|Year : 2020 | Volume
| Issue : 3 | Page : 323-328
Insights into IOLMaster®: A useful tool for cataract surgeons
Bharat Gurnani, Kirandeep Kaur, Poulomi Das
Cataract, Cornea and Refractive Services, Aravind Eye Hospital, Pondicherry, India
|Date of Submission||14-May-2020|
|Date of Acceptance||27-May-2020|
|Date of Web Publication||23-Dec-2020|
Dr. Kirandeep Kaur
Fellow Pediatric Ophthalmology and Squint Services, Aravind Eye Hospital, Pondicherry
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Gurnani B, Kaur K, Das P. Insights into IOLMaster®: A useful tool for cataract surgeons. Kerala J Ophthalmol 2020;32:323-8
| Introduction|| |
The IOL Master (Carl Zeiss Meditec AG, Goeschwitzer Strasse 51-52 07745 Jena, Germany) was first launched in 1999 and revolutionized ocular biometry principles due to its superior repeatability, reproducibility, and accuracy in results., The principle involved is Michelson's partial coherence interferometry to measure axial length (AL). It also uses the innovative lateral slit illumination of the crystalline lens and the cornea to measure anterior chamber depth (ACD) and autokeratometry to determine corneal curvature. Currently, it is the gold standard for preoperative computation of intraocular lens (IOL) power in patients undergoing cataract surgery. Being a noncontact noninvasive technique, it avoids the use for topical anesthesia, making it particularly profitable in studying ocular growth, myopia, and interventions in both children and adults. Since then, several other instruments have been introduced. They all utilize the same Michelson optical low-coherence interferometry to determine the AL and a range of other technologies to measure other components.
The other instruments include:
- Lenstar (Haag-Streit, Mason, OH, USA),
- Oculus Pentacam® AXL
- AL Scan (Nidek, Inc., Fremont, CA, USA)
- Aladdin (Topcon Corp., Oakland, NJ, USA)
- Galilei G6 (Ziemer Ophthalmic Systems AG, Switzerland), and
- OA-2000/OA-1000 (Tomey Corp., Phoenix, AZ, USA).
| Study Devices|| |
The IOLMaster 500 is a noncontact, high-resolution biometric device that measures AL using the principle of partial coherence interferometry, keratometry using reflected light spots on the surface of the cornea, ACD from optical sections of the lens and the cornea, and white-to-white distance based on a scleral and iris image.
The IOLMaster 700 is another noninvasive, noncontact, high-resolution biometric device that employs swept-source technology (laser with a tunable wavelength centered on 1055 nm) and generates ultrasound B-scan images to determine the biometric data of the eye. These B-scans are displayed as a full-length optical coherence tomography (OCT) image showing anatomical details on a longitudinal section through the entire eye. Thus, unusual ocular features, such as a tilt or decentration of the crystalline lens, can be detected. It measures AL, ACD, central cornea thickness, and lens thickness using swept-source OCT technology, corneal curvature using reflected light spots on the surface of the cornea, pupil diameter, visual axis (line of sight), and white-to-white measurement based on a scleral and iris image. The device acquires multiple measurements for each of the parameters in a single measurement-capture process. Axial measurements are based on swept-source frequency-domain OCT enabling a 44-mm scan depth with 22-mm tissue resolution. Note that corneal and lens thickness has been added to the already-existing measurement modes of the IOLMaster 500, and the technology used to measure ACD is different. The Sweptsource OCT machine scans sequentially through a series of wavelengths to enhance the interference pattern, which is then decoded with Fourier transformation into an Ascan trace. Multiple adjacent A-scans can be combined to form a B-scan image. The advantages of swept-source OCT include a deeper range of imaging into the eye, less sensitivity reduction with depth, and faster scanning speeds [Figure 1].
Lenstar LS 900
The Lenstar LS 900 is another noninvasive, noncontact, biometric device that measures AL, ACD, central corneal thickness, and lens thickness using the principle of partial coherence interferometry, corneal curvature using reflected light spots on the surface of the cornea, pupil diameter, visual axis (line of sight), and white-to-white measurement based on a scleral and iris image. All the above axial measurements are obtained by optical low coherence reflectometry using a broadband light source (20–30 nm) with a center wavelength of 820 mm. From a single measurement, all of these mentioned parameters can be assessed. The Lenstar LS 900 allows the user to view a clearly defined display represented in the style of an immersion A-scan. The measurement calipers can be adjusted if necessary, in the presence of artifacts, for example, vitreous floaters.,,,
[TAG:2]Components of Iolmaster, [Figure 2]a and [Figure 2]b, [Figure 3]a and [Figure 3]b[/TAG:2]
|Figure 2: (a) Various components of IOLMaster, (b) description of various components of IOLMaster|
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|Figure 3: (a) Lateral view image of IOLMaster showing various components, (b) description of various components of image 3a|
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- Instrument table IT 3 L
- Holding bar for securing the IOLMaster on the instrument table
- Keyboard support
- Narrow holding bracket for securing the IOLMaster on the keyboard support
- Paper pads for patient chinrest
- Power isolation transformer for connection of external accessory units
- Network isolator
- Software option A plus
- Software option B
- Connecting cable for coupling with PC.
The various parts are highlighted in the line diagrams.
Adjust the instrument table, the headrest, and IOLMaster according to the patient's height so that they can sit comfortably in a stable position. The whole procedure is explained in detail to the patient. It is necessary not to move the head or blink during the procedure. Let the patient blink before starting the measurement. They should fix steadily to the fixation light. The eyelid or eyelashes must not obscure light spots seen on the screen. If necessary, the patient should be asked to open the eye widely or assist them manually to keep it open. Measurement started with a centered and focused beam. If the signal quality is worse, then decentering and defocussing should be done and try again. A total of 3–5 measurements can be done per eye. If IOLMaster does not display a mean value or inconsistent reading is measured, find any posterior segment pathology or verify each measurement trace. Eventually then, zoom in and shift manually the measurement cursor for obtaining a correct signal [Figure 4].
|Figure 4: Image depicting patient positioned comfortably in front of IOL Master|
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[TAG:2]Uses of Iolmaster,[Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10][/TAG:2]
|Figure 7: Fine alignment with correct keratometry and anterior chamber volume. Also, the signal-to-noise ratio is within normal limit|
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|Figure 8: Another example of the fine alignment of IOLMaster when a patient is being evaluated|
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- AL measurements
- Keratometry (corneal curvature)
- ACD measurements
- White-to-white diameter (WTW) (optional)
- Calculation of IOL power.
The measurement is based on the patented interference optical method known as partial coherence interferometry.
- Silicone-filled eyed.
It is determined by measuring the distance between reflected light images projected onto the cornea.
The IOLMaster requires three keratometry readings for the final calculation. The IOLMaster reflects six points of light, arranged in a 2.3-mm diameter hexagonal pattern (measured by digital calipers) from the air/tear film interface. The instrument's internal software measures the separation of opposite pairs of lights objectively and hence, the toroidal surface curvatures are calculated.
Anterior chamber depth
It is measured as the distance between the optical sections of the crystalline lens and the cornea produced by lateral slit illumination.
When ACD mode is turned on, the lateral slit lamp is automatically turned on. This illumination thus produced subjectively appears to be very bright to the patients.
The ACD can be measured only on phakic eyes. If ACD measurements are performed on pseudophakic eyes, it will result in measuring errors and/or incorrect readings. The readings for pseudophakic eyes do not reflect the ACD correctly.
Eventually, five ACD readings will be taken together with the calculated mean.
It is determined from the image of the iris.
Method of measurement:
- The patient is asked to look at the fixation point
- The point of focus is iris and not the spotlight
- The validity of the WTW determination depends on this check of the correct recognition of the iris edge image
- The WTW is the horizontal diameter of the iris
- In addition to the measured WTW value, the deviation of the visual axis from the center of the iris (X, Y) will also be displayed.
Intraocular lens power calculation
- Depending on the IOL calculation formula, once all the measurements have been taken, options can be generated for IOL to be implanted
- Click on the appropriate option to select the desired formula
- The IOL Haggis, Hoffer Q, Holladay, SRK 2, and SRKT formulas are implanted as standard
- After refractive corneal surgery, the Haggis-L formulae may be selected.
Another important entity to be kept in mind during the measurement is SNR (signal to noise) ratio. In that:
- Red indicates incorrect value
- Yellow indicates uncertain value/borderline
- Green indicates good value.
| Advantages of Iolmaster|| |
- Easy to learn and quick time measurements can be done
- Noncontact measurements
- Ultra-high-resolution biometry
- Extensive integrated safety features which make measurements less cumbersome
- It has the advantage of imaging fovea in case of posterior staphyloma
- It gives the true refractive length rather than anatomical AL
- The accuracy of IOLMaster is 0.02 μm which is operator dependent
- Highly ametropic patients with refractive error can wear glasses while sitting on the IOLMaster which aids in fixation.
- In patients with media opacities such as corneal opacities, dense cataract, NS iv grade, and posterior polar cataract, it cannot measure AL
- In case of vitreous hemorrhage, again AL cannot be measured
- Difficulty in measuring AL in infants, small children, and mentally disabled patients.
| Intraocular Lens Power Calculation with Ultrasound (A-Scan) Biometry|| |
The three major aspects of intraocular lens power calculation are:
- IOL power calculation formulae
- Clinical variables – IOL power calculations in special situations.
- AL measurement
- Keratometry (K-reading) – corneal power
- Effective IOL position.
While performing A-scan biometry, a thin parallel sound beam is emitted from the probe tip at a given frequency of approximately 10 MHz, with an echo bouncing back into the tip of the probe as the sound beam strikes each interface.
An interface is defined as the junction between any two media of different densities and velocities.
In the eye, these interfaces can be divided into:
- Anterior corneal surface
- Aqueous/anterior lens surface
- Posterior lens capsule/anterior vitreous
- Posterior vitreous/retinal surface
- Choroid/anterior scleral surface.
The process of measuring the curvature of the cornea is called keratometry.
By definition, a keratometer (ophthalmometer) is a noninvasive diagnostic instrument for assessing the curvature of the anterior surface of the cornea. It is used particularly for assessing axis and the extent of astigmatism [Figure 11].
|Figure 11: Mid-level ophthalmic personnel performing keratometry on the patient|
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| Keratometry can be performed by:|| |
- Manual topography
- IOLMaster/Lenstar 900.
A-scan measurement techniques can be divided into:
By the applanation biometry method, an ultrasound probe is placed directly on the cornea which is attached to a slit lamp.
After explaining the procedure to the patient under topical anesthesia, clean the probe. The probe is placed on the patient's cornea which is attached to a device that delivers adjustable sound waves. The measurements are displayed as spikes on the screen of the monitor. The appearance of the spikes and the distance between them can be correlated to structures within the eye and the distance between them. The operator while performing the handheld method aims the probe toward the macula of the eye. The alignment with the optical axis is indicated by high lens spikes and a high retina spike on the scan graph [Figure 12]a, [Figure 12]b, [Figure 12]c, [Figure 12]d.
|Figure 12: (a) Image of the patient depicting the positioning of the probe on the ocular surface, (b) handheld method of A scan – the probe is placed on the patient's cornea which is attached to a device that delivers adjustable sound waves, (c) display monitor on which readings are noted for axial length and keratometry, (d) the alignment with the optical axis is indicated by high lens spikes and a high retina spike on the scan graph|
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Immersion A-scan biometry
The immersion technique employs the use of a Prager Scleral Shell ( ESI, Inc, 2915 Everest lane N, Plymouth, Mn 55447).The immersion technique is accomplished by placing a small scleral shell between the eyelids of the patient, filling it with saline and then immersing the probe into the fluid, being careful to avoid contact with the cornea. It is more accurate than contact method because corneal compression is avoided. The eyes measured with this method are, on an average, 0.1–0.3 mm longer.
[TAG:2]Intraocular Lens Power Calculation Formulae[/TAG:2]
- Theoretical formulae – Binkhorst, Modified Binkhorst, Colenbrander–Hoffer, Gill's, Clayman's, and Fyodorov formulae
- Regression formulae – SRK-I, SRK-II, Modified SRK-II, Holladay-I, Holladay-II, and Hoffer-Q
- Most commonly used- SRK II formula: P= A-2.5 L-0.9 K.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]