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 Post subject: IOL power calculation after LASIK is inaccurate
PostPosted: Sat Mar 04, 2006 1:40 pm 
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http://www.ophthalmologytimes.com/ophth ... ?id=302330

Excerpt:

The topic of IOL implantation after refractive surgery and power calculations arose a few years ago. However, Dr. O'Brien noted that now this situation is arising on a daily basis in practices as the number of refractive procedures increases each year and as the population ages.

"There has been a question of whether laser refractive surgery is accelerating the development of cataract. This is controversial, but I see it more and more often in my practice," he said. Dr. O'Brien is professor of ophthalmology and director of the Refractive Surgery Service, Bascom Palmer Eye Institute, Miami.

The problem, he pointed out, is that the patients who have undergone refractive surgery and then develop cataract are different in mindset from the traditional patient with cataract in that they have extraordinarily high expectations; they want immediate results; and they want no surgical discomfort, sutures, or downtime after the procedure.

"These patients who develop cataract after refractive surgery want a perfect outcome. They are potentially frustrated and angry because their quality of vision has suffered as the result of the refractive surgery, and the results may be unpredictable. Interestingly, incorrect power is the most common reason for IOL explantation. This is the result of our not being able to determine the power as accurately as we would like," he stated and advised exercising extra care with these patients.

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Cataract & Refractive Surgery Today
April 2006
Cataract Surgery in Postrefractive Surgery Patients
By Eric D. Donnenfeld, MD


"Another concern is that IOL power calculation in eyes with previous refractive surgery is notoriously inaccurate".

http://www.crstoday.com/PDF%20Articles/ ... 06_13.html

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http://www.revophth.com/index.asp?page=1_697.htm

Excerpts:

"However, as more individuals have refractive surgery, the number of cataract patients with this in their history continues to increase, and calculating IOL power in these eyes can be quite problematic."

?The problem is that we can?t do nearly as well with people who have had LASIK, which alters the refractive index of the cornea,? he continues. ?We?re getting an increasing number of post-LASIK patients, and I consider that I?ve done well if I get within a diopter of the intended outcome.?

"Most of the surgeons we spoke to agree that the uncertain outcomes that still occur with post-refractive surgery patients necessitate warning these patients in advance. ?Despite our good outcomes to date, refractive surprises may still occur,? says Dr. Wang. ?It?s important to advise patients of the risk of unacceptably high postoperative myopia or hyperopia, and the possible need for glasses, contact lenses, or additional surgery. Additional surgery could mean corneal refractive surgical enhancement, IOL exchange, or a piggyback IOL.?

?Right now our methods are not accurate enough,? agrees Dr. Packer, ?especially since these patients have already paid a lot of money to have refractive surgery. Now they have a cataract and they expect a good refractive result. With current outcomes, 40 to 50 percent of these patients could need a piggyback lens to correct residual refractive error so they can be emmetropic. So, we warn all of them that it?s likely that they?ll need a piggyback implant.?

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http://www.eyeworld.org/article.php?sid=3307

EyeWorld
August 2006

Intraocular Lenses
Powering up new IOL calculations for post excimer laser patients


--------------------------------------------------------------------------------
by Maxine Lipner EyeWorld Senior Contributing Editor

Two new methods of calculating IOL power post LASIK are put to the test

An increasingly common breed of cataract patient is populating most offices?those who have undergone previous laser refractive surgery. Prior excimer laser surgery can make determining the proper IOL power difficult in these patients. Two new approaches, however, may help practitioners attain more accurate results in crunching the numbers.
The problem faced in treating cataract patients who have previously gone under the laser stems from the inability to use ordinary devices to accurately measure the corneal curvature in these patients, said Samuel Masket, M.D., in private practice in Los Angeles and clinical professor of ophthalmology, University of California at Los Angeles.
?The underlying reason for this is that all existing standard keratometers and topographers assume a certain relationship between the anterior and posterior corneal curvature,? Dr. Masket said. ?Following excimer laser photoablation, that relationship is changed, and the curvatures are no longer near parallel, as they are in the untreated eye.?
This situation is unique to excimer laser surgery.
?It is significantly different [because] in radial keratotomy (RK) surgery, the relationship between the anterior and posterior corneal curvature is unchanged,? Dr. Masket said. ?With RK, if one can read central or near central corneal power, one can achieve accurate readings, but that?s not the case in the post excimer cornea.?
Ordinarily, IOL power calculations rely primarily on two figures?the length of the eye and the power of the cornea. It is this second factor that presents a challenge in post-excimer laser cases.
?The problem is that after laser photoablation all standard keratometers erroneously overestimate the power of the cornea after myopic ablation and underestimate the cornea after hyperopic ablation,? Dr. Masket said.

Masket regression formula

Dr. Masket first recognized the need for a correcting factor early on, when he saw that his IOL power calculations were far afield in the first two myopic LASIK patients. At the time there was nothing in the literature to offer help.
?I thought about how we determine power calculations, and as I thought about what laser vision correction does to the cornea, it became apparent to me that we had to correct for the change in that relationship,? he said.
As it happened, the first two cases seemed to indicate the same type of error. ?There was approximately a one-diopter error in IOL power for each 3D of myopic LASIK,? Dr. Masket said.
After using this rule of thumb in 30 post-excimer laser patients Dr. Masket was able to refine the calculation and develop a regression formula to adjust the final IOL power: IOL Adjustment (from IOL Master Calculation) = D (prior laser treatment) x (-0.326) + 0.101.
Dr. Masket has found the formula to be remarkably accurate. In a study published in the March 2006 issue of the Journal of Cataract & Refractive Surgery, results showed that 28 of 30 patients were within .5 D of intended correction. The remaining two eyes were both -.75 D from the targeted correction. Emmetropia was achieved in 14 of the eyes. The mean error for the 30 eyes was -0.15 D.

Reviewing the approach

While there are a variety of methods available, Elizabeth A. Davis, M.D., adjunct assistant clinical professor, University of Minnesota, Minneapolis, said Dr. Masket?s regression formula is a simple, very accurate method.
?You don?t need to know any pre-operative measurements of the corneal curvature,? she said. ?You just need to know how much ablation was performed.?
She said this method is easier to use than some other approaches such as the contact lens method.
?That can be challenging because usually it?s done in the face of a cataract,? Dr. Davis said, ?so it?s difficult to get a precise measurement because the patient?s vision is blurred by the cataract.? With other methods, which rely on corneal topography, it can be difficult to determine where on the cornea to take the measurements, she pointed out.
If there is any question, Dr. Davis suggested practitioners use several formulas to confirm results. ?I usually try two formulas and compare them,? she said. ?Dr. Masket?s formula is definitely one of the ones that I look at.?
Most cataract surgeons rely on the traditional clinical history method of Jack Holladay, M.D. (Baylor College of Medicine, Houston), said David F. Chang, M.D., clinical professor, University of California, San Francisco. With his method the true central corneal power is estimated based on the surgical change in subjective spherical refraction.
?Although accurate pre-operative data are usually available, an accurate post-LASIK refraction is often missing,? Dr. Chang said. ?Some happy patients do not return for their one-to-three month follow-up.? In other cases, a careful post-operative refraction was not performed and was never done with cycloplegia, he said.
?The Masket regression formula is also a clinical history method, but it only requires the amount of dioptric correction programmed in the excimer laser,? Dr. Chang said. ?This presumably could be obtained from the laser facility even if the physician records are not available.?
One potential problem is that the formula assumes that the cornea actually has changed by the amount that was intended. To avoid this problem Dr. Chang suggests that practitioners still calculate the spherical change based on actual pre- and post-op refractions, if available.
Jack A. Singer, M.D., president, Singer Eye Center, Randolph, Vt. likewise sees a potential weakness with the Masket regression formula. He pointed out that Dr. Masket has done a linear regression analysis based on observed outcomes and found that there?s a 0.326 D correction to the lens implant power for 1 D of LASIK correction.
?There could be some complicated non-linear relationship, and we?d only know that as more cases were observed,? Dr. Singer said. This is particularly true for hyperopic eyes, with only seven included in the study, he observed.
While additional confirmation would be helpful, Dr. Singer found the Masket regression formula to be spot on in a prior hyperopic LASIK patient of his who had undergone phacoemulsification. Dr. Singer used the clinical history method on the patient?s first eye, done two years earlier, and the patient ended up with a myopic surprise.
?Then I looked at how the outcome would have been if I used the Masket regression formula, and it would have been right on,? he said. ?That?s just one case, but I?m going to use the Masket formula for her second eye.?

Corneal bypass method

Another method gaining ground for myopic patients who?ve gone under the excimer laser is the corneal bypass method. The method was introduced by Keith A. Walter, M.D., assistant professor or ophthalmology, Wake Forrest University Eye Center, Winston-Salem, N.C.. This technique circumvents the difficulties in measuring the cornea after LASIK.
?With our method what we?ve done is figure out a way to bypass reading the patients? current corneal measurements, [because] we know that they?re wrong,? Dr. Walter said. ?We tried this in the past by doing sort of a historical method [in which] we can subtract the patients? prior corneal values and diopter for diopter they?ll subtract how much ablation was done, but that has errors, too.?
Instead, the corneal bypass method circumvents the inaccuracies of the post-LASIK cornea and relies on pre-LASIK keratometery and post-LASIK axial length. The correct IOL power is obtained by plugging in the original K values, original target myopic refraction, current axial length, and the amount of cornea removed to the practitioner?s myopic formula of choice. Practitioners aim for the pre-LASIK refraction with the IOL calculation.
The premise of the method is that if the desired results are the same, the order of events does not matter. It is the communicative power of addition in which both a + b = c and b + a = c, which is the cornerstone here, said Dr. Walter. It doesn?t matter which the patient underwent first, LASIK or cataract surgery.
Dr. Walter explained the logic this way: ?We have a patient who?s an artist, who wants to remain nearsighted,? he said. ?He wants to have his cataracts taken out, but he doesn?t want to be distance corrected; he wants to be the same -3.?
In a case such as this, practitioners would plug -3 D into their formula of choice as the patient?s target acuity, measure his K values if he hasn?t had any refractive surgery, and put in his axial length. The proper IOL power for the patient would then be determined and the lens inserted.
If after a few years the patient decided that he wanted to be Plano, he then could undergo PRK or LASIK. This would not change the lens power, which would remain correct, Dr. Walter explained?only the corneal power would change.
If the same artist wanted LASIK to start with to get rid of his -3 D of myopia and then later underwent cataract surgery, the IOL power would remain the same. ?The true IOL power doesn?t change for that person,? Dr. Walter said. ?Either way it?s still the same.?
A study of the corneal bypass method, which was recently published in the March 2006 issue of the Journal of Cataract & Refractive Surgery, has shown it to be very accurate. In the study, which included nine eyes, the mean spherical equivalent was + 0.03 D post-op with a standard deviation of just 0.42.
?We were pretty much right on the money,? Dr. Walter said.

Weighing in

Richard S. Hoffman, M.D., clinical associate professor of ophthalmology, Casey Eye Institute, Oregon Health and Science University, Portland, Ore., said the idea has merit.
?I think that it?s really brilliant because it bypasses the inconsistencies of measuring a cornea after keratorefractive surgery,? Dr. Hoffman said. ?The only issue with this is that you have to have the pre-operative measurements, and that can sometimes be a problem if someone has had keratorefractive surgery 10 years earlier?the chart may no longer be available, and you may not have those measurements.?
Another issue to consider is whether the patient was in the midst of developing a cataract when he underwent refractive surgery. Dr. Hoffman cited the example of a 55-year-old who has a little bit of a cataract at the time of refractive surgery.
?It?s possible that six months later he could be -1, and one year later he could -2,? he said. ?They never quite stabilize because the cataract is changing their refractive power, not the corneal change.? Ultimately, for the corneal bypass method to work practitioners need a stable endpoint.?
Likewise, Douglas D. Koch, M.D., professor and the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, said that the corneal bypass method has some nice elements.
?It doesn?t rely on trying to estimate corneal power at the time the patient presents for cataract surgery, which is a clear benefit,? Dr. Koch said. ?It also doesn?t rely on the need to do some form of a double-K calculation to account for effective lens position.?
However, he said there a couple of downsides.
?The potential disadvantage of it is the fact that you depend on measurements that someone else made before the patient underwent LASIK Surgery?specifically, the refraction and more importantly the K readings,? he said.
The second problem he sees is that practitioners are dependent on knowing how much refractive change was caused by the LASIK. ?That means you have to have a very accurate value for the change in manifest refraction after LASIK but before the cataract began to affect the vision and the refraction?two pieces of historical information that are not always accurate or available,? Dr. Koch said.
Overall, Dr. Koch said this is yet another tool in the calculation armamentarium. ?If you have the data, it?s a worthwhile method to try and factor it in with all the others.?
He said he is hopeful that in the future devices such as the Pentacam (Oculus, Dutenhofen, Germany) will accurately measure posterior corneal curvature?allowing practitioners to forgo special calculations altogether in these patients. ?We actually will be able to truly measure corneal power, which would really take us a major step ahead,? he said.

Editors? note: Drs. Chang, Davis, Hoffman, Koch, Masket, and Walter have no related professional affiliations. Dr. Singer is a consultant for eyeonics.

Contact Information
Chang: 650-948-9123, dceye@earthlink.net
Davis: 952-885-2467, eadavis@mneye.com
Hoffman: 541-687-2110, rshoffman@finemd.com
Koch: dkoch@bcm.tmc.edu
Masket: 310-229-1220, avcweb@aol.com
Singer: 802-728-9993, jack@singereye.com
Walter: 336-403-2236, kwalter@wfubmc.edu

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EyeWorld
August 2006

Intraocular Lenses
New study compares leading formulas for determining IOL power


--------------------------------------------------------------------------------
by Maxine Lipner EyeWorld Senior Contributing Editor

Physician develops new ?straightforward? formula

There?s a relatively new breed of patients that cataract surgeons must deal with more frequently?those who previously have undergone refractive surgery.
Robert L. Latkany, M.D., assistant professor, New York Medical College, and director and founder, Center for Ocular Tear Film Disorders, New York, has seen the number of these patients grow.
?One year ago we used to see one case every four months; now we probably see two a month. Next year, we?ll probably see two a week,? Dr. Latkany said.
With these cataract patients practitioners can have a more difficult time determining the correct IOL power. Many calculations have appeared over the last few years to help remedy the problem.
To help evaluate the effectiveness of such methods, including one newly emerging formula for IOL power calculation after refractive surgery, Dr. Latkany published the findings from a study he performed on this subject.
The study, which appeared last year in Journal of Cataract & Refractive Surgery, was a retrospective, non-comparative case series that involved 21 cataract patients who previously had undergone myopic refractive surgery. Investigators compared IOL power calculations and used six methods:
The first was the clinical history method IOLHisK, in which an adjustment for corneal plane was made using a standard vertex distance of 12 mm.
The second calculation used, another clinical history method, was(IOLHisKs, in which the spherical equivalent after refractive surgery was left at the spectacle plane.
The third technique was the IOLvertex method in which IOL power was determined based on pre-LASIK keratometry readings.
The fourth method was the IOL back-calculated formula (IOLBC), which uses the pre-LASIK spherical equivalent.
Using the fifth technique, known as the IOL average K method (IOLavek), IOL power was based on average post-LASIK keratometry readings.
With the sixth technique, known as the IOLflatk method, IOL power was chosen using the flattest post-LASIK keratometry readings. A new formula (called the Latkany Regression Formula) also was tried with the IOLflatk. With this method an adjustment to the IOL power is made using the formula -(0.47x + 0.85), in which x is the value of the pre-op spherical equivalent.

The right formula

?The nice thing about the Latkany Regression Formula as opposed to other formulas is that it?s pretty straightforward and doesn?t require a lot of data,? Dr. Latkany said. ?All you need is the current keratometry readings and to take the flattest value.?
For example, with keratometry readings of 39 and 41, the flatter 39 reading is typed into the A-scan unit twice. Practitioners then can use whatever method they are most comfortable with to determine what lens implant to put inside the eye.
?Whether it?s the Holladay formula, the Hoffer formula, SRKT?you name it?you can use that formula,? Dr. Latkany said. ?You plug in the two flattest K values and you come up with your lens number.? Then, with the Latkany Regression Formula, an adjustment to the lens power is made.
The six methods were all compared with an IOLexact formula, which indicated which lens would make the patient happy. Results showed that the most accurate methods used were the IOLHisKs method and the IOLflatk used in conjunction with the Latkany Regression Formula.
?We looked at the data afterward and my formula showed no statistically significant difference from the IOLexact formula,? Dr. Latkany said.
The only other formula that statistically was significant was the IOLHisKs, in which historical K readings were used at the spectacle plane.
?The problem with using this formula is that you need to know the pre-laser surgery K readings, and those aren?t easy to obtain,? Dr. Latkany said. With the Latkany Regression Formula fewer data are needed and also are easier to obtain.
?Certainly, no one is going to remember in 1998 what his K readings were, and maybe he won?t be able to recover his charts,? he said. ?However, some people just know their old prescription or could bring in an old pair of glasses and I could just read them.?

Considering the calculations

Richard L. Lindstrom, M.D., adjunct professor emeritus, Department of Ophthalmology, University of Minnesota, Minneapolis, said the study has tapped into a growing need.
?It?s an important topic that people are always interested in and even more interested in now because there are more patients coming in for cataract surgery who have had previous refractive surgery,? he said.
Whenever he can, Dr. Lindstrom uses what he called the ?basic refraction technique,? in which he considers the patient?s original refraction and the stabilized refraction following procedures such as LASIK. When he has all the prerequisite data, he has found this to be very successful.
To adjust the IOL power with something like the IOLflatk Dr. Lindstrom has found that a rule of thumb suggested by Samuel Masket, M.D., president of American Society of Cataract and Refractive Surgery (ASCRS), and clinical professor of ophthalmology, Jules Stein Eye Institute, University of California at Los Angeles seems to work well.
?For every 3 D that you?ve corrected with refractive surgery, you need to adjust the IOL lens power by 1 D and basically use more power than you thought you otherwise would have because there is less power in than you would measure,? Dr. Lindstrom said.
Louis E. Probst, M.D., regional medical director, TLC Vision, Chicago, Madison, Wis., and Greenville, S.C., uses a slightly different rule of thumb.
?You simply add 1 D of IOL power for every 4 D of LASIK correction,? Dr. Probst said. ?That?s based on the fact that the Ks have been shown in studies to underestimate the central flattening of the cornea by 25%?so it?s about one-fourth off.?
Dr. Probst said that one important contribution the paper makes is to show the importance of not vertexing the calculation to spectacle plane with the clinical history method?something that never has been shown before.
Dr. Probst also said the investigators? method of using the flat K calculation with the Latkany Regression Formula is important.
?What that tells us is that you don?t necessarily need the K readings; you just need the pre-op spherical equivalent,? he said.
However, the two methods that were most effective?IOLHisKs and IOLflatk?still had a very large standard deviation at 1.59 D and 2.19 D, respectively.
?That means a lot of people aren?t going to be anywhere near emmetropia,? Dr. Probst said. The smart thing to do is to always err on the side of the more powerful lens.
?Maybe add a ?0.5 D or a 1 D to the IOL power to make sure that the patient ends up dead-on or, at least if there is going to be an error, it?s going to be a myopic one, not hyperopic,? he said.

Editors? note: Dr. Probst has a financial interest in Advanced Medical Optics (AMO, Santa Ana, Calif.). Dr. Lindstrom has a financial interest in AMO, TLC Vision (Chicago), and Bausch & Lomb (Rochester, N.Y.). Dr. Latkany has no related professional affiliations.

Contact Information
Latkany: 212-832-2020, relief@drydoctor.com
Lindstrom: 612-813-3633, rllindstrom@mneye.com
Probst: 608-249-6000, louisprobst@comcast.net

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EyeWorld
August 2006

IOL Calculation Using the Flattest K Method
with the Latkany Regression Formula

--------------------------------------------------------------------------------


? A patient presents post-LASIK surgery with a cataract.
? Their pre-LASIK prescription was 5.00-2.00x180.
? Their pre-LASIK spherical equivalent is 6.00.
? Their current K readings are 39.00, 40.25.
? Their current axial length is 23.50.

? Using an SRK-T formula and entering K readings of 39.00 twice (flattest K) an A constant of 118.4 yields a 25.5 Diopter lens for a -0.27 Diopter result.
? An adjustment is then made using the Latkany Regression Formula: -(0.47x + 0.85)
x= pre LASIK spherical equivalent of -6.00 gives an adjustment of +1.97
? You then add the adjustment of +1.97 to the 25.5 Diopter lens obtained with the flattest K calculation.
? This gives you a 27.5 Diopter lens for such a patient.

Source: Robert L. Latkany, M.D.

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EyeWorld
August 2006

Intraocular Lenses
Here?s another way to calculate IOL power after LASIK


--------------------------------------------------------------------------------
by Vanessa Caceres EyeWorld Contributing Editor

Technique involves manifest refraction measurement after cataract removal


A new way to calculate IOL power in patients with previous LASIK had accurate results in a group of 12 eyes, according to a recent study.
The technique involves taking patients out of the operating room (OR) to perform a manifest refraction after cataract removal but before IOL implantation.
The study was published in the March issue of the Journal of Cataract & Refractive Surgery. The lead author was Richard J. Mackool, M.D., senior attending surgeon, New York Eye and Ear Institute, New York.
In their retrospective study the investigators analyzed the results from 12 eyes in nine patients that had previous LASIK. The surgeon performed cataract removal under topical anesthesia. About 30 minutes later the patient underwent automated retinoscopy and manifest aphakic refraction, IOL power calculation with the Mackool algorithm (below) from Dr. Mackool, and returned to the operating room (OR) for IOL implantation.
The Mackool algorithm for patients with an aphakic refraction between +8.00 and +13.00 diopters is: Aphakic refraction (Spherical equivalent) x 1.75 = IOL Power.
Dr. Mackool developed his algorithm based on experience with secondary IOL implantation and assumed an A-constant of 118.84 for emmetropia.
All patients in the study had a spherical equivalent aphakic refraction between +8.50 and +12.375 D.
?It was pretty simple for me to figure out that when we started to take patients? lenses out and we couldn?t calculate their IOL power, we could use this technique,? Dr. Mackool said.
At two weeks post-op investigators found that patients? refractive error ranged from 0.50 D of unintended hyperopia to 0.75 D of unintended myopia. The mean of the absolute refractive error was 0.30 D, and the average refractive error was ?0.18 D. The results demonstrate the technique?s accuracy, the investigators wrote.

Take-home value

Physicians said Dr. Mackool?s approach in the study is simple, accurate, and contributes a solution to the growing challenge of calculating IOL power in LASIK patients.
?There aren?t a lot of these patients out there, but that will change 10 years down the road, when more LASIK patients come up for cataract surgery,? said Richard S. Hoffman, M.D., clinical associate professor of ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, Ore.
?This adds to information that we all need,? said Y. Ralph Chu, M.D., Edina, Minn.
IOL power calculations after LASIK are rarely easy to make, Dr. Chu said. Surgeons will use the historical method (based on a patient?s previous history), the hard contact lens method, the topography method, and other approaches reported recently to calculate IOL power.
Still, a surgeon may feel that selecting the right IOL for a patient based on variable results from IOL formulas is more of an art than a science, said Jack L. Weiss, M.D., San Diego.
An approach such as Dr. Mackool?s may work best in patients in whom no information is available regarding how much laser was used, said Samuel Masket, M.D., clinical professor of ophthalmology, Department of Ophthalmology, University of California Los Angeles. Although those patients are infrequent, they are part of the patient mix, he said.
Still, surgeons expressed concern about interrupting the sterile field for the manifest refraction and then returning to the OR, increasing the risk for infection.
?If you?re going into the eye twice, you increase the risk for endophthalmitis,? Dr. Chu said. ?I?d rather have an IOL calculation surprise than endophthalmitis.?
Certain patients would not be well suited for manifest refraction measurement immediately after cataract removal, said Jorge L. Ali?, M.D., Ph.D., professor and chairman, Ophthalmology Department, Miguel Hernandez University, Alicante, Spain. The situation of the cornea, the pupil, and the stability of the anterior chamber are not always optimal to guarantee a precise refraction,? he said. ?We have performed this technique in several cases, and the variability in our hands is far from ideal.?
?This mandates a second operation in every patient, whereas if you use other predicting formulas, if you place the lens in and it?s correct, you don?t need a second operation,? Dr. Masket said.
Patients may not like the interruption in the flow of the procedure.
?I think you?d need a very understanding patient in terms of getting him off the table with a fresh incision and getting him to a room where he?ll be examined, and then back into the OR and prepped for IOL implantation,? said Steven Dewey, M.D., Colorado Springs, Colo.
Performing the procedure in the way the study describes it requires access to an Ambulatory Surgery Center (ASC), which isn?t practical for physicians in a hospital setting, Dr. Hoffman said. Access to an ASC does indeed make the technique easier to use, Dr. Mackool said.
?Having my own ASC made it pretty easy to go to the examining lane and get a refraction,? he said.
However, surgeons also can insert the IOL in the capsular bag up to three weeks after cataract extraction, Dr. Mackool said, making the technique easier for other surgical settings or in corneas that are not clear enough to perform a refraction immediately.

Future data and directions

Physicians said they would like to see results from Dr. Mackool?s approach tracked in a larger group of patients, although they realize there isn?t a huge representative patient population to pull from yet.
?Possibly, with more patients they could refine their algorithm and make it slightly more accurate,? Dr. Hoffman said.
In response, Dr. Mackool stated that he has now performed the aphakic refraction technique in more than 50 post-LASIK eyes and 150 other eyes with conditions such as posterior staphylomata that made standard IOL calculations problematic. Our results continue to me extremely accurate, and complications have been non-existent, he added.
Dr. Hoffman also would like to see formulas that consider where the lens will position itself in the eye using anterior chamber depth and white-to-white diameter measurements.
With the challenges presented by IOL calculations after LASIK surgeons should caution this subgroup of patients that the end result may need some tweaking, Dr. Dewey said.
?I advise them that because their cornea has been altered, we can?t measure them with the same accuracy, and they may need an enhancement,? he said.
Physicians also may want to consider power calculation using a variety of formulas before they decide on the right IOL, said Dr. Weiss, who said 20% to 30% of his cataract patients have had previous LASIK. He will review up to five calculations performed by his technicians and choose the IOL that seems to best fit the patient?s visual needs.
Dr. Weiss has added a piggyback IOL in less than 1% of these patients and performs post-op LASIK in 5% to 8% of his cataract surgery patients, usually for residual astigmatism.

Editors? note: The physicians interviewed have no financial interests related to their comments.

Contact Information
Ali?: +3465154062, jlalio@vissum.com
Chu: 952-835-1235, yrchu@chuvision.com
Dewey: 719-471-4139, deweys@prodigy.net
Hoffman: 541-687-2110, rshoffman@finemd.com
Mackool: 718-228-3400, mackooleye@aol.com
Masket: 310-229-1220, avcmasket@aol.com
Weiss: 858-455-9972, jacklweiss@aol.com

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PostPosted: Wed Nov 22, 2006 1:54 pm 
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Review of Ophthalmology

Wills Eye Resident Case Series

A LASIK patient?s seemingly uncomplicated cataract surgery leads to a disappointing outcome.

Bradley T. Smith, MD, and Sadeer B. Hannush, MD


Quote:
The patient experienced a ?refractive surprise? due to a miscalculation of IOL power. Unfortunately, this is not uncommon due to the limitations of keratometry and IOL calculation formulas in post-LASIK eyes.

Keratometry assumes the posterior radius of curvature is 80% the anterior radius of curvature. This ratio changes after LASIK, since only the anterior radius of curvature is altered during photoablation.3 Also, keratometry measures the power of the cornea in the paracentral area, not the visual axis. In normal eyes the cornea is relatively spherical and this off-axis measurement is of little consequence. However, this approximation is less accurate after LASIK due to the flattened central cornea. These keratometric assumptions overestimate the true corneal power and result in implantation of an IOL of insufficient power.




Read the entire article at:
http://www.revophth.com/index.asp?page=1_1009.htm

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PostPosted: Sun Dec 03, 2006 2:42 pm 
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EyeWorld Supplement
2006-05-30

Determine IOL Power Three Years After LASIK

By Warren E. Hill, M.D.

Excerpt:


Quote:
Although understanding LASIK calculations for the post-keratorefractive eye is an enormously complex topic, most ophthalmologists will soon be faced with this type of patient. The issue is a growing one for ophthalmology because an increasing number of patients who have had LASIK surgery are aging and will need to have cataract surgery.


Quote:
Three common IOL calculations were used: Holladay's historical method (one of the first IOL power calculation methods described for the post-keratorefractive eye), the Masket method (soon to be published in the Journal of Cataract and Refractive Surgery), and the Holladay equivalent K readings from the Holladay Report of the Pentacam (Oculus, Dutenhofen, Germany).


Quote:
The take-home message for IOL power calculations following keratorefractive surgery is to look to devices that have the potential to provide direct measurements rather than base one's conclusions on broad assumptions. If you go by assumptions alone then you may get burned with the refractive outcome.




Read the entire article at:
http://www.eyeworld.org/ewweeksupplemen ... php?id=164

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PostPosted: Tue Nov 27, 2007 12:59 am 
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http://www.revophth.com/index.asp?page=1_13610.htm

"Determining IOL power for a cataract patient has always been challenging, but it?s gotten a lot tougher lately. Thanks to refractive surgery, more and more individuals with cataracts have altered corneas that make it difficult to ensure an acceptable outcome for cataract surgery. Many surgeons are trying to solve this problem, but so far the result has been a proliferation of formulas and methods with no one solution rising to the top."


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PostPosted: Tue Nov 27, 2007 1:29 am 
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From the link provided above by kaleyedoscope:

Quote:
?Calculating IOL power after refractive surgery is tough for several reasons,? says Dr. Hoffer. ?First, most keratometers don?t measure the central cornea where the effective corneal power is. Second, LASIK and PRK change the index of refraction between the front and back surfaces of the cornea. Third, many modern formulas predict IOL position using the axial length and corneal curvature. That won?t work in this situation because these procedures change the corneal curvature?but not the expected position of the IOL". Dr. Hoffer says there are currently about 24 formulations in use.


This would be funny if it weren't so pathetic! I know, I have the solution! Warn the patients BEFORE they have refractive surgery. Then they won't be in this position years down the road!

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 Post subject: well....
PostPosted: Tue Nov 27, 2007 1:36 am 
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In the words of my FORMER refractive surgeon, JOSEPH KIERNAN of PREMIER EYE CARE AND SURGERY, "Don't worry, I'm sure they'll figure something out [for cataract surgery, post-lasik]." :shock:

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http://www.osnsupersite.com/view.asp?rID=26483

Surgeons: Previous refractive surgery presents challenges in cataract

?The most important thing, clearly, is to warn the patient that this is a far less accurate process than is standard for regular cataract surgery on a virgin eye,? Dr. Reeves said."


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http://www.star-telegram.com/health/story/507006.html

Cataract patients have plenty of options

Quote:
The new lenses aren't completely out of the question if you've had LASIK surgery. But Shingleton says it's a "relative contraindication," because LASIK makes it more difficult to pick lenses with the right corrective power.

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PostPosted: Sun Apr 06, 2008 12:51 pm 
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OCULAR SURGERY NEWS U.S. EDITION April 10, 2008

Approaches to cataract surgery vary in patients with previous refractive surgery

Quote:
Dr. Koch: There are two reasons that it is difficult to obtain good corneal measurements in patients who have had prior refractive surgery. We must remember that the cornea has two curvatures, a front and a back, and when we use a standard keratometer or topographer, we are only measuring the front, but we are giving an overall value for corneal power. That overall value is based on some assumption about what the back corneal power is. If you subject a patient to PRK or LASIK, you change the front corneal power but not the back, and so you alter the relationship. These devices use a different index of refraction to compensate for the back corneal power, but if you change the relationship between front and back, that index of refraction is no longer valid.

Unfortunately, because different levels of correction change the front in different magnitudes relative to the back, there is no new single value for refractive index you can just plug into those machines to get your true corneal power. So what we are doing is measuring the front and having to make a sophisticated guess about the back, and that guess is often not as accurate as we would like it to be.


Quote:
Dr. Hovanesian: If you see a patient who has previously had refractive surgery, what sort of discussion do you have with them? Let?s assume that we are talking about a monofocal IOL.

Dr. Koch: The first thing you want to figure out is what type of refractive correction the patient desires. Presumably, because these patients have undergone refractive surgery, they want at least one eye to be corrected for distance. On the rarest of occasions that will not be true, but in general, they are going to want to have good distance vision in one eye. And then you have to make the decision about whether or not to do monovision and whether they have had monovision and how to go through that whole exercise.

It is essential to point out to them the inaccuracy of the calculations. You could even potentially print out the results from the Web site of the calculations and show the patient the variability of the lenses that are recommended by the formulas so the patient understands the extent of the uncertainty here. And then you have to go over carefully with the patients the options that will exist postoperatively, and I think it is helpful to do sufficient preoperative testing to know whether or not additional corneal refractive surgery is an option. Is the cornea thick enough? Is the tear film adequate? Are there any other problems that might preclude additional corneal refractive surgery?

So you can tell the patient that there is the likelihood that something will be needed, what the options will be and, importantly, what the cost will be to the patient so that the patient does not expect a free postoperative laser procedure to correct for the residual refractive error ? unless you are offering it to them.


Quote:
Dr. Hovanesian: And certainly the older patients with flaps are more prone toward difficulties with ocular surface dryness and epithelial ingrowth, as well as other irregularities of those flaps.

Dr. Koch: I completely agree.


Quote:
Dr. Hovanesian: Any final advice for approaching these patients?

Dr. Koch: They take more time. They are more demanding. They have already indicated that they want perfect vision, and it is a real source of frustration for these patients if it is not provided. You have to be sympathetic with them, even though they seem demanding, but they went into this whole process expecting high-quality vision for their lifetime. So it is our responsibility to try to give them the best correction possible, as well as a level of understanding and education that helps them go through this process as comfortably and happily as possible.


Read entire article at:
http://www.osnsupersite.com/view.asp?rID=27382

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