The Influence of Subscapularis Muscle Split Location on Subscapularis Function After the Latarjet Procedure

Background: Splitting the subscapularis in the Latarjet procedure is known to influence subscapularis muscle mechanics postoperatively; however, the influence of split level on postoperative muscle and joint function remains poorly understood. Purpose: To assess the effects of midlevel, lower-third, and upper-third subscapularis split levels in the Latarjet procedure on subscapularis lines of action and moment arms in the shoulder abduction, abduction and external rotation (ABER), and apprehension positions. Study Design: Controlled laboratory study. Methods: The Latarjet procedure was performed on 8 fresh-frozen human cadaveric upper extremities with a simulated 20% anteroinferior glenoid bone defect. A midwidth subscapularis muscle belly split was first performed on all specimens in which the conjoint tendon was routed. Lines of action and moment arms of 4 subregions of the subscapularis muscle (superior, mid-superior, mid-inferior, and inferior) were quantified radiographically with the conjoint tendon unloaded and loaded and the glenohumeral joint positioned in (1) 0° of abduction, (2) 90° of abduction, (3) 90° of abduction and full external rotation (ABER), and (4) the apprehension position, defined as ABER with 30° of horizontal extension. Testing was then repeated in random order after rerouting the conjoint tendon through both an upper- and then lower-third subscapularis split. Utmost care was taken to ensure that the subscapularis muscle integrity was not disrupted during the rerouting process. Results: Subscapularis splitting in Latarjet surgery deformed the muscle fibers below the split level, significantly increasing the inferior inclination of subscapularis muscle lines of action, but only for the midlevel and lower-third subscapularis split levels (P < .001). This increased inferior inclination was significantly greater in the ABER and apprehension positions compared with those at 0° and 90° of abduction (P < .05). In the ABER and apprehension positions, the adduction moment arms of the mid-superior subscapularis muscle subregion were also significantly larger for the midlevel split compared with the lower-third and upper-third split (P < .05), indicating greater depressor capacity. Conclusion: Latarjet surgery deforms subscapularis muscle fibers below the level of the split, changing subscapularis leverage and line of force. The midlevel subscapularis muscle split in the Latarjet procedure confers greater mechanical advantage in terms of shoulder depressor function and stabilizing potential than that associated with an upper-third or lower-third split, particularly in the ABER and apprehension positions. Clinical Relevance: Subscapularis muscle leverage and force potential are significantly influenced by split location in Latarjet surgery. A midlevel subscapularis split is likely to provide the greatest mechanical stability, particularly in positions of shoulder instability.