Tendon motion and strain patterns evaluated with two-dimensional ultrasound elastography

Abstract

The purpose of this study was to evaluate the use of 2D ultrasound elastography to assess tendon tissue motion and strains under axial loading conditions. Four porcine flexor tendons were cyclically loaded to 4% peak strain using a servo hydraulic test system. An ultrasound transducer was positioned to image a longitudinal cross-section of the tendon during loading. Ultrasound radiofrequency (RF) data were collected at 63 frames per second simultaneously with applied force and crosshead displacement. A grid of nodes was manually positioned on an ultrasound image of the unloaded tendon. Small kernels (2 × 1 mm) centered at each node were then cross-correlated with search regions centered at corresponding nodal locations in the subsequent frame. Frame-to-frame nodal displacements were defined as the values that maximized the normalized cross-correlations. This process was repeated across all frames in the loading cycle, providing a measurement of the 2D trajectories of tissue motion through out the loading cycle. The high resolution displacement measures along the RF beam direction were spatially differentiated to estimate the transverse (relative to tend on fibers) tissue strains. The nodal displacements obtained using this method were very repeatable, with average along-fiber trajectories that were highly correlated (r2>0.98) with the prescribed crosshead displacements. The elastography transverse strains were also repeatable and were consistent with average transverse strains estimated via changes in tendon width. The apparent Poisson’s ratios (0.82-1.64) exceeded the incompressibility limit, but are comparable to values found for tendon in prior experimental and computational studies. The results demonstrate that 2D ultrasound elastography is a promising approach for noninvasively assessing localized tissue motion and strain patterns.

Non-uniform displacements within the Achilles tendon observed during passive and eccentric loading

“…The goal of this study was to use ultrasound elastography (Chernak and Thelen, 2012; Slane and Thelen, 2014) to measure Achilles tendon displacement patterns under both passive and eccentric loading conditions in two knee postures. We hypothesized that eccentric loading would induce more non-uniform tendon motion than observed under passive stretch.…”

Negative Poisson’s ratios in tendons: An unexpected mechanical response

“…A few studies have also investigated the Poisson's ratio of tendons, both experimentally [2,[14][15][16][17][18][19] and numerically [20,21]. The Poisson's ratio [22] is a fundamental material property in its own accord and describes the change in size of a system in a direction perpendicular to an applied stress.…”

“…Here it must be noted that unlike stiffness, the Poisson's ratio is a two-dimensional property and in anisotropic materials, its sign and magnitude may depend not only on the direction of stretching, but also on the orthogonal direction being measured. Unfortunately, in view of the complexity associated with studying the Poisson's ratios, studies reporting this property in tendons have been limited in number [2,[14][15][16][17][18][19], and normally make various assumptions, which may have resulted in incomplete reporting of the Poisson's ratio. For example, in most ex vivo studies on the Poisson's ratios of tendons it had been assumed that the tendon exhibits transverse isotropy, with reported values typically ranging between 0.4 and 4.3 [2,[14][15][16][17] when the Poisson's ratio is measured in the elastic region.…”

High variability in strain estimation errors when using a commercial ultrasound speckle tracking algorithm on tendon tissue

“…It is independent of anatomical landmarks and can therefore be applied to free tendon where much of tendon pathology occurs. A number of ultrasound speckle tracking algorithms, both in-house developed (4)(5)(6) and commercially available (7) have been validated for assessment of displacement in tendons. It would be desirable to measure strain rather than displacement in tendons as strain is more likely to be a cause of injury.…”