Insufficient accuracy of the ultrasound-based determination of Achilles tendon cross-sectional area
Introduction
The accurate measurement of the Achilles tendon cross-sectional area (CSA) is a crucial prerequisite to investigate tendon plasticity in response to loading (Arampatzis et al., 2007, Arampatzis et al., 2010, Bohm et al., 2014, Kubo et al., 2007, Kubo et al., 2012), immobilization (Kinugasa et al., 2010, Kubo et al., 2004, Shin et al., 2008) and pathologies (Gao et al., 2011, Kongsgaard et al., 2005, Zhao et al., 2009). Besides the quantification of tendon morphology, precise tendon CSA values are necessary in order to calculate tendon stress and, further, to evaluate the tendon material properties (i.e. Young׳s modulus). Due to an adequate resolution and good contrast between surrounding tissues, magnetic resonance imaging (MRI) enables the reliable and objective segmentation of the identified tendon CSA and, thus, is currently the preferred gold standard imaging methodology (Couppé et al., 2014, Magnusson et al., 2003). Accordingly, the MRI-based CSA assessment has been widely used in human tendon research (Arampatzis et al., 2010, Bohm et al., 2015, Hansen et al., 2003, Kongsgaard et al., 2005, Kubo et al., 2007, Magnusson et al., 2003, Magnusson and Kjaer, 2003, Westh et al., 2008).
The high expenses and limited accessibility of MRI scanners promoted the application of ultrasonography as an alternative imaging technique for the assessment of tendon dimensions (Dudley-Javoroski et al., 2010, Finni et al., 2009, Foure et al., 2011, Foure et al., 2013, Houghton et al., 2013, Kallinen and Suominen, 1994, Maganaris and Paul, 2002, Urlando and Hawkins, 2007, Ying et al., 2003, Zhao et al., 2011). However, there is evidence that the ultrasound-based patellar tendon CSA measurement does not adequately fulfill the test quality criteria of reliability and objectivity (Ekizos et al., 2013). As suggested, the low clarity of the connective tissue structures and unclear visibility of tissue boundaries in the ultrasound images, especially of the acuminate medio-lateral tips of the patellar tendon, affect the segmentation process and account for the low reliability and objectivity of the ultrasound-based methodology (Ekizos et al., 2013). However, the shape of the Achilles tendon CSA features less eccentricity (i.e. less ovality) compared to the very pointed oval-shaped patellar tendon CSA and, therefore, it can be assumed that the tissue boundaries are easier to identify, allowing for a more precise segmentation. Hence, Brushoj et al. (2006), Finni et al. (2009) and O׳Connor et al. (2004) reported greater test-retest reliability of the Achilles tendon CSA and size measurements using ultrasonography and no statistically significant observer effects, indicating a more reliable assessment and higher objectivity compared to the measurements at the patellar tendon (Ekizos et al., 2013).
However, evidence for reliability and objectivity alone is not sufficient for the establishment of a new methodology. Validity, which refers to the degree of how exact a test measures what it claims to measure (Heyward, 2006, Morrow et al., 2011), is another important quality criterion for the evaluation of test accuracy. Since tendon dimensions are very visible in ultrasound images as well (Rasmussen, 2000), validity of this method could be expected. To the best of our knowledge, the validity of the ultrasound-based methodology for the determination of the human Achilles tendon CSA has not been investigated yet. From a methodological point of view, the adequate application of the ultrasound-based methodology requires evidence of all three mayor test quality criteria (i.e. objectivity, reliability and validity). Despite this issue to be still unsolved, a lot of recent studies used the ultrasound methodology to measure the Achilles tendon CSA (Houghton et al., 2013, Kunimasa et al., 2014, Milgrom et al., 2014, Stenroth et al., 2016, Waugh et al., 2012, Waugh et al., 2014).
The purpose of the present study was to assess the accuracy, i.e. objectivity, reliability and validity of the ultrasound-based methodology for the measurement of the Achilles tendon CSA. The objectivity and reliability was investigated using an experimental test-retest design. The CSAs in the proximal, medial and distal part of the free Achilles tendon from 17 males were captured in five separate trials on two different days while all images were analyzed by three independent observers. The ultrasound-based CSA determination was validated with respect to the established MRI methodology, whose reliability and objectivity has been proved. Based on the aforementioned literature reports of tissue visibility of tendon dimensions using ultrasonography and reliability of Achilles tendon CSA and, further, our assumption of more reliable and objective CSA assessments of the Achilles tendon compared to the patellar tendon due to its lower eccentricity, we hypothesized an adequate accuracy of the ultrasound-based CSA determination for the Achilles tendon.
Section snippets
Participants
Seventeen male adults (age: 27.8±3.9 yrs., height: 180.3±6.7 cm, mass 76.3±8.5 kg) devoid of musculoskeletal impairments in the lower limbs participated in the present study after giving informed consent to the experimental procedure, which was approved by the local scientific board. None of the participants took part in physical activity twelve hours prior the ultrasonography and MRI tendon CSA measurements, respectively, which were conducted in a random order.
Ultrasonography-based tendon CSA assessment
The participants lay supine on a
Results
Average tendon CSA values of the five trials of day 1 and 2 at the three positions (proximal, medial and distal) are presented separately for the three observers in Table 1.
Discussion
The present study provides a systematic and detailed analysis of the accuracy of the Achilles tendon CSA assessment by means of ultrasonography. Significant effects of the observer, measurement day and method on the CSA assessment were observed, indicating limitations regarding all three main test quality criteria objectivity, reliability and validity. The results contradicted our hypothesis and question the applicability of ultrasound for a precise and acceptable determination of the human
Conflict of interest statement
All authors disclose any financial and personal relationships with other people or organizations that could inappropriately influence (bias) the study.
Acknowledgments
This work was financially supported by the Federal Institute of Sport Science (BISp) Germany (IIA1-070501/11-13). Further, we thank the foundation Stiftung Oskar-Helene-Heim for providing access to the MRI scanner. The supporting institutions were not involved in any process of the study.
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