The changing face of Refractive Surgery by dfhrf555fcg


More Info
									Refractive Surgery Draft                                                                   Bradley

       The Changing Face of                            of the procedure in advertising and the
        Refractive Surgery                             popular media”.
                   by                                  In spite of much positive marketing of
      Arthur Bradley, Ph.D.                            refractive surgery lingering doubts exist
                                                       about its reliability, safety and stability.
Preface:                                               For example, Professor of
                                                       Ophthalmology, Leo Maguire, has
The recent diversification and                         referred to patients who have undergone
availability of refractive surgery has                 refractive surgery as the “refractive
initiated the most significant change in               underclass”3. Also, there are sufficient
refractive technique since the                         numbers of patients dissatisfied in their
popularization of the contact lens during              refractive surgery results that they have
the 1960s. Just as the contact lens freed              their own web page. This web page
the myope from the spectacle, refractive               ( is full of
surgery may free the myope from                        testimonials and even some computer
spectacles and contact lenses.                         simulations of post-refractive surgery
                                                       vision which are worth seeing.
In spite of its rapid development and
coverage in the popular press (e.g.                    In spite of the lingering concerns about
articles in Time magazine and Consumer                 refractive surgery it continues to be
Reports), it is not easy to keep abreast of            promoted and has become a real option
the data on and changes in refractive                  for many patients, some of whom will
surgery. Optometrists are often provided               seek advice from their Optometrist prior
with pseudo-scholarly publications that                to deciding on surgery. This article is
are actually promotional literature1                   designed as an up-to-date short review of
published by those marketing refractive                this field to help our readers understand
surgery. It is this environment of biased              the benefits, shortcomings, and possible
and difficult to access information, that              future of this approach to correcting
motivated Dr. Bradley, who is a member                 ametropia.
of the FDA Ophthalmic Devices panel
and Professor of Optometry and Vision                  1. The optometrists role in laser vision
Science at Indiana University to write a                  correction: TLC, Laser Eye Centers,
short summary of the recent history and                   1999.
new developments in this field.                        2. Waring G III, Future developments
                                                          in LASIK. In: Pallikaris I, Siganos,
Some prominent ophthalmologists such                      D, eds. LASIK, Thororofare, NJ:
as George Waring III are concerned                        Slack , 1998, pp 367-370.
about the mismatch between the reality                 3. Maguire L, Quoted in Consumer
of refractive surgery and the promotional                 Reports article on LASIK titled “Zap
marketing literature2: “the patient must                  your myopic eyes”, June, 1999.
have realistic expectations of the
procedure based on honest
communication from the surgeon and
professional staff, regardless of portrayal

                                              Page 1
Refractive Surgery Draft                                                                  Bradley

Introduction:                                           treating myopia, RK and PRK differ in
In spite of the fact that most spherical                both the site of intervention and the
refractive errors are caused by eyes                    surgical method. RK makes incisions
having anomalous axial lengths (too                     deep into the peripheral cornea, while
long in myopes and too short in                         PRK removes tissue from the anterior
hyperopes), there is a long history of                  central cornea using a high-energy
correcting for this anatomical defect by                ultraviolet laser.
introducing optical changes at the
anterior eye. For centuries, spectacle                  Photoablative Refractive Surgery
lenses were the only option available to                Most photoablative corneal reshaping
make this change, but during the last half              techniques employ UVB lasers, e.g., an
of the 20th century, contact lenses                     argon fluoride excimer laser (=193
became a convenient alternative and are                 nm), to produce high-energy radiation
currently worn by over 20 million                       which is highly absorbed by the corneal
Americans.      These lenses work by                    stroma. This energy is sufficient to
changing the curvature at the air                       break the chemical bonds that form the
interface, where the refractive index                   collagen fibers and effectively remove
difference is large and most of the eye’s               this tissue from the cornea.
optical power exists. A similar and more
permanent strategy is to change the                     Initial attempts to use UV lasers were
curvature of the anterior corneal surface               based upon the RK radial incision
directly.                                               technique. However, the UV laser failed
                                                        as a “knife” because it created wider
Although          refractive        surgery             incisions than the scalpel and produced
(Keratotomy) was pioneered during the                   more significant scars. More recently,
nineteenth century, it was not widely                   the UV excimer laser has been modified
available until the last quarter of the 20th            to ablate stromal tissue within the optical
century.      Several      methods       for            zone and thus reshape the optical surface
implementing corneal curvature changes                  directly. Two manifestations of this
were developed during the last 1/4 of the               approach       have    been    developed,
20th century and continue to be                         Photorefractive Keratectomy (PRK) and
developed today. Early methods, e.g.,                   laser in situ keratomileusis (LASIK), and
radial keratotomy (RK) in the 1970’s                    both share a common goal, to reshape
and      80’s      and      photorefractive             the anterior corneal surface by ablating
keratectomy (PRK) in the 1990’s, had                    stromal tissue. However, the methods
serious shortcomings and they are now                   for achieving this goal are quite
being replaced.                                         different.
For example, RK, in addition to poor                    In PRK, anterior stromal tissue is ablated
predictability, produced eyes with                      after the corneal epithelium has been
unstable refractive errors that varied                  scraped away (although in rare cases
diurnally and with altitude and on                      transepithelial PRK was performed). Of
average shifted towards hyperopia after                 course, this method also ablates the
surgery (e.g., almost 50% shifted by 1                  basement membrane (Bowman’s Layer)
diopter). This article will describe some               upon which the epithelium grows, and
of the more recent surgical approaches                  thus has a number of undesirable
and in particular will examine the                      complications associated with loss of
refractive success and the safety issues                epithelial       function        including
associated with each.                                   susceptibility to infection, post-surgical
                                                        pain, abnormal epithelial growth, and
Refractive surgeries designed to reshape                reduced optical transparency. These
the cornea can be grouped by either the                 problems are most pronounced in the
site of surgical intervention or the                    period after surgery, and thus patients
surgical method.      For example, in                   did not generally have bilateral PRK, but

                                               Page 2
Refractive Surgery Draft                                                                Bradley

had to maintain one untreated eye during             example, a patient may elect to have a
the epithelial recovery period. In spite             small amount of myopia to aid in
of this protracted recovery period, PRK              reading.
surgery has been performed on both eyes
simultaneously.                                      Many studies report and plot the average
                                                     post-surgical refractive error, and in
The      problems      associated    with            general with more recent technology this
destruction of the epithelium in PRK                 approaches the target indicating an
have been largely eliminated by                      almost perfect outcome.         However,
implementing a different pre-ablation                individual eyes do not achieve the mean
surgical procedure. Instead of scraping              post-op Rx, and therefore, in order to
off the epithelium, a deep cut into the              assess efficacy, the post-surgical
stromal lamellae is made approximately               refractive errors of individual eyes must
parallel to the corneal surface using a              be considered.
micro-keratome (LASIK).          The cut
begins temporally or inferiorly and cuts             In order for the FDA to approve a
across the central cornea but leaves the             photoablative laser for LASIK, it must
nasal or superior edge uncut. This                   be able to demonstrate efficacy by
method produces an anterior corneal flap             having a high percentage of the post-
(70-160 microns thick), which can be                 surgical refractions within some range of
folded back to expose the corneal                    the intended or target refraction (e.g.,
stroma. At this point a photoablative                75% must be within 1 diopter of
method, the same in principle to that                intended and 50% within 0.50 diopters).
used in PRK, is employed to remove                   Most current systems achieve this goal,
stromal tissue and thus reshape the                  with about 60-70% of the eyes ending up
corneal stroma without destruction or                within 0.50 D of the target and
removal of the epithelium. Once the                  sometimes more than 90% within 1
ablation is complete, the flap can be                diopter. However, some studies still
repositioned over the remaining stroma               report only succeeding in getting 70%
resulting in a cornea with a mostly                  within 1 diopter of target.
functioning epithelium (some sensory
nerve damage and associated corneal                  In general, the anticipated residual
insensitivity occurs, which remediates               refractive errors increase with the
after about two weeks). The flap is a                magnitude of the pre-surgical refractive
non-rigid      structure    and     when             error. However, although approximate
repositioned its shape is affected by the            emmetropia may not be achieved in
underlying stromal re-shaping which is               some highly myopic eyes, it can be
transferred to the anterior corneal                  argued that converting a –10 diopter
surface thus changing the optical power              myope into a –2 D myope is an effective
of the cornea.                                       procedure since their level of visual
                                                     disability while uncorrected will be
LASIK is currently the most widely used              greatly reduced.
surgical method for correcting refractive
errors and several commercial lasers                 It is important, therefore, that patients be
have received FDA approval.                          fully aware of the likely refractive
                                                     outcome prior to opting for surgery.
LASIK Efficacy                                       Realizing that a patient will typically
If refractive surgery is effective, the              expect to leave their eye-care
post-surgical refractive errors should be            practitioner’s office seeing “perfectly”,
the same as the targeted or intended                 clinicians counseling patients about
refractive error. The reason to use                  refractive surgery should emphasize that
targeted or intended instead of                      this will probably not happen. Typical
emmetropia       is    that    sometimes             results in recent studies indicate about
emmetropia is not the target.         For            80% to 90% of patients end up with

                                            Page 3
Refractive Surgery Draft                                                                 Bradley

uncorrected VA (UCVA) of 20/40 or                       seem to have an effect. For example,
better, and between 40 and 70% with                     bowing of the posterior corneal surface
20/20 or better UCVAs. The FDA                          has been reported and this may reflect
requires a new laser system to                          structural changes caused by the removal
demonstrate 20/40 UCVA in at least                      of more than 100 microns with the
85% of treated eyes to qualify as                       keratome and up to 200 microns with
effective. That is, perhaps 50% of                      photoablation, reducing the 500 micron
LASIK patients will have to tolerate                    thick cornea to approximately only 200
uncorrected VAs poorer than 20/20 or                    mechanically integrated microns. A
wear a spectacle or contact lens to                     significant correlation between bowing
achieve their pre-surgical VA. As many                  and residual stromal thickness has been
patients with low levels of refractive                  observed when the thickness is less than
error now do, these post LASIK patients                 290 microns. The same study concluded
with small residual refractive errors                   that inaccuracies in the refractive
generally choose to leave them                          outcome stem primarily from a
uncorrected making the clear choice of                  combination of secondary bowing and
convenience over vision quality.                        epithelial thickness changes that develop
                                                        post-surgically. Leaving less than 250
There is one significant complication                   microns intact is generally felt to be
associated with efficacy.             Since             unsafe.
photoablation removes tissue, there will
always be some wound healing process,                   The primary determinant of efficacy is
and this can and does lead to post-                     the amount and spatial distribution of
surgical refractive instability.      Since             tissue ablated. This often depends upon
PRK removed the entire epithelium and                   proprietary algorithms, which can be
Bowman’s layer, the healing process                     updated to improve efficacy if a
was very active, and this was the likely                procedure has been shown to either
cause of much of the post surgical                      under or over correct. Very simply, if
instability. The reduced wound healing                  the pre-ablation anterior corneal
response experienced with LASIK                         curvature is known, the desired change
results in less post-surgical instability in            in refraction determines the required
Rx, most eyes (e.g. 95%) experiencing                   new curvature and the amount of tissue
less than 1 diopter change during the                   to be removed. Studies have shown how
year post surgery. Recent protocols                     much tissue will be ablated by a given
have reduced the population mean                        amount of laser energy (e.g. 0.1 microns
change in Rx to almost zero. However,                   can be removed by a 50 mJ/cm2 excimer
some individual eyes do experience                      laser pulse), but these values vary
changes during the 6 months post-                       slightly from eye to eye depending upon
surgery.                                                such things as stromal hydration. An
                                                        additional source of variability is eye
Although LASIK does not require                         position and eye movements during
complete re-growth of the corneal                       surgery. In response to this concern,
epithelium and the wound healing is                     some laser systems (e.g. Autonomous
reduced, recent studies have observed                   flying spot laser) include an eye position
increased epithelial thickness anterior to              tracking system to effectively stabilize
the ablation indicating some epithelial                 the eye with respect to the laser. This
response to the surgery or the ablation.                system corrects for any eye movements
                                                        during the procedure, which can last
Of course, efficacy will be compromised                 from few seconds to 60 seconds
by any change in corneal structure                      depending on the amount of tissue to be
following        keratomileusis        or               ablated.
photoablation, and the significant
reduction in the thickness of the                       One major advantage of PRK over RK is
remaining structurally intact cornea does               that, unlike RK, it did not suffer from

                                               Page 4
Refractive Surgery Draft                                                                Bradley

significant diurnal fluctuations or the               The incidence of infections caused by
significant hyperopic shifts associated               LASIK is very low, and includes
with high altitudes that plagued RK.                  bacterial keratitis due to poor ocular
Recent studies by the US military at                  hygiene combined with imperfect
14,000 ft. have confirmed that LASIK                  epithelial coverage along the flap
eyes do not suffer from the 1.5 diopter               incision. Vitreous hemorrhage and
hyperopic shifts seen in RK eyes, but if              retinal detachments following
an eye has had LASIK recently, a                      corneoscleral perforations resulting from
hyperopic shift of about 0.5 diopters was             the surgical microkeratome have also
observed. However, after six months, no               been reported, but again, the incidence is
such shift was observed.                              very low (e.g., 2 eyes out of 29,916).
                                                      Other vitreoretinal pathologies in the
Since the mean post-LASIK Rx has                      post-surgical LASIK patients were also
approached zero, it appears that the                  very rare and may reflect typical levels
tissue ablation algorithms have been                  experienced by highly myopic eyes.
optimized. The fact that the majority of              This emphasizes that, although LASIK
eyes do not end up emmetropic results                 may correct the myopic refractive error,
from the eye-to-eye variability in such               it does not treat or prevent the other
factors as epithelial growth, corneal                 problems associated with and caused by
bowing and reaction to the laser.                     increased axial length in myopic eyes.
Therefore, in order to improve the                    Dry eye is a very common complaint
efficacy still further, a two step surgery            following LASIK, possibly due to
may have to be implemented. The                       cutting the corneal nerves and
second ablation will fine tune the small              decreasing the primary signal that
errors left after the first LASIK.                    produces normal tear levels. Dry eye
However, the second procedure is nearly               complaints persist for a long time and
as costly as the first and reduces profit             individuals with dry eye prior to surgery
margins. Such an approach is already                  should be counseled that LASIK may
used to correct “poor outcomes” after the             exacerbate their existing problem.
initial LASIK procedure.                              Those without dry eye should be
                                                      counseled that dry eye complaints are
LASIK Safety                                          relatively common and can last for
Evaluation of safety is more complicated              several months to a year following
than assessing efficacy of refractive                 surgery.
surgery. We can consider any change to
the eye which compromises vision as a                 2.     Wound      healing    response:
safety problem. There are five general                Diffuse interface keratitis, with an
categories of such problems following                 accumulation of inflammatory cells at
LASIK: (1) infections and pathology in                the flap interface has been observed
response to the surgical or/and ablative              presumably due to a wound healing
procedures, (2) undesirable wound                     response. Also, unusual epithelial
healing responses, (3) photoablative                  growth has been observed when trauma
changes that cannot be corrected with                 dislodges the flap. Recent evidence
standard spectacle or contact lenses, (4)             from animal studies indicates that the
effects of the high energy laser on other             healing process at the flap interface
ocular tissues, and (5) optical problems              continues for about 9 months after
associated with the pre-ablation surgery              LASIK.     The consequences of this
(e.g. flap irregularities). Due to the                prolonged wound healing are unclear.
invasive nature of this surgery, it is not
surprising to find that problems                      3. Optical changes uncorrectable
associated with the flap surgery are the                 with standard ophthalmic lenses:
most significant.                                     There is a genuine concern that
                                                      photoablative procedures will result in
1.     Post-surgical pathology:                       reduced optical quality of the cornea due

                                             Page 5
Refractive Surgery Draft                                                                  Bradley

to either a loss of transparency and                    higher order aberrations such as
optical scatter or irregular changes in the             spherical aberration and coma, which
shape of the optical surface. Both of                   limit retinal image quality in pre-surgical
these optical changes are uncorrectable                 eyes.     Autonomous Technologies is
with standard spectacle lenses.                         pioneering this concept, which requires
                                                        measurement of the eye's aberrations in
Aberrations exist in an optical system                  addition to the refractive error typically
when, even with an optimum sphero-                      measured.       We expect to see this
cylindrical correction, the rays forming a              approach, referred to as “custom cornea”
point image will not focus to a single                  to develop rapidly in the next few years.
point.    Increased optical aberrations                 Of course, in order to correct for the
reported in post-PRK and post-LASIK                     aberrations, they must first be measured.
eyes4 may reflect the algorithms used to                New technology borrowed from
create the ablations, but other factors                 astronomy has been successfully
must also be involved. For example,                     employed to measure ocular aberrations5
myopic “islands” are often reported after               and these can be used to guide
PRK or LASIK and for some reason                        photoablative surgeries.        The term
these local under-corrected areas seem to               “wave-guided corneal surgery” was
disappear over time. The cause of these                 recently coined to describe this
myopic islands and the mechanisms                       procedure.
behind their remediation are well
understood.                                             We shall soon see if wave-guided
                                                        corneal surgery can succeed. McDonald
As a check for such detrimental changes                 presented some of the first data earlier
in the cornea, the FDA requires that post               this year and showed that the increase in
LASIK VAs be determined with the                        aberrations and thus reduction in retinal
optimum spectacle correction in place                   image quality associated with the
(Best Spectacle Corrected Visual Acuity:                standard LASIK procedure may not
BSCVA). If an eye can no longer be                      occur following a “custom cornea”
corrected to its pre-surgery levels of VA,              approach. Currently, it is not clear how
it is likely that one or both of the above              successful this approach will be. It may
optical changes have occurred. The                      be a way to maintain optical quality at
FDA requires that less than 5% of eyes                  pre-surgical levels, but the potential is
lose more than 2 lines of BSCVA, and                    there for actual improvement.
less than 1% end up with BSCVA of
worse than 20/40. One might argue that                  Although the ablation algorithms may be
any loss of BSCVA is unacceptable                       perfect and corneal transparency
since it is essentially an untreatable                  maintained, there is another factor that
vision loss. It is, however, disappointing              will lead to significant loss of retinal
that after centuries of striving to improve             image quality in LASIK or PRK. In
retinal image quality, we are now willing               order to maintain a monofocal optical
to accept reduced retinal image quality                 system, the reshaped cornea must be
and significant loss of vision all in the               larger than the eye’s entrance pupil.
name of convenience.                                    However, there are limits to the
                                                        maximum size of the ablation zone
Although current standards tolerate                     because increased ablation zone size
reduced retinal image quality and the                   requires deeper ablations. For example,
current LASIK protocols increase the                    by increasing the ablation zone from 4
eye’s aberrations, the potential is there to            mm to 7 mm approximately doubles the
actually improve retinal image quality                  necessary ablation depth in the central
and reduce aberrations. In principle,                   cornea when correcting myopia. Thus,
photoablative techniques can be used to                 correction of large refractive errors
correct not only the eye’s spherical and                requires more tissue ablation. Larger
cylindrical refractive errors but also                  ablation zones also require deeper

                                               Page 6
Refractive Surgery Draft                                                                Bradley

ablations. For example, Sher calculated                flattening of the central cornea by
that 300 microns of tissue would have to               LASIK actually leads to steepening of
be removed to correct a –12 diopter                    the peripheral cornea potentially
myopia over a 7 mm diameter area.                      exaggerating simultaneous bifocal effect
Approximate corneal thinning caused by                 for larger pupils. Also, by adding
photoablation for myopia is 12, 18 and                 transition zones into the surgical
25 microns per diopter with 5, 6, and 7                procedure, a dilated pupil produces
mm ablation zones, respectively.                       multifocal optics.

The problems associated with leaving                   The       impact      of     post-surgical
too little attached stroma after ablation              simultaneous bifocal or multifocal optics
are exaggerated with LASIK since up to                 would only be manifest at low light
150 microns of the anterior cornea has                 levels, and studies from Europe seem to
been removed already in the flap.                      indicate that night vision can be
Ablating significant amounts of the                    significantly compromised by PRK and
remaining       stromal    tissue    may               LASIK. Visible halos and glare at night
compromise the structural abilities of the             are often reported, and increase in
remaining stroma and result in the                     frequency with increased myopic
observed “bowing” of the posterior                     correction, and cases have been reported
corneal surface after surgery.                         in which post LASIK and post PRK
                                                       night vision is so poor that night driving
Since there are limits to how much                     has to be eliminated. It would be wise
corneal tissue can be safely removed,                  therefore, as Applegate6 has been
ablation zone size has typically been                  emphasizing for many years now, to
smaller than necessary to cover the                    discourage individuals with large night-
entire dilated pupil present at night.                 time pupils from undergoing this
Current standards try to maintain at least             procedure. Simulations of these night
250 microns of intact stroma after                     vision problems can be visualized on the
photoablation.      Given this type of                 web at
constraint, the photoablation zone size is
limited. Early PRK photoablations were                 4. UV damage to other ocular tissue:
performed with 4 mm and 5 mm zones,                    The introduction of a high intensity UV
but the standard now is about 6 mm with                radiation source into the eye produces
perhaps a 1-2 mm “transition” zone.                    obvious concerns for other ocular tissue
Because the pupil of many young eyes                   since UV is known to cause
will be larger than 6 mm under low light               cataractogenesis and may be a
conditions, the effective optical system               significant factor in age related
creating the retinal image will be bifocal.            maculopathy. However, 193 nm UV
The central zone will be near to                       radiation does not penetrate more that a
emmetropic and the marginal zone near                  few microns. This is why it is so
to the pre-ablation refractive error.                  effective at stromal ablation.
Although this has obvious parallels to
simultaneous bifocal contact lenses or                 5.      Problems with the flap.
IOLs, it is not an effective bifocal                   The major concern with LASIK stems
correction since the additional add                    from the radical surgery preceding the
power in the peripheral optics will vary               photoablation.      The entire anterior
from eye to eye and will be too                        cornea (epithelium and part of the
peripheral to be effective. This bifocal               stroma) is removed across the central
problem cannot be corrected with a                     cornea exposing the central stroma.
spectacle lens or easily corrected with a              Problems develop due to poor quality of
contact lens and bifocal optics are                    the keratome blade, poor control of the
known to produce significantly reduced                 cutting speed, failure to complete the
image quality, halos and glare. Data                   cut, leaving tiny metal fragments from
over the last few years indicate that the              the blade on the flap, deposition of other

                                              Page 7
Refractive Surgery Draft                                                                  Bradley

material (e.g., surgical glove powder)                  the reduced sensitivity following surgery
within the wound, and movement of the                   (sensory nerves have been cut) the
tissue during the cut. Expert use and                   normal feed-back that controls corneal
maintenance of the micro-keratome is                    insult has been seriously compromised
essential to reduce the incidence of these              which must increase the chances of
vision-compromising complications.                      elevated mechanical forces on the cornea
                                                        due to trauma or lid friction.
It is important to realize that cutting
corneal tissue requires much greater                    In addition to flap displacement, the
precision and better quality cut surfaces               structural weakness of the flap and its
than cutting tissue in other parts of the               attachment can lead to structural changes
body. Errors, such as the micro-chatter                 within the flap. Small scale “ripples” or
marks seen post LASIK, on the scale of                  “wrinkles” in the flap have been
the wavelength of light, can become                     reported, as have larger folds. Flaps are
significant. Also, since the stroma is                  sometimes detached and reattached to
avascular, there is little opportunity for              try and remedy flap irregularities. There
debris to be removed by phagocytic                      is also the problem of accurately
inflammatory cells. Reports of tiny                     realigning the flap and replacing it in the
metal fragments from the micro-                         correct location. Flap decentration has
keratome blade, powder from the                         been reported. As with flap wrinkling, it
surgical gloves, small pieces of sponge                 will lead to reduced optical quality.
as well as corneal tissue remnants have
been seen under the flap post surgically.               The final complication associated with
All of these reduce transparency, and                   the flap surgery stems from the pre-
can require a second procedure in which                 incision protocol. In order for the
the flap is opened up and the tissue                    keratome to make a precise cut, the
cleaned.                                                corneal tissue must be held firmly by a
                                                        vacuum ring. During this procedure, the
LASIK has a unique safety issue not                     intraocular pressure spikes to above 60
present with other refractive surgical                  mm of Hg. There is some concern that
procedures, which stems from the                        this IOP spike, particularly if it is
structural weakness of the corneal flap                 maintained for more than a few seconds,
and its poor adhesion to the underlying                 can lead to retinal damage. Suction
corneal stroma. In some ways it is                      duration depends upon the speed of the
remarkable that the flap can “reattach”                 procedure and can vary significantly
so easily without sutures.           Initial            (e.g., from 6 to 80 seconds). Changes in
reattachment results from hydrostatic                   retinal blood flow and visual function
pressure due to the hydrophilic nature of               following this transient elevated IOP
the     inner     cornea.         Primary               have been reported. In addition to the
“reattachment” forces may result from                   IOP spike, there is some globe
capillary surface tension. It is therefore              deformation associated with the vacuum
quite easy to remove the flap for                       ring.
additional photoablation, if the initial
surgery was not as effective as desired.                LASIK Summary
However, the flap can also become                       The overall picture emerging from the
dislodged accidentally.      Remarkably,                LASIK literature indicates that it is a
this is very rare, but it can and does                  largely safe and effective treatment for
happen, usually following some ocular                   myopia, hyperopia and astigmatism.
trauma. A notable concern exists for                    However, LASIK is not risk free, and
patients with dry eye who may                           final vision quality will probably be
experience adhesion forces between the                  slightly inferior to pre-surgical vision.
anterior corneal surface and the lid. This              Night vision may be significantly
has led to a patient waking to find the                 impaired. There are many stories of
flap stuck to the lid. Also, because of                 post-PRK and post-LASIK patients

                                               Page 8
Refractive Surgery Draft                                                                Bradley

having to modify their night driving                  photoablative techniques which calculate
behavior because of seriously reduced                 the desired tissue to be removed, LTK
vision at night. For the patient, the very            must rely on empirically determined
small risk of serious complications and               nomograms. Predictability with this
the likely small reduction in vision and              approach has not been established and
night driving problems must be balanced               dosimetry studies continue to examine
against the obvious convenience of                    the impact of wavelength, temperature,
never having to worry about contact                   penetration of the radiation, beam
lenses or spectacles. Perhaps more                    profile, and spatial pattern and duration
significantly, highly myopic patients will            of radiation. There is also concern that
never have to suffer the serious handicap             the thermal effects cannot be confined to
that exists when their high myopia is                 the stroma, and damage to the
uncorrected.       For many patients,                 epithelium and endothelium may occur.
particularly those who are seriously
handicapped by their myopia, and those                Surgical Implants
for whom highest quality vision is not                In addition to the methods just described
required, this may be the surgical                    in which the cornea is reshaped by
treatment of choice at this time.                     removing tissue or reshaping the cornea,
However, it is imperative that all                    two new surgical approaches are being
patients are made aware of the risks,                 developed that insert foreign bodies into
particularly the commonly occurring                   the eye. The first inserts a ring deep into
reduced quality of vision and night                   the peripheral corneal stroma and the
driving problems.                                     second     involves      implanting      an
                                                      intraocular lens (IOL) into a phakic
Thermokeratoplasty                                    ametropic eye.
In addition to the photoablative use of
short wavelength UV lasers, corneal                   1.       Intrastromal Corneal Rings
irradiation using long wavelength (1.5 –              Just as RK and LTK change the
2.0 micron) lasers has been developed to              curvature of the central cornea by
create thermally induced changes in the               changing the structure of the peripheral
corneal stroma. This method, Laser                    cornea, intrastromal corneal rings (ICR)
Thermokeratoplasty (LTK), has some                    or intrastromal corneal ring segments
obvious parallels to radial keratotomy,               (ICRS) are inserted into the peripheral
and it is sometimes referred to as radial             cornea to treat myopia. The ring or ring
thermokeratoplasty. Unlike RK, which                  segments are inserted through a small
treated myopia by introducing deep                    incision and threaded circumferentially
incisions to allow the peripheral cornea              into the deep stromal lamellae. The
to stretch and thus reduce central corneal            structural changes that are produced
curvature,      LTK causes peripheral                 translate into curvature changes in the
corneal shrinkage due to thermally                    central cornea. Inserting PMMA annular
induced shrinkage of individual collagen              rings into the deep stromal lamellae of
fibers. Thus, LTK has the opposite                    the corneal periphery changes the
effect on the peripheral cornea, and                  already prolate elliptical cornea into an
therefore induces myopic shifts in the                even more prolate cornea, reducing the
central cornea. It has been suggested                 overall corneal curvature and thus
and actually tested as a treatment for                producing a hyperopic shift. Studies
hyperopia (either naturally occurring or              indicate that myopia of up 3 or 4
secondary to over-correction by PRK or                diopters can be treated with this method.
LASIK), but it is still in the                        The biggest advantage of this approach
investigational stage and has not                     is that, unlike PRK, RK or LASIK, it is
received FDA approval.         There are              largely reversible by simply removing
major concerns about its ability to                   the ring (segments). Thicker rings (0.45
produce a stable refractive change since              mm diameter) introduce large changes
large regressions occur. Also, unlike                 and thus can correct for more myopia

                                             Page 9
Refractive Surgery Draft                                                         Bradley

while thinner rings (0.25 mm) are used         foreign body into the eye. For example,
to correct lower levels of myopia.             the    acceptably    low    levels     of
BSCVAs seem to remain high and thus            complications associated with cataract
the method must not introduce large            surgery may be unacceptably high for
amounts of aberrations or turbidity in the     phakic IOL refractive surgeries. Also,
central cornea. There is some concern          recurring problems with lenticular and
that significant refractive instability        corneal physiology, the development of
exists with this method including diurnal      cataracts, and reduced endothelial cell
variations.    Peripheral corneal haze,        counts cast doubt on the acceptability of
small lamellae deposits adjacent to the        this approach for routine refractive
ring, deep stromal neovascularization,         surgery.
and pannus are also associated with the
ring insertions. Currently the FDA has         The efficacy of this approach hinges on
approved one ICR (Keravision’s Intacs).        the application of thick lens optics and
                                               accurate biometric data on the eye.
2.      Phakic Intraocular Lenses              There is still some uncertainty in
Unlike the previous methods, which all         calculating the required PIOL power and
required the development of new                therefore the post surgical refractions are
technology, IOL implantation has a long        not very accurate with residual errors of
and successful history as a treatment for      up to 6 diopters. These inaccuracies are,
cataract. The major difference with            of course, also determined by the precise
phakic IOL implantation is that the            position of the lens in the eye, and this
natural lens is left in place. The general     can vary significantly from eye to eye.
principle of using an IOL to correct for
ametropia has of course been part of the       There are two primary safety issues that
typical cataract lens replacement regime       continue to compromise this approach.
for many years. By manipulating the            First, posterior chamber PIOLs that are
curvature, refractive index and thickness      typically in contact with both the lens
of an IOL, significant refractive errors       and the iris, routinely lead to cataract
can be corrected by the cataract surgery.      development. Incidence rates of up to
                                               80% have been reported, but some
A phakic IOL (PIOL) is placed in either        studies report zero incidence of cataract.
the anterior or the posterior chamber and      Anterior chamber PIOLs seem to lead to
anchored in a similar way that traditional     reduced endothelial cell counts and thus
IOLs are implanted. PIOLs are made of          compromise the physiology of the
flexible materials such a silicone and         cornea, and in some cases (20% of eyes
hydrogel-collagen, and can be anchored         in one study) have lead to the surgical
with nylon haptics or other mechanical         removal of the PIOL. Also, the posterior
anchors. The anterior chamber PIOLs            chamber PIOLs push the iris forward
typically anchor in the angle between the      and thus lead to reduced anterior
cornea and iris while posterior chamber        chamber depth (and volume) and
PIOLs anchor around the zonules. One           narrower angles with the associated
beneficial effect of transferring the          elevated chance of angle closure
myopic correction from the spectacle to        glaucoma. Also, oval pupils and glare
the iris plane is that there will be           problems have been reported following
significant image magnification which is       insertion of anterior chamber PIOLs.
responsible       for    the     observed
improvements in VA after this                  The major advantage of this approach
procedure.                                     over the corneal reshaping techniques
                                               described previously is that it can correct
The primary concerns with phakic IOLs          for very large refractive errors, and has
stem from the intrusive nature of the          been used to correct eyes with up to –30
surgery in an eye that does not need to        D of myopia and +10 of hyperopia. One
be opened and the introduction of a            interesting combination therapy for the

                                         Page 10
Refractive Surgery Draft                                                        Bradley

very high myopes has been to implant a         were published on LASIK during the last
PIOL to correct most of the myopia and         five years. I used over 50 such articles
then use the more predictable LASIK to         identified by searching through the
further reduce the myopia towards              National     Library    of   Medicine’s
emmetropia.                                    MEDLINE system to write this article. I
                                               have not included all of these citations,
One solution to the cataract development       but a comprehensive bibliography on
complication associated with posterior         these topics can be located at
chamber PIOLs is to remove the natural
lens and replace it with one that will         simply by searching for PRK, LASIK,
correct the refractive error.                  PIOLs, etc. Also, the year 2000 abstract
                                               listings from the annual meeting of the
PIOLs have not received FDA approval           Association for Research in Vision and
although several are in the last phases of     Ophthalmology (ARVO) proved to be a
FDA approved clinical trials.                  valuable resource (

Summary:                                       Acknowledgements:
Refractive surgery has been widely             Earlier drafts were improved with help
available for about three decades now,         from Raymond Applegate, O.D., Ph.D.
and      it   has     undergone      many      (Indiana Alumnus), Professor of
transformations.     Overall, the newer        Ophthalmology, University of Texas,
techniques have improved accuracy,             San Antonio; Michael Grimmett, M.D.
stability and reliability, but continue to     Assistant Professor of Ophthalmology,
be plagued by biological variability           University of Miami; and by Carolyn
leading to small errors in correction.         Begley, O.D., M.S., and David Goss,
Although serious problems rarely occur         O.D., Ph.D. from the Indiana University
with PRK or LASIK, minor problems              Optometry faculty.
associated with reduced optical quality
are routinely produced.         Eye care       Three important papers published by
practitioners should advise patients of        I.U. faculty and alumni on refractive
the small risks of serious complications       surgery:
and the high risk of slight daytime vision
problems and possible serious night            4. Oshika, T, Klyce, SD, Applegate,
driving problems. These risks must be             RA, Howland, HC, El Danasoury,
balanced with the tremendous increase             MA, Comparison of corneal
in convenience of reducing or                     wavefront       aberrations     after
eliminating dependence on spectacle or            photorefractive keratectomy and
contact lenses.                                   laser    in   situ    keratomileusis,
                                                  American Journal of Ophthalmology,
The costs associated with excimer lasers          127:1-7, 1999.
and the imperfect results observed with        5. Thibos, LN and Hong, Clinical
PRK and LASIK are the primary driving             applications of the Shack-Hartmann
forces behind the continued development           Aberrometer, Optom. Vision Sci.,
of novel refractive surgical techniques           76, 817-825, 1999.
and products, and we can expect to see         6. Applegate, R.A., Gansel, K.A., "The
more developed in the future.                     Importance of Pupil Size in Optical
                                                  Quality Measurements Following
                                                  Radial Keratotomy", Corneal and
Post-script                                       Refractive Surgery 6:47-54, 1990.
Most of the information reported in this
review article comes directly from the
primary literature. Refractive surgery
has proliferated a large number of
publications. For example, 330 articles

                                         Page 11

To top