Dynamic changes in anterior third supraspinatus tendon—relations of a muscle and muscle-tendon transition at rest and in abduction

Abstract

Background: It is said that the anterior third of the supraspinatus tendon controls the upward movement of the humeral head, and we have reported that the anterior third is changed by disease or symptoms. However, there are few studies demonstrating dynamic changes in the shoulder joint. Studies using ultrasonography allow real-time evaluation of soft tissue. Purpose: To clarify the change in thickness of the anterior third of the supraspinatus tendon and the thickness of supraspinatus muscle during rest and exercise, and the relationship between these parts in healthy individuals. Methods: The study assessed 30 shoulders of 15 healthy volunteers (15 males, mean age 23.4± 3.3 years, height 172.7± 5.5 cm, body weight 67.1± 11.1 kg). One person was excluded with a history of fracture of the shoulder girdle. The thickness of the anterior third of the supraspinatus tendon and the thickness of the supraspinatus muscle were measured in 30◦ shoulder joint abduction and at rest. The measurement position at rest was chair sitting, 0◦ shoulder flexion, 0◦ abduction, internal and external rotation midposition, 90◦ elbow joint flexion, and forearmmid-pronation. One assessor measured using a 7.5MHzB-mode linear ultrasonograph (VIAMO, Toshiba, Japan). The probe was placed on the anterior third of the supraspinatus tendon and the direction of the long axis of the supraspinatus muscle fibers of the scapular spine upper 30 cm for measurement site. To determine the reliability of the measurements, we calculated an intraclass correlation coefficient (ICC 1.1) from the four measurements at each site. For the statistical treatment, we used the Wilcoxon rank sum test to compare the difference between the thickness of the supraspinatus tendon at rest and in abduction, and the paired t-test for the difference in muscle thickness. The Pearson product-moment correlation coefficient was used to look for relationships between increases in the thickness of the supraspinatus muscle and the thickness of the anterior third of the supraspinatus tendon. Results: The mean anterior third of the supraspinatus tendon thickness was 4.9± 0.6mm (ICC= 0.91) at rest, 5.6± 0.6mm (ICC= 0.81) in abduction. The muscle thickness was 17.5± 2.8mm (ICC= 0.82) at rest, 19.0± 3.1mm (ICC= 0.87) in abduction. Comparing abduction and rest, there was a significantly increase in the thickness of both the tendon (p< 0.01) and the muscle (p< 0.01) during abduction. There was no correlation between the increases in tendon and muscle thickness (r= 0.01, p= 0.63). Conclusion(s): The thickness of the anterior third of the supraspinatus tendon increased with abduction means that a rotator cuff, joint capsule, and ligament act complexly. Then that the increase in thickness reduced the space between the humeral head and acromion is considered. Implications:Bymeasuring the thickness of anterior third supraspinatus tendon in abduction, it may become to understand that the supraspinatus tendonworks as a spacer. And the abduction of the shoulder may contribute toward the development or exacerbation of symptom due to compression.