Acute ankle sprain is one of the most common injuries to the lower extremities, with an incidence rate of 11.88%, and it occurs more frequently during sports activities [5, 12, 13]. Acute ankle sprain in children is often accompanied by lateral ankle ligament injuries and avulsion fractures of the lateral ankle [14, 15]. The anterior drawer test and inversion stress test are classic methods for examining whether the anterior talofibular ligament (ATFL) is injured, but it is usually difficult for children with acute sprains to complete these tests [16, 17]. Surgeons typically use the “Ottawa Ankle Rules” to determine whether radiological examination is necessary [18]. However, in cases of acute ankle sprain in children, lateral ankle fractures are often avulsion fractures involving cartilage, and radiography of the ankle usually cannot adequately demonstrate cartilage damage.

Schutze and Maas reported that in children with ankle sprains, one-third of avulsion fractures cannot be identified through radiography [19]. However, Haraguchi et al. reported that 76% of avulsion fractures in children can be detected via radiography, as the authors reported that most chondral avulsion fractures are associated with bony tissue avulsion, which can be detected via radiological imaging [20]. In our study, we found that while some children with ankle sprains do have bony avulsions (41%) based on X-ray results, simple chondral avulsion fractures (59%) are more common. For this reason, a total of 17 false-negative results were produced by ankle radiography in our study, with a sensitivity of only 41% and a negative predictive value of 63%.

Ultrasound examination has unique advantages in detecting cartilage injuries. Simanovsky et al. [21] found that ultrasonography has high sensitivity and specificity in the diagnosis of chondral avulsion fractures of the ankle. In our study, we compared 79 cases of lateral ankle fractures via radiography and ultrasonography. We found that 29 cases of avulsion fractures of the lateral ankle were detected via ultrasonography but were missed via radiography. Interestingly, while it is commonly believed that avulsion fractures of the lateral ankle are typically distal fibular chondral avulsion fractures, our study revealed that 31% (9 out of 29) of the cases were actually avulsion fractures of the lateral talus process. However, it is important to note that pain at the level of the distal fibular physis should not be overlooked, as a physeal injury can lead to epiphysiolysis, which requires a different diagnostic and therapeutic approach.

In this study, 59 cases were evaluated with MRI as the gold standard, and the sensitivity of ankle ultrasound in diagnosing lateral ankle fractures reached 97%, whereas the sensitivity of radiological diagnosis for lateral ankle fractures was only 41%. This indicates that if the presence of lateral ankle fractures in children with acute ankle sprain is determined only based on radiography results, a significant number of false-negative cases may occur. Additionally, five false-positive cases were identified in the ultrasonography diagnosis in this study, which may be due to the avulsion fractures being too small to be detected by MRI or possibly due to oedema at the fracture site, leading to missed diagnoses in the MRI results. If avulsion fractures are difficult to definitively diagnose during ultrasound examination, dynamic stress ultrasonography can be performed, and partial stress can be applied to the fracture site to assess the stability of the fracture, which is a unique advantage of ultrasonography [22]. Ultrasound examination can assist orthopaedic surgeons in making more accurate diagnoses, but this does not imply that the treatment plan will be altered. In fact, most acute ankle sprains in children achieve good treatment outcomes without requiring surgical intervention.

The lateral ligaments of the ankle in children are often injured following acute sprains, with over 80% of ankle sprain patients experiencing anterior talofibular ligament injury [16, 23]. However, ankle radiography cannot accurately determine whether the anterior talofibular ligament is damaged. Since both the anterior talofibular ligament and the calcaneofibular ligament are located relatively superficially, ultrasonography of the ankle can clearly detect injuries to these ligaments [24,25,26]. Hosseinian et al. reported in their study of adult ankle sprains that ultrasonography for diagnosing anterior talofibular ligament injuries achieved 100% sensitivity, specificity, positive predictive value, and negative predictive value, with a kappa value of 1 [24]. Our results are similar: ultrasonography yielded two false-positive cases and one false-negative case, with a sensitivity of 96%, specificity of 94%, positive predictive value of 92%, negative predictive value of 97%, and a kappa value of 0.894.

Unlike previous studies on adult ankle ligament injuries, we found that only 5 cases involved calcaneofibular ligament injuries in children, which is significantly fewer than the number of anterior talofibular ligament injuries [27]. However, the accuracy of musculoskeletal ultrasound in diagnosing calcaneofibular ligament injuries is still high, with a sensitivity of 83%, a specificity of 98%, and a kappa value of 0.814. Our study results show that the agreement between ultrasound and MRI in diagnosing anterior talofibular ligament and calcaneofibular ligament injuries is very high, indicating that ultrasonography can be a reliable alternative to MRI.

The main limitation of this study was the lack of interobserver and intraobserver reliability of the ultrasound examination results, as these results may be related to the experience of the sonographer [4]. However, in our study, the interpretation of the ultrasound results was performed by a senior sonographer with extensive experience in musculoskeletal ultrasound, which should largely mitigate the bias that might arise from an inexperienced sonographer.