Due to the inherent interaction with each other, the four rotator cuff muscles serve as the main dynamic stabilizers of the humeral head [1-3]. The antagonist subscapularis and infraspinatus/teres minor muscles build a force couple that centers the humeral head in an anteroposterior direction [4,5]. Any asymmetric tonus of the agonists results in altered glenohumeral shear force leading to humeral head displacement from its center of rotation in the glenoid cavity, thereby resulting in the other antagonist muscle weakness and atrophy [5,6]. Also, changes in muscle size and joint strength are known to occur throughout the adult lifespan. Muscle volume, especially of the shoulder, decreases with age. The linear relationship, however, between muscle volume and age remains uncertain [7-9].

The subscapularis muscle is the largest and most powerful muscle among the rotator cuff muscles; it is also named the “forgotten tendon” as it rarely tears. This muscle plays a key role as an anterior structure of the transverse force couple along with the infraspinatus and teres minor muscles, which are posterior to the shoulder joint [10,11]. Among the posterior structures of the transverse force couple, the importance of the infraspinatus and teres minor muscles are widely recognized; however, significance of the supraspinatus muscle as the posterior structure of the shoulder joint remains unclear.

Recently, magnetic resonance imaging (MRI) has been widely applied to evaluate the shoulder joint, and has been highly useful in understanding anatomical structures [12-15]. Several methods have been introduced to measure the volume of the rotator cuff muscles. Tingart et al. [16] separated each muscle of the rotator cuff in cadaver experiments and then measured the amount of overflowing water when each muscle was soaked in water. Juul-Kristensen et al. [17] evaluated the muscle volume in 20 patients using MRI. According to the data collected by Juul-Kristensen et al. [17] and Tingart et al. [16], no differences were observed between the volume of muscle obtained by soaking in water in the cadaver experiments and the volume of muscle obtained by three-dimensional (3D) reconstruction with MRI. Currently, rotator cuff atrophy is assessed using a multi-layered approach involving these methods.

As a posterior structure of the shoulder, the supraspinatus tear affects the posterior shoulder force couple, and could also affect the anterior shoulder muscle volume (subscapularis). This retrospective study was undertaken to evaluate if the supraspinatus tendon tear affects the subscapularis muscle volume. We compared the subscapularis muscle volume between groups, with and without supraspinatus tendon tear, by considering sex and age.

No significant differences were detected between the groups with respect group I and group T among participants in their 40s (p>0.05). However, a significant difference was observed between both groups when considering male in the 50s and 60s (p<0.05). Subscapularis muscle volume loss tended to increase with age in the group I, as indicated by linear regression analysis (Fig. 4, 5). However, this linear decrease was not observed in the group T (Table 2, 3).

The primary goal of this study was to explore the relationship between the subscapularis muscle volume and supraspinatus tendon tear. Results of this study indicate no significant relationship between supraspinatus tendon tear and volume of subscapularis muscle in females. However, a higher subscapularis muscle volume was observed in group T than group I of males in their 50s and 60s.

We hypothesized that the posterior section of the supraspinatus interacts with the subscapularis muscle as a posterior structure which contributes to some degree of anterior-posterior stability. Furthermore, the supraspinatus tendon tear affects the volume of the subscapularis muscle loss, thereby leading to a tear [19].

According to Bergin et al. [19], subscapularis tendon abnormality is related to chronicity of supraspinatus tendon tears. In patients with supraspinatus tear, the dynamic anterior instability increases due to the elevated posterior shoulder force couple weakness, resulting in sub-coracoid impingement and consequently a subscapularis tendon tear [19]. MRI scans reveal the existence of bone marrow edema of lesser tuberosity; however, the study of bone marrow edema was not included in the current study.

Contrary to our hypothesis, our study revealed different outcomes. We initially hypothesized that supraspinatus tendon tear affects the reduction of the subscapularis muscle volume. However, we did not account for the degree of supraspinatus tendon tear size, tear thickness, torn tendon retraction degree, occupation, activities, and body skeletal mass [20]. Hence, our data revealed no significant relationship between supraspinatus tendon tear and volume of subscapularis muscle in the group T of males in their 40s. Since the supraspinatus tendon tear might be small in the 40s with a shorter duration of tear, its effect on the volume of the subscapularis muscle may be negligible. Furthermore, in the group comprising males in their 50s and 60s, group T had higher subscapularis muscle volume than group I. We also considered that males included in the group T had a higher level of labor intensity or sports activities, and these factors might affect in increasing the volume of subscapularis muscle before onset of supraspinatus tear. However, we also considered that this result could be due to the small number of samples, thereby resulting in selection bias which plays a role in enhancing the subscapularis muscle volume in group T than in group I. Furthermore, our data showed that the subscapularis muscle volume in group T of female was lower than in group I, but was statistically not significant. We believe this could be that most female are housewives or working with lower labor intensity jobs. When considering sex of the female, the subscapularis muscle volume seemed more affected by the supraspinatus tendon tear than by life activities.

In this study, we found that although the subscapularis muscle volume reduces with aging, it was not related to cuff tear. This finding is supported by previous muscle studies that demonstrate that the aging process results in a loss of muscle mass, with subsequent replacement by fat and connective tissue [21,22]. Regardless of the sex, reduced muscle volume and aging were correlated for subjects in their 40s, 50s, and 60s. With the onset of advancing age, muscle tissue is gradually lost, resulting in diminished mass and strength; this condition is referred to as sarcopenia [21,23]. One study presented that patients with cuff tears have sarcopenia, and patients with large to massive tears have a significantly inferior sarcopenic index than those with small to medium tears [24]. They measured the whole-body muscle mass as per the Janssen [25] method of bioelectrical impedance analysis equation, which is different from the analysis used in our current paper, in that we only calculated the subscapularis muscle [24]. Another paper explains that rotator cuff atrophy is related to increasing age and not to tear severity. For patients without rotator cuff tears, the prevalence of fatty infiltration and atrophy increased with aging [26]. Further evaluation is required to determine the pathophysiology and progression of muscle atrophy in rotator cuff muscles.

Although cadaver studies provide a detailed understanding of the effects of supraspinatus tendon tear on the subscapularis muscle atrophy or tendon tear, it is necessary to achieve meaningful results. In this study, we measured the muscle volume of the rotator cuff (such as subscapularis muscle) through MRI. Since only the muscle part is measured (except for the surrounding fatty tissue), the accuracy is relatively high [27]. Considering various other factors such as tear size, jobs, sports activities, height, and weight that influence the muscle mass will yield more meaningful results.

This study has some limitations. Most notably, being retrospective in design, we are unable to make any causal claims explaining the relationship between supraspinatus tear and subscapularis muscle volume. Second, this study has a small sample size and does not exclude factors that might affect muscle volume, such as height, weight, occupation, and sports activities. Third, we did not account duration of the disease and size (i.e., full-thickness tear or partial thickness) of the supraspinatus tendon tear. Fourth, muscle volume measurements using the MIMIC program did not consider fatty infiltration in the muscles, hence the measured volume value does not reflect the intrinsic atrophy of the muscles accurately.

Analysis applying 3D reconstruction by MRI reveals decreasing subscapularis muscle volume with aging, regardless of the sex. The supraspinatus tear was seen to affect the subscapularis muscle volume in a certain group of age and sex.