Principles and practice of corneal astigmatic surgery

Dr Laurie Sullivan FRANZCO
Melbourne, Australia
Corneal Clinic, RVEEH, East Melbourne
Bayside Eye Specialists
Lasersight

 To you, for your interest
 To Gerard and the University of Sydney for asking
me to contribute
There is a commentary for this presentation so I
hope you have your sound working
Dr Laurie Sullivan 2009
lsulliva@optusnet.com.au 2

 No financial interest, apart from the odd free travel
to the B&L Technolas user group meeting
 I use the B&L Technolas Z 100 excimer laser, and
the Amadeus mechanical microkeratome, and
Intralase femtosecond laser keratome

I understand that by now, you have already covered:
 Overview of Refractive Surgery: History, classification, terminology
 Anatomy, Physiology, Pharmacology, Pathology ,Corneal Wound Healing
 Corneal Imaging: Topography, Orbscan, Pentacam, Aberrometry
 Ocular and Systemic Disease relevant to Refractive Surgery
 Patient Assessment and Evaluation
 Principles of Laser Technology including Excimer, Femtosecond, Thermal
and Conductive Keratoplasty
 Principles and Practice of LASIK
 Principles and Practice of PRK/LASEK
So I am going to assume a fair bit or prior knowledge on your part.

= Differing refractive powers at differing axes
(meridia)
 Due to non-spherical (often “toric”) refractive
surface(s)
 Synonyms: “cylinder”, “toric”

 Corneal astigmatism
◦ Regular or Irregular.
Only regular astigmatism is remediable with refractive
surgery. Irregular corneal astigmatism requires rigid
contact lens or surgery to regularise the surface.
◦ Symmetrical or non-symmetrical
 Intraocular (“lenticular”) astigmatism
◦ Refractive astigmatism without corresponding corneal
curvatures

Cornea:
 Manual or automated keratometry
 Corneal topography (videokeratography)
Refractive:
 Subjective and objective refraction
 Wavefront analysis

 Computerised video-keratography
 A means of describing, depicting (“mapping”), and
quantifying corneal shape and/or power
 Placido disc imaging the basis of many
topographers.
 Also slit scanning and Scheimpflug imaging in
more recent machines.
 These later technologies include posterior corneal
shape and thickness maps

 Warm (redred) colours indicate a relatively
steepersteeper curvature or more anterioranterior elevation
 Cool (blueblue) colours indicate a relatively flatterflatter
curvature or more posteriorposterior elevation
 Corneal curvatures are numerically described in
dioptres (which are derived mathematically from
the radii of curvature which are what is actually
measured)

 Keratoconus (KCN) and pellucid marginal corneal
degeneration (PMCD) will often evidence
astigmatism
 Ectasia is a risk if these conditions are operated
upon
 Beware asymmetric corneas, drooping against-Beware asymmetric corneas, drooping against-
the-rule astigmatism (“C sign” or “pinch sign”), andthe-rule astigmatism (“C sign” or “pinch sign”), and
exaggerated posterior corneal surface elevationexaggerated posterior corneal surface elevation

 A means of obtaining a detailed refractive map of
the entire optical system through the pupil
 Can quantify higher order optical aberrations
(especially coma and spherical) as well as sphere
and cylinder
 Most excimer lasers may use this information to
drive the refractive correction, but it should be used
only if it corresponds to the patient’s subjective
refraction.
 Iris registration (imaging) improves the accuracy of
the alignment of the laser’s astigmatic correction

 Toric (“cylindrical”) spectacle lens
 Contact lenses
◦ Rigid: imposes a spherical surface over the cornea +/-
toric surface of the contact lens itself
◦ Soft: toric contact lens surface(s)
 Refractive surgery: mainly excimer laser,
intraocular lenses, or corneal incisions.

 Corneal options
◦ Steepen the flat axis (“remove tissue”; excimer laser,
corneal shrinkage techniques E.g. thermal keratoplasty)
◦ Flatten the steep axis (“add tissue”; astigmatic
keratotomy incisions, intracorneal ring segment insertion

 Prior to the advent of the excimer laser AK was
the main method of correcting astigmatism, often
in association with Radial Keratotomy (RK) for
correction of myopia
 Early excimer lasers were only able to treat
spherical error so AK was often used in
conjunction

AK
incision
RK
incision

 AK nomograms were developed to improve predictability.
 Increased patient age, incision length, depth and number are
important determinants of increased effect
 Incisions closer to the corneal apex have more effect
 Incisions near the limbus are called Limbal Relaxing Incisions
(LRIs) and are used in cataract surgery
 “Coupling” describes the observation that an incision will
cause flattening in the axis of the incision and also steepening
in the axis at 90° to the incision. This occurs because the
cornea is a closed physical system, limited by the limbus
Lindstrom RL, Lindquist TD. Surgical correction of postoperative astigmatism. Cornea 1988;
7:138–148

 A guarded micrometer diamond blade is
generally set at 95-100% of thinnest local
pachymetry in virgin corneas – always cuts less
deep than intended
 AK in corneal transplants is unpredictable – one
should always aim for significant
undercorrection
Lindstrom RL, Lindquist TD. Surgical correction of postoperative astigmatism. Cornea
1988; 7:138–148
Price FW, Grene RB, Marks RG, Gonzales JS. Astigmatism reduction clinical trial: a
multicenter prospective evaluation of the predictability of arcuate keratotomy;
evaluation of surgical nomogram predictability. Arch Ophthalmol 1995; 113:277–282;
correction, 577

Astigmatism (D) Incision Type Length (mm) Optical Zone
1.00 One LRI 6.0 At limbus
1.00 to 2.00 Two LRIs 6.0 At limbus
2.00 to 3.00 Two LRIs 8.0 At limbus
>3.00 Two LRIs 8.0 and CRIs as indicated at
3 months postop
LRI = limbal relaxing incision; CRIs = corneal relaxing incisions

 “Always cut on the red” (steep axis)
 Always plan your surgery before you get to the
operating room, and draw a diagram on the
patient’s topographic map for intraoperative
reference
 Mark the patient’s eye preop at the slit lamp with
the eye in the primary position

102° = steep
axis
Paired AK incisions
planned @ 7mm optical
zone. Blade set at 640
microns

 Excimer laser (PRK or LASIK) is effective,
particularly if there is a spherical refractive error
as well
 Most lasers can sculpt either plus or minus
cylinder, or a mixed correction, depending on the
starting refraction
 This flexibility allows planning for minimum tissue
removal with adequate refractive effect

 Most surgeons are happy treating 4 or 5
dioptres of regular cylinder, more in corneal
transplants
 Regression of effect may occur with astigmatic
corrections, just as with spherical corrections
 Epithelial hyperplasia and subepithelial haze are
the main causes of regression
 Many surgeons use Mitomycin C in PRK to
minimise regression

This is absolutely critical for optimum results
 Cornea or limbal conjunctiva can be marked at the
slit lamp with 25g needle and gentian violet ink
 Iris registration images (in some laser platforms)
enable the most accurate alignment

 Point spread
function
 Simulated
snellen letter

 Higher astigmatic corrections may be required
 Retreatments are more common
 Astigmatic keratotomy is unpredictable
 PRK seems to get better results than LASIK
(Lawless M, unpublished data 2008)

 Cytotoxic, crosslinks DNA
 Kills keratocytes, amongst other cells (which later
repopulate the stroma over 6-12 months)
 Prevents haze formation after PRK
 Common dose is 0.02% soaked on a sponge and
applied to the stroma for 10-20 seconds

 Intraocular option
◦ Insertion of toric intraocular lens, either phakic or
pseudophakic
 Use nomograms provided by lens manufacturers
Toric Visian phakic IOL for myopic astigmatism

 The Visian TICL* is available for patients with
myopia between -4.0 and -20.0 and astigmatism of
1D to 4D.
 Posterior chamber insertion, anterior to crystalline
lens
 3.0 mm ACD required (crystalline lens growth
throughout life)
 1-2% rate of anterior cortical cataract formation
 Eyes treated with LASIK on average have three
times more spherical aberration and two times more
coma than the Visian ICL eyes
*Sarver EJ, Sanders DR, Vukich, JA. Image quality in myopic eyes corrected with laser in situ
keratomileusis and phakic intraocular lens. J Refract Surg. 2003;19(4):397-404.

 Relevant because of the increasing use of clear
lens extraction (“refractive lens exchange”) for
treating high hyperopic refractive error

Steep axis of
postoperative
corneal
astigmatism

Alcon Toric IOL calculation
Sheet (online)
 Recommends power &
axis of placement
 Considers surgically-
induced astigmatism
 Again, place the IOL marks
on the steep (red) axis

 Ocular astigmatism of 0.75 D or more will
decrease VA.
 Most surgeons will treat the refractive
astigmatism rather than the corneal astigmatism
(unless lens extraction is part of the surgery).
 Patients often dislike having their astigmatism
over-corrected (axis reversal), or having a
significant axis change.
 Small amounts of astigmatism (<= 0.5D) may
improve depth of focus (and reading ability) in
presbyopes and pseudophakes.

 Any further questions can be directed to the email
address below

Principles and practice of corneal astigmatic surgery

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