Transverse US scan of the medial compartment of the ankle. The retinaculum (R) is the thin hyperechoic line that lies over the tendons. PT = posterior tibial; CFD = communis flexor dig-itorum, NVB = neurovascular bundle
Retinacula at ultrasound appear as thin hyper-echoic structures located more superficially than the sliding tendons, in very critical areas from a bio-mechanical point of view (Fig 3.15). Annular pulleys are biomechanical devices made of fibrous connective tissue, which keep the flexor digitorum tendons in position during flexion-extension movements.
For this reason, the sonographic assessment of the pulleys has to be performed with a dynamic method; the US dynamic analysis should be obtained during flexion-extension movements of the fingers and, if a tendon tear is suspected, it should be supplemented with contrasted flexion. The transducer should always have a perpendicular and transverse position over the flexor tendons, with a high amount of gel used as a spacer in order to avoid any pressure on the tissue. In longitudinal views of flexor tendons, the pulley appears as a thin oval structure lying superficially compared with the tendon sheath (Fig. 3.16) [30,31].
The structure of ligaments is very similar to that of tendons; the main differences are reduced thickness and a less regular arrangement of structural elements; for this reason, it is harder to study ligaments with US than tendons.
The US examination of ligaments, unlike tendons, is mainly performed using long axis views, the transducer being aligned on the ligament's major axis. Transverse views (short axis) have poor diagnostic value. With US, ligaments appear as homogeneous, hyperechoic bands, 2-3 mm thick, lying close to the bone (Fig 3.17) .
The easiest ligaments to assess with US are those of the medial and lateral compartments of the ankle (deltoid, anterior talofibular and fibulocalcaneal), the collateral ligaments of the knee, the collateral and annular ligaments of the elbow, the coraco-
acromial and coraco-humeral ligaments of the shoulder and the ulnar collateral ligament of the thumb [30-32].
The medial collateral ligament of the knee (MCL) has a very complicated structure that deserves detailed description. The MCL is a flattened, large structure extending from the distal
extremity of the medial femoral condyle to the proximal tibial extremity; it is about 9 cm long and it is divided into two components, deep and superficial, which are separated by a thin layer of loose connective tissue. The deep component is then divided into two small ligaments that fix the medial meniscus respectively to the femur (meniscofemoral ligament) and to the tibia (menisco-tibial ligament). Sonographically the MCL appears as a trilaminar structure consisting of two hyperechoic layers, separated by a central interleaved hypoe-choic area. The hyperechoic bands correspond to deep and superficial fiber bundles; whereas the loose areolar tissue constitutes the hypoechoic central area that divides the superficial component from the deep one (Fig. 3.18) .
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