IC17: Intracorneal rings for Keratoconus and post-LASIK ectasia
INTRODUCTION
Keratoconus is a progressive ectatic disorder of the cornea that
generally affects younger individuals. As the disease progresses, the
central cornea thins and the cornea assumes a conical, rather than
spherical, shape. The apex of the cone is usually decentered inferiorly.
This leads to progressive myopia and astigmatism, usually irregular,
leading to a loss of best spectacle-corrected visual acuity.
Treatment of early keratoconus includes the use of glasses or
soft contact lenses to improve visual acuity. When irregular astigmatism
precludes adequate correction with glasses or soft lenses, rigid gas
permeable contact lenses, which mask the surface irregularities, can
often restore vision. Some patients, however, cannot be comfortably fit
with a rigid lens, and as the disease progresses, the cornea may
become too steep to maintain a contact lens on the eye. In addition,
central corneal scarring can limit vision despite the use of any optical
device. Traditionally, when patients could not get adequate vision with
either glasses or contact lenses, the only surgical option has been
penetrating keratoplasty (PK).
Recently, new advances have added to the surgical options for
keratoconus. In addition, they also can be applied to two closely related
diseases, pellucid marginal degeneration and post-LASIK ectasia.
Intracorneal ring segments can improve uncorrected and best corrected
visual acuity, or allow for successful contact lens fitting in a previously
unfittable patient, avoiding the need for keratoplasty. Collagen cross
linking (CXL) can increase the biomechanical rigidity of the cornea,
flatten the cornea, and prevent progression of the disease. While laser
remodeling of the cornea would threaten to worsen the disease in an
untreated cornea, it might be successfully and safely performed in a
cross-linked cornea. PK has generally been a very successful technique
for restoring vision in patients with advanced disease. Graft rejections,
however, are common, and threaten graft survival. Recent advances in
lamellar surgery techniques that enable near-equivalent visual outcomes
to PK have made deep anterior lamellar keratoplasty (DALK) a viable
option to PK, sparing the risk of endothelial graft rejection.
INTRACORNEAL RING SEGMENTS
Intracorneal rings affect the keratoconic cornea in two ways. First,
the ring effectively adds tissue and elevates the anterior surface of the
cornea overlying the ring. Since the central cornea flattens in the
direction of an elevated area, the 360o placement of the ring causes a
generalized flattening of the central cornea, reducing the amount of
myopia. The flattening effect of the ring segments is proportional to the
thickness of the ring, and inversely proportional to the diameter of the
ring segments. Asymmetric placement of rings of different thicknesses
can reduce astigmatism and centralize an eccentric cone.
Secondly, in keratoconus, the corneal elastic modulus is reduced
due to pathology in the corneal stroma. This causes the cornea to be
more deformable in response to stress, leading to thinning and
protrusion (increased curvature), which are the hallmarks of
keratoconus. By shortening the path length of the collagen lamellae
central to the segments, there is a redistribution of corneal stress,
interrupting the biomechanical cycle of keratoconus disease progression
and in some cases, reversing the process.
The goal of ring segment surgery is to reduce the degree of
myopia and astigmatism, improving uncorrected and best corrected
visual acuity. Shifting the position of the cone more centrally within the
cornea may also improve higher order aberrations, such as coma. Unlike
purely refractive procedures, however, the goal is not emmetropia;
rather, a successful prodecure is one that improves vision enough that
the patient can be comfortably corrected to an adequate level of vision
with spectacles or contact lenses, avoiding riskier procedures such as
keratoplasty.
Intacs Ring Segments
While there are a variety of intracorneal rings used worldwide, including
the Keraring, and the Ferrara Ring Segment, the most widely used, and
only ring segment used in the U.S., is the Intacs® Ring Segment
(Addition Technology Inc, Sunnyvale, California). The Intacs segments
consist of a pair of PMMA semicircular pieces, each having an arc length
of 150o and a hexagonal cross-sectional shape. When implanted
surgically, the Intacs rings have an external diameter of 8.10 mm and an
internal diameter of 6.77 mm. The refractive effect is determined by the
ring thickness. In the U.S., the FDA-approved rings are available in sizes
ranging from 0.21 mm to 4.5 mm. Current designs have a predicted
myopic range of correction from -1.00 D to -4.10 D. Recently a new
Intacs design (Intacs SK®) with an inner diameter of 6 mm and an oval
cross-sectional shape has been introduced.
Indications
Intacs can be used in patients with keratoconus, pellucid marginal
degeneration, and post-LASIK ectasia who are not adequately and
comfortably corrected with gas-permeable rigid contact lenses. They
should not be used in patients with significant central corneal scarring.
They should also not be used in patients with untreated atopic disease
or evidence of local or systemic infection. While there are no absolute
guidelines for the maximum corneal steepness to be treated with Intacs,
some surgeons in the US have suggested 58D as an upper limit.
Surgical Technique
Intacs procedures are generally performed using topical
anesthesia. Surgery is performed around the geometric center of the
cornea, which is marked with ink. The incision axis is determined, and,
using a specially designed marker, the incision site and location of the
rings is delineated. In the US, the steep axis technique (placing the
incision at the topographic steep axis of the cornea) is the most popular,
but a temporal incision can also be used. After measuring the corneal
thickness at the incision site (minimum cornea thickness is 480µ), a
guarded diamond knife is used to make the incision at approximately
70% cornea depth. After initiating a stromal channel at the base of the
incision, a complete circular stromal channel is created with a stromal
spreader. The Intacs segments are then inserted. The choice of Intacs
segments, and whether done symmetrically or with different sized
segments, is based on mean refractive error, amount of astigmatism,
and location of the cone. After correct placement of the Intacs segments,
the incision is closed with tight 10-0 nylon sutures with the knots buried.
Patients are treated with antibiotic and steroid eyedrops and oral
analgesics. The incision and stromal tracks can also be created with
most of the different femtosecond lasers currently on the market .With
this technology , it is very simple to select the inner and outer diameter
of the channel dissection , its depth and to perform the radial incision on
the selected axis in less than 15 seconds .Great care should be taken to
the centration of the procedure because of the frequent shift of the
central cornea due to the applanation of the laser .
Results
Several studies have reported the results of the implantation of
intracorneal rings for keratoconus , pellucid marginal degeneration and
post-LASIK ectasia . Using the mechanical dissection method, Colin et
al reported that 78% of eyes gained uncorrected visual acuity (UCVA)
with improvement in the mean refractive spherical equivalent (MRSE)
and mean keratometry (K) value of 3.1 ± 2.5 D and 4.3 ± 2.8 D ,
respectively. Alio et al. found that best corrected visual acuity (BCVA)
improved from 20/50 to 20/30 and remained constant at 36 and 48
months, and MRSE improved from -5.40 D to -3.95 D . Although they
found fluctuation in K-values, refraction remained constant at 36 and 48
months.
Complications
Implantation of intracorneal ring segments is generally a very safe
surgical procedure. Most significant intraoperative complications occur
while dissecting the channel for the rings. These can include creating too
shallow a channel or uneven channel depth, anterior or posterior
perforations, and decentration of the channels. These complications
might be avoided with the use of the femtosecond laser to create the
channels. Postoperatively, infections are rare but do occur. In addition,
sutures can loosen and lead to secondary infection. Segments can
migrate, especially if patients rub their eyes, leading to unwanted optical
effects. Overlapping segments can cause pressure necrosis of the
cornea and corneal ulceration. In the event the ring segments cause
significant unwanted effects, they are easily removed with the cornea
reverting to very near the pre-operative topography. One advantage of
ring segment surgery is that the central cornea is not violated. Later
keratoplasty can be performed, either after ring segment removal, or
around the ring segments.
COMBINED COLLAGEN CROSS LINKING AND INTRASTROMAL
RINGS
CXL and intracorneal rings have complementary effects on the corneal
biomechanics and refractive parameters. CXL has its effect mainly on
the anterior cornea, and intracorneal ring segments provide flattening
effects and redistribution of stress on deeper layers Sequential or
simultaneous treatment with both ICRs and CXL appears to have an
additive effect over either procedure alone. Combined treatment results
in greater improvement of UCVA and BCVA, more reduction of myopia
and astigmatism, and a greater than two fold reduction of mean and
steepest keratometry readings. Postulated explanations for the
additional effects include: 1) a simple addition of the effect of the two
procedures, 2) coupling from cross-linking of collagen around the ring
segments, and 3) greater effect of crosslinking due to pooling and
concentration of riboflavin in the channel around the ring segments.