###### Abstract  
Medial collateral ligament (MCL) injuries are the most frequent ligamentous injuries of the knee. Focused extracorporeal shock wave therapy (f-ESWT) is progressively expanding its field of application to many musculoskeletal pathologies. Although there is evidence surrounding the efficacy of f-ESWT in tendinopathies, no studies have described the usefulness of ultrasound (US)-guided f-ESWT in the treatment of ligament injuries. Herein, we report a case of a MCL injury treated with f-ESWT. Moreover, our case shows the importance of using ultrasonography in determining the effect of treatment. A 60-year-old man with a focal area of lesion in the deep fibers underwent 4 weekly sessions of US-guided f-ESWT to the injured ligament area. His pain decreased to a visual analog scale (VAS) of 3 at the end of the treatment and was completely resolved at the 1-month follow-up visit, with these results being maintained at 4-month follow-up. The US examination showed an initial deposition of \"newly formed tissue\" at the site of previous injury of the proximal MCL insertion, and a reduction in MCL thickness together with an improvement in echostructure. Based on this result, we speculate that non-surgical ligament injuries could be a new indication for f-ESWT. However, further investigation on the effects of f-ESWT for ligament injuries is needed. focused extracorporeal shock therapy; ligament injury; medial collateral ligament; knee; ultrasonography +
Footnote †: journal: Medical Collateral Ligament Injuries  
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Footnote 1: _Applied_  
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24 November 2021  
22 December 2021  
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **1. Introduction**  
The medial collateral ligament (MCL) is a very complex apparatus connecting the medial surface of the femoral condyle to the tibia. Its function is to resist forces applied from the outside of the knee preventing the medial or inner part of the joint from widening. Moreover, the MCL is considered a static stabilizer. Its structure is triangular and slightly expansible. Its origin is located proximally on the medial epicondyle of the femur next to the adductor tubercle, whereas its distal attachment lies below the medial condyle of the tibia on its medial surface [1]. MCL injuries are the most frequent type of ligamentous injury of the knee, with a reported annual incidence of 0.24-7.3 per 1000 and a male-to-female ratio of 2:1 [1; 2]. The primary mechanism of injury to the MCL results from a valgus force with the foot being fixed to the ground [3]. The severity of the damage can be subdivided into three grades, depending on the state of laxity highlighted by the valgus stress test at 30\({}^{\circ}\) knee flexion [4]: 3-5 mm (grade 1), 6-10 mm (grade 2), and >10 mm (grade 3). A further classification from the American Medical Association subdivided the MCL injuries into three grades based on the severity of ligament damage: grade I with minimal torn fibers without loss of integrity, grade II with a partial MCL tear, and grade III with disruption of the ligament [5]. MCL injuries often present as isolated lesions; however, they may be associated with other ligamentous injuries, more frequently involving the anterior cruciate ligament [1]. The diagnosis of an MCL lesion is mainly based on clinical evaluation and imaging. In particular, ultrasounds (USs) are a safe, non-invasive and fast procedure with very high reliability. USs can be easily used to study the ligament and characterize its impairment, evaluating the morphological features of the damage. Moreover, USs are useful in the classification of the lesion severity and are consequently very helpful in guiding the therapeutic decision. Finally, USs may be used as an outcome measure to evaluate the efficacy of treatment. Several nonoperative or operative treatment options have been reported.