Despite the general acknowledgement that measurement of tendon thickness by ultrasonography (US) is an integral part of clinical examination of tendons in both symptomatic and asymptomatic athletes, there is no consensus on where and how the tendons should be measured.
Purpose: This study aims to evaluate the Achilles and patellar tendons by ultrasonography with the intention of establishing a consensus for measuring the thickness of Achilles and patellar tendons in future studies.
Materials and methods: This study includes three sub-studies, evaluating: 1. Achilles and patellar tendon thickness in relation to the distance from the attachment at the calcaneus or patella, 2. longitudinal versus transversal US scan for measurement of the tendon thickness by examining the tendons in both longitudinal and transversal scan planes twice by the same observer, and 3. differences in tendon thickness using three different US measurement methods, when measuring both the sagittal AP thickness and the "true" thickness (measured perpendicular to the greatest width) twice by the same observer. A total of 209 tendons were included.
Results: Normal Achilles tendons have the same thickness in the distal 5 cm-long section. Patellar tendons are more cone-shaped proximally. There is no significant difference between the longitudinal and transversal scan except when applied on abnormal patellar tendons. The tendon thickness and coefficient of variation is smaller when measuring the true thickness compared to the AP thickness.
Conclusion: The true tendon thickness is less than the AP thickness, because the AP-thickness is dependent upon the rotation of the tendon. Moreover, the true thickness is a more precise measurement. In future measurements, the true thickness of tendons could be measured in either transversal or longitudinal scan. When measuring abnormal patellar tendons, however, it is necessary to apply a longitudinal scan as this is the only method allowing the examiner to record the distance from the point where the thickness is measured to the bony attachment. The measurement can thereby be repeated at exactly the same point during subsequent controls.