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To evaluate the reliability of ultrasonographic (US) elastography of the sternocleidomastoid (SCM) muscle and to define normal strain ratio and shear wave velocity (SWV) values in healthy infants. Methods:Two hundred healthy infants (mean age ± standard deviation, 1.64±1.78 month; 113 boys and 87 girls) were included in this prospective study. The thickness of bilateral SCM muscles was measured by B-mode ultrasonography, and the stiffness of SCM muscles was measured in both the longitudinal and transverse sections, symmetrical and extensional supine position, by using strain and shear wave elastography. The correlation between thickness and elastic values of the SCM muscle and the following possible influential factors were evaluated: sex, different sides of SCM muscle, different ultrasonic sections and different infant positions.Results: Both sex and the side of SCM muscle did not show significant correlation with the thickness or stiffness of the SCM muscle (P > 0.05). The stiffness of SCM muscle in the longitudinal section was significantly greater than in the transverse section (P < 0.05). The measurements of the SCM muscle in the stretching position were significantly greater than those in the symmetrical position (P < 0.05). Conclusion:The stiffness of SCM measured by US elastography is affected by relative positions of the infants. Therefore, the factor should be taken into account when measuring the stiffness of SCM by US elastography. US elastography can evaluate the stiffness of SCM, which is helpful for clinical diagnosis and treatment of children with torticollis.

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