| |
These FAQs are not intended as a substitute for proper training and the User Manual, and are provided only as a guideline for correct and safe instrument operation. Contact your local Carl Zeiss Meditec representative for instrument training. Refer to the User Manual for detailed operating instructions.
What should I keep in mind while taking measurements?
General:
- Adequate tear film: Let patients blink before taking each measurement.
- Position the instrument table, headrest and instrument properly to avoid tensed up patients and unnecessary eye movements.
- When positioning the instrument to the patient (Overview menu), a coarse focus will do.
- Ask patients to focus steadily on the orange/red fixation point.
- For axial length measurements, ask whether the patient sees the fixation point.
Axial length measurement:
- Measure cataract patients with a larger light spot, almost they size of the green circle. (Measurement is independent of distance.)
- It is advisable to measure nuclear cataracts slightly off axis—shift vertically, not laterally—using a small laser spot.
- Patients with an "error" message (in case of dense cataract/strong opacity of posterior capsule) and a signal-to-noise ratio between 1.4 and 1.6 often still yield reliable results displayed beside the graph. (If the display shows five consistent values, they should be entered manually in the calculation table by double-clicking the dot in the graph.)
- In case of double or multiple peaks, adjust the dot in the graph if necessary. (For details, see user manual.)
- In patients with ametropia of more than about 6 D, measurements through the spectacle give better results. It improves SNR and reduces standard deviation. Measurement through contact lenses gives incorrect values.
How comparable are ultrasound and optical biometry measurements? How do different methods of measurement contribute to measurement differences?
Ultrasound instruments measure the distance between the anterior surface of the cornea and the limiting membrane, whereas the IOLMaster measures the distance between the anterior cornea (strictly speaking, the anterior surface of the tear film) and the pigmented epithelium.
A great number of comparative measurements have been performed regarding these methodological differences. The "correction factors" resulting from these measurements were integrated in the device- internal software so that the well-known biometrical formulas with customized IOL constants for immersion technique can be used without any problems.
In measurements using the ultrasound contact technique, the amount of applanation (shortening of anterior chamber due to the pressure exerted on the cornea by the ultrasound probe by approximately 100-300 µm) also must be considered. Measurement deviations vary from patient to patient due to differences in anatomy and, in the case of ultrasound measurements, they also depend on the examiner. To an extent, however, correction factors already have been integrated in some ultrasound systems, so that on these instruments the values obtained with the contact technique do not differ in mean from those obtained with immersion instruments. At any rate, the IOLMaster measures the exact distance existing in the viewing process, whereas ultrasound instruments determine only the approximated distance. With the IOLMaster the measured lengths are operator-independent.
Due to appropriate factory-set defaults on the IOLMaster, you can expect to obtain results that correspond to those measured with the ultrasound immersion technique.
How do axial length results compare for ultrasound biometry and optical biometry?
The IOLMaster delivers measurement results adapted to ultrasound biometry with immersion technique. Compared to contact ultrasound instruments, longer axial length results are to be expected with the IOLMaster.
What is to be taken into account in the case of double or multiple peaks?
If the reflection at the RPE is strongest, the measurement is correct. For instance, if in a series of five measurements there are 4 neat peaks and a double peak with a shorter axial-length value, it can be assumed that the limiting membrane causes the stronger reflection of the double peak, thus presenting a too short axial-length value. In this case, delete or correct the inappropriate 5th measurement following the instructions in the user manual (by zooming in and manually shifting the measuring point). After the correction, you should find that the value fits very well in the measurement series.
If a too large axial-length value turns up in the measurement series, it is caused by a strong reflection at the choroid. In this case, follow the procedure described for too short values (delete or correct the reading). In the case of multiple peaks, proceed analogously. You should always make sure you obtain consistent measurement series. An extra look at the peak structure will prevent erroneous measurements.
What is the reason for the generation of fine structures of peaks or multiple peaks?
So far, the reasons haven not been investigated sufficiently. In practice, there are reflections—possibly caused by nystagmus—that are additional to those of the layer reflecting most strongly, the retinal pigmented epithelium (RPE). These reflections are produced by the inner limiting membrane and/or the choroid.
This results in additional peaks, most of whose origin can be clarified only by several measurements. In the case of double peaks, they may be produced by both the limiting membrane/RPE and the RPE/choroid. Additional reflections may be produced even by retina ablation. In that case you will probably get an incorrect reading.
Can I also measure patients with staphyloma (protrusion of sclera) precisely?
Yes. It is just these eyes that often cannot be measured reliably and accurately by ultrasound, where the IOLMaster achieves clearly better results. This is due to the measuring principle that simulates the normal process of vision—measurement along the visual axis at a small measuring area.
As staphylomas (posterior) cannot be exactly measured by means of ultrasound A scans, additionally a B scan is necessary, if a staphyloma is suspected. As a rule, this is not done, thus resulting in erroneous results. Using the optical method, measurements are automatically correct. In this way, gravely misfitted IOLs will not occur.
In a lecture at the DGII 2000 (German-speaking Society for Intraocular Lens Implantation) in Luzern/Switzerland, B. Lege (University Eye Hospital, Wuerzburg/Germany) reported a case where a misfit of about 5D was avoided by optical measurement. Contrary to what is often assumed, irregular shapes of the eye's fundus exist not only with long eyes, but also with short eyes. The latter, however, usually are not diagnosed.
What is the reproducibility on the eye?
Better than ±30 µm, this corresponding to a mean postoperative error of less than 0.1 D. In clinical use, mean standard deviations of 23 µm were obtained for 5 successive measurements (Dr. Haigis, University Eye Hospital, Wuerzburg/Germany) and 25.6 µm for 20 measurements (Dr. Vogel, University Eye Hospital, Mainz/Germany). The variability between 5 different examiners was 21.5 µm (Dr.Vogel, University Eye Hospital, Mainz/Germany).
Safety criterion:
If the results of individual measurements differ by more than 200 µm (minimum to maximum), no mean value will be displayed, and the individual measurements must be checked for consistency.
Return to the top of the page
When do I get faulty or no measurement results with the keratometer?
You will get faulty or no measurement results if:
- the instrument is not properly focused on the eye
- measuring marks are partly obscured by eyelid or eyelashes
- the eye is being closed during the measurement
- strong reflections are additionally produced at the anterior surface of the IOL with pseudophakic eyes
- the anterior surface of the cornea is affected by scars, local or other irregularities
- the tear film is strongly impaired
What can I do to nevertheless obtain results after an error message is displayed?
- Focus the instrument exactly onto the light dots (usually a fine circle is visible around the dots).
- Ask the patient to open the eye wide, then repeat the measurement.
- Let the patient close and open the eye several times to improve the tear film, then quickly start the measurement.
- In case of dry eyes, use of artificial tears is recommended to improve results.
- If measuring marks are obstructed, carefully hold the eyelid.
- Slightly decentre the instrument by moving the instrument toward the eye to measure, for instance, beside local scars.
However, note that with very irregularly formed corneas, or after refractive surgery of the cornea, you must not use the measured values for the calculation without thorough consideration. In this respect, the built-in keratometer does not differ from any commercially available manual keratometer or autokeratometer.
How do I recognize "appropriate" or "inappropriate" reflections (risk of erroneous measurements)?
If you can see six peripheral circular light dots on the screen, everything is OK. If one of the dots is irregularly shaped, this might be caused, for instance, by a local corneal scar. If several dots are deformed, this might be due to a poor tear film. In this case, tell the patient to briefly close and open her or his eyes again. If the top dot is missing, you should consider an obstructing eyelid or eyelashes.
Is it possible to measure pseudophakic eyes with the keratometer?
This depends on the intensity of reflections additionally produced at the IOL. If the additionally visible six light dots are faint, you can measure the eye without any problems. If they are strong, measurements are impossible. This depends on the curvature of the implanted IOL. If the additional light dots are so strong that the measurement is impossible, you can slightly defocus the IOLMaster (approximately 1-2 mm). In this way, the additional reflections become so faint that they do not disturb the measurement. Meanwhile, defocusing the instrument by 1 to 2 mm will affect the results only insignificantly.
What is to be observed for patients after refractive surgery (PRK, LASIK, ...)?
In the calculation for the future IOL, enter at least the measured axial length and the corneal curvatures (refractive power of cornea). The changed corneal geometry does not affect the axial length measurement, but does affect the keratometer values. The formulas presently used for IOL calculation are based on the "Gullstrand Eye," and thus presuppose defined conditions of the radii of curvature of the cornea's anterior and posterior surfaces. This "geometry," however, has been changed by refractive surgery. For this reason, the data obtained with the keratometer cannot be included without adjustment in the IOL calculation (exception: Haigis-L formula).
What should I bear in mind when I compare corneal power (K values) obtained with keratometers of different manufacturers?
Keratometers/ophthalmometers always measure the radius of the cornea. On one and the same eye (equal radius), different diopter values will be displayed due to the different eye models used by different manufacturers. (Refer to the table below for corneal refractive indices.) This may result in deviations of 0.5 to 0.8 D, which are to be attributed only to the eye model used, not to the actual measuring accuracy.
Influence of the measurement with different keratometers using different corneal refractive indices:
 | Corneal refractive index | Resulting deviation of refraction
(referred to 1st line) |
| Topcon, Carl Zeiss | 1,332 | 0 D |
| Haag-Streit, American Optical | 1,336 | 0,5 D |
| Rodenstock, Humphrey | 1,3375 | 0,7 D |
| Hoya | 1,338 | 0,8 D |
If you want to compare the results of keratometers of different manufacturers with the IOLMaster, you will obtain comparable diopter results, if you enter the same corneal refractive indices in the menu "Options → setup → program settings → Keratometer".
Software version V2.0 to V3.02—List of measurements
You may choose between "one measurement" and "list" of measurements in menu "Options → Setup → program settings → Keratometer". "List" of measurements allow you to display 3 measurements. The blue highlighted values will be used for IOL calculation.
New in software version V4.0 and higher
After 3 measurements a mean is calculated which will be used for the IOL power calculation.
Return to the top of the page
What should I keep in mind while taking measurements?
- Focus on the focus point (the smallest dot).
- Make sure the dot is within the green square.
- Avoid too strong extraneous reflections and room light.
- The image of the lens should be well visible on the screen.
- The front surface of the lens should be visible inside the pupil.
Why should the ACD be measured?
Most formulas use axial length and corneal power for IOL calculation only and calculate a predicted ACD. This may cause problems in case of short or long eyes. According to J. Holladay, 10% of short eyes have longer than predicted ACD and 10% of long eyes have shorter than predicted ACD. Therefore, especially for short or long eyes, use of a biometric formula with measured ACD should be better.
What is the principle of anterior chamber depth measurement?
The measuring principle of anterior chamber depth is based on the "optical section" through the anterior chamber by means of a slit lamp with subsequent image analysis. The right eye is illuminated from the right; the left eye is illuminated from the left at an angle of approximately 30 degrees relative to the optical axis.
The measurement is performed by image analysis. The instrument measures the anterior-chamber depth (ACD) as it is usually measured in biometry. Anatomically, this is the anterior chamber depth plus corneal thickness.
What should I think of, if in successive anterior chamber depth measurements large deviations in the range of 0.1 mm occur?
Naturally, patient accommodation affects anterior chamber depth measurement and lens thickness. The patient should be told to look relaxed at a distant point. To eliminate accommodation and make measurements with small anterior chamber and small pupils easier, it is possible to measure under a mydriatic.
Is it possible to measure pseudophakic eyes with the anterior chamber depth meter?
No! An implanted IOL does not scatter the slit beam of the slit lamp because of its polished optical surfaces. The illumination from the side causes bright reflex images (Purkinje images of the IOL). The software interprets and evaluates these wide and very bright images as the anterior IOL surface. This leads to erroneous measurements. Therefore this measuring method must not be used on pseudophakic patients.
The measurement requires scattered light from the lens. The IOL of just-operated patients is hardly scattering light. That's why measurements are not possible in this case. The ACD measurement is interesting especially in pseudophakic patients with accommodative lenses. The front and back side of IOLs are strong reflecting surfaces. It is possible to measure the position of the reflexes and therefore the relative movement of the IOL.
How often may I measure an eye per day using the device that measures anterior chamber depth?
The number of measurements is not limited. The light load on the patient remains beneath critical even with a great number of measurements. A few patients, however, find the white flickering light too glaring.
What is the reproducibility on the eye?
Reproducibility is better than  50 µm. This corresponds to a mean postoperative error of less than 0.05D. In clinical use, mean standard deviation of 31,9 µm was obtained for 15 measurements (Dr. Haigis, Univ.-Augenklinik Wuerzburg/Germany) and 33,4 µm was obtained for 20 measurements (Dr. Vogel, University Eye Clinic, Mainz/Germany). The variability between 4 different examiners was 37,7 µm (Dr. Haigis, Univ.-Augenklinik Wuerzburg/Germany) and between 5 different examiners was 29.8 µm (Dr. Vogel, University Eye Clinic, Mainz/Germany).
Safety criterion:
If the results of the individual measurements differ by more than 100 µm, no mean value will be displayed, and the measurement must be repeated.
Return to the top of the page | |
|