Depressor Function of the Biceps to Superior Migration of the Humeral Head in Shoulders with Rotator Cuff Tears

Abstract

The purpose of this study was to determine the in vivo function of the biceps as an active depressor of the humeral head. Thirty-eight shoulders were examined: 19 shoulders with rotator cuff tears (RCT, avg age 59 yrs) and 19 shoulders with normal MRI findings of the rotator cuff (Control, avg age 55yrs). A special arm brace which was fixed to the upper arm was used to keep the biceps in active contraction (biceps loading) during arm elevation. The subjects were asked to grasp a rubber band attached to the distal end of the brace, pull it proximally, and keep the elbow flexed against the resistant force producted by the band. The x-ray were obtained with the arm elevated at 0,45,90 degrees in the scapular plane with and without pulling the band. The center of the humeral head was determined using NTH image and compared between the groups with and without biceps loading. Without biceps loading, the positions of the humeral head at 0,45 degrees were significantly higher in RCT than those in Control (p=0.0101, p=0.0020, respectively). After loading the biceps, the humeral head was depressed significantly at each degree in RCT (P=0.0082, p=0.0259, p=0.0059, respectively). As a result of biceps loading, there were no more significant differences in the positions of the humeral head between RCT and Control (p=0.3285). The positions of the humeral head in RCT with biceps loading were similar to the positions in Control without biceps loading. This is the first study to show the in vivo function of the biceps as an active depressor. From these results, we conclude that the biceps is an active depressor of the humeral head in cuff-defficient shoulders.