The infraspinatus muscle: a proposed classification system

The infraspinatus muscle, a key component of the rotator cuff, consists of two parts: the superior (transverse) and the inferior (oblique). Both originate from the infraspinous fossa of the scapula. The oblique part inserts onto the greater tubercle of the humerus, while the transverse part attaches to the tendinous portion of the oblique part. Previous studies have described variations in the anatomy of the infraspinatus muscle; however, the present study specifically highlights morphological variability in the oblique portion.

Ninety-one upper limbs (46 left and 45 right) fixed in 10% formalin solution were examined.

Two distinct morphological types were identified. Type I, observed in 76.92% of specimens, consisted of a single muscle belly originating from the infraspinous fossa and inserting onto the greater tubercle of the humerus. Type II, present in 23.08% of specimens, displayed two separate bellies: a superior belly arising from the upper half of the infraspinous fossa and inserting onto the greater tubercle of the humerus, and an inferior belly originating from the lower half of the infraspinous fossa and inserting onto the greater tubercle, inferior to the superior belly insertion.

The infraspinatus muscle demonstrates morphological variability, particularly in the number and arrangement of its bellies. Recognition of these variants may be valuable for surgeons operating in the rotator cuff region and for anatomists studying shoulder morphology.