Viscoelastic properties of healthy achilles tendon are independent of isometric plantar flexion strength and cross-sectional area

Abstract

Changes in tendon viscoelastic properties are observed after injuries and during healing as a product of altered composition and structure. Continuous Shear Wave Elastography is a new technique measuring viscoelastic properties of soft tissues using external shear waves. Tendon has not been studied with this technique, therefore, the aims of this study were to establish the range of shear and viscosity moduli in healthy Achilles tendons, determine bilateral differences of these parameters and explore correlations of viscoelasticity to plantar flexion strength and tendon area. Continuous Shear Wave Elastography was performed over the free portion of both Achilles tendons from 29 subjects. Isometric plantar flexion strength and cross sectional area were measured. The average shear and viscous moduli was 83.2 kPa and 141.0 Pa-s, respectively. No correlations existed between the shear or viscous modulus and area or strength. This indicates that viscoelastic properties can be considered novel, independent biomarkers. The shear and viscosity moduli were bilaterally equivalent (p = 0.013, 0.017) which allows determining pathologies through side-to-side deviations. The average bilateral coefficient of variation was 7.2% and 9.4% for shear and viscosity modulus, respectively. The viscoelastic properties of the Achilles tendon may provide an unbiased, non-subjective rating system of tendon recovery and optimizing treatment strategies.

Keywords: biomarkers; continuous shear wave elastography; tendon mechanics; ultrasound elastography; viscoelastic moduli.

Conflict of interest statement

Conflict of interest

Stephen M Suydam and Daniel H Cortes are pursuing a patent for the Continuous Shear Wave Elastography protocol.

© 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

Figures

Figure 1

The elastography arrangement. The ankle is at 10° of dorsiflexion, the probe is placed at the soleus myotendinous junction and the actuator is placed approximately 3 cm proximal to probe, along the Achilles tendon.

Figure 2

The average velocity at each of the frequencies applied from the external actuator. Note the consistent increase in velocity with the frequency applied, showing the need for the optimization with a viscoelastic model.

Figure 3

The shear modulus (A) and viscosity modulus (B) maps overlayed on the gray scale image of the Achilles tendon. The area evaluated is the center region of the tendon due to the processing technique requiring data before and after the area of interest.

Figure 3

The shear modulus (A) and viscosity modulus (B) maps overlayed on the gray scale image of the Achilles tendon. The area evaluated is the center region of the tendon due to the processing technique requiring data before and after the area of interest.

Figure 4

Tendon cross sectional area viewed at the soleus myotendinous junction in the transverse plane. The tendon is traced in the thick black line and the area was calculated using ImageJ.