Transcript refractive surgery

REFRACTIVE SURGERY 
Refractive surgical techniques have evolved rapidly 
over the past three decades .
Emerging as safer and more reliable means of 
treating myopia , hyperopia and astigmatism ,thus
reducing the need for corrective lenses
Although a wide range of refractive procedures 
exists , we will discuss the following refractive
procedures :
. 
A) Radial keratotomy . 
B) Excimer laser photorefractive keratectomy ( PRK ) 

C) Laser assisted in situ keratomiliuses (LASIK) 
D) Laser assisted subepithelial keratomiliuses ( LASEK ) 

E) Laser thermokeratoplasty (LTK ) 
D) Intracorneal ring segments (ICRS) 
G) Phakic intraocular lenses (IOLs ) 
History of keratorefractive surgery 
Radial keratotomy to correct myopia was 
introduced in Japan in the 1940s using anterior
and posterior corneal incisions . This approach
fell into disfavor 10 to 20 years later when
resulting endothelial cell injury led to irreversible
corneal oedema .
Russian investigators improved the procedure 
in the 1970s by using only anterior incisions.
Since it's introduction to the United States in 
1978, RK underwent continual refinement,but
long term studies showed significant instability
and progressive hyperopic shifts that led to
virtual abandonment of this surgical procedure
in the late 1990s .
Excimer laser which makes pulses of invisible 
ultraviolet light of 193 nm ,each pulse of light
can removes a microscopic layer from the front
surface of the cornea , was introduced in
ophthalmology to reshape the anterior corneal
surface .
Early excimer laser was used in the PRK 
operation, establishing it's efficacy for the
correction of myopia, hyperopia , and
astigmatism .later excimer laser was used in
the operations of LASIK,LASEK and LTK.
The preoperative evaluation is of great 
importance in these circumstances , like patient
selection ,motivation , contraindications
,physical and ocular examination ,cycloplegic
refraction , and screening tests like pachymetry
, keratometry and computerized
videokeratograohy .
We will discuss now the main refractive 
procedures according to it's development :
1) Radial Keratotomy (RK ): Refers to the 
placement of deep paracentral and peripheral
incisions in the cornea ,producing central
corneal flattening and thus reducing the central
refractive power and myopia .
RK is best for low myopia ( -2.00 TO -3.00 D.) 
The procedure has the advantage of being safe 
,with rapid results and high predictability in the
low range of myopia .
Incisions are ideally 80% to 90% of corneal 
depth , and should not extend to Descement
membrane to avoid the danger of mechanical
instabilita and perforation .
In general no more than eight radial cuts was 
done with a 3.0 mm central clear zone
2) Astigmatic keratotomy : constitutes the 
placement of transverse or arcuate incisions
perpendicular to the steepest corneal meridian
to correct astigmatism , the incised meridian
flattens while the meridian 90 degrees away
steepens by nearly the same amount .Incisions
are ideally between 5 and 7mm from the pupil
center .
3) Excimer laser PRK : 
Photorefractive keratectomy uses the cool 
beam of the excimer laser to reshape the front
of the eye by treating the surface of the cornea
so that the rays focus more sharply on the
retina .
The outer surface of the cornea , called 
epithelium , must be removed , this is
accomplished by carefully scraping it away .
The laser is then applied directly to Bowman 
membrane and anterior corneal stroma to
remove microscopic amounts of these tissues
measured in microns .
The epithelium grows back over the treated 
area within several days .
A (bandage ) contact lens is placed on the eye 
to minimize discomfort during this healing
period . Complete healing and visual recovery
can take as long as 3-6 months with PRK .
RESULTS : In FDA Phase III trials for PRK in 
low myopia (-1 to -6 D),88 to 92% of patients
had uncorrected vision of 20/40 or better at 1
year with 50 to 60% better than or equal to
20/20, and 70 to 80% within 1.0 D of the
intended refraction .
Excimer laser PRK for higher levels of myopia 
is somewhat less predictable .
LASIK has virtually replaced PRK for the 
treatment of hyperopia and higher degrees of
myopia .
4) LASIK( Laser assisted in-situ Keratomileusis 
Is a refractive surgical procedure that results in rapid 
recovery of vision and has the capability to benefit
patients with myopia , hyperopia , and astigmatism .
Millions of people worldwide have already this exciting
and marvelous procedure . Candidates must be at
least 18 years of age .
In this procedure the eye surgeon creates a thin 
surface flap of the cornea using a microkeratome to
expose the underlying tissues ( stromal bed ) in which
the surgeon applies the excimer laser beam to create
the refractive ablation .
Once the laser ablation is completed , the surgeon 
gently replaces the corneal surface flap to restore
surface integrity of the eye .
The results are often dramatic and very rapid , 
with most patients seeing well enough to drive a
car without correction the very next day .
However , the best post-operative visual acuity 
may not be obtained until 2 to 3 weeks , or in
some cases , even a few months after the
procedure .
Visual results with LASIK : 
LASIK nearly always results in improved vision 
without correction .However LASIK is an
imperfect procedure , and does not always
result in 20/20 or even 20/40 visual acuity .
Patients in the extreme range of myopia 
generally have less favorable outcomes with
LASIK .
In a study results of the American Society of 
Cataract and Refractive Surgery from 1736
eyes with myopia and myopic astigmatism that
underwent LASIK operation , they showed the
following results :
Post-op
1 month
3 months
6 months 
20/20 or better 45%
46%
50% 
20/40 or better 87%
89%
93% 
The best candidate for LASIK is an individual 
who desires to be less dependant on glasses or
contact lenses , is willing to accept the risk of the
procedure , and understands that an
enhancement procedure may sometimes be
required .
We now know that dry eye syndrome commonly 
follows LASIK procedures , at least transiently .
This make the patient feels dry or gritty or even
blurring vision . Many surgeons advise using nonpreserved artificial tears for the first few weeks
after LASIK .
5) LASEK(Laser Subepithelial Keratomileusis ) 
LASEK is a relatively new procedure that sits 
somewhat between LASIK and PRK ,is used
mostly for people with corneas that are too thin
or too flat for LASIK .
It was developed to reduce the chance of 
complications that occur when the flap created
during LASIK is not of the ideal thickness or
diameter .
In LASEK instead of using a microkeratome , a 
blade called a trephine is used
LASEK is a procedure that takes about the 
same amount of time as LASIK . A topical
anesthetic is applied to the eye , then the
trephine is used to create the epithelium flap .
After the flap is made , the surgeon covers the
eye with a dilute alcohol solution (20% ) . The
solution loosens the edges of the epithelium .
After the eye has been soaked for roughly 30 
seconds , the surgeon uses a tiny hoe to peel
back the epithelium and fold it back so it is out
of the way .
At this point the doctor employs an excimer 
laser in the same way as in LASIK or PRK ,and
molds the corneal tissue to the desired shape .
After the shaping is finished, the surgeon 
replaces the epithelium flap .
The recovery time for LASEK is slightly longer 
than it is for LASIK , as it takes approximately 4
to 7 days to recover good vision .The flap itself
heals within a day , but the eye can still feel
irritated for up to three days .
6) Thermokeratoplasty : Consists of heating the 
cornea to shrink the peripheral and paracentral
stromal collagen to produce a peripheral
flattening and a central steepening of the
cornea to treat HYPEROPIA . Solid state
infrared lasers , such as the
holmium:yttrium,aluminum , and garnet
(Ho:YAG ) laser of 480-530nm,have been used
in a peripheral intrastromal radial pattern (
Laser Thermokeratoplasty ), to treat hyperopia
of 2.50D.and less .
7) Another form of thermokeratoplasty is the so 
called ( Conductive Keratoplasty ) which used
a handheld radiofrequency probe to shrink the
peripheral collagen .
The shrinkage temperature of the corneal 
collagen is between 55-58 degrees .
Despite the instability of corrections and 
regressions , laser thermokeratoplasty and
conductive keratoplasty have the major
advantage of untouched the central cornea .
8) Intracorneal ring segments (ICRS): 
ICRS are placed in the peripheral cornea and 
act by compressing the peripheral cornea and
changing the radius of curvature of the central
cornea . When the anterior surface of the
cornea is lifted focally over the ring , a
compensatory flattening of the central cornea
occur for treatment of myopia .ICRS are
threaded into a peripheral midstromal tunnel
A potential advantage of intracorneal segments 
over other refractive surgical techniques is
reversibility .
The main drawback is the limited range of 
correction ( up to -3D )in myopia and ( up to
+2D ) in hyperopia .
There are another new approaches to refractive 
surgery like :
1) Refractive lensectomy : The extraction of the 
clear lens in the correction of high myopia ,
originally performed in 1890, was later
abandoned because of an unacceptable high
rate of complications . With more recent
operative techniques, such as
phacoemulsification , and better IOLs , there
has been renewed interest in managing high
refractive errors by clear lens extraction .
One drawback of this procedure is the loss of 
accomodation .The use of accommodating or
multifocal IOLs could obviate this problem .
2) Phakic intraocular lenses : 
--Anterior chamber phakic IOL , named as iris claw 
lens to correct high myopia in phakic patients ,
enclaved in the midperipheral iris . Long term followup has reported progressive pupil ovalization with an
older model.
-- Posterior chamber phakic IOL , it must 
accommodate to the space between the posterior iris
and the crystalline lens . If it vaults too much , pigment
dispersion and even pupillary block glaucoma could
result .If it lies against the anterior surface of the
crystalline lens , cataract could result .
3) Gel injection adjustable keratoplasty : 
Consist in injecting gel in the paracentral 
corneal stroma in order to flatten the central
cornea and reduce myopia . The gel was
extremely well tolerated with no evidence of
inflammation .It can correct myopia above 5
or 6 diopters as well as astigmatism .
What is most exciting about this procedure is 
that it make it possible to adjust patients,
refractive errors throughout their lives . Gel
can either be added or removed
4) Procedures that modify the axial length of the eye either by 
resection of the sclera or by reinforcement of the posterior
pole in case of high myopia ,have had a greater role in the
management of staphyloma than in the management of the
patient's refractive error .
5) Customized corneal ablation through Wavefront mapping : 
This is accomplished by mapping the profile of the whole eye
through wavefront sensing devices, this sophisticated method
identifies abberations in the entire optical system and not
only the corneal surface . With the use of advanced lasers
and wavefront deformation measuring devices , the
correction of these distortions of the human eye will continue
to gain widespread use .