More than Meets the Annulus: Identification and Quantification of the Inferomedial Extraocular Muscle Raphe.

Purpose: During endoscopic endonasal access to small intraconal masses deep in the orbital apex, a line of fusion between inferior and medial recti is encountered distal to the termination of the common tendinous ring. The intraoperatively viewed length of this segment has not been quantified. To assist clinical recognition of this structure, our study quantifies its length and proposes the standardized nomenclature term of inferomedial extraocular muscle raphe (IM-EOMR).

Methods: A cadaveric experimental study was performed utilizing 8 fresh-frozen adult human specimens (16 orbits). Endoscopic endonasal bilateral middle meatal antrostomy, followed by posterior septectomy and spheno-ethmoidectomy were performed. The periorbita and optic nerve sheath dura were divided to define the muscular origin of the medial and inferior recti. From this origin, the distance to termination of the line of fusion between the medial and inferior recti was measured in triplicate. Our primary outcome was median length quantified from all 48 observations. Wilcoxon Signed-rank tests were employed to examine for differences related to lateralization.

Results: The IM-EOMR was observed in all dissections, with a median length of 6 mm (interquartile range: 1 mm). No differences between left and right sides was observed (6.0 vs. 6.3 mm, p = 0.20).

Conclusions: Endoscopic endonasal access to small intraconal lesions of the orbital apex may be limited by a 6-mm IM-EOMR distal to the common tendinous ring. Further histological analysis is required to delineate the composition of the IM-EOMR.