principles and practice of corneal astigmatic surgery

Dr Laurie Sullivan FRANZCOMelbourne, AustraliaCorneal Clinic, RVEEH, East MelbourneBayside Eye SpecialistsLasersight

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 [email protected] 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 Beware asymmetric corneas, drooping against-the-rule astigmatism (“C sign” or against-the-rule astigmatism (“C sign” or “pinch sign”), and exaggerated posterior “pinch sign”), and exaggerated posterior corneal surface elevationcorneal 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 limbus1.00 to 2.00 Two LRIs 6.0 At limbus2.00 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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