The Evaluation of Shoulder Abduction with and without Rotation on the Supraspinatus Tendon and Labrum: A Finite Element Study

OBJECTIVES: Subacromial Impingement Syndrome (SIS) is thought to be the most common cause of isolated shoulder pain. SIS is the abnormal contact of the soft tissues of the subacromial space with the inferior aspect of the anterolateral acromial arch. The actual in vivo pathophysiology of subacromial impingement is still not well-understood. Current biomechanical studies have evaluated different shoulder motions causing impingement, but they are limited to clinical subacromial impingement tests and evaluated strains at the coracoacromial ligament. In the present study, we provided an in-depth analysis of various shoulder motions on the soft tissues of the shoulder, particularly the supraspinatus tendon and labrum, through finite element analysis (FEA) utilizing a novel three-dimensional shoulder model. We hypothesize that the results will confirm previous mechanical studies on impingement and labral pathology, but bring a greater in vivo understanding of the stresses on these soft tissues. METHODS: The geometry of the shoulder model was created from CT and MRI scans, and consisted of bones, ligaments, cartilage, labrum, and supraspinatus muscle and tendon (Figure 1a). Isotropic elastic material properties were used for bones and cartilage, and other soft tissues were modeled as Ogden hyperelastic material properties [1]. We then simulated 90⁰ of abduction followed by 30⁰ of internal rotation, 90⁰ abduction with 30⁰ of external rotation, 90⁰ abduction, and 90⁰ forward flexion. These movements have been theorized to result in the most contact between the supraspinatus and the acromial arch. Hwang et al previously conducted a similar study to provide initial validation [1]. The maximum von Mises stresses at the supraspinatus tendon and labrum were calculated and compared in all tested motions. RESULTS: The stresses at the supraspinatus muscle were similar for 90⁰ abduction with 30⁰ of internal rotation, 90⁰ abduction with 30⁰ of external rotation, and 90⁰ abduction, (28 to 33 MPa) but were less for 90⁰ forward flexion (22 MPa) (Table 1). Stress values at the labrum were highest for 90⁰ abduction with 30⁰ of external rotation (22 MPa), but stresses for 90⁰ abduction, 90⁰ abduction with 30⁰ of internal rotation, and 90⁰ forward flexion were 22%, 50% and 90% less compared to abduction with external rotation (17 and 2 MPa), respectively. The stresses at the labrum were concentrated at superior and posterior labrum (Table 1, Figure 1b and 1c). Current studies suggest different causal mechanisms of SIS. It is not clear whether it is primarily due to mechanical or intrinsic degeneration Our results show higher stresses on the supraspinatus tendon at 90⁰ abduction with and without + 30⁰ of internal and external rotation. Our analyses pertain to quasi static motions but the biomechanical effects may increase under repetitive motion. Our results also show that increased superior labrum stresses take place during 90⁰ abduction, and 90⁰ abduction with 30⁰ of external rotation, which could have clinical relevance for understanding SLAP (superior labrum anterior posterior) pathology. CONCLUSION: This study provides insight in understanding the various shoulder motions that may lead to SIS and SLAP pathology. To date there is no such study to our knowledge that utilizes FEA to analyze such in vivo properties of the rotator cuff and labrum. This novel model will be applied to future studies assessing the effect of various acromion morphologies on SIS.