Tendons are flexible and inelastic bands of strong fibrous connective tissue that transmit force from muscle to bone and form an integral part of the musculoskeletal system. They can be intraarticular, coated in a tendon sheath or remain extrasynovial, coated by paratenon. Tendons are considered part of the soft tissues of the body.
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Gross anatomy
Tendons are formed by dense connective tissue and connect muscle at the myotendinous junction to bone at the enthesis 1,2. They are surrounded by paratenon or by a synovial tendon sheath 3. With respect to their form, they feature several lengths and their cross-section might be round oval or flat 3. They are characterized by a rich neural network 3. Blood circulation in tendons is not as ample as in muscle or bone but is important for their metabolism. Extrinsic blood supply varies on whether they are intrasynovial or extrasynovial, the first ones are relatively avascular and diffusion through the synovial fluid presents an important source of nutrition, whereas unsheathed tendons receive blood supply at different locations along the tendon 4.
Function
The main function of tendons is the transmission of loading forces including storage and release of elastic energy 1-3.
They also have an important role in proprioception and nociception of joints 3.
Histology
Histologically tendons consist of closely packed collagen fibers embedded in an extracellular matrix with tendon cells as well as non-fibrillar proteins as proteoglycan and elastin. The fibers are organized in fascicles that are surrounded by connective tissue the endotenon which is continuous with the epitenon covering the tendon surface 1-4.
Radiographic features
Plain radiograph
On plain radiographs, tendons are only rarely visible if large in size and/or surrounded by fatty tissue and/or air like the Achilles tendon or patellar tendon.
Ultrasound
On ultrasound tendons usually display a hyperechoic fibrillar pattern in both short and long axes. When they are not perpendicular to the insonation angle anisotropy can cause artifactual appearance resembling tendinosis or tendon tears 4,5.
CT
On CT tendons are characterized by soft tissue density and are difficult to distinguish from other soft tissues as e.g. muscle.
MRI
Normal tendons appear hypointense on all sequences. However, they are susceptible to magic angle artifacts especially if depicted on sequences with lower echo times ≤35-40 ms, where they display a high signal when oriented approximately 55° in relation to the magnetic field 4,5.
Related pathology
Pathologies associated with tendons include the following:
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tendon injuries (acute and overuse)
tendon tear (longitudinal, transverse)
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tendon associated tumors
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