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Satoshi Hasebe, Shin Morisawa, Tomoki Tokutake; Corrective effect of Fresnel membrane prisms in the secondary and tertiary gaze positions. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2928.
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© ARVO (1962-2015); The Authors (2016-present)
Fresnel membrane prisms (Press-On OpticsTM, 3M) are widely used in strabismus examination and treatment. They are usually used to correct an angle of strabismus in the primary position, but the prismatic effect is affected by the gaze position and/or side of the spectacle lens on which they are applied. This study aimed to clarify these characteristics of prisms to make better use of them in clinical practice.
The prismatic effect of Fresnel prisms of 5-25 prism diopters at different gaze positions (+/-45 degrees horizontally and vertically) were calculated by Snell’s law and presented on two-dimensional contour maps. Experimentally, the prism was placed on a precision rotation stage, and the relationship between the deviation of a light ray produced by a laser pointer on a tangent screen and its rotation angle was evaluated.
The experimental data corresponded with the theoretical estimation. They indicated that the effect becomes highly asymmetric and exponentially increases as the gaze moves toward the direction of the apex when a prism of greater than 15 prism diopters is monocularly applied on the back surface of the lens (Figure, left). In contrast, when the prism is applied on the front surface (center), or two prisms with equally-divided power are applied symmetrically over both eyes (right), the effect is symmetrical and fairly stable regardless of gaze direction. However, the prism power seemed to be calibrated based on an assumption that it is applied on the back surface, and thus a power error induced by the difference in angle of the incident ray should be corrected. The effect of a horizontally placed prism slightly increases as the gaze moves vertically from the primary position regardless of application method.
Unilateral prescription of a Fresnel prism on the back surface sometimes introduces a large binocular disparity in the secondary and tertiary gaze position even if the angle of strabismus is well corrected in the primary position. The application method of the prism(s) should be individually chosen while considering the power of the prism and incomitant nature of strabismus.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Two-dimensional representation of prismatic effects. Broken lines indicate a typical spectacle frame.
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