Summary

This study is the first to explore the relationship between moment arm and excursion in rotator cuff muscles. It introduces a muscle efficiency index that quantifies efficient muscle function in both healthy and torn states. This novel index may help improve clinical understanding of shoulder mechanics, potentially guiding better treatment strategies for rotator cuff injuries.

Abstract

Introduction

The rotator cuff (RC) consists of four muscles: subscapularis, supraspinatus, infraspinatus, and teres minor. These muscles stabilize the glenohumeral joint and enable shoulder movement. This study examines how the RC moment arm and muscle excursion change during shoulder abduction and the effects of a supraspinatus tear on these properties. The goal is to develop a muscle efficiency index to evaluate recovery post-RC repair.

Methods

Six fresh-frozen cadaveric shoulders (female, 60–79 years) were dissected to preserve key tendons and ligaments. The scapulae were fixed, and the glenohumeral joint was stabilized using Nylon strings and weights per biomechanical loading protocol. Shoulder abduction from 0° to 90° was tracked under three conditions: intact supraspinatus, 50% partial tear, and complete tear (CT). Muscle excursions and moment arms were measured using a 3D coordinate system. Data were analyzed with repeated measures ANOVA and Pearson’s correlation.

Results

The supraspinatus is crucial for shoulder abduction, particularly in the initial 0-20 degrees, and provides posterior joint stability. Tear severity significantly affects muscle function. As abduction continues, muscle excursion decreases, with complete tears showing higher excursion before and after 20 degrees, indicating increased stretching capability with tear severity. The supraspinatus moment arm increases significantly after 20 degrees of abduction across all conditions, with the CT group showing the largest moment arm. This suggests a greater lever arm for torque generation, though compromised muscle function may limit effective torque. A negative correlation between muscle excursion and moment arm, especially at 20 degrees, indicates altered muscle efficiency with abduction. The muscle efficiency index is higher in the CT group, suggesting that the muscle must generate more torque for the same level of abduction.
The teres minor, aiding in external rotation and joint stability, also shows increased excursion and moment arm with abduction. The CT condition results in a significantly higher muscle efficiency index (0.806) compared to intact (0.457) and partial tear conditions, reflecting compensatory efforts. Similarly, the infraspinatus shows decreased excursion with abduction, particularly in the CT condition, and a reduced moment arm. The correlation between excursion and moment arm varies, with intact muscles showing a positive relationship, while partial and complete tears show a negative correlation, indicating that the infraspinatus compensates initially but struggles as abduction progresses.

Discussion

This study explores how muscle excursion and moment arms of the supraspinatus, infraspinatus, and teres minor interact to stabilize the glenohumeral joint and facilitate shoulder abduction. Supraspinatus tears impact the efficiency and function of the infraspinatus and teres minor, compromising shoulder stability. The teres minor compensates with increased efficiency under CT conditions, while the infraspinatus shows decreased efficiency with greater tear severity. Limitations include the experimental setup, simplified vector assumptions, and focus on acute tears.
Significance/Clinical Relevance: This study is the first to analyze the relationship between moment arm and excursion in rotator cuff muscles and introduces a muscle efficiency index. This index may enhance clinical understanding of shoulder mechanics and aid in developing better treatment strategies for rotator cuff injuries.