LASIK for Correction of Myopia Workup

Updated: Dec 03, 2018
  • Author: Michael Taravella, MD; Chief Editor: Douglas R Lazzaro, MD, FAAO, FACS  more...
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Ultrasound corneal pachymetry

Pachymetry is an important part of the refractive surgery workup.

The FDA has mandated that 250 µm of untouched cornea remain in the bed following LASIK. This is calculated as follows: initial pachymetry minus calculated (or measured) flap thickness minus ablation depth must be greater than or equal to 250 µm. [7] Ectasia, which represents a biomechanical weakening of the cornea (see Complications), is risked when the residual bed is less than 250 µm. Note that leaving 250 µm in the residual bed does not guarantee that ectasia will not occur; this is simply the current FDA guideline.

Wavefront analysis

Some basic concepts are useful in understanding wavefront analysis customized corneal ablations. [8, 9]

Wavefront technology is an offshoot of astrophysics and was initially developed to help obtain undistorted telescopic images of the night sky. The current technology used in refractive surgery examines what happens as light interacts with the optical system of the eye.

A wavefront represents a locus of points that connects all the rays of light emanating from a point source that have the same temporal phase and optical path length. The optical path length specifies the number of times a light wave must oscillate in traveling from one point to another point. Light propagation is slower in the refractive media of the eye than in air, so that more oscillations will occur in an optical system, such as an eye, than in air for light to travel the same distance. If the optical system of the eye is perfect, a point source of light emanating from the back of the eye will create a locus of points with the same optical path length exiting the pupillary plane in the form of a flat sheet. This represents an unaberrated wavefront. When the cornea or lens has imperfections, optical aberrations are created, causing the wavefront to exit the eye as curved or bent sheets of light.

Aberrations can be defined as the difference in optical path length (OPL) between any ray passing through a point in the pupillary plane and the chief ray passing through the pupil center. This is called the optical path difference (OPD) and would be 0 for a perfect optical system.

Another way of characterizing the wavefront is to measure the actual slope of light rays exiting the pupil plane at different points in the plane and compare these to the ideal; the direction of propagation of light rays will be perpendicular to the wavefront. This is the basic principle behind the Hartman-Shack devices commonly used to measure the wavefront. Wavefronts exiting the eye are allowed to interact with a microlenslet array. If the wavefront is a perfect flat sheet, it will form a perfect lattice of point images corresponding to the optical axis of each lenslet. If the wavefront is aberrated, the local slope of the wavefront will be different for each lenslet and result in a displaced spot on the grid as compared to the ideal. The displacement in location from the actual spot versus the ideal represents a measure of the shape of the wavefront.

Once the wavefront image is captured, it can be analyzed. One method of wavefront analysis and classification is to consider each wavefront map to be the weighted sum of fundamental shapes. Zernike and Fourier transforms are polynomial equations that have been adapted for this purpose. Zernike polynomials have proven especially useful since they contain radial components and the shape of the wavefront follows that of the pupil, which is circular. Fourier transforms, however, may prove to be more robust and allow mathematical description of the wavefront with less smoothing effect and greater fidelity. Illustrations of the basic Zernike shapes are appended.

The term higher order optical aberration has begun to replace the older term irregular astigmatism as wavefront analysis has become more accepted. This simply refers to the mathematical term used to describe the aberration and its place in the polynomial expansion. Lower order aberrations, such as sphere and cylinder, require lower order mathematical terms within the polynomial expansion to characterize them and are commonly referred to as second order aberrations. The most important higher order terms are spherical aberration (a fourth order term) and coma (a third order term).

Fortunately, spherical aberration is relatively easy to understand. Light rays entering the central area of a lens are bent less and come to a sharp focus at the focal point of a lens system; however, peripheral light rays tend to be bent more by the edge of a given lens system so that in a plus lens, the light rays are focused in front of the normal focal point of the lens and secondary images are created. This is why many lens systems incorporate an aspheric grind, so that the periphery of the lens system gradually tapers and refracts or bends light to a lesser degree than if this optical adaptation was not included. See the image below.

Spherical aberration: a schematic diagram for the Spherical aberration: a schematic diagram for the human eye.

Traditional myopic LASIK patterns tend to induce spherical aberration; the higher the degree of correction, the greater the induction of this optical error. During the day, the pupil size tends to limit the effect of spherical aberration, since peripheral light rays are blocked. At night, as the pupil enlarges in dark or scotopic conditions, these light rays enter the eye and can create a blurred focal point and secondary images.

Custom laser treatments incorporate a specific algorithm to help limit the induction of spherical aberration. This algorithm is based on a patient's unique wavefront measurement of their individual eye to some extent. However, the most important aspect of treatment is a blend or tapering of the peripheral treatment zone. Some lasers have incorporated a noncustom approach to this problem and create the transition zone at the edge of the ablation based on an empirical approach that takes into account the patient's prescription glasses and corneal curvature readings instead of using unique patient wavefront data. The best approach to limit this problem is under investigation. Note that coma and other preexisting aberrations would only be corrected by using data from an individual patient's unique wavefront error to plan and determine the shape of the laser ablation pattern. This approach is used in custom treatments.

Wavefront maps are commonly displayed as 2-dimensional maps. Just as interpretation of corneal topography has been greatly aided by the use of color maps, so too has wavefront mapping. The color green indicates minimal wavefront distortion from the ideal, while blue is characteristic of myopic wavefronts and red is characteristic of hyperopic wavefront errors.

Remember that wavefront maps are a 2-dimensional attempt to display 3-dimensional shapes. See the images below.

Spherical aberration post-LASIK. The original refr Spherical aberration post-LASIK. The original refractive error was -10.00 diopters.
Coma in a patient with mild ectasia. This higher o Coma in a patient with mild ectasia. This higher order optical aberration is also characteristic of decentered ablation zones and ectasia.
Postoperative ectasia: Orbscan. Note the elevation Postoperative ectasia: Orbscan. Note the elevation on anterior and posterior floats and the thinning of the central cornea on the pachymetry map.
Ectasia post-LASIK: Tracey WaveScan. Note the prep Ectasia post-LASIK: Tracey WaveScan. Note the preponderance of higher order aberrations, including spherical aberration and coma. The Orbscan of this same patient appears in the image above.

The Root Mean Square (or RMS) value has proven to be a useful way of quantifying the wavefront error and comparing it to normal. This number can be calculated for the wavefront as a whole or as individual components of the wavefront when displayed as Zernike polynomials. See the image below.

Zernike polynomials: pictorial representation. Zernike polynomials: pictorial representation.

Corneal topography

Corneal topography is a method of measuring and quantifying the shape and the curvature of the corneal surface. Most topographers consist of a placido disc made up of multiple circles, which is backlit or projected onto the corneal surface. The resultant circular images are reflected and captured with a video camera and digitized. [10]

Using the mathematics of convex mirrors and proprietary mathematical algorithms, the image size is measured and quantified. The resulting data are displayed as a corneal curvature map.

The maps consist of colors corresponding to corneal power and curvature; steep contours are displayed as warm colors (eg, red), while flat contours correspond to cool colors (eg, green, blue).

Both absolute and normalized maps can be displayed. Absolute maps always assign the same color to the same power, and normalized maps take into account the range of power over a given cornea, ascribing red and yellow colors to the steepest contours and blue and green colors to the flattest contours for that particular cornea.

Many factors can affect the accuracy and reproducibility of corneal topography maps; these factors include quality of the tear film, the ability of the patient to maintain fixation, and operator experience.

Corneal topography is used primarily as a screening tool to evaluate prospective refractive surgery candidates and a diagnostic aid in evaluating refractive surgery patients with poor outcomes. [11, 12] Irregular corneas are poor candidates for refractive surgery since results with current lasers can be unpredictable. Keratoconus and contact lens warpage are the most common causes of irregular corneas in the screening population. Steep (ie, red) areas isolated to the inferior cornea suggest keratoconus, and many topographers come equipped with programs to alert the clinician when a diagnosis of keratoconus is likely. Postoperative patients with poor vision should have topography; such problems as central islands, irregular ablation profiles, and decentered laser ablations can be assessed with these devices.

Normal astigmatism pattern with corneal topography Normal astigmatism pattern with corneal topography.
Normal corneal topography spherical pattern. Normal corneal topography spherical pattern.
Keratoconus suspect; inferior and asymmetric corne Keratoconus suspect; inferior and asymmetric corneal astigmatism pattern.
Keratoconus with elevation map; asymmetric and irr Keratoconus with elevation map; asymmetric and irregular astigmatism with inferior corneal elevation and steep area of inferior cornea.