Anatomy of Inferior Oblique - The Private Eye Clinic

12/14/2007AnatomyAnatomy Nasal to the IRcrossing, IO is engulfedin orbital fat, is oval inshape with a diameterof 4-6mm. Lateral to the crossing itflattens and widens to8-10mm and is coveredby Tenon’s capsule. Between the Tenon’s capsule penetrationand insertion, IO muscle capsule contributesto the intermuscular septum, which providesinterconnection between IR, LR and IO.AnatomyAnatomy The inferotemporal vortex vein exits from thesclera at the temporal border of IR, 12mmposterior to its insertion, then continues on acircuitous upward course on the innersurface of Tenon’s capsule just posterior toIO before penetrating the muscle cone.Unless visualised at the time IO is hooked,the vein may be damaged. IO inclinedposterolaterally at ~45 owith AP plane, almostparallel with SO tendon. The nerve and bloodsupply enter the IO inthe undersurface of the“dogleg” turn, 15mmfrom the insertion, justlateral to IR.ActionsAnatomical Variations IO elevates the visual axis because itdepresses the posterior aspect of the globe. Primary action, elevation, increases inadduction, and is nil in abduction. It is theonly elevator in adduction. Subsidiary actions, abduction and extorsion,increase with abduction and decrease withadduction. Stephen Kraft, Toronto, 1999, Am J Oph 100 cadaver “virgin” orbits:– # of divisions at insertion– Variation in anatomy 10 and 12mm from insertion– Total width of muscle belly2