Ankle sprains are common in daily clinical practice with a high incidence of 40 % in sports injuries, and they result in chronic lateral ankle instability (CLAI) in 20 % of these patients [[1], [2], [3], [4]]. Since CLAI has the potential to develop into osteoarthritis (OA), precise diagnosis of the ankle lateral ligament, anterior talofibular ligament (ATFL), and calcaneofibular ligament (CFL) are required to prevent OA progression by appropriate treatment. While arthroscopic ATFL repair has become the gold standard for CLAI treatment, the treatment of CFL remains controversial because the diagnosis of CFL injury is vague. Therefore, it is important to establish an accurate diagnosis of CFL injuries in the decision of the treatment plan.

Magnetic resonance imaging (MRI) is recognized as a reliable tool for the diagnosis of ATFL injury with high sensitivity, specificity, and accuracy [5]. On the other hand, it has been reported that diagnosing CFL injury on the conventional coronal, sagittal, and axial planes is difficult because of the tortuous course of the CFL, its angle to the plantar fascia, and anatomical variations [6,7]. A recent meta-analysis revealed that the sensitivity of CFL injury on MRI was 0.56, which concluded that conventional MRI cannot accurately diagnose CFL injury in patients with CLAI [8]. Therefore, previous reports have demonstrated the usefulness of oblique coronal images parallel to the CFL to visualize the entire CFL length and entire width, as well as its margin sharpness [9,10]. Although observation of the CFL on the oblique CFL view has improved the diagnosis of CFL injury, a method to quantitatively assess the degree of CFL damage is desirable. The CFL arises from the tip of the lateral malleolus and connects to a tubercle on the lateral aspect of the calcaneus, traversing superficially by the peroneal brevis tendon. A previous cadaveric study [11] demonstrated that the angle formed between the CFL and calcaneal axis was approximately 13° in the neutral position. We hypothesized that CFL injury could be quantitatively evaluated by assessing the changes in the angle between the CFL and calcaneus angle (CFLCA) due to the loss of tension caused upon CFL deficiency. This study aimed to examine the relationship between morphological changes in the CFLCA and the CFL injuries in patients with CLAI.