The influence of ultrasound measurement techniques on the age-related differences in Achilles tendon size

Abstract

Achilles tendon (AT) properties change with age, however the extent to which aging influences the size of the AT may be influenced by the measurement technique. The purpose of this study was to 1) examine the age-related differences in AT size using measures of cross-sectional area (CSA) and thickness, and 2) to determine the test–retest reliability and minimum difference (MD) values of these measures. The size of the AT was examined in 19 young (19.79 ± 2.3 years) and 18 older (69.4 ± 3.1 years) healthy men on two separate occasions using an ultrasonography. The CSA and thickness of the AT were determined by separate transverse and longitudinal scans on the right leg at the same distance from the medial malleolus, respectively. Prior to and after accounting for body mass, AT CSA was larger (P ≤ 0.01) in the older men (64.49 ± 13.87 mm²) when compared to the younger men (44.12 ± 16.04 mm²), however there were no differences in AT thickness between age groups (P = 0.96). In addition, there was no systematic error (P > 0.05) between testing days, and the intraclass correlation coefficients, standard error of measurement (expressed as a % of the mean), and MD values for CSA and thickness ranged from 0.80–0.98, 5.46–8.68%, and 5.65–7.94 mm² and 0.51–0.86 mm, respectively between both groups. These findings suggest that the reliability of the CSA and thickness measurements were similar, however, CSA was a more sensitive measure to detect the age-related changes of AT size.

Introduction

The Achilles tendon (AT) is a large viscoelastic tissue that transmits force from the plantarflexor musculature to the calcaneus resulting in joint and limb movement. It undergoes large loads during activities of daily living (Magnusson et al., 2008) and has shown to be a metabolically active tissue that responds to chronic loading and unloading (Magnusson et al., 2008). Furthermore, the AT is susceptible to a high rate of injury which may be due to its “low safety factor” during high-intensity activities (Magnusson et al., 2003).

Aging is accompanied by changes in maximal and rapid strength (i.e. rate of force development) (Thompson et al., 2014) resulting in impaired locomotor function which may subsequently lead to a higher risk of falls and poorer quality of life (Beaudart et al., 2015). Previous studies have indicated that the age-related changes in tendinous tissues have a significant influence on muscular function (Stenroth et al., 2012) and injury risk (Magnusson et al., 2003). Animal models have demonstrated aging results in tendon hypertrophy (Tsuchida et al., 1997, Kotani et al., 1998), however these findings are inconsistent in humans (Magnusson et al., 2003, Pang and Ying, 2006, Carroll et al., 2008). It is possible that these discrepancies may be due to the measurement technique of AT size. For instance, Pang and Ying (2006) recently demonstrated that older participants (> 50 years) had greater AT cross-sectional area (CSA) when compared to younger participants using ultrasonography (US), however there were no age-related differences reported in AT thickness. Previous authors (Ekizos et al., 2013) have also questioned the reliability of US imaging to examine tendon morphology, which may indicate that future studies are needed to determine the most appropriate and reliable measure for investigating AT size. Thus, the purpose of the present study was to 1) examine the age-related differences in AT size using CSA and thickness in young and older adults and 2) determine the test–retest reliability and minimal difference values to determine a difference or change in AT size that can be considered real (Weir, 2005).