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Post Info TOPIC: Different methods of corneal thickness evaluation: a comparative study for KC eyes


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Date: Tue Jan 24 10:22 AM, 2006
Different methods of corneal thickness evaluation: a comparative study for KC eyes
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The below study concenrs a question I tried to understand some time ago: several methods give different results. Which one to believe more? Our corneas are liable to change their thickness as function of multiple factors. Even using the same method one may find different results during different hours of a single day. This is important for patients. I am sure the doctors that are used to such measurments and KC may give us some useful remarks. Comments are welcome! 
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Central Corneal Thickness Measurements Using Orbscan II Scanning Slit Topography, Noncontact Specular Microscopy, and Ultrasonic Pachymetry in Eyes With Keratoconus.
Cornea. 24(8):967-971, November 2005.
Kawana, Keisuke MD *; Miyata, Kazunori MD +; Tokunaga, Tadatoshi COT +; Kiuchi, Takahiro MD *; Hiraoka, Takahiro MD *; Oshika, Tetsuro MD *

Abstract:
Purpose: To compare corneal thickness measurements using Orbscan II scanning slit topography, Topcon SP-2000P noncontact specular microscopy, and ultrasonic pachymetry in eyes with keratoconus.

Methods: Central corneal thickness was measured in 22 eyes with keratoconus. Eyes with apparent corneal opacity were excluded. Scanning slit topography, noncontact specular microscopy, and ultrasonic pachymetry were used in this sequence. The acoustic equivalent correlation factor (0.92) was used for Orbscan readings.

Results: Three devices gave significantly different corneal thickness readings (P < 0.001, repeated-measure analysis of variance). Measurements with Orbscan scanning slit topography (449.5 +/- 43.2 [SD] [mu]m) were significantly smaller than those of ultrasonic pachymetry (485.0 +/- 29.3 [mu]m; P < 0.001, Tukey multiple comparison) and SP-2000P noncontact specular microscopy (476.7 +/- 28.3 [mu]m; P = 0.002). There were significant linear correlations between ultrasonic pachymetry and scanning slit topography (Pearson correlation coefficient r = 0.741, P < 0.001), between scanning slit topography and noncontact specular microscopy (r = 0.880, P < 0.001), and between noncontact specular microscopy and ultrasonic pachymetry (r = 0.811, P < 0.001).

Conclusion: In eyes with keratoconus, Orbscan II scanning slit topography system gave significantly smaller corneal thickness readings than the other 2 devices. Measurements taken by noncontact specular microscopy and ultrasonic pachymetry were comparable. Three devices showed significant linear correlations with one another.

(C) 2005 Lippincott Williams & Wilkins, Inc.



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yarsky
Anonymous

Date: Tue Jan 24 1:30 PM, 2006
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Yarsky,


Here's a link to work on how to get in more detail finer measurements of the cornea... Its very important to detect "sub-clinical keratoconus" before laser treatment and also so that in the future x linking can be used to avoid problems sooner. I suspect the end result being that kc is found to be more common.


http://www.escrs.org/Publications/Eurotimes/05November/Pdf/ESCRSSymposiumReport.pdf



-- Edited by QuintriX at 17:49, 2006-01-24

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Anonymous

Date: Sun Mar 19 3:17 AM, 2006
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Yarsky, out of the studied machines which one is the most accurate? or should two differant types of machines be used so that a mean average can be worked out between the two differing readings so that it can be relied upon or with in a good enough range, not to worry about?


I take it that the Orbscan failed sort of the rest... but would you say that because only "spots" on the cornea are tested for with these machines... that areas are being missed when taking calculations?


I think the thickness readings of the cornea at all points is lacking with these machines... if you are broader line above the 400 thickness mark then its worth knowing what the true readings are... as the minimum thickness at any one given point should be known, instead of going with readings from machines which have a margin of error greater than thought before... and so you would not know where you are at with your kc... basically at the moment, for most people to be sure, you need the same machine and the same person testing you, and then you need a completely different machine but the same person as before to know if your kc is getting worse or not ...so that you can act... if you choose to do so...



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Executive

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Date: Sun Mar 19 9:00 AM, 2006
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I still do not know which one is more exact. But I think even KC corneas have a "regular", "smooth" shape: no jumps int it. It is always thinner in the center and thicker on the periphery. Collagen fibers themselves are thinner in the center. In my case, the difference between the two methods (Orbscan and Ultrasonic) for central parts is about, or lets say, looks like, 50-100 µm. Orbscan gives 315µm, ultrasonic 415 for the right eye, and 479-532 for the letf! Decision on x_linking was made however on the basis of ultrasonic results. I think surgeons know a bit more about the coherence of the results. We should ask them this question.

However, another likely hypothesis is that for normal clear and not deformed corneas teh results of different methodes are quasi the same. In deformed and sick corneas, with haze, we have what I described. Read the following and it will partially answer the question. The thing is that I think KC corneas are corneas with "haze" (Vogt striae, etc. etc.) Look below, please.

1) Comparison of corneal pachymetry using ultrasound and Orbscan II
Mohamed A. Fakhry, MD, Alberto Artola, MD, PhD, José I. Belda, MD, PhD, Ma José Ayala, MD, PhD, Jorge L. Alio, MD, PhD

Purpose:
To compare Orbscan II pachymetry with ultrasonic pachymetry in eyes having corneal refractive surgery.

Setting:
Instituto Oftalmolo´gico de Alicante, Alicante, Spain.

Methods:
In this comparative prospective study, 72 eyes (36 patients) were divided into 4 groups: Group 1, 16 eyes with haze after photorefractive keratectomy (PRK); Group 2, 12 eyes with no haze after PRK; Group 3, 24 eyes that had uneventful laser in situ keratomileusis; Group 4, 20 normal eyes. The corneal thickness was measured in all eyes by the Orbscan II (version 3.27.10, Orbtek Inc.) and then by ultrasonic pachymetry (DHC 500, Technology Inc.).

Results:
The results between the 2 systems of pachymetry were significant in Group 1 (P < .0001) and not significant in Groups 2, 3, and 4 (P = .056, P = .051, and P = .68, respectively). A statistically significant correlation existed between the grade of haze and the decrease in Orbscan readings (C = −0.821, P < .0001).

Conclusion:
The Orbscan II pachymetry measurements correlated with the ultrasound measurements in eyes with clear corneas. Haze after PRK decreased the Orbscan pachymetry. J Cataract Refract Surg 2002; 28:248-252

2) Accuracy of Orbscan optical pachymetry in corneas with haze
Francesco Boscia, MD, Maria Gabriella La Tegola, MD, Giovanni Alessio, MD, Carlo Sborgia, MD

Purpose:
To compare corneal thickness measurements with the Orbscan instrument (Orbtek, Inc.) and an ultrasonic pachymeter (Alcon Surgical) in corneas with haze.

Setting:
Department of Ophthalmology and Otolaryngology, University of Bari, Bari, Italy.

Methods:
Six eyes of 6 patients having phototherapeutic keratectomy for treatment of grade 2+ haze were assessed preoperatively and postoperatively by Orbscan optical pachymetry and ultrasonic pachymetry. Contralateral transparent untreated corneas of the same patients were used as controls. Two Orbscan and 5 ultrasound pachymetry measurements were performed at each visit and the mean values recorded. The accuracy and repeatability of the measurements were evaluated.

Results:
Preoperatively, the median corneal thickness in the 6 eyes with haze was 512.7 μm by ultrasonic pachymetry and 375.0 μm by optical pachymetry. The mean difference of 143.1 μm was statistically significant (Wilcoxon signed rank test, P=.0277). Postoperatively, the median pachymetry was 439.0 μm and 433.0 μm, respectively; the mean difference was 10.7 μm (P= .0277). In the control group, the median ultrasonic pachymetry was 530.9 μm and the median optical pachymetry, 529.0 μm; the mean difference was 4.7 μm (P= .1730). Precision was good in all groups; the standard deviation was less than 6.0 μm.

Conclusion:
The Orbscan system showed a decreased accuracy in measuring corneal thickness when clinically significant haze was present. The repeatability of the measures was good, suggesting that the cause of error lies in the optical acquisition process. J Cataract Refract Surg 2002; 28:253-258

Abstracts taken from
http://www.ascrs.org/publications/jcrs/jcrsfeb02.html

There is another study done on post LASIK eyes, that says the same thing: Orbsacn significantely underestimates corneal thickness for post LASIK eyes. Here

http://bjo.bmjjournals.com/cgi/content/full/88/4/466

So my conclusion is: KC eyes ressemble post LASIK eyes with ectasia and vice versa. SO for us, the KC, Orbscan gives results less acurate than ultrasonic...The sereverer the KC the less acurate Orbsacn (the reason being that KC corneas are optically inhomogeneous, and the Orbscan method takes the inhomogenuity (for instance Vogt striae) as being the posterior surface of the cornea, effectively decreasing the thickness). BUT, it is good to do both from time to time for they are linearly correlated: if the one goes down, the other certainly does it too, if one goes up the other does the same...SO GUYS DO BOTH IF YOU CAN BUT HAVE IN MIND THAT ULTRASONIC IS BETTER FOR ADVANCED CASES. WELL, IT SEEMS SO....




-- Edited by Yarsky at 09:14, 2006-03-19

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yarsky


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Date: Sat Dec 16 7:46 PM, 2006
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I recieved the following news on the on-going discussion about the problem outlined in this topic.
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OCULAR SURGERY NEWS U.S. EDITION December 1, 2006
Corneal measurement technologies for refractive surgery advance; validation needed
New technologies reflect the importance of corneal thickness measurements for refractive surgeons.
By Daniele Cruz
As corneal thickness measurements have become more important to refractive surgeons, the technologies used to make these measurements have evolved.
Although these new technologies can deliver reproducible measurements for a range of applications, ultrasonic pachymetry still remains the gold standard for corneal thickness measurement for many refractive surgeons, said Perry S. Binder, MD.
“Ultrasound will ultimately be supplanted by future technologies,” Dr. Binder told Ocular Surgery News. “It is just going to take time for us to take a look and see what these can and cannot do. ”
Currently, some of the new technologies for measuring corneal thickness show differences from one another when used to measure the same corneas. (See accompanying article below.) Dr. Binder said it will be some time before ophthalmologists learn the best ways to use each of these technologies in clinical practice.
“There are lots of reasons why these technologies do not agree with each other, because they are all different in the way they analyze their data,” he said.
“They [the new technologies] make for great images, and I think there are other applications that we need them for, in addition to LASIK,” he continued. “We are putting plastics in the cornea, and it would be nice to be able to quantitatively measure how deep we actually placed them. As with any new technology, we do not know all of their capabilities. There may be other applications we can use them for that we currently are not aware of.”
Dr. Binder said he currently uses 50 MHz ultrasonic pachymetry for corneal thickness measurement and has recently begun using the Oculus Pentacam. He said he is studying these and other technologies to determine which might be the most accurate and useful diagnostic tools. Dr. Binder reviewed some of the available technologies for measuring corneal thickness in an interview with Ocular Surgery News.
Different devices record different corneal measurements in same eyes
LAS VEGAS — Several diagnostic devices reported different corneal measurements from the same patients, according to a presentation here.
Canan A. Utine, MD, and colleagues compared pachymetric and keratometric measurements taken by the Oculus Pentacam, the Bausch & Lomb Orbscan and the Carl Zeis Meditec IOLMaster to measurements using ultrasonic pachymetry and standard keratometry. They presented the results of their study in a poster presentation at the American Academy of Ophthalmology meeting.
The researchers compared central corneal thickness (CCT), thinnest corneal thickness (TCT) and keratometry in five patient groups: 49 myopic eyes, 29 keratoconic eyes, 24 emmetropic eyes, six hyperopic eyes and seven eyes that had undergone LASIK.
In keratoconic eyes, the Orbscan reported CCT and TCT as significantly thinner than ultrasonic pachymetry and Pentacam (P < .005). In myopic and hyperopic eyes, CCT and CTC measured by Orbscan were thicker than Pentacam.
In myopic, keratoconic and emmetropic eyes, the Pentacam reported keratometry as lower than the IOLMaster. In emmetropic and myopic eyes, the Pentacam reported keratometry as higher than the Orbscan (P < .005). The IOLMaster consistently reported the steepest keratometry.
Pentacam and Orbscan measurements were comparable in keratoconic eyes, and there were no significant differences in eyes with previous LASIK among all machines, according to the poster.
The authors suggested that the differences in measurements could be due to the way the devices take their measurements: the Orbscan scans the central cornea twice with a scanning slit technique, whereas the Pentacam scans 360· circularly.
“The knowledge about the study principles of the devices, in association with the clinical findings of the patients, will be helpful in the correct evaluation of the measurement results,” the authors said.


Ultrasound
Ultrasonic pachymetry uses sound waves to measure the thickness of the cornea, Dr. Binder explained.
The device sends out a wave of sound that bounces back when it encounters a change in substance, such as from solid to liquid. A probe picks up the signal and the device computes the difference.
“It does a differential measurement by measuring several hundred times a second to give you an average of those readings,” Dr. Binder said. “I will take two or three readings until I come up with one that is within 2 µm of the previous measurement, and I use that one.”
Ultrasound is probably the most widely used device for measuring corneal thickness, Dr. Binder said, but it has some drawbacks.
“One shortcoming is that you have to put the probe back in the same spot to get your readings,” he said.
Scheimpflug technology
The Pentacam uses a rotating Scheimpflug camera to capture images in three dimensions and compute the thickness of the cornea as well as other structures in the eye.
“It projects a slit beam across the cornea and then looks at the two sides of the beam and the difference in the width of the beam in several areas,” Dr. Binder explained. “Unlike ultrasound, which only can measure one spot, this measures across an 8 mm or 9 mm area.”
The device computes thickness at several spots on the cornea, including central, superior, inferior, nasal and temporal, and it ultimately creates a color printout that shows a diagram of the cornea from limbus to limbus. The printout is color coded for corneal depth — for instance, greens and blues indicate thinner areas and reds and oranges are thick.
“It gives the surgeon an overall appreciation for the thickness of the cornea in all areas,” Dr. Binder said, “whereas with ultrasound, you have to physically put the probe in these different areas.”
Corneal topography
Most corneal topographers map only the front surface of the cornea, but one topographer, the Bausch & Lomb Orbscan, estimates the back surface of the cornea by increasing its readings by 8%, Dr. Binder said. This gives an approximation of the readings that ultrasound technology produces, he said.
Confocal microscopy
Another technology that can be used to measure corneal thickness is confocal microscopy.
“This uses a high magnification lens, like a giant microscope that you focus through the tissue,” Dr. Binder said. “The operator first looks at the endothelium of the cornea and changes the depth of focus of the confocal microscope until it hits the front of the cornea, the epithelium.”
The difference between those two points is the corneal thickness, he said.
“That has been compared to ultrasound, and there have been differences as well, with that measuring thicker than what ultrasound tends to read,” he said.
High-frequency ultrasound/laser
The Ultrasound BioMicroscope (UBM, Paradigm Medical Industries), uses high-frequency ultrasound to create a cross-section image of the tissues in the anterior segment, Dr. Binder said.
“The surgeon looks at the cross image of the cornea and puts a bar where he thinks the epithelium is and another bar where he thinks the endothelium is, and the program calculates what the thickness is,” he said. “The advantage there is, like the Scheimpflug, you can get multiple thickness readings across the cornea in a given meridian. The drawback is that the surgeon has to place the cursor where he thinks the thickness is. The machine just does not necessarily do it automatically, so there is potential for error.”
Similar images can be created with laser technology rather than ultrasound using the Visante OCT (Carl Zeiss Meditec), a version of optical coherence tomography that was specifically designed for imaging the anterior segment. The Visante uses a different frequency of laser from Zeiss’ Stratus OCT, which is used to create retinal images and measure retinal thickness.
Very-high-frequency ultrasound
The Artemis very-high-frequency ultrasound, from Ultralink, is an even more accurate and high-speed probe that uses an immersion water bath to interface with the cornea.
“This gives some very nice high-resolution images of the cornea,” Dr. Binder said. “The negatives are that if the same operator puts his dots on the same spot several times, and comes back again and does it the next day on the same image, you get a pretty wide variation of 10 µm or 20 µm, which is not sensitive enough for a lot of our needs.”
Dr. Binder recommended that surgeons in private practice may want to wait until tests comparing these technologies have been published in peer-reviewed journals before investing in one or another.




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yarsky
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