Contributions By Mario Nubile, MD; Leonardo Mastropasqua, MD; Omid Kermani, MD; Georges Baikoff, MD; H. Burkhard Dick, MD; Mana Tehrani, MD; Carlo Francesco Lovisolo, MD; and Christopher Wirbelauer, MD
It is an exciting time for refractive surgery. Next-generation diagnostics will significantly impact the level of patient treatment that is becoming available to physicians. Improvements in anterior segment imaging, using high-frequency ultrasound and optical coherence tomography (OCT), are providing ophthalmologists with tools that allow them to evaluate pathologic corneas, eyes with corneal or anterior disease and the results of anterior segment surgery.
Several recently developed or evolving tomographers will prove advantageous for ophthalmologists. The instruments offer a higher degree of detail because the wavelength of light is much shorter in the anterior segment. These tomographers can detect minute changes in corneal thickness, thus allowing successful identification and disqualification of patients who may have early keratoconus or other corneal-thinning disorders that would contraindicate corneal refractive surgery. Additionally, the devices permit ophthalmologists to (1) determine the exact position of phakic and aphakic IOLs in the anterior segment and (2) evaluate the fixation, centration and location of these lenses within the eye. The tomographers can also calculate curvatures in the central cornea in a 1.5-mm diameter, a measurement not provided by topographers or keratometers.
Ophthalmologists are able to achieve a higher level of accuracy with IOL power calculations by means of new devices that facilitate precise optical axial length measurements, even in eyes that have fairly transparent media. Combination units now provide topography, wavefront analysis and refractions. By ensuring that the patient's fixation is the same for all three tests, these devices eliminate the problems of alignment and differences between the measurements that would make comparability inadequate. These technologies provide anterior segment surgeons with imaging instruments that better monitor each patient's optical system.
One deterrent to the clinician's adoption of new diagnostic technologies is cost. The devices represent significant investments, and physicians should weigh this consideration with the benefits of such devices (eg, obtaining useful information not previously provided). In my experience, although it may be a slow process, quality of care always prevails.
— Jack T. Holladay, MD
HRT CORNEAL MODULE
By Mario Nubile, MD, and Leonardo Mastropasqua, MD
There have been recent developments for next-generation in vivo confocal microscopes (IVCMs). Physicians may now use high-resolution images of the microscopic architecture of the transparent corneal tissues as well as nontransparent structures like the limbal and conjunctival surface in both healthy and diseased eyes. Consequently, a new evolution is occurring with these diagnostic tools in clinical practice. The Heidelberg Retina Tomograph (HRT; Heidelberg Engineering GmbH, Dossenheim, Germany) is one of the well-established in vivo confocal imaging systems, with its Rostock Corneal Module, a high-resolution laser scanning microscope for the visualization of the anterior segment of the eye (Figure 1).1-7
Over the past decade, IVCMs have offered multiple clinical applications for (1) the evaluation of corneal tissue micromorphology associated with corneal diseases and (2) follow-up of corneal surgical and refractive procedures.8,9 All corneal layers (ie, epithelium, subbasal nerve plexus, Bowman's membrane, stromal keratocytes, Descemet's membrane and endothelium), may be imaged without invasiveness and with high magnification (Figure 2). IVCM offers the opportunity to evaluate altered structures of the cornea in several pathologies and surgical interventions; these methods are validated by scientific literature in several applications.
But, the new challenge involves the imaging of tissues located peripherally to the cornea. The imaging of the limbus at a microscopic level (Figure 3) represents a challenging advance in ocular surface diagnostic procedures because it is a specialized region that is highly vascularized and innervated. It harvests corneal epithelial stem cells and antigen-presenting cells. Particularly, the fact that corneal epithelial stem cells (ie, limbal stem cells) reside outside of the cornea proper suggests that studying the corneal epithelium without taking the related limbal issue into account will produce only partial pictures. Ocular surface epithelia show a typical microscopic pattern of transition, moving from conjunctiva toward the limbus (with its unique structures such as Vogt's palisades and crypts) and the cornea. This shift needs to be considered when analyzing in vivo limbal epithelial features. Laser scanning IVCMs provide images of the limbus and related epithelial transition zones that correspond with impression cytology of the limbal area (Figure 4). Limbal stem cells located in protected basal crypts, however, cannot be discerned by morphology. Although confocal microscopy is limited in terms of real phenotypic cell and tissue characterization, it is a powerful morphological analysis tool. This technology provides indications and mechanistic explanations for the effects of limbal disease (eg, chemical burns, infections, chronic limbitis and ulcers) on limbal stem cell function. Identifiable microscopic signs of limbal damage often precede the formation of corneal conjunctivalization. For example, inflammation, necrosis and loss of the transition pattern are commonly observed prior to its onset (Figure 5). Moreover, the HRT Corneal module or other IVCMs may help ophthalmologists evaluate cellular changes after limbal or amniotic membrane transplantation or other ocular surface surgeries involving the limbus.
Finally, the limbal zone is a specialized site containing a high number of resident dendritic cells and blood capillaries. Therefore, the confocal microscopic investigation of these structures is particularly useful in understanding the immune response of the ocular surface10 (ie, in graft rejections, herpetic keratitis and immune-mediated ulcers) and different types of neovascularization. The future applications of IVCMs in limbal analysis will be based on a greater comprehension of the complex network influencing corneal and limbal function.
NIDEK OPD-SCAN
By Omid Kermani, MD
The Nidek OPD-Scan (Nidek Co, Gamagori, Japan) is an aberrometer that can be used for routine clinical screenings, intraocular surgery screenings and can aid in the custom ablation of normal to extremely aberrated eyes (Figure 6). Incorporating approximately 7,000 data points, the OPD-Scan is a combination unit that measures wavefront, corneal topography, refraction, keratometry and pupillometry (Figure 7). Surgeons can examine virtually all patients. A corneal navigator screening software is integrated into the OPD station's add-on software package, facilitating automatic screening of patients for various corneal conditions (eg, pellucid marginal degeneration and keratoconus) (Figure 8). Furthermore, this software is useful in identifying keratoconic suspects, coupled with the surgeon's clinical impressions.
In addition to traditional wavefront maps, the OPD-Scan provides wavefront data in clinically relevant wavefront refractive formats including OPD, OPD higher order and internal OPD maps. The information provided is useful for immediately evaluating the refractive effect of aberrations. For example, spherical aberration is a common cause of night myopia. The magnitude of wavefront error (in µm) for spherical aberration alone may not provide the full clinical picture. Physicians must consider that various aberrations interact differently and may benefit or deleteriously affect visual quality. The OPD map addresses this issue by plotting a diagram in diopters to demonstrate how the various aberrations interact to cause a refractive error gradient across the mesopic pupil. It also plots the overall magnitude of refraction for different pupil diameters. In the case of spherical aberration, if the OPD map reveals a significant increase in myopia peripherally, then the patient may be more prone to night myopia and halos because the myopic periphery will come into play as the pupil expands at night.
Another useful feature of this instrument is the ability to provide information on corneal aberrations and internal aberrations. The effect of corneal or internal aberrations can be separated out, and the resulting effect on visual quality may be simulated. This aids the surgeon in determining whether corneal or intraocular surgery is warranted. The generation of corneal and internal wavefront-error values permits the surgeon to select the type of customized IOL that suits best. The use of wavefront error values coupled with the OPD higher order maps, which clarify the refractive effect of higher order aberrations, allows the optimal selection of custom or conventional treatments. The ability to use the point spread function and letter charts that simulate visual quality is an excellent patient teaching tool. It helps in the explanation of why the surgeon is opting for one type of surgery over the other. The OPD's unique method of measurement also evaluates extremely aberrated eyes that have undergone previous surgery, whether intraocular or corneal. This enables the treatment of a subset of patients who cannot be measured with most other aberrometers.
Providing supplementary clinical information, the combination of features incorporated in the Nidek OPD-Scan provides valuable clinical information to aid in the diagnosis of corneal pathology and the identification of the appropriate treatment for a variety of patients.
CARL ZEISS MEDITEC'S VISANTE OCT
By Georges Baikoff, MD
OCT applications for studying the eye were first published in 1991 by David Huang et al.11 Since then, the technology has improved and routinely explores the posterior pole of the macular area and the optic nerve. More recently, Carl Zeiss Meditec AG (Jena, Germany) developed a prototype and commercialized the device (Visante OCT) to explore the anterior segment (Figure 9). In comparison with the Stratus OCT (wavelength of 810 µm), which the company developed for posterior segment imaging, the Visante OCT's wavelength capabilities (1,310 nm), acquisition software and image treatment are slightly modified. The Visante OCT gives cross-sectional images of the anterior segment along any of the meridians (ie, vertical, horizontal or oblique). All the anterior chamber distances and angles can be evaluated with callipers. Furthermore, corneal thickness is indicated in a pachymetric map of 10 mm in diameter.
The axial resolution is 18 µm and the transverse resolution is 60 µm. High-resolution acquisition systems can improve resolution by ≤8 µm. Examination is simple; there is no contact with the eye and no need for a water or gel bath. Scanning is fast and may be performed manually or automatically, along one or all four meridians. Additionally, the operator can modify the images with the software. The images may be registered digitally or printed on paper. With today's technology, the only drawback is the wavelength and the power of the infrared light beam, which cannot explore behind the iris pigments. Therefore, it is impossible to view the posterior chamber precisely behind the iris — unless the iris is light in color with few pigments.
The media behind the opaque nonpigmented structures (eg, sclera or an oedematous cornea), however, are easily observed. There are many applications, and each anterior segment specialist who uses the Visante OCT can define new applications corresponding to their needs and area of research. One advantage of the device is the ability to simulate natural accommodation using positive or negative lenses in front of the fixation target. The Visante OCT is currently one of a few devices that features imaging capabilities for the analysis of anterior segment distortions during natural accommodation.
The position of an artificial pseudophakic crystalline lens can be observed with the device as well as any movements of accommodative implants. Some investigators reportedly believe that the accommodative implants move, and others maintain that there is insignificant or no movement at all. In our experience, no shifting of implants has occurred, however, some colleagues and I believe that the way in which these accommodative implants function is still uncertain. In corneal surgery, the high-resolution images may be useful in (1) measuring corneal thickness, (2) specifying the depth of corneal opacities and (3) ascertaining the state of the anterior segment behind an opaque cornea. Thus, the utilization of the Visante OCT serves both a strategic and a diagnostic interest during corneal surgery. LASIK flaps can be visualized with precision to check their regularity as well as the thickness of the residual stromal bed and whether there is enough tissue left for an enhancement.
Another application in refractive surgery is the definition of safety criteria for inserting iris-fixated or angle-supported anterior chamber phakic implants. Evaluation of the internal diameter of the anterior chamber is easily accomplished. My research demonstrates that the internal circumference of the anterior chamber is more often an oval with a large vertical axis.12 When inserting the Artisan (Ophtec, Groningen, Netherlands) or the Verisyse (Advanced Medical Optics, Santa Ana, California) lens, the objective measurement of iris convexity is crucial, as it enables the surgeon to define exclusion criteria and therefore avoid pigment dispersion syndrome. This results when the crystalline lens' forward thrust is above 600 µm.13
Finally, glaucoma specialists can define the iridocorneal angle and objectively specify the degree of closure. Exploration of the subconjunctival area can be done without difficulty to postoperatively check filtrating surgery.
The Visante OCT opens a door for in vivo anterior segment exploration. This diagnostic tool's contribution to the field of anterior segment analysis is likely to be as great as its impact on the examination of the macula and papilla.
OCULUS PENTACAM HR
By H. Burkhard Dick, MD, and Mana Tehrani, MD
The Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany) (Figure 10) is an automatic rotating Scheimpflug camera for noncontact optical analysis of the anterior eye segment. It uses blue LED light. The device captures ≤50 Scheimpflug images in <2 seconds. Through the rotating measurement, the center of the cornea is fine meshed. A second pupil camera detects possible eye motions and automatically corrects them. The device is user- and patient-friendly and can image even irregular corneas.
The Pentacam HR provides topography and elevation data of corneal surfaces and an overall pachymetry map. The pachymetry- and topography-based keratoconus detection can screen patients, identifying those who may not be good candidates for refractive surgery. The modified version of this high-resolution device features improvements that allow for greater performance reliability and obtaining more detailed information on intraocular structures. The University Eye Hospital in Bochum, Germany is currently conducting scientific trials to evaluate the efficacy of the Pentacam HR.
The anterior chamber analyzer displays measurements of chamber depth, angle and volume. Combined with the pachymetry-based correction of the tonometrically measured IOP, it is a helpful tool for glaucoma screening. Alternatively, the device provides indispensable information for preoperative planning of phakic IOL implantation. Each captured Scheimpflug image can be viewed, which allows the practitioner to obtain manual individual measurements at any location. All aberrations of the anterior and posterior corneal surface can be displayed.
The density of the cornea and the lens cataract is analyzed and quantified to assess its progression. Today, one of the key concerns is IOL power calculation after corneal refractive surgery. The integrated calculation software provides helpful information to choose the right IOL power. The Scheimpflug images afford the surgeon good control in the positioning, tilt and centering of IOLs in the anterior segment, but the main advantage is the comprehensive analysis of the anterior eye segment. One examination provides all of the necessary information.
The software allows surgeons to monitor the positioning of phakic IOLs (eg, Artiflex-Veriflex [Ophtec, Groningen, Netherlands]) following implantation, which improves the safety of postsurgical outcomes. In a recent clinical trial, the safety distances were measured postoperatively.14 The results confirmed appropriate preoperative counseling and the decision for surgery in each single eye. The relevant data regarding the positioning and location of the optic edge of the phakic IOL was collected, including the iris convexity, central and peripheral anterior chamber depth and the distance between the phakic IOL and the crystalline lens. This tool can also be used for toric phakic IOLs because images are captured in all angular directions. The Pentacam HR offers an objective basis, particularly for borderline patients, to make the right decision to increase patient safety. The first clinical trials demonstrated a good correlation between the preoperative analysis and the postoperative measurement of the phakic IOL position in the anterior chamber.14
ULTRALINK ARTEMIS 2
By Carlo Francesco Lovisolo, MD
After almost 10 years, a distinct change concerning the need for high-resolution biometry in refractive surgery has occurred. Ophthalmologists worldwide no longer debate whether it is necessary to accurately measure anatomic structures and intraocular distances before planning LASIK retreatments or implanting phakic IOLs. Instead, the question has shifted to which imaging technology is best suited for these purposes and how it is incorporated into their practices.
The Artemis 2 (A-2; Ultralink LLC, St. Petersburg, Florida) (Figure 11) is a very-high frequency (VHF) imaging device that uses a broadband 50-MHz ultrasound transducer to acquire B-scan arcs. These arcs follow the curvatures within the globe (ie, cornea, iris plane, sclera and retina). The technology maintains maximum perpendicularity when scanning, thus achieving an optimal signal-to-noise ratio. A proprietary digital signal technology calculates and stores ultrasound data. It has twice the degree of resolution and measurement precision compared with conventional analog VHF data.15 As a result of coaxial simultaneous image-capture capabilities at each scanning position, correlated measurements may enable accurate 3-D reconstructions from multiple meridional scans.
At my practice, we use the A-2 high-resolution scans and differential maps to image each layer of the cornea (epithelium, flap, residual stromal bed, effective ablated tissue) with 1-µm–precision following excimer laser procedures (Figure 12). Precise information about anatomic structures aids in etiologic diagnosis and increases the safety and efficacy of retreatments to perform an anatomical diagnosis with increased precision. An A-2 scan is always performed prior to enhancing patient outcomes.
An interestingly growing application of the A-2 is keratoconus screening. The compensatory interaction between the epithelium and stromal mechanics may prevent accurate tissue effects analysis with topography alone. The thinning epithelium absorbs any steep curvature change and masks the distortion of the front surface of the cornea. Therefore, during its early stage, naturally occurring or iatrogenic keratoconus may be detected on the back surface of a cornea. Accuracy of Orbscan (Bausch & Lomb, Rochester, New York) back surface elevation maps has not been completely validated, however, they are frequently used to detect suspect cases before surgery. Following excimer laser surgery, the reliability of the maps may be compromised. Only A-2 maps can combine epithelial profiles, point-by-point stromal pachymetry and true back surface curvatures to effectively determine the development of an ectatic disorder of the cornea.
To reverse presbyopia with scleral expansion procedures, the A-2 measures scleral thickness and identifies the lens equator. Locating the optimal implantation site for bands or laser sclerotomies dramatically increases safety and efficacy.
The real benefit of the machine is for phakic IOL size selection, particularly regarding the Visian ICL (STAAR Surgical Company, Monrovia, California). Maximizing the safety of phakic IOLs is the surgeon's responsibility; we expect the device to remain pristine in eyes of young patients for several decades — if not a life span — without serious side effects. Measuring the white-to-white with the caliper and adding 0.5 mm is an inadequate sizing method, as there is no statistically significant correlation between the horizontal external white-to-white distance and the internal dimensions of the anterior zonules or the ciliary sulcus diameter. This has been proven by my retrospective study involving postoperative A-2 scanning,16 cadaver eye studies17 and MRI measurements.18 In addition, A-2 scans display hidden anatomic spaces, preventing other postoperative surprises including cysts or tumors of the ciliary body or unexpected complications.
Currently, I am coupling A-2 anterior segment biometry with the Lovisolo Phakic IOL Sizer (ie, software I developed that is based on finite element analysis and checked with retrospective regression analysis of more than 1,000 implanted eyes) to predict tissue-to-lens implant clearances in a theoretical eye model (Figure 12). Additionally, the program takes into account patient age by incorporating a 0.015-mm reduction in anterior chamber depth per year. In our hands, postoperative complication rates (eg, endothelial cell loss, iris pathology, blood-aqueous barrier disruption and crystalline lens opacity) at the 5-year mark have been reduced to almost zero.
HEIDELBERG'S SL-OCT
By Christopher Wirbelauer, MD
My work with the Slit Lamp-Mounted Optical Coherence Tomography device (SL-OCT; Heidelberg Engineering GmbH, Dossenheim, Germany) began in 1996, during the technology's early developmental phases. The objective of our research was to improve the anterior segment imaging of a noncontact high-resolution technology by coupling it with the capabilities of a traditional slit lamp. The SL-OCT, modeled after this concept, received CE Marking in 2003 (Figure 13). This device is more cost effective than other systems because the OCT is mounted on the slit lamp.
OCT in the anterior segment has developed substantially from the early prototypes, and the features of today's adaptation offer clinicians an elevated level of functionality. The speed of image acquisition has been significantly enhanced — due to the incorporation of improved scanning technology — and it enables the display of the entire anterior segment in <1 second.
Presently, the SL-OCT's primary clinical applications involve refractive surgery and glaucoma screening. In the anterior eye segment, the device may measure corneal thickness and visualize the flap, as well as assess residual stromal thickness during LASIK screening or anterior chamber depth and width prior to phakic IOL implantation.19-21 Glaucoma specialists may perform noncontact screenings and pre- and postoperative examinations with the SL-OCT to determine anterior chamber angles.22 Furthermore, OCT imaging may be used in lieu of gonioscopy.
The SL-OCT achieves precise cross-sectional imaging of the anterior segment (Figure 14). The device's high-resolution and noncontact imaging distinguish it from ultrasound technologies. Unlike other optical diagnostic tools, images are produced perpendicularly to the eye rather than with oblique illumination and scanning, which may create distortions and optical artifacts. Thus, the SL-OCT facilitates the acquisition of more valid data and does not require complicated mathematical corrections.
Diagnosis through utilization of the SL-OCT is comparable with conventional slit-lamp evaluation; physicians should not notice a learning curve. The combination unit provides convenience during examination. Patient tolerance and feedback have been positive. Educating patients by showing them images of their eyes (captured by the SL-OCT) is valuable for clinician and patient alike, enhancing the consultation and treatment process.
Two drawbacks of the OCT technology are (1) displaying transparent structures (eg, IOLs) is not always possible and (2) deep structures behind the iris or ciliary body are not as well delineated as a result of the absorption of infrared OCT light. Technological modifications in development should address these limitations.
Although the SL-OCT is a refined diagnostic tool, future advancements are inevitable. Improvements will include a higher degree of resolution, a more compact design and a more affordable cost. It will also be possible to further increase image-capture speed, quickening the reconstruction of topographical values and 3-D images of the anterior segment. These factors will contribute to the rate of industry acceptance.
Georges Baikoff, MD, is a partner and professor of eye surgery at the Ophthalmology Centre of the Monticelli Clinic, in Marseilles, France. He states that he is a paid consultant for Carl Zeiss Meditec AG. Dr. Baikoff may be reached at g.baik.opht@wanadoo.fr or +33 491 16 22 28.
H. Burkhard Dick, MD, is chairman of the University Eye Hospital, in Bochum, Germany. Professor Dick is a member of the CRSToday Europe Editorial Board. He states that he has no financial interest in any of the products or companies mentioned. Professor Dick may be reached at burkhard.dick@kk-bochum.de or +49 234 299 3101.
Jack T. Holladay, MD, is clinical professor of ophthalmology at Baylor College of Medicine, in Houston, and founder and medical director of the Holladay LASIK Institute, in Bellaire, Texas. Dr. Holladay may be reached at docholladay@docholladay.com or +1 713-668-7337.
Omid Kermani, MD, is leading consultant for cataract and refractive surgery at the Laser Eye Center, in Cologne, Germany. He states that he holds no financial interest in any of the products or companies mentioned. Dr. Kermani may be reached at o.kermani@augenportal.de or +49 221 650 722 0.
Carlo Francesco Lovisolo, MD, is founder and medical director of the Quattroelle Eye Center, in Milan, Italy. He states that he is a paid consultant for STAAR Surgical. Dr. Lovisolo may be reached at loviseye@fastwebnet.it or +39 02 8057388.
Leonardo Mastropasqua, MD, is director of the Regional Center of Excellence in Ophthalmology at the University G. D'Annuzio, in Chieti-Pescara, Italy. He states that he has no financial interest in any of the products or companies mentioned. Professor Mastropasqua may be reached at mastropa@unich.it or +39 0871 358 410.
Mario Nubile, MD, is head of the cornea and ocular surface service at the Regional Center of Excellence in Ophthalmology at the University G. D'Annuzio, in Chieti-Pescara, Italy. He states that he has no financial interest in any of the products or companies mentioned. Professor Nubile may be reached at m.nubile@unich.it or +39 0871 358 410.
Mana Tehrani, MD, is a researcher in the department of ophthalmology at the Johannes Gutenberg-University, in Mainz, Germany. She states that she has no financial interest in any of the products or companies mentioned. Dr. Tehrani may be reached at tehrani@augen.klinik.uni-mainz.de or +49 6131 172589.
Christopher Wirbelauer, MD, is attending physician at the Klinik für Augenheilkunde, Vivantes Klinikum Neukölln, in Berlin. He states that he holds no financial interest in any of the products or companies mentioned. Dr. Wirbelauer may be reached at christopher.wirbelauer@vivantes.de or +49 30 6004 3131.
