To explore the feasibility of using high-frequency ultrasound to observe the internal structure and composition of the sternocleidomastoid muscle, in order to provide a basis for the diagnosis of sternocleidomastoid myopathy and the selection of sternocleidomastoid muscle flaps.

High-frequency ultrasound was used to obtain long-axis and short-axis sonograms of 182 sternocleidomastoid muscles in 91 healthy volunteers from May to June 2022. The shape, internal echo, and anatomical variation were observed. The total thickness of the sternocleidomastoid muscle and the thickness of muscle bundles at the midpoint of the line connecting the mastoid and the sternoclavicular joint, the width at the upper edge of the clavicle and the insertion point of the upper edge of the sternum, the width and thickness of the internal aponeurosis 1 cm above the upper edge of the clavicle and the upper edge of the insertion of the sternum, the thickness of the tendon at the insertion point of the clavicle head and the upper border of the sternum, and the length of the aponeurosis within the muscle bundle of the clavicle head and the sternum head were measured. The t-test was used to compare the mean values and differences between the left side and right side and between males and females. The thickness difference of each part of the sternocleidomastoid muscle was compared by analysis of variance, and the correlation between the total thickness of the sternocleidomastoid muscle and the thickness of each part and BMI was analyzed by Pearson correlation.

High-frequency ultrasound can clearly display the whole course of the sternocleidomastoid muscle and its four-part structure, as well as the variations. The cephalic end of the sternocleidomastoid muscle inserts on the upper nuchal line and the mastoid process, and the foot end inserts on the sternum and the anterior superior border of the clavicle. During the dynamic scanning, it can be seen that the sternocleidomastoid muscle is composed of four parts, namely, the mastoid-clavicular bundle, the mastoid-sternal bundle, the occipital-clavicular bundle, and the occipital-sternal bundle. At the midpoint between the mastoid process and the sternocleidomastoid joint, the average thickness of the whole sternocleidomastoid muscle and its four parts was (0.90±0.16) cm, (0.54±0.13) cm, (0.68±0.14) cm, (0.40±0.10) cm, and (0.28±0.09) cm, respectively. There was no statistically significant difference in the muscle thickness between the two sides (P＞0.05), but there was a statistically significant difference between males and females (P＜0.05). There were statistically significant differences in the measured thickness of each part (P＜0.05). Pairwise comparisons of the measured values of these thicknesses showed that the differences were statistically significant (P＜0.01). Seven of the 91 cases (7.69%) showed the variation of the clavicle end, all of which occurred unilaterally, with two clavicle heads. The total thickness of the sternocleidomastoid muscle at the midpoint of the line between the mastoid and the sternoclavicular joint, the thickness of each muscle bundle, the width at the upper edge of the clavicle and the sternum, and the width of the internal aponeurosis 1 cm above the upper edge of the clavicle and the sternum showed no significant difference (P＞0.05). There was no significant difference in the bilateral measurement of the length of the aponeurosis within the clavicular head and the sternal head (P＞0.05). Among the above measurements, except for the thickness of the internal aponeurosis 1 cm above the upper border of the clavicle and sternum insertion, and the length of the aponeurosis wthin the muscle bundle of the sternum head, the difference between males and females was statistically significant (P＜0.05). The Pearson correlation coefficients between BMI and the total thickness of sternocleidomastoid muscle and the thickness of each part were 0.286-0.371 (P＜0.01).

High-frequency ultrasound is an effective imaging tool for evaluating the fine internal structure of the sternocleidomastoid muscle and identifying its variation, which provides a reference for the diagnosis of sternocleidomastoid myopathy and the precise selection of muscle flaps.