The Effect of Hinge Position and Hinge Width on Corneal Sensation and Dry Eye After IntraLase LASIK Procedure

Corneal innervation/sensation is supplied by the long ciliary nerves which branch from the
trigeminal nerve. The nerves enter the cornea in the mid-stroma at the nasal and temporal
limbus. They then branch and turn anterior to form a dense plexus sub-Bowman's layer. The
nerves finally terminate in the wing cell layer from where they enervate the epithelium.
Corneal sensation has been shown to be decreased after all corneal surgeries, including
LASIK.

Corneal sensation is known to play a role in tear secretion. Decreased sensation leads to
decreased tear production; and when bilateral, leads to a decreased blink rate as well. Dry
eyes are a common side effect of LASIK. Incidence rates vary widely, but have been reported
in as many as 59% of patients. And almost all patients have some transient dry eye symptoms
immediately following LASIK.

Previous studies have looked at the effect of hinge position (superior vs. nasal) and hinge
width on corneal sensation and dry eye after LASIK performed with a mechanical
microkeratome.1,2 Corneal sensation was decreased and dry eye signs and symptoms increased
immediately following LASIK in all eyes. These parameters then improved at all time periods
between 1 week and 6 months post-operative. Loss of corneal sensation and dry eye signs and
symptoms were greater in eyes with superior-hinge flap than nasal-hinge flap, and in eyes
with narrower hinge flap rather than wider hinge flap IntraLase LASIK, using the IntraLase
femtosecond laser rather than a mechanical microkeratome to cut the corneal flap, has become
an increasingly popular procedure. It provides several advantages over mechanical
microkeratomes, including reduced surgical complications, more predictable flap thickness,
better astigmatic neutrality, decreased epithelial injury,3 and an ability to operate on a
wider range of patients.

We propose this study to evaluate the effect of flap hinge position and size in corneal
flaps created with the IntraLase laser. We would like to determine if there is a difference
from the previously discussed results found when using the mechanical microkeratome. Also,
with the increased ease of programming alternate hinge position and width with IntraLase, if
we find a significant difference with an alternate flap configuration, it might translate to
a feasible change in clinical practice.

Specific Aims: To evaluate the effect of hinge position (superior vs. temporal) , hinge
width (45 vs. 90 degrees), and flap thickness on corneal sensation and dry eye after
IntraLase LASIK.

Inclusion Criteria:

- Refractive error: Myopia < 12 D, with astigmatism < 3 D,

- Age > 20 y/o

- Regular corneal curvature, sufficient corneal thickness, appropriate pupil size,
normal slit lamp examination

- Informed consent to permit us to use their records for this study without using name,
medical record number, or date of surgery.

Exclusion Criteria:

- Pregnant/nursing

- Systemic collagen vascular disease

- Autoimmune disease

- Severe dry eyes