Imaging of tendons--adaptation, degeneration, rupture.

Tendon imaging is mainly performed with ultrasonography (US) and magnetic resonance imaging (MRI) and has been improved within the last years because of technical advancements and a better understanding of tendon pathology. Several concepts concerning the etiology and the course of tendon diseases have influenced image interpretation and vice versa. Adaptive mechanisms within the tendon tissue against stress can be observed mainly on histologic specimens and not macroscopically or with in-vivo imaging. Degeneration may occur in the form of tendinitis, peritendinitis, enthesitis, or myotendinal junction abnormality. Distinct imaging findings exist for most of these forms. Many concepts that have been developed to explain tendon degeneration have been applied on virtually all tendons in the human body. They can be grouped into those which focus on hypovascularization, on biomechanical overload, and on degeneration secondary to other underlying disease. Tendon rupture seems, in many cases, to be the final stage of tendinitis. From this point of view, imaging may be used to predict the risk of tendon rupture together with other intrinsic and with extrinsic parameters. These considerations result in the concept of the "vulnerable zone" and of the "critical phase" in which tendon ruptures may predominantly occur.