Extraocular geometry in SEE++ defines muscles and their directions of pull. A muscle’s geometric description is based on definition of important reference points (e.g. origin, pulley, insertion), while the muscle’s direction of pull is determined by the definition of a rotation axis, around which the muscle would rotate the eye.
The muscle force distribution shows these relative rotational components for selected eye muscles along a horizontal view range in a certain level of elevation or depression. The rotational components are indicated in standardized values between -1 and 1 for ab-/adduction, elevation/depression and in-/extorsion. The illustration shows the force distributions of the superior oblique muscle in the SEE-KID Pulley Model with no elevation or depressen (0 degrees) along the horizontal field of vision with up to 40 degrees of abduction/adduction of a left eye. The diagram clearly shows that the primary function of the superior oblique is intorsion in abduction (green curve) while it is depression in adduction (red curve). Moreover, the superior oblique has nearly constant abductional function (blue curve).
The “functional topography” in SEE++ makes it possible to transform the muscle force distribution for a muscle and its possible insertions on the globe into a color coded 3D visualization. Therefore, the insertion of a selected muscle is “virtually” moved around the globe and at each location, the directional components (ab-/adduction, elevation/depression, in-/extorsion) are projected onto the surface as color-coded magnitude (darker colors indicate the dominant direction of pull). Additionally, the visualization of coordinate axes and eye poles, the derivation of an “ideal muscle path” or main-equal line with the possibility to represent intersection points of different components or zero muscle effects give important guidelines for surgical interventions. In the illustration on the right, the functional topography for horizontal action (blue color) for the superior oblique of a left eye is visualized. Dark blue colors show insertion locations for the superior oblique muscle with large abductional pulling effect. Light blue and white areas indicate no abducting effect of the muscle. Especially important for the oblique muscles are the intersection of muscle force direction components and zero effect of ab-/adduction at the superior and inferior functional pole (intersection of Z-axis with the globe).