Introduction
The purpose of the eye’s optical system is to form an image on the retina. For a perfect eye, all rays of light from a single point in space will create a single image point on the photoreceptor layer to form a “sharp” image. This is called an “aberration-free eye.” However, human eyes suffer from three types of optical imperfections, namely, diffraction, scatter, and aberrations, all of which degrade image quality, blurring the image and lowering retinal image contrast. This chapter will focus on optical aberrations and their impact on vision.
Optical aberrations are generally divided into lower- or higher-order aberrations. Lower-order aberrations include myopia, hyperopia, and astigmatism. We have been successfully providing excellent vision of 20/20 to the population by simply correcting the lower-order aberrations with spherocylindrical lenses for the past 200 years, so why should we even try to correct “higher-order aberrations” (HOAs)? Porter showed that over 90% of the total aberrations in eyes are due to lower-order aberrations. There are the two main reasons to consider correcting HOAs. First, by correcting HOAs, vision quality of our patients could be further improved depending on the magnitude of the HOAs, which can be large in pathologic eyes, (i.e., keratoconic eyes). Secondarily, because the clinician’s view of a patient’s eye is created by the eye’s optics, the image quality will improve if HOAs of the examined eye are corrected, even allowing a cell-level retinal fundus examination of retinal pathology , or a cell-level view of trabecular meshwork under gonioscopy.
This chapter intends to provide the reader with a basic understanding of optical aberrations and the impact and clinical relevance of these aberrations on vision, with special attention paid to spherical aberration (SA).
Optical aberrations
Optical aberrations include chromatic aberrations, monochromatic aberrations (lower-order aberrations and HOAs), and off-axis aberrations. All three play a significant role in ophthalmology.
Chromatic aberrations
Because the human eye is sensitive to a wide spectral range of light (approximately 400–750 nm) and because many optical instruments such as autorefractors and aberrometers employ infrared radiation, changes in optical properties of the eye across this large spectral range can affect retinal image quality, objective measures of refractive state, and measures of axial length by partial coherence interferometers, a technology employed in the clinical instrument called IOL Master. As is common to all transparent optical materials, the refractive indices of ocular tissues vary with wavelength, which creates a wavelength-dependent change with short wavelengths focusing in front of the long wavelengths along the horizontal optical axis (longitudinal chromatic aberration [LCA], Fig. 2.1A ), as well as wavelength-dependent variations in image size and location (transverse chromatic aberration [TCA], Fig. 2.1B ). A generally well-centered pupil that lies close to the eye’s nodal plane (which sits approximately just posterior to the lens ) minimizes the magnitude and impact of TCA. Although LCA is large in dioptric terms (e.g., 2.5 diopters [D] across the visible spectrum ), its impact on quality of vision is small because visual sensitivity at both ends of the visible spectrum is very low. Also, there are reports that the impact of LCA on polychromatic image quality or contrast sensitivity is further reduced by the presence of monochromatic aberrations. However, instruments that measure refractive state of the eye using infrared (IR) radiation must include a correction factor to convert a near IR 850 nm measure to one that is visually relevant (e.g., 550 nm).
Although there are no simple ways to correct ocular chromatic aberrations, several clinical interventions have been implemented that either reduce or potentially increase chromatic aberrations. For example, different materials used for intraocular lenses (IOLs) have different refractive indices, and generally those with the higher refractive indices will have a larger change in index across the visible spectrum, but these modest changes in chromatic dispersion have no measurable impact on polychromatic image quality. A more significant manipulation of LCA is generated by a type of lens called a “diffractive lens,” which exhibits LCA opposite in sign to that of the human eye, and if the optical power of the diffractive lens is about +3 D, it can be used to correct the eye’s LCA. Conveniently, +3 D is a typical add power used for a bifocal, and diffractive optics are routinely included in bifocal or trifocal IOLs and contact lenses. There is one optical correction that can amplify TCA by moving the entrance pupil anterior to the corneal plane (corneal inlays ), because TCA will scale with distance of the pupil from the eye’s nodal point.
Aberrations that vary across the visual field
Although it is common to refer to the “image plane,” such a plane only really exists for a small region of the visual field, and the true shape of the image surface is a three-dimensional curved surface (a “Petzval surface” ). Because the axial location of the image surface changes, a focused image can only be present across the retina if it too has a curved shape, which, conveniently, it does. Therefore, the curved retina generally matches the curved image surface over the central +/– 40 degrees of the visual field, conveniently providing a focused image across a large area of the retina once the foveal image has been focused by standard refractive correction. Although eye shape can mostly correct for this “curvature of field,” aberrations and off-axis astigmatism can be very large in the peripheral retina. Although large optical aberrations are present in the peripheral image, the neural insensitivity to defocus means that these off-axis aberrations likely play a minor role in peripheral vision. When we talk about defocus (either hyperopic or myopic), it simply means the image is out of focus. Hyperopic defocus means image focuses behind the retina as in uncorrected hyperopes, whereas myopic defocus means image focuses in front of the retina as in uncorrected myopes. Recent animal studies suggest that hyperopic defocus in the peripheral retina may play a significant role in the development of myopia. Studies on interruption of hyperopic defocus or inducing myopic defocus peripherally have shown promising results on myopia prevention.
Monochromatic aberrations
We usually describe myopic or hyperopic refractive errors of the eye with three numbers (sphere, cylinder, axis). Myopia, hyperopia, and astigmatism are refractive errors known as second-order aberrations. In myopia ( Fig. 2.2B ), light rays entering the eye focus anterior to the retina plane. This is most often seen in an elongated eye. In contrast, hyperopia ( Fig. 2.2C ) occurs in a less-powered eye where light rays tend to focus behind the retina plane. Astigmatism happens when the uneven curvatures of the cornea and lens cause light rays to focus at two different points. The location of two different points relative to the retina determines the type of astigmatism. When one focal line is located anterior to the retina and the other on the retina, it is termed simple myopic astigmatism ( Fig. 2.2D ). When one focal line is located on the retina and the other behind it, it is termed simple hyperopic astigmatism. When both focal lines are before the retina but at two different locations, it is termed compound myopic astigmatism. When both focal lines are behind the retina but at two different locations, it is termed compound hyperopic astigmatism. When one focal line is in front of the retina and the other is behind the retina, it is termed mixed astigmatism. Both myopia and hyperopia can be corrected with spectacles, contact lenses, intraocular lenses, or refractive surgery. Astigmatism can be corrected with a cylindrical lens, a toric contact lens or IOL, or refractive surgery by counteracting the uneven curvature of the cornea or lens ( Fig. 2.2 ).
For an aberration-free system, all rays at distance form at a single point (focal point) on the image plane ( Fig. 2.3A ). However, in the presence of optical aberrations, rays from a single object point intersect the image plane at different locations, causing the light to spread out over an area. As a result, there is no single focal point, as illustrated in Fig. 2.3B .
Clinicians often umbrella HOAs, including SA and coma, under “irregular astigmatism.” When parallel light rays refract through a spherical lens with SA, the central rays will converge at the focal point of the lens, but the peripheral light rays fail to converge at the same focal point. Instead, in the presence of positive SA, the peripheral rays will refract more relative to the central rays ( Fig. 2.3C ), and the image of the point light source appears as concentric rings surrounding a central point. Coma is another example of an HOA, and causes light rays from an off-axis point of light to form an image with a comet-like tail ( Fig. 2.3D ).
The traditional sphere and cylinder lenses can no longer correct the optical error of the eye with these HOAs. A more complex numerical description is required to capture these HOAs.
Representation of aberrations
There are many ways to describe the shape of the wavefront, as schematically illustrated in Fig. 2.4 : (1) we can draw rays perpendicular to every point on the wavefront to indicate the direction of propagation; (2) if we know the direction of each ray, we know the slope of the wavefront at every point, which means we can use the mathematical process of integration to determine the wavefront shape; (3) we can describe the extra distance light rays must travel when getting to each point on the wavefront, using the pupil plane as a reference—this extra distance is called the “optical path length difference”; (4) if light travels along paths of unequal length, then the phase will be different. This is very useful mathematically because a description of the phase of light at every point in the pupil plane is a major component of a pupil function (Pr). The pupil function is the product of amplitude and phase (Pr = P amp × P phase ). The amplitude factor of the pupil function is determined by the effective pupil area. The phase portion of pupil function is derived from the wavefront error (WFE) map, where WFE is relative to the perfect spherical wavefront required to focus an image on the retina.
Zernike polynomials
To reduce the dimensionality of an aberration map, we can mathematically fit the aberration map with a relatively small number of Zernike radial polynomials and describe aberrations in terms of the coefficients of each polynomial. Fig. 2.5 shows the first four radial orders of the Zernike pyramid used to describe the eye’s wave aberrations. Zernike polynomials are the preferred method of representing wavefront aberrations for a couple of reasons. They are continuously orthogonal to or independent of one another, which will allow us to multiply one polynomial by another polynomial. They are reported as a single- or double-indexing scheme. The double-index notation is preferred because it minimizes the chance for confusion. The single-index notation is handy when making some types of plots, but it does not hold nearly as much information. For example, primary SA can be described either as Z12 in the single index notation, or as Z subscript 4 superscript 0 (Z 4 0 ) in double index notation ( Fig. 2.5 ).
In overview, each color map is a specific mode in the Zernike expansion through the fourth radial order. The second radial order defines the aberrations associated with defocus and astigmatism, in ophthalmic terms, sphere, cylinder, and axis. The third-order aberrations and above are collectively referred to as the HOAs. There are an infinite number of modes. The location of each mode in the tree is designated by the combination of a radial order and an angular frequency. When referring to a specific mode, a double index system is used. The Z is short for symbolizing all Zernike modes. The subscript and superscript designate each mode. The radial order is the subscript, and the angular frequency is the superscript. For example, Z subscript 4 superscript 0 (Z 4 0 ) refers to term number 12 in the single index notation, which is SA. The C is shorthand for the coefficient for each mode. The Zernike coefficients reveal how much of each mode is contributing to the total WFE, and these coefficients are typically specified in microns (short for micrometers; μm), or sometimes they are reported in the units of the wavelength of the light.
One drawback of the above standard Zernike polynomials is that higher-order modes involve lower-order modes (e.g., Z 4 0 contains an r 4 term and an embedded –r 2 term). To avoid this confusion, Gatinel et al. proposed a new polynomial decomposition method by a low-order decomposition and higher-order decomposition that separates the orthogonality between low and high orders while maintaining orthogonality.
Standard metrics of optical quality
Although the Zernike coefficients are simpler than the wavefront map for describing the whole eye’s optical imperfections, they remain a complex description and cannot directly correlate with vision. Therefore, we need standard metrics to summarize the overall optical quality and to simplify the complexity of the wavefront map. In the field of refractive ophthalmology, the most used metrics are root-mean-squared (RMS) WFE, point spread function (PSF), and modulation transfer function (MTF).
RMS
Computationally, RMS is simply the square root of the variance, that is, the standard deviation of the values of the WFE over the whole pupil. The unit of RMS is microns (micrometers; μm).
PSF
PSF is defined as the distribution of light intensity in the image plane for a point light source. To compute PSF, we first Fourier-transform the pupil function and then square it to get the PSF (PSF = FT (P r ) 2 ). Because the PSF is derived from the pupil function (phase × amplitude), both the pupil size and aberrations can influence the point source. The bigger the pupil and the larger the aberrations, the more irregular the shape of the point source imaged on the retina and therefore the worse the PSF ( Fig. 2.6 ).
MTF
MTF is essentially the ratio of the image contrast to the object contrast. Fourier transformation of the PSF is known as the optical transfer function, which is composed of both MTF (the contrast attenuation) and phase transfer function (the amount of phase shift). Both modulation and phase transfer functions are spatial frequency dependent.
Based on the above principal components, the team at Indiana University has developed 31 image quality metrics (i.e., area under MTF) to quantify the quality of the optical wavefront in either the pupil plane or retina plane. These metrics have been widely used to compute the peak image quality and depth of focus on a variety of new intraocular lenses, , and in the field of myopia control and accommodation. ,
The impact of HOAs on vision
In the previous section we reviewed the basic concepts and fundamentals of aberrations. How much do HOAs affect human vision?
In normal eyes, wavefront aberrations are not a significant source of image quality degradation. Without correction of lower-order aberrations (sphere and astigmatism), only about 7% of RMS comes from HOAs in the normal population, and 70% of these eyes have refractive error of 0.25 D or less after correction of lower-order aberrations. This is considered clinically nonsignificant because patients can tolerate 0.25 D with no noticeable perceptional difference.
In contrast to normal eyes, visual quality can be significantly degraded by the elevated HOAs in some pathologic eyes; for examples, severe dry eye patients, keratoconus, and some postrefractive surgery eyes. Dry eye disease, one of the most common ocular diseases, affects over 16 million adults in the United States, resulting in poor subjective , and objective optical quality. Owing to the irregular tear film, SAs and coma increase about 2.5 times compared with normal controls, which can be temporally improved with artificial tears. In keratoconic patients, the HOAs can be 10 times higher than the normal population, predominantly due to coma and SA, both of which can be significantly improved after successful treatment by crosslinking , or customized contact lens. In postrefractive surgery eyes, the induction of pre-existing HOAs worsens as pupil size increases, (e.g., SA up to 1.7 microns for a 7-mm pupil). The aberrated marginal optics around the edge of the ablation zone causes night vision disturbances or dysphotopsias such as “starbursts,” “glare,” and “halos.” SAs and coma are secondary effects of standard laser corneal ablations, which can be overcome with the optimized Femto-laser-assisted in situ keratomileusis (LASIK).
Spherical aberration
Because primary SA is one of the most significant HOAs in humans and it has been most widely used clinically, in this section we will review the concept of SA.
As described by Snell’s law, SA is generated as light refracts through any lens with a spherical surface resulting in a blur circle on the image plane. On-axis SA can be theoretically corrected with a perfect Cartesian oval where all on-axis rays form an aberration-free single point on the image plane. The SA becomes positive when the surface is more spherical or oblate compared with the perfect Cartesian oval, and negative if the surface is more prolate.
Unaccommodated human eyes typically have positive SA coming predominantly from the cornea, and become less positive (or more negative) during accommodation. Ocular SA is general quite small for typical pupil sizes, partly owing to the negative SA in the lens compensating for the positive SA in the cornea, and partly because of the aspheric form of the cornea. However, SA becomes much larger with larger pupils (SA expands with fourth power of pupil), which can be up to 2.8 D more myopic peripherally compared with the paraxial for an unaccommodated eye with a fully dilated pupil.
The impact of spherical aberration on vision
With the now routine control of SA levels by manipulating the wavefront profile on both IOLs and contact lenses, sometimes adding negative SA, sometimes removing SA, , , , and sometimes introducing large amounts of SA to achieve multifocality, , it is important to understand the impact of SA manipulation on vision. In this section, we will review the impact of SA on refractive error, visual acuity, contrast sensitivity, depth of focus, and night vision.
Spherical aberration impact on refractive error
As mentioned, the aberration caused by distortion of light as it passes through an imperfect optical system can be expressed mathematically as polynomials. SA is typically defined in Seidel form <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='S40=C405(6r4)’>𝑆04=𝐶045(6𝑟4)‾‾‾‾‾‾‾√S40=C405(6r4)
S 4 0 = C 4 0 5 ( 6 r 4 )
, which only contains the r 4 term, or Zernike form <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Z40=C405(6r4−6r2+1)’>𝑍04=𝐶045(6𝑟4−6𝑟2+1)‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾√Z40=C405(6r4−6r2+1)
Z 4 0 = C 4 0 5 ( 6 r 4 − 6 r 2 + 1 )
, which contains an r 4 term and a negativer 2 term (–defocus), where C 4 0 is the Zernike coefficient of SA, and r is the radius of the pupil in mm. The key difference between the Seidel SA and Zernike SA is the embedded negative defocus term in Zernike form. Although the r 4 term is the distinguishing feature of the primary SA, without removing the –r 2 term, Zernike SA primary effect on refractive error is caused by the lower-order r 2 term embedded in this polynomial. For example, by adding positive Zernike SA (+Z 4 0 ), refractive error becomes hyperopic due to the –r 2 . On the other hand, after adding the C 2 0 (r 2 term) to achieve paraxial focus, Seidel SA effect on refractive error is caused by the higher-order r 4 term, that is, by adding positive Seidel SA (+S 4 0 ), refractive error becomes myopic due to the +r 4 term. In the presence of positive Seidel SA, however, this myopic shift is mostly observed when viewing stimuli containing low spatial frequencies (e.g., contrast sensitivity charts with 2 cycles per degree grating stimuli). On the other hand, subjective refractions with high spatial frequency stimuli (e.g., high-contrast visual acuity charts) remain unchanged as the pupil expands. Clinical refractions using the “darker” criterion (“Lens #1 vs. #2, which one makes the letter darker to you?”) bias the patient to use lower spatial frequencies and thus prone to myopic bias, as most of unaccommodated human eyes have positive SA. On the contrary, the “sharper” criterion (“Lens #1 vs. #2, which one is sharper to you?”) favors the patient to use high spatial frequencies and generates reliable refractions that are minimally affected by the pupil size and the level of SAs.
Spherical aberration impact on visual acuity
In general, correcting aberrations with adaptive optics (a technology that uses deformable mirrors to correct aberrations) improves visual acuity by 0.1 logMAR relative to subjective distance refraction, and human eyes can tolerate up to 0.1 microns of RMS without noticeable blurry vision. However, by adding more Zernike SAs, visual acuity can be significantly affected even for high-contrast letters at photopic light level (retinal illuminance >50 Troland , ). In general, detrimental impacts of Zernike SA on visual acuity varies from 0.3 logMAR/microns of SA to 0.8 logMAR/microns of SA. , After removing the r 2 term by adding C 2 0 (Seidel SA), the impact of Seidel SA on visual acuity is much smaller. , For low-contrast letters, visual acuity is on average 0.2 logMAR worse than high-contrast visual acuity for a given SA.
Spherical aberration impact on contrast sensitivity
At photopic light levels, contrast sensitivity is maximal when aberrations are removed via adaptive optics, and contrast sensitivity decreases in the presence of SA. Similar reduction in contrast sensitivity is seen for the spherical Sensar AR40e IOL compared with the aspheric Tecnis Z9000 IOL. In post photorefractive keratectomy (PRK) eyes with significant amount of SAs, contrast sensitivity is much worse compared with the emmetropic control, especially at low light levels. The reduced image contrast is presumably caused by the increased SA associated with larger pupils at low luminance levels.
Spherical aberration impact on night vision
Patients with high ocular aberrations routinely complain of vision disturbance (e.g., halos, starbursts) while driving at night. Studies from Xu et al. showed that the starburst or halo size (in degrees) linearly correlates with the amount of SA (in microns) with a slope of 1.5 degrees/microns of SA for a fixed 7-mm pupil, a typical mesopic pupil size for the middle-aged population. They also showed that focusing near the pupil margin minimizes starburst size, but a typical subjective refraction focuses a near paraxial region of the pupil; therefore, starbursts exist in most eyes, especially those eyes with large amounts of SA. Also, significant positive SA generates myopic shift in refractive state when viewing high-contrast point sources at night ; this is commonly known as night myopia.
Spherical aberration impact on depth of focus
Depth of focus (DOF) simply means the dioptric distance between the closest image and farthest image of an object that appears acceptably focused. There’s no universal definition of DOF, and therefore the amount of DOF reported in the literature varies with the criteria used to define DOF. By convention, DOF can be defined by objective (e.g., the dioptric range over which a specific image quality exceeds certain criteria) or subjective criteria (e.g., the dioptric range over which visual acuity or contrast sensitivity meets a certain criterion level), both of which can be further grouped as relative (e.g., 50% of peak image quality) or absolute (e.g., 20/40 visual acuity). Although both primary (Z 4 0 ) and secondary SA (Z 6 0 ) can produce the double-peaked through-focus curves characteristic of true bifocals, this expansion of DOF is mostly absent if a strict image quality criterion is applied. The expansion of DOF observed with a low-image-quality criterion when opposite sign Z 4 0 and Z 6 0 are combined because of the elevated r 4 term. Although multifocality can be generated by adding primary SA alone or by adding opposite sign of Z 4 0 and Z 6 0 , the former is more effective than the latter.
Treatments to alleviate the impact of HOAs
Fundamentally, there are two ways to alleviate the impact of HOAs on vision, either through manipulating the pupil amplitude function or via the phase function.
Reducing pupil size
Mathematically, Schwiegerling proved that Zernike polynomials scale to the power of their radial orders as pupil sizes change. For example, 0.4 microns of SA (C 4 0 ) with a 6-mm diameter pupil will reduce to 0.4 × (3/6) 4 = 0.025 microns with a 3-mm diameter pupil. Clinically, reducing the impact of HOAs via reducing pupil size can be achieved with pinhole glasses, contact lenses containing small artificial pupils, , miotic ophthalmic solutions, , or diaphragm IOLs. All of these have proven successful at reducing night vision disturbance in highly aberrated eyes.
What is the optimal pupil size to minimize aberration effects associated with large pupils? This question becomes particularly interesting nowadays due to the ongoing development of multiple miotics as a treatment for presbyopia. , The traditional optical model taught us that a pupil size between 2 and 3 mm is optimal as a counterbalance between diffraction and aberration effects. However, this is only true under photopic light levels (where Weber’s law applies, meaning contrast sensitivity is unaffected by retinal illuminance ) and for a well-focused eye with average levels of ocular aberrations. The optimal pupil size is determined by the interactions between diffraction, amount of aberration, and photon noise effect. As a result, a smaller pupil (e.g., <2 mm) is optimal in the presence of significant lower-order aberrations or HOAs at high light levels, while a larger pupil (e.g., >3 mm) is optimal at mesopic light levels, and the exact size depends on the amount of aberration and/or retinal illuminance. ,
Reducing phase function from HOAs
Correcting HOAs with spectacles, contact lenses, and intraocular lenses
Spectacles
For wavefront-customized spectacles to work, they would have to be stable enough to not decenter with head movements, and they would have to be designed in such a way that the aberrations would be corrected in all fields of gaze and at different pupil sizes. Ophthonix, Inc. (Vista, CA) has designed a lens available for commercial use that consists of a three-layer structure with a refractive index of 1.6. The middle layer consists of a patented photopolymer in between two coated lenses. The lens corrects HOAs from the second to sixth order using wavefront-guided technology that corrects aberrations based on measurements from a built-in aberrometer.
Contact lenses
The use of customized contact lenses to correct HOAs is also possible. There are rigid gas permeable (RGP) and soft contact lenses. In a clinical setting, RGP lenses are helpful in correcting refractive errors in highly irregular corneas. The smooth contour of their anterior and posterior curvatures, coupled with the tear film formed within the interface, provides a new refracting surface for the eye. These lenses are designed to move freely, reaching up to 1 mm of decentration from the visual axis of the eye with every blink, which makes RGP lenses unable to correct HOAs, owing to decentration. Soft contact lenses, however, are much more stable and less likely to undergo misalignment relative to the visual axis. These lenses rotate less than 5 degrees with every blink, which is ideal for HOA correction because coma and astigmatism are tolerant to misrotations of up to 60 degrees.
IOLs
Unlike young eyes in which the negative SA produced by the lens can compensate for the positive SA from the cornea, old eyes and pseudophakic eyes with spherical IOLs have significant positive SAs, which has led to the dominance of aspheric IOLs. There are generally two types of aspheric IOLs: one with negative SAs and one with zero aberrations (aberration-free). The total aberration of a pseudophakic eye is the sum of the cornea aberration and the aberration from the inserted IOLs. Aspherical IOLs with negative SA are intended to neutralize the inherent positive SA of the cornea to achieve postoperatively aberration-free vision. Because SA can expand DOF, sometimes aspherical aberration-free IOLs are used to preserve the inherent aberrations of the cornea to expand DOF.
Some commercially available aspherical IOLs, such as the Tecnis-Z9000 (Pfizer, New York, NY) and SN60WF (Alcon, Fort Worth, TX), are designed with a prolate anterior surface, hence the term aspheric , and have the same radius of curvature at every point on the surface. The benefit of aspheric IOLs depends largely on their centration and stability within the capsular bag. The US Food and Drug Administration (FDA) approved the light adjustable IOL in 2017, which, once implanted in the eye, can be tuned to compensate for residual refractive errors, and even HOAs by irradiating the optic with blue light to polymerize monomers, setting up a diffusion gradient to alter lens shape. ,
Corneal ablations
How can ablating cornea correct ocular aberrations? Let us start with lower-order aberration correction (e.g., myopia as an example). Myopia happens when an image focuses in front of the retina. LASIK can reshape the myopic cornea by flattening the central cornea so that the image can sharply focus on the retina. Similarly, to correct the HOAs, the preoperative and/or intraoperative aberrations can be measured using an aberrometer. Then this measured aberration profile can be loaded into a computerized laser system to generate a precise ablating profile that compensates for the pre-existing aberrations. For example, an eye with +0.2 microns of SA preoperatively requires induction of –0.2 microns of SA with corneal ablation to achieve postoperatively aberration-free vision.
The main focus of laser refractive surgery nowadays is to not only correct refractive errors such as defocus (myopia or hyperopia) and astigmatism, but to create customized ablations that compensate for induced HOAs (optimized ablation) and also correct pre-existing aberrations (customized ablation). There are several laser platforms utilized for corneal ablations. These range from the simple conventional treatments to the more sophisticated topographical guided and wavefront-customized ablations. Differences in these treatment modalities lie in the type of beam delivery, spot size, treatment zone, and amount of tissue ablated for each diopter of refractive error treated. For the purpose of this chapter, we will only discuss wavefront-customized corneal ablations.
There are two types of wavefront-customized ablations: wavefront-guided ablations and wavefront-optimized ablations, both of which aim to correct for HOAs in the eye. In wavefront-guided ablations, the treatment is aimed to correct HOAs that exist preoperatively. The wavefront-optimized ablations profile corrects expected HOAs for an average eye and those that are anticipated as a result of the surgery. Therefore, unlike wavefront-optimized ablations, which apply population-based mean aberration levels to individual eye, wavefront-guided ablations may be better at correcting eyes with extremely high preoperative HOAs. Consistently, studies found that wavefront-guided ablations induced fewer HOAs and had better contrast sensitivity after surgery than wavefront-optimized ablations. However, a recent study showed that owing to the intraoperative induction of HOAs especially associated with LASIK, the beneficial effect of wavefront-guided ablations was mainly found in PRK-treated eyes, not LASIK-treated eyes.
Wavefront sensing
In the previous sections, we explained the concepts of optical aberrations and their impact on human eyes. In this section, we will review how we measure these aberrations in both laboratory and clinic settings.
Evolution of aberrometers
Wavefront sensing is a technique originally developed by astronomers to improve their vision to see stars in space. In the 1600s a German astronomer, Christopher Scheiner, set the stage for modern aberrometry and wavefront-sensing devices with the invention of the Scheiner disc. He reasoned that an optically imperfect eye would form two separate images on the retina when looking at a distant light source through a disc containing two pinholes ( Fig. 2.7 ). If the eye’s imperfection is a simple case of defocus, then the double retinal images can be brought into register by viewing through a spectacle lens of the appropriate power. However, if a simple lens cannot bring the two retinal images into coincidence, a more complex device is needed for quantifying the imperfection.
