Original Article
Stress Distribution in Superior Labral Complex and Rotator Cuff During In Vivo Shoulder Motion: A Finite Element Analysis

https://doi.org/10.1016/j.arthro.2015.04.082Get rights and content

Purpose

To quantitatively and qualitatively evaluate the impingement behavior between structures within the glenohumeral joint under simulated abduction–external rotation (ABER) motion using finite element analysis.

Methods

Computed tomography (CT) scanning of 1 shoulder in a volunteer was performed at 0° and 120° of shoulder abduction with external rotation (ABER position), followed by magnetic resonance imaging at 0° of abduction. The CT and magnetic resonance images were then imported into a customized software program to undergo 3-dimensional reconstruction followed by finite element modeling of the bone and soft tissue including the upper part of the rotator cuff and glenohumeral labral complex. Glenohumeral motion from 0° to the ABER position was simulated by CT images in 2 different humeral positions. On the basis of simulated humeral motion with respect to the scapula, we measured the stress value on the biceps-labral complex and upper part of the rotator cuff as a consequence of their structural deformation. In addition, we intended to design 2 types of labra—a normal stable labrum and an unstable posterosuperior labrum—to evaluate the geometric alteration and resulting stress change on the posterosuperior labrum against a compressive force from the humeral head and rotator cuff.

Results

In the ABER position, the posterosuperior labrum was deformed by the humeral head and interposed posterior part of the rotator cuff. When viewed from the rotator cuff, the posterior part of the rotator cuff came into contact with the posterosuperior labrum as external rotation increased. The measured peak contact stress values were 19.7 MPa and 23.5 MPa for the posterosuperior labrum and the upper rotator cuff, respectively. The stress values for both structures decreased to 5.8 MPa and 18.1 MPa, respectively, in the simulated SLAP model. The root of the long head of the biceps became compressed halfway through the range of motion by the humeral head, especially from the part involving horizontal extension and external rotation, resulting in a high stress of 22.4 MPa.

Conclusions

In this simulated SLAP model, the posterosuperior labrum was medially displaced by the humeral head and upper rotator cuff in the ABER position, causing a functional loss of the spacer effect.

Clinical Relevance

In SLAP lesions, the posterosuperior labrum loses its ability to function as a spacer in certain positions (especially ABER) and may decrease the important spacer effect between the humerus and the rotator cuff; this may lead to posterosuperior subluxation of the humeral head or rotator cuff abnormalities and tears during repetitive ABER tasks.

Section snippets

3D Bone Modeling and Motion Simulation Procedure

The right shoulder of a 29-year-old volunteer was scanned with a high-resolution CT scanner (Somatom Sensation; Siemens, Erlangen, Germany) with 1-mm slices taken at 2 different shoulder angles in the prone position: 0° (neutral rotation) and the ABER position. The prone position was selected because it was believed to allow appropriate scapular motion while positioning the shoulder at different abduction angles in limited space. The study was approved by our institutional review board

Results

In the ABER position, the posterosuperior labrum was deformed by the humeral head and interposed posterior part of the rotator cuff. When viewed from the rotator cuff, the posterior part of the rotator cuff came into contact with the posterosuperior labrum as external rotation increased, as depicted in Figure 4. In the designed SLAP model, the posterosuperior labrum moved medially rather than undergoing structural deformation during ABER movement.

The peak contact stress values at the ABER

Discussion

The principal findings of this study determined that the posterior part of the upper rotator cuff, LHB, and posterosuperior labrum were the structures at risk with the ABER position. These findings suggest that those structures might be damaged by repetitive impingement when the arm is in an abducted and maximally externally rotated position.14, 15, 25

High stresses especially on the undersurface of the posterior rotator cuff and posterosuperior labrum were caused by mutual compression between

Conclusions

In this simulated SLAP model, the posterosuperior labrum was medially displaced by the humeral head and upper rotator cuff in the ABER position, causing a functional loss of the spacer effect.

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The authors report the following potential conflict of interest or source of funding: Funded by the National Research Foundation of Korea, Ministry of Education (grant 2013R1A1A2011589 to Y-S.Y.).

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