Arm Position with Increased Risk of Partial Subscapularis Tear Progression Owing to Subluxation of the Long Head of Biceps Tendon: Cadaveric Biomechanical Study.

Backgroud: This study aimed to evaluate the differences in long head of biceps (LHB) motion between the normal and subscapularis intrasubstance partial tear conditions and identify the arm positions that exhibit the most significant LHB motion differences using a cadaveric biomechanical study.

Methods: The LHB tendons of 6 fresh-frozen cadaveric shoulders (2 men and 4 women; mean age, 68.4 ± 2.3 years; range, 65-71 years) were marked with metal beads and mounted in a custom-made shoulder testing system. Data for arm positions at 20° or 60° of forward flexion or abduction, with neutral rotation and maximum internal and external rotation with a torque of 1.5 N·m, were collected. Considering the scapulohumeral rhythm, 20° or 60° forward flexion or abduction in a cadaveric shoulder corresponds to 30° or 90° shoulder elevation in vivo . Mediolateral (subluxation) and inferosuperior (excursion) LHB motions were measured using a 3-dimensional digitizer, and the differences between normal and subscapularis partial tear conditions were analyzed.

Results: While the LHB mediolateral motion difference was the highest during 60° forward flexion with neutral rotation (1.2 ± 0.4, p = 0.042), 20° forward flexion with neutral rotation (0.9 ± 0.3, p = 0.024) and 60° abduction with maximum external rotation (0.9 ± 0.3, p = 0.036) also demonstrated high mediolateral LHB motion difference between the normal and subscapularis partial tear conditions. In contrast, the LHB inferosuperior motion difference was the highest during 20° forward flexion with neutral rotation (0.7 ± 0.3, p = 0.045) between the normal and subscapularis partial tear conditions.

Conclusions: Upon comparing normal and subscapularis partial tear conditions in this cadaveric study, high pathological movements of the LHB were observed during arm forward flexion with neutral rotation and abduction with external rotation. Repetitive activity in these arm positions could aggravate the condition in a partial subscapularis tear.