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New instrument uses laser, not ultrasound for axial length
More precise measurement helps target emmetropia.
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Introduction of a new axial length measuring device could make IOL power calculations more precise and help surgeons achieve emmetropia more often.
Zeiss’ partial coherence interferometer debuted in Germany in September 1999 and is poised for domestic distribution. The device, called the IOLMaster, uses laser interferometry instead of ultrasound to measure the eye.
According to R. Bruce Wallace III, MD, a 1992 study in Denmark showed that axial length measurements cause most IOL power calculation errors. Quality axial length measurements will reduce number and severity of refractive surprises, he said.
“If axial length measurements are extremely accurate, then cataract surgery becomes an even more powerful refractive procedure,” Dr. Wallace said. “The accuracy of the IOLMaster will encourage us to reduce error in all components of IOL calculations.”
The IOLMaster has sold more than 100 units since its debut at the European Society of Cataract and Refractive Surgeons in September 1999, said William Burnham, OD, marketing manager of the instrument.
New technology
---The IOLMaster
requires the patient to fixate on a light, so a technician knows
whether the patient is aligned with the visual axis.
The device
is a two-beam partial coherence interferometer that measures axial length in
0.4 seconds. It produces two beams by means of an optical delay path. These two
beams of short coherence light are reflected from the surfaces of the cornea
and then the retina. The delay path is compared to the optical pathway of the
eye and that interference pattern is evaluated. Corneal curvature and anterior
chamber depth are also measured by the IOLMaster.
“Axial length is the most important measurement of the IOL calculation process. Minimal errors in the axial length measurement correspond to significant errors in the postoperative refraction,” Dr. Burnham said. “With the IOLMaster axial length is measured objectively.”
Technicians can easily learn the procedure, which now works much like an autorefractor, he said. Also, the IOLMaster detects whether it is reading the right or left eye automatically.
The IOLMaster is accurate within 10 µm with a test eye and about 30 µm in the human eye, Dr. Burnham said (standard deviation ±25 µm). Ultrasound units studied for comparison were accurate to about 100 µm.
“If the accuracy of ultrasound is 100 µm, then that translates into 0.28 D of refractive variation compared with 30 µm, which is less than 0.1 D,” Dr. Burnham said. “We’re improving the end result, which is important to the doctors to target emmetropia and provide their patients with spectacle freedom.”
“The accuracy combined with the ease of use of the IOLMaster make it a great benefit to the ophthalmological practice,” Dr. Burnham said.
Test of technology
In the United States, as part of regulatory evaluation of the device, James Gills, MD, I. Howard Fine, MD, and Dr. Wallace have used the IOLMaster. John Retzlaff, MD, has analyzed their data in a comparative analysis between IOLMaster and conventional keratometry and ultrasound biometry. Kenneth Hoffer, MD, and Jack T. Holladay, MD, are doing their own analysis of the instrument.
According to Dr. Holladay, ultrasound typically is accurate to 100 µm, which corresponds to a possible refractive error of about 0.25 D. Because the IOLMaster achieves accuracy within 30 µm, refractive errors stemming from axial length mismeasurements are limited to 0.025 D, 10 times more accurate than ultrasound.
Also, ultrasound measurements stop at the internal limiting membrane of the retina, Dr. Holladay said. Partial coherence interferometry penetrates to Bruch’s membrane, where the photoreceptors are, and then into the choroid.
Ultrasound measurements assume the photoreceptors are between 150 µm and 250 µm behind the internal limiting membrane. Partial coherence interferometry penetrates this veil and finds out exactly how far back the photoreceptors lie.
“You’re measuring to the exact point that you want,” Dr. Holladay said.
New demands
The IOLMaster requires the patient to fixate on a light, so the patient is aligned with the visual axis.
“In ultrasound, you never can be sure of that because the patient is not looking at anything, so you’re just shooting toward the back of the eye,” Dr. Holladay said. “You don’t really know where the fovea is. Because the eye is not a perfect sphere, if someone is looking off to the right a little bit, it’s shorter toward the width, so you get a lot shorter axial length.”
Patients with high myopia often have staphylomas, which greatly distort measurements in the ultrasound, because their eye is nothing like a sphere posteriorly, he added.
However, the laser cannot penetrate 4+ brunescent cataracts to generate an interference pattern. But patients who have waited until their vision is counting fingers are usually not as demanding as to final refraction.
“Axial length is no longer going to be one of the limiting factors,” Dr. Holladay said.
“The position of the lens within the eye is going to become the limiting factor in the refractive outcome. In the past, the axial length has almost always been the variable that causes an error in the 0.25 D and 0.75 D ranges. In this instrument, the axial length error goes away and we’re down to the other variables: K, IOL power accuracy and predicting the axial position of the IOL within the eye.”
The IOLMaster is nearing regulatory approval in the United States The U.S. Food and Drug Administration (FDA) requested more documents about accuracy and calibration from Zeiss Humphrey in early January, so the approval is expected soon, according to Dr. Burnham.
Optical biometryPrinciple: dual beam partial coherence interferometer
Optical and acoustical biometry
Optical and acoustical axial length
For Your Information:
- William Burnham, OD, can be reached at Zeiss Humphrey Systems, 5160 Hacienda Drive, Dublin, CA 94568 U.S.A.; +(1) 877-486-7473; fax: +(1) 925-557-4775. Mr. Burnham is marketing manager for Zeiss Humphrey Systems.
- R. Bruce Wallace III, MD, can be reached at 4110 Parliament Drive, Alexandria, LA 71303 U.S.A.; +(1) 318-448-4488; fax: +(1) 318-448-9731. Dr. Wallace has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Jack T. Holladay, MD, can be reached at 5108 Braeburn Drive, Bellaire, TX 77401 U.S.A.; +(1) 713-668-6828; fax: +(1) 713-669-9153. Dr. Holladay did not disclose whether he has a direct financial interest in any of the products mentioned in this article or if he is a paid consultant for any companies mentioned.
- For more information on the IOLMaster, contact Carl Zeiss, D-73446, Oberkochen, Germany; +(49) 736-42-03372; fax: +(49) 736-42-04485.