Translation and eccentric rotation in ocular motor modeling

Area of Science:

• Ophthalmology
• Biomechanics
• Ocular motor system

Background:

• Existing ocular mechanics models inadequately represent globe translation during eye rotation.
• Combined rotational and translational eye movements are measurable via magnetic resonance imaging (MRI).

Purpose of the Study:

• To investigate the rotational center of the human eye during eccentric fixation.
• To assess the impact of a varying ocular rotational center on horizontal rectus muscle torque.

Main Methods:

• Magnetic resonance imaging (MRI) was used to capture eye movements in axial planes.
• Normal volunteers fixated horizontally eccentric targets to induce combined eye rotation and globe translation.
• Analysis focused on determining the effective center of ocular rotation.

Main Results:

• The human eye rotates eccentrically around a point that varies with gaze direction.
• This rotational center is typically anterior to the globe's geometric center, shifting laterally in abduction and medially in adduction.
• The medial rectus muscle's relative torque lever arm is smaller in adduction than predicted by oculocentric rotation models.

Conclusions:

• The concept of a fixed ocular rotational center is insufficient for accurate biomechanical modeling.
• A varying, eccentric rotational center influences ocular muscle torque without necessarily altering muscle tension.
• These findings may explain torque variations observed during convergence.