Spatial variations in Achilles tendon shear wave speed

Abstract

Supersonic shear imaging (SSI) is an ultrasound imaging modality that can provide insight into tissue mechanics by measuring shear wave propagation speed, a property that depends on tissue elasticity. SSI has previously been used to characterize the increase in Achilles tendon shear wave speed that occurs with loading, an effect attributable to the strain-stiffening behavior of the tissue. However, little is known about how shear wave speed varies spatially, which is important, given the anatomical variation that occurs between the calcaneus insertion and the gastrocnemius musculotendon junction. The purpose of this study was to investigate spatial variations in shear wave speed along medial and lateral paths of the Achilles tendon for three different ankle postures: resting ankle angle (R, i.e. neutral), plantarflexed (P; R - 15°), and dorsiflexed (D; R+15°). We observed significant spatial and posture variations in tendon shear wave speed in ten healthy young adults. Shear wave speeds in the Achilles free tendon averaged 12 ± 1.2m/s in a resting position, but decreased to 7.2 ± 1.8m/s with passive plantarflexion. Distal tendon shear wave speeds often reached the maximum tracking limit (16.3m/s) of the system when the ankle was in the passively dorsiflexed posture (+15° from R). At a fixed posture, shear wave speeds decreased significantly from the free tendon to the gastrocnemius musculotendon junction, with slightly higher speeds measured on the medial side than on the lateral side. Shear wave speeds were only weakly correlated with the thickness and depth of the tendon, suggesting that the distal-to-proximal variations may reflect greater compliance in the aponeurosis relative to the free tendon. The results highlight the importance of considering both limb posture and transducer positioning when using SSI for biomechanical and clinical assessments of the Achilles tendon.

Keywords: Noninvasive mechanics; Shear wave elastography; Shear wave imaging; Tendon mechanics; Ultrasound elastography.

Figure 1

Experimental methods. Shear wave speed was measured along two paths: medial (M) and lateral (L), for three different postures: plantarflexed (P), resting ankle angle (R), and dorsiflexed (D). Data were collected from an average of six transducer positions from the calcaneus insertion to 70 mm beyond the gastrocnemius muscle-tendon junction. Portions of this figure adapted from Healthwise, Incorporated.

Figure 2

Representative example of images obtained along a medial path at the resting ankle angle (R). A custom standoff pad was used for the first two transducer positions. Shear wave speed was measured within five boxes per transducer position (only one shown per position for clarity). After collection, an ROI (text shown in cyan, ROI drawn in black for clarity) was defined within the tendon using a custom GUI. Anatomical landmarks, i.e. the calcaneus insertion (CI), soleus junction (SJ), and aponeurosis (AP), were then used to divide the Achilles tendon shear wave speed data into five regions.

Figure 3

Shear wave speed in the free tendon varied significantly (†, p < 0.05; ‡, p < 0.001) between plantarflexed (P) and resting ankle angle (R) postures. Data from the dorsiflexed (D) posture were omitted because of excessive data saturation (>10%). Note that because the first transducer position was the same for the medial and lateral paths, all trials from both paths were averaged.

Figure 4

Average shear wave speeds (with standard deviations) from plantarflexed (P), resting ankle angle (R), and dorsiflexed (D) postures. Both medial and lateral paths exhibit a distal to proximal reduction in shear wave speed, with the transition occurring more distally along the lateral path. Note that data for the dorsiflexed posture in regions 1 and 2 had greater than 10% pixel saturation (D SAT), which results in underestimation of the true shear wave speed.

Figure 5

Spatial variations in shear wave speed in the resting ankle angle posture (R) for medial and lateral paths for regions 3 to 5. Data from regions 1 and 2 were omitted because the transducer paths had not sufficiently diverged to consider the sides distinct (see Fig. 1). Shear wave speed decreased significantly moving from the calcaneus insertion to the gastrocnemius aponeurosis (*, p < 0.05; **, p < 0.001). Note that only adjacent regions were compared for clarity. Medial and lateral paths were also compared in each region. Shear wave speed was significantly higher along the medial path (s, p < 0.05; ss, p < 0.001) in the soleus and gastrocnemius aponeuroses.

Figure 6

Tendon thickness and depth varied along the length of the Achilles tendon for plantarflexed (P), resting ankle angle (R), and dorisflexed (D) postures. The thickness decreases proximally, with the thickest region of the tendon occurring in the free tendon between the calcaneus insertion (CI) and the soleus junction (SJ). Depth increases after the CI, with a distinct change in the rate of depth increase beyond the gastrocnemius muscle-tendon junction (AP).

Figure 7

Univariate regression analysis showing the relationship between shear wave speed and depth or thickness along each path (medial or lateral) for plantarflexed (P) and resting ankle angle (R) postures. The dorisflexed (D) posture was omitted because of excessive (>10%) saturation in regions 1 and 2. Least-squares fits were significant (p < 0.001) but exhibited relatively weak correlations.

Figure 8

Quality and percent saturation from the calcaneus insertion (CI) to the medial gastrocnemius aponeurosis (AP) for plantarflexed (P), resting ankle angle (R), and dorisflexed (D) postures. The quality of the shear wave speed estimates was relatively constant, with the exception of measurements taken near the CI. There was little measurement saturation in the plantarflexed posture. However, in the dorsiflexed posture, over 10% of pixels registered the maximum measurable shear wave speed of 16.3 m/s in regions 1 and 2. Note that shear wave speed estimate quality and percent saturation were similar along the lateral path.