Difference in orientation of the talar articular facets between healthy ankle joints and ankle joints with chronic instability

Abstract Since both the talocrural and subtalar joints can be involved in chronic ankle instability, the present study assessed the talar morphology as this bone is the key player between both joint levels. The 3D orientation and curvature of the superior and the posteroinferior facet between subjects with chronic ankle instability and healthy controls were compared. Hereto, the talus was segmented in the computed tomography images of a control group and a chronic ankle instability group, after which they were reconstructed to 3D surface models. A cylinder was fitted to the subchondral articulating surfaces. The axis of a cylinder represented the facet orientation, which was expressed by an inclination and deviation angle in a coordinate system based on the cylinder of the superior talar facet and the geometric principal axes of the subject's talus. The curvature of the surface was expressed as the radius of the cylinder. The results demonstrated no significant differences in the radius or deviation angle. However, the inclination angle of the posteroinferior talar facet was significantly more plantarly orientated (by 3.5°) in the chronic instability group (14.7 ± 3.1°) compared to the control group (11.2 ± 4.9°) (p < 0.05). In the coronal plane this corresponds to a valgus orientation of the posteroinferior talar facet relative to the talar dome. In conclusion, a more plantarly and valgus orientated posteroinferior talar facet may be associated to chronic ankle instability.


| INTRODUCTION
Identifying factors that are related to chronic ankle instability (CAI) can decrease unnecessary delay in diagnosis and treatment, increase the effectiveness of treatment as well as reduce the direct costs of medical care, and decrease the high socioeconomic burden associated with ankle sprains. 1 CAI is defined as instability of the ankle joint with the sense of giving way, episodes of recurrent ankle sprains, with or without the presence of joint laxity. 2,3 Varus malalignment of the lower leg or the hindfoot has shown to increase the risk of CAI. [4][5][6][7][8][9][10][11] The malalignment of the hindfoot is radiographically defined as the calcaneal offset in relation to the longitudinal axis of the tibia. A varus malalignment shifts the axial load laterally compared to a neutral alignment, which subsequently creates an inversion moment that can result in a lateral ankle sprain if not timely countered at foot landing. 12 While the correlation of tibial malalignment and ankle joint instability has been extensively analyzed, 5 only few studies assess the importance of hindfoot varus alignment on the development of CAI.
The alignment of the hindfoot is influenced by the shape of the bones. In particular, the orientation of the articulating surfaces as well as the deformation in the mid-and forefoot induced by foot loading play an important role in this alignment. Both the talocrural and subtalar joints can be involved in CAI. 2 The talus is a key player between both joint levels and therefore also regarding the alignment of the hindfoot. Malalignment can be present at any level in the mechanical chain from tibia to the ground. 7,13 The influence of the morphology of the talocrural joint on the development of CAI is well documented. For example, the radius of the talar dome was shown to be larger in patients with CAI than in controls. 14 However, controversies exist on the role of the morphology of the subtalar joint in the relationship with CAI. Furthermore, abnormal orientation at one joint level in the mechanical chain of the hindfoot can be compensated by an adaptation at another joint level. 7,14 This makes it important to determine the morphology per joint level and the interrelationships in orientation at these joint levels.
A three-dimensional (3D) analysis of the orientation of the articulating facets in the mechanical chain of the hindfoot is currently missing in present literature. Previously, the determination of subtalar joint orientation was reported in a two-dimensional (2D) manner (e.g., 4,13,15,16 ), however, this is prone to errors and has limitations. 9,17,18 As a result, the present study was designed to address the potentially present differences in the morphology of the talar facets between subjects with CAI and those without (i.e., healthy controls).
A comparison was made between healthy controls and patients with CAI to assess the differences in orientation of the posteroinferior subtalar joint relative to the talar dome in 3D space, and of the curvature of these facets. The hypothesis is that this orientation and curvature is significantly different between patients with CAI and healthy controls.

| Datasets
For this retrospective case-controlled study, three datasets from three previous studies were used. [18][19][20] Each study was approved by the local Institutional Review Board. In all three studies, the distal tibia, talus, and calcaneus of the participants were imaged in supine position with computed tomography (CT) (0.3 <voxel size <0.4557 mm, tube charge of 120 kV, and radiation dose of 150-160 mAs, Brilliance 64 CT scanner, Philips Healthcare, Amsterdam, The Netherlands). A total of 40 ankles from two groups of 20 healthy volunteers each with non-symptomatic feet/ankles were included to form the control group for the present study. 18,19 In case both sides were scanned, an arbitrary choice was made for one of the sides assuming that the differences within a subject is smaller than the differences between subjects. 18 The CAI group included 12 patients with CAI (Table 1). 20 If both sides of the patient were affected, the side with the highest incidence of previous ankle sprains was selected. No a-priori power analysis was performed since there were no data available to perform such analysis.

| Hanalysis
The talus was segmented from the 3D-CT image and modeled as 3D polygons using custom made software, 21 which was developed and validated at our academic hospital. 22 The following subchondral ar-

| Data processing
The facets were modeled as a segment of a cylinder. 18 Hereto, a cylinder was fit to the points in each selected polygon surface using a nonlinear least-squares optimization process ( Figure 1).
The cylinders were named TalusSF and TalusIF Another outcome of the present study was the interrelationship between the two facets of the talus. The orientation of TalusIF in respect to TalusSF describes (a part of) the talar morphology. To align all tali uniformly and reproducibly, a local orthogonal right-handed coordinate system was defined with the following steps. All left tali were mirrored to right tali for further analyses in a corresponding orientation. The X-axis was set parallel with the cylinder's axis of TalusSF and was in a medial-lateral direction. Next, the geometric principal axes of the subject's talus were determined. The Y-axis was defined using the Gram-Schmidt process with the determined X-axis and the talar principal axis in antero-posterior direction. 23 The Z-axis was defined perpendicular to the X-and Y-axis with the positive direction orientated superiorly. The orientation of a cylinder was defined by a direction vector, which was chosen in approximately the medial direction in this study. From this, it follows that the direction vector of TalusSF was parallel to the X-axis, pointing in the negative direction.
The orientation of TalusIF was defined by two angles

| Intraobserver and interobserver analysis
The intraobserver reliability in cylinder orientation and radius, introduced by manual selection of the facet surfaces, was evaluated using three randomly chosen feet; one per study cohort. Surface selection was done three times with at least 1 week in between.
Interobserver reliability was evaluated based on manual selection of the facet surfaces by two observers (RPK and JD; both researchers have several years of experience in segmentation and 3D software) of nine randomly selected hindfeet (three from the CAI group and six from the control group). The two observers were blinded to each other's selections.

| Statistical analyses
The orientation angles of TalusIF, the radii of the cylindrical fits, and the root mean square errors were analyzed for normal distribution with the Shapiro-Wilk test. For normally distributed data, outliers were identified as exceeding the value of three times the standard deviation from the mean. For non-normally distributed data, outliers were identified as exceeding the value of 1.5 times the interquartile range from the median. An outlier in one of the orientation angles was removed from further analyses for both angles. An outlier in the root mean square error or radius of a cylinder facet was removed from further analyses for that facet. The differences between the control group and the CAI group were tested with an independent Student t test in case of normally distributed data and with a Mann-Whitney U test for non-normally distributed data. The Hegdes' g value for effect size was calculated. The Hedges' g value was interpreted as small effect (closest to 0.2); medium effect (closest to 0.5); or large effect (closest to 0.8). 24 The intraclass correlation coefficient (ICC) was calculated as a measure of intraobserver and interobserver reliability. 3 | RESULTS

| Facet orientation
The orientation angles were normally distributed. One outlier was

| Cylinder radius
The cylinder radii were normally distributed. One outlier was identified for TalusSF in the control group, and one outlier in the CAI group for the root mean square error. The cylinder radii of TalusSF were not statisti-

| Root mean square errors of the fits
The root mean square errors of the cylinder fit were normally distributed, except for TalusIF in the CAI group. After removal of this outlier, the remaining data were normally distributed. The root mean square errors of TalusSF were not statistically significantly different

| Intraobserver and interobserver analyses
The intra-and interobserver reliability showed excellent reliability for each of the outcome measurements ( Table 2). The differences in the intra-observer measurements were smaller than the interobserver measurements. The largest differences were found in deviation angle of TalusIF.

| DISCUSSION
The present study investigated the orientation and curvature of the superior and posteroinferior talar articulating surfaces of nonsymptomatic individuals and of patients with chronic ankle instability. Previous reports have identified a varus hindfoot deformity to be a risk factor in the development of CAI after a first ankle sprain. [4][5][6][7][8][9][10][11] Since both the talocrural and subtalar level can be involved in CAI, 2 the present study assessed the talar morphology as this bone is the key player between both joint levels.
The most important finding was that the mean inclination angle of the cylinder's axis of the posteroinferior talar facet was statistically significantly more plantarly orientated, by 3.5 degrees, in the CAI group compared to the control group. In a coronal plane this corresponds to a talus with a slight valgus of the posteroinferior facet relative to the talar dome ( Figure 4).
We expected to find a varus orientation of the subtalar joint in patients with CAI when comparing them to the healthy population.
This would have been in accordance with earlier studies who found a correlation between varus malalignment and CAI. 4,5,9 In our cohort, Note: Also, the average ± 1 SD and range (minimal-maximal) of the absolute differences are presented for the intra-and inter-observations. TalusSF: cylinder of superior facet of the talus. TalusIF: cylinder of posteroinferior facet of the talus. Absolute differences angles in degrees, and radius in mm.

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No statistically significant differences were demonstrated between the groups for the radii nor root mean square errors of the cylinder fits. The root mean square error indicates how well the facets fit to a cylinder. These results imply that the morphology of the two facets was comparable between the two study groups.
Frigg et al. 14 demonstrated an inverse relationship between the curvature of the talar dome and the stability of the ankle joint. This larger curvature alone cannot explain a more instable joint configuration. The coverage of the tibial surface is also important. The most instable joint configuration would be a combination of a small tibial coverage and a large curvature. 14  surface fit possibly results in a larger radius than the 2D radiographically determined radius. 14 Another explanation is that in our study groups we had a larger percentage of male subject that resulted in the larger average radius.
The present study focused on the bony morphology of the talus as a possible risk factor for CAI. Caution is required when interpreting the results of this one factor, namely the bony morphology, when in fact CAI is multi-factorial. 2 There is an overlap in individuals with and without CAI (Figures 2 and 3). Subjects that served as controls may in the future acquire CAI. These could be the individuals that also have slightly more valgus in the talus, but that with what can be predicted from our cylinder model of the posterior subtalar joint. 28 The subtalar joints comprise of more than only the posterior articulating surfaces. The whole subtalar joint is more complex. Our method could be used in determining the 3D orientations of the hindfoot bones, and does not reflect the full complex subtalar joint morphology.
The strength of this present study was that the interrelationship of the superior and posteroinferior talar facets could be determined in 3D without influence of the position of the foot in the CT scanner.
The applied method for selecting the articular surface on 3D surface models proved to be very reliable based on the results of the intraobserver and interobserver analyses, but was laborious. A semiautomatic procedure can make this step more efficient in the future.
The present study provides a next step in identifying morphology of bony structures in the hindfoot being associated with CAI. It is the first study that analyzed the interrelationship of the talocrural and subtalar joints in 3D. Future studies should focus on a method to determine the orientation at each joint level in the chain from the knee to the ground in 3D with a golden standard for a frame of reference, preferably in a weight bearing condition.
In conclusion, it was found that the morphology of the talus itself does not necessarily need to be in "varus" to contribute to CAI. It was demonstrated that CAI patients have a more valgus position of the posteroinferior talar facet that may be a risk factor for CAI by varus compensation. The relationship between the superior and posteroinferior facets of the talus in relation to the lower leg axis in varus and valgus deformities, the overall alignment, needs further research.