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Slide 1 |
Reprinted with permission from Hill WE. The IOLMaster. Techniques in Ophthalmology. 2003; 1:62-67.
By optical coherence biometry (OCB), axial length measurements with the IOLMaster are the equivalent of upright, ultra-high-resolution immersion A-scan ultrasonography, approximately five times more accurate than standard applanation A-scan ultrasonography. The physics of OCB are discussed and several special applications of this device reviewed. Intraocular lens constants for OCB are typically higher than applanation A-scan ultrasonography but are very close to those for immersion A-scan ultrasonography. Because the IOLMaster is an optical device, measurements may not be possible in the presence of significant axial opacities, such as a central corneal scar, mature cataract, vitreous hemorrhage, or dense PSC plaque.
Cataract Surgery as a Refractive Procedure
The last few years have been witness to amazing advances in both the art and science of the refractive component of cataract surgery. Increasingly sophisticated intraocular lens (IOL) designs, IOLs in 0.25-diopter steps, aspheric designs for powers as high as +40.0 diopters, and the limited correction of higher-order aberrations are now available options. However, without the ability to consistently calculate the correct IOL power, an appreciation of the elegance of these advancements is lost within the surgical exercise.
It is helpful to remember that every aspect of cataract surgery will have an impact on the final postoperative refractive result. For example, if keratometry is off by 0.50 diopters, then the final refractive outcome will be off by that same amount. If the capsulorhexis is made larger than the optic of the IOL, an anterior shift in position may occur during capsular bag contraction, resulting in more postoperative myopia than anticipated. Other items, such as careful optimization of IOL constants and the IOL power calculation formula used, will also in.uence the refractive outcome. And, of course, errors in axial length measurement can have a profound effect on the refractive outcome, especially for high axial hyperopes.
Both our patients and our peers have come to view cataract surgery as not only a rehabilitative procedure, but as a refractive procedure as well. It is helpful to think of IOL power calculations as a chain of interconnected nuances. If one item is incorrect, the final outcome will be less than optimal.
Ideal Axial Length Measurement Technique
It has been well reported that the most common reason for incorrect IOL power calculations is an error in the measurement of axial length. Until recently, 10-MHz A-scan ultrasonography was the measurement technique most commonly available and limited the resolution of the exercise to approximately 0.10 mm. This translates to about ±0.25 diopters under optimal conditions, as when axial length is measured using an immersion technique.1 However, the accuracy of A-scan ultrasonography is less when carried out by the applanation technique, which produces a falsely short axial length and sometimes widely variable results due to varying degrees of corneal compression. The ideal axial length measurement technique should be one that can be carried out in an upright position, without corneal contact or compression, and with a level of accuracy high enough for outcomes consistently within 0.25 diopter of the target refraction.
The ideal measurement technique should also be accurate enough to satisfy future requirements, such as the more demanding IOL power calculations necessary to correct for any number of higher-order aberrations.
The Zeiss IOLMaster was approved for use in Europe in 1999 and in the United States in 2000. By employing a partially coherent light source rather than ultrasound, the IOLMaster appears to have fulfilled all of the most important objectives for measuring the axial length of the human eye with a level of accuracy unimaginable just a few years ago.
During testing, the patient is seated upright and there is no corneal contact. Because optical coherence biometry (OCB) uses an infrared light source rather than a 10- MHzsound beam, measurement accuracy is increased from 0.10 mm to between 0.02 mm and 0.01 mm, an improvement of approximately five times.2 With the introduction of OCB as part of the preoperative evaluation, axial length measurement errors are no longer a limiting factor, and the era of high-resolution IOL power calculations has begun.
The use of OCB to measure the axial length of the human eye was first reported in 1986.3 Since that time there have been many refinements, culminating in the introduction of the IOLMaster, the first commercial version of OCB for use in general ophthalmology.
The IOLMaster uses a modified Michelson interferometer to measure axial length with unprecedented accuracy. Interestingly, the use of OCB in ophthalmology is based on optical principles laid down more than 100 years ago by the German-American physicist Albert Michelson (1852-1931). Dr. Michelson’s original work in interferometry was so important that in 1907 he received the Nobel Prize in physics.
The Michelson interferometer portion of the IOLMaster is used to create a pair of coaxial 780-nm infrared light beams with a coherence length of approximately 130 nm. Unlike the classic Michelson interferometer, for which the eye would have to be kept perfectly still, the use of a dual coaxial beam allows the IOLMaster to be insensitive to longitudinal movements and makes axial length measurements mostly distance-independent.
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While one mirror of the interferometer is fixed, the other mirror is moved at a constant speed by a small motor. This process takes one of the light beams out of phase with the other by twice the displacement of the moving mirror. Both beams of light then illuminate the eye to be measured and are reflected at the level of the cornea and the retinal pigment epithelium. After passing through a polarizing beam splitter, all light beam components are combined together, producing interference fringes of alternating light and dark bands. The constant speed of the measuring mirror causes a Doppler modulation of the intensity of the interference pattern. An optical encoder is then used to sense the position of the moving mirror with great precision, which is then translated into an axial length figure (Slide 1).4-6
Because OCB measures from the corneal vertex to the retinal pigment epithelium, and A-scan ultrasound measures from the corneal vertex to the vitreoretinal interface, some method of calibration is necessary to avoid a measurement error of 0.20 mm, or the approximate thickness of the retina at the center of the macula.
At the Eye Clinic at the University of Würzburg, Haigis and others calibrated axial length readings from the IOLMaster against immersion technique measurements with the exquisitely accurate Grieshaber Biometric System (GBS). The GBS is an ultra-high-resolution ultrasound biometer that employs four 40-Mhz counters and is capable of an astonishing accuracy of 20 µm.4
Based on a comparison of the measurements obtained with the GBS, an internal algorithm for the IOLMaster was developed such that the axial length displayed by the IOLMaster mirrors that of the GBS. In essence, the IOLMaster is the equivalent of an upright, noncontact, ultrahigh-resolution immersion A-scan, consistently accurate to within 0.01 mm.
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Axial length measurements with the IOLMaster are straightforward and surprisingly quick. Although mostly operator-independent, some degree of interpretation is still necessary for optimal refractive outcomes (Slide 2).
The patient is seated comfortably and positioned in a chin rest similar to what is typically used for a slit lamp. The overview mode is used for course alignment; the patient looks at a small, yellow fixation light. Once the video image of the eye is centered, the operator switches to the axial length measurement (ALM) mode. The patient then views a small red light and the image of the eye is enlarged, with the iris filling most of the video screen. It is best if nothing has touched the corneal surface prior to axial length measurements (e.g., an applanation tonometer or contact lenses).
Measuring axial length with the IOLMaster allows a high degree of flexibility. Rather than simply positioning a small, in-focus image in the middle of a set of video screen cross hairs, the operator can instead maneuver the focusing spot anywhere within the measurement reticule, and even focus in or focus out. In this way it is possible to sample different areas around the visual axis until the best axial length display is obtained. Then, once that best area is discovered, all subsequent measurements are taken from that location.
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This technique is especially useful for eyes with small corneal scars, anterior cortical spokes, posterior subcapsular plaques, or other localized media opacities (Slide 3).
If the signal-to-noise ratio (SNR) of the eye to be measured is borderline (2.0-1.6), focusing in or focusing out in such a way that the focusing spot enlarges to about the same size as the measurement reticule often improves the quality of the axial length display. This is possible with the IOLMaster because axial length measurements by OCB are distance-independent.
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It is useful to take all 20 measurements. At least four of these measurements should be within 0.02 mm of one another and should exhibit the characteristics of an ideal display. In terms of accuracy, an ideal axial length display is far more important than a high SNR (Slide 4).
During axial length measurements it is important for the patient to look directly at the small red fixation light. In this way, axial length measurements will be made to the center of the macula, giving the refractive axial length rather than the anatomic axial length. For eyes with high to extreme myopia and a posterior staphyloma, being able to measure to the fovea is an enormous advantage over conventional A-scan ultrasonography.
One other interesting and helpful feature of the IOLMaster is if there is a high refractive error (more than ±4.00 D), measurement5 can also be taken with the patient’s glasses in place to ensure adequate fixation. Measurements with and without glasses are usually identical.
The axial length can be determined in most eyes with a high degree of precision, including extreme axial hyperopes and myopes, aphakes, pseudophakes, and even for eyes filled with silicone oil.
The characteristics of an ideal axial length display by OCB are the following: SNR ratio greater than 2.0; tall, narrow primary maxima, with a thin, well-centered termination; and at least one set of secondary maxima. However, if the ocular medium is poor, secondary maxima may be lost within a noisy baseline and not displayed. At least 4 of the 20 measurements taken should be within 0.02 mm of one another and show the characteristics of a good axial length display.
If given a choice between a high SNR and an ideal axial length display with a lower SNR, the quality of the axial length display should always be the determining factor for measurement accuracy.
Because the IOLMaster is an optical device, any significant axial opacity has the potential to be a problem. Clinical situations such as a mature or darkly brunescent lens, central posterior subcapsular plaques, anterior cortical spokes, corneal scars that pass through the visual axis, and vitreous hemorrhages may interfere with the partially coherent light beams and decrease the SNR to the point that may preclude a meaningful measurement. On the other hand, very difficult immersion ultrasonography measurements, such as eyes with posterior staphylomata or eyes in which the vitreous cavity has been temporarily filled with silicone oil, are almost routine with the IOLMaster. In the typical North American ophthalmology practice, approximately 90% of patients can be measured successfully using the IOLMaster. The remaining 10% of patients must be measured by A-scan ultrasonography for the reasons outlined above.
Caution Measuring Pseudophakic Eyes
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On rare occasions, reflections from the surface of an IOL in the pseudophakic eye may produce an axial length reading falsely short by as much as 4.0 mm. This sometimes occurs if the measurement is taken directly through a reflection off the IOL, which is typically visible on the video monitor. This phenomenon has been seen in pseudophakic eyes with PMMA, silicone, and acrylic IOLs and should be suspected if there is a large difference in axial length between the right and the left eyes. This can easily be avoided by taking measurements from multiple areas within the measurement reticule and with the focusing spot moved away from any IOL reflection (Slide 5).
The IOLMaster is pretty much an "all-in-one" IOL power calculation device. Not only will it do axial length measurements with great precision, but it will also measure the central corneal power by automated keratometry. The instrument takes five keratometry measurements within 0.5 seconds and averages them. The latest software revision (version 3.01) has an improved keratometry algorithm and will alert the operator if a keratometry measurement is questionable. Some operators have found that central corneal power measurements with the automated keratometry feature of the IOLMaster may run anywhere from 0.25 D to 0.50 D steeper than with manual keratometry.
The IOLMaster will also measure the anterior chamber depth (the distance between the optical section of the cornea and the anterior surface of the crystalline lens) using a lateral slit illumination at approximately 30 degrees to the optical axis. This measurement is helpful for IOL power calculation formulas, such as Haigis and Holladay 2, which require a measured anterior chamber depth.
An additional software option can be used to measure the horizontal corneal diameter to within 0.10mm, which is very useful for estimating the haptic diameter of backup anterior chamber IOLs and for inputting ACD data into the Holladay 2 formula.
Included in the standard IOLMaster software package are five popular IOL power calculation formulas (Holladay, SRK/T, Haigis, SRK II, and Hoffer Q). IOL power calculations can be carried for four IOLs at a time and to a precision of either 0.50 or 0.25 diopters. The IOLMaster software will accommodate as many as 20 surgeons, each with up to 20 preferred IOLs and corresponding personalized lens constants.
IOL constants for the IOLMaster will be closer to those normally seen for the immersion technique and are typically higher than what would normally be used for the applanation technique, which is based on a falsely short axial length due to corneal compression. In making the transition from applanation IOL constants to IOLMaster IOL constants, a good place to begin would be to increase already optimized applanation IOL constants by 0.50 for the SRK/T formula and by 0.29 for the Holladay and Hoffer Q formulas. Failure to make this initial adjustment may result in approximately 0.50 diopters of initial postoperative hyperopia. The IOLMaster software comes with an IOL constant optimization feature that can subsequently be used to refine postoperative outcomes. Some lenses, like the Alcon SA60AT, show very little difference when compared to immersion A-scan ultrasonography, while others, like the Bausch & Lomb U940A, show a larger difference.
To determine the best initial IOLMaster constant, Dr. Wolfgang Haigis at the University of Würzburg has recommended the following approach for calculating the initial IOLMaster SRK/T A-constant:
AIOLMaster = AUltrasound + 3 x (ALIOLMaster - ALUltrasound)
AIOLMaster = Optimized A-constant for IOLMaster
AUltrasound = Optimized A-constant for ultrasonography
ALIOLMaster = Average IOLMaster axial length
ALUltrasound = Average ultrasound axial length
One additional advantage of the IOLMaster is a significant increase in office efficiency. With manual keratometry and immersion A-scan ultrasonography, the average time it takes to do axial length measurements, corneal power determination, and IOL calculations using the Holladay IOL Consultant in our office was typically 20 minutes. Using the IOLMaster and an optimized version of the Haigis formula, this time has been reduced to approximately 4 minutes. For any busy surgical practice, reducing the time needed to complete a common measurement by 80% is of enormous benefit.
Think of the IOLMaster as the equivalent of an ultra-high resolution immersion A-scan ultrasonography, giving the refractive axial length rather than the anatomic axial length. Because the IOLMaster is an optical device, measurements may not be possible in the presence of significant axial opacities, such as a central corneal scar, mature cataract, vitreous hemorrhage, or dense PSC plaque. IOL constants for the IOLMaster are often slightly higher than the manufacturer's suggested numbers for A-scans carried out by the applanation technique, but they are very close to those for the immersion technique. It is suggested that IOLMaster-specific IOL constants be used with the various popular IOL power calculation formulas.
With the introduction of the IOLMaster, the era of high-resolution IOL power calculations has begun. Ophthalmology now has an indispensable tool for a time in the not-too-distant future when IOL-based higher-order aberration correction may become commonplace.
