Abnormal axial rotation of the Talus on weight-bearing computed tomography in patients with micro-instability of the ankle

Abstract

Background

While diagnosing mechanical chronic ankle instability (CAI) is often straightforward, identifying subtle micro-instability remains challenging. A tear of the superior bundle of the lateral ligament complex has been proposed as a contributor to micro-instability, potentially causing increased anterior translation and internal talar rotation under load. Weight-bearing computed tomography (WBCT) offers valuable insight into hindfoot alignment and load-induced deformation, making it a promising tool for assessing suspected micro-instability. This study aims to compare talar axial rotation between symptomatic patients reporting subjective micro-instability and asymptomatic controls.

Methods

Forty ankles from patients with osteochondral lesions of the talus (OLT) and signs of micro-instability were compared to asymptomatic controls. WBCT and image analysis software were used to generate 3D models and perform semi-automated hindfoot alignment measurements. Inter- and intra-observer reliability was also assessed.

Results

The OLT group showed a mean axial rotation difference of −4.5 ± 4.5 degrees compared to controls (P < 0.001), indicating increased external rotation of the talus. Intra-observer reliability was good to excellent (ICC 0.88, 0.92), and inter-observer agreement was excellent (ICC 0.93, 0.90).

Conclusion

WBCT did not reveal abnormal internal rotation in OLT patients with subjective micro-instability. Instead, significant external rotation of the talus was observed. These findings suggest that external rotation may predispose patients to micro-instability. Further studies are needed to clarify its role in functional ankle instability

Level of evidence

III

1

Background

Chronic lateral ankle instability (CAI) has traditionally been classified into mechanical and functional types , . Mechanical CAI refers to pathological laxity of the ankle joint and will have a positive anterior drawer test and/or a positive talar tilt test ,,, , whereas functional CAI involves a subjective sense of instability despite normal ligamentous integrity on physical examination . However, this dichotomy has become increasingly outdated. Recent studies, particularly those by Vega and colleagues, have introduced the concept of ankle micro-instability: a condition characterized by subtle capsuloligamentous insufficiencies, involving the superior fascicle of the anterior talo-fibular ligament (ATFL), which may not be detectable through standard imaging or clinical tests , . Diagnosing tears of the superior ATFL can be challenging, particularly when the tears are partial. In these instances, patients often do not exhibit classic symptoms of mechanical instability, and diagnostic tools such as Magnetic Resonance Imaging (MRI) and ultrasound (US) frequently produce inconclusive results. Arthroscopy is currently considered the gold standard for diagnosing superior ATFL tears , . However, the fact that a definitive diagnosis is often made only during surgery adds complexity to preoperative planning.

Dalmau-Pastor et al., in a cadaveric study, investigated the mechanical effects of ATFL tears on ankle stability . Their findings revealed that superior fascicle tears were associated with increased anterior translation and internal rotation of the talus, particularly during plantarflexion. In contrast, Yuan et al. analyzed axial rotation of the talus using non-weight-bearing imaging and found that excessive internal rotation is characteristic of Mechanical CAI, but not Functional CAI . Weight-bearing computed tomography (WBCT) addresses this limitation by providing imaging in a weight-bearing position, allowing for more accurate demonstration of hindfoot alignment . As a result, WBCT holds potential as a tool for evaluating abnormal axial rotation in cases of microinstability. Identifying abnormal internal rotation in these cases using WBCT could be especially valuable, as it may indicate axial instability of the tibio-talar joint, allowing surgeons to address and potentially correct this issue during surgery, which could improve patient outcomes.

In the current study, WBCT, 3D modules, and semi-automatic measurement techniques were used to analyze the axial rotation of the talus in patients experiencing microinstability. The primary objective of the study is to determine if abnormal axial rotation is present in this cohort. A secondary objective is to assess the reliability of these measurements.

2

Material and methods

2.1

Patient population

This retrospective study was conducted in accordance with the Declaration of Helsinki and the Guidelines for Good Clinical Practice. All the patients included in this study were evaluated in our outpatient clinic between January 2021 and December 2023, where WBCT is performed routinely. Two groups are included in the study. The study cohort comprised 40 patients (40 ankles) diagnosed with posttraumatic osteochondral lesions of the medial talus, scheduled to undergo arthroscopic surgery osteochondral lesion of the talus (OLT) group. A control group consisted of 40 patients (40 ankles) with asymptomatic ankles, who underwent imaging for unrelated pathologies in the contralateral limb. The inclusion criteria were: (1) patient with posttraumatic medial OLT ( Image 1 ) (2) positive history of Deep ankle pain for more than six months (3) patient report of subjective ankle instability feeling or recurrent ankle sprains with or without lateral ankle pain (4) negative anterior drawer in a neutral and plantarflex position and negative talar tilt tests. The exclusion criteria were (1) patients younger than 18 years, (2) patients with mechanical instability, (3) damaged inferior ATFL bundle on standard MRI or US, and (4) a history of ankle fracture or lateral OLT. All the patients were evaluated for instability in our outpatient clinic by an orthopedic resident and two fellowship-trained orthopedic surgeons specializing in foot and ankle surgery. Patients’ images were extracted from the medical center’s picture archiving and communications system (PACS) database. Patient demographics were obtained through the electronic medical records scanned for relevant surgical and orthopedic history.

Image 1

Three-dimensional model of the talar dome of the talus generated by WBCT in a patient exhibiting medial OLT on the medial side.

2.2

WBCT scan protocol

A WBCT was performed in a uni pedal single stance position. The Following image acquisition protocol was implemented for all patients: 0.2-mm-slice thickness, 1-mm slice, and 0.014-mSv effective radiation dose per scan (Planmed Verity, Planmed Oy, Helsinki, Finland). The forefoot and hindfoot (low and high) must be acquired separately according to the system. Next, a stitching algorithm was utilized to combine several volumes into one. Image analysis was performed semiautomatically by DISIOR Bonelogic Ortho Foot and Ankle Software (version 2.0, Helsinki, Finland). Initially, a 3 dimensional model (3DM) was reconstructed from the WBCT-generated DICOM files. An orthopedic surgeon marked each bone in a designated maker point. The software then creates a 3D module, and the predetermined landmark is used to represent the bone axis and calculate angles semiautomatically visually ( Image 2 A and B).

Image 2

Depicts the segmentation process. During step II A, an orthopaedic surgeon meticulously selects every bone in the ankle and foot, emphasizing the boundaries of each joint. Moving to step II B, the Artificial Intelligence-driven segmentation process outlines each bone, constructing a three-dimensional model of the foot.

2.3

Alignment analysis

WBCT-generated 3DM was used to analyze the ankle alignment in the coronal, sagittal, and axial plains. Angles were measured semiautomatically utilizing the software’s preexisting configurations or manually using the semiautomatic generated bone axis as a reference point.

The coronal alignment was assessed using four semiautomatically generated angles; the Salzmann 20 degrees set the overall alignment of the hindfoot. The Alfa and Talar Surface Angle (TSA) evaluated the tibiotalar joint alignment. The Talo Calcaneal Angle (TCA) assessed the subtalar alignment ( Image 3 ).

Image 3

Illustration of an example of semi-automatic angle measurements of the: A) Tibio-Talar Surface angle (TSA); B) Talo-Calcaneal coronal angle; C) Saltzmann view angle at 20°; D) Beta angle.

The sagittal alignment was assessed using the Betta angle, calculated semiautomatically.

The axial rotation was assessed using the posterior talar rotational angle (PTARA). It was measured manually and was defined by the intersection of the interaxial line, which connects the anatomical axis of the tibia and fibula and the transverse axis of the talar body ( Image 4 ).

Image 4

Illustration of the manual assessment of axial rotation in the talus using the Posterior Talar Rotational Angle (PTARA) angle. The axial view of the hindfoot displays the calcaneus three-dimensional model in grey, with the tibia, talus, and fibula excluded, showcasing only the semi-automatic visual representation of the axis. Point A represents the longitudinal axis of the tibia, and Point B represents the axis of the fibula, establishing the transverse axis between them. Line C parallels the transverse axis of the talus (depicted as the red transverse line). The PTARA angle (Angle D) is determined by a perpendicular line between AB and line C.

2.4

Data comparison

Two fellowship-trained orthopedic surgeons specializing in foot and ankle surgery performed the measurements. To ensure the use of standardized language and grading scales, a pilot evaluation of 10 cases was performed. The two raters segmented, analyzed, and measured each case independently. The foot and ankle alignment was measured for all the OLT; each rater collected alfa, TSA, Salzmann 20°, and BETA measurements generated by the image processing software. Each rater also calculated the PTARA angles using the computer-generated bone axis. Two months after the initial reading, a second reading followed the identical steps. Consequently, the OLT group’s coronal, axial, and sagittal alignment were compared to the control group. An interrater agreement between the two raters and an intra-rater analysis between the two readings was also calculated.

2.5

Statistical analysis

Shapiro-Wilk test was used to evaluate the normal distribution of the data. Normally distributed data are presented as mean ± standard deviation; abnormally distributed data are presented with median and interquartile range (IQR). Compression between the groups was made using a two-sided paired sample T-test for the normally distributed data and a related sample Wilcoxon signed rank test for the not normally distributed data. The significance level was set to P < 0.05. ICC was calculated for a single measure using a two-way random effect model of absolute agreement. Interclass correlation coefficient (ICC)< 0.5 was interpreted as poor agreement, ICC 0.5–0.75 as moderate agreement, ICC 0.75–0.9 was considered good agreement, and ICC 0.9–1 as excellent agreement. Statistical analysis was performed using the IBM SPSS statistical software (IBM Corp. Released 2013. IBM SPSS Statistics, Version 29.0. Armonk, NY: IBM Corp.)

3

Results

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Sep 5, 2026 | Posted by in ORTHOPEDIC | Comments Off on Abnormal axial rotation of the Talus on weight-bearing computed tomography in patients with micro-instability of the ankle

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