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IOL Master for Determining the IOL Power at the Time of Surgery

Hampton Roy

Warren E Hill

Introduction

The Zeiss IOLMaster is a noncontact optical device that measures axial length of the eye by partial coherence interferometry, with a consistent accuracy of 0.02 mm (less than 0.10 diopter), or better. It also does automated keratometry, measures anterior chamber depth, the horizontal corneal diameter, and calculates intraocular lens powers, all in a single sitting.1–6

The IOLMaster employs a modified Michelson interferometer to divide, and phase delay, a 780 nm partially coherent beam of light. One beam is reflected from the corneal surface, while the other is reflected from the retinal pigment epithelium. A photodetector and on-board computer translate the interference pattern produced by the two beams into a highly accurate measurement of axial length. Calibrated against the ultra-high resolution 40-MHz Greishaber Biometric System, an internal algorithm then approximates the distance to the vitreo-retinal interface, for the equivalent of the ultrasonic axial length. Considering the fact that axial length measurements by A-scan ultrasonography (using a standard 10-MHz transducer) have a typical resolution of 0.10 mm to 0.12 mm, axial length measurements by the IOLMaster represent a fivefold increase in accuracy.

Using the instrument is straightforward. The patient is placed in the chin rest and looks straight ahead at a small red fixation target. The eye is viewed on a video screen by the technician during all phases of measurement, allowing for proper alignment.

Modes

The following modes are useful:

Overview mode

This allows the technician to grossly align the instrument.

Axial length mode

The axial length can be determined in most eyes with a high degree of precision, including high myopes with posterior staphyloma, aphakia, pseudophakia and even for eyes filled with silicon oil. The machine displays a signal-to-noise ratio for each

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measurement, as one indication of reliability, and also compares multiple measurements. If the measurements are all within 0.1 mm, the machine displays an average axial length. If the measurements fall outside this range, the technician is instructed to evaluate the series of measurements before concluding the examination.

The characteristics of a proper axial length display are the following:

•Signal-to-noise ratio greater than 2.0.

•Tall, narrow primary maxima, with a thin, well-centered termination.

•At least one set of secondary maxima. However, if the ocular media is poor, secondary maxima may not be displayed.

•At least four of the 20 measurements taken should be within 0.02 mm of one another and show the characteristics of a good axial length display.

Automated keratometry mode

IOLMaster uses an integrated autokeratometer to determine the corneal curvature of the principal meridians with corresponding axes, displayed in diopters, or in millimeters. The instruments take five 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.

Anterior chamber depth mode

The distance between the optical section of the cornea, and the crystalline lens, is measured using a lateral slit illumination at approximately 30 degrees to the optical axis. This measurement is helpful for intraocular lens power calculation formulas, such as Haigis and Holladay 2, which require a measured anterior chamber depth.

Intraocular lens power calculation mode

The collected data can be transferred to the intraocular lens power calculation area. Five intraocular lens power calculation formulas (Haigis, Hoffer Q, Holladay 1, SRK II, SRK/T) are included with the IOLMaster software. The surgeons selects the calculation formula that he wishes to use, the target refraction, and the IOLMaster will calculate the power of upto four intraocular lenses in the physician database.

The IOLMaster can accommodate as many as 20 surgeons, each with upto 20 preferred intraocular lenses, and corresponding personalized lens constants.

The IOLMaster is easy to use, accurate and has excellent reproducibility.

New Intraocular Lens Constants

Some lenses, like the Alcon SA60AT, show very little difference when compared to immersion A-scan ultrasonography, while others, like the Bausch and Lomb U940A show a larger difference. In order to determine the best initial IOLMaster constant, Dr. Wolfgang Haigis, at the University of Würzburg in Germany has recommended the following approach for calculating the initial A-constant.

AUltrasound = Optimized A−constant for ultrasonography

ALIOLMaster = Average IOLMaster axial length

ALUltrasound = Average ultrasound axial length

Advantages of Using the IOL Master

•No topical anesthetic is needed.

•Multiple measurements, at different instrument stations, are not necessary.

•Patients sit upright.

•Using the IOLMaster is quick, accurate, and requires minimal training, although some Interpretation by the operator is necessary.

•Noncontact technique precludes the occurrence of corneal epithelial injuries, and the transmission of infections.

Disadvantages

•Unable to use for dense nuclear cataracts, posterior subcapsular plaques, corneal scars and vitreous hemorrhages. In any case in which the axial opacity interferes with the partially coherent light beams, IOL master cannot be used.

•Unable to use on patients that cannot fully cooperate because of physical or psychological reasons.

Approximately 95 percent of patients can be measured successfully using the IOL Master.

Results of the IOL Master

The accuracy of intraocular lens power predictions from the IOLMaster measurements have been found to be as good, or better, than immersion A-scan ultrasonography. With a combination of the IOLMaster, and the Holladay 2 formula, Warren E. Hill, M.D. has been able to consistently achieve refractive outcomes with a mean absolute prediction error of better than ±0.25 diopters. This approaches the theoretic limit of the exercise, given the fact that intraocular lens implants come in 0.50 diopter steps.

Summary

Think of the IOL Master (Fig. 6.1) as a form of 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 corneal scar, mature cataract, vitreous hemorrhage, or dense PSC plaque, etc.

IOL constants for the IOLMaster will often be slightly higher than the manufacturer’s suggested numbers and are very close to those used for

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FIGURE 6.1 IOL master

immersion A-scans. It is suggested that IOLMaster-specific intraocular lens constants be used with the various popular intraocular lens power calculation formulas.

The IOLMaster is a highly reliable tool for determining the intraocular lens power prior to surgery.

References

1.Vogel A, Dick B, Krummenauer F: Reproducibility of optical biometry using partial coherence interferometry. Intraobserver and interobserver reliability. J Cataract Refract Surg 27:1961–68, 2001.

2.Schachar RA, Levy NS, Bonney RC: Accuracy of intraocular lens powers calculated from A- scan biometry with the Echo-Oculometer. Ophthalmic Surg 11:856–58, 1980.

3.Drexler, W, Findl O, Menapace R et al: Partial Coherence Inferometry: A Novel Approach to Biometry in Cataract Surgery. Am J Ophthalmol 126:524–34, 1998.

4.Holladay JT, Musgrove KH, Praeger TC et al: A three-part system for refining intraocular lens power calculations. J Cataract Refractive Surgery 14:17–24, 1988.

5.Wallace RB: IOLMaster Optical Coherence Biometry: Accurate Axial Length Measurement for Cataract Surgery and Refractive Lensectomy. Refractive Eyecare for Ophthalmologists 4:17– 20, 2000.

6.Retzlaff J, Sanders DR, Kraff MC: Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refractive Surgery 16:333–40, 1990.