Subject-Specific Finite Element Modeling of the Human Shoulder Complex Part 2: Quantitative Evaluation of the Effect of Rotator Cuff Tear Propagation on Glenohumeral Joint Stability

The objective of this paper is to quantitatively evaluate the effects of rotator cuff tear propagation on glenohumeral joint stability in a previously constructed and validated finite element shoulder model. Rotator cuff tears with a sequence of increasing sizes were created from the anterior portion of the supraspinatus osseous insertion site and propagated posteriorly through the infraspinatus tendon until a complete tear extended through the entire teres minor tendon. Finite element simulations were performed in the same physiological loading and boundary conditions as in the original model. A novel integrative stability index was proposed and used for quantitative analysis of the simulated results. By defining the healthy condition as the baseline (100%), the stability index decreased slightly with small tear sizes but declined suddenly after half tear of the infraspinatus accompanied by a complete tear of the supraspinatus tendon until the full tear condition, when the index reached 0.41%. These results confirm the clinical and cadaveric findings that glenohumeral joint stability generally decreases as the size of the rotator cuff tear increases and that the critical tear size which leads to the loss of normal shoulder biomechanics was half tear of the infraspinatus accompanied by a complete tear of the supraspinatus tendon. It is concluded that the finite element shoulder model and the proposed novel stability ratio can accurately predict shoulder biomechanics in the investigated rotator cuff condition. Both the model and ratio may have the potential to be used to improve diagnostic and therapeutic strategies for clinicians.