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Differences Between Subtalar Instability and Lateral Ankle Instability Focusing on Subtalar Ligaments Based on Three Dimensional Isotropic Magnetic Resonance Imaging

Yoon, Deuk Young, MD*; Moon, Sung Gyu, MD, PhD*; Jung, Hong-Geun, MD, PhD; Kim, Na Ra, MD, PhD*

Journal of Computer Assisted Tomography: July/August 2018 - Volume 42 - Issue 4 - p 566–573
doi: 10.1097/RCT.0000000000000717
Musculoskeletal Imaging

Objective The purpose of this study was to assess the differences between subtalar instability (STI) and lateral ankle instability (LAI) focusing on subtalar ligaments using 3-dimensional (3D) isotropic magnetic resonance imaging (MRI).

Methods Preoperative MRIs of 10 patients with STI who failed nonoperative treatment and consequently underwent arthroscopic subtalar reconstruction were compared with preoperative MRIs of 23 patients with LAI who underwent ligament repair or reconstruction. Dimensions of anterior capsular ligament (ACL), interosseous talocalcaneal ligament (ITCL), calcaneofibular ligament (CFL), and anterior talofibular ligament (ATFL) were measured. Tears of ACL, ITCL, CFL, ATFL, cervical ligament, and inferior extensor retinaculum were analyzed.

Results Patients with subtalar instability had significantly lower ACL thickness and width than patients with LAI (thickness: 1.48 vs 2.12 mm, P = 0.045; width: 7.30 vs 8.64 mm, P = 0.029). An ACL thickness of 1.8 mm or less had sensitivity and specificity both at 75.0%, and an ACL width of 8 mm or less had sensitivity of 75.0% and specificity of 85.0% for discriminating STI from LAI. Absence or complete tear of ACL was more frequent in patients with STI than in patients with LAI (60.0% vs 13.0%, P = 0.010). The ATFL thickness was significantly greater in patients with LAI (P = 0.001). Complete tear of ATFL was more common in patients with LAI (P = 0.008). Complete tear of CFL was common in both the STI and LAI groups without significant difference (20.0% vs 21.7%). There was no significant difference in thickness and width of ITCL and in CFL thickness. Complete tear of ITCL, cervical ligament, and inferior extensor retinaculum were rare without significant difference.

Conclusion In patients with STI, the ACL is thin and narrow and more commonly absent or torn compared with patients with LAI. Complete tear of ATFL was more common in patients with LAI. Complete tear of CFL was commonly encountered in both the STI and LAI groups.

From the *Department of Radiology, and

Department of Orthopedic Surgery, Konkuk University Medical Center, Konkuk University School of Medicine, Gwangjin-gu, Seoul, Korea.

Received for publication September 15, 2017; accepted December 13, 2017.

Correspondence to: Sung Gyu Moon, MD, PhD, Department of Radiology, Konkuk University Medical Center, Konkuk University School of Medicine, 120-1 Neungdong-ro, Gwangjin-gu, Seoul 05030, Korea (e-mail: sgsgmoon@gmail.com).

The authors declare no conflict of interest.

This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.

Lateral ankle instability (LAI) occurs when the healing of lateral collateral ligament is inadequate after an acute sprain or when the ligament is deteriorated by repeated sprains. Subtalar instability (STI) is represented by varus tilting in combination with anterior and medial subluxation of the calcaneus relative to the talus.1,2 Subtalar instability is often misdiagnosed as LAI because both are usually caused by acute inversion injury with similar symptoms.3

Clinical diagnosis of STI and LAI is usually established on patient's history, physical examination, and radiographic evaluation. The hindfoot motion perceived by a physical examination is the sum of movements occurring in both ankle and subtalar joint, making it difficult to diagnose STI by physical examination alone.1,4 Furthermore, because of relatively small magnitudes of subtalar motion, any additional significant changes would likely be subtle with limited clinical relevance.5 Stress radiography is the most popular method to assess mechanical instability, and many authors have published criteria of pathological instability.6–9 Modified stress radiography, such as Broden view, which is the most widely used for identifying STI, may also be adopted to investigate the talocalcaneal tilt angle. However, the sensitivity and specificity of stress radiography were unsatisfactory to assess the degree of injury of individual ligament. Stress radiography may be insufficient to investigate STI because a wide range of overlap has been reported between symptomatic and asymptomatic ankles, and its output depends on the direction of the radiograph due to the complexity of subtalar joint.4,8

Ultrasound and magnetic resonance imaging (MRI) are sometimes beneficial from the standpoint of detailed description of morphologic findings of lateral ankle ligaments and subtalar ligaments.10,11 Ultrasound has limitations in evaluating structures that are deeply located, such as subtalar ligaments. Recently, 3-dimensional (3D) isotropic MRI can facilitate the evaluation of small structure and thus may allow easy tracking for ligaments. The use of 3D isotropic sequences can enable us to obtain images with thinner section and multiplanar reformation in arbitrary planes that might be tailored to anatomical structures of interest, thus allowing the reduction of partial volume averaging effects.12,13 However, it is difficult to interpret STI based on MRI alone because the lack of MRI studies regarding the subtalar ligament may not confirm instability in the presence of its partial or complete tear.

It has been widely accepted that anterior talofibular ligament (ATFL) and calcaneofibular ligament (CFL) can provide anatomical stability to lateral ankle joint.14 On the other hand, previous anatomic studies have indicated that interosseous talocalcaneal ligament (ITCL), cervical ligament (CL), inferior extensor retinaculum (IER), and CFL could be stabilizers to subtalar joint.15,16 Some of these ligaments might cause confusion due to their adjacent positions. According to cadaver studies, there are 2 distinct ligaments in the tarsal sinus: ITCL and anterior capsular ligament (ACL).3,15,17 Because ITCL and ACL have unique insertion and running patterns, it has been proposed that they should be regarded as 2 different ligaments. However, there is no consensus on the relative contribution of these ligaments to stabilization of the subtalar joint. Some subtalar joint constraints might also contribute to LAI to some degree.

Despite frequent coexistence of STI and LAI,18,19 few studies have focused on detailed imaging features to determine the difference between the two on MRI. Therefore, the objective of this study was to retrospectively compare morphologic characteristics between STI and LAI using 3D isotropic MRI. Magnetic resonance imaging findings of STI and LAI were compared in this study, focusing on subtalar ligaments. In addition, the diagnostic value of the obtained imaging feature for distinguishing between the two was determined in this study. The hypothesis of this study was that difference in subtalar ligamentous morphology could be used to distinguish between the 2 instabilities (STI and LAI).

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MATERIALS AND METHODS

Study Population

Institutional review board approval was obtained for this retrospective study. The requirement for informed consent was waived due to its retrospective nature. Between January 2015 and December 2016, 82 patients were diagnosed with STI or LAI based on clinical chart review of electrical medical and radiological records. Patients were divided into 2 groups: LAI and STI. The LAI group included patients with isolated LAI who did not respond to nonoperative treatment and were consequently treated with lateral ligament repair or reconstruction but did not have STI. The STI group included patients with isolated STI who had failed nonoperative treatment and consequently underwent arthroscopic subtalar reconstruction but did not have LAI. A total of 10 patients were included in the STI group, whereas 23 patients were included in the LAI group based on the following criteria: (a) clinical and operative diagnosis of STI alone or LAI alone; (b) MRI performed at our institution according to a standardized protocol; (c) arthroscopic surgery performed less than 6 months after MRI; and (d) no history of ankle surgery. A total of 49 patients were excluded, including 18 patients who had diagnosis of both STI and LAI, 21 patients who underwent preoperative MRI at an outside institution, 2 patients who did not undergo surgery within 6 months after MRI, and 8 patients who had history of prior ankle surgery.

In our institution, if ankle instability was suspected, MRI was performed and nonoperative treatment was attempted for more than 3 months. Surgery was considered if conservative treatment failed. Magnetic resonance imaging was usually performed before the start of nonoperative/surgical treatment. However, most patients in our study group had long-lasting symptoms, and some had already started receiving rehabilitation treatment at outside clinics before visiting our institution. The mean interval between MRI and surgery was 65 days (1–178 days).

Of the 23 patients with LAI alone, 12 were female and 11 were male. Their mean age was 30.2 years (range, 16–56 years). A total of 10 right ankles and 13 left ankles were included for this group. The mean duration of symptoms was 39 months (range, 4 months–10 years). Surgical treatment of LAI was performed under the clinical diagnosis based on the following inclusion criteria: a persistent unstable feeling symptom lasting for at least 3 months with repeated episodes of recurrent inversion sprain and evidence of LAI based on physical and radiologic examinations (talar tilt, >10°) without responding to peroneal muscle strengthening exercise.20 Varus stress and anterior drawer test radiographs were obtained in a neutral position at 150 Newtons of stress force using a stress device (Telos SE 2000, ARD Medizin Produkte GmbH, Germany). Preoperative ankle MRI was performed to evaluate thicknesses and contour of ATFL and CFL as indicators of ligament condition to plan surgical options. Intraoperative C-arm stress fluoroscopy under anesthesia demonstrated tibiotalar joint width widening and anterior translation of the joint without subtalar joint width change. Cases with intraoperative findings of attenuated or deficient ATFL and CFL were selected for modified Broström procedure (20 ankles) or ligament reconstruction using semitendinosus allografts (3 ankles). Debridement and synovectomy were performed for 16 patients with synovitis. Osteochondral lesion of the talus was observed for 2 ankles. It was treated with arthroscopic debridement and microfracture. Os fibulare excision was performed for 8 ankles, whereas open cheilectomy was performed for 1 ankle that had anterior ankle impingement. For 7 cases, subtalar arthroscopic examination was also performed. Arthroscopic signs of STI were not observed.

Of the 10 patients with STI alone, 5 were female and 5 were male. Their mean age was 31.1 years (range, 15–57 years). There were 6 right ankles and 4 left ankles. All ankles had history of previous ankle sprains with symptomatic and recurrent ankle sprain. The mean duration of symptoms was 26 months (range, 4–72 months). Surgical treatment was considered for patients whose symptoms were not improved after rehabilitation treatment for more than 3 months. Clinical diagnosis of STI was based on the following diagnostic criteria provided by a senior orthopedic surgeon in our institution.21 Patients need to meet at least 4 of 5 characteristics of preoperative diagnostic criteria: (1) recurrent ankle sprain, (2) sinus tarsi pain and tenderness, (3) hindfoot looseness or giving way, (4) hindfoot instability on physical examination, and (5) radiographic STI on ankle and Broden varus stress views (ipsilateral subtalar tilt >10° or contralateral subtalar tilt difference >5°).9 Ankle and Broden varus stress radiography was performed with a stress of 150 Newtons. Preoperative MRI was performed to determine additional pathologic conditions that could affect operative procedure such as lateral ankle ligament tear and osteochondral lesion of the talus. Surgical diagnosis of STI was confirmed by remarkable subtalar joint widening and medial displacement of the calcaneus to the talus on intraoperative C-arm stress fluoroscopy under anesthesia. In addition, subtalar arthroscopy was performed to investigate the presence of subtalar joint laxity, chronic tear of the vertical segment of the interosseous ligament (ACL) in the tarsal sinus, synovitis, and other features for all cases. An intact ligament was diagnosed when the ligament showed preserved continuity. Ligament dysfunction due to chronic tear was defined as definite discontinuity of the ligament and adhesion of adjacent tissue. All patients with STI underwent subtalar reconstruction surgery. Anterior and posterior CFL were reconstructed using semitendinosus allografts.21 Debridement and synovectomy were performed for 4 patients with synovitis. One ankle had an osteochondral lesion of the talus. It was treated by arthroscopic debridement and microfracture.

To assess clinical symptoms, patients in both groups were asked to rate pain levels using a 10-point visual analog scale (VAS). The American Orthopedic Foot and Ankle Society (AOFAS) and Karlsson-Peterson ankle instability scores were checked before surgery and at 6 months after the surgery. In addition, the presence of tarsal sinus tenderness on physical examination was recorded from a medical chart.

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Magnetic Resonance Imaging Protocol

Magnetic resonance images were acquired using two 3.0 Tesla scanners: a Magnetom Skyra (Siemens Healthcare Diagnostics, Erlangen, Germany) using a 16-channel phased array coil (n = 22) and a Signa HDxt (GE Healthcare, Milwaukee, Wis) using an 8-channel coil (n = 11). Three-dimensional imaging data were obtained from the sagittal plane using T2-weighted fast spin-echo sequence without fat suppression. The parameters used for the 3D imaging sequence are listed in Table 1. For orthogonal axial and coronal image reconstruction, a slice thickness of 0.6 mm without interslice gap was applied. In addition, routine MRI protocol at our hospital for ankle joint included axial and coronal T2–, sagittal T1–, sagittal T2– with fat suppression, and axial, coronal, sagittal T1–weighted image with contrast enhancement.

TABLE 1

TABLE 1

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Magnetic Resonance Imaging Analysis

Two radiologists with 17 and 2 years of musculoskeletal MRI experience analyzed these images. At the time of review, the readers were informed that the patient had an arthroscopically confirmed instability, but they were blinded to the final diagnosis, either STI or LAI, and other arthroscopic findings. Each reader independently assessed the morphologic characteristics of the ligament, that is, ligament dimension and ligament tear, and then reviewed them in consensus to determine their status. All measurements were performed separately and provided identically for each reader. Measurements were taken using measurement tools of GE PACS software (GE Healthcare, Mt. Prospect, Ill).

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Anatomy of Subtalar Ligaments

Ligaments in the tarsal sinus were illustrated in Figure 1. The ITCL and ACL were located in the posterior aspect of the tarsal sinus. Anterior capsular ligament was defined as the thick part of the anterior capsule of the posterior talocalcaneal joint. Anterior capsular ligament was started at the anterior margin of the posterior facet of the talus and extended vertically across the subtalar joint, attaching to the calcaneus.17 Interosseous talocalcaneal ligament was located inside the ACL. It ran diagonally from the talus in the tarsal canal to the calcaneus in the tarsal sinus. Alternatively, ACL may be referred to as the vertical or lateral segment of the interosseous ligament, and ITCL may be referred to as the oblique or medial segment of the interosseous ligament in the tarsal sinus. The space between ITCL and ACL was filled with adipose tissue. Therefore, ACL and ITCL could be distinguished from one another. The CL was located in the anterior aspect of the tarsal sinus, extending from the undersurface of the talar neck to the back of the calcaneus. The IER was more laterally positioned with respect to the CL and ITCL. The medial root of the IER blended with the fibers of the ITCL into a common insertion.17

FIGURE 1

FIGURE 1

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Quantitative Analysis of Ligaments

Ligament dimensions were measured on the plane with the best representation of the structure. In the case of ACL, thickness and width were measured on sagittal and axial isotropic 3D T2-weighted images, respectively. Measurements of ITCL thickness and width were obtained from sagittal and coronal isotropic 3D T2-weighted images, respectively. The ATFL thickness was also measured on axial isotropic 3D T2-weighted images. All ligament dimensions except CFL thickness were measured at the midpoint of the ligament. The CFL thickness was measured in the middle part between the peroneal intersection and calcaneal attachment. Quantitative measurements were made 3 times, and their mean value was recorded in millimeters.

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Qualitative Analysis of Ligaments

For qualitative analysis, the following MRI findings were evaluated and designated as present or absent: (a) abnormalities of ACL and ITCL characterized by the absence of ligaments or complete tear, (b) abnormalities of CFL and ATFL characterized by complete tear, (c) abnormalities of CL characterized by complete tear, and (d) abnormalities of IER characterized by partial or complete absence of 3 roots of inferior extensor retinaculum. The MRI findings of complete tear were designated as nonvisualization, interruption of the entire ligament, and highly irregular thin appearance suggesting scar tissue.22

Supplementary MRI findings known to be related to LAI or STI include synovitis, osteochondral or chondral lesion of the talus, ossification such as os fibulare, peroneal tendon pathology, and tarsal sinus fat obliteration.23–25 They were evaluated with 2D imaging sequences with or without contrast enhancement.

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Statistical Analysis

Continuous data were analyzed using the Mann-Whitney test. To determine cutoff values of statistically significant quantitative criteria, receiver operating characteristic analysis was used to calculate sensitivity and specificity. Fisher exact test was used to compare qualitative criteria between the 2 groups. Interobserver reliability was evaluated using interclass correlation coefficient and kappa statistics. It was interpreted as follows: less than 0.2, slight; between 0.2 and 0.4, fair; between 0.4 and 0.6, moderate; between 0.6 and 0.8, substantial; and greater than 0.8, almost perfect. SPSS Version 21.0 (SPSS Institute, Chicago, Ill) was used for all statistical analyses. A p value of less than 0.05 was considered to be statistically significant.

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RESULTS

Underlying Group Characteristics

There was no significant difference in age (P = 0.982), sex distribution (P = 0.722), right/left (P = 0.465), or duration of symptom (P = 0.515) between the 2 groups. Tarsal sinus tenderness was significantly more frequent in the STI group than in the LAI group (50.0% vs 13.0%, P = 0.036). Preoperatively, mean VAS, AOFAS, and Karlsson-Peterson scores for the STI group were 6.1 ± 2.6, 66.6 ± 13.1, and 42.6 ± 21.1, and those for the LAI group were 4.6 ± 1.9, 73.3 ± 10.5, and 60.3 ± 16.1, respectively. Postoperative mean VAS, AOFAS, and Karlsson-Peterson scores for the STI group were 0.8 ± 1.0, 92.0 ± 5.9, and 82.8 ± 12.8, and those for the LAI group were 1.0 ± 1.3, 89.9 ± 7.4, and 86.6 ± 11.6, respectively. Before surgery, scores of VAS or AOFAS were not significantly different between the 2 groups either. However, the Karlsson-Peterson score was lower (P = 0.022) in the STI group than in the LAI group. After surgery, all the clinical scores including VAS, AOFAS, and Karlsson-Peterson score improved significantly (P < 0 .001) compared with those preoperatively in both groups. Examples of a patient in the LAI group and a patient in the STI group are provided in Figures 2 and 3, respectively.

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

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Comparison of Ligament Dimensions Between STI and LAI

The STI group had significantly smaller ACL thickness and width than the LAI group (thickness: 1.48 ± 0.39 vs 2.12 ± 0.62 mm, P = 0.045, Mann-Whitney test; width: 7.30 ± 0.93 vs 8.64 ± 1.38 mm, P = 0.029). The thickness of the ATFL was significantly larger in the LAI group compared with that in the STI group (4.53 ± 1.13 vs 2.81 ± 0.70 mm, P = 0.001). However, the thickness of ITCL, the width of ITCL, or the thickness of CFL did not differ significantly between the 2 groups. Receiver operating characteristic analysis of the ACL showed a cutoff of 1.8-mm thickness with a sensitivity of 75.0% and a specificity of 75.0% (area under the curve, 0.819; P = 0.048) for distinguishing patients with STI from patients with LAI. A cutoff of 8 mm for ACL width showed a sensitivity of 75.0% and a specificity of 85.0% (area under the curve, 0.844; P = 0.033) to distinguish patients with STI from patients with LAI. The interobserver variability evaluated by intraclass correlation coefficient was considered moderate to substantial as follows: 0.687, thickness of ACL; 0.640, width of ACL; 0.617, thickness of ITCL; 0.577, width of ITCL; 0.839, thickness of CFL; and 0.738, thickness of ATFL (Table 2).

TABLE 2

TABLE 2

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Comparison of Ligament Abnormalities Between STI and LAI

In the STI group, there were 6 cases without ACL. In the LAI group, there were 3 cases without ACL. Absence or complete tear of the ACL was significantly more frequent in the STI group compared with that in the LAI group (60.0% vs 13.0%, P = 0.010). Complete tear of ATFL was less common in the STI group compared with that in the LAI group (10.0% vs 60.9%, P = 0.008). On the other hand, frequencies of abnormalities of ITCL, CL, IER, or CFL were not significantly different between the 2 groups. There was no case without ITCL in either group. Band shape of the ITCL accounted for the most (87.9%). Fan-shaped or split-shaped ITCL was rare. Complete tear of the CL was found in 1 case in the STI group and 2 cases in the LAI group (P = 0.675). In remaining cases of both groups, fan-shaped or band-shaped striated fiber bundles were found. Medial roots of IER were not seen in 2 cases in the LAI group. However, partial or complete absence of IER was not found in the STI group (P = 0.479). Complete tear of the CFL was not significantly different between the 2 groups (20.0% in STI vs 21.7% in LAI, P = 0.648), although it was more common in the LAI group. Interobserver agreement between the 2 readers was considered substantial with κ values of 0.921 for abnormalities of ACL, 0.814 for ATFL, 0.678 for CFL, 0.784 for CL, and 0.653 for IER. For ITCL, both readers reported that there was no absence or complete tear (Table 3).

TABLE 3

TABLE 3

Os fibulare was significantly more frequent in patients with LAI compared with patients with STI (0% in STI vs 52.2% in LAI, P = 0.004). However, there was no significant difference in synovitis (30.3% vs 60.9%, P = 0.105), osteochondral or chondral lesion of the talus (10.0% vs 13.0%, P = 0.649), peroneal tendon pathology (10.0% vs 26.1%, P = 0.294), or tarsal sinus fat obliteration (40.0% vs 26.1%, P = 0.343) between the STI and LAI groups.

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DISCUSSION

In the LAI group, complete tear of ATFL was very common (61%), followed by complete tear of CFL (22%). These outcomes correspond to results of previous studies.26,27 In the STI group, absence or complete tear of ACL was very common (60%), followed by complete tear of CFL (20%). Even when ACL was visible in the STI group, ACL was thinner and narrower than that in the LAI group. Ligamentous attenuation of ACL might be due to repeated varus stretching injuries with failure of self-healing processes.27 A notable point of this study was that ACL might have played the most important role in subtalar stability. In addition, complete tear of CFL was common in both STI and LAI groups, suggesting that CFL might play an important role in the stability of the subtalar joint and the lateral ankle joint as reported previously.5

Subtalar instability is defined as chronic functional instability characterized by a combination of anterior or medial translation and varus tilting of the calcaneus to the talus.1,2 However, discrimination of STI from LAI is challenging due to their similarities in injury mechanism and symptoms.3,8 Although it is believed that STI can occur as an isolated event with subtalar inversion and ankle locked in dorsiflexion, subtalar ligament injuries are often associated with ankle inversion sprains.2 Subtalar instability and LAI may coexist to contribute to hindfoot instability.2,5 It has been reported that STI occurs in 10% to 25% of patients with chronic LAI.18,19 However, treatment varies depending on the kind of instability.

Currently, direct repair of torn ligament and ligament reconstruction using auto/allograft are the standard for surgical treatment of symptomatic LAI.28,29 In the modified Broström procedure, ATFL and CFL repair can be enhanced by suturing the extensor retinaculum to the distal fibula. This step serves to reinforce the repair and limit inversion. It may correct coronal and subtalar components of the instability.29 If the torn ligament remnant has poor quality, the ligament reconstruction may be performed using a graft, preferably in an anatomical manner. Arthroscopic repair or reconstruction is more favorable in comparison to an open repair procedure and has been reported more feasible in clinical settings.30

On the other hand, chronic tear of the CFL, CL, or interosseous ligament has been proposed as etiologies of STI. Several reconstructive surgical procedures have been reported to be useful for treating STI. Recently, good results have been reported in patients with STI who underwent a novel method of simultaneously reconstructing anterior and posterior subtalar ligaments using an allograft.21,31 With this method, the anterior CFL graft plays the roles of the interosseous talocalcaneal and CLs, whereas the posterior CFL graft anatomically reconstructs the original CFL. In cases where STI and LAI are combined, the Broström procedure can be performed in addition to subtalar reconstruction procedure.21 If the 2 instabilities could be distinguished preoperatively and the condition of subtalar ligaments could be predictably evaluated by MRI, it may help surgeons determine the surgical plan before operation.

It has been well established that LAI is associated with ATFL and CFL injuries.14 However, there is a lack of agreement concerning the association between STI and subtalar ligaments. Subtalar ligaments are known to consist of CL, ITCL, and 3 roots of the IER.32,33 Controversy remains as to which ligaments are more important stabilizers of the subtalar joint. Some reports have claimed that ITCL or CL is the most important major stabilizer of the subtalar joint, whereas others have indicated that CFL is the most important stabilizer.1,4,5,15,16,34,35 Anatomical structure and function of ligaments remain controversial because there are subjective differences in the understanding of their anatomical structures and variations in shapes.36 Some ligaments might cause confusions due to their adjacent positions. According to cadaver studies, there are 2 distinct interosseous ligaments in the tarsal sinus. It has been suggested that ACL and ITCL should be regarded as 2 different ligaments because their origin and insertion are different.15,17 The ACL was first called interosseous ligament. It was named ACL due to its position along the anterior surface of the posterior talocalcaneal facet.37,38 In the present study, we investigated the interosseous ligament by separating ACL and ITCL. Alternatively, ACL may be referred to as the vertical or lateral segment of the interosseous ligament, and ITCL may be referred to as the oblique or medial segment of the interosseous ligament in the tarsal sinus.

Absence or complete tear of the ACL was significantly more common in the STI group compared with that in the LAI group. However, there was no case where ITCL was absent in either group. The thickness and width of the ACL showed significant differences between the STI and LAI groups. The ACL of the STI group had a width of 7.30 mm and a thickness of 1.48 mm, whereas that of the LAI group had a width of 8.64 mm and a thickness of 2.12 mm. Our observations on LAI were similar to the results of a previous cadaver study showing a width of 10.1 mm and a thickness of 2.4 mm.15 In the present study, a cutoff of 1.8 mm in thickness and 8 mm in width may be helpful for distinguishing between STI and LAI. The presence of ACL was 40% in the STI group. Absence or complete tear of ACL was significantly more common in the STI group. In the LAI group, the ACL was present in 87.0% of cases, which was within the prevalence range of 78% to 95% according to previous cadaver studies.15,17 On the other hand, there was no case without ITCL in either group. The thickness or width of ITCL showed no significant difference between the 2 groups.

There was no significant difference in abnormality of CL or IER between the 2 groups either. Only a fraction of the study population showed complete tear of the CL and partial absence of the IER. However, CL and IER were intact in most of these patients. Our results may suggest that ITCL, CL, and IER are not primary stabilizers of subtalar joint. Instead, ACL might play a more important role in maintaining subtalar stability. The ACL seems to work more directly to restrain the posterior talocalcaneal joint due to its course. The ACL is located close to the posterior talocalcaneal joint posterior to the CL and runs outside the ITCL. In contrast, ITCL seems to have little effect on varus tilt of the talocalcaneal joint, as it is located inside the tarsal sinus. As is well known, complete tear of CFL may cause LAI.16 Complete tear of CFL in the STI group was common, similar to that in the LAI group in the present study. This suggests that CFL may play a critical role in posterior-lateral stabilization of the subtalar joint, whereas ACL may play a role as an anterior-medial stabilizer to the posterior subtalar joint. According to our study of patients with STI, ACL deficits were significantly more common and CFL deficits were equally encountered compared with patients with LAI. In a clinical setting with ankle instability having CFL completely torn, ACL deficits on MRI may suggest STI requiring subtalar ligament reconstruction procedures rather than LAI requiring lateral ligament treatment.

Several authors have emphasized that intra-articular pathology and peroneal tendon tears are frequently associated with LAI.23,24 In our study, there was no significant difference in synovitis, osteochondral or chondral lesion of the talus, peroneal tendon pathology, or tarsal sinus fat obliteration between the 2 groups, whereas ossification including os fibulare was significantly more frequent in patients with LAI. On the other hand, tarsal sinus tenderness was more frequent in patients with STI, consistent with previous reports that STI was frequently associated with tarsal sinus syndrome.25

Our study has several limitations. First, because of its retrospective nature, clinical information and results of radiological evaluation might have introduced bias. Second, the number of patients with STI was relatively small. However, studies comparing LAI with STI confirmed by surgery were lacking. Third, only ligamentous structures were considered in this study, although other factors such as bony configuration and musculature might also play a role in maintaining the stability of lateral ankle and subtalar joint. Nevertheless, for the comparison between STI and LAI, all candidates of ligamentous subtalar stabilizers, that is, ACL, ITCL, CL, IER, and lateral collateral ligaments were investigated. Fourth, chronicity of ligament tear was not considered in this study. Despite the appearance of undamaged ligament, it may become thinner or thicker due to previous partial tears that go undetected. It may also have added bias to the ligament diameter. Last, MRI data were obtained from 2 different magnetic resonance devices with different imaging parameters that could affect the size of the ligament.

In conclusion, in patients with STI, the ACL is thin and narrow and more commonly absent or torn compared with patients with LAI. In addition, a complete tear of the ATFL was more commonly encountered in the LAI group. Complete tear of CFL was common in both STI and LAI groups, suggesting that CFL might play an important role in stabilizing lateral ankle and subtalar joint.

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Keywords:

instability; subtalar; ligaments; ankle; MRI; 3D

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