Inferior oblique anteriorization

The neurovascular bundle entering the posterior muscle acts as a fibrous tether, and becomes a secondary origin once the distal muscle is carried forward. Anteriorized, the muscle loses most of its elevating action; fully anteriorize the posterior pole fibers and it can become a net depressor.

Selection

Overaction is graded +1 to +5. Primary overaction of +1 needs nothing, +2 or more is operative; secondary overaction from a superior oblique palsy is worth operating on at even +1. The atlas tabulates which weakening procedure suits which grade (Table 7-1, p. 77); ranked weakest to strongest, the options run 8 mm recession, 10 mm, 14 mm, denervation with extirpation, then anteriorization.

Laterality. Primary overaction is usually bilateral and often asymmetric early — search hard for even +1 on the other side before operating unilaterally, and if you find it, operate bilaterally using the procedure the worse eye needs. If the other side is genuinely clean, suspect a superior oblique palsy, which is corrected by unilateral surgery and can be made worse by bilateral. Avoid unilateral full anteriorization except in DVD or palsy: it can limit elevation and produce a hypotropia.

Anatomy to respect

• The insertion is the most posterior part of the muscle and lies over the macula. No careless needle there — which is the argument for clamping and cutting before suturing.
• The inferotemporal vortex vein exits just behind the posterior border. Identify it before you hook anything.
• The distal muscle runs against the intraconal fat pad; entering it invites fat adherence.

Key steps

1. Inferotemporal fornix incision, with headlamp illumination — overhead light doesn’t reach these landmarks.
2. Isolate the lateral rectus and pass a 4-0 silk under it, out through superior conjunctiva and clamped across the drape, holding the eye in maximum adduction and elevation.
3. With a Stevens hook laterally and a large hook below, have the assistant lift both away from sclera to display the anterior and posterior borders within Tenon’s. Locate the vortex vein, then slide a Stevens hook posteriorly parallel to sclera, rotate the tip to catch the posterior border, engage the belly and deliver it anteriorly with the tip elevated.
4. Off-load Tenon’s and septum from the hook tip, taking care not to off-load muscle fibers — a missed posterior slip leaves residual overaction. When the posterior border shows a clean pink-white junction, incise the septum at the muscle border.
5. Expose the insertion with a second large hook and clean it of septum and Tenon’s, cutting close to the muscle until the whole insertion is visible and bare.
6. Clamp before you cut. Place a small clamp across the insertion right at sclera, visualizing both tips to confirm the entire insertion and nothing else is captured, then shave the insertion off with curved scissors following the globe’s curvature.
7. Secure the insertion on the clamp with a double-armed 6-0 Vicryl: half-thickness across the belly, then a locking bite at each edge. Keep track of which pole is which — reattaching the posterior pole posteriorly is what stops the belly twisting and cancelling the effect.
8. Remove the silk, hook the inferior rectus to hold the eye in elevation, and uncover sclera just behind and temporal to its insertion.
9. Carry both poles fully forward and sew them alongside the temporal edge of the inferior rectus. The posterior pole fibers in particular must be fully advanced and maximally tightened — that is what converts elevating force into depressing force.

Carrying the posterior fibers a millimeter or two in front of the inferior rectus insertion increases the effect, at the cost of postoperative hypotropia with limited elevation — especially unilaterally.

Pearl: Bunch the new insertion; don’t fan it. Kushner described a syndrome after anteriorization in which elevation is restricted in abduction, producing a Y or V pattern and an upshoot of the other eye that looks exactly like residual inferior oblique overaction. In his 77 bilateral cases it developed in only two — the two in whom the muscle was placed 2 mm in front of the inferior rectus insertion and spread out laterally. Keep the posterior fibers tight against the temporal border of the inferior rectus, go no more than 1 mm in front of its insertion, and if overaction seems to come back after a well-done anteriorization, think of this before you plan more weakening.

Pearl: Look for a second belly. In 100 cadaver orbits, 17% of inferior obliques had more than one division at the insertion and 8% had two bellies at the point where the muscle is normally hooked. Once you think you have the whole muscle, look again behind the posterior border before you accept it — an unhooked slip is a common reason a weakening procedure fails.

Pitfalls to avoid

• A needle pass at the original insertion, over the macula, when clamping first would avoid it.
• Off-loading muscle instead of septum, leaving an unoperated posterior slip.
• Losing track of the posterior pole and reattaching the muscle twisted.
• Unilateral full anteriorization in a patient who actually has bilateral overaction.

Further reading

• Del Monte MA, Archer SM. Atlas of Pediatric Ophthalmology and Strabismus Surgery. Churchill Livingstone, 1993. Ch. 7, “Inferior Oblique Muscle Surgery” (Del Monte and Archer), pp. 75–92.
• Kushner BJ. Restriction of elevation in abduction after inferior oblique anteriorization. J AAPOS. 1997;1(1):55–62. doi:10.1016/s1091-8531(97)90024-0
• Stager DR. The neurofibrovascular bundle of the inferior oblique muscle as its ancillary origin. Trans Am Ophthalmol Soc. 1996;94:1073–1094. PMC1312118
• De Angelis D, Makar I, Kraft SP. Anatomic variations of the inferior oblique muscle: a potential cause of failed inferior oblique weakening surgery. Am J Ophthalmol. 1999;128(4):485–488. doi:10.1016/s0002-9394(99)00225-1