The Superior Fascicle of the Anterior Talofibular Ligament Functions as a Restraint Against Internal Rotation: A Cadaveric Biomechanical Study

Abstract

Background:

Lateral ankle sprains frequently result in anterior talofibular ligament (ATFL) dysfunction, leading to chronic lateral ankle instability. The ATFL consists of superior and inferior fascicles; however, the stabilizing role of the superior fascicle remains unclear. We biomechanically evaluated its function using cadaveric models.

Methods:

Nine frozen cadaveric ankles were tested using a robotic system (FRS2015). The superior fascicle was sequentially transected arthroscopically to simulate cumulative 3 injury types: type 1, partial detachment at the medial margin of the fibular attachment; type 2, proximal detachment; and type 3, complete detachment. Anterior talar translation, internal rotation, and inversion angles relative to the tibia were measured as displacements under the anterior drawer, internal rotation, and inversion loads, respectively, at 15° dorsiflexion and 15° and 30° plantarflexion. In situ forces and ankle kinematics were also measured.

Results:

Internal rotation displacement at 30° plantarflexion was significantly greater in type 3 than in the intact (mean difference, 2.9°; 95% CI, 0.6°-5.0°; P = .01), type 1 (mean difference, 2.6°; 95% CI, 0.5°-4.8°; P = .02), and type 2 (cumulative with type 1; mean difference, 2.5°; 95% CI, 0.4°-4.5°; P = .02). With the numbers available, no significant differences were found among intact, type 1, and type 2, nor in anterior or inversion displacement across any conditions. The in situ force was greater in type 3 than in type 1 during anterior drawer and internal rotation loading, and ankle kinematics did not differ.

Conclusion:

Partial injury to the superior fascicle of the ATFL was not associated with a statistically significant difference in ankle instability under the tested conditions, whereas complete injury was associated with significantly greater internal rotational displacement.

Clinical Relevance:

Injury to the superior fascicle of the ATFL is associated with increased internal rotational laxity, supporting the use of internal rotation testing, such as the anterolateral drawer test, when evaluating suspected ATFL injury.

Visual Abstract.

This is a visual representation of the abstract.

Introduction

Ankle sprain is a common musculoskeletal injury, accounting for approximately 30% of all sports-related ankle injuries, of which 80% to 90% are classified as lateral ankle sprains (LASs).^1,2 The incidence of LASs in athletes has been reported to be 4.95 per 10 000 athlete-exposures. ³ In addition, LAS has a high recurrence rate, and many cases are reported to progress to chronic lateral ankle instability (CLAI).^{4 -6} CLAI causes ankle pain, persistent instability, and functional limitations.^7,8 In patients with CLAI, arthroscopic findings have revealed injury or elongation of the anterior talofibular ligament (ATFL) in 86% of cases and of the calcaneofibular ligament (CFL) in 64%. ATFL dysfunction is considered a primary contributor to the symptoms and instability associated with CLAI. ⁹

The ATFL is a component of the lateral ligament complex and has traditionally been reported to resist anterior translation of the talus.^10,11 Additionally, inversion sprains are the most common type of ankle sprain, and the ATFL is the most frequently injured ligament in the ankle.^2,10 Various anatomical configurations of the ATFL have been reported, among which the double-bundle structure consisting of the superior and inferior fascicles is the most frequently described.^{12 -14} The 2 fascicles differ in both anatomy and function. The superior fascicle is considered an intra-articular ligament that is in contact with the joint capsule, whereas the inferior fascicle may be either capsular or extracapsular.^14,15 The inferior fascicle is continuous with the CFL via arciform fibers, forming the lateral fibulotalocalcaneal ligament complex.^12,16 Owing to these structural differences, the inferior fascicle exhibits minimal length changes and functions as an isometric stabilizer together with the CFL. ¹³ In contrast, the superior fascicle becomes taut during plantar flexion and relaxed during dorsiflexion.^13,14,17 Consequently, the superior fascicle is considered the first structure to be tensioned and injured in ankle inversion sprains. ¹⁴

Recently, Vega et al^14,18 reported that isolated injury to the superior fascicle may cause subtle ankle instability. They proposed the term “ankle microinstability” to describe this condition. ¹⁸ Microinstability, previously described in the shoulder and hip joints, refers to excessive joint motion beyond the physiological range that can cause pain. ¹⁹ However, its clinical relevance in the ankle remains unclear, and a standardized definition has not yet been established. Although injury to the superior fascicle has been suggested as a cause of microinstability, ²⁰ detailed descriptions of the specific injury patterns remain limited. Vega et al ²⁰ classified arthroscopic injury to the superior fascicle into 5 grades in patients with ankle microinstability. Sugino et al ²¹ classified ATFL injuries into 4 grades and evaluated the degree of instability using stress radiography. They observed partial injuries even with minimal talar tilt, suggesting that such lesions may correspond to microinstability.

Dalmau-Pastor et al ²² partially transected individual ATFL bundles and analyzed the biomechanical consequences. This study was the first to quantitatively assess the reduction in resistance to motion following partial transection of the superior fascicle. However, this model does not accurately reflect the actual injury patterns observed in clinical patients, and there have been no detailed biomechanical reports comparing the different actual injury patterns involving the ATFL. Furthermore, displacement in multiple directions has not been measured. Therefore, this study aimed to evaluate ankle joint instability using cadaveric models that closely simulated actual injury patterns, with joint displacement as the primary outcome. We hypothesized that transection of the superior fascicle would result in increased anterior and internal rotational displacement.

Methods

Specimen Preparation

Nine freshly frozen cadaveric lower limbs (8 males and 1 female) donated to the Department of Anatomy at our institution were used in this study. The mean age at death was 88 years (range, 80-97 years). Approval for specimen use was obtained from the institutional ethics committee. Specimens with previous ankle fractures, osteoarthritis of the tibiotalar joint, or evident soft tissue injuries around the ankle were excluded. The absence of ankle osteoarthritis was confirmed radiographically prior to testing. In addition, specimens exhibiting detachment at the fibula or elongation of the ATFL, as confirmed by arthroscopy, were excluded.

All specimens were stored at −20 ℃ and thawed at room temperature for 24 hours before testing. Each limb was cut 15 cm proximal to the lateral malleolus. The distal tibiofibular syndesmosis was fixed with two 4.5-mm cortex screws (DePuy Synthes) placed 6 cm proximal to the lateral malleolus, while maintaining the ankle and subtalar joints in neutral alignment. The fibula was cut 3 cm proximal to the screw fixation, and the syndesmosis was reinforced with acrylic resin (Ostron II; GC, Japan). The subtalar joint was stabilized in a neutral position with 2 screws, a Schanz screw and a wooden screw, and the posterior calcaneus was embedded in an acrylic mold. The molded cylinders of the syndesmosis and calcaneus were mounted on robotic clamps.

Robotic System

A 6–degrees of freedom (DOF) robotic testing system (FRS2015; Technology Service, Japan) was used (Figure 1). The system consisted of a 6-DOF manipulator, servo motor controllers, and a control computer. ²³ The manipulator incorporated 3 translational actuators (SGMP Series; Yasukawa Electric) and 3 rotational actuators (FHA Series; Harmonic Drive Systems). The positional accuracy of the manipulator was approximately 120 nm under a 500-N load, with a clamp-to-clamp stiffness greater than 312 N/mm. ²³ Real-time control of the displacement, force, and torque along each axis was achieved using a custom LabVIEW-based program (v.12.0.1; National Instruments). Bone positions and the neutral reference position were defined according to the joint coordinate system described by Wu et al. ²⁴ The maximum clamp-to-clamp compliance of the robotic system was 3 mm/N, and the fluctuations in force control were within 10 N for force and 0.4 Nm for moment. ²³

Figure 1.

The specimens were attached to the robotic system in a neutral position. Anterolateral (AL) and accessory anterolateral (AAL) portals were created and arthroscopic procedures were performed with the specimen fixed to the robot.

Arthroscopy

Before biomechanical testing, ankle arthroscopy was performed with the ankle positioned at 20° plantarflexion and distracted 2 mm from the neutral position. A 2.7-mm, 30° arthroscope (HD3 Synergy; Arthrex) was introduced through 2 portals: an anterolateral portal for visualization and an accessory anterolateral portal for instrumentation (Figure 1). ²⁵ After confirming the absence of cartilage damage, the synovium around the ATFL was carefully debrided using a motorized shaver (Synergy Resection Shaver; Arthrex) and an aspirating ablator (ApolloRF; Arthrex) to clearly expose the ligament. Specimens with detachment of the fibula or damage of the ATFL were excluded. Specimens without ligament damage were considered intact. All arthroscopic procedures were performed by a foot and ankle surgeon specializing with more than 3 years of experience in ankle arthroscopy.

Testing Protocol for Intact State

In the intact condition, passive dorsiflexion–plantar flexion (DF-PF) motion from 15° dorsiflexion to 30° plantarflexion was applied under a constant axial load of 5 N, and 3-dimensional ankle kinematics (anterior-posterior translation, internal-external rotation, and inversion-eversion) were recorded (Passive DF-PF test). Subsequently, the anterior drawer, internal rotation, and inversion moments were applied under the same axial load of 5 N at 15° dorsiflexion, neutral position (0°), 15° plantarflexion, and 30° plantarflexion (multidirectional loading test). During the anterior drawer tests, an anterior force of up to 60 N was applied, whereas for internal rotation and inversion tests, torques of up to 1.7 Nm were applied at a constant rate. ²⁶ For each test, the 4 DOFs other than DF-PF were set under force control at zero force or moment. The displacement–anterior translation of the talus relative to the tibia (mm), laxity angle of internal rotation, and inversion angle (degrees) were measured.

Arthroscopic Transection

After testing for intactness, the superior fascicle of the ATFL was sequentially transected arthroscopically. Based on the classification reported by Vega et al, ²⁰ 3 cumulative injury patterns were reproduced sequentially: type 1, partial detachment at the medial margin of the fibular attachment (Figure 2B); type 2, proximal detachment (Figure 2C); and type 3, complete detachment (Figure 2D). The superior fascicles were released using a radiofrequency device. In type 1, approximately 2 mm from the medial edge of the fibular attachment was detached; in type 2, 5 mm from the superior border of the attachment was released; and in type 3, detachment was extended until the inferior fascicles of the ATFL were exposed. The extent of detachment was verified by the operating surgeon and 1 additional foot and ankle surgeon. At the conclusion of testing, the skin, subcutaneous tissue, and extensor retinaculum were removed to visually confirm the transection site (Figure 3). All specimens had a double-bundle configuration, as confirmed both arthroscopically prior to transection and by anatomical dissection following the procedure, consistent with previously described bundle variations. ²⁷

Figure 2.

Arthroscopic findings. (A) Intact: No injury to the anterior talofibular ligament (ATFL) was observed, and both the superior fascicle (SF) and inferior fascicle (IF) were confirmed. (B) Type 1: The medial portion of the fibular attachment of the ATFL was partially detached (arrowheads). (C) Type 2: The proximal portion of the fibular attachment of the ATFL was detached (arrowheads). (D) The entire SF was detached, whereas the inferior fascicle remained visible posteriorly.

Figure 3.

Anatomical photograph from the lateral aspect. After completion of all experiments, the skin, subcutaneous tissue, and extensor retinaculum were removed to expose the ligaments. Only the superior fascicle (SF) was completely detached, whereas the inferior fascicle (IF) remained intact.

Testing Protocol for Transected State

For each injury type, a replay test was first performed, which included passive DF-PF test and multidirectional loading tests. During these tests, the forces and torques generated by the robotic system were recorded, and the in situ force of the transected portion was calculated by subtracting these values from those obtained in the preceding condition. ²⁸ The in situ force was defined as the force applied to each transected portion of the ATFL during anterior drawer, internal rotation, and inversion loading in the intact ankle. The in situ force measured for each injury type was expressed as ΔF (Type k) = F (Type k) – F (Type k – 1). For example, ΔF (Type 2) represents the difference between the force measured after transection (type 2) and that measured in the preceding condition (type 1), which represents the force applied to the ligament that was transected to create the type 2 condition. Subsequently, multidirectional loading tests identical to those performed under the intact condition were repeated. Anterior talar translation, internal rotation, and inversion angles were measured and compared across different injury states. All tests were performed sequentially at 0° and 15° dorsiflexion, 15° plantarflexion, and 30° plantarflexion. Manual examination and radiography were performed after testing to confirm that joint fixation remained secure.

Statistical Analysis

The displacement and in situ forces between the intact and transected states were compared using a repeated measures 2-way analysis of variance, with cutting condition and ankle position as fixed factors and specimen treated as a repeated measure. Post hoc pairwise comparisons were adjusted using the Bonferroni test. The same statistical approach was applied to ankle kinematics during the passive dorsiflexion-plantarflexion test. All analyses were conducted using GraphPad Prism v.10 (GraphPad Software), and statistical significance was set at P <.05.

Results

Displacement (Multidirectional Loading Test)

The angle of internal rotation at 30° plantarflexion was significantly increased in type 3 compared with intact (mean difference, 2.9°; 95% CI, 0.6°-5.0°; P = .01), type 1 (mean difference, 2.6°; 95% CI, 0.5°-4.8°; P = .02), and type 2 (mean difference, 2.5°; 95% CI, 0.4°-4.5°; P = .02) (Figure 4B). With the numbers available, no significant differences in internal rotation were observed between type 1 and intact or between type 2 and intact at any ankle position. The anterior translation and inversion angles did not increase across all conditions (Figure 4A and C). Specifically, at 30° plantarflexion, anterior translation in type 3 did not differ significantly from intact (mean difference, 0.2 mm; 95% CI, −0.5 to 0.8; P = .85), type 1 (mean difference, 0.3 mm; 95% CI, −0.2 to 0.9; P = .28), or type 2 (mean difference, 0.5 mm; 95% CI, −0.1 to 1.0; P = .09). Similarly, inversion at 30° plantarflexion did not differ significantly between type 3 and intact (mean difference, 0.2°; 95% CI, −0.7 to 1.2; P > .99), type 1 (mean difference, 0.5°; 95% CI, −0.3° to 1.2°; P = .38) and type 2 (mean difference, 0.5°; 95% CI, −0.1° to 1.1°; P = .13). During the anterior drawer test, the coupled internal rotation angle was significantly greater in type 3 than in type 1 (mean difference, 1.2°; 95% CI, 0.7° to 1.8°; P < .001) and type 2 (mean difference, 1.0°; 95% CI, 0.4° to 1.7°; P = .003) at 30° plantarflexion (Figure 4D).

Figure 4.

Displacement of the talus relative to the tibia during multidirectional loading test. (A) Anterior translation (mm). Although statistically significant, the biomechanical impact appears limited. (B) Angle of internal rotation (IR, degrees). The angle at 30° plantarflexion was significantly increased in type 3 compared with the other conditions (*P < .05). (C) Angle of inversion (IV, degrees). The angle did not increase across any condition. (D) Angle of internal rotation during the anterior drawer test (degrees). The angle was significantly larger in type 3 than in types 1 and 2 at 30° plantarflexion (†P < .05, ††P < .01, †††P < .001 vs type 1, **P < .01 vs type 2). DF, dorsiflexion; PF, plantarflexion.

In Situ force (Replay Test)

ΔF (Type 3) in the internal rotation direction was significantly greater than ΔF (Type 1) at 30° plantarflexion (mean difference, 17.7 N; 95% CI, 4.2-31.1; P = .01) (Figure 5B). In the anterior direction, ΔF (Type 3) was significantly greater than Δ(Type 1) at 30° plantarflexion (mean difference, 15.1 N; 95% CI, 4.2-26.0; P = .009) and at 15° plantarflexion (mean difference, 10.4 N; 95% CI, 2.0-18.9; P = .02). In addition, ΔF (Type 3) was significantly greater than Δ(Type 2) at 30° plantarflexion (mean difference, 12.9 N; 95% CI, 0.5-25.3; P = .04) (Figure 5A). In the inversion direction, with the numbers available no significant differences were found among any injury types at any ankle position (Figure 5C, supplementary file).

Figure 5.

In situ force of the transected portion (N). The force in each injury type was expressed as ΔF (Type k) = F (Type k) – F (Type k – 1). (A) Anterior drawer: ΔF (Type 3) was significantly greater than ΔF (Type 1) at 15° and 30° plantarflexion, and greater than ΔF (Type 2) at 30° plantarflexion. (B) Internal rotation: ΔF (Type 3) was significantly greater than ΔF (Type 1) at 30° plantarflexion. (C) Inversion: the force of each transected portion was not significantly different. †P < .05, ††P < .01 vs type 1, *P < .05. DF, dorsiflexion; PF, plantarflexion.

Ankle Kinematics (Passive DF-PF Test)

In the DF-PF test, with the numbers available, no significant differences in displacement were found at any angle, and the ankle kinematics demonstrated similar motion patterns (Figure 6, supplementary table).

Figure 6.

Ankle kinematics during passive dorsiflexion-plantarflexion (DF-PF) test. (A) Anterior-posterior translation (mm). (B) Internal rotation (IR)–external rotation (ER) (degrees). (C) Inversion (IV)–eversion (EV) (degrees). With the numbers available, no significant differences in displacement were found at any angle or in any direction.

Discussion

The most important finding of this study was that partial injury to the superior fascicle of the ATFL was not associated with any ankle instability under the tested conditions with the numbers available, whereas complete injury was associated with internal rotational instability only at 30° plantarflexion. No anterior instability measurable by our device was observed with isolated superior fascicle injury. In addition, in situ force analysis indicated that the superior fascicle contributes substantially to anterior and internal rotational stability, whereas overall ankle kinematics were not affected by the extent of injury. These findings were confirmed using an arthroscopically created model.

In previous cadaveric investigations, the ATFL was transected following removal of the capsule and inferior extensor retinaculum, which differs from clinical conditions. These structures are considered to play a secondary, although limited, role in maintaining ankle stability. ²⁹ In this study, arthroscopic transection of the ATFL allowed us to more closely replicate the actual clinical situation.

Although the ATFL generally consists of 2 distinct fascicles, most previous studies have reported transection of the ligament in its entirety.^26,30,31 Dalmau-Pastor et al ²² separately evaluated the fascicles and reported that partial injury of the superior fascicle did not produce instability, whereas complete injury reduced resistance to anterior and internal rotational loading. Edama et al ³² demonstrated increased anterior displacement after superior fascicle transection using ultrasonography. In the present study, the in situ force in the anterior and internal rotational directions was significantly greater in the complete injury condition, consistent with those findings. In contrast, anterior translation did not differ among injury types. These results indicate that isolated superior fascicle injury primarily results in internal rotational displacement without increasing anterior translation; the increase in internal rotation is consistent with previous reports. ³¹

Although the ATFL functions as a restraint to anterior translation, anterior displacement did not increase, possibly because the joint capsule and extensor retinaculum were preserved. Additionally, ligament injury is associated with increased internal rotation displacement during the anterior drawer test. Constrained motion during anterior drawer testing may have redirected load toward the internal rotation axis, indicating a compensatory motion pattern associated with ligament insufficiency. This indicates that rotational restraint is required immediately after an ATFL injury, and consideration of internal rotational instability is essential during manual examination. Incorporating an internal rotation component, as in the anterolateral drawer test,^33,34 may enhance the diagnostic accuracy for such instabilities.

Our model reflects acute ligament injury; however, the findings may inform the understanding of chronic instability. Vega et al^18,35 referred to microinstability as an isolated injury to the superior fascicle. However, in the present study, instability was observed even with an isolated injury that was measurable by our device. Thus, the concept of microinstability, if applied, may only pertain to cases with partial injury to the superior fascicle. In contrast, partial injury has been associated with anterior instability in clinical cases. ²¹ This is presumed to result from ligamentous degeneration and elongation caused by recurrent ankle sprains, leading to CLAI.^36,37 Accordingly, anterior translation may be more pronounced in acute-on-chronic injury, whereas isolated acute injury may not produce instability. Therefore, microinstability may be more appropriately considered as partial injury of the superior fascicle with minimal ligamentous degeneration. However, the source of lateral ankle pain observed in cases of microinstability remains unclear, and further studies are warranted.

In the present study, ankle kinematics did not differ by injury severity, whereas Sakakibara et al ²⁶ reported greater anterior translation and internal rotation during plantar flexion after complete ATFL transection compared with the intact condition. The difference from the present study lies in whether the inferior fascicle was also injured. Early dorsiflexion-focused range-of-motion exercises are recommended after mild acute ankle sprains.^38,39 The present findings indicate that isolated superior fascicle injury does not compromise stability during dorsiflexion-plantarflexion motion, thereby supporting these recommendations.

The sequential cutting design may theoretically introduce creep due to repeated loading. To mitigate this effect, a standardized protocol was implemented at each stage with a constant axial load, controlled loading conditions, and consistent joint positioning. Each condition was tested immediately after transection using an identical loading sequence, thereby limiting time-dependent mechanical effects. Although cumulative mechanical effects cannot be completely excluded, the observed increase in internal rotation after complete transection is unlikely to be explained solely by sequential loading.

This study had certain limitations. First, injury types were not quantitatively classified. Although detachment was reproduced according to prior classifications, the extent of injury was assessed independently by 2 surgeons to enhance reproducibility. Second, only immediate post-injury conditions could be evaluated. In patients with CLAI, ligaments frequently undergo degeneration; however, such chronic changes cannot be replicated in a cadaveric model. Furthermore, the mean specimen age of 88 years may limit extrapolation of these findings to younger or athletic populations. Third, the distal tibiofibular and subtalar joints were fixed. Although subtalar motion contributes to ankle kinematics, its impact is likely minimal given that the ATFL primarily stabilizes the tibiotalar joint. Fourth, the robotic system required fixation of the distal lower leg, necessitating detachment of surrounding muscles and tendons and precluding evaluation of dynamic stabilizers such as the peroneal tendons. Fifth, the deltoid ligament was not evaluated, despite its secondary contribution to anterior ankle stabilization.^40,41 Sixth, the clinical relevance of the observed difference of approximately 2.5° to 2.9° remains to be determined, as the minimum detectable threshold for internal rotation instability in clinical practice has not been established. Finally, a limited number of specimens were used.

Conclusion

Complete injury to the superior fascicle of the ATFL was associated with significantly greater internal rotational displacement under the tested conditions. In contrast, partial injury was not associated with a statistically significant difference in ankle instability with the numbers available. These findings suggest that the superior fascicle of the ATFL is associated with limiting talar internal rotation in a cadaveric setting, although whether this translates to clinically meaningful instability under physiological conditions requires further investigation. Anterior translation and inversion were not significantly altered by isolated superior fascicle injury.