Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Masanao Fujieda MA,

Mukesh Jain, PhD.,

Peter Keller, PhD.

Introduction

NAVWave is a coupling of two brilliant technologies, to deliver customized ablation for the correction of refractive error. The OPD-ScanR system, combining wavefront analysis with corneal topography to map the aberrations of the entire optical system, is linked with Nidek's unique Final Fit Software to evaluate and convert the data to produce the precise ablation parameters for customized excimer laser ablation of the cornea.

OPD-Scan

The OPD-Scan (Optical Path Difference Scanning System) is equipped with corneal topography, aberrometry, refractometry, keratometry and pupilometry functions within a single device overcoming the potential problems of inter-device misalignment. The OPD-ScanR measures both corneal topography using Placido disk reflection method as well as total ocular aberrations using dynamic skiascopy technology. Dynamic skiascopy currently provides the most direct measurement of any wavefront system without using a lenslet array or the projection of a grid target onto the retina. The technique of skiascopy has been successfully used in auto-refrac- tometers but has now been modified to permit the

additional measurement of higher order aberrations. The major difference between the dynamic skiascopy based OPD-ScanR and other skiascopy based autorefractometers is that the OPD-ScanR can measure the distribution of refractive error in a wider corneal zone as it provides internally with a photo diode array at a conjugate position with the cornea.

A constant-speed single direction slit shaped infrared ray is projected towards the entrance pupil center as a thin beam and onwards to strike the retina. An aperture and a photo diode array that is conjugate with the retina for emmetropia collect the reflected light from the retina. There are four photodetectors above and four photodetectors below the optical axis. There are also two photodetectors, one of each side, which detect the center of the photodetector pairs. The basic principle is in many ways similar to confocal microscopy in that only the light reflected form the retina that passes through the aperture would reach the photodiode array. How the reflected light reaches the photo diode array depends on the positional relationship between the retina and aperture. We have just said only the reflected light that passes through the aperture will be collected. In the case of emmetropia eye, the aperture is positioned on the retina, and during the slit shape light goes across the aperture, the reflected light hits the all photo-diode cells simultaneously. Therefore, there is no time lag between different detection time

and the time difference should be zero. In hyperopia, the aperture is positioned behind the retina and hence there is a time lag between different diodes detection time.

In the myopic eye, the aperture stop is located in front of the retina. The incoming slit-shaped light bundle bounces off the retina and the reflecting slit moves in an opposite direction compared to the incoming slit. This causes the some photodetectors cells to be stimulate earlier that the others. By correlating the time differences with refractive errors, a refractive error at each position on the cornea (photo diode position) with respect to the corneal center can be derived. By simple rotation of the measurement system through 180 degrees, the distribution of refractive errors can be obtained through 360-degree meridians1-2. The OPD scan also measures corneal topography using a Placido ring method and which has been widely utilized for almost two decades.

Topography & Wavefront Analyzer

The OPD-Scan is a combined corneal topography device and wavefront analyzer. The purpose of laser keratorefractive surgery is to photo ablate the cornea to modify its refracting power, and thereby improve unaided vision. The early systems were effective at reducing primary defocus and astigmatism but often led to increased higher order aberrations. Customized ablation now offers the potential to correct not just the lower order errors but also the majority of significant higher-order aberrations to achieve better vision. Generally, the Treatment Area consists of an Optical Zone and a Transition Zone. The optical zone plays the largest role in optical performance and should be larger than the pupil diameter whereas the Transition Zone blends the OZ with the untreated cornea to avoid abrupt changes in shape that are thought to trigger unwanted healing effects. The Transition Zone is also thought to play an impor-

tant role in improving visual performance under Scotopic conditions as the pupil enlarges however, it is impossible to design the Transition Zone from Wavefront Analyzer information alone without thought to the cornea over relying areas outside the entrance pupil.

A major functional difference between Dynamic-Skiascopy and Hartmann-Shack type devices lies in the range of measurements capability:

Hartmann-Shack principle: Sphere: –15D to +7D, Cylinder: 0 to +6D

OPD Power Map

The data generated by the OPD is typically displayed in map form showing the distribution of wavefront errors of the entire eye (D). Figure 1a illustrates the OPD map of a -4.75 D spherical model eye. In this map, the refractive error at a desired position on the cornea within a 6mm diameter is displayed. Since the map is of a spherical model eye uncorrected for spherical aberration, the measured errors vary with each concentric zone; the outermost concentric zone being more negative (reddish) than the central zone. In this figure, the terms, Sphere, Cylinder, and Axis have their conventional meaning and are displayed in the middle left for the central zone as: S: -4.75, C: -0.00, A: 0.0. This is repeated for 3mm and 5mm annulus zones immediately below the central data and show increasingly negative defocus errors in the periphery. Figure. 1b illustrates the OPD map of a 3D cylindrical model eye. In this map of against the-rule astigmatism the steeper meridian (in the 0º to 180º direction) and flatter meridian (in the 90º to 270º direction) are observed as a colorcoded butterfly pattern. It is also clear from the map that even in this cylindrical model eye there is significant spherical aberration.

The OPD-ScanR also allows the mapping of various individual aberrations described in terms of Zernike coefficients from the wavefront data. The low-order components (up to 5) that can be corrected with spectacle lenses and some high-order components (6 and higher) can be displayed. Figure 2a illustrates an OPD map of an emmetropic human eye. This map is color-coded in three greenish colors for simple pattern recognition. Figure. 2b shows an eye with myopic astigmatism and Figure 2c shows a hyperopic astigmatism eye. The OPD-ScanR produces maps that are easily interpreted, using display

techniques familiar to corneal topography maps. Just as computer assisted corneal topography devices raised the standard of keratometry, so too do wavefront devices provide us with a wealth of information over and above sphere, cylinder and axis.

In addition, it is easily perceived whether the map has a butterfly pattern and illustrates symmetric or asymmetric astigmatism from the symmetry of the butterfly pattern. The OPD map provides us with more visual and multifactorial refractive information compared to a conventional auto refractometer that provides S, C and A data only.

OPD Power Map and Wavefront Map

To explain how the OPD-ScanR measures optical aberrations begins with the duality of light, and that light can be thought to propagate in waves. The wavefront is defined as a plane perpendicular to the direction in which light travels and deviations from the ideal wavefront are termed wavefront errors and these errors tell us about the optical properties of the system. To illustrate this point consider the following diagram, Figure 3, of a simplified schematic eye with light reflected back out of the eye from a single point P on the retina. For a perfectly emmetropic eye the exiting wavefront will be a plane

wave perpendicular to the Z-axis. However in this case the eye is myopic so the wavefront converges to a point P' where it intersects the reference axis. The difference in slope between the wavefront and the ideal wavefront allows the construction of the wavefront error map for multiple discrete points. The difference is often described using Zernike polynomials with each coefficient relating to a particular type of aberration such as Defocus, Astigmatism, Coma, Trefoil and so on4. Thus, one can tell the type and amount of aberrations that the eye produces. This information is useful for comparing preoperative and postoperative changes in ocular aberration quantitatively.

OPD Map and Cornea Topography

Map

This section describes one example in which a comparison between corneal topography and OPD map provides new information. Figure 4 illustrates the OPD map of total ocular refraction and the corneal refractive power map among corneal topographic maps for the same eye. The Corneal refractive map illustrates the distribution of corneal refractive powers calculated by Snell’s law showing corneal astigmatism of about 2D. The OPD map on the left reveals total refractive astigmatism of 0.5 D at most with the difference being most likely due to crystalline lens astigmatism perpendicular to and therefore canceling the corneal astigmatism of 2D. Quite clearly the full corneal astigmatism of 2D

should not be corrected whether it be by contact lenses or refractive surgery. To surgically correct refractive error demands an understanding of both the total ocular refractive status and the corneal surface ensuring a quick and correct diagnosis. The change in total refraction can be converted into an equivalent change in corneal surface shape and power. The Target Refractive map, method of calculating a shape of ablation, is obtained by adding the ocular refractive error map as measured by the OPD, to the corneal refractive power map. Either of them can be optically calculated from each other, but Axial and Instantaneous maps among topography maps cannot be calculated directly from the OPD map.

Relational expression: Refractive + OPD = Target Refractive

Aligning Topography Data with OPD Data

The abovementioned Target Refractive maps merge corneal information (Refractive) and total refractive information (OPD). It is important to avoid problems due to any change in eye alignment when moving from one device to another when collecting total ocular refraction data and corneal topography data. This potential problem is overcome with the OPD-scan by incorporating the two devices within the one instrument. The OPD-ScanR compensates for any positional shift between the Refractive map and OPD map by determining the location of the First Purkinje images its reference point.

Measurement of Pupillary Diameter

The OPD-ScanR can measure the diameter and center position for both Photopic and Scotopic pupils as shown in Figure 5. To prevent halos and glare, the ablated area should be outside the nighttime dilated pupil. It would be ideal to compare a pupil image exposed to the laser with Photopic and Scotopic pupil diameters and respective pupil centers, and to align the laser axis with Scotopic pupil center. To avoid halo and glare due to poorly sized optical zones, the ablated area should be larger than the pupil under Scotopic conditions.

Final Fit Software: Outline and

Features

The Final Fit Software is designed to calculate ablation profile through analysis of both topography and OPD data obtained by the OPD-ScanR and generation of exact laser pulse shot data that controls the laser delivery system such as the movable apertures with the Nidek EC5000CX II refractive laser. With the Final Fit Software, the ablation profile data is divided into three components, as shown in Figure 6, so that they correspond to the ablation mechanism of the EC-5000CXII. Radially symmetric components are ablated by scanning and rotating

the rectangular laser beam controlled by the opening and closing of a diaphragm, linearly symmetric components are ablated by the laser beam controlled by the opening and closing of a slit mask, and irregular components are divided into small spots of 1 mm in diameter and ablated respectively, Multi Point Ablation.

The Final Fit software also displays preoperative and postoperative topography maps at the upper part of the screen, allowing the operator to optimize the conditions for operation such as the amount of correction, Optical Zone (OZ) and Transition Zone (TZ) for each eye (Figure. 6).

Customized Ablation

With the Final Fit software, the following ablation modes are available:

Spherical with OATZ Ablation

The Instantaneous corneal topography map is ideal for grasping subtle changes in corneal shape because the map better represents local corneal shape. Figure 7 shows that the position of the red ring representing the area with abrupt change in curvature changes with each power profile. As the profile moves up from a to c, the red ring moves to the periphery of the cornea, increasing the OZ and increasing the depth of ablation.

Topo-guided Customized Ablation

Spherical with OATZ ablation allows for customized ablation surgery in the periphery of the cornea to reduce nighttime halo, glare and the possible resultant decrease in contrast sensitivity. With the customized ablation method, spherical lenses and cylindrical lenses are ablated in accordance with the amount of correction in the optical zone (OZ), and the ablated surface is smoothly connected with the non-ablated periphery of the cornea through the transition zone (TZ), This is to reduce nighttime halo, glare and a decrease in contrast sensitivity thought to be caused by abrupt changes in shape in the TZ5. The TZ shape can be determined by selecting the appropriate profile in Final Fit Software.

Topo-guided with OATZ ablation allows customized ablation for correcting pathological or post-operative corneal irregularities, such as decentered ablations. With the Topo-guided ablation method the larger irregularities are removed first. This ablation method allows the operator to ablate corneal irregularities with the minimum ablation depth. In these cases manifest refraction results can be unreliable but by removing much of the irregularities and then re-measuring and re-analyze the eye with the OPD-Scan superior results can be achieved.

Wavefront-Guided Customized Ablation

Unlike some other wavefront analyzers, which calculate ablation depth directly from the amount of aberration, the OPD-Scan calculates ablation depth from what is required to change corneal surface power. As mentioned earlier, the preoperative refractive map and OPD map are used to generate the Target Refractive map, as well as corneal elevation data and the difference between the data used to determine the shape of ablation.

Wavefront-guided ablation allows for the display of an aberration-free area (0D) and to optimize the transition zone. The wavefront-guided ablation method is intended to reduce the amount of aberrations in the entire treatment zone (OZ + TZ). Although the above explanation presumes a target refractive endpoint of emmetropia this can be programmed to achieve various levels of emmetropia as desired for example -1 D of myopia.

Features of the Final Fit Software

The features of the Final Fit are listed below:

-Preoperative and postoperative simulated Topography data allows an estimation of postoperative outcome. -Safety

-The three ablation modes, Normal (Spherical lens)/ Topo-guided/ Wavefront-guided ablation are selectable according to the eye to be operated on. -Quality of vision

-Vision quality can be enhanced by a smooth TZ with an OATZ (Optimized Aspherical Transition Zone). -Quality of vision

-The comparison function allows the operator to display preoperative, and postoperative topography map, the OPD map, and differential maps. - Evaluation of postoperative outcome

- The topography data of the entire cornea and pupil contour displayed allows the operator to easily check the positional relationship between the treatment area and pupil, and distribution of powers. In addition, the OPD map allows the operator to check the distribution of refractive errors within a pupil. (This is also available with the OPD-Scan.) Evaluation of postoperative outcome

Correction of Cyclo-Torsion

Little attention has been previously given to the possible shift of cylinder axis due to cyclotorsional rotation of the eye and many ophthalmic devices measure patients in the upright sitting position whereas during refractive surgery, the patient is in a recumbent position6. For this reason, when using the data obtained by the OPD-Scan in corneal refractive surgery, the eye rotation caused by this counter-rolling of the eyeball must be compensated. The OPD-ScanR analyzes iris pattern, pupil contour, pupil center and pupil size by capturing anterior eye images just after topography measurement. The pupil position information is then used for image registration and alignment as part of the Final Fit calculations. The Final Fit Software generates ablation profiles and sends this together with the iris image registration data to the excimer laser system.

The iris image of the patient's eye lying on a bed will be captured by CCD camera installed in EC-5000CXII laser system and then pupil center is calculated. Pupil center will also be determined against the iris image obtained by OPD Scan. Similar distinctive iris patterns will be detected from both iris images. The angle for some distinctive iris pattern will be measured from the baseline, which will go through each Pupil center and the angle difference between both images will be torsional angle error for as shown in Figure 8.