IOL UPDATE
| HIGHLY ACCURATE IOL POWER CALCULATIONS —Warren E. Hill, MD, Medical Director, East Valley
Ophthalmology, Mesa, AZ
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| Goals: to make people “20/happy”; discuss objectives with patient; remember that cataract surgery now refractive
as well as rehabilitative procedure; with enough effort, can come within 0.25 diopter (D) of what patient wants;
measurements must be easy to perform; outcomes must be consistent
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| Components of accurate calculations: good patient evaluation (highly accurate calculations not possible in
some individuals); precise keratometry, done properly; accurate and consistent biometry; validation guidelines;
up-to-date formulas for calculating lens power; optimal surgical technique; outcomes tracking and error analysis
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 | Obstacles to highly accurate result: triple procedures; history of keratorefractive surgery; nanophthalmos or macrophthalmos;
indwelling silicone oil
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| Keratometry: have one instrument assigned to keratometry; recalibrate regularly; with autokeratometry, should
have 3 measurements within 0.25 D in each principal meridian; do not use topographer as keratometer; autokeratometer
or Javal-Schiotz keratometer recommended
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| Validation guidelines: have second person check measurement if K readings very flat or steep (<40 D or >48 D;
make sure they sign chart; average power between corneas of >1.5 D nonphysiologic and requires double-checking);
astigmatism >4 D may signal early keratoconus and indicates immediate topography; if something does not
seem right, double-check measurement before moving on
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| Ultrasonography-based: best devices permit adjustment of sound velocity for all gates, have solid probe tip with
central fixation light, good real-time echo display, immersion capability that uses cornea as second spike, and
printouts of the echo display and measurements; applanation A-scan creates artifact through corneal compression,
but amount of compression varies, reducing accuracy; both types of A-scan use sound waves of 10 MHz,
but immersion A-scans more consistent (not more accurate) because they use anterior corneal surface as second
spike, measuring from there to retina; make sure gates aligned correctly; immersion A/B biometry—consists of
simultaneous horizontal B scan and vector A scan; permits measurement from corneal vertex to fovea under direct
visualization (mandatory for patients with staphyloma)
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| IOLMaster: noncontact laser device; can measure phakic, pseudophakic, and polypseudophakic eyes with silicone
oil; however, until recently, less effective on eyes with dense cataracts or certain other types of lens opacities;
uses light at 780-nm wavelength (yields high resolution); measures distance from corneal vertex to retinal pigment
epithelium, making it impervious to variations in retinal thickness; currently, some operator interpretation
necessary, but next generation of machines will overcome; situations that require IOLMaster—nanophthalmos;
extreme axial myopia, especially if patient has staphyloma; presence of silicone oil in vitreous cavity; and presence
of pseudophakia or phakic intraocular lens (IOL); tips for using—axial length display more important than
signal-to-noise ratio; display should have tall slender well-defined primary maxima; ensure there are no double
peaks; take all 20 measurements and sample lens widely to find “sweet spot” with best axial length display; validation
criteria 4 measurements within 0.02 mm of each other on either side; delete all other measurements; measure
around plaque or corneal scar or spoke, staying within measurement reticule; new-generation software will
permit signal processing to remove background noise and produce composite measurement; will allow for measurement
of very dense opaque cataracts
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| Validation guidelines for measuring axial length: have another person double-check measurements and sign
chart if eye unusually long or short, if difference between eyes >0.3 mm, if axial length does not correlate with
refractive data, or if between-eye IOL power calculation >1 D; whenever measurement problematic, “do not
soldier on”; have patient return for repeat measurement when necessary
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| IOL calculation formulas: normal axial length 22 to 24.5 mm; long eye—24.5 to 30 mm; short eye—20 to <22
mm; extreme axial myopia—>30 mm (suspect staphyloma); extreme axial hyperopia—axial length <20 mm
(associated with small corneal diameter, possible nanophthalmia); different formulas suit different types of
eyes; only variation calculating lens position relative to cornea; 2-variable formulas assume anterior and posterior
segments proportional, and keratometry always related to anterior chamber depth; assumptions often
wrong, but axial length and anterior segment size normal in most patients
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| Holladay 1 formula: best for normal to long eyes
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| Hoffer Q formula: recommended for normal to short eyes
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| SRK-T formula: most commonly used formula in North America, but also associated with most mathematical errors;
works well on normal to moderately long eyes, but introduces large refractive error when used on eyes
with short axial length or steep K values plus long axial length
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| Haigis formula: uses a0, a1, and a2 as constants, plus anterior chamber length; designed for use with IOLMaster;
however, only speaker or Dr. Haigis can do required regression analysis
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| Holladay 2 formula: possibly best formula; requires 7 variables, but is “hands-down champion for the unusual
eye”; expensive; optimization requires ≥50 patients
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| Capsulorrhexis: defining portion of surgical procedure; rhexis must be round, centered, and smaller than optic
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| Mean absolute error: absolute error divided by number of patients
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| Calculation components: biometry, keratometry, rhexis configuration, calculation formula, variations in retinal
thickness around fovea, and tolerance of IOL itself; for 0.25 D outcomes with IOLMaster, all components
must be changed accordingly; rhexis as important as keratometry because it keeps lens at plane of zonules;
take-home message—optimize every component to hit preoperative refractive targets consistently
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| IOL POWER CALCULATIONS FOLLOWING KERATOREFRACTIVE SURGERY —Dr. Hill
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| Limitations of power calculation formulas: no way to measure cornea directly (warn patients that corneal
power calculations estimates); unusually steep or flat central corneal power introduces artifact into formula (most
instruments measure intermediate cornea and extrapolate central corneal power)
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| Methods of estimating central corneal power
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| After radial keratotomy (RK): normal ratio between posterior and anterior cornea 82% to 83%; increases post-
RK; incisional myopic procedure and ablative hyperopic procedure (hyperopic laser in-situ keratomileusis
[LASIK]) affect cornea similarly, require same calculations; after RK, central cornea continues flattening
throughout patient’s life (renders historical method useless); best approach to average 1-, 2-, and 3-mm annular
power values from numerical view of Zeiss-Humphrey Atlas topographer, using that as estimate of central
corneal power; or use Holladay diagnostic summary of intraoperative suture adjustment (ISA) system; patient
will have transient hyperopia immediately postoperatively (may shift from 3 D immediately after surgery to -
1 D or -2 D few weeks later; patient should have at least 1 D to 1.5 D hyperopia during first postoperative
weeks); rule of twos—2 stable refractions on 2 consecutive visits 2 mo after surgery; indicates cornea stable
enough for power calculation
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| Myopic LASIK: ratio between posterior and anterior cornea decreases; historical method of power calculation can
be used, but preoperative cycloplegic refraction and K values plus stable refractive error 4 to 6 mo postoperatively
necessary; most methods in current use estimates, not precise calculations; Holladay equivalent K feature
of Pentacam uses Scheimpflug image to measure central corneal power to within 4 mm (effective but
should not be only method in ophthalmologist’s arsenal); calculate IOL power using historical methods, objective
methods, and combination of these, and see how they compare; Holladay 2 formula probably single best
currently available way of neutralizing extremely flat central corneal power; if Holladay 2 formula unavailable,
Aramberri double K formula works well in conjunction with SRK-T, Hoffer Q, or Holladay 1 formulas;
Haigis L formula objective and part of software package with IOLMaster; Masket et al demonstrated linear relationship
between error returned by simulated keratometry and amount of laser vision correction (holds up for
both hyperopic and myopic LASIK)
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| Maximizing accuracy: Feiz-Mannis calculation overcorrects (use to estimate absolute upper limit of lens power);
manual or simulated Ks undercorrect (use as lower limit); use as much information as possible to calculate actual
power
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| Hyperopic LASIK: relationship between posterior and anterior cornea similar to that achieved with RK; average
annular power values taken at 1, 2, 3, and 4 mm to estimate corneal power (correction also necessary); standard
spherical IOL or ordinary IOL good lens choices; with wavefront-guided LASIK or RK involving <8 incisions,
aspheric or IQ lenses appropriate; Technis lens recommended for patients who have had more extensive LASIK
or RK procedures
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| When to recheck measurements: if power difference between eyes >2 D; calculated lens power unusually high or
low; average K between eyes differs by >3 D
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| THE ACRY S OF R E STOR LENS: WHAT CAN IT DELIVER ?—Martin G. Edwards, MD, Staff Ophthalmologist,
Hartford Hospital, Farmington, CT
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| Description of lens: multifocal, with apodized diffractive central component and refractive peripheral component
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| Multifocal: lens optic has 2 primary focal points (one for distance, one for near; patient perceives only focused
image); adverse visual effects include glare, halo, and starburst
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| Diffraction: occurs when light waves encounter irregularity in medium through which they travel (eg, scratch in
windshield); diffractive lenses with stepped structures divide light between 2 images
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| Apodization: refers to change in lens property (from center to periphery) that occurs radially
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| ReSTOR lens components: optic has 3.6-mm central diffractive structure, with +4 correction at center (produces
≈3.2-D addition at spectacle plane); step height decreases gradually from 1.3 µ in center to 0.2 µ at periphery,
which is completely refractive; end result to direct more light to distant foci when light low (with corresponding increase
in pupil size); periphery has no diffractive rings; goal to reduce glare and halo at low light; system’s energy
balance varies with pupil size (in bright light, when pupil small, lens directs more light to near foci); system mimics
natural pupillary response to light
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| Clinical indications and pearls: desire for spectacle independence (expectations must be realistic; patients must
understand risks and benefits of lens extraction and IOL implantation); no other ocular pathology; <1 D of corneal
astigmatism; lens should center well intraoperatively and remain centered after surgery; relative contraindications
include previous refractive surgery and occupational night driving; success greatest when bilateral
implants used; -2 D myope who wears glasses only for night driving may not be good candidate; accurate biometry
and careful monitoring of refractive outcomes important; surgeons should have low rate of complications
and ability to spend extra time educating patients; also should be prepared to address refractive “surprises” with
lens exchange or refractive surgery
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| Outcomes: according to data submitted to Food and Drug Administration (FDA) by manufacturer, 80% of patients
no longer needed glasses following bilateral implantation; 17% used glasses occasionally; 3% required glasses always;
13% experienced moderate-to-severe problems with night vision; 26% experienced moderate-to-severe
glare; and 24% of patients reported halos; all problems more frequent than in control group with monofocal lenses;
other studies have yielded similar results
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Suggested Reading
Aramberri J: Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract
Surg 29:2063, 2003; Chiam PJ et al: ReSTOR intraocular lens implantation in cataract surgery: quality of vision.
J Cataract Refract Surg 32:1459, 2006; Koch DD, Wang L: Calculating IOL power in eyes that have had
refractive surgery. J Cataract Refract Surg 29:2039, 2003; Kohnen T et al: European multicenter study of the
AcrySof ReSTOR apodized diffractive intraocular lens. Ophthalmology 113:584, 2006; Latkany RA et al: Intraocular
lens calculations after refractive surgery. J Cataract Refract Surg 31:562, 2005; Masket S, Masket SE:
Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser
photoablation. J Cataract Refract Surg 32:430, 2006; Narvaez J et al: Accuracy of intraocular lens power prediction
using the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas. J Cataract Refract Surg 32:2050, 2005;
Olsen T: Improved accuracy of intraocular lens power calculation with the Zeiss IOLMaster. Acta Ophthalmol
Scand 85:84, 2007; Souza CE et al: Visual performance of AcrySof ReSTOR apodized diffractive IOL: a prospective
comparative trial. Am J Ophthalmol 141:827, 2006; Vogel A et al: Reproducibility of optical biometry using
partial coherence interferometry: intraobserver and interobserver reliability. J Cataract Refract Surg 27:1961, 2001;
Wang L et al: Comparison of intraocular lens power calculation methods in eyes that have undergone laser-assisted
in-situ keratomileusis. Trans Am Ophthalmol Soc 102:189, 2004; Wang L et al: Methods of estimating corneal refractive
power after hyperopic laser in situ keratomileusis. J Cataract Refract Surg 28:954, 2002.
Educational Objectives
| The goal of this program is to help ophthalmologists calculate intraocular lens (IOL) power with maximum accuracy,
and to provide a comprehensive review of the advantages and disadvantages of the AcrySof ReSTOR lens. After
hearing and assimilating this program, the listener will be better able to:
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| 1. Discuss the goals of highly accurate IOL measurement.
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| 2. List the components of an accurate IOL power calculation.
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| 3. Recognize the limitations of formulas used for calculating IOL power.
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| 4. Describe the basic design of the ReSTOR lens.
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| 5. Name the most common adverse effects associated with the ReSTOR lens.
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Faculty Disclosure
In adherence to ACCME guidelines, the Audio-Digest Foundation requests all lecturers to disclose any significant financial
relationship with the manufacturer or provider of any commercial product or service discussed. The following
has been disclosed: Dr. Hill is a consultant for Alcon Laboratories, Carl Zeiss Meditec, and Santen.
Acknowledgements
Drs. Hill and Edwards were recorded at New Options for Treatment of Refractive Error: Biometry and Intraocular
Lens Selection, held December 8, 2006, in Boston, MA, and sponsored by the New England Ophthalmological Society.
The Audio-Digest Foundation thanks the speakers and the sponsor for their cooperation in the production of this
program.
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