Ultrasonographic tissue characterisation of human Achilles tendons: quantification of tendon structure through a novel non-invasive approach

OBJECTIVE: To asses if three-dimensional imaging of the Achilles tendon by Ultrasonographic Tissue Characterisation (UTC) can differentiate between symptomatic and asymptomatic tendons. DESIGN: Case-control study. SETTING: Sports medical department of The Hague medical centre. PATIENTS: Twenty-six tendons from patients with chronic midportion Achilles tendinopathy were included. The "matched" control group consisted of 26 asymptomatic tendons. INTERVENTIONS: Symptomatic and asymptomatic tendons were scanned using the UTC-procedure. One researcher performed the ultrasonographic data-collection. These blinded data were randomised and outcome measures were determined by two independent observers. Main outcome measurements: The raw ultrasonographic images were analysed with a custom-designed algorithm that quantifies the three-dimensional stability of echopatterns, qua intensity and distribution over contiguous transverse images. This three-dimensional stability was related to tendon structure in previous studies. UTC categorizes four different echo-types that represent: I) highly stable; II) medium stable; III) highly variable and IV) constantly low intensity and variable distribution. The percentages of echo-types were calculated and the maximum tendon-thickness was measured. Finally, the inter-observer reliability of UTC was determined. RESULTS: Symptomatic tendons showed less pixels in echo-types I and II than asymptomatic tendons (51.5% versus 76.6%, p<0.001), thus less three-dimensional stability of the echopattern. The mean maximum tendon thickness was 9.2 mm in the symptomatic group and 6.8 mm in the asymptomatic group (p<0.001). The Intra-class Correlation Coefficient (ICC) for the inter-observer reliability of determining the echo-types I+II was 0.95. The ICC for tendon thickness was 0.84. CONCLUSION: UTC can quantitatively evaluate tendon structure and thereby discriminate symptomatic and asymptomatic tendons. As such UTC might be useful to monitor treatment protocols.