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

Both corneal topography and wavefront analysis provide useful information to the refractive surgeon. Unlike corneal topography, wavefront analysis provides us with information about the overall refractive status of the eye, including the cornea, lens and retina. It also demonstrates aberrations that occur with a dilated pupil.

The greatest benefit of wavefront technology is in custom ablation planning, but it is also useful in screening patients for refractive surgery. Wavefront analysis helps explain visual aberrations when they are not obvious on corneal topography. Patients with visual aberrations following refractive surgery may have unremarkable corneal topography maps, but their symptoms can usually be explained on wavefront analysis1. In such cases, wavefront analysis will commonly show elevated values for vertical coma and trefoil.

The Nidek OPD Scan is a scanning slit refractometer using skiascopic technology with simultaneous Placido disc corneal topography (Fig 1). It is the first diagnostic instrument to combine corneal topography, autorefraction, and wavefront analysis. It takes the autorefraction, keratometry, and wavefront measurements simultaneously in 0.4 seconds. The Nidek OPD Scan measures the largest range of spherocylinder refractive error in the industry. It measures between –20D and +22D of sphere and 12 D of cylinder. Following those measurements the technician performs the corneal topography (also in less than one second). Unlike Hartmann-Shack systems, the Nidek OPD-Scan uses

dynamic retinoscopy to measure aberrations through 1,440 data points (highest number of all wavefront machines)2. These points create refractive power maps with more data than on other systems. Unlike the grid on Hartmann-Shack wavefront systems, there is no mixing of data points with the scanning slit technology.

The purpose of this chapter is to highlight important features of the Nidek OPD Scan to help surgeons interpret the data more accurately. The following guide is helpful in examining data obtained with this instrument.

GUIDE TO CLINICAL INTERPRETATION WITH THE NIDEK OPD SCAN

NIDEK OPD SCAN SIX MAP DISPLAY

Refractive (Snell’s Law) Map

Look at the color and topography patterns, keratometry, and Sim K values, as you would normally do for corneal topography maps. The scale should be set so that green represents 44 diopters in the scale settings section of the software. The dioptric power steps should be set to either 0.5 diopter or 1 diopter steps.

IROC (Instantaneous Radius

of Curvature) Tangential Map

Look at peripheral color patterns to look for peripheral changes beyond the 4 mm zone. The scale should be set so that green represents 44 diopters in the scale settings section of the software. The dioptric power steps should be set to either 0.5 diopter or 1 diopter steps.

OPD (Optical Path Difference to Emmetropia) Map

Look at color patterns and overall dioptric power error scale for the 6mm optical zone. ARK (Autorefractor Keratometer) values are displayed at the 2.5 mm, 3 mm and 5 mm zone. The irregular component of S/C/A at the 3 mm and 5mm ring zone are quantified using RMS (Root Mean Square) values in diopters. RMS diopter values below 0.5 on the OPD map are regular or near normal sphere and

Section IV: Aberrations and Aberrometer Systems

cylinder patterns (radially linear and radially symmetric). The components of S/C/A become more irregular at values greater than 0.5 RMS (diopters). Look for irregular patterns associated with double vision (two or three power lobes similar to a three leaf clover pattern) or ghosting (significant variation in central power from nasal to temporal within 4mm zone). The scale should be set so that green equals "0" diopters in the scale settings section of the software. The dioptric power steps should be set to either 0.5 diopter or 1 diopter steps.

Total Order Aberration Map

This map displays how an emmetropic wavefront deviates through the entire optical system. Both lower and higher order aberrations are displayed in this map. The scale should be set so that green equals "0" microns in 0.5 µm or 1.0 µm steps in the scale settings section of the software.

Higher Order Aberration Map

Determine the Higher Order (HO) wavefront percent by dividing Higher Order RMS wavefront error by Total Order RMS wavefront error (HO WF RMS error/TO WF RMS error = % of HO aberrations). This map displays only the irregular S/C/A or Higher Order aberrations of the optical system. The scale should be set so that green equals "0" microns in 0.5 µm or 1.0 µm steps in the scale settings section of the software.

The percentage of HO Aberrations that is significant will vary for preoperative and postoperative patients. Preoperative HO/TO percent aberrations are abnormal after 20%, and postoperative HO/TO percent aberrations are suspect after 50%. However, this is a subjective observation that does not have clinical meaning until one reviews all of the available corneal topography, ARK, and OPD wavefront data.

Examine the coefficients 6 through 27 and look for values of 0.5 RMS µm or higher for clinical significance. As for Zernike, the irregular (higher order) components of S/C/A can be measured and described optically as coma, trefoil and pentafoil and irregular (higher order) sphere can be described and measured by spherical aberrations (12th and 24th coefficient). RMS µm values below 0.5 µm are considered low and not clinically significant. RMS Values above 0.5 µm generally correlate to subjective visual complaints. This would apply to all higher order Zernike coefficients starting from 6 to 27 as displayed on the Zernike Graph. The Zernike Graph coefficient values are in microns of light (not tissue).

RMS error is the best measure of dispersion around the best-fit sinusoid (Fig 2). It expresses a degree of irregularity or reliability of S/C/A values. Emmetropic patients will generally have very uniform topography maps, normal keratometry, low RMS values and low Zernike values. The magnitude of error increases for conditions such as asymmetrical astigmatism and keratoconus.

Examine the simulated keratometry and patterns on the topography maps, the ARK Data and RMS values and patterns on the OPD map, the ratio of HO/TO WF error from the wavefront maps and the HO Zernike RMS coefficients from 6 to 27th to determine:

1.If the visual errors are caused by irregular astigmatism or by irregular spherical error, and to what degree by looking at Zernike coefficients.

2.Compare the RMS OPD values to a known normal range (0.0 to 0.5 diopters) to identify irregular vs. regular profiles.

3.Evaluate whether HO/TO WF error % aberration value is clinically significant by examining the Zernike RMS coefficient values. A value of

0.0to 0.5 µm is not clinically significant. Values above 0.5 µm are clinically significant. This would apply to all higher order Zernike coefficients starting from 6 to 27 as displayed on the Zernike Graph.

CLINICAL EXAMPLES

Keratoconus

This 20 year-old male presented for refractive surgery. Manifest refraction was –10.00 -8.00 x 165 (20/40). The refractive map revealed elevated

keratometry readings with asymmetric astigmatism. OPD Map readings showed the irregular component of S/C/A at the three and five-millimeter zone using RMS values in diopters. RMS dioptric values measured greater than 0.50 D, which is associated with irregular astigmatism (Figure 4).

Pellucid Marginal Degeneration

Corneal topography demonstrates classic loop cylinder (Butterfly Image) associated with

Section IV: Aberrations and Aberrometer Systems

Pellucid Marginal Degeneration. Wavefront RMS values are elevated (2.72D @ 3mm and 3.10 @ 5mm) and consistent with irregular astigmatism (Figure 5).

Decentered LASIK Ablation

This 40 year-old patient was treated at another laser center and presented to the Feinerman Vision Center for correction of her LASIK complication. Her main complaint was decreased best-corrected vision and visual aberrations that worsened at night. BCVA was 20/60 in her right eye.

The corneal topography and OPD maps of the right eye demonstrate an inferotemporally decentered ablation. RMS diopter values far exceed 0.5 D, which correlate to the patient’s visual complaints.

Significant spherical and coma aberrations are present. Individual Zernike RMS values greater than 0.5 D in either direction correlate with the patient’s visual problems (Figure 6).

Refractive laser surgery techniques such as laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) are effective methods of treating spherical myopic errors up to 12D and hyperopic errors up to 6D, with good visual outcomes. However, generally more than half of the people who are suitable candidates for refractive surgery have enough astigmatism to warrant its inclusion in the surgical correction. As astigmatism has both direction and magnitude, its incorporation into the treatment makes planning more complex. It has been shown that vector analysis could likely improve the visual outcome of sphero-cylindrical treatments by combining the topographic and refractive astigmatic components to target a reduced level of corneal astigmatism compared to using refractive parameters alone. (1)(2)(5)

Measurement of Astigmatism

There are three differing categories of astigmatism; naturally occurring regular astigmatism, naturally occurring irregular astigmatism and secondary irregular astigmatism associated with ocular trauma, disease, infection or previous ocular surgery.(1) There are many different ways to measure astigma-

tism, some assessing corneal astigmatism only, and the others measuring refractive astigmatism including the internal optics of the eye. It is important in routine clinical practice to utilize more than one method in the pre-operative examination.

The manifest subjective refraction is a measure of the sphero-cylindrical correction required for the patient’s perception of their best vision. The principal contribution to the cylindrical error is the corneal astigmatism, but also includes astigmatism from the internal optics of the eye (such as the crystalline lens) as well as the interpretation of the image by the cerebral cortex. The measured result depends on many variables such as chart illumination and contrast, test distance and room lighting. The newer technology of wavefront analysis provides a spatially oriented refractive map of the pathway of light through the eye, which provides a greater amount of information on the refractive system than the manifest refraction data alone. It too includes the internal optics of the eye, but unlike subjective responses does not include the conscious percept of the cerebral cortex, thus giving no information regarding the nonoptical interpretation of astigmatism images on the retina and visual cortex. This subjective value conventionally forms part of the ablative treatment and is an important component for patient satisfaction.

The application of wavefront analysis in the treatment plan is discussed further on.

Keratometry is a useful objective test to measure average corneal curvature at the paracentral region of the cornea. However, as it requires the manual alignment of optical mires to identify the steepest and flattest corneal meridian, there is a potential problem with reproducing reliable results due to variability between different observers. Corneal topography, or computer assisted videokeratography (CAVK), provides a more detailed quantified view of the corneal astigmatism displayed as a map based on the measurement of refractive power of thousands of separate points over the entire cornea. Average topographical astigmatism can be represented by a simulated keratometry value, which is a mean value derived from a number of constant reference points. It is a best fit compromise, and determined in various ways by the different types of topographers.

Surgical Planning ~ Refraction, Topography, or Both?

In an ideal world the goal of astigmatic refractive surgery is to completely eliminate astigmatism from the eye and its optical correction. However, it has since been recognized that this is not possible in the majority of cases due to the inherent differences between corneal astigmatism (represented by the simulated keratometry value from topography) and refractive astigmatism (represented by lower [second] order aberrations from aberrometry) correcting the eye.(4)(5) Most surgeons traditionally treat the refractive value alone based on the principle that treating what the patient perceives to give their best corrected vision will provide a superior visual outcome.

However, this is not necessarily the case. Disregarding the shape of the cornea while changing it flies in the face of the fundamental principles of corneal surgery. In fact, simple arithmetic analysis shows that an excessive amount of corneal astig-

Section IV: Aberrations and Aberrometer Systems

matism may be left if treatment is applied exclusively based on the parameters derived from the refractive cylinder magnitude and axis.(2)(3) This occurs because failing to align the maximum ablation closer to the flattest corneal meridia results in off-axis loss of effect when reducing corneal astigmatism. Consequently, lower (second) order astigmatic aberrations and (third order) coma would not be minimized, with more remaining than otherwise necessary.(1) This may result in post surgical symptoms such as reduced visual acuity and contrast sensitivity, creating difficulty with night driving and thus actually diminishing satisfaction in a proportion of patients.

As it becomes more widely recognized that a zero overall astigmatism is mostly unattainable, effective contemporary methods target astigmatism outcomes that combine both the refractive and topographic measurements in the analytical planning process. This should ensure the distribution of the remaining astigmatism to achieve the optimal outcome. That is, choosing a maximal treatment that leaves the minimum amount of astigmatism and in the most favorable orientation. With-the-rule astigmatism is more prevalent in the younger population undergoing laser vision correction, and is thought to be more visually tolerable to refractive perception than against-the-rule astigmatism (Javals rule). (3)(5)(8)

Vector Analysis by the Alpins Method

The surgical planning and analysis process is expedited by the implementation of computer and software technology. Calculations performed for the publication of this chapter utilized the ASSORT program developed by the first author (the Alpins Statistical System for Ophthalmic Refractive surgery Techniques). It employs the principles of vector planning and analysis(1)(2)(3)(4)(5)(6) and can utilize a paradigm that favors with-the-rule astigmatism that minimizes measurable postoperative refractive astigmatism quantified as second order aberrations.