Ординатура / Офтальмология / Английские материалы / Corneal Endothelial Transplant (DSAEK, DMEK & DLEK)_John_2010

Figure 4-15: Double anterior chamber after deep anterior lamellar keratoplasty in a case of corneal stromal dystrophy. Patient’s Descemet’s membrane that is not adherent to the donor corneal stroma is clearly visible.

Figure 4-16: Same eye as shown in Figure 4-15 after an anterior chamber injection of C3F8. Note the total collapse and disappearance of the double anterior chamber and good adherence of the patient’s Descemet’s membrane to the donor corneal stroma. Also seen is the resolution of the corneal edema.

Figure 4-17: Focal area of corneal stromal edema is visible, that was secondary to a dislocated Artisan phakic implant. The phakic implant was repositioned.

has a high resolution software giving an axial resolution of 8 µm. Images of the corneal stroma are more precise and with this new software a 10 mm diameter pachymetric mapping is available. Studying the thin or thick zones of the cornea is possible and this will probably be very helpful for an early diagnosis of keratoconus (Figure 4-18). However, great care is needed during the testing process, as the use of automatic software can lead to potential errors. In an ongoing study on postoperative LASIK flaps, the automatic method used to measure the corneal flaps (Figure 4-19), occasionally produced significant errors. The enveloping curve that reconstructs the anterior surface of the cornea has a serious defect leading to important errors, especially, if only taking into account the automated figures

Figure 4-18: Pachymetry mapping of an eye with keratoconus.

Figure 4-19: LASIK flap measurement using OCT imaging.

produced by the device’s software. The advances in technology definitely make things simpler, but it is necessary to keep an open and critical mind, and personally check the results provided by the technicians. It is essential to compare the complimentary examinations with the clinical evaluations.

Conclusions and the Future of

Anterior Segment OCT

This general overview should give the reader an idea of the importance of this imaging technique in clinical practise. It is important to note that the equipment is fairly simple to use. Once the patient has fixated on the target, manipulation is as easy as with a corneal topography unit. It is a noncontact device, with quick image capture and the technician decides which axis he wishes to explore. Image resolution is similar to the ultra-high-frequency scanning devices. However, with the OCT the explored zones are easier to find because, the fixation point is on the optical axis. The

irido-corneal angle is clearly visible in the OCT. When studying the measurements, or checking the evolution of the anterior segment, either the irido-corneal angle area or the scleral spur area, can be used as a reference point, since both these areas remain constant in the anterior chamber anatomy during various modes of testing using the OCT.

Finally, in the laboratory, with a more appropriate wavelength, and/or a modification of the power of the light ray it has been possible to obtain images that are close in comparison to tissue histology. On pseudophakic cadaveric eyes, Linnola et al10 was able to demonstrate cell proliferation on the posterior capsule. The images obtained with the high resolution OCT,10 were similar to histopathologic sections performed on the same cadaveric eyes.

Future technological evolution of the Visante™ OCT for exploring the anterior segment is something to look forward to. Hopefully, in the near future, these improvements will be similar to those of the OCT for exploring the posterior segment of the eye, namely, more precise images, resolution to a few microns and a 3D image reconstruction of the anterior segment structures. All of these are under study. It is quite certain that this OCT imaging system will, in daily practice, replace ultrasound equipment for anterior segment exploration.

References

1. Huang D, Swanson EA, Lin CP, et al. Optical Coherence Tomography. Science 1991;254:1178-81.

2. Puliafito C, Hee MR, Schuman JS, et al. Optical Coherence Tomography of Ocular Diseases, Slack Inc, 1996.

3. Izatt JA, Hee MR, Swanson EA, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmology 1994;112:1584-9.

4. Baikoff G, Lutun E, Ferraz C, et al. Analysis of the eye’s anterior segment with an optical coherence tomography: Static and dynamic study. J Cataract Refract Surg 2004;30:1843-50.

5. Baikoff G, Jitsuo Jodai H, Bourgeon G. Evaluation of the measurement of the anterior chamber’s internal diameter and depth: IOLMaster vs AC OCT. J Cataract Refract Surg 2005;31 (9):1722-8.

6. Baikoff G, Lutun E, Ferraz C, et al. Refractive Phakic Iols: Contact Of Three Different Models With The Crystalline Lens, An Ac Oct Study Case Reports. J Cataract Refract Surg 2004;30: 2007-12.

7. Baikoff G, Bourgeon G, Jitsuo Jodai H, et al. Pigment Dispersion and Artisan Implants. The crystalline lens rise as a safety criterion. J Cataract Refract Surg 2005;31:674-80.

8. Baikoff G, Lutun E, Ferraz C, Wie J. Anterior chamber optical coherence tomography study of human natural accommodation in a 19-year-old albino. J Cataract Refract Surg 2004;30:696-701.

9. von Helmholtz H. Uber die akkommodation des auges. Albrect von Graefes Arch Klein Exp Ophthalmol 1855;1(2):1-89.

10.Linnola R, Findl O, Hermann, B Sattmann H, Unterhuber A, Happonen RP, et al. Intraocular lens-capsular bag imaging with ultrahigh-resolution optical coherence tomography. Pseudophakic human autopsy eyes. J Cataract Refract Surg 2005;31: 818-23.

Principles of Optical Coherence

Tomography

Optical coherence tomography (OCT) utilizes multiple pulses of light to create a cross-sectional image [See also Chapter 4, Optical Coherence Tomography (OCT) of the Anterior Segment, and Chapter 6, Use of Optical Coherence Tomography in Descemet’s Stripping with Endothelial Keratoplasty (DSEK) and Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK)], somewhat analogous to the manor in which a B- scan ultrasound cross-section image is created by the feedback signals of multiple pulses of sound waves. OCT performs cross-sectional imaging by measuring the light echo time delay and the intensity of the back reflected light from structures within the eye.

Because OCT uses light, one of the essential requirements in developing an OCT image is that the structure be partially transparent to the wavelength of light. In ophthalmology, OCT is particularly well suited because of the optical properties of the eye. In addition, OCT can be performed without physical contact to the eye, unlike ultrasound. This improves patient comfort and ease of application.

The light energy directed into the eye by OCT is reflected back from boundaries between different tissues. The light scatters differently in tissues with different optical properties. The distance and dimension of the different tissue structures are determined by the time delay of the “echo” of light that is back reflected or back scattered. Because the speed of light is almost one million times faster than the speed of sound, measurements involving light require sophisticated ultrafast time resolution. The ultrafast and high resolution measurements in OCT are possible because of an optical technique known as low-coherence interferometry.

The physical technique of white-light interferometry was first described by Sir Isaac Newton.1 The fundamental principle is that of splitting the optical beam at its origin, and then comparing the downstream beam that is passed through the eye to a reference optical beam that is not passed through the eye structures. In the ophthalmic version of OCT, the light source is a laser with the ability to emit low energy short pulses of light. The optical beam from the laser is directed onto a partially reflecting mirror, commonly known as a beamsplitter. This mirror splits the light into two parts, the reflected beam and a transmitted beam. The beam that passes into the eye and then is reflected back is compared to the beam that is not passed through the eye. The disparity of these two beams creates the interferometry signal. Optical interference occurs when the pulses

coincide, and the interference is measured and quantitated by a photodetector.2

In order to measure the time delays of the light reflection (echoes) from the various structures within the eye, the reference mirror within in the OCT device can vary its position. The key principle is that the interferometer measures the time delays of the optical echoes by utilizing the comparison of the light reflected from the eye structure to the reference beam that has traveled the reference path within the device. Figure 5-1 shows the basic schematic principle of the ophthalmic OCT as developed by Zeiss, Inc. (Visante OCT, Zeiss-Meditec, Dublin, CA).

Figure 5-1: Schematic diagram of the optics of the Visante anterior segment OCT (Courtesy Zeiss-Meditec, Dublin, CA).

The images created by OCT can be displayed as a gray scale or a false-color scale. The gray scale image represents the intensity of the back reflected optical signal, where, a bright white color represents a strong return signal and black represents no return signal. The shades of gray are levels of back return signal in between. The gray scale is useful in anterior segment OCT, in particular, because it allows a finer definition than the representation of the falsecolor scale on a computer screen. In posterior segment OCT, the false-color scale is more commonly used because the colors can bring out subtle differences in the return signal, corresponding to different tissues. In anterior segment OCT, the color scale therefore has some utility in imaging iris structures, in particular.

In retinal OCT imaging, the wavelength employed is typically 800 nm. In contrast, a wavelength of 1310 nm is used for the Visante OCT (Zeiss-Meditec, Dublin, CA).

This is because longer wavelengths are scattered less and penetrate deeper, this is particularly important for penetrating the sclera in order to image the anterior angle and the anterior sclera. In addition, the safe level of light exposure is higher at a longer wavelength, and therefore more power can be employed. This results in the ability to

increase the speed of capturing, with more frames per second. This speed helps avoid artifact from movement of the eye during the capture of an image. 3,4

OCT Imaging of Lamellar Corneal

Dissection

The appearance of a normal cornea is shown in Figure 5-2A. In addition to the frequently employed gray scale, the two choices for color scale are also shown. Figure 5-2B uses the same false-color scale that is familiar from posterior segment OCT, whereas Figure 5-2C uses a color scale that has better resolution of fine detail. Note that all of these images represent high resolution captures of the cornea. The vertical line and bright reflex in the center indicate that the image capture was well centered; this artifact occurs when the OCT is well centered on the vertex. In the upper right hand corner, one can see a circle with “OD” highlighted, indicating that the image is from the right eye.

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B

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Figures 5-2A to C: OCT images of a normal cornea. (A) Gray scale; (B) False color scale; (C) Rainbow scale.

The triangle represents the nose. The direction of the arrow shows the orientation of the image plane. In this case, the image is a horizontal section. Just below that, on the right, is a marking for 0 degrees, and on the left, 180 degrees. These numbers change as the operator selects different orientations, either automatically or manually. The orientation of the section can be changed in steps as fine as one degree. HIPPA-compliant patient information is displayed at the top of the image. The operator can deselect the computer generated boundaries laterally and at the anterior and posterior corneal levels, if desired.

A LASIK flap is shown in Figure 5-3A. Note that this particular section runs from inferotemporal to superonasal in the right eye at a 45 degree angle. The interface of the LASIK flap is best seen on the right side of the image as a slightly brighter line, representing increased light signal return from the interface. Figure 5-3B shows the addition of measuring devices to the image of a LASIK flap. Shown in orange, the “flap tool” allows the operator to measure both the overall thickness of the cornea and the thickness of the flap and underlying residual stromal bed. Up to seven measurements can be made simultaneously on the same cornea. Associated with each of these measurements are three numbers. At the top, the number anterior to the cornea is the distance from the corneal vertex. One can see that the central measurement is perfectly centered at the vertex, represented by 0.00 mm. The two numbers posterior to the cornea show the distance from the anterior surface to the horizontal mid-stromal line, and the second number

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B

Figures 5-3A and B: (A) IntraLase LASIK flap; (B) IntraLase flap measured with the flap tool and calipers.

represents the distance from the horizontal line to the posterior cornea. In this case, the central flap thickness is 155 microns and the residual bed is 477 microns. The operator selects the position of the interface marker, where the software automatically selects the anterior and posterior placements of the flap tool, based on the detection of the corneal boundaries. Therefore there is some element of subjectivity. In this particular example, on the left side at the -2.39 mm location, one can see that the flap location has not been accurately identified, with the horizontal line being slightly anterior to the interface. In addition to the flap tools in this example, in blue one can see a caliper set at 1.55 mm. This caliper is frequently used to measure structures within the eye. In this example, however, it has been placed to serve as a scale.

Measuring the depth and placement of the lamellar dissection of the cornea is usually a critical element in corneal inlay surgery. It is critical for the surgeon to know the accuracy of the dissecting instrument used to create the pocket or flap. Moreover, the tolerance and optical performance of many inlays is dependent upon the anteroposterior location of the implant. Furthermore, the biocompatibility of a foreign element within the cornea usually requires placement of the element within a defined limited depth. The Visante OCT is uniquely capable of providing this information accurately and easily.

Refractive Inlays

Studies are underway with refractive inlays under cornea flaps. Hydrogel corneal inlays were investigated in the past as a means to correct aphakia, hyperopia, and myopia. Early efforts in this area were optically unsatisfactory for a variety of reasons, particularly because of the inadequacies of early-generation microkeratomes as well as the attempt to create anterior corneal contours that were either optically unstable or associated with major aberrations.5

More recently, ReVision Optics (Lake Forest, CA) has been investigating a proprietary high water content material for correction of hyperopia and presbyopia. Figure 5-4A shows a 5 mm diameter positive power inlay. Note the thicker center and fine taper of the inlay in the periphery. In Figure 5-4B, the flap tool has been used to define the depth and thickness of the inlay, and a horizontal caliper has been used to measure the diameter. The diameter of 4.17 mm indicates that the cross-section, in this case vertical, is near but not exactly through the center of the inlay, which is 5.0 mm in diameter. The flap tools show that the flap has an anterior thickness between 114 and 122 microns, an important demonstration of uniformity with the flap created by the IntraLase laser (IntraLase Corporation, Irvine, CA).

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Figures 5-4A to E: (A) 5 mm ReVision Optics intracorneal inlay. (B) Inlay measured with flap tools and caliper. (C-E) Three different power ReVision Optics implants.

However, one of the flap tools, positioned at 0.40 mm, has been placed with the horizontal bar at the posterior side of the inlay, compared to the flap tool at -0.08 mm which is positioned anteriorly. The difference between these two readings of 166 microns and 121 microns shows that the central thickness of the implant in this case is approximately 45 microns. This technology allows, for the first time, a direct measurement of the thickness of the high water content material once it has been placed and it has been stabilized. The thickness, of course, directly correlates to the optical power of the inlay. Figure 5-4C shows another case with a lower hyperopic correction. The flap tools show difference in thickness centrally of 25 microns. In yet another case, Figure 5-4D shows a center thickness of 32 microns, while Figure 5-4E shows a much higher powered

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Figures 5-5A to C: (A) 1.5 mm presbyopic correcting ReVision Optics implant (left); illustration of resultant multifocal cornea (right). (B) 1.5 mm ReVision Optics presbyopic correcting inlay. (C) Flap tool and caliper measurement of the presbyopic inlay (images Courtesy of ReVision Optics).

implant, with a central thickness of 74 microns. The Visante OCT is demonstrated to be a critical tool in analyzing the postoperative position and characteristics of this type of corneal inlay.

ReVision Optics (Lake Forest, CA) is currently investigating a 1.5 mm implant, designed to be placed either primarily or secondarily under a previous LASIK flap for the correction of presbyopia. The implant itself has positive power. It is centered on the pupil. A multifocal corneal optic is created as the corneal contour transitions from the area of the implant itself out to the original contour in the periphery. Figure 5-5A shows a slit lamp photograph of the implant on the left, and, on the right, the multifocal optic created in the transition zone labeled “2” in between the central power (“1”) and the more peripheral underlying corneal power (“3”). Figure 5-5B shows how thin and delicate this implant is, with the implant being seen as a thin separation under the LASIK flap centrally. In another case, shown in Figure 5-5C, the caliper shows the 1.5 mm implant at a depth of 159 microns.

Aperture Inlay for Presbyopia Correction

A non-refractive intracorneal inlay is under investigation for unilateral implantation in the non-dominant eye to increase depth of focus and thereby correct presbyopia utilizing the well known pinhole effect. The technology is being developed by AcuFocus Corporation (Irvine, CA). The clinical appearance of this device is shown in two representative slit-lamp photographs (Figures 5-6A and

B).

The implanted material consists of a ring of a proprietary polymer. The polymer is extremely thin (on the order of 10 microns) and, as seen in Figure 5-6B, has multiple micropores to allow transmission of corneal nutrients while minimizing any light scattering that would degrade the optics. The manufacturer’s specifications are for an aperture of 1.60 mm centrally and an outer diameter of 3.80 mm. The central aperture diameter was selected as the optimum diameter to give maximum depth of focus while minimizing negative diffraction effects.

The inlay is shown in Visante cross-section on a gray scale in Figure 5-6C. The section is not through the precise center, and therefore the caliper showing the aperture reads 1.39 mm and the overall diameter is read as 3.67 mm. The implant has been placed under a LASIK flap, as shown by the flap tool that is over the peripheral skirt. Note that the peripheral skirt blocks nearly all of the OCT light, and therefore the cornea appears dark under the skirt. However,

a small amount of reflected signal can be seen as vertical lines. In Figure 5-6D, this affect is even more easy to see, with the colored signal against the dark background. The return signal under the skirt is because of the presence of the micropores, which are allowing a small amount of light to pass and be reflected back.

Conclusions

The Visante OCT technology brings a critical new tool in anterior segment imaging. Knowledge of the performance of a microkeratome or the flap creation laser is critical, as the flap depth and uniformity will have a significant impact on the optical performance, and biological compatibility of intracorneal implants.

The centration and optical characteristics of intercorneal implants are both quantitatively and qualitatively documented by the OCT technology. As shown in the

examples of the ReVision Optics (Lake Forest, CA) refractive implants and the AcuFocus (Irvine, CA) aperture inlay, the OCT technology plays an important role in optimizing intracorneal inlay procedures.

References

1. Born M, Wolf E, Bahatia AB. Principles of optics: Eectromagnetic theory of propagation, interference and diffraction of light. 7th ed. New York: Cambridge University Press, 1999.

2. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science 1991; 254:1178-81.

3. Huang D, Wang J, Lin CP, Puliafito CA, Fujimoto JG. Micronresolution ranging of cornea an anterior chamber by optical reflectometry. Lasers Surge Med 1991; 11:419-25.

4. Radhakrishnan S, Rollins AM, Roth JE, et al. Real-time optical coherence tomography of the anterior segment at 1310 nm. Arch Ophthalmol 2001; 119:1179-85.

5. Steinert RF, Storie B, Smith P, McDonald MD, Van Rig G, Bores LD, et al. Hydrogel intracorneal lenses in aphakic eyes. Arch Ophthalmol 1996; 114:135-41.

Introduction

Optical coherence tomography (OCT) is a high resolution imaging modality that is noninvasive and uses lowcoherence interferometry to provide in vivo cross-sectional images of tissue with a spatial resolution of 10 to 20 microns1 [See also Chapter 4, Optical Coherence Tomography (OCT) of the Anterior Segment, and Chapter 5, Optical Coherence Tomography in Corneal Implant Surgery]. Examination of the retina and posterior segment with OCT has been studied extensively. Recently, a high resolution anterior segment OCT (AC-OCT) has been made commercially available (VisanteTM OCT, Carl Zeiss Meditec, Dublin, CA). This instrument uses 1310 nm infrared light to visualize the anterior chamber to a resolution of 10 μm. Infrared-beam penetration at this wavelength is blocked by pigments, preventing examination behind the iris, but it can be used through an opaque cornea. AC-OCT provides easy, noncontact, and real-time images of the cornea, iris, lens, and iridocorneal angle.2 This instrument has been used to evaluate the static and dynamic properties of the anterior segment,2 the anterior chamber depth, corneal curvature and corneal thickness.3 There are many potential applications of the AC-OCT. Changes in the anatomical configuration of the angle can be examined in such conditions as primary and secondary angle closure3 and analysis of the potential for occlusion of the anterior chamber angle.

Real time imaging of the corneal layers is invaluable in corneal and refractive surgery. AC-OCT has been used to image corneal flap thickness after laser in situ keratomileusis (LASIK).4 Furthermore, the AC-OCT can be used in the preoperative and postoperative evaluation of patients undergoing phakic intraocular lens implantation.5 In planning lamellar keratoplasty, the depth of scarring can be assessed by OCT. Advances have occurred in the treatment of corneal endothelial disease, namely, deep lamellar endothelial keratoplasty (DLEK) and more recently Descemet’s stripping with endothelial keratoplasty (DSEK) and Descemet’s stripping with automated endothelial keratoplasty (DSAEK) (See also Chapter 13, Definition, Terminology and Classification of Lamellar Corneal Surgery). This chapter reviews the promising application of AC-OCT for DSEK and DSAEK.

AC-OCT for DSEK and DSAEK

The AC-OCT is useful in the preoperative and postoperative imaging of DSEK corneas. The Visante OCT (Carl Zeiss Meditec, Dublin, CA) has been used in all our patients undergoing DSEK. Before surgery, the recipient cornea can

Figure 6-1: AC-OCT image of recipient cornea. Note thickening of the stroma.

be imaged to assess preoperative corneal thickness across all radii and to assess details of the cornea and anterior chamber that may be obscured by corneal edema (Figure 6-1). For preparation of the donor lamellar button, the donor corneoscleral button is mounted in an artificial anterior chamber (See also Chapter 12, Artificial Anterior Chambers) which dovetails with a blade microkeratome (ALTK system, and CB microkeratome, Moria S.A., Antony, France). The donor corneoscleral button is mounted in the ALTK artificial anterior chamber unit with the lamellar plane created by the microkeratome (Figures 6-2A and B, arrows). The thickness parameters of the donor cornea are shown in Figure 6-2C. Although the ideal thickness of the donor lenticule has not been determined, we have found that donor corneal buttons measuring between 125 and 225 μm are

Figures 6-2A to C: Donor cornea on anterior chamber after microkeratome application. Arrows show the lamellar plane of the dissection.