Medial and lateral ligaments injuries can be diagnosed on sonography [30,31]. A tear of the medial collateral ligament has a similar appearance on sonography to that described for MRI . The sonographic appearance of a Grade I injury is that of hypoechoic fluid, caused by edema and hemorrhage, parallel to the medial collateral ligament (MCL). A Grade II injury at sonography is seen as hypoechoic fluid around the MCL as well as ligament thickening. Finally, Grade III injury is seen as hypoechoic fluid or hematoma filling the site of the tear, with disruption of both superficial and deep components of the MCL. A thickened, well-defined, or calcified ligament indicates old injury . The same principles apply in sonographic evaluation of the lateral collateral ligament.
Sonographic evaluation of the anterior cruciate ligament is limited because of difficulty in direct visualization. In contrast, the posterior cruciate ligament (PCL) can be directly visualized sonographically from a posterior approach . At sonography, the PCL is normally hypoechoic. PCL injury is manifested sonographically either as enlargement of the ligament when compared with the contralateral normal PCL, or by focal disruption of the ligament . Although MRI remains the imaging gold standard for clinically suspected internal derangement, sonography could be considered for those patients who cannot undergo an MRI examination.
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