Videokeratograph (VKS) for monitoring corneal curvature during surgery

Abstract

Arq Bras Oftalmol 2002;65:37-41 Purpose: We have developed an instrument for computerized surgical videokeratography. A corneal central region of approximately 7.00 mm in diameter may be analyzed, providing the surgeon with information of the power and the astigmatism. Methods: The system is based on a fiber optic Placido disc projecting cone, which is attached to the objective lens of a Zeiss compatible surgical microscope. At the beam splitter we installed a monochromatic high resolution camera. A frame grabber is installed on a PC and images are digitized at a 480x640 resolution. Image processing is used for edge detection of rings. Results: Calibrating curves based on 4 spherical surfaces were generated and approximately 3600 points are calculated for each examination. Preliminary measurements on 10 healthy corneas were compared with those of an EyeSys System 2000 Corneal Topographer. Mean deviation was 0.05 for radius of curvature, 0.24 D for the power and 5.0 degrees for the cylinder. Conclusions: This surgical VKS, with some hardware and software improvements, may be used to reduce residual astigmatisms in conventional cataract and keratoplasty. It could also be used to gather preoperative data in corneal topography assisted LASIK.Videoceratógrafo para monitoramento da curvatura da córnea durante a cirurgia * Mantém vínculo com a EyeTec Equipamentos Oftálmi-cos Ltda.(www.eyetec.com.br), empresa que fabrica a versão comercial do instrumento apresentado.

Open-access operating algorithms for commercial videokeratographer and improvement of corneal sampling

“…Thus, from the labeled rings, a point-to-point curvature map of the cornea will be calculated. Some authors describe this method as the look-up table or calibrated-spheres algorithm [8][9][10]. In [6], the authors used eighteen steel spheres of known curvature radius with an accuracy of 10 µm to obtain a set of calibration curves that describe each of the reflected rings.…”

“…Therefore, results are not always reproducible by the user. Some works in literature [6][7][8] try to improve videokeratography algorithms avoiding proprietary limitations. In [6], the authors detail the algorithms and procedures to re-calibrate a topographer and obtain the raw data of corneal curvature.…”

Resultados preliminares de um sistema de topografia de grande-ângulo usando videoceratógrafo com discos de Plácido

“…Para tal minimizamos o termo: (17) onde o termo à esquerda dentro do colchete representa a elevação da córnea obtida através do videoceratógrafo e o termo à direita representa a somatória de cada Polinômio de Zernike (Z i ) multiplicado por seu respectivo coeficiente (C i ). Como pode ser visto, estamos utilizando os 36 primeiros termos de Zernike, embora estes polinômios possam ser gerados até a ordem que se desejar (38) .…”

“…O surgimento de técnicas para correção de aberrações oculares, como a ceratotomia radial (1)(2) em meados dos anos 80 e o surgimento de lasers para cirurgia refrativa no início dos anos 90 (3)(4) , motivou o desenvolvimento de técnicas mais precisas para a topografia de córnea (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) , sem falar em instrumentos para ceratometria que já haviam sido desenvolvidos muitos anos antes das cirurgias refrativas, como aquele desenvolvido por Bicas (18) em 1967. Devido a estas necessidades e, ao mesmo tempo, o barateamento dos micro-computadores com processadores mais potentes, surgiram equipamentos sofisticados que automatizaram todo o processo de mapeamento da córnea.…”

Custom designed dynamic videokeratometer

“…The camera can work as a static system, with one single capture, or in continuous video capture mode, which allows dynamic measurements of the 95 corneal topography. The method of calibrating a videokeratometer using steel balls has been already implemented in [16], but the authors used a projection head with fewer Placido rings and less calibration balls which limited the obtained accuracy. Furthermore, the 100 authors presented the methodology, but no quantitative results were given.…”