Accessory infraspinatus muscle in a male cadaver: a case report

Abstract Purpose The infraspinatus muscle is a component of the rotator cuff that has relatively few known variants. Understanding variation in this structure is critical for clinicians and may have implications for understanding human development. Methods One 83-year-old male cadaver was dissected as part of the graduate anatomy education curriculum at Johns Hopkins University School of Medicine in fall 2022. Dissections were guided using Grant’s Dissector, 17th edition as a reference. Results An accessory infraspinatus muscle as well as other muscular and neurological variations were discovered in the cadaver. The variant muscle is thin and flat with parallel fibers originating on the medial dorsal surface of the scapula, running under the scapular spine, and inserting on the greater tubercle of the humerus. The accessory infraspinatus muscle is separated from the infraspinatus muscle and deltoid muscle by fascial planes, and its muscle body extends past the tendinous portion of the underlying infraspinatus muscle; we suggest these as additional criteria for the identification of the accessory infraspinatus. Conclusion This is the second reported occurrence of an accessory infraspinatus muscle and the first known occurrence in a male. The co-occurrence of this rare variant with other variations in the limbs, back, and head suggests that variation in development may be affecting the entire body, ultimately being responsible for the presence of an accessory infraspinatus in this cadaver.