Aberrometric System for Surgery of Eyes
Aberrometric System of New Generation Based on Adaptive Optics for Diagnostics of Human Eye Aberrations for Carrying out Operations in Refractive Surgery
Tech Area / Field
- PHY-OPL/Optics and Lasers/Physics
8 Project completed
Senior Project Manager
Malakhov Yu I
VNIIEF, Russia, N. Novgorod reg., Sarov
- Moscow State University / International Laser Center, Russia, Moscow
- University of Göteborg / Institute of Neuroscience and Physiology, Sweden, Molndal\nNational University of Ireland, Ireland, Galway\nCity University, UK, London
Project summaryRefractive surgery is a young and rapidly growing field. In 1998, 400,000 Americans underwent refractive surgical procedures. This represents an increase over 1997 of approximately 100%. In 2003 in USA there have been performed over 1.5 million operations and over 40 million people in USA need this operation. Currently, more then 3,000 ophthalmologists are being trained to perform these operations. The number is increasing at a rate of 500 to 1000 surgeons per year. It is being predicted, that the number of trained specialists in refractive surgery will approach 50% of the specialists in ophthalmology.
Simplicity in carrying out operation, high satisfaction of results, the minimum of complications and surgical comfort - these are the reasons making now LASIK (abbreviation LAser in SIutu Keratomileusis) the most popular among the other ways of corneal refractive surgery. During this operation the microceratome or laser radiation is used to create a hinged corneal flap, then the laser ablates (evaporates) internal corneal tissue and then the flap is repositioned back. So the cornea is shaped to compensate for eye aberrations. Advantages of LASIK in comparison with other ways of vision correction are the smaller pain syndrome and faster period of rehabilitation. Already next day patients feel themselves comfortably. However, LASIK has several aftereffects. For example, postoperative vision worsens during twilight and night, halo and glare appearing. These symptoms appear mainly in the darkness, when an eye pupil widens and peripheral regions of the eye start to play a role in image formation. These aftereffects are due to high-order eye aberrations that have not been compensated, or have been induced during a surgery operation. That is why, as ophthalmologists confirm, it is necessary to measure both low and high order aberrations of the patient's eye before an operation to calculate LASIK ablation pattern (cornea region to be evaporated by laser radiation) correctly. Unfortunately in the world practice, ablation pattern is calculated taking into account mainly low order aberrations (defocus and astigmatism) only.
It is known, that there is a natural balance between aberrations of a cornea (which, as a rule, contribute most of all) and aberrations of a crystalline lens and vitreous. The last ones partially compensate for corneal aberrations. Reshaping the cornea during LASIK destroys this natural balance, resulting in rise of unpredictable high order aberrations sometimes. The problem is caused first of all by measuring total aberrations of human eye but not the aberrations of each element. That is why preoperative procedure should include measurement of each eye layer aberrations and further calculation of ablation pattern taking into account both measured aberrations of eye layers and diffraction effects caused by presence of different layers of an eye with different forms of their surfaces.
This project is intended to carry out the investigations resulted in the development of a technique for measuring the total eye aberrations and aberrations of separate eye layers (we call the last “layer-by-layer technique”). Testing is planned to be carried out first on the model of artificial eye and then on a real eye. All real eye measurements will correspond to international eye safety standards because device prototype is equipped with extremely powerless light source in the safe bandwidth. The model of artificial eye will include bimorph flexible mirror to reproduce spatial-temporal aberration statistics both of normal human eye and eye with aberrations.
The measurements of aberrations are planned to be based on double-pass measurements scheme where the beam passes through the same aberration media twice. That is why in the beginning we plan to investigate validity of such aberration measurement approach. For that purpose we will numerically evaluate the dependence of retinal scattered light spatial coherency on the eye pupil diameter and on value of eye aberrations. While measuring eye aberrations we will benefit our knowledge of manufacturing, calculating and creating software of the Shack-Hartmann wavefront sensors that measure low and high order aberrations of the laser beams. Shack-Hartmann principle of measuring aberrations is the following: the incident beam is split into subapertures with the help of microlens array and then the focal points displacements give possibility to reconstruct incident wavefront. Moreover, we intend to combine in the project the wavefront sensor with bimorph piezoelectric mirror. Our group has an unbeatable experience in modeling and manufacturing such mirrors. This mirror is planned to correct for eye aberrations in real time adaptive closed loop system. Thus the optical system with the bimorph mirror will give the patient possibility to observe the distant object as though he had perfect vision without any eye aberrations (except a negligible error of bimorph mirror wavefront reproducing). This will give patient possibility to see, whether he really needs to perform refractive surgery. To obtain the good quality correction of aberration by adaptive mirror it is planned first of all to estimate isoplanatic patch size for each particular human eye. Isoplanatic patch is the object area where difference between the phase of the light beam that emerges from different points of object is negligible. Therefore such phase distortions can be efficiently compensated for by means of just one corrector. We plan experimental investigations on compensating eye aberrations within isoplanatic patch and on the possibility of enlarging isoplanatic patch size to improve human retina resolution. Since we consider the eye that consists of several layers with each layer contributing to aberration pattern (for example cornea, crystalline lens and vitreous), the isoplanatic calculations should take lamellar eye structure into account.
There are well known papers (Pablo Artal et. al.) where birefringent properties of human eye have been shown. That is why we plan to investigate and each time to check the existence of birefringence of human eye and especially its response on trial linear polarized faint beam used for human eye aberrations measurements. In the case of human eye aberrations value dependence on the polarization status of light source the corresponding error should be included into calculation of ablation package. Probably considerations of polarization properties of human eye may result in increasing accuracy of ablation package calculation and therefore in avoidance of postoperative aftereffects. We plan not only to develop “layer-by-layer” technique and to perform measurements, but also to create software that most efficiently creates aberration-diffraction model of human eye and calculates for a surgeon the most precise ablation pattern minimizing residual eye aberrations.
Thus project investigations should result in:
- Development of theoretical and experimental technique of measuring aberrations of each element of an eye separately (“layer-by-layer” technique)
- Development of polarization measurement technique of human eye.
- Development of “live artificial eye model” based on flexible mirror for the calibration purposes.
- Performing calculations of isoplanatic patch of a human eye including the consideration of the lamellar structure of the eye, investigations of possible increasing isoplanatic patch size.
- Developing numerical aberration-diffraction eye model including results of layer-by-layer and polarization measurements.
- Creating the software minimizing eye aberrations that takes into account diagnostic layer-by-layer and polarization measurements and as a result swiftly and efficiently calculating ablation pattern for refractive surgery.
- Manufacturing of the prototype of the adaptive system based aberrometer and its calibration with the help of the created artificial eye.
This project will be based on the knowledge and experience of the scientists from RFNC-VNIIEF (Sarov) previously involved in development of nuclear weapons, Moscow Lomonosov State University (Moscow) and Institute of Physics of Semiconductors of Siberian Branch of Russian Academy of Sciences (Novosibirsk). The scientists, to be involved in the project have huge experience in the investigations of wavefront control and adaptive optical systems creation, laser beam propagation in the presence of linear and nonlinear light and matter interaction. Adaptive optical systems have been developed by the scientists are widely used in Max Plank Institute and Max Born Institute (Germany), École Polytechnic (France) and other laboratories all over the world. In the frame of the project the wide experience of the involved specialists will be transferred for solving the urgent practical problems of ophthalmology and creating the new generation of diagnostic devices for people who need to improve their vision.
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