Tendons are better visualized with US than with MR imaging (MRI). Tendons consist of bundles of linear arranged fibrils of type I collagen with a supporting matrix. The fibrils are oriented in a direction specific to the forces applied from the interaction between a tendon and its muscle and skeletal attachment.⁴, ⁵ During movement, tendons shorten and lengthen as springs do, transmitting and absorbing forces. When the US beam is perpendicular to the tendon, the energy of the beam is reflected back to the transducer resulting in a fibrillar pattern of alternating hyperechoic and hypoechoic lines (Fig. 14.4). The fibrillar pattern is unique to tendons. If the beam loses this perpendicular orientation, the beam is reflected away from the transducer and the tendon will appear hypoechoic within a hyperechoic paratenon. This phenomenon is known as anisotropy and can be avoided by keeping the beam perpendicular to the tendon.

Ligaments also contain longitudinally arranged parallel fibers of type I collagen.⁶ However, their architecture is not as fibrillar (Fig. 14.5). Although anisotropy is observed, it is not a prominent imaging feature, but it can be used to find ligaments and distinguish them from surrounding connective tissue. Like tendons, ligaments must be imaged longitudinally and transversely.

Peripheral nerves can also be accurately identified using US imaging. Nerves are composed of multiple axons that are bundled together in neuronal fascicles. The fascicles are held together by loose connective tissue: the epineurium.⁷, ⁸ Normal peripheral nerves typically appear as echogenic fascicular structures and tend to be slightly less echogenic than tendons or ligaments (Fig. 14.6). This appearance is somewhat variable depending on the location and orientation of the nerve but can usually be identified by the nerve distribution.

Capsular tissue and fascia appear hyperechoic.

Cortical bone is hyperechoic, reflecting the insonating beam and producing a black acoustic shadow; hence, precluding visualization of the osseous medulla.

Hyaline articular cartilage is anechoic and homogenous and can only be visualized in areas free of acoustic shadowing from neighboring bony structures—among them, the dorsal articular surfaces of most of the forefoot and midfoot joints.

Muscle, like tendons and ligaments, has an organized structure. The epimysium is identified as an echogenic envelope surrounding the muscle belly, whereas the perimysium, enveloping muscular fascicles, is seen as short echogenic lines or dots on a hypoechoic background (Fig. 14.7). Intramuscular tendon extensions and bright intermuscular septae and aponeuroses are also easily identified on sonography.⁴

Bland uncomplicated fluid and synovium appear uniformly anechoic. Posterior acoustic enhancement, the absence of vascularity on Doppler imaging, and the swirling movement of debris on intermittent pressure with the probe may differentiate simple fluid from synovium.

The ankle is the most frequently injured major joint in the body. It is also involved by degenerative and rheumatology-related diseases. US has become an increasingly important imaging modality in the assessment and diagnosis of tendons, muscles, ligaments, nerves, and the soft tissues.⁹, ¹⁰, ¹¹ However, radiographs of the ankle should be obtained to evaluate for bone lesions that can be overlooked or not visualized using US.