Unlike the palpebral opening which differs significantly according to age, the extraocular muscle size, or at least width at insertion, is closer to constant throughout life. Achild with a tiny palpebral opening is likely to have a medial rectus whose insertion width is very close to the adult average measurement of 10 mm. This means that the timing of strabismus surgery is not determined by either the size of the palpebral opening or of the extraocular muscles. Globe size, on the other hand, is significant in the design of strabismus surgery. Comparing globe size and extraocular muscle location, Swan and co-workers pointed out that in newborns, the posterior part of the globe is relatively smaller than the anterior part, meaning that a recession of 3 mm could place the medial rectus at the equator. This is important information but not for strabismus surgery, which is not indicated anyway in the newborn because of immaturity of the binocular system. The globe in the 4-month-old, an age some consider the earliest appropriate for strabismus surgery, is 19.5 mm in axial length. This is sufficient size to allow an appropriate recession of the medial rectus in congenital esotropia. For example, a recession of 9.5 mm measured from the limbus places the new insertion of the medial rectus at the equator in a 4-month-old with an axial length of 19.5 mm (Figure 5). The insertion of the medial rectus in an infant can be closer than 5.5 mm from the limbus. This means that a medial rectus recession, if measured 9.5 mm from the limbus, will be at least equal to a 4 mm recession, if measured from a medial rectus insertion that was 5.5 mm from the limbus. It could also be as large as 6 mm considering measurement from the insertion, if the medial rectus inserts 3.5 mm from the limbus as it does in some infants. With continued growth of the globe taking place primarily in the posterior part, the relative position of the medial rectus insertion will move anteriorly, and therefore safely, with maturity.

A

B

Figure 5
A The medial rectus of a 4-month-old measures approximately 10 mm at the insertion.
B The muscle has been recessed 9.5 mm from the limbus using a limbal incision.

The point of measuring from the limbus is that this provides a safe technique for performing a larger recession of the medial rectus, especially in those cases with medial rectus insertions closer to the limbus (Figure 6). Measurement from the limbus begs the question, "Is the significant factor in correction of esotropia the size of medial rectus recession or the new position of the muscle on the globe?" The answer may be that both play a role. We do know that undercorrection of congenital esotropia occurred in nearly 50% of cases when the maximum for medial rectus recession was on the order of 5.5 mm from the insertion as was the ‘rule’ in the 1960's. This undercorrection rate reduced immediately to approximately 10% in a series when medial rectus recession was measured from the limbus. This technique of recession measured from the limbus, allowing larger but safe recessions, is now joined by larger recession measured from the insertion. These larger recessions, some 7 mm or more, have been implicated in a higher overcorrection rate.