Orthokeratology: Shaping Up
Succeeding with orthokeratology is all about understanding corneal shape and how to change it.
By Eef van der Worp, BSc, FAAO, FIACLE
|Eef van der Worp is affiliated with the University of Maastricht in the Netherlands and specializes in corneal topography, GP lens wear and corneal desiccation. He is lecturing intensively worldwide, and he resides both in Amsterdam - the Netherlands and in Washington, DC.|
Orthokeratology, or overnight corneal reshaping, keeps intrigue on the practitioner's side and on the industry's side, and it definitely keeps intriguing patients. Every patient successfully fit with an orthokeratology lens is almost guaranteed to be a walking bulletin board, promoting the modality and the practice where the lenses were fit. Orthokeratology in different regions of the world is very well received, and in some parts of Asia and Europe it has evolved into a very successful and popular mode of vision correction.
But at the same time, some practitioners are not as comfortable as others with the modality, for a variety of reasons. No matter how much promotion this still new modality receives, it is practitioners who need to be involved and to be comfortable with it for the modality to be successful. While orthokeratology will most likely not take over the entire contact lens market, it's a very good solution for specific types of patients. Contact lens wearers who experience corneal dryness symptoms with existing hydrogel lenses can especially benefit from this modality. In terms of ocular comfort and quality of life, nothing beats lens-free vision during daytime hours.
This article will discuss what tools you need to succeed with this modality, how to determine the shape of the cornea and how to evaluate corneal changes that occur during the fitting process.
Evaluating Corneal Shape
The goal in orthokeratology is to temporarily change the shape of the cornea so that patients can see clearly without their lenses, hence the term corneal reshaping. But what is the shape of the cornea to begin with?
By far the best instrument to analyze the shape of the cornea is the corneal topographer. Several attempts to modify manual keratometers to carry out this analysis have proven to be of limited use. A keratometer typically measures the central 3mm of the cornea, which is only approximately 8 percent of the total corneal surface. Corneal topographers cover a much larger surface, although they typically aren't able to measure the far periphery of the cornea. Most corneal topographers extrapolate the 'missing' parts of the cornea that could not be measured, but you should interpret these data with care because assumptions are made about the shape of the cornea that do not necessarily reflect the real corneal shape.
The shape of a normal corneal surface is usually described as a prolate ellipse, indicating a gradual flattening from center to periphery. This is one of first things that is obvious when looking at an average corneal topography map — relatively cooler colors in the periphery of the corneal map represent this flattening. The amount of flattening is traditionally designated as eccentricity, mostly noted as the e-value. The e-value of an ellipse can be calculated from the central curvature and the peripheral curvature plus the distance (angle) from the center where that peripheral curve was measured. Many have described the average cornea, and its e-value is thought to be somewhere between 0.40 and 0.57. The evalue varies widely among individuals, and ideally you should measure and evaluate it on every single eye before fitting contact lenses. As an example, at The Ohio State University, Walline et al (2004) analyzed corneal shape in 683 children's eyes (aged 8 to 15) and found that while the vast majority flattened towards the periphery, two corneas actually steepened. There is weak correlation between ametropia and eccentricity (higher myopia, reduced eccentricity).
Figure 1. Corneal height map generated with a placido disk corneal topographer showing the total corneal sagittal height.
Not all meridians of the cornea have the same evalue. Most corneal topographers and automated keratometers will provide the average value of all meridians, although some will give e-values per meridian or quadrant. A major drawback with regard to e-value measurement is that manufacturers of topographers do not disclose the way they calculate the e-value: they usually reveal neither the distance from the center nor the meridian used, which means that e-values among topographers should not be used interchangeably. Unfortunately, there is no standard for defining corneal shape with corneal topography.
Besides this, another disadvantage of the e-value is that it can describe only prolate shapes. In corneas that are steeper in the periphery by default or that are reshaped that way by orthokeratology or laser surgery, a more oblate shape is present. In these cases the e-value is useless, because mathematically it can only define shapes larger than zero (spherical in shape).
We can overcome this problem by using what has been called the p-value, which is derived directly from the e-value: p = 1 – e2. Using the p-value describes the exact same shape, but keep in mind that a circle's value is now 1 instead of 0. The p-value of all prolate shapes is less than 1, and in the case of peripheral steepening the pvalue is larger than 1, which is exactly the opposite of the e-value. You may come across p-values as an alternative to e-values in the international literature and should be aware of the differences compared to the e-value.
Another approach to describe the asphericity of the cornea is to use Q. To derive this value, Q = p – 1, or Q = – e2. A negative Q-value describes a prolate shape and a positive Q-value describes an oblate shape, but the Q-value of a sphere remains 0, the same as the e-value. Therefore, the Q-value has advantages over both of the other shape descriptors, and Swarbrick (2004) has suggested considering it as a standard index for describing corneal shape, especially in orthokeratology and refractive surgery.
In reality, however, the actual average corneal shape is not as easily defined as a standard ellipse; most notably toward the periphery it becomes more complex and less predictable. The corneal shape is usually more spherical near the apex, and it may flatten at a variable (usually progressive) rate towards the periphery. Zernike polynomials can often describe the corneal shape in more detail, but even these complex mathematical formulations have their limitations. Newer mathematical definitions of the corneal shape are being developed, but the clinical usefulness of these complex formulations when manually fitting GP lenses is very limited. Therefore, to describe corneal eccentricity, shape parameters are still preferred and provide a good representation of the flattening of the cornea. The e-value is still the parameter most used in contact lens practice, and most topographers and automated keratometers currently use this value.
As a rule of thumb, you may square the e-value that the topographer or automated keratometer provides to have some idea of the amount of flattening in the periphery of an average cornea. An e-value of 0.4 means that the flattening in the periphery at 30 degrees from the center is about 0.16mm. An e-value of 0.6 describes a flattening of 0.36mm, and a cornea with an e-value of 0.8 is 0.64mm flatter in the periphery. From this we can conclude that as the e-value goes up, flattening increases progressively. This also means that small e-values are clinically of little importance, but the significance of large e-values increases rapidly as the number goes up.
The bottom line is that the cornea flattens towards the periphery no matter what value is used to describe it. During orthokeratology lens wear, the shape of the cornea will change from a prolate shape to a spherical shape or even to an oblate shape (steepening towards the periphery from center). E-value is also clinically relevant in orthokeratology with regard to the desired refractive effect. Mountford (2004) found that there can be a refractive effect of approximately 0.75D before any change in keratometry values occurs, attributing this to the changes in corneal shape (the e-value change). It's believed that more myopia correction can be achieved in corneas that have larger e-values than in corneas that have low e-values.
Figure 2. A bull's eye pattern in the difference map (right picture), which represents the difference between the initial corneal topography data (top left picture) and the newly created corneal topography map after orthokeratology treatment (bottom left picture).
Figure 3. An apparent central island pattern appears in the difference map (right picture), but this will most likely resolve over time into a bull's eye pattern.
But the most important benefit of knowing the rate of corneal flattening is that, combined with the known curves of the cornea and the cord (the total corneal diameter), it's easy to generate corneal sagittal height information (Figure 1). Modern projection-based topographers, rather than reflection systems such as placido disk systems, may help in directly measuring the actual corneal sagittal height. Such systems include the Orbscan (Bausch & Lomb), Pentacam (Oculus) and Galilei (Ziemer) systems.
Another advantage of projection systems is that they measure a larger area of the cornea, typically out to the limbus (the entire corneal surface). If the sagittal height of the cornea is known, it's fairly easy to create a reverse geometry lens that will cause the desired central pressure on the cornea to correct the desired amount of ametropia. With orthokeratology, the aim is to always create a thin tear layer over the apex of the cornea. Unfortunately, corneal topographers aren't always able to give an exact estimate of corneal height. They can over-or underestimate the sagittal height. Some topographers are better at producing reliable and reproducible data than others. You should perform several corneal topography measurements regardless of what instrument you use to minimize potential errors.
Using Topography to Evaluate an Ortho-k Fit
To evaluate an orthokeratology lens fit, and to determine whether the sagittal height of the cornea was calculated correctly, you need to manually evaluate the fluorescein pattern and/or the post-treatment topographical data. With regard to topography evaluation, you need to create a difference map between the initial corneal topography and the newly created corneal topography under the orthokeratology lens. One of three classical patterns will typically result. A bull's eye pattern, in which a central flat area is visible surrounded by a ring of relative steepening (Figure 2), is the desired outcome. The size of this central flat area tends to increase with time (after the first night it doesn't necessarily cover the entire pupil area).
The second possible pattern you may observe in orthokeratology is a central island, which is usually fairly simple to recognize: a small area of relatively steep curves (lighter colors) appears within the optical zone of the eye. It's important to evaluate whether this area is flatter than the original curves of the central cornea. If this small island is flatter than the original cornea, it most likely will resolve in a matter of days (Figure 3). In other words: keep using the current lens and it will evolve into a bull's eye. However, if the island has steeper curves than the original cornea, this is most likely a true central island (Figure 4). The topographer overestimated the sagittal height of the cornea — the cornea is in reality 'flatter' than calculated. A new, flatter lens will resolve this situation.
The third possibility is a smiley face pattern (Figure 5). In this case the lens is actually too flat for the cornea or, in other words, the lens sagittal height is too low. Lens decentration, mostly superiorly, in this case is common, and the flattest area of the cornea is displaced superiorly. A partial ring representing steeper areas can cover a portion of the optical zone of the cornea, resulting in decreased visual acuities. The cornea is steeper here than the topographer estimated, and a steeper lens fit with a greater sagittal height is required.
Figure 4. True central island pattern in the difference map (right picture).
Figure 5. A smiley face pattern in the difference map (right picture).
Know Your Topography Scales As Mountford (2004) described, you can and should use different topography scales to evaluate the orthokeratology outcome. An axial map is best used to evaluate the refractive effect — it corresponds best with the subjective refraction.
A tangential map is essential to evaluate the centration of the lens, which is crucial when fitting orthokeratology lenses. Ideally, a full ring is visible to ensure that the lenses have remained centered during the overnight portion of the procedure. Patrick Caroline, FAAO, has said that the goal with regard to orthokeratology lens fitting is to be 'lord of the rings,' and the only tool to evaluate this properly is a tangential map.
The refractive power map best estimates the size of the treatment zone (the axial and tangential maps can over- or underestimate the effect).
Take care when taking corneal maps: the tear film can break up, which can result in abnormal outcomes. In addition, corneal staining can cause erroneous measurements. It's advised to always take more than one picture to overcome this problem, and always check the cornea for staining prior to taking the topography map.
A Different Fitting Approach
While many orthokeratology systems are topography based, the CRT corneal refractive therapy system (Paragon Vision Sciences) allows practitioners to manually change the sagittal height of the lens to best match the cornea based on fluorescein evaluation. Alteration of the base curve radius causes a 7 micron change in sagittal height of the cornea, but typically you should handle this parameter with care as it simply resembles the refractive change that needs to be achieved. Changes to the base curve radius will cause a change in the refractive outcome. The second, or reverse, curve of the CRT lens can be changed using 25-micron steps to either increase or decrease the overall sagittal height. You can also alter the most peripheral zone of the lens, the tangent angle, which causes a 12-micron change in sagittal height. The angle that the periphery of the cornea makes with a horizontal reference line should meet this peripheral tangent angle of the lens. The tangent angle is a key factor for the success of the orthokeratology fit, and you can assess this parameter manually by looking at the fluorescein pattern.
Until recently, no effective measurement technique for this part of the cornea was available, but a new technique originally developed for retinal imaging has become available to image the anterior eye. The technique, called ocular coherence tomography (OCT) performed with the Visante (Carl Zeiss Meditec) is able to image the periphery of the cornea (and even parts of the sclera). With this technique (Figure 6), it seems that we can now easily and accurately determine the tangent angle of the peripheral cornea. This could allow for better CRT lens selection and potentially higher success rates.
Figure 6. Anterior segment OCT using the Visante can make it easier to determine the tangent angle of the peripheral cornea.
Get in Shape
Many patients today benefit from orthokeratology. These patients have been fitted in numerous practices worldwide, simply because practitioners offered orthokeratology as one of their vision correction modalities. It seems hard to believe that the same would not apply for many patients of other contact lens practices that currently do not utilize orthokeratology. With better understanding of the shape of the cornea and the mechanisms behind this modality, it will become more widely available to patients. Using new technology such as the OCT technique to image the periphery of the cornea (and even parts of the sclera) may further aid in this process.
For those of you who are really interested in orthokeratology and want to get fully in shape for it, the best equipped gym you can go to is the orthokeratology meeting organized by the Orthokeratology Academy of America, which will take place from April 17 to 20 in San Diego. Visit www.okglobal.org, and prepare to get in shape. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #148.