of 0.3 at an alpha level of 0.05. The minimum number of subjects needed to satisfy this condition in the control group was found to be 71. The lateral talar translation group had the smallest sample size among the groups compared before TAR, with 32 patients; the a priori power analysis indicated a power of 0.9 for an effect size of 0.5 at an alpha level of 0.05. All of the groups had a normal distribution according to the Shapiro-Wilk normality test.
Radiographic measurements were performed on 2 separate occasions by 2 independent investigators in a blinded manner. The investigators were simultaneously instructed by a surgeon who was trained in foot and ankle surgery. Each investigator then practiced taking the measurements on sample radiographs while being observed. The investigators were blinded to one another’s measurements and to their own previous measurements; each set of measurements was obtained with a minimum interval of 3 months to ensure that the previous alignment angle measurements would not be remembered. Interobserver and intraobserver reliability was determined by calculating the intraclass correlation coefficients for continuous data; the interobserver and intraobserver class measurements of all of the radiographic parameters were satisfactorily correlated (Table II).
The TCM values among the follow-up points were compared with use of the repeated-measures analysis of variance (ANOVA) test for each group, and the mean value of the TCM in each group was compared with use of 1-way ANOVA at each time period. Bonferroni multiple comparison procedures were used for post-hoc comparisons. Correlation analysis of the preoperative and postoperative TCM, which were normally distributed, was performed with use of the Pearson correlation coefficient.
Multinomial logistic regression analysis was used to model the effects of radiographic parameters in the coronal plane on talar translation. The ankles in the medial talar translation group and lateral talar translation group were compared with those in the neutral talar group with use of logistic regression analysis to calculate the odds ratios of the radiographic parameters in the coronal plane. P values of <0.05 indicated significance in the logistic regression analysis. All statistical analyses were performed with use of SPSS version 16.0.
The mean TCM result of the neutral talar group showed no meaningful changes at any time period. However, the TCM value showed significant changes from the 6-month to the 36-month follow-up in the other 2 groups, and the TCM values among the 3 groups became significantly different at the 36-month follow-up (Fig. 4). In fact, the TCM values at the 36-month follow-up showed a significantly strong relationship (R = 0.766, p < 0.05).
The TCM at the 36-month follow-up showed a significant positive correlation with the radiographic parameters of proximal alignment (MAD and LDTA; p < 0.001 for both), and showed a significant negative correlation with hindfoot alignment (hindfoot alignment angle and hindfoot moment arm; p < 0.001 and p < 0.01, respectively) (Table III).
On logistic regression analysis, the odds ratios of the MAD and LDTA were found to be significantly larger in the medial talar translation group at 3 years postoperatively (p < 0.001 for both) and smaller in the lateral talar translation group at 3 years postoperatively (p = 0.014 and p = 0.005, respectively) as compared with the neutral talar group at 3 years postoperatively (Table IV).
However, the odds ratios of the hindfoot alignment angle and hindfoot moment arm were significantly smaller in the medial talar translation group at 3 years postoperatively than in the neutral talar group at 3 years postoperatively (p < 0.001 and p = 0.011, respectively). Although the odds ratios of the hindfoot alignment angle were significantly larger in the lateral talar translation group at 3 years postoperatively than in the neutral talar group (p < 0.001), the odds ratios of the hindfoot moment arm in the lateral talar translation group at 3 years postoperatively were not significantly different (p = 0.333) (Table IV).
The LDTA, tibiotalar angle, and talar tilt angles were significantly corrected postoperatively (p < 0.001 for all) (Table V). In fact, all radiographic parameters around the ankles showed favorable results postoperatively. Of 8 cases with implant overhang, 5 showed medial malleolar impingement. Among these cases, 2 had delayed medial malleolar fracture and were treated conservatively. Moreover, 1 case with insert dislocation and 2 cases with edge-loading had initially presented with implant overhang (Table VI).
The mobile-bearing design of the entire ankle allows motion at the interface between the tibial component and polyethylene insert1, which could lead to coronal and sagittal translations of the talus relative to the tibia. Coronal and sagittal plane alignment should be restored after TAR12 to avoid asymmetric contact pressure between the component and the polyethylene insert17. Although few studies have focused on sagittal translation of the talus after mobile-bearing TAR2-4, to our knowledge, no study has been conducted on translation of the talus in the coronal plane after mobile-bearing TAR.
A significant correlation was observed between the preoperative and postoperative talar position in the coronal plane at the 36-month follow-up. The ankles were preoperatively divided into 3 groups according to the direction of the talar displacement in the coronal plane. The TCM values did not differ among the groups at the 6-month follow-up, gradually returned to the preoperative direction after TAR, and eventually showed a significant difference among the groups after 3 years (Figs. 4 and 5). However, the TCM values showed only a minimal change from 3 years until the final follow-up. As the patients could stand and walk comfortably at approximately 6 months postoperatively, we assumed that the deforming force was only minimally active until 6 months postoperatively. Moreover, we assumed that the deviation of radiographic alignment at 6 months postoperatively would cause the subsequent translation of the talus.
We also sought to clarify whether MAD and hindfoot alignment are involved in the repositioning of the talus to its preoperative positon. Therefore, we determined the correlation of the radiographic parameters at 6 months postoperatively with the coronal plane talar position at 3 years postoperatively. We noted that the MAD, hindfoot alignment, and LDTA were correlated with the TCM values at 3 years postoperatively.
As MAD and LDTA directly influence the slope of the tibial plafond in the coronal plane, it appears reasonable that both of these radiographic parameters are correlated with the TCM value. MAD was used to assess the deviation of the tibial axis from the mechanical axis because the MAD can be a translating force even though the tibial cutting was done perpendicular to the tibial axis. However, the relationship between hindfoot alignment and TCM values is not easily understood; therefore, we assessed the directional relationship between these radiographic parameters by dividing the ankles into 3 groups at the 3-year follow-up. As expected, varus slope of the tibial plafond was related to medial translation of the talus, and valgus slope was related to lateral translation. However, translation of the talus showed a significant negative correlation with hindfoot alignment, which suggests that a valgus hindfoot was related to medial translation of the talus. Moreover, the presence of a significant odds ratio of valgus hindfoot alignment in the medial talar translation group suggests that most ankles with medial translation of the talus were associated with valgus hindfoot alignment. In contrast, lateral translation of the talus was not significantly related to hindfoot varus. Therefore, we suggest that medial displacement calcaneal osteotomy should be considered in ankles with a large medial translation and a large degree of hindfoot valgus.
Medial malleolar impingement6-8, delayed malleolar fracture6, implant overhang9,10, insert dislocation, and edge-loading were associated with talar translation and are known causes of prosthetic failure after TAR. In the present study, medial malleolar impingement and overhang were present in both the medial and lateral talar translation groups. In their cadaveric study, Athanasiou et al.18 reported that the normal cartilage thickness of the medial gutter was 2.3 mm. Moreover, Park et al.19 reported that the mean medial clear space was 2 ± 0.4 mm during neutral ankle flexion on valgus-stress radiographs. Therefore, the neutral range of talar translation (−1.4 to 2.1 mm) in the present study would not lead to implant overhang or malleolar impingement; however, a larger translation would cause direct bone-to-bone contact.
Henricson and Ågren20 reported that secondary procedures are more common in TAR with preoperative hindfoot malalignment in order to ensure the stability of the TAR. In some cases in the present study, the patient with preoperative hindfoot malalignment had an unstable ankle joint after TAR (Figs. 6-A and 6-B). We believe that some cases of translation of the talus may represent a manifestation of the tilt force that arose from hindfoot malalignment. Hence, in cases with a preoperative medial or lateral translation of the talus along with a large degree of hindfoot malalignment, the simultaneous correction of the hindfoot malalignment should be considered.
This study has some limitations. First, the sample size of the lateral translation group was smaller than that of the other groups. Second, the other factors potentially influencing the position of the talus in the coronal plane include intraoperative factors such as the cutting angle of the talar dome, position of the talar component relative to the talus, and ligament balancing. However, both the cutting angle of the talus and the stability after ligament balancing are difficult to quantitatively assess.
In conclusion, the talus progressively translates to its preoperative position after mobile-bearing TAR through coronal alignment of the lower limb and hindfoot. The MAD and hindfoot alignment are factors strongly associated with talar translation.
Translation of the talus affects the radiographic outcome and is accompanied by various complications, as noted on coronal radiographs. Therefore, if needed, additional realignment procedures for coronal malalignment should be considered to prevent talar deviation after mobile-bearing TAR.
Note: The authors thank Kyoung Min Lee (Seoul National University Bundang Hospital) and Dong Il Chun (Soonchunhyang University Hospital) for giving us statistical advice to analyze data and complete this report.
Investigation performed at the Seoul Foot and Ankle Center, Inje University Seoul Paik Hospital, Seoul, Republic of Korea
Disclosure: This work was supported by the Inje Research and Scholarship Foundation (Seoul Foot and Ankle Center, Inje University Seoul Paik Hospital, Seoul, Republic of Korea). The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article.
- Copyright © 2017 by The Journal of Bone and Joint Surgery, Incorporated