Electromyography Evaluation of Subscapularis Function After Total Shoulder Arthroplasty

Overview

The primary objective is to document function of the structurally intact subscapularis after total shoulder arthroplasty by using electromyography. Structural integrity of the subscapularis will be confirmed using ultrasound. The secondary objective is to determine the diagnostic accuracy of the belly compression and lift off tests in evaluating the integrity and function of the subscapularis when compared to the ultrasound and electromyography (EMG). This will be accomplished by enrolling and evaluating patients with a primary diagnosis of osteoarthritis who have had a total shoulder replacement preformed by the PI from September 2003 through December 2007.

The functional status of the subscapularis following total shoulder arthroplasty has not been well documented. There is evidence of loss of subscapularis function shown through physical exam in up to 2/3 of patients following total shoulder arthroplasty. \[1\] Multiple techniques for subscapularis repair have been demonstrated with varying degrees of post operative function, ranging from 11-66% abnormal function on physical exam. \[1-3\] The belly compression test and the lift off test, which are the main physical exam tests used for evaluation following surgery, are of questionable reliability as indicators of the functionality of the subscapularis. \[4,5\] Through comparing ultrasound evidence of an intact subscapularis to physical exam findings it has been recognized that the belly compression test has low sensitivity and specificity for subscapularis integrity. \[5\] Internal rotation is often limited in many postoperative total shoulder patients, which could lead to false positive results. This questions the validity of the outcomes of the studies in which the standard of testing function was through exam. Ultrasound is a reliable exam for illustration of structural damage to the subscapularis tendon but additional testing is needed to evaluate the function of the structurally intact subscapularis. \[5,6\] Irreparable changes in the muscle function or nerve denervation could also result in false positive belly compression and lift off tests. The ability to accurately determine the functionality of the subscapularis following total shoulder replacement is imperative to clinic management. Although studies have shown functional abnormalities through physical exam, none have documented the functional status of the subscapularis using objective means such as EMG.