Ординатура / Офтальмология / Английские материалы / Modern Cataract Surgery_Kohnen_2002

In the last decade, a new noninvasive optical biomedical imaging technology, called optical coherence tomography (OCT), has been developed. It is analogous to conventional ultrasonic pulse-echo imaging (US A- and B-mode), except that OCT does not require direct contact with the tissue being investigated and that it measures echo delay and intensity of infrared light reflected back from internal tissue interfaces rather than using acoustic waves. OCT is based on an optical measurement technique known as partial coherence interferometry (PCI). Since the velocity of light is high, echo delay times cannot be measured directly and interferometric techniques have to be employed. The first medical application of this technique was biometry of the eye described by Fercher and Roth [11] in 1986. Since then, two related versions of this technique have been developed for noninvasive high-precision and high-resolution biometry and tomography in ophthalmology [12–15].

A special version of this interferometric technique, called dual beam PCI that eliminates any influence of longitudinal eye motions during measurement by using the cornea as a reference surface, was used to perform first axial eye length measurements in vivo of normal eyes [16], as well as corneal thickness and thickness profile measurements [17, 18].

Recently, Zeiss has developed a commercially available biometry equipment using this optical biometry technique (IOL Master, Zeiss). Apart from the high precision of measurement of axial length with this modality, this unit is supposed to be easy to use for clinical routine.

This chapter gives a short overview of the technique and applicability of the novel optical biometry technique. Results of clinical studies on refractive outcome after optical biometry for cataract surgery and on IOL position in pseudophakic patients will be summarized.

Measurement Technique

The principle of the dual beam version of PCI has been described in detail in previous chapters [11, 16, 19]. Briefly, an external Michelson interferometer splits an infrared light ( 855 nm) beam of high spatial coherence but very short coherence length into two parts, forming a coaxial dual beam (fig. 1). This dual light beam, containing two beam components (1, 2) with a mutual time delay of twice the interferometer arm length difference (2d) illuminates the eye and both components are reflected at several intraocular interfaces which separate media of different refractive indices. For the measurement of axial eye length, for example, reflection sites are the anterior surface of the cornea (C) and the retinal pigment epithelium (R). If the delay of these two light beam components – produced by the interferometer – equals an intraocular distance within the coherence length of the light source, an interference signal (called PCI signal) is detected, being similar to US A-scans, but with a very high resolution ( 12 m) and precision (0.3–10 m), the latter being more than one order of magnitude better than that of US biometry.

Fig. 1. Sketch of the scanning version of the dual beam partial coherence interferometer. The eye is illuminated via an external interferometer, that produces a coaxial, dual beam. Reflected signals from the eye, for example (C1, C2, R1, R2) are superimposed on and detected by a photodetector. A PCI signal of the optical distance (OL) indicating the optical axial eye length is depicted. Measurements at a specific angle between vision axis and measurement direction, or along a completely linear or circular scan are performed using a computer-controlled scanning mirror and special scanning optics [from 22].

Since PCI yields optical distances, these need to be divided by the group refractive indices of the respective ocular media to obtain geometrical distances [16, 20]. A single A-scan to perform an axial eye length measurement takes about 0.5 s.

Refractive Outcome after Cataract Surgery

To demonstrate the applicability of PCI for biometry in cataract patients, a first study was performed on a total of 196 eyes of 100 patients measuring axial eye length [21]. That study showed that measurement of axial length is possible in eyes with cataract of varying intensity. In a further study, this technique was shown to perform accurate biometry in 85 cataract eyes, having the potential to improve the refractive outcome of cataract surgery by 27% using the SRK II power formula when compared to applanation ultrasound [22] (fig. 2). The possible mean absolute error for postoperative refraction achieved with PCI

Fig. 2. Percentage of eyes with refractive error of less than 0.5 D, 1.0 D, etc. for PCI (solid line) and US (dashed line) biometry using the SRK II formula in 85 eyes.

biometry was 0.49 D, compared to 0.67 D with US biometry. Further improvement of refractive outcome could be obtained by using third-generation IOL power formulas [23] (fig. 3). Precision of PCI biometry was better than that of applanation US by a factor of more than 10. Axial eye length measured with the two techniques differed by a mean of 460 m. These differences were probably due to the indentation of the cornea by the applanation US technique and the different reflection sites of sound and light in the retina. We also found a difference between PCI and US in measured crystalline lens thickness. This difference was significantly correlated with cataract intensity as graded semiquantitatively. In those eyes with high-grade nuclear cataract, US lens thickness measurements were significantly thinner than with PCI. The optical biometry technique seems to be less influenced by intensity, or hardness, of nuclear cataract than the US technique. All these studies were performed with a laboratory prototype of PCI.

In a recent study, we compared the laboratory prototype of dual beam PCI with the commercial prototype of this novel optical biometry technique [24]. We could show that both optical devices measure similar axial eye lengths. The correlation between both optical techniques was excellent (r2 0.99). The precision, however, was significantly better with the laboratory prototype. The reason for this might be the different bandwidths of the superluminescent diodes used.

In comparison to immersion ultrasound, the optical technique measures axial eye length longer by approximately 200 m. The median difference

0.65

SRK II

0.47

0.57

Olsen

0.49

0.56

SRK/T

0.44

0.55

Holladay

0.44

Fig. 3. Mean absolute error for each biometry method (dark gray (conventional ultrasound); light gray (PCI)) and each intraocular lens power formula. Actual values (in D as spherical equivalent) are indicated.

between PCI and IUS was 186 m. The systematic differences between optical and acoustic techniques are mainly due to the different reflection site in the retina [21, 22]. Ultrasound measures up to the inner limiting membrane, whereas with PCI the maximum interference pattern from the retina is detected at the interface of the photoreceptor layer to the retinal pigment epithelium. Furthermore, the mismatch of the beam axis and the visual axis during US measurements may cause a deviation of axial eye length measurements between biometry methods. This is in accordance with the results of our previous study comparing PCI to applanation and immersion ultrasound techniques. Such a mismatch does not happen in optical biometry since the patient fixates the measuring beam. The precision of PCI was also significantly better than that of the ultrasound technique. Though, with PCI, outliers have also been detected. The reason could be fixation problems of the patients, so that measurements were not repeated at exactly the same point on the retina.

We were not able to attain measurements with the commercially available equipment in several eyes (6 of 55, i.e. 11%). Five patients had dense cataracts or fixation problems because of macular degeneration. The presence of dense posterior subcapsular cataract seems to be a special problem for measurements with the Zeiss equipment. We have not had difficulties measuring these eyes with the laboratory prototype. Possible reasons are the shorter wavelength and lower power of the laser light used in the Zeiss IOL Master.

In a recent randomized trial, we compared the refractive outcome after biometry with optimized immersion ultrasound and the prototype of the commercially available IOL Master [25]. Ninety eyes of 45 patients with age-related cataract were included. For each patient the first eye was randomly assigned to receive an IOL using the Holladay intraocular lens power formula based either on optical or immersion-US biometry. The alternative biometric technique was used for the contralateral eye. Refractive outcome as assessed 3 months postoperatively was not significantly different between the two biometry techniques. Refractive outcome in cataract patients using the prototype of the commercially available laser interferometer is as good as that achieved with optimized immersion ultrasound.

To summarize, the optical technique has a higher precision of measurement than the US technique. Measurements can be carried out with more comfort for the patient, without contact to the cornea and therefore minimizing the risk of infection. The assessment of axial eye length with this technique is time-saving, easy to use, quick to learn and appears adequate for clinical routine.

Pseudophakic Measurements

When performing US measurements of pseudophakic eyes, the implant material causes multiple artifacts at the posterior lens surface and in the vitreous, disturbing the interpretation of the A-scan, and therefore the precise determination of intraocular distances [26]. In contrast to US, biometry of pseudophakic eyes performed with PCI does not suffer from this problem, since it uses light instead of sound. Attached to the interferometer is a fully computer-controlled scanning unit which makes it possible to direct the measurement beam at various angles with respect to the vision axis [27] (fig. 1). To stabilize the visual axis, a fixation light is offered directly to the investigated eye.

Typical PCI signals – so-called optical A-scans – of the anterior segment of the same eye before and after cataract surgery are shown in figure 4. These optical A-scans are similar to those obtained by conventional ultrasound, but show very narrow signal peaks, allowing much higher precision and resolution of measurement. In figure 4, the PCI-signal intensity is plotted versus the optical distance to the anterior corneal surface. Four main peaks, arising from light reflected at the anterior and posterior corneal and anterior and posterior crystalline lens (top) and IOL (bottom) surfaces can be distinguished, indicating the optical corneal thickness, anterior chamber depth, crystalline lens (top) and IOL (bottom) thickness, respectively. In the cataract lens (top) peaks probably caused by light reflected at the cortex-nucleus interfaces are also detected. In contrast to ultrasound biometry, no artifacts due to the IOL material are seen with the PCI technique in the pseudophakic eye (bottom).

Fig. 4. Measurement of the anterior segment of eye 1 day before (top) and 12 weeks after (bottom) cataract surgery. The interference fringe contrast (intensity) is plotted as a function of the optical distance to the anterior corneal surface. Signal peaks indicating the corneal thickness, anterior chamber depth, crystalline (cataract) lens as well as intraocular lens are depicted. In the cataract lens (top) peaks probably caused by light reflected at the cortex-nucleus interfaces are also detected. In contrast to ultrasound biometry, no artifacts due to the implant material are detected by the PCI technique in the pseudophakic eye (bottom) [from 30].

Another example of the anterior segment measurement of a pseudophakic eye is depicted in figure 5 (top). Due to the high resolution of the scanning version of PCI, an additional peak, indicating a signal arising from the posterior lens capsule, enables the detection and precise quantification of the lenscapsule distance (LCD) [28].

The precision and resolution of PCI is more than 20 times better than conventional ultrasound. Therefore, accurate determination of the effective IOL position after cataract surgery is possible. Hence, the IOL-dependent constants, needed for IOL power calculation formulas in cataract surgery, can be determined more precisely. The high precision (4 m) and high resolution ( 12 m) of the scanning version of PCI allows highly accurate measurement of effective lens position of different IOLs and a precise quantification of a LCD when present [28].

Fig. 5. Partial coherence interferometric A-scan of anterior segment of a pseudophakic patient before (top) and after (bottom) Nd:YAG capsulotomy. Note the additional lens capsule peak (arrow) before capsulotomy which disappears after capsulotomy [from 29].

The presence of a LCD is a possible risk factor for after-cataract, since it allows migration of lens epithelial cells and formation of Elschnig pearls between IOL and capsule (no space – no cells hypothesis). In many instances a LCD can be detected by careful slit lamp examination, but quantification is not possible. Also, detection may be difficult or impossible with small LCDs, or with some IOL styles depending on the optical characteristics of the optic material. PCI can detect and very precisely quantify a central LCD, if present. The incidence of a LCD was roughly the same for different IOL designs studied, including 1-piece as well as 3-piece silicone and acrylic IOLs, and amounted to approximately 20% for each IOL style [28]. By quantifying LCD, future modifications of IOL design may be assessed more accurately to help reduce the incidence of a lens-capsule gap, and consequently reduce lens epithelial cell migration and, therefore, regeneratory after-cataract.

In another study, we could show that Nd:YAG capsulotomy caused a backward movement of the IOL in all 32 patients studied [29] (fig. 5). The extent of IOL movement was small with a mean of 25 m. However, some eyes showed a greater extent of movement with an accompanying slight hyperopic shift in

refraction. One-piece plate haptic IOLs showed significantly more backward movement than 3-piece IOLs.

Currently, one of our main interests is the effect that capsule shrinkage has on IOL anterior chamber depth after surgery. An aim is to find IOL designs that show the least variability in postoperative change of anterior chamber depth, and can therefore be predicted the best in terms of effective lens position, or postoperative refraction. Another interest is the movement of IOLs induced by contraction of the ciliary muscle, or pseudophakic accommodation. Several prototype ‘accommodating’ IOL designs are under development that are supposed to allow reading with distance correction because of a forward movement of the IOL optic with ciliary muscle contraction. PCI is a powerful tool for such studies, because of its high reproducibility for measuring IOL position in pseudophakic patients.

Concluding, biometry using PCI can be performed in cataract and pseudophakic eyes with a precision and resolution that is much better than that of conventional ultrasound. Therefore, accurate determination of the effective IOL position after cataract surgery is possible. Hence, IOL-dependent constants, needed for most of the IOL power calculation formulas in cataract surgery, can be determined more precisely. Furthermore, LCD, a possible risk factor for aftercataract, can be detected and quantified with this novel technique. Finally, optical biometry is a novel examination technique that offers a high degree of comfort to the patient and examiner, since biometry can be performed within a short time. In addition, it is a noncontact method with no need for local anesthesia and a reduced risk of corneal infection.

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Dr. Oliver Findl, Universitätsklinik für Augenheilkunde, Allgemeines Krankenhaus Wien, Währinger Gürtel 18–20, A–1090 Wien (Austria)

Tel. 431 40400 7901, Fax 431 40400 7881, E-Mail oliver.findl@akh-wien.ac.at