Anatomy of Inferior Oblique - The Private Eye Clinic

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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
12/14/2007Anatomical Variations - KraftAnatomical Variations - Kraft 17 multiple insertions, ranging from 2 to 4 8 had divisions at surgical capture site.Among these 8:– 4 had dehiscences within the muscle that resultedin 2 distinct bellies at 10 and 12mm positions, butthe bellies rejoined at the insertion.– remaining 4 had bifid muscle bellies that insertedseparately into the sclera. Muscle width at 10mm:– 8.4mm in 8 double bellies vs 7.7 in 92 other (notstat sig) Muscle width at 12mm:– 7.8mm vs 7.3 (not stat sig)Anatomical Variations - KraftAnatomical Variations – Kraft 2 These results are for “normal” patients The incidence of anatomical variations couldbe higher in those with IOOA - ? Doublebellies have greater vertical action Failure to detect duplications at time ofsurgery incomplete weakening andrecurrence/persistence. JAAPOS 2001 – prospective survey over 7years. Compared eyes with double vs single belliedIO muscles:– Gradings of pre-op IO & SO actions– Presence of fundus excyclotropia– A/V patterns– Presence and size of primary positionhypertropias– Post-op IO actionAnatomical Variations – Kraft 2 27 of 247 (11%) eyes operated on haddouble bellied IO. Only fundus excyclotorsion incidence differedbetween groups – 48% with double bellies vs27% with single bellies. The efficacy of weakening surgery inreducing IOOA was similar in both groups.3
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Anatomy of Inferior Oblique - The Private Eye Clinic

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