Ординатура / Офтальмология / Английские материалы / Modern Cataract Surgery_Kohnen_2002

ALac (mm)

Fig. 3. Optical and acoustical biometry: PCI axial length ALop (Zeiss) vs. immersion US axial length ALac (GBS).

optical biometry (and vice versa), it was necessary to determine the relationship between optical path lengths measured by PCI and the respective ultrasound axial lengths. In a pilot study with one of the IOLMaster’s prototypes (‘ALM’) comparing axial lengths of more than 600 eyes, the following relation was found [5, 7, 13, 14]:

ALop OPL/1.3549 0.9571 ALac 1.3033

As an ultrasound reference instrument, a high precision Grieshaber Biometric System (GBS) was used at 10 MHz in immersion technique which is known to be superior in accuracy to the commonly applied contact coupling method. This instrument allows simultaneous segmental measurements with a spatial resolution of 22 m and a reproducibility of 22 24 m. The correlation between optical and acoustical eye lengths is excellent (99%) as can be seen from figure 3. Optical axial lengths, as expected, were longer than acoustical ones (by 0.30 0.17 mm on an average [7]). The difference was found to be more pronounced in short eyes which can be explained by an underestimation of the lens thickness in these eyes as a consequence of using an average refractive index. Today, the regression line shown above is wired into the market version of the Zeiss IOLMaster which thus emulates an immersion ultrasound instrument – as far as the displayed axial length values are concerned – with the high precision of PCI technology.

In a follow-up study [unpubl. data] an IOLMaster individual out of the regular production line was rechecked with 101 patients against our high precision immersion ultrasound system. With a correlation coefficient of

98.8%, the following dependance between indicated axial lengths ALIOLMaster on the IOLMaster and immersion ultrasound reference values ALimmUS from the

GBS was found:

ALIOLMaster 1.0006 ALimmUS 0.0337

If the average standard deviation for five consecutive axial length measurements is taken to be a measure for reproducibility, we obtained values of 22 24 m for the GBS ultrasound immersion measurements and 23 15 m for the IOLMaster [6, 8, 9]. In another study [8], based on 146 comparative axial length measure-

ments between IOLMaster and GBS, a mean difference ALIOLMaster – ALimmUS of10 19 m (median 10 m, range 770 to 420 m) was found.

Keratometry and ACD Measurement with the IOLMaster

As an all-in-one-instrument, the Zeiss IOLMaster also features a keratometry module as well as the facility to measure anterior chamber (ACD) depth. For these two measurements, however, classical optical techniques are applied.

Corneal curvatures are conventionally deduced from the positions of the images of 6 infrared light-emitting diodes (LEDs) illuminating the cornea in a hexagonal pattern. ACD is determined from a slit image of the anterior ocular segment with the help of sophisticated image analysis software. It is measured from the anterior corneal vertex to the anterior vertex of the lens, just like an ultrasound ACD would be measured. In fact, IOLMaster ACDs are calibrated against immersion ultrasound ACDs on the basis of more than 800 comparative measurements which have been carried out in our laboratory. Thus, with respect to an ACD measured ultrasonically in contact coupling mode, the IOLMaster ACD is likely to be a bit longer (0.1–0.2 mm), since it is not affected by a possible globe impression as might be the case in contact ultrasound.

In an already mentioned study [8], IOLMaster keratometry results were compared to those obtained with an Alcon (Renaissance Series) handheld keratometer. A mean difference (IOLMaster – handheld keratometer) of the average corneal radius of 10 50 m was found for 154 patients (median 10 m, range 200 to 130 m). Additionally, a comparison between IOLMaster ACDs (n 151) and the respective immersion ultrasound data obtained with the GBS was carried out yielding a mean difference (IOLMaster – GBS) in ACD values of 30 180 m (median 0 m, range 400 to 680 m).

Fig. 4. ‘Learning curve’ for axial length measurement with the Zeiss IOLMaster: improvement of signal-to-noise ratio (SNR) as time progresses.

Observer Dependance and Learning Curve

In contact echography, which is widely used for axial length determination, the measured value depends, inter alia, on the experience of the examiner. An experienced examiner will e.g. exert less pressure on the eyeball than a beginner; hence, he or she will produce slightly longer axial lengths with less data scatter when repeating the measurement. To check the interand intra-examiner variability for the IOLMaster measurement modes, 4 examiners (2 experienced ones, 2 beginners) measured axial length, anterior chamber depth and mean corneal radius of 29 volunteers at three different times. Results for repeated measurements by one and the same examiner (intra-examiner variability) were 10.9 m for axial length, 31.9 m for ACD and 11.3 m for corneal radius. For different examiners measuring one and the same patient/volunteer (inter-examiner variability), the respective values were 11.8 m for axial length, 37.7 m for ACD and 13.4 m for corneal radius. Similar results have been published by Vogel et al. [19]. In terms of reliability, the following results were deduced: 100.0% for axial length, 97.8% for ACD and 99.6% for corneal radius measurements.

A criterion for measurement quality in optical coherence biometry is the ratio of the usable interference signal relative to background noise (signal- to-noise ratio – SNR). The higher the SNR, the better the measurement. Learning to apply this new biometry technology thus implies trying to achieve high SNR values. An example for a ‘learning curve’ in terms of mean SNR of five consecutive measurements on a test sphere, repeated on subsequent days by an absolute novice, is shown in figure 4.

Optical Biometry and IOL Calculation

Optical biometry with the Zeiss IOLMaster – as has already been mentioned – produces axial lengths as if stemming from an immersion ultrasound measurement. However, although known to be less precise, the contact ultrasound method is the procedure which is mostly used for axial length determination. Accordingly, manufacturers’ constants for the calculation of intraocular implant lenses are meant for and adapted to contact ultrasound data. Therefore, it is of utmost importance to adjust the published IOL constants (like e.g. the A constant or the ACD constant) to optical biometry – individually for any given intraocular lens type. This can be done on the basis of preand postoperative clinical data. We have shown [9] that after proper individualization of lens constants there is virtually no difference between refractive results based on optical coherence biometry and high precision immersion ultrasound.

Optimization of IOL constants for optical biometry is one of the main concerns of EULIB – the European User Group for Laser Interference Biometry. EULIB is an independent interest group of scientists and users, working in the field of optical biometry or applying this technique clinically. Founded in autumn 1999, EULIB can be contacted through its website at www.augenklinik.uni-wuerzburg.de/eulib.

From the EULIB site, general information regarding PCI biometry as well as the clinical application of the Zeiss IOLMaster can be obtained. Also, a spreadsheet form designed to accept preand postoperative clinical data for the purpose of constants’ optimization can be downloaded [20]. Patient data sent back via this form are processed in our laboratory to produce optimized IOL constants for all popular IOL formulas. The results are then published on the EULIB site [21] (see fig. 5).

The necessary adjustments e.g. in A constants for the SRK/T formula are typically of the order of 0.6 D, ranging from 0.2 to 1.3 D. This can be seen from figure 5, if only lens type results for n 50 are considered. Generally, immersion-based IOL constants are higher than constants for contact ultrasound. This is due to the fact that a ‘contact’ axial length which would lead to a correct IOL power will be measured longer in immersion which then would call for a weaker IOL if the IOL constants were not set to higher values.

Advantages and Disadvantages of Optical Biometry

Optical biometry is definitely advantageous over ultrasound biometry in cases of staphylomatous ocular backwalls [12, 16]. With ultrasound it is often difficult to decide among different axial length results from e.g. a highly

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SNR 7.5 AL 29.19 n 1.3549

Fig. 6. Ultrasound B scan and optical A scan of a staphylomatous eye: an intraocular lens calculated from ultrasound biometry produced a 4 D refractive surprise.

myopic eye. Since optical biometry measures along the visual axis, the PCI results are more reliable as long as the patient is able to fixate. An example is shown in figure 6. A patient presented a myopic refractive surprise of 4 D in the right eye. His axial length was 27.06 mm by ultrasound, 29.19 mm by PCI. IOL calculation had been based on the ultrasound length. The refractive surprise is fully explained by the difference between acoustical and optical axial lengths.

Another application where optical biometry is superior to classical ultrasound is the measurement of pseudophakic and silicone oil filled eyes. Every medium along the propagation path of light affects the optical path length by its individual propagation velocity (expressed in its group refractive index). Compared to a normal phakic eye, a pseudophakic eye will thus have a different optical path length. In ultrasound, opposite to PCI, propagation velocities of IOL materials are considerably different from those of ocular tissues. Therefore, considerable correction factors are needed for measuring e.g. a pseudophakic axial length by ultrasound, ranging typically from 0.6 mm for silicone to 0.4 mm for PMMA lenses [4]. For optical biometry, on the other hand, typical pseudophakic correction factors [10] are of the order of 0.1 mm and nearly independent of IOL material.

The same applies to silicone oil filled eyes: a normal eye with its vitreous cavity completely filled with silicone oil, measured as phakic eye will seemingly be some 0.7 mm too long when using PCI. The same eye measured with ultrasound in phakic mode will also appear to be too long – this time however by some 8.9 mm [unpubl. data].

The evident advantage of optical biometry is of course its ease of use both for patient and examiner due to its noncontact mode of operation. No topical anesthesia is needed, no possible infection hazard involved. (Infrared) light, however, must be able to pass through the eye and return back to the PCI instrument. Therefore, a certain amount of transparency along the propagation path is mandatory with no obstructions blocking out the light. Furthermore, a minimum in fixation is needed. This requires cooperation on the side of the patient. Sometimes a measurement may not be possible due to very dense cataracts as well as general inabilities to cooperate.

From our experience with more than 2,500 eyes (mostly unpublished data yet) some 5–15% of patients in a university hospital surrounding cannot be measured optically. In one study [14], no PCI measurements were possible in 58 eyes out of 678 (9%). Similar results between 7 and 12% are reported in the literature [18]. Among the reasons for optical biometry to fail were inability to cooperate (fixate), tremor, respiratory distress, severe tear film problems, keratopathy, corneal scarring, mature cataract, nystagmus, lid abnormalities, vitreous hemorrhage, membrane formation, maculopathy and retinal detachment.

Finally, some possible pitfalls in optical biometry should also be mentioned. An A-scan from an ultrasound biometry device – although the instrument was not designed for ultrasound diagnosis – still carries some diagnostic information, since echoes of neighboring structures and tissues along the path of the sound beam are also displayed. The IOLMaster interferogram, however, shows no such information but rather a small window into retinal reflectivity. Thus, without careful interpretation, optical signals may hide possible pathologies. It may e.g. happen, as we have recently demonstrated, that good quality signals of high SNR acceptable as good axial length measurements turn out to actually stem from a detached retina [15]. It takes a trained person and clinical background information to avoid traps like this.

In summary: Optical coherence biometry as available today in the Zeiss IOLMaster is easy to use for the operator and well acceptable for the patient since it is a noncontact procedure without the need for local anesthetics and without possible hazards which are characteristic for contact methods. By means of its wired-in calibration curves, the instrument simulates precise segmental immersion ultrasound measurements. Its accuracy is equivalent to high precision immersion ultrasound and superior to the commonly used applanation method. The innovative technique may well become a routine method for IOL calculation in cataract surgery in cases of ‘normal’ cataract eyes without additional pathologies with visual acuities 0.1. For some 5–15% of cataract patients, PCI fails out of different reasons. In these cases ultrasound will continue to be the method of choice. The same is true for all other biometrical applications apart from axial length determination.

References

1Drexler W, Baumgartner A, Findl O, Hitzenberger CK, Sattmann H, Fercher AF: Submicrometer precision biometry of the anterior segment of the human eye. Invest Ophthalmol Vis Sci 1997;38: 1304–1313.

2Fercher AF, Roth E: Ophthalmic laser interferometer. Proc SPIE 1986;658:48–51.

3Fercher AF, Mengedoht K, Werner W: Eye length measurement by interferometry with partially coherent light. Optics Lett 1988;13:186.

4Haigis W: Biometrie; in Straub W, Kroll P, Küchle HJ (eds): Augenärztliche Untersuchungsmethoden. Stuttgart, Enke, 1995, pp 255–304.

5Haigis W, Lege B: Optical and acoustical biometry. ASCRS/ASOA Meeting, Seattle, April 1999.

6Haigis W, Lege B: First experiences with a new optical biometry device. XVIIth Congress of the European Society of Cataract and Refractive Surgeons, Vienna, September 1999.

7Haigis W, Lege B: Ultraschallbiometrie und optische Biometrie; in Kohnen T, Ohrloff C, Wenzel M (eds): 13. Kongress der Deutschsprachigen Gesellschaft für Intraokularlinsen-Implantation und refraktive Chirurgie, Frankfurt 1999. Köln, Biermann, 2000, pp 180–186.

8Haigis W, Lege B: Akustische und optische Biometrie im klinischen Einsatz; in Wenzel M, Kohnen T, Blumer B (eds): 14. Kongress der Deutschsprachigen Gesellschaft für IntraokularlinsenImplantation und refraktive Chirurgie, Luzern, February 2000. Köln, Biermann, 2000, pp 73–78.

9Haigis W, Lege B, Miller N, Schneider B: Comparison of immersion ultrasound biometry and partial coherence interferometry for IOL calculation according to Haigis. Graefes Arch Clin Exp Ophthalmol 2000;238:765–773.

10Haigis W, Lege B: Konstanten für die optische Biometrie. 98. Tagung der Deutschen Ophthalmologischen Gesellschaft DOG, Berlin, September 2000.

11Hitzenberger CK: Optical measurement of the axial eye length by laser Doppler interferometry. Invest Ophthalmol Vis Sci 1991;2:616–624.

12Hoffmann PC, Schulze KC: IOL-Berechnung mittels Laserinterferenzund Ultraschallbiometrie bei hochmyopen Augen. Klin Monatsbl Augenheilkd 2001;218(suppl 1):8.

13Lege B, Haigis W: Optical biometry – First clinical experiences. ASCRS/ASOA Meeting, Seattle, April 1999.

14Lege B, Haigis W: Erste klinische Erfahrungen mit der optischen Biometrie; in Kohnen T, Ohrloff C, Wenzel M (eds): 13. Kongress der Deutschsprachigen Gesellschaft für IntraokularlinsenImplantation und refraktive Chirurgie, Frankfurt 1999. Köln, Biermann, 2000, pp 175–179.

15Lege B, Haigis W: Probleme der optischen Biometrie in Fällen gravierender Pathologie entlang der visuellen Achse. Klin Monatsbl Augenheilkd 2001;218(Suppl 1):9.

16Lege B, Haigis W: Laserinterferenzbiometrie und konventionelle Ultraschallbiometrie in staphylomatösen Augen; in Wenzel M, Kohnen T, Blumer B (eds): 14. Kongress der Deutschsprachigen Gesellschaft für Intraokularlinsen-Implantation und refraktive Chirurgie, Luzern, February 2000. Köln, Biermann, 2000, pp 92–94.

17Pancharatnam S: Partial polarisation, partial coherence and their spectral description for polychromatic light. II. Proc Indian Acad Sci 1963;57:231.

18Schrecker J, Strobel J: Optische Achsenlängenmessung mittels Zweistrahl-Interferometrie; in Kohnen T, Ohrloff C, Wenzel M (eds): 13. Kongress der Deutschsprachigen Gesellschaft für IntraokularlinsenImplantation und refraktive Chirurgie, Frankfurt 1999. Köln, Biermann, 2000, pp 169–174.

19Vogel A, Dick HB, Krummenauer F, Pfeiffer N: Reproduzierbarkeit der Messergebnisse bei der optischen Biometrie: Intraund Interuntersucher-Variabilität; in Wenzel M, Kohnen T, Blumer B (eds): 14. Kongress der Deutschsprachigen Gesellschaft für Intraokularlinsen-Implantation und refraktive Chirurgie, Luzern, February 2000. Köln, Biermann, 2000, pp 85–91.

20www.augenklinik.uni-wuerzburg.de/eulib/dload.htm

21www.augenklinik.uni-wuerzburg.de/eulib/const.htm

Dr. rer. nat. Wolfgang Haigis, Universitäts-Augenklinik, Josef-Schneider-Strasse 11, D–97080 Würzburg (Germany)

Tel. 49 931 201 5640, Fax 49 931 201 2454, E-Mail w.haigis@augenklinik.uni-wuerzburg.de