The internal structure of the infraspinatus muscle: a magnetic resonance study

Abstract

Purpose: This study aimed to describe the internal structure of the infraspinatus muscle. A secondary aim was to explore differences in internal structure between genders, sides, and correlations to demographic data.

Methods: In total, 106 shoulder MRI examinations of patients between 18 and 30 years of age seeking care in 2012-2020 at The Sahlgrenska University Hospital in Gothenburg, Sweden were re-reviewed.

Results: The number of intramuscular tendons centrally in the infraspinatus muscle varied between 3 and 8 (median = 5). Laterally, the number of intramuscular tendons varied between 1 and 5 (median = 2). There was no difference in the median between the genders or sides. No correlations between the number of intramuscular tendons and demographic data were found. The muscle volume varied between 63 and 249 ml with a median of 188 ml for males and 122 ml for females. There was no significant difference in volume between the sides. The muscle volume correlated with body weight (Pearson's correlation coefficient, r = 0.72, p < 0.001) and height (r = 0.61, p < 0.001).

Conclusion: The anatomical variations of the infraspinatus muscle are widespread. In the medial part of the muscle belly, the number of intramuscular tendons varied between 3 and 8, while the number of intramuscular tendons laterally varied between 1 and 5. Results of our study may help to understand the internal structure of the infraspinatus muscle and its function in shoulder stabilization.

Keywords: Anatomy; Infraspinatus muscle; Magnetic resonance imaging; Rotator cuff.

Fig. 1

The intramuscular tendons of the infraspinatus muscle were counted in a vertical line (dashed) under the deltoid tubercle of the scapular spine (arrow) in the coronal view. This line was labeled "the central measuring point". T2-weighted image, spectral adiabatic inversion recovery. Coronal section

Fig. 2

The intramuscular tendons of the infraspinatus muscle were counted in a vertical line (dashed above the arrow) midway between the glenohumeral joint space (left-most line) and the insertion of the muscle (right-most line) in the coronal view. The line above the arrow was labeled "the lateral measuring point". T1-weighted image, turbo spin echo. Coronal section

Fig. 3

The anteroposterior diameter (A) of the infraspinatus muscle measured in the axial view. Proton density-weighted image, spectral adiabatic inversion recovery. Axial section

Fig. 4

The craniocaudal diameter (B) of the infraspinatus muscle measured in the sagittal view. T2-weighted image, turbo spin echo. Sagittal section

Fig. 5

The lateromedial diameter (C) of the infraspinatus measured in the axial view. Proton density-weighted image, spectral adiabatic inversion recovery. Axial section

Fig. 6

The angle between the longitudinal axis of the scapular spine and the lower portion of the most inferior tendon of the infraspinatus muscle seen in the coronal view. T2-weighted image, spectral adiabatic inversion recovery. Coronal section

Fig. 7

The position of the intramuscular tendons of the infraspinatus muscle seen in the sagittal view. In this case, they were considered entirely intramuscular. T2-weighted image, spectral adiabatic inversion recovery. Sagittal section

Fig. 8

Illustrations of the ten most frequently occurring anatomical variants of intramuscular tendons in this study. For each variant, the number of intramuscular tendons are denoted as T: number of intramuscular tendons at the central measuring point, number of intramuscular tendons at the lateral measuring point. O, occurrence; n, number of examinations. The illustrations were drawn by PS

Fig. 9

Example of a muscle with many intramuscular tendons at the central measuring point. T2-weighted image, spectral adiabatic inversion recovery. Coronal section

Fig. 10

Example of a muscle with few intramuscular tendons at the central measuring point. T2-weighted image, spectral adiabatic inversion recovery. Coronal section

Fig. 11

Boxplot illustrating the muscle volume distribution for males and females in the reviewed examinations. The difference in median volume between the genders was 68 ml (95% CI 44.1–91.0), p < 0.001

Fig. 12

Scatter plot illustrating the correlation between body weight (kg) and muscle volume (ml). Pearson's correlation coefficient, r = 0.72

Fig. 13

Example of an examination with a narrow angle between the most inferior tendon of the infraspinatus muscle and the scapular spine. T2-weighted image, spectral adiabatic inversion recovery. Coronal section

Fig. 14

Example of an examination with a wide angle between the most inferior tendon of the infraspinatus muscle and the scapular spine. T2-weighted image, spectral adiabatic inversion recovery. Coronal section

Fig. 15

Example of intramuscular tendons that were judged as partly superficial, T1-weighted image. Sagittal section

Fig. 16

Example of intramuscular tendons that were judged as entirely intramuscular. T2-weighted image, spectral adiabatic inversion recovery. Sagittal section