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Real-time scanning of the injured knee is becoming increasingly available with improvements in ultrasound technology, which provides visualization for both ligaments and tendons of the knee (see supplemental video; http://links.lww.com/PHM/A103). The most important knee ligaments to examine for potential injury are the two ligaments at the medial and lateral aspects and the two cruciate ligaments deep in the knee joint. Because of their extra-articular locations, the medial collateral ligament (MCL) and lateral collateral ligament (LCL) can be easily viewed by ultrasound. The two intra-articular ligaments, namely, the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), indwell deeply in the joint and therefore can only be partially visualized, or else assessed by indirect signs (Fig. 1).
Medial Collateral Ligament
The MCL is a two-layer structure. The deep layer is composed of meniscofemoral and meniscotibial ligaments tightly attached to the medial meniscus.1 The superficial layer is composed of broad and flat connective tissues, running from the medial femoral epicondyle to its tibial attachment (Fig. 2).
To examine the MCL, the patient is positioned supine on the examination table. The transducer is placed on the medial aspect of the knee where the MCL is located. The most common signs of an injured MCL are thickening and swelling of the ligament, with heterogeneously hypoechoic changes.
In cases of severe injury, echogenic foci superficial to the bony cortex of the femur may be noted in the proximal part of the MCL. This particular type of injury is known as a Pellegrini-Stieda lesion.2
Lateral Collateral Ligament
LCL injuries are much more infrequent than MCL injuries. The LCL lies between the apex of the fibula and the lateral femoral epicondyle. This ligament’s properties and its oblique orientation make it hypoechoic under ultrasound.3 At its distal attachment, the LCL is conjoined with the tendon of the biceps femoris muscle. At its proximal part, the popliteus tendon runs deeply across the LCL as an echogenic oval structure in a bony groove (Fig. 3).
The LCL is more likely to be injured in a varus, or twisting, injury. Under ultrasound, an injured LCL appears thickened and/or uneven, or as a disrupted band.
Anterior Cruciate Ligament
The ACL can be partially seen from the anterior side with extreme flexion of the knee4,5 or evaluated by scanning the intercondylar fossa from the back of an extended knee to access the associated hemarthrosis. However, because it is deeply imbedded in the joint, it is impractical to examine the entire ligament by ultrasound. To allow access to the knee’s popliteal area, the transducer should be placed in the transverse position.
In cases of ACL tears, the presence of hemarthrosis, up to 70% of which may be caused by the tearing of the ligament, is helpful for diagnosis of acute injury. Hematoma on the posterolateral aspect of the intercondylar notch may be detected with the injured knee in extended position.
In general, fluid accumulation in or around the knee is usually associated with an injured ligament. The techniques of ultrasound-guided aspiration and/or injection may therefore be used to confirm acute ACL injury as well as treat it. The suprapatellar recess is the most convenient site to aspirate a knee effusion.
Evaluation of the tibial portion of the ACL can be processed from the anterior aspect of an extremely flexed knee. The tibial insertion is approximately 1 cm away from the anterior border of the tibial plateau, and the ACL runs deeply and obliquely into the knee joint. The anisotropic effect causes the ligament to be depicted as a hypoechoic band. Suzuki et al.4 reported a successful inspection of the ligament using this method, and Chen et al.5 proved it by using a Kelley to pinpoint the ACL during arthroscopic examination.
Posterior Cruciate Ligament
The PCL is stronger and less frequently injured than the ACL. Ultrasound provides visualization of only its distal half. To scan the PCL, the transducer is placed on the long axis of the midline of the popliteal fossa in an extended knee; then, the proximal part of the transducer is rotated toward the direction of the ligament. Fine-tuning of the transducer is necessary to get a more satisfactory image. The visible part of the PCL is a hypoechoic bandlike structure, which inserts at the echogenic tibia.6 The PCL’s hypoechoic appearance is mainly attributed to the anisotropic effect (Fig. 4). The average thickness of a normal PCL is less than 1 cm.6–8 An injured PCL tends to be thicker than usual, and a torn PCL is usually heterogeneously hypoechoic with an indistinct or wavy posterior margin.
Ultrasonography is a useful tool for evaluating the MCL and LCL. Regarding the ACL and PCL, ultrasound provides partial visualization as well as indirect signs of injuries, offering valuable information that can be used in diagnoses of traumatic lesions.
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