Ultrasonography (US) is used to visualise tendon structure. However, its capacity to detect early disintegration, to reproducibly monitor progress of pathology or repair, and to objectively evaluate effect of therapies and exercise is poor. Furthermore, US is not able to reliably assess tissue integrity as its limits of resolution means that every US image is a mixture of reflections (of relatively large structures like secondary tendon bundles, fasciculi) and interfering echoes (generated by smaller entities like fibrils and cells) that cannot be discriminated by the eye.
Therefore, a method for computerised ‘ultrasound tissue characterisation’ (UTC) was created. A high-resolution probe is fixed in a tracking device and moves automatically along a tendon's long axis, collecting transverse images at even distances of 0.2 mm. By compounding contiguous transverse images, a 3-D ultrasound data-block is created that can be used for (A) tomographic visualisation in 3 planes of view plus a 3-D rendered view, and for (B) tissue characterisation and quantification of 3-D architecture of the tendon matrix.
Dedicated UTC-Algorithms can discriminate four different echo-types, namely: echo-type I, generated by reflections at intact and aligned tendon bundles, echo-type II, generated by reflections at discontinuous or wavy tendon bundles, echo-type III, generated by interfering echoes from mainly fibrillar components, and echo-type IV, generated by mainly cellular components and fluid in amorphous tissue. Initially, UTC was validated by precisely matching UTC-processed images with corresponding tendon specimens, sampled from isolated flexor tendons in the horse. These four echo-types appeared to be highly correlated with histo-morphological characteristics, showing the discriminative power of UTC for tissue characterisation.
Since 2005, UTC is implemented for human Achilles and patellar tendons and has been used in multiple research projects too. Some relevant observations in these studies are:
Inter-and intra-observer reproducibility of both data-collection and analysis appeared to be high (ICC over 0.90), indicating an excellent reliability for longitudinal monitoring.
Observational studies of UTC parameters related to age revealed that Achilles tendons of young persons (24–30 years of age) are characterised by 80–85% echo-type I, 10–15% echo-type II and only 2–5% type III plus IV echoes. Initial deterioration, mostly asymptomatic, was characterised by increasing percentages of echo-types II (remodelling or fibrosis) and/or III plus IV (fibrillar and cellular disintegration), mainly localised in the postero-medial quadrant of the Achilles tendon. In patients with diabetes type II and in persons with lateral foot-landing pattern, the Achilles tendons demonstrated significantly more changes on UTC compared to matched controls.
Patients suffering tendinopathy (avg. age 45 years) have significantly higher percentages of echo-types II, III and/or IV, compared with a matching group of asymptomatic control persons.1
Significant increases of echo-type II and III can be observed in normal tendons 24–48 h after maximal loads: these changes appear reversible within 4 days.2 On the other hand, preliminary results in a group of high-performance athletes indicate that athletes with persistent changes on UTC are at risk of developing symptoms within the same season.
Ultra-structural effects of rehabilitation exercises can be quantitatively evaluated within 14 days. It is concluded that UTC is utmost suitable for (a) detection of in-situ tendon responses to load, (b) early diagnosis of injury or degeneration, (c) staging of histo-pathological stage of lesions, and for (d) objective evaluation of longitudinal effects of regenerative therapies.
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