Anatomy of human extraocular muscles: Description of an unusual variation associated with smooth muscle

Anatomical variations of the extraocular muscles are uncommon; reports in the literature mention the appearance of additional bellies, muscle slips or bridges between these muscles. Such variations may alter the normal anatomical relationships of orbital structures. This research aims to inquire about anatomical variations of the human eyeball, for this purpose, a search of the scientific literature was performed in the following databases: Pubmed, Scopus, Web of Science, Proquest and Scielo, using keywords without time or language restriction. The exploration was performed accordingto the protocol of the orbit dissection guide, in a male cadaveric specimen of unknown age. The skull was sectioned transversely, and the encephalon was removed until the roof of the orbit was reached, the periorbit was removed with a Stille-Luer Gouge forceps and a bone chisel, bordering the eyeball; then the superior orbital fissure and the optic canal were opened, and the contents of the orbit were extracted in blocks. This allowed seeing a structure like a muscular belly coming from the tendinous ring with a posterior trajectory, leaving the orbit towards the middle fossa of the skull, inserting itself in the sphenoid bone’s greater wing. In situ measurements were performed on the tissue with a fine-tip caliper and the sample was sent to histological study, where standard staining was performed with hematoxylin and eosin, the samples obtained present smooth muscle fibers in their majority composition. In the literature reviewed from 1921 to 2022 no descriptions similar to this variation were found. This finding provides relevant information for diagnostic imaging, surgical treatment and future studies.