Prediction of anterior tibiotalar ligament injury: measurement of the angle between the deep posterior tibiotalar ligament and talus on MRI

Abstract

Background

Identification of anterior tibiotalar ligament (aTTL) injury is essential because it influences the surgeon's treatment option and patient prognosis.

Purpose

To assess the diagnostic accuracy of the angle measurement between the talus and posterior tibiotalar ligament (talus-pTTL) on magnetic resonance imaging (MRI) in patients with arthroscopically proven aTTL injuries.

Material and Methods

Ankle MRI scans of 67 patients who underwent arthroscopic examination were retrospectively reviewed. The talus-pTTL angle on axial T2-weighted MRI and the medial clear space (MCS) on mortise ankle radiograph were measured. Inter-observer agreement of the measurements was calculated. Also, sensitivity, specificity, and area under the receiver operating characteristic (ROC) curve (AUC) were the metrics of diagnostic accuracy.

Results

AUC was 0.90 for observer 1 with 78.6% sensitivity, 97.4% specificity, 88% accuracy, and 54.7° cutoff value for the talus-pTTL angle. AUC was 0.87 for observer 2 with 85.7% sensitivity, 84.6% specificity, 85.2% accuracy, and 53.7° cutoff value for the talus-pTTL angle. AUC was 0.86 with 82.1% sensitivity, 79.5% specificity, and 80.8% accuracy for observer 1 and 0.79 with 57.1% sensitivity, 92.3% specificity, and 74.7% accuracy for observer 2 for the MCS. Different MCS values and additional capabilities when complemented with the angle measurement showed an increase in diagnostic performances. Intra-observer reliability of MCS and talus-pTTL angle of the two radiologists was excellent. Inter-observer reliability of the two radiologists was excellent for both the talus-pTTL angle (0.95) and the MCS (0.85).

Conclusion

Measurement of the talus-pTTL angle showed good sensitivity, specificity, and accuracy for the evaluation of aTTL injury with excellent inter-observer reliability.

Keywords

Ankle injury anterior tibiotalar ligament magnetic resonance imaging angle measurement

Introduction

In patients with chronic ankle instability (CAI), damage to the lateral ligament has been widely described in the literature. In recent cases of CAI, there is evidence suggesting an injury to the deltoid ligament (DL) (1 –5). The DL provides support to the ankle, subtalar, and talonavicular joints by spanning the medial malleolus, talus, calcaneus, and navicular bones (6 –8). The DL is a strong multibanded complex composed of superficial and deep DL. Superficial DL includes tibionavicular, tibiospring, tibiocalcaneal, and tibiocalcaneonavicular spring ligaments, and deep DL consists of anterior tibiotalar ligament (aTTL) and posterior tibiotalar ligament (pTTL) (9 –11). The main restraint to hindfoot eversion is the superficial deltoid, and that to talar external rotation is the deep deltoid (12). As the DL is the main stabilizer of the axially loaded ankle (13), DL repair surgery can prevent lateral excursion of the talus (14). Therefore, assessing the integrity of the ligament is crucial in establishing a plan for the surgical treatment of ankle fractures (13).

In the clinical field, plain radiography has been used widely for the assessment of ankle sprains. Examination of >4 mm of the medial clear space (MCS) on a mortise view is a simple method that has been employed in identifying DL ruptures (15,16). However, a normal MCS measured on a non-stress view is likely not sufficient to exclude DL, which may result in false-negative cases (17,18). On the other hand, a coronal plane of ankle magnetic resonance imaging (MRI) is suitable to depict the full length of the pTTL, whereas the axial plane can be used to visualize the anterior tibiotalar ligament (aTTL) (19). In the majority of cases, it is difficult to obtain a clear view of the aTTL because the aTTL is very thin and lacks fascicles (19). Therefore, the inter-reader reliability of MRI findings for assessment of the DL was much lower compared to that of stress test findings (20).

Given the high prevalence of aTTL injury and the functional aspect of the aTTL withstanding excessive external rotation (4,6,18), the role of a radiologist in preoperative imaging is very important. Therefore, the authors adopted a new indirect method of measurement for the diagnosis of aTTL injury on ankle MRI. The aim of the present study was to propose a new diagnostic method for aTTL injury using a measurement of the talus-pTTL angle on routine ankle MRI.

Material and Methods

We obtained approval for the retrospective review of patients’ records from the local institutional review board, and the requirement for patients to provide informed consent was waived.

Patient population

We retrospectively reviewed the electronic medical records of patients at our single institution who underwent arthroscopic examination (n = 731) of the ankle from December 2012 to March 2020. A physical examination and ankle injury adjustment was performed on all the participants according to a standard evaluation in the outpatient clinic or emergency department.

The inclusion criteria were the presence of pre-procedure ankle MRI scans and ankle radiograph. Patients with pTTL injury on ankle arthroscopy or no documented DL, a history of previous surgery, or other pathological conditions including diabetes, septic arthritis, and non-infectious osteoarthritis were excluded (n = 619). We then excluded patients who had no history of acute ankle injury within the previous month (n = 26) and presence of medial malleolar fracture (n = 19). A total of 67 patients remained and were enrolled, including 28 patients with aTTL injury after experiencing an acute ankle injury (range = 1–28 days; mean duration = 5.4 days) and 39 patients with neither aTTL injury nor superficial DL injury at arthroscopic examination (range = 1 week–1 month) as a comparison group (Fig. 1).

MRI techniques

Imaging was performed using 3.0-T MRI (Magnetom Skyra; Siemens Healthcare, Erlagen, Germany) with an eight-channel boot coil. The feet of the participants were placed in a neutral position. Axial, sagittal, and coronal images were obtained to detect disorders of the ankle joint. To evaluate the DL, axial T2-weighted (T2W) turbo spin echo (TSE) sequence and coronal T2 fat-suppressed TSE sequence were used with cross-matching (Fig. 2). The parameters of coronal T2 fat-suppressed TSE sequence were as follows: TR / TE = 2970 / 53 ms; matrix = 168 × 384; echo train length = 9; number of signal acquisitions = 2; field of view = 16 cm; section thickness = 2 mm; and interslice gap = 0.2 mm. The parameters of T2W TSE sequence were as follows: TR / TE = 4590 / 88 ms; matrix = 384 × 384; echo train length = 15; number of signal acquisitions = 2; field of view = 14 cm; section thickness = 2 mm; and interslice gap = 0.5 mm.

Image analysis

Two musculoskeletal radiologists (with 19 and 3 years of experience, respectively), blinded to clinical and MRI information, measured the talus-pTTL angle on axial T2W MRI independently. Serial guidelines for the angle measurement were established through preliminary measurement from the patient population as follows (Fig. 2). First, we selected a coronal image with a parallel talar dome and roof of sulcus tali (Fig. 2a). Next, we used a cross-matching tool that provided a local PACS system (Deja-View, version 3.0; Dongeun IT, Bucheon, Republic of Korea) to fit the middle of the pTTL talar attachment site (Fig. 2b). Next, we drew a Cobb line 1 connecting the anterior and posterior margins at the talar attachment site of the pTTL on the selected T2 axial image (Fig. 2c). After this, we drew a Cobb line 2 on the anterior and posterior margins of the medial-most pTTL (Fig. 2d). Then, we reduced the Cobb line 2 by a half-length (Fig. 2e) and rotated the Cobb line 2 counterclockwise around the center of the ligament and stretched it out to place it in the center of the Cobb line 1 (Fig. 2f). The calculated Cobb angle was then recorded. In addition, we measured the MCS, which is defined as the space between the lateral edge of the medial malleolus and the medial side of the talus, on a mortise ankle radiograph (21).

Statistical analysis

The values of all variables were tested with the Kolmogorov–Smirnov test for homogeneity of distribution. The continuous variables of age, talus-pTTL angle, and MCS were expressed as mean ± standard deviation, and the relationships between the presence of a ligament tear and continuous variables were analyzed using an independent t-test. The only categorical variable was sex, which was analyzed using Pearson's chi-square test to determine a statistically significant difference. A P value <0.05 was considered to indicate statistical significance.

Intra- and inter-observer agreement of the measurements was calculated using the intraclass correlation coefficient (ICC). Intra-observer reliability was evaluated independently at intervals of four weeks. Agreements were classified as poor if the ICC was <0.40, moderate if the ICC was in the range of 0.40–0.75, and excellent if the ICC was ≥0.75 (22). From the receiver operating characteristic (ROC) curve, the sensitivity, specificity, and area under the ROC curve (AUC) were the metrics of diagnostic accuracy. Determination of the most appropriate cutoff value for the talus-pTTL angle was analyzed using Youden's index. Diagnostic performance was calculated using both of MCS and angle measurement to evaluate additional value of the talus-pTTL. Post hoc power analysis was used to determine the power for AUC comparison using an alpha value of 0.05. Statistical analyses were performed with Rex ver. 3.0.3 (RexSoft, Seoul, Republic of Korea).

Ankle arthroscopy

In the present study, ankle arthroscopy was used as a reference of standard. The indication for arthroscopy is acute or chronic injury with ankle instability and ligament injury on ultrasound or MRI. Arthroscopy was performed by an orthopedic surgeon with 10 years of experience in the foot and ankle subspecialty to whom the MRI reports were provided. The mean time between the MRI and arthroscopic surgery was 3.1 ± 5.8 days (range = 1–29 days). Arthroscopic findings were recorded as the presence or absence of superficial DL, aTTL, and pTTL injury. We defined the DL injury as partial tear or complete tear on arthroscopy.

Results

All of the variables showed normality of distribution. There was no significant difference in the patients’ age between the two groups, but there was a significant difference in the sex of the patients (Table 1). The intra-observer reliability of an author (JGC) was excellent in MCS (0.99, 95% confidence interval [CI] = 0.988–0.995) and talus-pTTL (0.99, 95% CI = 0.991–0.996), respectively. In addition, the intra-observer reliability of the other author (YSY) was excellent in MCS (0.98, 95% CI = 0.968–0.988) and talus-pTTL (0.97, 95% CI = 0.958–0.984), respectively. The inter-observer reliability of the two radiologists was excellent for the talus-pTTL angle (0.95, 95% CI= 0.93–0.97) and the MCS (0.85, 95% CI = 0.76–0.90). The mean value of the MCS from observer 1 was 3.5 ± 1.8 mm (range = 1.2–10.9), while the mean MCS from observer 2 was 3.7 ± 1.8 mm (range = 2.0–10.8). The mean value of the angle as measured by observer 1 was 54.2°±11.1° (range = 38.6–86) and that measured by observer 2 was 54.7° ± 11.2° (range = 36.4–83.9).

Table 1.

Participant demographics.

Characteristic	aTTL injury (n = 28)	No aTTL injury (n = 39)	P value
Sex			0.009*
Male	15	9
Female	13	30
Age (years)	41.5 ± 13.5 (16–61)	37.6 ± 15.0 (15–62)	0.232^†
Symptom duration (days)	5.4 ± 7.0 (1–28)	NA	NA

Values are given as n or mean ± standard deviation (range).

aTTL = anterior tibiotalar ligament; NA = not available.

*Chi-square test.

^†

Independent t-test.

On ROC curve analysis (Fig. 3a), the AUC was 0.90 (95% CI = 0.80–0.96) for observer 1, with a sensitivity of 78.6%, specificity of 97.4%, accuracy of 89.5%, and a cutoff value of 54.7° for the talus-pTTL angle, according to Youden's index. For observer 2, the AUC was 0.87 (95% CI = 0.76–0.94), with a sensitivity of 85.7%, specificity of 84.6%, accuracy of 85.1%, and a cutoff value of 53.7° for the talus-pTTL angle (Table 2). For the MCS, the AUC for observer 1 (Fig. 3b) was 0.86 (95% CI = 0.75–0.93), with a sensitivity of 82.1%, specificity of 79.5%, accuracy of 80.6%, and a cutoff value of 3.2 mm, and for observer 2, the AUC was 0.79 (95% CI = 0.67–0.88), with a sensitivity of 57.1%, specificity of 92.3%, accuracy of 77.6%, and a cutoff value of 3.8 mm (Table 3). When the 5-mm cutoff value and the two readers' optimized MCS cutoff values of 3.8 mm and 3.2 mm, respectively, were used with the talus-pTTL angle, sensitivity was dramatically increased (5 mm: from 28.6% to 82.1%; 3.8 mm: from 57.1% to 85.7%; 3.2 mm: from 82.1% to 92.9%), the accuracy also increased as follows (5 mm: from 70.1% to 92.5%; 3.8 mm: from 77.6% to 94%; 3.2 mm: from 80.6% to 97.0%), and the positive predictive value (PPV) also increased to 100% in all groups (Table 4). Post hoc analysis of the diagnostic performance of the different MCS with angle measurement showed a statistical difference only in the comparison between MCS (3.8 mm) with angle measurement versus MCS (3.2 mm) with angle measurement (sensitivity: P = 0.0044, accuracy: P = 0.0009, negative predictive value [NPV]: P = 0.0211) (Table 5). Comparing the ROCs for the MCS and the talus-pTTL angle showed no significant difference between the two observers (P = 0.228–0.447) (Fig. 4). All of the patients who had aTTL injury showed concomitant superficial DL injury on arthroscopic records. Post hoc power analysis of the comparison of AUCs yielded a power in the range of 3.2%–17.9% (Tables 4 and 5).

Table 2.

Diagnostic performance of angle measurement between talus and posterior tibiotalar ligament.

Parameters	Observer 1	Observer 2
Sensitivity (%)	78.6 (59–91)	85.7 (67–96)
Specificity (%)	97.4 (86–99)	84.6 (69–94)
Accuracy (%)	89.5 (79–95)	85.1 (74–92)
AUC	0.90 (0.80–0.96)	0.87 (0.76–0.94)
Cutoff value (°)	54.7	53.7
PPV (%)	95.7 (78–99)	80 (61–92)
NPV (%)	86.4 (72–94)	89.2 (74–96)
P value	<0.001	<0.001

Values in parentheses are 95% confidence intervals.

AUC, area under the receiver operating characteristic curve; NPV, negative predictive value; PPV, positive predictive value.

Table 3.

Diagnostic performance of measurement of the medial clear space.

Parameters	Observer 1	Observer 2
Sensitivity (%)	82.1 (63–94)	57.1 (37–76)
Specificity (%)	79.5 (64–91)	92.3 (79–98)
Accuracy (%)	80.6 (69–89)	77.6 (66–87)
AUC	0.86 (0.75–0.93)	0.79 (0.67–0.88)
Cutoff value (mm)	3.2	3.8
PPV (%)	74.2 (55–88)	84.2 (60–97)
NPV (%)	86.1 (71–95)	75 (60–86)
P value	<.001	<.001

Values in parentheses are 95% confidence intervals.

AUC, area under the receiver operating characteristic curve; NPV, negative predictive value; PPV, positive predictive value.

Table 4.

Comparison of diagnostic performance between different MCS values and when the angle measurement is added.

Parameters	MCS (5 mm)	+ Angle measure	P value	Post hoc power analysis (%)	MCS (3.8 mm)	+ Angle measure	P value	Post hoc power analysis (%)	MCS (3.2 mm)	+ Angle measure	P value	Post hoc power analysis (%)
Sensitivity (%)	28.6 (13–49)	82.1 (63–94)	0.0003	17.9	57.1 (37–76)	85.7 (67–96)	0.0133	7.2	82.1 (63–94)	92.9 (77–99)	0.2482	8.1
Specificity (%)	100 (91–100)	100 (91–100)	–		92.3 (79–98)	100 (91–100)	0.2482		79.5 (64–91)	100 (91–100)	0.0133
Accuracy (%)	70.1 (58–81)	92.5 (83–98)	0.0003		77.6 (66–87)	94.0 (85–98)	0.0026		80.6 (69–89)	97.0 (90–100)	0.0026
PPV (%)	100 (63–100)	100 (85–100)	–		84.2 (60–97)	100 (86–100)	0.1569		74.2 (55–88)	100 (87–100)	0.0159
NPV (%)	66.1 (53–78)	88.6 (75–96)	0.0161		75.0 (60–86)	90.7 (78–97)	0.0914		86.1 (71–95)	95.1 (83–99)	0.3295

Values in parentheses are 95% confidence intervals.

MCS, medial clear space; NPV, negative predictive value; PPV, positive predictive value.

Table 5.

Post hoc analysis of diagnostic performance of the different MCS with angle measurement.

Parameters	MCS (5 mm) + angle measure	MCS (3.8 mm) + angle measure	P value	Post hoc power analysis (%)	MCS (5 mm) + angle measure	MCS (3.2 mm) + angle measure	P value	Post hoc power analysis (%)	MCS (3.8 mm) + angle measure	MCS (3.2 mm) + angle measure	P value	Post hoc power analysis (%)
Sensitivity (%)	82.1 (63–94)	85.7 (67–96)	1	3.2	28.6 (13–49)	92.9 (77–99)	0.2482	5.1	85.7 (67–96)	92.9 (77–99)	0.0044	4.1
Specificity (%)	100 (91–100)	100 (91–100)	–		100 (91–100)	100 (91–100)	-		100 (91–100)	100 (91–100)	0.2482
Accuracy (%)	92.5 (83–98)	94.0 (85–98)	1		70.1 (58–81)	97.0 (90–100)	0.2482		94.0 (85–98)	97.0 (90–100)	0.0009
PPV (%)	100 (85–100)	100 (86–100)	–		100 (63–100)	100 (87–100)	-		100 (86–100)	100 (87–100)	0.1356
NPV (%)	88.6 (75–96)	90.7 (78–97)	1		66.1 (53–78)	95.1 (83–99)	0.4889		90.7 (78–97)	95.1 (83–99)	0.0211

Values in parentheses are 95% confidence intervals.

MCS, medial clear space; NPV, negative predictive value; PPV, positive predictive value.

Discussion

In recent years, orthopedic evidence-based research on ankles and foot injuries has frequently mentioned the DL, highlighting the anatomical importance of ankle joint stability in DL injuries and the importance of the DL in relation to unstable type-IV supination-external rotation fractures (2,23). DL injury subsequently causes degeneration or attenuation of this ligament, leading to acquired flatfoot anomalies in adults, which then results in valgus talar tilt (2). For this reason, there is an increasing trend in performing primary repair of concomitant ligament injuries or reconstruction with a graft associated with ankle injury (2,7,9,14,24). The deep DL is the primary restraint to external rotation of the talus, while the superficial DL resists eversion of the hindfoot. Furthermore, a review of the literature shows that the major contributor to ankle stability against a valgus load is the deep DL, whereas the superficial DL is considered only a minor contributor (2,8,9). For this reason, proper preoperative diagnosis of aTTL injury or suspicious stationary morphology of medial ankle instability (MAI) is needed because it affects the surgeon's choice to operate with or without deltoid reconstruction. A recent study also insisted that the anterior DL repair under arthroscopy would be sufficient treatment for MAIs that are due to DL rupture. In addition, MAI due to DL rupture could be detected and easily repaired using arthroscopy (14).

In the present study, external rotational instability occurs due to injury to the anterior tibiotalar ligament. As a result, the anterolateral talar displacement and the posterior tibiotalar ligament rotate counterclockwise around the talar attachment site resulting in an increased talus-pTTL angle. The angle measurement between the talus and pTTL on the axial MRI scan of the ankle showed good diagnostic performance in determining the presence of aTTL injury. In our study, the talus-pTTL angle showed a sensitivity of 78.6%–85.7%, specificity of 84.6%–97.4%, and accuracy of 85.1%–89.5%. However, a prior report described a sensitivity and specificity for deep DL tears of 96.3% (26/27) and 97.9% (46/47), respectively (24). However, this study was focused on the evaluation of pTTL injury. The main aim of the present study was to determine the diagnostic performance for aTTL tears instead of assessment of pTTL injury. In all patients who were enrolled, the posterior tibiotalar ligament was intact on ankle MRI and arthroscopy, because the angle can be accurately measured only when the posterior tibiotalar ligament is intact.

The most widely accepted values associated with incompetence of the DL are an absolute MCS ≥4–5 mm (25,26). However, the optimized cutoff value obtained in the present study was lower than that of previous literature and a lower value of AUC than angle measurement. Comparing the diagnostic abilities of MCS measurement and MRI qualitative evaluation with intraoperative visualization as the gold standard, the measurements of the MCS were found to be much less accurate than MRI in predicting deltoid ruptures (46% vs. 79%, respectively) (27). The widening of the medial clear space in the mortise view does not sufficiently reflect the deep DL integrity, and most of the criteria presented in the previous literature were cadaver experiments (13). On the other hand, the MR-based indirect angle measurement presented by the authors can be evaluated as showing better diagnostic performance with the integrity of the posterior tibiotalar ligament confirmed in the image and arthroscopically.

Diagnostic performances using different MCS values and the angle measurement showed an increase in sensitivity, specificity, accuracy, PPV, and NPV. Among them, sensitivity, accuracy, and NPV showed statistically significant differences only in the comparison of diagnostic performance of MCS 3.8 mm with angle measurement and 3.2 mm with angle measurement. Therefore, angle measurement for aTTL injury should be performed on MRI of the ankle when there is eccentric widening of the MCS in the ankle mortise radiograph of a patient with clinical ankle instability.

The role of radiology is important in the preoperative evaluation of ankle injury, but in the case of evaluation for medial stabilizer injury, many clinicians have relied on initial ankle radiographs with MCS widening and clinical signs such as ecchymosis, swelling, and tenderness to evaluate the integrity of the DL. Furthermore, some authors insist that integration of the DL is possible through MRI, for example, by checking with a special oblique view (28), However, in the MRI evaluation for MAI, there are limitations, such as limited sequences of routine MRI, limited time, and anatomical structure attributed by location, course, fan-shape, and short length of the native structure. In particular, the deep aTTL has an extremely short course and small volume. Not only will that lead to a misdiagnosis by the radiologist as a complete tear with residual atrophy, but stationary MAI will naturally not be assessed. Even if MRI is able to detect ligament injury, there is no correlation with stress views indicating an inability of MRI to predict instability. In addition, the inter-observer agreement between radiologists when grading the severity of ligamentous injury was only fair to moderate (25). These reasons necessitate the need for appropriate and early diagnosis in MRI, and prevention of MAI, osteoarthritis, and medial ankle impingement syndromes. Therefore, we think that the 54.7° cutoff value of the talus-pTTL angle with good sensitivity, specificity, and accuracy will be helpful in diagnosing aTTL injury. In accordance, inter-observer reliability was excellent for the evaluation of aTTL tears in our study. Excellent inter-observer agreement means that aTTL injury could be evaluated in an objective manner using this angle measurement. In contrast, a previous study conducted by Nortunen et al. (20) reported that inter-observer agreement for aTTL tears was very low (kappa = 0.22). This poor inter-observer agreement is not surprising because even an intact aTTL is not fully visible on routine ankle MRI (29).

The present study has some limitations. First, it was a retrospective study, so it was susceptible to selection bias and included no external validity. Therefore, a multicenter analysis will be needed in a future study. Second, the relatively small sample size may reduce statistical significance. Third, we did not analyze ankle injuries as either acute or chronic. Finally, we did not analyze the degree of ligament injury divided by stretched but still intact, partial tear, or complete tear.

In conclusion, measurement of the talus-pTTL angle showed good sensitivity, specificity, and accuracy for the evaluation of aTTL injury, with excellent inter-observer reliability. These findings could be a helpful and simple method in practical situations of ankle injury. However, further prospective research is needed to determine whether quantitative measurement of ankle MRI with this cutoff value can improve the accuracy of the diagnosis of aTTL injury.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received the following financial support for the research, authorship, and/or publication of this article: Supported by the Soonchunhyang University Research Fund.

ORCID iD

Jang Gyu Cha

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