Subacromial injection improves deltoid firing in subjects with large rotator cuff tears

Abstract

Previous studies demonstrate that scapulohumeral mechanics improve after subacromial injection. However, it is unclear how injection affects muscle firing. Forty-one subjects with two-tendon rotator cuff tears and 23 volunteer subjects with normal rotator cuffs documented by ultrasonography were examined. Electromyographic activity from 12 muscles was collected during ten functional tasks. Nine symptomatic subjects with rotator cuff tears underwent subacromial injection of anesthetic and underwent repeat electromyographic examination. Subjects with rotator cuff tears demonstrate global electromyographic differences when compared to normal controls. Asymptomatic subjects with rotator cuff tears had significantly increased anterior deltoid firing when compared to symptomatic counterparts during forward shoulder elevation. After subacromial injection, symptomatic subjects demonstrate increased anterior deltoid firing. Previous in vitro and in vivo studies have suggested that pain leads to deltoid inhibition and that subacromial injection leads to improved deltoid firing and, subsequently, improved shoulder function. This study provides direct evidence that subacromial injection improves deltoid firing in symptomatic subjects with rotator cuff tears. These findings reinforce the concept that deltoid inhibition resulting from pain is an important component of the motor disability associated with rotator cuff tears.

Fig. 1

Illustration of how subjects were segregated among normal, symptomatic, and asymptomatic groups based on imaging, range of motion, and visual analog scale

Fig. 2

Electromyography of selected muscles during elevation of an 8-lb weight overhead in normal subjects, asymptomatic subjects, symptomatic subjects pre-injection and symptomatic subjects post-injection. Globally, symptomatic patients/post-injection appear to demonstrate global improvement of muscle firing patterns

Fig. 3

In symptomatic subjects, the %MVC of the anterior deltoid improved in both the 8- and 1-lb overhead lift after subacromial injection