Summary

The stiffness of the middle scalene muscle should be considered for conservative and surgical treatment of throwing athletes with thoracic outlet syndrome.

Abstract

Introduction

As a type of thoracic outlet syndrome (TOS), scalenus anticus syndrome involves the compression of the brachial nerves as they pass through the interval surrounded by the anterior and middle scalene muscles, and the first rib bone or cervical rib. Exercise-induced TOS is becoming more common in athletes, especially for those who perform repetitive overhead and hyperabduction maneuvers with upper limbs, such as baseball players. However, the effect of throwing on the stiffness of the scalene muscles is unknown. Thus, the purpose of this study was to quantitatively measure the stiffness of the scalene muscles using real-time shear wave elastography (SWE).

Methods

Thirty college baseball players (age range 19 to 21 years) were included for this study. Ultrasonic SWE with a 2-10 MHz linear array probe transducer was used to assess the muscle stiffness. The measurements were performed in two arm positions: 1) adducted and neutral rotation of the shoulder, and 2) 90 degrees of abduction and external rotation of the shoulder with the elbow flexed. The transducer was positioned just superior to the clavicular bone, parallel to its axis. The transducer was moved superiorly and tilted to visualize the superior surfaces of the anterior and middle scalene muscles parallel to the surface of the fifth cervical nerve simultaneously. In this position, SWE was performed to measure the stiffness of each scalene muscle. Each muscle was divided into superior and deep areas. For both areas of each muscle, stiffness was defined by taking the average of three 3 mm-diameter circles. A repeated-measures analysis of variance (ANOVA) was used to compare the stiffness of each area in the anterior and middle scalene muscles for the throwing and non-throwing side. Values of p<0.05 were considered statistically significant.

Results

For the throwing side, higher stiffness was found in the deep part of the middle scalene muscle compared to the superior and deep parts of the anterior scalene muscle with an adducted and neutrally rotated shoulder (p=0.0433). Moreover, the muscle stiffness was significantly higher in the superior and deep part of the middle scalene muscle than in the superior and deep parts of anterior scalene muscle in an abducted and externally rotated position of shoulder (p =0.00187). Meanwhile, no significant difference was found in the anterior and middle scalene muscles for the non-throwing side in both arm positions.

Discussion

Moderate to severe hypertrophy of the anterior scalenus muscles has been reported to be found in professional athletes with TOS. Meanwhile, although the stiffness of the scalene muscles can also be related to compression of the brachial plexus in the thoracic outlet, its quantitative measurements in the scalene muscles have not been reported. In this study on the throwing side, the muscle stiffness significantly increased in the middle scalene muscle, while no contribution was identified in the scalene muscles on the non-throwing side. As a result, repeated throwing motion can increase the stiffness of the middle scalene muscle.