Biomechanical effects of teres minor trophicity on external rotation following lower trapezius transfer for posterosuperior massive rotator cuff tears.

Background: Lower trapezius tendon (LT) transfer has shown promise in managing irreparable posterosuperior massive rotator cuff tears (PSMRCTs) with weakness of active external rotation (ER). However, the influence of teres minor (TM) trophicity on restoring ER strength has not been thoroughly investigated. This cadaveric biomechanical study evaluated the effect of atrophic, normotrophic, and hypertrophic TM on humeral head stability and ER recovery following LT transfer in PSMRCTs.

Methods: Eight fresh-frozen cadaveric shoulders were tested under 5 conditions: (1) Intact rotator cuff, (2) PSMRCT condition, (3) LT transfer with atrophic TM (0 N), (4) LT transfer with normotrophic TM (5 N), and (5) LT transfer with hypertrophic TM (10 N). Superior translation and subacromial contact pressure were assessed at 0°, 20°, and 40° of glenohumeral abduction in each TM trophicity state. Resting internal rotation (IR) and rotational torque measurements were evaluated by digital torque wrench at 30° and 60° ER for each abduction position. A linear mixed-effects model with Tukey post hoc analysis was used for statistical comparisons (P < .05).

Results: Superior translation and subacromial contact pressure significantly improved after LT transfer compared to PSMRCT condition (P < .007 and P < .028, respectively), but no differences were observed among different TM trophic states. Regarding resting IR, atrophic TM showed no significant improvement at any abduction angle (P > .05), while normotrophic TM demonstrated significant reductions at 0° (P = .049) and 40° (P = .006) abduction. Notably, hypertrophic TM exhibited marked reductions across all abduction positions (all P's < .001). In rotational torque, atrophic TM yielded no significant improvement (P > .169), whereas normotrophic TM significantly increased torque only at 60° ER positions (P < .037). Considerably, hypertrophic TM significantly enhanced rotational torque across all abduction positions at both 30° (P < .002) and 60° ER (P < .001), with significant differences compared to atrophic conditions (P < .040).

Conclusion: LT transfer reduced superior humeral translation and subacromial contact pressure across all TM trophicity conditions. Hypertrophic TM yielded the greatest ER torque and least resting IR, suggesting that TM status influences the biomechanical effect of the transfer. Although these findings clarify the association between TM condition and ER restoration, their clinical implications require further validation.