Ultrasonic evaluations of Achilles tendon mechanical properties poststroke

Abstract

Spasticity, contracture, and muscle weakness are commonly observed poststroke in muscles crossing the ankle. However, it is not clear how biomechanical properties of the Achilles tendon change poststroke, which may affect functions of the impaired muscles directly. Biomechanical properties of the Achilles tendon, including the length and cross-sectional area, in the impaired and unimpaired sides of 10 hemiparetic stroke survivors were evaluated using ultrasonography. Elongation of the Achilles tendon during controlled isometric ramp-and-hold and ramping up then down contractions was determined using a block-matching method. Biomechanical changes in stiffness, Young's modulus, and hysteresis of the Achilles tendon poststroke were investigated by comparing the impaired and unimpaired sides of the 10 patients. The impaired side showed increased tendon length (6%; P = 0.04), decreased stiffness (43%; P < 0.001), decreased Young's modulus (38%; P = 0.005), and increased mechanical hysteresis (1.9 times higher; P < 0.001) compared with the unimpaired side, suggesting Achilles tendon adaptations to muscle spasticity, contracture, and/or disuse poststroke. In vivo quantitative characterizations of the tendon biomechanical properties may help us better understand changes of the calf muscle-tendon unit as a whole and facilitate development of more effective treatments.

Figures

Fig. 1.

A custom knee-ankle joint-driving device used for the study. This knee-ankle evaluation device consisted of 2 motors and a linkage in between. The torque sensors were mounted on the motor shaft at both joints to measure the ankle and knee joint torques. With the knee flexion axis aligned with the knee motor, the ankle motor can be adjusted along the leg linkage to align it with the ankle flexion axis.

Fig. 2.

Achilles tendon length and moment arm measurements. Top: a picture of a human foot and ankle. Line A was drawn along the Achilles tendon from the calcaneus to soleus-Achilles muscle-tendon junction (MTJ). Line B was drawn from the inferior tip of malleolus perpendicular to the tendon line of action. The intersection of line A and line B is also shown. Bottom: a LOGIQView image of Achilles tendon in sagittal plane. The image was acquired by scanning the probe along line A. The tendon length was defined as the distance from the MTJ to the calcaneus notch along the tendon line of action. The tendon moment arm was calculated by subtracting the perpendicular distance between the intersection and the tendon LOA (mt; in bottom) from the distance between the center of rotation and the intersection mg in sagittal plane.

Fig. 3.

An ultrasound image of Achilles tendon on transverse plane. The region inside the brighter boundary shows the cross section of Achilles tendon.

Fig. 4.

Top: an ultrasound image of soleus-Achilles MTJ at rest. The zoomed block shows the region to be tracked during ramp-and-hold task. Bottom: an ultrasound image during the holding phase from the same task. The arrow is pointing from the original position to the tracked position after the tendon elongation took place. The zoomed blocks shows similar pattern as the block in the top.

Fig. 5.

The Achilles tendon force and MTJ displacement estimated during a ramping up-then down task. The force and displacement data displayed were decimated to 5 samples/s. From the trend of the curves, we can see how tendon force and MTJ displacement correlated.

Fig. 6.

Force-displacement curves collected from both sides of a subject in ramping up then down task. From the figure, we can conclude that 1) the tendon in the impaired side was more compliant than the unimpaired side and 2) the impaired side had higher mechanical hysteresis compared with the unimpaired side.

Fig. 7.

Mean and standard deviation of Achilles tendon length (A), Achilles tendon cross-sectional area (CSA; B), Achilles tendon stiffness (C), Achilles tendon Young's modulus (D), and Achilles tendon mechanical hysteresis in the impaired (G1) and unimpaired (G2) sides, respectively (E).

Fig. 8.

Illustration of the changes of the calf muscles and Achilles tendon (simplified and modeled as 2 springs in series, with the spring width representing its stiffness value) poststroke. As the calf muscles become stiffer and shorter poststroke, the middle point between the two springs (the soleus-Achilles MTJ) may shift toward the calf muscles with the Achilles tendon elongated.