* How much surgery is the right amount?
* How many millimeters should each muscle be recessed, resected, or shifted for a given deviation?
* A paradoxical but supportable statement is the following: It will probably cause less damage doing the "wrong" operation well than doing the "right" operation poorly. In cases where the plan is apparently correct or at least supportable but the surgery is done poorly, the resulting new strabismus may be worse than before surgery and the new problem is unfixable.
* Are numbers important? Of course, but they are only a part of the equation. A disappointing result should not be rationalized by the defense that the "numbers" were correct according to some "experts" table. Most experts have succumbed to the temptation or more accurately the pressure to publish surgery "numbers" usually with appropriate disclaimers. However, each surgeon must be accountable for his/her own results - "numbers".
* Since the issue cannot be avoided we must use some "numbers". Let's look at how the numbers are determined - but first we must agree that the medial rectus is weakened in esotropia, "strengthened" in exotropia and that appropriate "weakening" - "strengthening" strategies are used with the other extraocular muscles. This basic principle must be accepted before consideration of numbers.
Adjustable sutures theoretically obviate the need for numbers, but there are limitations to the effectiveness of this technique ensuring the need for "numbers".
Some rules: 1) Smaller deviations require smaller numbers - larger deviations larger numbers; 2) Resections usually require larger numbers than recession for a given angle; 3) Lateral recti usually require larger numbers than medial recti; 4) Vertical recti usually require smaller numbers than horizontal recti (the huge superior rectus recession in DVD done by some (>10 mm in some cases is an exception)
The Anatomy of the Numbers Rectus Muscles
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The S.O. muscle is "off limits". The superior oblique tendon is nonstretchable. Surgery on the superior oblique is limited to the tendon only - except in rare cases of trochlea fracture for S.O. tendon transfer. |
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The I.O. is probably the least important muscle, but is subject to the most adventuresome surgery. Maybe the surgeon gets away with so much because this muscle does not make that much difference. |
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In the usual clinical situation the only time small globe size is a factor in recession is in surgery for congenital ET* |
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Measure medial rectus recession from the limbus - 8.5 mm minimum to 11.5 mm maximum
Very young children, under a year, with 19-20 mm axial length or under 6 month with < 19 mm axial length should not have medial rectus recession in excess of 10.0 mm to 10.5 mm. |
* In microphthalmos there is more effect per millimeter - large eyes with axial myopia need larger numbers because of larger globe size and because of lateral rectus weakness from orbital crowding pressure.
The limbal measurement is a logical way of arriving at the maximum possible recession. This was begun by me (EMH) in the early 70's while the "insertion" measurers were gradually and steadily increasing recession amount, eventually up to 7.0 mm because of a 50% undercorrection rate which occurred when 5.5 mm was the accepted maximum for medial rectus recession. Going to limbal measurements produced 85% alignment on the first try. While useful, limbal measurement has not achieved widespread use.
The numbers for obliques are loosely adhered to. Weakening is usually disinsertion or myectomy (tenectomy). Measured recession with attachment to the globe has very little validity for quantification.
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S.O.
 Tenectomy (closer to the trochlea results in more weakening)
Disinsert or tenectomy close to the insertion results in less weakening |
I.O.
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Myectomy - makes Lockwood new "insertion"
The neurovascular bundle of the inferior oblique "stabilizes" Lockwood connection (Stager) Recession of the S.O. and I.O. will be discussed later. |
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