Lower trapezius and latissimus dorsi transfer relieve teres minor activity into the physiological range in Collin D irreparable posterosuperior massive rotator cuff tears: a biomechanical analysis

Background: Tendon transfers are established techniques to regain external rotation mobility in patients with an irreparable, posterosuperior massive rotator cuff tear (MRCT). Posterosuperior MRCT with intact teres minor (type D MRCT) can lead to excessive teres minor loading to maintain external rotation. We hypothesize that tendon transfers are effective in relieving teres minor loading in type D MRCTs. Our aim was to biomechanically assess muscle synergism with latissimus dorsi (LD transfer) and lower trapezius (LT transfer) tendon transfer during external rotation at different abduction heights. Methods: Using musculoskeletal modeling, we analyzed and compared the moment arm, muscle torque, and muscle activity between a healthy and type D MRCT pathologic model with and without the LD- or LT transfer at infraspinatus and teres minor insertion sites. Output measures were analyzed during external rotation at different abduction angles and 10-50 N resistance against external rotation. We assessed its impact on teres minor loading in a type D MRCT. Morphologic variations were parameterized using the critical shoulder angle and the acromiohumeral distance to address variations among patients. Results: Both transfer types reduced teres minor torque and activity significantly, reaching physiological state at 40 N external resistance (P < .001), with insertion to infraspinatus site being more effective than teres minor site (P < .001). External rotation moment arms of LD transfer were larger than LT transfer at 90° abduction (25.1 ± 0.8 mm vs. 21.2 ± 0.6 mm, P < .001) and vice versa at 0° abduction (17.4 ± 0.5 mm vs. 24.0 ± 0.2 mm, P < .001). Although the healthy infraspinatus was the main external rotator in all abduction angles (50%-70% torque), a type D MRCT resulted in a 70%-90% increase of teres minor torque and an up to 7-fold increase in its activity leading to excessive loadings beyond 10 N resistance against external rotation. Varying the critical shoulder angle and the acromiohumeral distance led to minor variations in muscle moment arm and muscle activity. Conclusion: We identified biomechanical efficacy of both tendon transfers in type D MRCT regarding teres minor load relief and superior performance of the transfers at the infraspinatus insertion site.