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IOLs for Cataracts after LASIK - A Big Bad Surprise
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| gospa68@aol.com 2005-08-15, 10:55 pm |
| Accurate lens position, corneal power needed for calculations in
post-refractive surgery patients
Surgeons implanting IOLs in patients who have undergone corneal
refractive surgery must be precise in their measurements and
calculations.
Nicole Nader
---------------------------------------------------------------------------=
-----
Calculating the correct IOL power for a patient who has undergone
previous corneal refractive surgery is a challenge. Miscalculations in
these patients have led in some cases to large "refractive
surprises" requiring lens exchange or a secondary piggyback IOL, said
Jack T. Holladay, MD, MSEE, FACS.
To prevent these surprises from occurring, surgeons must be precise in
obtaining preoperative measurements and exact in their prediction of
the effective lens position (ELP) in order to achieve the best surgical
outcome, Dr. Holladay said.
"Carrying out successful cataract surgery, refractive lens exchange
or phakic IOL implantation after LASIK, PRK, RK or LASEK requires the
right ingredients," he said. "You need to know the ELP and the net
corneal power. These two variables are the most difficult to determine
precisely of the four preoperative variables (axial length,
keratometry, ELP and desired postoperative refraction) necessary to
calculate the proper IOL power."
Average ELP
The average ELP, or manufacturer's lens constant, for each IOL is
listed on the product packaging, along with the A-constant and the
surgeon factor. Dr. Holladay noted that most IOL manufacturers
misleadingly refer to the ELP as the "anterior chamber depth," or
ACD. He said this label is an antiquated misnomer because it refers to
the anterior chamber depth of the eye, when the measurement actually
represents the effective lens position of the IOL relative to the
corneal vertex.
The ELP measurement provided by lens manufacturers is the average value
of the position of the IOL within the eye when measured from the
corneal vertex, Dr. Holladay said. The ELP value for each lens model or
style is averaged from data collected from thousands of patients, he
said.
The ELP and surgeon factor measurements are expressed in millimeters,
while A-constants are expressed in diopters. Although these factors are
measured in different units of measurement, all are equivalent in
value. Each can be converted to the other types of measurements, like a
linear distance converted from feet to meters, Dr. Holladay said.
Conversion formulas from A-constant to ELP and surgeon factor are as
follows:
ELP =3D (Aconst * 0.5663) - 65.600 + 3.595
0=2E9704
SF =3D (Aconst * 0.5663) - 65.600
Dr. Holladay noted that inconsistent values for A-constant and ELP
occasionally appear on IOL packaging because the manufacturer has not
updated the lens constants with a conversion formula that he developed
in 1997.
"When the converted lens constants are inconsistent, it is usually
because one value (usually ACD/ELP) is for in-the-sulcus positioning
and the other is for in-the-bag. If you find a set of lens constants
for an IOL that are not consistent, then the higher value is usually
the best value and is for in-the-bag placement," he said.
Determining individual ELP
IOL power calculation formulas use the manufacturer's ELP to help
predict the specific ELP for each patient, he said. Individual ELP is
determined by starting with the manufacturer's ELP for the average
patient and then using preoperative biometric measurements to determine
the value for the specific patient.
The Holladay II formula, a fifth-generation IOL formula introduced by
Dr. Holladay in 2000, factors in the patient's axial length,
keratometry, horizontal corneal diameter, lens thickness, preoperative
refraction and age, in addition to the lens model's ELP, to determine
the individual ELP for the patient.
In the course of developing the Holladay II formula, Dr. Holladay found
that the horizontal corneal diameter (commonly called the
white-to-white measurement) is a key anatomical factor that is helpful
in predicting individual ELP.
"The corneal white-to-white measurement is probably the most
important element in judging the size of the anterior segment and
indicating the depth of the IOL within the eye," Dr. Holladay said.
The average horizontal white-to-white measurement in a normal eye is
11.7 mm, he said.
"Ninety-five percent of people have a white-to-white measurement
between 12.5 mm and 10.8 mm," Dr. Holladay said. "Patients with a
measurement of 12.5 or greater have a large anterior segment, while
patients with 10.5 mm or less have a small anterior segment."
Nine types of eyes
In doing the research that led to the Holladay II formula, Dr. Holladay
found that there is little correlation between the size of the anterior
segment - small, normal or large - and the patient's axial
length.
"We used to think that these two factors, anterior segment and axial
length, were proportional," Dr. Holladay said. "But we determined
that the size of the anterior segment and the length of the eye in the
posterior compartment are far enough apart that they only correlate
about 10% to 20% of the time."
This finding led to the conclusion that, instead of three types of eyes
(small, normal or large), there are nine possible types of eyes, with
three sizes of anterior chamber and the additional independent variable
of short, normal or long axial length. (See accompanying chart.)
"With this system, we determined that 80% of short eyes and 90% of
long eyes have normal anterior segment sizes," Dr. Holladay noted.
Building these differentiations in the types of eyes into the Holladay
II enabled the formula to predict ELP more accurately in shorter eyes,
Dr. Holladay said. This helped surgeons avoid the"5 D surprise"
that was often caused by ELP formulas that preceded the Holladay II
formula, he said.
The ELP, the predicted position of the IOL within the eye, is an
important factor in modern IOL formulas because it is the only variable
that cannot be measured or chosen by the surgeon, Dr. Holladay said.
The surgeon has no control over the prediction of the ELP for a
specific patient other than choosing the formula for the calculation,
he said.
The Holladay II formula uses seven variables to predict the ELP (axial
length, keratometry, horizontal white-to-white, anterior chamber depth,
lens thickness, age and current refraction of the patient). The
original Holladay I uses two (axial length and keratometry) as do other
third-generation formulas such as the Hoffer Q and SRK/T, he said.
The additional five measurements are especially helpful in precisely
predicting the ELP in short eyes (< 22 mm), he said.
Once the IOL formula has been chosen; the corneal power, axial length,
white-to-white, ACD and lens thickness have been measured; age and
current refraction have been determined; and the desired postoperative
refraction chosen, all of the necessary ingredients are ready to be
entered into the vergence formula. The accurate determination of the
net corneal power of the front and back surfaces of the cornea can be
achieved with tomography and calculations detailed in this article.
"The vergence formula (shown below), relating the targeted
refraction, IOL, corneal power, individual ELP and the retina is more
than 140 years old," Dr. Holladay said. "The only difference in
today's theoretical formulas is the method of predicting the ELP."
Axial length in long eyes
Dr. Holladay said the surgeon must be "extra cautious" in measuring
axial length in long eyes.
"Eyes with axial lengths that are 26 mm or longer can have a myopic
staphyloma. This means that the scleral fibers in the back of the
sclera, where the fovea is, are weakened and bulge out," he said.
Traditional ultrasonic measurements, which measure axial length to the
deepest point where the ultrasound wave is perpendicular to the retina,
can scan deep into a staphyloma and may miss the fovea completely, he
said.
"In highly myopic patients with staphyloma, the fovea can be mid-way
up the staphyloma or on the rim," Dr. Holladay said. As a result, he
said, the anatomic axial length (at the posterior pole) can be up to 3
mm longer than the optical axial length (at the fovea).
"For every millimeter difference between the optical axial length and
the anatomic axial length we make a 2.5 D to 3 D surprise," Dr.
Holladay said.
In the Journal of Cataract and Refractive Surgery in 2000, Dr. Holladay
and Roberto Zaldivar, MD, reported on this finding. They found that
patients with axial length measurements greater than 26.5 mm (up to 35
mm) had anatomic and optical axial lengths that differed by up to 3 mm,
which would cause a 9 D error in the power of their IOL.
To avoid this problem, Drs. Holladay and Zaldivar said surgeons should
measure patients with long eyes (26 mm or more in length) with light
instead of ultrasound. A partial interferometry device such as the Carl
Zeiss Meditec IOLMaster can accurately measure axial length to the
fovea by allowing patients to fixate on a target, he said.
"It is crucial that you measure highly myopic long eyes with light.
The IOLMaster is the only currently available technology that uses
light and not ultrasound," Dr. Holladay said. He noted, however, that
the IOLMaster cannot measure eyes with dense cataract, because
opacification prevents the coherent light from forming a measurable
interference pattern. In these patients, ultrasound is the only option.
Central corneal measurement
After the anatomic factors explained above have been accurately
measured, corneal power must be determined before the IOL power can be
calculated correctly, Dr. Holladay said. He said it is particularly
difficult to determine the corneal power of an eye that has undergone
corneal refractive surgery such as LASIK, PRK, or RK because the
traditional tools surgeons have to measure corneal power are
inadequate; they were originally created to measure the corneal power
of an unaltered cornea.
"Our current instruments don't give us an accurate measurement of
corneal power," Dr. Holladay said. Keratometers and topographers are
limited in their ability to measure surgically treated corneas because
they take paracentral measurements and do not truly measure the center
of the cornea.
"There is very little correlation between the paracentral measurement
and what's going on in the center of the cornea," Dr. Holladay
said. "Topographers and keratometers have a central scotoma from 1.5
to 3 mm in diameter where no measurements are taken, and this central
area is the most important in the patient's vision and the true
corneal power."
The center of the cornea is the most critical area for calculating the
corneal power of a patient who has had refractive surgery, he said, and
yet it is the one area that is not truly measured by the available
technologies. These tools miss this critical zone, which increases in
size with the amount of refractive surgical correction.
"On the average patient with a 44 D cornea, the keratometer measures
3=2E2 mm apart in diameter," Dr. Holladay said. "This means, at the
corneal center, everything less than 3.2 mm in diameter is lost. This
isn't a problem in a patient who hasn't had corneal refractive
surgery, but in a refractive patient not measuring the central area
causes a significant error."
For example, he said, a patient with a cornea that measures at 36.5 D
after -10 D laser surgery actually has a central anterior corneal
power that is 15% of his refractive change flatter (-10 =D7 15% =3D
-1.5 D) than a patient with a 36.5 D cornea who has not had surgery.
The refractive surgery patient who has had the -10 D LASIK and
measures 36.5 D is actually about 35 D (36.5 - 1.5) in the center of
the anterior cornea, Dr. Holladay said.
Deriving net power
A second limitation of keratometers and topographers is that they fail
to measure the posterior surface of the cornea, which is needed for
calculating net corneal power.
"Topographers and keratometers only measure the front surface of the
cornea," Dr. Holladay said. "They assume that the back and front
surface power of the cornea are equal. This isn't true, of course.
The back radius of the cornea is steeper than the front, approximately
82% of the front radius of the cornea."
Most IOL formulas and programs use a keratometric index of 1.3375 when
converting from corneal power to radius using the keratometric formula
below:
337.5/radius of curvature of the cornea (in mm) =3D power of the cornea
Being aware of net power, the authors of IOL formulas have compensated
for the front-and-back corneal ratio by reducing the keratometric power
(1.3375) of the cornea to a net power by using a "net" index of
refraction, Dr. Holladay said.
"Unfortunately, there is no exact agreed-upon value for this," he
said, "so the net index refraction varies from 1.3315 to 1.3333
depending on the IOL formula."
This results in reducing the keratometric power of the cornea by
0=2E3315/0.3375 or 0.3333/0.3375, depending on the formula, to 98.22% or
98.76% of the measured power. For a keratometric power of 44 D, the net
value would be from 43.22 to 43.45 D, which is 0.55 to 0.78 D less than
the measured keratometric power, he said.
"The power has to be reduced, otherwise the power of the cornea will
be overestimated," Dr. Holladay said.
After corneal refractive surgery, the back surface of the cornea is no
longer 82% of the front surface, as it is in a normal cornea.
Therefore, there is a second error in the net corneal power, which is
10% of the refractive change (from the above example, -10 D =D7 10% =3D
1 D).
The peripheral sampling error outlined above equals 15% of the
refractive surgery treatment, and the change in the ratio of the back
to front surface adds another 10%, so the total error in the
keratometric reading is 25% (15% + 10%) of the effect of the refractive
surgery. So a patient with a keratometry reading of 36.5 D after a
-10 D LASIK is actually 34 D (36.5 - 25% =D7 -10).
Corneal power calculations
Because, on their own, keratometric and topographic measurements of the
cornea are inadequate after corneal refractive surgery, Dr. Holladay
has published four mathematical methods to determine and validate the
true power of the cornea in these patients.
"The four methods are the historical method, two modified historical
methods and the hard contact lens method," Dr. Holladay said.
"While none of these is perfect, they are better than using the
keratometric or topographic reading."
In the examples that follow, the same corneal power measurement (39.50
D) is derived as the answer by each of the four methods. In a
"perfect world," Dr. Holladay said, the answers would always be
equivalent, thereby making it easy for the surgeon to choose the
corneal power to plug into the vergence formula. However, he said, in
real life the corneal powers derived from the historical and contact
lens methods are "rarely" the same numeric value when calculated
for a single patient.
"After myopic surgery, the corneal measurements are never the
same," he said. "So you should always use the flattest power that
you get. This avoids getting a refractive surprise later on, because
you avoided believing that the cornea was steeper than it really
was." After hyperopic refractive surgery the opposite is true,
because the true corneal power is higher than the measured value, he
said.
Historical methods
Using the historical method, the surgeon subtracts the patient's
surgically induced refractive change from the preoperative keratometry
reading to determine the current corneal power.
"So if the patient was a -4 D myope with 44 D of corneal power
before surgery and turned out to be +0.50 D after surgery, he underwent
a -4.5 D change," Dr. Holladay explained. "You should be able to
subtract this change from the preoperative K-reading, which would give
you a corneal power of 39.50 D."
In the first modified historical method, the keratometry reading is
used, along with the surgeon's "best guess" at the refractive
change, if preop keratometry readings are not available. As in the
example above, where 25% of the power of the refractive change was used
to compensate for paracentral sampling and the change in front and back
ratio of the corneal surfaces, Dr. Holladay said 25% of the power of
the refractive change is subtracted from the keratometric measurement.
"The keratometer after myopic refractive surgery makes an
overestimate of the corneal power by 25% of the refractive change,"
he said. "If you get a measurement of 40.58, with 4.5 D as the
refractive change, you take 25% of that value, which is 1.08. Subtract
this value from 40.58 and you again get 39.50 D," Dr. Holladay said.
"This is the corneal power that you enter into your IOL program."
In the second modified historical method, topography is used instead of
keratometry.
"Topography gets closer to the center of the cornea than does
keratometry, so the topographer reading needs to be reduced by only 15%
of the refractive change rather than 25%," Dr. Holladay said.
"Therefore, if the topographer central refractive power were 40.18,
then 15% of the -4.50 D refractive change is -0.68 D, so the
calculated power would be 39.50 D."
When obtaining the topography measurements, Dr. Holladay said, the
surgeon should be careful not to use the simulated Ks from the
topographer. These measurements are identical to keratometry
measurements.
"Use the central refractive power as reported by the topographer,"
he said. "Even though it doesn't truly know what the central
corneal measurements are, it extrapolates them. If the measurements
don't automatically come up on the map, click on the central area
with your mouse a few times and take an average of those values."
Contact lens method
In the fourth calculation method, a rigid contact lens is used. Dr.
Holladay gave the example of a patient whose refraction is =EC0.50 after
refractive surgery. If this changes to a refraction of -0.5 D with a
41 D contact lens in place on the cornea, the front corneal curvature
must be 1 D flatter than 41 D or 40 D (41 D - 1 D =3D 40 D).
The contact lens method does not compensate for the change in
back-to-front surface ratio, Dr. Holladay said, "so once you get this
sum, you still need to reduce by about 10%, just like with topography,
because the back surface of the cornea is not consistent with the
front. This leaves you with a value of 39.55. That's the calculated
power."
Measuring front and back
Until better ways of measuring and calculating the total net power of
the cornea are available, surgeons must rely on one or more of these
calculations to determine corneal power. But Dr. Holladay said that in
the very near future surgeons may have a tool available that will
eliminate the need for calculations by accurately measuring the front
and back surfaces of the cornea.
The Pentacam, from Oculus, "has the potential to be four to five
times more accurate in measuring corneal power than any of the
historical or contact lens methods, because it measures the back radius
curvature of the cornea as well as the front," Dr. Holladay said.
The Pentacam measures the tomography of the cornea by taking 50
meridional Scheimpflug images. Dr. Holladay explained.
"The device eliminates eye movement by having a consistent central
point, so it makes the precision of the instrument far more accurate
than any other tomographer I've ever seen," he said.
In a study he conducted, Dr. Holladay determined that the net power of
the cornea can be measured by the Pentacam to within =B10.55 D.
"We can measure a cornea that has undergone corneal refractive
surgery to within =B10.5 D of its actual power," he said.
Dr. Holladay is currently working with Oculus to incorporate a display
into the Pentacam called the Holladay Report, which he said will
accurately calculate the front and back surface powers of the cornea,
adjust for any power overestimate, and report a term called the
equivalent keratometric reading, or EKR.
When this report is incorporated into the Pentacam software, he said,
surgeons will be able to use the EKR in IOL calculation software just
as they would a standard keratometry reading.
For Your Information:
Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK
Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200,
Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail:
docholladay@docholladay.com; www.docholladay.com.
References:
Holladay JT. Standardizing constants for ultrasonic biometry,
keratometry, and intraocular lens power calculations. J Cataract
Refract Surg. 1997;23(9):1356-1370.
Zaldivar R, Shultz MC, et al. Intraocular lens power calculations in
patients with extreme myopia. J Cataract Refract Surg. 2000;26;
668-674.
Holladay JT. Intraocular lens power calculations for the refractive
surgeon. Operative Techniques in Cataract and Refractive Surgery.
1998;1:105-117.
Carl Zeiss Meditec, maker of the IOLMaster, can be reached at 5160
Hacienda Drive, Dublin CA 94568; 877-486-7473; fax: 925-557-4778; Web
site: humphrey.com.
Oculus Inc., maker of the Pentacam, can be reached at Oculus
Optikger=E4te GmbH, M=FCnchholzh=E4user Str. 29, D-35549 Wetzlar, Germany;
49-641-2005-0; fax: 49-641-2005-255; e-mail: sales@oculus.de.
Nicole Nader is an OSN Correspondent based in Philadelphia, who is
writing the QOV series.
| |
| Glenn - USAEyes.org 2005-08-15, 10:55 pm |
| That is pretty funny WizKid. Your subject line has "Big Bad Surprise"
in it, and then the article you post shows the formula to best
determine the correct intraocular lens power for cataract surgery
after refractive surgery. I would think a "Big Bad Surprise" would not
have such a ready solution.
Glenn Hagele
Executive Director
USAEyes.org
"Consider and Choose With Confidence"
Email to glenn dot hagele at usaeyes dot org
http://www.USAEyes.org
http://www.ComplicatedEyes.org
I am not a doctor.
| |
| EyesRBAD 2005-08-17, 10:57 pm |
| Dry eye misery now, misery later from wrong IOL power calculations. LASIK is
such a great
surgery. Even if the surgeon knows the patient has had LASIK, the cornea is
so complex and
aberrated it is really tough for them to get the cateract surgery power
prediction right.
They just can't plan or TRY to be precise, it isn't within their ability.
LASIK is a wildcard.
<gospa68@aol.com> wrote in message
news:1124150379.514592.167880@g49g2000cwa.googlegroups.com...
Accurate lens position, corneal power needed for calculations in
post-refractive surgery patients
Surgeons implanting IOLs in patients who have undergone corneal
refractive surgery must be precise in their measurements and
calculations.
Nicole Nader
--------------------------------------------------------------------------------
Calculating the correct IOL power for a patient who has undergone
previous corneal refractive surgery is a challenge. Miscalculations in
these patients have led in some cases to large "refractive
surprises" requiring lens exchange or a secondary piggyback IOL, said
Jack T. Holladay, MD, MSEE, FACS.
To prevent these surprises from occurring, surgeons must be precise in
obtaining preoperative measurements and exact in their prediction of
the effective lens position (ELP) in order to achieve the best surgical
outcome, Dr. Holladay said.
"Carrying out successful cataract surgery, refractive lens exchange
or phakic IOL implantation after LASIK, PRK, RK or LASEK requires the
right ingredients," he said. "You need to know the ELP and the net
corneal power. These two variables are the most difficult to determine
precisely of the four preoperative variables (axial length,
keratometry, ELP and desired postoperative refraction) necessary to
calculate the proper IOL power."
Average ELP
The average ELP, or manufacturer's lens constant, for each IOL is
listed on the product packaging, along with the A-constant and the
surgeon factor. Dr. Holladay noted that most IOL manufacturers
misleadingly refer to the ELP as the "anterior chamber depth," or
ACD. He said this label is an antiquated misnomer because it refers to
the anterior chamber depth of the eye, when the measurement actually
represents the effective lens position of the IOL relative to the
corneal vertex.
The ELP measurement provided by lens manufacturers is the average value
of the position of the IOL within the eye when measured from the
corneal vertex, Dr. Holladay said. The ELP value for each lens model or
style is averaged from data collected from thousands of patients, he
said.
The ELP and surgeon factor measurements are expressed in millimeters,
while A-constants are expressed in diopters. Although these factors are
measured in different units of measurement, all are equivalent in
value. Each can be converted to the other types of measurements, like a
linear distance converted from feet to meters, Dr. Holladay said.
Conversion formulas from A-constant to ELP and surgeon factor are as
follows:
ELP = (Aconst * 0.5663) - 65.600 + 3.595
0.9704
SF = (Aconst * 0.5663) - 65.600
Dr. Holladay noted that inconsistent values for A-constant and ELP
occasionally appear on IOL packaging because the manufacturer has not
updated the lens constants with a conversion formula that he developed
in 1997.
"When the converted lens constants are inconsistent, it is usually
because one value (usually ACD/ELP) is for in-the-sulcus positioning
and the other is for in-the-bag. If you find a set of lens constants
for an IOL that are not consistent, then the higher value is usually
the best value and is for in-the-bag placement," he said.
Determining individual ELP
IOL power calculation formulas use the manufacturer's ELP to help
predict the specific ELP for each patient, he said. Individual ELP is
determined by starting with the manufacturer's ELP for the average
patient and then using preoperative biometric measurements to determine
the value for the specific patient.
The Holladay II formula, a fifth-generation IOL formula introduced by
Dr. Holladay in 2000, factors in the patient's axial length,
keratometry, horizontal corneal diameter, lens thickness, preoperative
refraction and age, in addition to the lens model's ELP, to determine
the individual ELP for the patient.
In the course of developing the Holladay II formula, Dr. Holladay found
that the horizontal corneal diameter (commonly called the
white-to-white measurement) is a key anatomical factor that is helpful
in predicting individual ELP.
"The corneal white-to-white measurement is probably the most
important element in judging the size of the anterior segment and
indicating the depth of the IOL within the eye," Dr. Holladay said.
The average horizontal white-to-white measurement in a normal eye is
11.7 mm, he said.
"Ninety-five percent of people have a white-to-white measurement
between 12.5 mm and 10.8 mm," Dr. Holladay said. "Patients with a
measurement of 12.5 or greater have a large anterior segment, while
patients with 10.5 mm or less have a small anterior segment."
Nine types of eyes
In doing the research that led to the Holladay II formula, Dr. Holladay
found that there is little correlation between the size of the anterior
segment - small, normal or large - and the patient's axial
length.
"We used to think that these two factors, anterior segment and axial
length, were proportional," Dr. Holladay said. "But we determined
that the size of the anterior segment and the length of the eye in the
posterior compartment are far enough apart that they only correlate
about 10% to 20% of the time."
This finding led to the conclusion that, instead of three types of eyes
(small, normal or large), there are nine possible types of eyes, with
three sizes of anterior chamber and the additional independent variable
of short, normal or long axial length. (See accompanying chart.)
"With this system, we determined that 80% of short eyes and 90% of
long eyes have normal anterior segment sizes," Dr. Holladay noted.
Building these differentiations in the types of eyes into the Holladay
II enabled the formula to predict ELP more accurately in shorter eyes,
Dr. Holladay said. This helped surgeons avoid the"5 D surprise"
that was often caused by ELP formulas that preceded the Holladay II
formula, he said.
The ELP, the predicted position of the IOL within the eye, is an
important factor in modern IOL formulas because it is the only variable
that cannot be measured or chosen by the surgeon, Dr. Holladay said.
The surgeon has no control over the prediction of the ELP for a
specific patient other than choosing the formula for the calculation,
he said.
The Holladay II formula uses seven variables to predict the ELP (axial
length, keratometry, horizontal white-to-white, anterior chamber depth,
lens thickness, age and current refraction of the patient). The
original Holladay I uses two (axial length and keratometry) as do other
third-generation formulas such as the Hoffer Q and SRK/T, he said.
The additional five measurements are especially helpful in precisely
predicting the ELP in short eyes (< 22 mm), he said.
Once the IOL formula has been chosen; the corneal power, axial length,
white-to-white, ACD and lens thickness have been measured; age and
current refraction have been determined; and the desired postoperative
refraction chosen, all of the necessary ingredients are ready to be
entered into the vergence formula. The accurate determination of the
net corneal power of the front and back surfaces of the cornea can be
achieved with tomography and calculations detailed in this article.
"The vergence formula (shown below), relating the targeted
refraction, IOL, corneal power, individual ELP and the retina is more
than 140 years old," Dr. Holladay said. "The only difference in
today's theoretical formulas is the method of predicting the ELP."
Axial length in long eyes
Dr. Holladay said the surgeon must be "extra cautious" in measuring
axial length in long eyes.
"Eyes with axial lengths that are 26 mm or longer can have a myopic
staphyloma. This means that the scleral fibers in the back of the
sclera, where the fovea is, are weakened and bulge out," he said.
Traditional ultrasonic measurements, which measure axial length to the
deepest point where the ultrasound wave is perpendicular to the retina,
can scan deep into a staphyloma and may miss the fovea completely, he
said.
"In highly myopic patients with staphyloma, the fovea can be mid-way
up the staphyloma or on the rim," Dr. Holladay said. As a result, he
said, the anatomic axial length (at the posterior pole) can be up to 3
mm longer than the optical axial length (at the fovea).
"For every millimeter difference between the optical axial length and
the anatomic axial length we make a 2.5 D to 3 D surprise," Dr.
Holladay said.
In the Journal of Cataract and Refractive Surgery in 2000, Dr. Holladay
and Roberto Zaldivar, MD, reported on this finding. They found that
patients with axial length measurements greater than 26.5 mm (up to 35
mm) had anatomic and optical axial lengths that differed by up to 3 mm,
which would cause a 9 D error in the power of their IOL.
To avoid this problem, Drs. Holladay and Zaldivar said surgeons should
measure patients with long eyes (26 mm or more in length) with light
instead of ultrasound. A partial interferometry device such as the Carl
Zeiss Meditec IOLMaster can accurately measure axial length to the
fovea by allowing patients to fixate on a target, he said.
"It is crucial that you measure highly myopic long eyes with light.
The IOLMaster is the only currently available technology that uses
light and not ultrasound," Dr. Holladay said. He noted, however, that
the IOLMaster cannot measure eyes with dense cataract, because
opacification prevents the coherent light from forming a measurable
interference pattern. In these patients, ultrasound is the only option.
Central corneal measurement
After the anatomic factors explained above have been accurately
measured, corneal power must be determined before the IOL power can be
calculated correctly, Dr. Holladay said. He said it is particularly
difficult to determine the corneal power of an eye that has undergone
corneal refractive surgery such as LASIK, PRK, or RK because the
traditional tools surgeons have to measure corneal power are
inadequate; they were originally created to measure the corneal power
of an unaltered cornea.
"Our current instruments don't give us an accurate measurement of
corneal power," Dr. Holladay said. Keratometers and topographers are
limited in their ability to measure surgically treated corneas because
they take paracentral measurements and do not truly measure the center
of the cornea.
"There is very little correlation between the paracentral measurement
and what's going on in the center of the cornea," Dr. Holladay
said. "Topographers and keratometers have a central scotoma from 1.5
to 3 mm in diameter where no measurements are taken, and this central
area is the most important in the patient's vision and the true
corneal power."
The center of the cornea is the most critical area for calculating the
corneal power of a patient who has had refractive surgery, he said, and
yet it is the one area that is not truly measured by the available
technologies. These tools miss this critical zone, which increases in
size with the amount of refractive surgical correction.
"On the average patient with a 44 D cornea, the keratometer measures
3.2 mm apart in diameter," Dr. Holladay said. "This means, at the
corneal center, everything less than 3.2 mm in diameter is lost. This
isn't a problem in a patient who hasn't had corneal refractive
surgery, but in a refractive patient not measuring the central area
causes a significant error."
For example, he said, a patient with a cornea that measures at 36.5 D
after -10 D laser surgery actually has a central anterior corneal
power that is 15% of his refractive change flatter (-10 × 15% =
-1.5 D) than a patient with a 36.5 D cornea who has not had surgery.
The refractive surgery patient who has had the -10 D LASIK and
measures 36.5 D is actually about 35 D (36.5 - 1.5) in the center of
the anterior cornea, Dr. Holladay said.
Deriving net power
A second limitation of keratometers and topographers is that they fail
to measure the posterior surface of the cornea, which is needed for
calculating net corneal power.
"Topographers and keratometers only measure the front surface of the
cornea," Dr. Holladay said. "They assume that the back and front
surface power of the cornea are equal. This isn't true, of course.
The back radius of the cornea is steeper than the front, approximately
82% of the front radius of the cornea."
Most IOL formulas and programs use a keratometric index of 1.3375 when
converting from corneal power to radius using the keratometric formula
below:
337.5/radius of curvature of the cornea (in mm) = power of the cornea
Being aware of net power, the authors of IOL formulas have compensated
for the front-and-back corneal ratio by reducing the keratometric power
(1.3375) of the cornea to a net power by using a "net" index of
refraction, Dr. Holladay said.
"Unfortunately, there is no exact agreed-upon value for this," he
said, "so the net index refraction varies from 1.3315 to 1.3333
depending on the IOL formula."
This results in reducing the keratometric power of the cornea by
0.3315/0.3375 or 0.3333/0.3375, depending on the formula, to 98.22% or
98.76% of the measured power. For a keratometric power of 44 D, the net
value would be from 43.22 to 43.45 D, which is 0.55 to 0.78 D less than
the measured keratometric power, he said.
"The power has to be reduced, otherwise the power of the cornea will
be overestimated," Dr. Holladay said.
After corneal refractive surgery, the back surface of the cornea is no
longer 82% of the front surface, as it is in a normal cornea.
Therefore, there is a second error in the net corneal power, which is
10% of the refractive change (from the above example, -10 D × 10% =
1 D).
The peripheral sampling error outlined above equals 15% of the
refractive surgery treatment, and the change in the ratio of the back
to front surface adds another 10%, so the total error in the
keratometric reading is 25% (15% + 10%) of the effect of the refractive
surgery. So a patient with a keratometry reading of 36.5 D after a
-10 D LASIK is actually 34 D (36.5 - 25% × -10).
Corneal power calculations
Because, on their own, keratometric and topographic measurements of the
cornea are inadequate after corneal refractive surgery, Dr. Holladay
has published four mathematical methods to determine and validate the
true power of the cornea in these patients.
"The four methods are the historical method, two modified historical
methods and the hard contact lens method," Dr. Holladay said.
"While none of these is perfect, they are better than using the
keratometric or topographic reading."
In the examples that follow, the same corneal power measurement (39.50
D) is derived as the answer by each of the four methods. In a
"perfect world," Dr. Holladay said, the answers would always be
equivalent, thereby making it easy for the surgeon to choose the
corneal power to plug into the vergence formula. However, he said, in
real life the corneal powers derived from the historical and contact
lens methods are "rarely" the same numeric value when calculated
for a single patient.
"After myopic surgery, the corneal measurements are never the
same," he said. "So you should always use the flattest power that
you get. This avoids getting a refractive surprise later on, because
you avoided believing that the cornea was steeper than it really
was." After hyperopic refractive surgery the opposite is true,
because the true corneal power is higher than the measured value, he
said.
Historical methods
Using the historical method, the surgeon subtracts the patient's
surgically induced refractive change from the preoperative keratometry
reading to determine the current corneal power.
"So if the patient was a -4 D myope with 44 D of corneal power
before surgery and turned out to be +0.50 D after surgery, he underwent
a -4.5 D change," Dr. Holladay explained. "You should be able to
subtract this change from the preoperative K-reading, which would give
you a corneal power of 39.50 D."
In the first modified historical method, the keratometry reading is
used, along with the surgeon's "best guess" at the refractive
change, if preop keratometry readings are not available. As in the
example above, where 25% of the power of the refractive change was used
to compensate for paracentral sampling and the change in front and back
ratio of the corneal surfaces, Dr. Holladay said 25% of the power of
the refractive change is subtracted from the keratometric measurement.
"The keratometer after myopic refractive surgery makes an
overestimate of the corneal power by 25% of the refractive change,"
he said. "If you get a measurement of 40.58, with 4.5 D as the
refractive change, you take 25% of that value, which is 1.08. Subtract
this value from 40.58 and you again get 39.50 D," Dr. Holladay said.
"This is the corneal power that you enter into your IOL program."
In the second modified historical method, topography is used instead of
keratometry.
"Topography gets closer to the center of the cornea than does
keratometry, so the topographer reading needs to be reduced by only 15%
of the refractive change rather than 25%," Dr. Holladay said.
"Therefore, if the topographer central refractive power were 40.18,
then 15% of the -4.50 D refractive change is -0.68 D, so the
calculated power would be 39.50 D."
When obtaining the topography measurements, Dr. Holladay said, the
surgeon should be careful not to use the simulated Ks from the
topographer. These measurements are identical to keratometry
measurements.
"Use the central refractive power as reported by the topographer,"
he said. "Even though it doesn't truly know what the central
corneal measurements are, it extrapolates them. If the measurements
don't automatically come up on the map, click on the central area
with your mouse a few times and take an average of those values."
Contact lens method
In the fourth calculation method, a rigid contact lens is used. Dr.
Holladay gave the example of a patient whose refraction is ì0.50 after
refractive surgery. If this changes to a refraction of -0.5 D with a
41 D contact lens in place on the cornea, the front corneal curvature
must be 1 D flatter than 41 D or 40 D (41 D - 1 D = 40 D).
The contact lens method does not compensate for the change in
back-to-front surface ratio, Dr. Holladay said, "so once you get this
sum, you still need to reduce by about 10%, just like with topography,
because the back surface of the cornea is not consistent with the
front. This leaves you with a value of 39.55. That's the calculated
power."
Measuring front and back
Until better ways of measuring and calculating the total net power of
the cornea are available, surgeons must rely on one or more of these
calculations to determine corneal power. But Dr. Holladay said that in
the very near future surgeons may have a tool available that will
eliminate the need for calculations by accurately measuring the front
and back surfaces of the cornea.
The Pentacam, from Oculus, "has the potential to be four to five
times more accurate in measuring corneal power than any of the
historical or contact lens methods, because it measures the back radius
curvature of the cornea as well as the front," Dr. Holladay said.
The Pentacam measures the tomography of the cornea by taking 50
meridional Scheimpflug images. Dr. Holladay explained.
"The device eliminates eye movement by having a consistent central
point, so it makes the precision of the instrument far more accurate
than any other tomographer I've ever seen," he said.
In a study he conducted, Dr. Holladay determined that the net power of
the cornea can be measured by the Pentacam to within ±0.55 D.
"We can measure a cornea that has undergone corneal refractive
surgery to within ±0.5 D of its actual power," he said.
Dr. Holladay is currently working with Oculus to incorporate a display
into the Pentacam called the Holladay Report, which he said will
accurately calculate the front and back surface powers of the cornea,
adjust for any power overestimate, and report a term called the
equivalent keratometric reading, or EKR.
When this report is incorporated into the Pentacam software, he said,
surgeons will be able to use the EKR in IOL calculation software just
as they would a standard keratometry reading.
For Your Information:
Jack T. Holladay, MD, MSEE, FACS, can be reached at Holladay LASIK
Institute, Bellaire Triangle Building, 6802 Mapleridge, Suite 200,
Bellaire, TX 77401; 713-668-7337; 713-668-7336; e-mail:
docholladay@docholladay.com; www.docholladay.com.
References:
Holladay JT. Standardizing constants for ultrasonic biometry,
keratometry, and intraocular lens power calculations. J Cataract
Refract Surg. 1997;23(9):1356-1370.
Zaldivar R, Shultz MC, et al. Intraocular lens power calculations in
patients with extreme myopia. J Cataract Refract Surg. 2000;26;
668-674.
Holladay JT. Intraocular lens power calculations for the refractive
surgeon. Operative Techniques in Cataract and Refractive Surgery.
1998;1:105-117.
Carl Zeiss Meditec, maker of the IOLMaster, can be reached at 5160
Hacienda Drive, Dublin CA 94568; 877-486-7473; fax: 925-557-4778; Web
site: humphrey.com.
Oculus Inc., maker of the Pentacam, can be reached at Oculus
Optikgeräte GmbH, Münchholzhäuser Str. 29, D-35549 Wetzlar, Germany;
49-641-2005-0; fax: 49-641-2005-255; e-mail: sales@oculus.de.
Nicole Nader is an OSN Correspondent based in Philadelphia, who is
writing the QOV series.
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| serebel 2005-08-17, 10:57 pm |
| Here we go with the reposting of someone elses by a nut who has nothing
informative at all to add.
SErebel
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