Pectoralis major tendon transfers above or underneath the conjoint tendon in subscapularis-deficient shoulders. An in vitro biomechanical analysis.

BACKGROUND: Different operative techniques for transfer of the pectoralis major tendon have been proposed for the treatment of irreparable ruptures of the subscapularis tendon. The objective of this study was to compare the effects of two techniques of transferring the pectoralis major tendon (above or underneath the conjoint tendon) on glenohumeral kinematics during active abduction in a biomechanical model of a subscapularis-deficient shoulder.
METHODS: Six shoulder specimens were tested with a custom dynamic shoulder testing apparatus. After the kinematics of the intact shoulder were recorded, a complete tear of the subscapularis tendon was simulated surgically. A transfer of the clavicular portion of the pectoralis major muscle to the lesser tuberosity was then performed with the transferred tendon placed either above (tendon-transfer 1) or underneath (tendon-transfer 2) the conjoint tendon. For each condition, the maximum abduction angle as well as the external rotation angle and the superoinferior and anteroposterior humeral translations at the maximum abduction angle were recorded.
RESULTS: With the rotator cuff intact, the mean maximum glenohumeral abduction angle (and standard error of the mean) was 86.3 degrees +/- 2.1 degrees and the mean amount of external rotation at the maximum abduction angle was 5.5 degrees +/- 7.6 degrees . A complete tear of the subscapularis tendon decreased the mean maximum abduction angle to 40.8 degrees +/- 2.4 degrees (p < 0.001) and increased the mean external rotation to 91.8 degrees +/- 4.8 degrees (p < 0.001). The mean humeral translations in the anterior and superior directions (+3.4 +/- 0.5 and +6.3 +/- 0.3 mm, respectively) at the maximum abduction angle were also increased (p < 0.01 and p < 0.001) when compared with those in the intact shoulder. Significant differences were found in the mean maximum abduction angle as well as the mean external rotation angle and humeral translations (anterior and superior) at maximum abduction between the tendon-transfer-1 condition (63.2 degrees +/- 13.5 degrees , 82.4 degrees +/- 6.6 degrees , 4.0 +/- 1.8 mm, and 3.3 +/- 1.9 mm, respectively) and tendon-transfer-2 condition (89.5 degrees +/- 12.3 degrees , 45.7 degrees +/- 22.5 degrees , -0.6 +/- 2.0 mm, and 0.5 +/- 2.3 mm, respectively). The tendon-transfer-2 condition restored glenohumeral kinematics that were closer to those in the intact shoulder than were those resulting from the tendon-transfer-1 condition.
CONCLUSIONS: Transfer of the pectoralis major tendon in subscapularis-deficient shoulders partially restored the glenohumeral kinematics of the intact shoulder. One possible explanation for the superior effect of the tendon-transfer-2 condition is that, with a pectoralis major tendon transfer underneath the conjoint tendon, the line of action of the transferred tendon is closer to that of the subscapularis muscle.