In Fourier-domain OCT corneal mapping, corneal surface elevation profiles are very sensitive to eye movement. According to the Munnerlyn formula,7
for example, a 10 μm axial sag error within the central 3.0 mm diameter could produce an anterior corneal power calculation error of approximately 3.0 D. This was the primary reason for the poor repeatability of corneal power measurements by the lower-speed time-domain OCT system.4
A previous study by our group4
used a time-domain OCT system with a scanning speed of 2000 axial scans per second; the repeatability of direct net corneal power measurements was 0.71 D in normal eyes. By combining time-domain OCT pachymetry and Placido-ring topography, we were able to obtain composite power measurements with an acceptable repeatability of 0.24 D for net corneal power. However, it was awkward clinically to combine information from 2 unconnected machines. Therefore, we hoped that the development of significantly faster Fourier-domain OCT technology would facilitate direct, precise corneal power measurements.
The Fourier-domain OCT system used in this study has a scanning speed of 26
000 axial scans per second, 13 times faster than the speed of the time-domain OCT system used in our previous study.4
With less motion errors, the Fourier-domain OCT system directly measured net corneal power almost 4 times more precisely than time-domain OCT in normal eyes. In fact, direct measurement by Fourier-domain OCT was more precise than the combination of time-domain OCT and Placido-ring topography in normal eyes.
We compared the accuracy of Fourier-domain OCT measurement and standard keratometry measurement of anterior corneal shape. The anterior corneal power measurements by the 2 methods agreed well on the average, which was reassuring. There was still a considerable range of differences between the 2 methods for individual eyes. The differences were not due to error in the computerized detection of anterior and posterior corneal boundaries because our visual inspection showed the software to be accurate. The individual difference might be due to some basic differences between the 2 methods. Standard keratometry measures the slope of the cornea over an annular area (or a portion of) centered on the vertex, while Fourier-domain OCT measures the curvature of the cornea over a circular area centered on the pupil. These 2 methods should yield the same results if the cornea is perfectly spherical; however, any deviation from sphericity could produce measurement differences. Further investigations are needed to elucidate the source and implications of these differences.
In normal eyes, the repeatability of Fourier-domain OCT net corneal power measurements was slightly worse than manual keratometry measurements but was comparable with measurements by autokeratometry and simulated keratometry. The repeatability of corneal power of a Bausch & Lomb manual keratometer was reported to be approximately 0.08 D.8,9
For automated keratometry (IOLMaster), the repeatability was approximately 0.14 D.9,10
The simulated K values of a Scheimpflug imaging system (Pentacam, Oculus Optikgerate GmbH) are reported to have a repeatability of 0.10 D11
to 0.14 D.12
We did not find previous literature on the repeatability of standard keratometry and simulated keratometry in post-LASIK or keratoconus eyes. The Fourier-domain OCT net power measurement variability was negligibly small compared with the difference between Fourier-domain OCT and standard keratometry.
The net corneal power from Fourier-domain OCT was, on average, 1.21 D lower than that of standard keratometry in the normal group. This discrepancy could be attributed to 2 factors, both of which are related to the keratometric index of 1.3375. First, the standard keratometry power is defined for the back vertex of the cornea1
and the Fourier-domain OCT net corneal power for the second principal plane of the cornea, which is slightly anterior to the front vertex. That makes the Fourier-domain OCT net power roughly 0.8 D lower than the standard keratometry power. Second, the keratometric index is based on Gullstrand 1 schematic eye, which assumes an anterior corneal radius of 7.7 mm and a posterior radius of 6.8 mm. Thus, the anterior–posterior corneal curvature ratio is 6.8/7.7, or 0.883. The Fourier-domain OCT data in our present study found the average ratio in normal eyes to be significantly lower, accounting for the remaining difference. Because standard IOL power formulas (eg, Hoffer Q, Holladay II, SRK/T) are calibrated for standard keratometry, the offset of 1.21 D should be added to the Fourier-domain OCT net corneal power before plugging it into standard IOL power formulas.
In post-LASIK and keratoconus eyes, the difference between Fourier-domain OCT net power and standard keratometry power was even greater than in normal eyes. Two factors contributed to this. First, the anterior corneal power measured by Fourier-domain OCT was lower than that measured by standard keratometry. This was mostly likely due to the higher weight of the central region in our Fourier-domain OCT corneal power algorithm. The LASIK patients in this study all had myopic correction, which resulted in an oblate cornea that was flatter in the center than in the periphery.7
Keratoconic corneas tend to have the greatest curvature inferotemporally rather than at the very center.13,14
Second, both myopic LASIK and keratoconus lower the anterior–posterior curvature ratio because of central thinning. Thus, these corneas deviate even further from the ratio assumed by the keratometric index. Standard keratometry overestimates corneal power after myopic LASIK, which, in turn results in hyperopic surprises after cataract surgery.15–17
Our results confirm that this error in standard keratometry is mostly the result of the altered anterior–posterior corneal curvature ratio.1
Because Fourier-domain OCT can directly measure both anterior and posterior corneal curvatures, it may be a more valid instrument for use in post-LASIK and keratoconic eyes, which have lower than normal anterior–posterior curvature ratios.
The finding that keratoconic eyes had lower anterior–posterior curvature ratios may have diagnostic implications. At present, we use computerized corneal topography to detect the increased and distorted anterior corneal surface in keratoconus. Our finding suggests that the alteration in the posterior curvature is even greater. Thus, Fourier-domain OCT measurement of posterior corneal curvature and topography may enhance detection of forme fruste keratoconus.
There are 2 possible landmarks for centering the OCT corneal mapping scan: the pupil and the corneal vertex. We chose the pupil because its position could not be altered by surgery (eg, LASIK, photorefractive keratectomy, phototherapeutic keratectomy), by disease (eg, keratoconus), or by corneal scarring. Thus, Fourier-domain OCT scans can be taken at different times to be registered correctly. Pupil centration is also used in laser vision correction, another application for Fourier-domain OCT corneal measurements. We had a theoretical concern that pupil position might not be as repeatably established as the corneal vertex. However, our results show that pupil centration can provide repeatable corneal power measurements. Therefore, we recommend using pupil centration for all Fourier-domain OCT corneal mapping scans unless the pupil is pathologically eccentric.
One limitation of our method is that the OCT net corneal power was measured over a fixed analytic zone of 3.0 mm in diameter. It was a reasonable choice because the size is comparable to that of standard keratometry and simulated keratometry. In addition, we found that the OCT net corneal power within the 3.0 mm area accurately tracked post-LASIK refractive changes.4
It is also reported to provide accurate corneal power input for IOL power calculations for cataract patients without previous refractive surgery.18
The optimal analytic zone for calculating true corneal power, however, may vary by pupil size as well as by higher-order aberrations. In the future, OCT corneal power measurement might be further improved by ray-tracing calculations that take pupil size and higher-order corneal aberrations into account.
We used the parabolic fitting approach in the current study because the parabolic surface has a simple correspondence to focusing power. Therefore, parabolic fitting is a simple and robust way to extract the average focusing power of the central cornea. Parabolic fitting, however, smoothes the detailed topographic information. Therefore, a different approach is needed to provide a corneal topography map from the OCT images. A topography software will require motion compensation, interpolation, and surface reconstruction algorithms. We are developing topography software and will report the results separately.
Optical coherence tomography is not the only instrument that can directly measure the curvatures of both the anterior and posterior corneal surfaces. Slit-scanning instruments such as the Orbscan II, Pentacam, and the Galilei dual Scheimpflug camera (Ziemer Group) also have that capability. However, the Fourier-domain OCT system is faster with higher resolution. At a reasonable working distance, the 5 μm resolution of the Fourier-domain OCT device is much higher than that possible with slit scanning. Thus, the = Fourier-domain OCT system gives more accurate corneal thickness measurements than the Orbscan II in the presence of corneal haze or opacity.19–22
Theoretically, the higher resolution may also translate to higher accuracy in corneal power measurements. The current Fourier-domain OCT system mapped the cornea in 0.32 second, compared with 1.0 to 2.0 seconds with the slit-scanning devices. Comparative studies are needed to evaluate whether these technical advantages translate to better accuracy in corneal power measurement in the clinical setting.
It would be interesting to know whether the precision of Fourier-domain OCT corneal power measurement is limited by its speed or spatial resolution. The relationship between the repeatability of net corneal power compared with anterior power offers a clue. If speed were limiting the precision, the predominant cause of curvature variability would corneal movement during the scan. If centration were limiting the precision, the predominant cause of curvature variability would be the different centration between scans. In either case, because the anterior cornea and posterior cornea move together, the resulting error would have opposite signs that would partially offset each other because the anterior corneal boundary has positive power and the posterior cornea has negative power. Therefore the net corneal power should have less variability than the anterior corneal power. On the other hand, if spatial resolution were limiting precision, the anterior and posterior curvature measurement errors should be dominated by segmentation errors in the anterior and posterior corneal boundaries. These 2 variances would be independent and additive. In this case, the variance in the net corneal power would be greater than both the variance in the anterior–posterior corneal power. Our intravisit repeatability results suggest that speed or centration, not resolution, limited the precision of the Fourier-domain OCT system we used. Faster Fourier-domain OCT technology has been evaluated,23,24
and it might improve the precision of corneal measurements in the future.
000 axial scans per second and 5 μm, Fourier-domain OCT is fast enough to obtain direct corneal power measurement with acceptable repeatability. Because the technology does not rely on an assumed fixed geometric relationship between the anterior surface and the posterior surface, it may be a more robust method for measuring corneal power that is accurate in surgically modified (post-LASIK) eyes and pathologically distorted (keratoconus) eyes. It may have application in IOL power calculation and refractive surgical planning. Further clinical studies are needed to validate each application.