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Chapter 2: Surgical Anatomy -
Sclera
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Lecture 22 of 22 NEXT»
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The thickness of the sclera varies according to location (Figure 64).
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At the limbus, the sclera is 0.8 mm thick.
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Anterior to the rectus muscle insertions, it is0.6 mm thick.
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Posterior to the rectus muscle insertions, it is0.3 mm thick.
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At the equator, it is 0.5 to 0.8 mm thick.
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At the posterior pole, it is greater than 1 mm thick. The area of greatest surgical activity for the extraocular muscle surgeon coincides with the thinnest area of the sclera.
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B
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Figure 64
A The sclera varies in thickness according to location
B The sclera is thinnest, 0.3 mm, posterior to the rectus muscle insertion
Care must be exercised when placing a needle into the sclera (Figure 65). A reverse cutting needle should be used only while exercising extreme caution because such a needle may be as thick as or thicker than the sclera into which it is inserted. This could lead to scleral perforation, an event that undoubtedly occurs more often than is suspected or reported. Fortunately, most cases of inadvertent scleral perforation heal without incident. If such a cutting needle is used, it should be very fine (preferably less than 0.3 mm), if possible, and it should be inserted carefully with the top of the needle seen through the superficial sclera at all times. For added safety, the cutting edge can be directed sideways so that it cuts along the scleral lamellae rather than into the eye, as shown. A curved cutting needle is less likely to perforate the sclera than a reverse cutting needle, but the curved cutting needle is prone to ‘cut itself out’ of the sclera unless an excessively deep bite is taken.
Figure 65
A Keystone spatula, cutting tip down
B Keystone spatula, cutting tip up
C Hexagonal spatula, neutral cutting tip
D Reverse cutting - tends to cut in - can be placed sideways
E Curved cutting - tends to be cut out
A much safer needle to use is the spatula design. With such a needle, only the tip and sides are cutting edges. The sclera is displaced upward and downward away from the body of the needle and is cut laterally and ahead of the needle. This action makes the complication of scleral perforation less likely to occur with spatula needles than with cutting needles. The spatula needle’s widest dimension should remain parallel to the scleral surface. Needles with a wire diameter of .203 mm are both sufficiently strong and delicate enough to be inserted safely.
A keystone spatula, with cutting tip up, is safe but can ‘cut out’ of sclera. A keystone spatula, cutting tip down, produces a longer track but can also ‘cut in’ to the eye. A hexagonal spatula or ‘neutral’ tip needle must be guided to stay at mid-scleral level. The sclera is white and opaque when fully hydrated. If this tissue becomes dried out, it becomes dark amber-colored and translucent. Re-hydration rapidly restores the opaque whiteness of the scleral tissue.
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