More than 20 million people in the
United States have myopia between 1-3 diopters (D). Several options for
visual rehabilitation are available to each individual, including spectacle
correction, contact lens fitting, refractive keratotomy (RK),
photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis
(LASIK), and intracorneal rings or Intacs. Currently, nonsurgical approaches
are still the least expensive and safest. For the subset of patients who
desire freedom from glasses and lenses, refractive surgery is an option.
The ideal refractive surgery procedure is one that is
effective, predictable, safe, and potentially reversible. The goal of this
article is to introduce a fairly new refractive surgical procedure in which
implantation of a polymethyl methacrylate (PMMA) ring or ring segments in
the peripheral corneal stroma is used to correct myopia.
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Device description
The KeraVision intrastromal corneal ring (ICR) is a
device designed to correct mild-to-moderate myopia by flattening the
anterior corneal curvature without encroaching on the visual axis. The
device is an open-ended PMMA transparent ring with an outer diameter (R2) of
8.1 mm and an inner diameter (R1) of 6.8 mm. It is precision lathe-cut to
±0.01 mm as a hexagonal-shaped section of a cone with positioning holes for
manipulation. It is inserted through a peripheral radial incision made with
a diamond knife at two-thirds corneal depth in to a 360° peripheral
intrastromal channel created with specially designed instruments. Anterior
corneal curvature is changed by using rings of different thicknesses.
Intrastromal corneal ring segments (Intacs microthin
prescription inserts), or ICRS, are a more recent design modification of the
ICR. The ring segments split the ring into two 150° arcs. The use of ring
segments simplifies the implantation procedure. Furthermore, ring segments
can be placed away from the radial incision, thereby minimizing the
potential for incision-related complications.
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Surgical instruments
KeraVision has designed surgical instruments specifically
for use with Intacs insertion. These include the following: corneal
thickness gauges, glides, incision and placement marker, pocketing hook,
pocketing lever, ring forceps, stromal spreader, vacuum-centering guide with
vacuum system, and clockwise and counterclockwise dissectors.
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Advantages of Intacs inserts
Several benefits of using Intacs microthin inserts exist.
It is a minimally invasive outpatient procedure. Since the surgery is
completed in the peripheral cornea, the central optical zone is not
disturbed. Results are rapid and predictable. There is a reduced risk of
visual side effects and a long-term convenient refractive correction. It is
removable and exchangeable. Corneal asphericity (prolate) is maintained.
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Implantable corneal ring segments. 
