An experimental study was conducted with the aim of developing a method to measure strabismus angles larger than 50 prism dioptres (PD) using ‘piggyback prisms’ – stacking Fresnel and block prisms. The stacked prisms were placed 2cm from the light source (green laser – wavelength 532mm) and a tangent screen placed 100mm behind the prisms. A measurement was made of the deviation from the original location of the laser spot. Fresnel prisms ranging from 5 to 40PD in increments of 5 and block prisms ranging from 10 to 50PD in increments of 10 were used. This equated to a total of 71 combinations tested, including individual prisms. Repeatability of the measurements were assessed by repeating each combination on 2 separate days. A total of 11% of combinations could not be measured due to the laser point not being on the tangent screen. These were the higher combination 35 and 40 dioptre Fresnel prisms and 30–50 dioptre block prisms. A look-up table was created from the values of the 2 combined prisms as a quick reference for clinical use. These are required due to the observed deviation in PD being different to the sum of the labelled prisms; an additivity error, which increased exponentially with increase prism strength. The method of using ‘piggyback prisms’ demonstrated that it is possible to measure strabismus angles larger than 50 dioptres, up to a maximum of 156 dioptres (Fresnel 30 dioptre with block 50 dioptre). If the additivity error is not considered, this would result in underestimation of strabismus deviations. The authors also give practical tips on various methods in which prisms can be combined in clinical settings.
Using ‘piggyback prisms’ to measure large angle strabismus – don’t simply add them together
Reviewed by Lauren Hepworth
Combining Fresnel and block prisms to measure large angles of strabismic deviation.
CONTRIBUTOR
Lauren R Hepworth
University of Liverpool; Honorary Stroke Specialist Clinical Orthoptist, Northern Care Alliance NHS Foundation Trust; St Helen’s and Knowsley NHS Foundation Trust, UK.
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