Abstract
Background On the basis of the available literature, we proposed the hypothesis that the number of muscle bellies is morphologically constant. The main purpose of this study was to examine the morphological variability of the SM and to create a new classification of it based on number of muscle bellies. Methods Sixty-six adult cadavers of Central European population (45 females, 21 males) were obtained and fixed in 10% formalin before examination. Results The SM was found in all 66 specimens (45 females, 21 males, 31 left and 35 right sides). After meticulous dissection, we distinguished nine types on the basis of number of bellies. Type I was characterized by single belly and occurred in 1.5%. Type II had a double belly and was present in 3%. Type III, the most common type, occurring in almost 32% of the studied population, had three bellies. The frequency of type IV, characterized by four bellies, was also high, just over 30%. The following types were less frequent: type V with five bellies (18.2%), type VI with six bellies (7.6%), type VII with seven bellies (3%), type VIII with eight bellies (1.5%), and type IX with nine bellies (3%). All of the types had origin on the anterior surface of the scapula. Conclusions The SM is morphologically variable in the number of its bellies. Evolutionary changes are probably the reason. The most common type was the SM with three bellies, in line with Larson's model of the division of the SM into three parts. Subsequent studies should be carried out based on MRI or ultrasonography examination to confirm if it is possible to show all types (presented in this study) among group of patients during MRI.
Highlights
The subscapularis muscle (SM) is the most powerful and largest muscle in the rotator cuff
There were many muscle belly variants, and the following types were differentiated on the basis of meticulous dissection: Type I: this type is characterized by a single belly; its proximal attachment is located on the anterior surface of the scapula
Type II: this type is characterized by a double belly; the proximal attachment is located on the anterior surface of the scapula
Summary
The subscapularis muscle (SM) is the most powerful and largest muscle in the rotator cuff. The muscular part has origin on the anterior part of the scapula, called the subscapular fossa [2]. This muscle occupies the entire fossa, so it is recognized as the largest muscle attached to the scapula [3]. The upper subscapularis nerve (USN) and the lower subscapularis nerve (LSN) are responsible for innervating the SM They both arise from the posterior cord of the brachial plexus and receive contributions from C5 and C6. The main purpose of this study was to examine the morphological variability of the SM and to create a new classification of it based on number of muscle bellies. Subsequent studies should be carried out based on MRI or ultrasonography examination to confirm if it is possible to show all types (presented in this study) among group of patients during MRI
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