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Lid angles and aperture size are important factors for fitting and on-eye performance of contact lenses. In particular, toric and translating bifocal lenses rely on predictable interaction with the eyelid for successful positioning and orientation. Traditionally, lid assessment and fitting evaluation is performed under slit lamp observation. Observations are limited to primary gaze and nasal or temporal directions. Quantification of lid parameters and lens positioning is subjective and depended on the skill of the practitioner. No commercial instrument is available off the shelf, which would be suitable to measure objectively ocular parameters and/or contact lens positioning under different directions of gaze. This is the likely reason why only one study has been published in which ocular characteristics for different gaze angles were obtained [1]. However, only a limited number of parameters and gaze directions were investigated. Almost all contact lenses are designed for and assessed under primary gaze. Considering that patients constantly change their gaze while wearing contact lenses, the quantification of contact lens movement and changes in ocular parameters with eye movement can contribute substantially to the understanding of lens performance and thereby lead to improved lens designs. This is of particular importance for toric and translating bifocal lenses. Their complex optical and topographical design requires precise positioning, orientation and movement with changes in gaze direction in order to provide adequate vision. Baron [2] suggested that the lower eyelid is the most important factor for lens movement of translating bifocal lenses, but questions still remained on the complete dynamics of on eye behaviour. For toric contact lenses, the rotational orientation is more important than vertical translation to obtain optimum visual correction. Most toric lenses feature a prism ballast design. Blinking movements, mainly of the upper eyelid, interact with the wedge like shape and squeeze the thick part downwards. It has been observed that the upper eyelid does not move in a straight downward movement, but also twists slightly to close the temporal side first [3]. Precise quantification of this motion and how it affects the contact lens movement have not been reported. A new instrument was designed and constructed to provide objective measurements of ocular parameters.
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