Human orbital muscle in adult cadavers and near-term fetuses: its bony attachments and individual variation identified by immunohistochemistry

Purpose To compare fetal and adult morphologies of the orbital muscle (OM) and to describe the detailed topographical anatomy in adults. Methods Using unilateral orbits from 15 near-term fetuses and 21 elderly cadavers, semiserial horizontal or sagittal paraffin sections were prepared at intervals of 20-100 mu m. In addition to routine histology, we performed immunohistochemistry for smooth muscle actin. Results At near term, the OM consistently extended widely from the zygomatic bone or the greater wing of the sphenoid to the maxilla or ethmoid. Thus, it was a large sheet covering the future inferior orbital fissure. In contrast, the adult OM was a thin and small muscle bundle connecting (1) the greater wing of the sphenoid to the maxilla (11/19 cadavers), (2) the lesser wing of the sphenoid to the maxilla (5/19) or the greater wing (3/19). The small OM was likely to be restricted within the greater wing (5/19 cadavers) or the maxilla (3/19). Two of these five types of OM coexisted in eight orbits. OM attachment to the lesser wing was not seen in fetuses, whereas ethmoid attachment was absent in adults. Conclusions The lesser wing attachment of the OM seemed to establish after birth. A growing common origin of the three recti was likely involved in stealing the near-term OM attachment from the ethmoid. The strong immunoreactivity of remnant-like OM in the elderly suggests that OM contraction is still likely to occur against the increased flow through a thin vein. However, the contraction might have no clinical significance.

Automated summary

The study found that in near-term fetuses, the orbital muscle covered the inferior orbital fissure, while in adults, the orbital muscle was a smaller muscle bundle connecting different bones. The lesser wing attachment of the orbital muscle in adults likely established after birth. The strong immunoreactivity of remnant-like orbital muscle in elderly individuals suggests that the muscle may still react to increased venous flow, though it may not have clinical significance.