The Medical Model: Contact Lens Evaluation

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Imaging and Instrumentation in Contact Lens Practice By: David A. Berntsen, OD, MS, FAAO.

This article is published with permission from Review of Optometry and Jobson Publishing LL.C

Recent advances in technology have greatly increased the availability of clinical instruments that aid contact lens practice. Some of these instruments are not required to fit contact lenses, but they do provide new and useful information that is especially valuable when fitting and troubleshooting advanced contact lens patients. This article will review the keratometer, which is one of the most basic and forgotten tools available to contact lens practitioners. Next will follow a review of corneal topography, including the difference between various topographical maps and fundamental design differences between topography systems. Finally, it will discuss digital photography, aberrometry and anterior segment optical coherence tomography (OCT). Keratometry The value of the keratometer is often underrated. Keratometers measure four points on the central cornea that are separated by roughly 3mm. The index of refraction used by most keratometers is about 1.3375, similar to the index of refraction of the preocular tear film. The refractive index of the cornea is roughly 1.376. Of course, 1.3375 is also the refractive index used when converting between keratometric dioptric power and radius of curvature. The reason for using 1.3375 instead of the true corneal index of refraction is because it accounts for the negative power of the posterior corneal surface, which effectively reduces the total refractive power of the cornea. Thus, by using 1.3375, a keratometer provides the overall refractive power of the cornea. These instruments provide useful data for basic and advanced contact lens fitting and follow up.

Keratometry is very efficient when what you need is the curvature of the cornea to fit a soft or GP contact lens. It's also a quick way to monitor a typical contact lens patient for corneal curvature changes over time. Though many will argue that keratometry values aren't necessary to fit most soft contact lenses, especially for lenses available only in one base curve, keratometric values are useful when following your patients long term for corneal changes. Distortion of the keratometry mires can also be useful when trying to determine

whether irregular astigmatism or corneal distortions, such as with keratoconus, are present. In the absence of a topographer, this can be extremely valuable information when trying to determine reasons for reduced visual acuity. Corneal Topography A corneal topographer is an essential instrument in contact lens practice because it measures thousands of points across the cornea rather than the four points in the central 3mm of the cornea that a keratometer measures. While a topographer is not required to fit the typical contact lens patient, it provides valuable information when fitting specialty cases such as keratoconus, post-surgical and post-refractive surgery patients. A topographer is an absolute necessity when fitting orthokeratology lenses, in which you must assess the centration of the treatment zone (Figure 1). Maps Topographers offer various maps, such as axial, tangential and elevation maps, to help you interpret corneal surface data. Axial maps (also referred to as sagittal maps) provide corneal curvature in diopters. Because of the way topographers calculate these maps, an overall smoothing of the surface occurs and more subtle corneal changes are lost. When axial maps are calculated, it's assumed that the normal to the measured corneal point must cross the optical axis of the system. Tangential maps (also referred to as instantaneous curvature maps) are not restricted by the same assumptions used in axial map calculations. The normal to a point on the cornea doesn't have to cross the optical axis, which allows tangential maps to be more sensitive to sudden changes in corneal curvature. This is useful when trying to identify the location of corneal ectasias, such as with keratoconus and pellucid marginal degeneration. The location of a cone can be invaluable when trying to fit a keratoconic cornea and to understand the centration of the contact lens. Because tangential maps are more sensitive to sudden changes, the corneal power values are typically not as helpful as those on axial maps when fitting contact lenses. Elevation maps show the changes in corneal elevation as referenced to a sphere. Unlike other maps that report curvature in diopters, elevation maps report changes in corneal elevation in microns. After determining the best fit reference sphere, the map reports a positive or negative elevation value for the corneal surface referenced to that sphere. Elevation maps can be useful when predicting GP contact lens fluorescein patterns. Areas of the cornea with negative elevation values are more likely to show fluorescein pooling, whereas areas with positive elevation values will show touch. In general, it's best to not rely on one map. Topography software will allow you to create a customized display that combines several maps into one printed output. Utilizing the strengths and weaknesses of each of these maps allows for a better understanding of the shape characteristics of the cornea. Another useful feature, especially when a technician performs topography for you, is the ability to include the raw topography image on the output. Debris in the tear film or ocular dryness can produce inaccurate results. Tangential maps are

especially sensitive to tear film debris. Including the raw image of the topography rings on your output will allow you to quickly assess the quality of the measurement.

Figure 1. Topography of a patient fitted with orthokeratology lenses. The tangential map (bottom left) shows the centration of the treatment zone. Indices Depending on your topographer, many indices are meant to help describe the shape of the cornea and to aid in diagnosing and fitting contact lens patients. Although some indices are proprietary, one of the more universal indices is the shape factor, which describes corneal asphericity and is based on corneal eccentricity. Shape factors can range from –1.0 (oblate shape) to 1.0 (prolate shape). A shape factor of zero indicates a perfectly spherical surface. The shape factor is derived from eccentricity (SF = e2); for a normal cornea this value is more prolate, generally between 0.13 to 0.35. The shape factor can be used as an aid when interpreting topography maps. For example, negative shape factors (indicating oblate corneas) occur with pellucid marginal degeneration, refractive surgery and ortho-k patients. High positive shape factors are typical of keratoconus patients. Fundamental Design Differences Fundamental design differences exist between commercially available topographers. Most topographers use a digital camera to capture an image of a placido disk reflected off the cornea. The advantage of this method is that you can acquire data in less than a second once the patient is properly positioned. A rotating Scheimpflug camera and scanning slits are the other two methods of acquiring topography data with current commercially available topographers. These two methods take longer to

acquire data (from 1.5 seconds to 2 seconds) and are more sensitive to movement during the measurement; however, they offer advantages not provided by placido disk topographers. Many companies manufacture placido disk topographers. Large-cone placido disk systems may be more comfortable for patients because they have a longer working distance. However, these systems are also more likely to have areas of missing data for patients who have deep set eyes or a prominent nose or brow. Small-cone systems have a much shorter working distance and are therefore more sensitive to magnification errors; however, many commercially available topographers have addressed this issue with precise alignment systems and software. Small-cone designs allow the placido disk to be projected further onto the peripheral cornea so you obtain more data on patients who have deep set eyes because facial features do not block the reflection of the placido disk. The commercially available systems that utilize Scheimpflug cameras are the Pentacam (Oculus, Inc.) and the Galilei (Ziemer USA, Inc.). An advantage of this type of system is that it measures the entire anterior segment of the eye and, therefore, can provide cross-sectional views of the anterior segment as well as three-dimensional views. In addition to providing anterior surface corneal curvature, this technology also allows the topographer to measure the posterior corneal surface, total corneal pachymetry and anterior segment depth. The Orbscan IIz (Bausch & Lomb) is the only topography system that utilizes scanning slits to measure the cornea. Scanning slits also allow for measurements of the posterior corneal surface, full corneal pachymetry and anterior segment depth. The Orbscan IIz also includes a partial placido disk, which improves the accuracy of the topographical data versus using the slit image data alone; however, data for the most superior and inferior portions of the cornea aren't provided when the placido disk is used to generate the topographic data. A corneal topographer is a vital tool that can greatly improve your ability to fit more complex contact lens patients and to follow them over time. Many topographers include additional software features to aid in diagnosing corneal disease as well as GP lens fitting modules. Although not necessary to diagnose and fit contact lens patients, these modules can be useful when fitting more challenging patients. Digital Photography Another useful tool in contact lens practice is digital photography. Many practitioners routinely photograph the appearance of the optic nerve head — so why not do the same with your more complex contact lens fits? A photograph of the fluorescein pattern can aid in documenting how a new lens looked when dispensed versus when the patient returns for follow-up days or months later. Photographing a contact lens fit can also reduce your documentation time on complex fits.

Figure 2. Fluorescein pattern of an orthokeratology lens taken using a digital camera through a slit lamp ocular. There are many commercially available digital photography systems, many of which can be incorporated into paperless charting systems. Alternatively, you can obtain photographs by simply using your own digital camera to photograph a contact lens fit through one of the oculars of your slit lamp. For example, Figure 2 documents the fit of an ortho-k lens before the sagittal depth of the lens was reduced to increase the treatment area. If you choose to photograph contact lens fits with your own camera, I recommend purchasing a quality digital camera and experimenting with diffusers and filters to find the right combination to produce the desired results. Aberrometry Commercial aberrometers have been available for several years and provide information about higher-order aberrations (HOAs), such as coma, spherical aberration and trefoil. Hartmann-Shack aberrometers use a monochromatic, near infrared laser beam to create a point source of light on the retina. After bouncing off the retina, a wavefront of light travels through the eye and exits the pupil. The wavefront of light meets an array of uniformly spaced lenslets that sample the wavefront at hundreds of locations across the pupil and focus the light onto a sensor. Using the image captured by the sensor, the deviation of the light focused by each lenslet from the lenslet's optical axis is used to calculate the slope of the wavefront at each point. The slopes are then fitted using polynomials that can then be used to reconstruct the shape of the wavefront. The wavefront aberrations represent the difference between the measured wavefront and an ideal flat wavefront. While an aberrometer is not necessary to fit a contact lens, there are instances in which this technology can help explain vague visual complaints, especially from patients who have corneal irregularities. For example, this technology can help explain instances in which patients have 20/20 Snellen visual acuity, yet complain that their vision isn't clear. Because HOAs have a greater impact on visual quality under low-contrast viewing situations and mesopic lighting conditions when the pupil is dilated, measuring aberrations and comparing

them to published population averages can help quantitatively explain these visual complaints. Aberrometers are available that measure the aberrations of the entire eye. Additionally, some topographers have software modules that will calculate the HOAs created by the front surface of the cornea, which can be useful when treating patients who have irregular corneas. Research Aberrometers are also used in research to improve the optical quality of both GP and soft lenses. GP lenses reportedly provide better aberration profiles than do soft lens designs, although studies have noted individual variations between subjects. The superior optics is not a surprise to anyone who fits GP lenses. Various studies of soft contact lenses have reported that soft lenses provide optical quality similar to that of spectacles and that soft lenses can increase HOAs when compared to not wearing lenses on normal eyes. A recent study examining the on-eye optical quality of several major toric soft lenses found that prismballasted toric lenses induced more vertical coma than did a toric lens stabilized with thinzones, although the lens that induced less coma didn't improve vision. Aspheric Lenses Perhaps of greater interest are studies of aspheric lenses, which are touted as being able to improve optical quality by correcting spherical aberration. When measuring the effectiveness of these lenses on the eye, some studies have shown that aspheric soft lenses don't improve visual acuity. One likely reason is that as these contact lenses decenter on the eye, the spherical aberration correction also decenters and induces coma-like aberrations. Aspheric soft contact lenses reportedly provide worse visual acuity than do toric or spherical soft contact lenses when worn by low astigmatic patients who have large pupils. This can help explain why some patients report worse vision when wearing the aspheric version of a lens than when wearing the non-aspheric version of the same lens. Patients who have large pupils can end up with worse vision because of induced coma if the contact lens decenters on the eye.

Figure 3. Higher-order aberrations of a patient before being fitted with orthokeratology lenses (left, higher-order RMS=0.1265 microns) and one month after being fitted (right,

higher-order RMS=0.3318 microns). Note that the scale changes between the figures. All aspheric contact lenses are not created equal. Soft contact lenses advertised as aspheric have a pre-determined amount of spherical aberration correction incorporated into the design, which varies from manufacturer to manufacturer. It's important to know that different patients have different amounts of spherical aberration. If your patient doesn't have the amount of spherical aberration that the contact lens is designed to correct, it's possible for the patient to end up with more spherical aberration than they started with. The only way to determine this is to measure their HOAs with an aberrometer before and after the lens is fitted. Orthokeratology For patients fitted in ortho-k lenses, aberrometry can provide a wealth of information explaining associated visual phenomenon. Studies have shown that mesopic contrast sensitivity is reduced for patients wearing overnight ortho-k lenses. Patients often report halos around lights and increased glare under low lighting conditions. By measuring the aberrations of patients fitted with these lenses, we know that there is a significant increase in spherical aberration (Figure 3), which is often described as a Mexican hat-shaped aberration. Increases in spherical aberration have been associated with reduced mesopic, best-corrected visual acuity, as well as an association between losses of mesopic contrast sensitivity and the amount of myopia being treated. Similar to topography software, the software features available on aberrometers vary from manufacturer to manufacturer. Generally, aberrometry software will provide you with a list of Zernike polynomials that are used to reconstruct the wavefront and provide you with a threedimensional view of a patient's wavefront profile. You can typically choose to exclude specific aberrations, for example spherical aberration, to see the impact this has on the wavefront. Depending on the software, you can also view a point spread function or see a simulation of how an object such as an eye chart would appear with the measured aberrations. The point spread function is a representation of what a point of light looks like through the eye's optics. If the eye is free of all aberrations, a point of light will look like a small spot. However, when a patient has a greater than average amount of HOAs, the point spread function gives you an idea of the visual disturbance the patient is experiencing. Let's look at two cases in which aberrometry helped explain patients' visual complaints.

Figure 4. Point spread functions of a PMD patient before (top) and after (bottom) being fitted with a GP lens. Case Histories A 36-year-old female presented reporting decreased vision with spectacles in her left eye. Best-corrected visual acuity (BCVA) was 20/25 in each eye and subjective vision was much worse OS than OD. Corneal findings were normal with the exception of mild inferior thinning OS, and posterior segment findings were normal. We performed topography, complete corneal pachymetry and aberrometry and we diagnosed early pellucid marginal degeneration based on Orbscan topography findings of marked against-the-rule astigmatism OS (4.4D), the classic "kissing birds" keratometric pattern and inferior corneal thinning (496μm) located 2.4mm from the corneal apex. We fit this patient with a typical GP lens design OD and a large diameter GP lens OS, which resulted in 20/20 vision with each eye. The patient's unaided HOAs OS for a 5mm pupil diameter, as measured with the Bausch & Lomb Zywave II, decreased from 1.47μm to 0.30μm with the GP lens. The patient's HOA point spread function improved dramatically when wearing the GP contact lens (Figure 4).

A 34-year-old male presented wearing GPs with a previous diagnosis of mild keratoconus OU. He reported much better vision with his GPs than with spectacles; however, his BCVA was 20/20 regardless of whether he wore his spectacles or his GP lenses. Slit lamp examination revealed a small paracentral scar at 8 o'clock OD, Fleischer's rings in each eye and central corneal thinning in each eye. Although he could see 20/20 with each eye when wearing his GPs, he reported better vision OS. The patient's unaided HOAs for a 5mm pupil diameter decreased when wearing GP lenses in his right eye from 0.92μm to 0.68μm and in his left eye from 0.73μm to 0.54μm. His HOA point spread functions while wearing his GP lenses help explain why he noted that his vision OD was not as clear as OS when wearing GPs. The comet-shaped appearance of the OD point spread function (Figure 5a) when compared to his OS point spread function (Figure 5b) shows the influence that uncorrected coma has on his vision OD. These cases show that although aberrometry isn't essential to fit a contact lens patient, it can be a useful tool when troubleshooting visual complaints. In the future, patients may benefit from custom wavefront soft contact lenses. The patients most likely to benefit from custom lenses are those who have elevated HOAs, such as keratoconus patients. Because movement of the contact lens also moves the centration of the aberration-correcting optics, custom lenses aren't likely to benefit the general population. However, Marsack et al (2007) have demonstrated promising results. They recently produced a custom wavefront-guided soft contact lens for a moderately keratoconic eye and reported improved visual acuity with the custom lens. While lenses such as these are not currently available to contact lens practitioners, they may be in the future. Anterior Segment OCT Many people are familiar with the high-quality images that posterior segment ocular coherence tomography (OCT) instruments can produce. Carl Zeiss Meditec, Inc. recently used this technology to produce the Visante OCT, a non-contact anterior segment OCT instrument. This instrument has multiple scanning modes that allow users to obtain crosssections of the anterior segment, high-resolution corneal images and total corneal pachymetry.

Figure 5. Point spread functions of a keratoconus patient after being fitted with GP lenses. The comet-shape PSF of the right eye (top) explains why his vision was better OS than OD with GP lenses.

Figure 6. Visante OCT image of a keratoconic cornea. Note the corneal thinning immediately to the left of the corneal reflex beam. The Visante OCT uses low-coherence interferometry to create high-resolution cross-sectional

images of the eye. A low-coherence light source is split into two arms by a beam splitter. One arm of the light enters the eye and is reflected back into the instrument by the ocular surfaces, while the other arm of light is sent into a reference path within the instrument and reflected off of a reference mirror that moves axially to change the optical length of the reference arm. Light from both arms returns and enters a photo detector. Because low-coherence light is used, interference of the two arms of light occurs only within the narrow coherence length of the instrument's light source. The reference path is changed until interference occurs, which provides the axial depth of the reflected signal from a structure in the eye. This process is rapidly repeated by scanning the light across the eye to create a cross-sectional tomograph. As with aberrometry, contact lens practitioners can certainly fit and manage contact lens patients without having an OCT instrument. However, it does provide additional information that can be useful in practice. Perhaps the most direct contact lens application of this instrument was described by Eef van der Worp, BSc, FAAO, FIACLE in the March 2008 article titled, "Orthokeratololgy: Shaping Up." In the article, he describes how to use an anterior segment image from the Visante and the included software tools to measure the tangent angle of the peripheral cornea when fitting ortho-k patients. For example, when using the Paragon Vision Sciences Corneal Refractive Therapy (CRT) lens, one of the controllable factors is the Landing Zone Angle (LZA). The angle measuring tool in the Visante software can determine the LZA in the periphery of the cornea that is truly tangent to the cornea, potentially improving the CRT fit. The Visante can also help you identify and track changes in the cornea for various corneal conditions. For example, you can obtain full corneal pachymetry and high resolution corneal images to identify corneal thinning (Figure 6). The software will also allow you to measure the thickness of the cornea at any point on a corneal image. If a patient has had LASIK, the Visante allows you to not only measure total corneal pachymetry, but to visualize and measure the LASIK flap as well (Figure 7). In this particular patient, who underwent retreatment, the LASIK flap was measured at 194μm with a residual bed thickness of 233μm. In addition to ocular measurements, you can also use the Visante to visualize contact lenses on the eyes, which can be useful for evaluating on-eye contact lens curvature.

Figure 7. Visante OCT image of a LASIK patient in which the interface between the flap and residual bed is visible.

The Visante also has tremendous clinical research applications. Recent studies have reported using the Visante to measure crystalline lens thickness in children and crystalline lens thickness changes with accommodation as well as providing measurements of ciliary muscle size. Because this non-contact instrument produces excellent images and has superior repeatability and resolution, it's not surprising that this technology is being embraced similar to its posterior OCT counterpart. Conclusion While some of the most recent clinical instruments aren't necessary to fit routine contact lens patients, these instruments are useful when troubleshooting challenging patients who have unexplained complaints. More established instrumentation can help you better manage both routine and challenging contact lens patients. CLS Dr. Berntsen is a senior research associate at The Ohio State University College of Optometry, where he is currently working on a PhD in Vision Science. His research interests include juvenile-onset myopia and contact lens-related aberrations.

Reprinted by permission from Contact Lens Spectrum, published by Wolters Kluwer Pharma Solutions Vision Care Group © 2009-2010 All rights Reserved. Wolters Kluwer Pharma Solutions Vision Care Group is located 323 Norristown Road, Suite 200, Ambler, PA 19002 (USA). Please visit www.clspectrum.com for more information.