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Declan Kirk, Coby Ray, Kenn Freedman; A Reevaluation of Prism Position in the Measurement of Strabismus. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1026.
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© ARVO (1962-2015); The Authors (2016-present)
The aim of this study is to critically evaluate the traditional clinical standards for using prisms in measuring strabismus using optical calculations; with the specific goal being to increase the overall accuracy in measuring ocular misalignments
Using an analysis involving simple geometric optics, the effective prism power, measured in prism diopters (PD) was determined for standard acrylic and glass prisms as a function of rotational angle. These results were plotted graphically and were then also tabulated and compared for four standard clinical prism positions.
Plotting the effective prism power as a function of rotational angle produced curves demonstrating that the Posterior (Frontal Plane) and Midline Positions lie along the flatter portions of the curves and the Prentice and Anterior Positions lie along the steeper portions of the curve.
Acrylic prisms can be held in the posterior (frontal plane) or midline positions with acceptable amounts of error for prisms up to 40 PD. Glass prisms though, as currently calibrated are of questionable clinical value for any measurements much greater than 15 PD. The Prentice position is difficult to precisely achieve clinically and according to this analysis can result in significant errors and should be abandoned. It is recommended that a new set of clinical prisms be manufactured, either glass or acrylic, and calibrated to be held in a more easily achievable and natural position such as the posterior or midline position.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
Figure 1: Effective prism diopters plotted as a function of rotational angle for 40 PD glass (1.A) and acrylic (1.B) prisms. The four discussed prism positions are shown by vertical arrows.
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