The infraspinatus muscle: a proposed classification system.

Background: The infraspinatus muscle is a part of the rotator cuff. It has two parts: the superior - transverse and the inferior - oblique. Both parts originate from the infraspinous fossa. The oblique part inserts onto the greater tubercle of the humerus; the transverse part inserts onto the tendinous part of the oblique part. Our research shows that the oblique part of the infraspinatus muscle demonstrates morphological variability.

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

Results: Two types were identified. Type I (76.92%) was characterized by one belly with origin within the infraspinous fossa and insertion on the greater tubercle of the humerus. Type II (23.08%) was characterized by two bellies: the superior belly originating on the upper half of the infraspinous fossa and inserting onto the greater tubercle of the humerus, and the inferior belly originating on the lower half of the infraspinous fossa and inserting onto the greater tubercle of the humerus, inferior to the upper belly insertion.

Conclusions: The infraspinatus muscle is characterized by morphological variability in the number of bellies. This classification may be helpful for surgeons operating in the rotator cuff area and anatomists.