Background: There are 2 representative shapes of full-thickness rotator cuff tears commonly observed: crescent-shaped and L-shaped tears. To date, the exact process of tear propagation in each shape remains unknown. Purpose: To determine the stress distribution pattern in rotator cuff tendons with a crescent-shaped tear and an L-shaped tear. Study Design: Descriptive laboratory study. Methods: The computed tomography (CT) data of a normal cadaveric shoulder from a 69-year-old man were used to develop a 3-dimensional finite element model of the rotator cuff tendons. Three different sizes (1.0-, 2.0-, and 3.0-cm widths) of crescentshaped and L-shaped tears were created. Tensile load was applied to each cuff tendon that simulated shoulder abduction and external rotation in the hanging-arm position. Elastic analysis was performed, and distribution of the von Mises equivalent stress was calculated. Then, the stress distribution pattern was compared between the 2 tear shapes and among the 3 different tear sizes. Results: In the crescent-shaped tear models, stress concentration was seen in both the anterior and the posterior edges of the torn tendon stump. In the L-shaped tear models, stress concentration was seen in the posterior torn tendon edge as well as the bottom of the longitudinal tear. The areas with high stress concentration increased with increasing tear size. At the posterior torn tendon edge, a common site of delamination, the articular half represented a higher equivalent stress than the bursal half in all models. The highest equivalent stress increased with an increase in tear size for both the abduction and the external rotation loads. Conclusion: A high stress concentration was seen in both the anterior and the posterior tendon edge in crescent-shaped tears and in both the posterior tendon edge and the bottom of the longitudinal tear in L-shaped tears. Stress concentration inside the tendon increased with increasing tear size. Clinical relevance: Crescent-shaped tears may propagate in the anterior and posterior directions, whereas L-shaped tears may propagate in the posterior and medial directions. Tear propagation may accelerate with an increase in tear size.
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