Isolated Tears of the Superior Fascicle of Anterior Talofibular Ligament Can Be Identified on MRI: A Cadaveric Study

Abstract

Category: Sports; Ankle Introduction/Purpose: Vega et al. describe two components of the anterior talofibular ligament(ATFL), including the intracapsular superior fascicle(sATFL) and the extracapsular inferior fascicle(iATFL). Based on this anatomic distinction, ankle “microinstability” describes instability arising from injury to the sATFL alone. Because the sATFL resides within the ankle joint, synovial fluid limits its healing, potentially leading to pain and subjective ankle instability without a positive anterior drawer exam. Since physical examination is unreliable for microinstability, arthroscopic examination is considered the gold standard for diagnosing sATFL injury. A noninvasive approach for diagnosis would be ideal; however, the efficacy of MRI in detecting injury to the sATFL is not understood. Therefore, we aimed to assess whether MRI could identify the sATFL as an independent structure and diagnose an isolated sATFL tear. Methods: Fourteen mid-tibia cadaveric specimens (7 males, age: 33-78 years) with neutral alignment and no history of trauma or previous surgery were included in the study. A board-certified foot and ankle orthopedic surgeon dissected the lateral ankle ligament complex to confirm the independent presence of the sATFL and the iATFL. After initial dissection, all specimens underwent a clinical MRI scan sequence, and two musculoskeletal radiologists with > 15 years of experience read the images in consensus to identify the sATFL. Half of the specimens were randomly assigned to undergo bisection of the sATFL. All 14 specimens underwent a second MRI scan, and the same radiologists assessed the images while blinded to the sATFL condition. The percentage of specimens with identifiable sATFL during the initial MRI scan and identifiable injury of the sATFL during the second scan were calculated. Results: A total of 14 specimens were dissected down to the lateral ligament complex. The ATFL was observed as a two- fascicle(sATFL and iATFL) ligament in all 14 specimens. A gap between both fascicles was observed. The fibular insertion of the sATFL was located just distal to the fibular insertion of the anterior tibiofibular ligament. In contrast, the fibular insertion of the iATFL was located between the insertion of the sATFL and the fibular tip. In the initial MRI scans, radiologists identified both the sATFL and the iATFL in 10/14(71.4%) specimens. The distinction between the two fascicles was not possible in the other four specimens. In the second set of MRI scans, radiologists correctly identified 6/7(85.7%) specimens that underwent sATFL bibisection and 7/7(100%) specimens with intact sATFL. Conclusion: Our cadaveric dissection confirmed the presence of two distinct fascicles of the ATFL in all specimens. While the radiologists could only identify the discrete bundles of the sATFL and iAFTL in 71.4% of intact specimens, they distinguished the bisected sATFL in 6 out of 7 specimens. These findings suggest that MRI can be a potential tool for diagnosing microinstability. Since microinstability frequently involves intraarticular pathology, performing MRI can be considered if patients experience subjective ankle instability and pain but without signs of ankle laxity on clinical examination.