A first "attempt" for the development of a mathematical model of the human eye was formulated by Krewson in 1950. This model defined exclusively geometrical relations and was improved by Robinson in 1975. A few years later in 1984 a first complete biomechanical model was introduced by Miller and Robinson, which additionally supported muscle forces and kinematics as well. This model was called SQUINT and later it was refined in a second, very similar version. Based on the models described so far, Günther implemented a new biomechanical model in 1986 using geometrical formulations from Kusel and Haase (1977). This model was also implemented as a computer system, but the model predictions could not be correlated with clinical experience.

 

In 1995, Miller and Demer introduced a new biomechanical model and computer system called Orbit™ (further information can be found at www.eidactics.com). This system was the first computer simulation, which also considered anatomic elements like pulleys and so provided significantly better model predictions that were even comparable to clinical data. At the same time Orbit™ was the first computer system with a graphical user interface in this field.

 

In the year 2000, the biomechanical model "Eyelab" was introduced by Porrill, Warren and Dean. This implementation could only be used for research purposes and was implemented with Matlab. This model does not provide a clinical test and therefore cannot be used for the modeling of pathological situations.

 

The SEE-KID model, which is the model primarily used in SEE++, is partially based on the formulation of "Eyelab" and "Orbit". The Orbit model was additionally implemented in the SEE++ system in an unmodified form in order to enable comparisons between different simulations. The SEE-KID model was developed using modern methods of software engineering as well as object and component-oriented technologies. Compared to others, this model is characterized by the fact that it realizes an abstract definition of a biomechanical eye model. Thus, several instances of different models (SEE-KID model, SEE-KID active pulley model, Orbit model, string model and tape model) can be realized and subsequently compared. The biomechanical eye model formulated in the context of the SEE-KID research project nevertheless is a full, mathematical solution for the proposed problem and it uses non-linear optimization strategies for solving a kinematic system. Furthermore, it defines an independent geometrical model.

 

The SEE-KID active pulley model is also an independent model within the SEE++ system, which is based on the SEE-KID model. Additionally, this model implements the "Active-Pulley-Hypothesis" formulated by Demer et. al., which describes the active movement of the pulleys of the straight eye muscles in secondary and tertiary gaze positions.

 

In contrast to other simulation systems, SEE++ offers an intuitive graphical user interface, which allows the usage of nearly all functions of the system (parameterization, simulation, surgery) interactively with a 3D-model.

 

As mentioned before, the SEE++ system includes different mathematical models of research history and provides interactive comparability of simulation results among different model-based calculations. SEE++ implements five different models from which two models simulate purely geometrical characteristics of the oculomotor system.

 

Different Model Types Included in SEE++

 

The fact that the purely geometrical models do not contain force simulations and kinematics leads to the restriction that a binocular test of eye motility, like the Hess-Lancaster test, cannot be simulated with these models. Nevertheless, string and tape model offer an ideal introduction to a better understanding of the oculomotor system and its underlying geometrical characteristics.