Effective outcome of inferior oblique (IO) corrective surgeries demands a detailed knowledge of morphometry and variations of IO. Our aim was to study and morphometrically define the surgical anatomy of the IO muscle and its variations. Also to provide easily identifiable surgical coordinates to locate, the IO origin and the oculomotor nerve entry point into the IO. Dissection was performed on 16 cadaveric orbits. IO anatomy, variations, morphometry and relevant surgical distances were measured using digital caliper. IO with multiple bellies was found in five specimens. The IO mean length was 33.1 ± 3.3 mm, width at origin was 3.1 ± 0.6 mm, and width at insertion was 8.8 ± 1.5 mm. For easy localization of origin, its distance from the palpable landmarks, Zygomatico-maxillary suture and fronto-maxillary suture was measured. The mean distance between IO and the optic nerve was 10 mm. Distance of the nerve to inferior oblique entry point to the origin and insertion of the inferior oblique was measured. The nerve to IO was 28 mm long. The mean distance of the nerve entry point to IO origin was 15.5 ± 2.3 mm and distance to IO insertion was 15.2 ± 2.8 mm. A muscular bridge between the Inferior rectus (IR) & IO was found in one case, affecting ~¼ of the IO length; the distal end of the bridge was 5 mm from the IO insertion. Origin of the IO can be localized on the orbital surface of maxilla, 1-2 cm from the point where zygomatico-maxillary suture cuts the inferior orbital margin and 1-2 cm from the fronto-maxillary suture. In 19% of the orbits, the IO length was less than 30 mm, which may cause traction injury in muscle transposition procedures. The width at insertion is useful as most corrective surgeries are performed at the insertion site. The nerve to IO consistently entered at the center of medial border. The nerve entry point is important surgically as myectomy is performed between it and the insertion point. The safe distance available from the optic nerve was 7 mm. Detailed morphometry of IO may aid surgeons in better surgical planning and execution.
Keywords: Inferior oblique muscle; Strabismus surgery; morphometry; surgical anatomy; variations of surgical importance.