Intraocular Lens Power Calculation After Laser Refractive Surgery Based on Optical Coherence Tomography

This study is about an imaging method called Optical Coherence Tomography (OCT) which provides detailed cross-sectional (layered) views of structures in the eye. The OCT system scans a beam of light across the eye to take a picture. OCT provides a more detailed image than other imaging methods of the eye such as ultrasound, CT scan (computed tomography) and MRI (magnetic resonance imaging). In addition, OCT imaging does not touch the eye. OCT is routinely used in imaging structures in the back of the eye (retina) and cornea. This study uses high-speed FDA approved OCT systems. The proposed research plan is a combination of clinical studies and software development to be performed synergistically. Clinical studies will provide OCT images for image processing software development and testing. The image processing software will provide automated measurement of anatomic parameters essential for clinical use.

Cataract extraction and IOL implantation is the most common eye surgery. The power of the IOL implant is calculated from 2 measurements: the axial eye length (AL) and keratometric power (K). The Holladay II formula also uses the external corneal diameter ("white-to-white" or WTW) and anterior chamber depth (ACD). These formulae work well (±0.5D) in normal eyes. However, these formulae can leads to biased and unpredictable refractive results in eyes that had refractive surgery procedures such as LASIK, PRK, and RK. With a large number of patients undergoing refractive surgery every year, the problem is becoming more severe.

The conventional IOL formulae fail because several inherent assumptions are no longer true in the eye that had refractive surgery. These assumptions are: 1) The corneal refractive power is uniform. 2) The anterior and posterior corneal power has a fixed relationship such that the overall corneal refractive power can be calculated from the anterior keratometry (or topography) using the keratometric index. 3) The position of IOL can be predicted by K with or without additional information such as WTW and ACD.

Relative to the posterior curvature, the anterior curvature becomes flatter after myopic correction and steeper after hyperopic correction with LASIK or PRK. To adapt the conventional IOL formulae to this situation, most surgeons use rigid contact lens over-refraction to calculate an "effective K." However, the accuracy of refraction in cataract patients is poor due to poor vision. Alternatively, one could use a historical method to calculate the effective K from pre-refractive surgery values. However, those measurements are often no longer available. If many years have lapsed, the historical value may no longer accurately reflect the current shape of the cornea.

The axial position of the IOL is determined by the positions of lens zonules and capsule which is in turn related to the corneal curvature (K) in the normal eye. A flatter cornea (lower K) is usually associated with a larger anterior segment, where the lens apparatus is located further back. A more complex model that also uses a separately measured white-to-white corneal diameter may be even more accurate. In post-refractive surgery eyes, however, K is altered and no longer has the normal relationship with the size of the eye. One way to get around this is to enter the pre-refractive surgery K. However, this historical information is not always available. We believe that a better solution would be use an entirely different approach that does not depend on the 3 above assumptions at all. Since OCT can separately measure the corneal anterior and posterior surfaces and AC and lens dimensions, we believe it has the potential of being the basis of a much better IOL calculation formula.

Previously we developed a method to measure both anterior and posterior corneal surface curvatures and obtain more accurate corneal power measurements than conventional keratometry, which only measures the anterior surface. This was the basis of an OCT-based intraocular lens (IOL) formula that showed better results than other formulas for post-myopic LASIK cataract surgery. However, this formula was only on par with the best regression-based formulas in eyes with previous hyperopic LASIK or radial keratectomy (RK), because IOL position prediction error and higher-order corneal aberration limited the accuracy of optical calculations in in these cases. We propose to improve IOL position prediction by using the long-range OCT that can image the entire anterior eye and accurately measure the lens equatorial position. We propose to more accurately determine corneal power in the presence of high aberration by developing ray tracing techniques based on ultrahigh-speed OCT mapping of corneal surfaces. Together, these new methods should improve IOL selection and refractive outcome for all types of eyes. In addition, we will develop a new OCT-based toric IOL formula that takes into account posterior corneal astigmatism, which is ignored in conventional keratometry.