ANALYSIS OF MECHANICAL STRESSES IN THE FAILURE OF THE SUPRASPINATUS TENDON

Abstract

Supraspinatus tendon (rotator cuff) tearing and failure is a fairly common injury, which leads to reduced mobility and shoulder pain. The purpose of this study is to determine the critical tear dimension that causes the induced mechanical stress to exceed the tendon's failure strength. This dimension is defined as the critical failure width, measured at the tendon's insertion. Therefore, surgical treatment of tendon's failure must be performed so the postoperative failure size remains below the critical failure width. Methodology: Longitudinal and transversal mechanical stresses in the supraspinatus tendon and anterior part of the infraspinatus tendon were found by means of finite element analysis. The geometrical (humeral head radius, width and thickness of tendon), and mechnical properties (elasticity modulus and failure stress) were found by means of experimentation in 10 postmortem human specimens. Shoulder functionality was defined as the ratio between the induced longitudinal stress with failure to the nominal working stress of the healthy tendon. Functionality was considered normal when this ratio was above sixty percent. Critical failure width was calculated as the failure that reduced functionality below sixty percent. Results: Elasticity modulus in the direction was found to be 18.50 ± 15 MPa (longitudinal) and 3.70 ± 2.90 MPa (transversal). Failure stress was 4.50 ± 1.30 Mpa (longitudinal) and 3.05 ± 10 Mpa (transversal). Failure width at which the induced longitudinal stress was higher than the tendon failure stress was calculated. Average width of normal supraspinatus tendon was 23.2 ± 2.5 mm. For such a tendon, the calculated critical failure width was 8.3 mm. Conclusions: A finite element model for the mechanical stresses in the rotator cuff tendon was implemented. Geometrical and mechanical properties for the model were expermentally obtained from postmortem human specimens. The model was used to calculate the critical failure width that reduced estimated shoulder functionality below sixty percent. The results suggest that, for a normal 23.2 mm wide tendon, failure width must be surgically reduced below 8.3 mm, so the induced mechanical stresses are safely below the mechanical strength of the tendon.