Refractive Surgery at the Virtual Eye
Within the framework Medicine Technology and in cooperation with the company Dr. Bal-deweg GmbH, Dresden we develope a system for refractive surgeries at a Virtual Eye.
1.
Objectives and Principles
The fundamental principle of the Virtual Eye is the 3D-representation of individual human eye data inside a modern computer environment as a World Wide Web service for the ophthalmology. That means the presentation of 3D-images of the eye in autostereoscopic sys-tems using cylindrical lenses and the function 3D-interaction. Additionaly the simulation of the biomechanical and the optical behavior of the human eye is essential.
The benefits for the ophthalmology are the following services: • the training of surgical operations ( e.g. for students),
• the preparation of surgical operations for individual human eyes and
• the test of new surgical operation methods (particularly for special ametropies). An abstract idea of the Virtual Eye shows Figure 1.
Figure 1: The main functions of the Virtual Eye
The in-vivo interface produces the geometrical data of the human eye (e.g. from a patient of a hospital) as input for the 3D-image function of the Virtual Eye (on line). The ex-vivo inter-face produces material data of preparated eyes as input for the simulation part biomechanics of the Virtual Eye (off line).
The 3D-image/interaction functions provides 3D-geometry data for the simulation part bio-mechanics which simulates the biomechanical behavior of the eye model in dependance of the material data, the virtual surgery via 3D-interaction and further in-vivo dates (e.g. intraocular pressure).
At last, the simulation part optics simulates the changed optical properties of the Virtual Eye, depending of the new geometry (result of the biomechanical behavior).
In this cycle, an arbitrary number of virtual surgical operations will be possible.
2.
Simulation
The goal of the simulation is to find an improved vision of the respective human eye in de-pendance of geometrical changes (e. g. variation of the cornea).
It is not possible and also not necessary to build a simulation model which contents all details of a human eye. Rather it is sufficient to build first a simplified model, which contents only the significant features of the human eye.
Such a model is the so called Gullstrand Eye which is shown in Figure 2.
The relation between the optical and the biomechanical behavior of the human eye is evident. Therefore both parts must be simulated. The biomechanical simulator produces a new geo-metry after a variation (via 3D-interaction as a virtual surgery) of the Gullstrand Eye. After this procedure the optic simulator produces, based on the new geometry data, the new vision of the Gullstrand Eye.
2.1
Biomechanics
For the biomechanics modeling and the simulation of the Gullstrand Eye the Finite Element Method (FEM) will be used.
The first description of the FEM model is a coherent volume model. The 3D-geometry data input comes from the 3D-image function inside the Virtual Eye. A simplified 3D-volume mo-del is shown in Figure 3.
Figure 3: A simplified 3D-volume model of the Gullstrand Eye
Additional to the volume model different net topologies has to be considered (structured, unstructured and mixed forms) as a very important precondition for the simulation (s. Figure 4).
Figure 4: The 3D-volume model with nets
Finaly this model can be simulated based on different material parameters and further variabel physical parameters (from the ex-vivo/in-vivo-interface).
Starting from this fundamental structure of the Gullstrand Eye extensions of the biomechani-cal model are necessary, e.g. refinements of the cornea and the lens (different layers) and mo-deling of muscles.
2.2
Optics
For the simulation of the optical behavior of the Gullstrand Eye ray tracing methods will be used (s. Figure 5).
Figure 6: Object images at the retina of the Gullstrand Eye
For the objective interpretation of these images a metric of the sharpness of the distribution on the retina was developed.
Like the biomechanics modeling further extensions of the optical modeling are necessary. That means e.g. the consideration of different layers and nonspherical geometries of the cor-nea and of the lens.
Figure 5: Ray tracing inside the Gullstrand Eye
Starting with a spherical description of the cornea and the lens it is possible to produce images by simulation. Figure 6 represents an example of such images at the retina depending on dif-ferent object distances.
Figure 7: The common stucture of the distributed functions of the Virtual Eye and the in-vivo/ex-vivo interface
In this context it is necessary to support a common data structure and a complex communica-tion protocol. Embedding these facilities in the World Wide Web guarantees a flexible hand-ling of the Virtual Eye. The end users of the Virtual Eye (e.g. hospitals) do not need complex and expansive simula-tors and databases. Personal computers or workstations including a connection to the internet is a standard in the most institutions today or will be in the next future. For using the services of the Virtual Eye only a autostereoscopic display including a 3D-interaction System will be necessary to.
3.
Distributed simulation and data service via the World Wide Web
The nature of the Virtual Eye and the in-vivo/ex-vivo interface is distributed.
Only hospitals can support the in-vivo/ex-vivo interface. The developement of the 3D-image/interaction functions is a typical task of the IT-research and the IT-industry. Modeling and simulation is rather a task for engineers.
For the realization of the Virtual Eye it is necessary to bring togehter these different tasks. A solution is possible using a central database server ( s. Figure 7).
