Corneal and Anterior Segment Optical Coherence Tomography - Optovue
The objective of the study is to further develop clinical applications for the high speed corneal and anterior segment optical coherence tomography (CAS-OCT) system developed by Optovue, Inc. (Fremont, CA, USC). The systems are currently designated ACVue and RTVue with corneal lens adapted for the anterior segment.The specific aim is to develop OCT for intraocular lens (IOL) implant power calculation.
OCT is a non-contact imaging technology that provides detailed cross-sectional images (tomography) of internal structures in biological tissues. Its principle is similar to that of radar or ultrasound imaging, in which the round-trip delay time of the reflected wave is used to probe the target structure in depth. An image is achieved by scanning the wave laterally and combining a series of axial (depth) scans. In OCT, a beam of infrared light is used, instead of radio or ultrasound wave. The beam is directed at a sample and the delay of reflected light is measured. Because light travels extremely fast (300,000 kilometer/sec), it is impossible to directly measure the travel time of light over a distance of few micrometers. To overcome this limitation, OCT measures the delay indirectly, by comparing the sample reflection with a reference reflection in an interferometer. The axial resolution of OCT is determined by the coherence length of the light, hence the name "optical coherence tomography." The resolution of OCT is very high, ranging from 2 to 20 μm full-width-half-maximum (FWHM), making it ideal for imaging and measuring small eye structures. Though very-high frequency ultrasound and confocal microscopy could achieve similar resolution, OCT is more practical in many situations because the imaging is non-contact, wide-field, and can be done at a great working distance.
The RTVue OCT is a very high speed FD-OCT system designed for retinal imaging. It was approved by the Food and Drug Administration (FDA) in 2006. The RTVue OCT uses light at the 830nm wavelength and scans at 26,000 axial scans (A-scans) per second. The very high speed allows accurate measurement of in vivo tissue dimensions without significant motion error. The RTVue OCT has axial resolution of 5 µm (in tissue). An adaptor lens is is mounted on the front objective lens to adapt the RTVue for corneal and anterior segment imaging.
An anterior segment OCT system called ACVue (Optovue, experimental system) will also be used in this study. ACVue is based on time-domain OCT technology and operates at 2000 A-scans per second. Although it has slow speed and lower resolution, ACVue is capable of wider and deeper scan ranges. It will be used in parts of the study that require deeper or wider scans than the RTVue can provide.
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.
Control: Historical Control, Endpoint Classification: Efficacy Study, Intervention Model: Single Group Assignment, Masking: Open Label, Primary Purpose: Treatment
USC Doheny Eye Institue
University of Southern California
Results (where available)
- Source: http://clinicaltrials.gov/show/NCT00532051
- Information obtained from ClinicalTrials.gov on July 15, 2010
Medical and Biotech [MESH] Definitions
Partial or complete opacity on or in the lens or capsule of one or both eyes, impairing vision or causing blindness. The many kinds of cataract are classified by their morphology (size, shape, location) or etiology (cause and time of occurrence). (Dorland, 27th ed)
Reproducibility Of Results
The statistical reproducibility of measurements (often in a clinical context), including the testing of instrumentation or techniques to obtain reproducible results. The concept includes reproducibility of physiological measurements, which may be used to develop rules to assess probability or prognosis, or response to a stimulus; reproducibility of occurrence of a condition; and reproducibility of experimental results.
The removal of a cataractous CRYSTALLINE LENS from the eye.
Presence of an intraocular lens after cataract extraction.
Absence of the crystalline lens resulting from cataract extraction.
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