Reevaluation of Current Prism Standards With Recommendations to Increase Accuracy in the Measurement of Strabismus.

PURPOSE: To critically evaluate the traditional standards for holding single prisms in measuring strabismus, with the specific goal being to increase the overall accuracy of clinical measurements.

DESIGN: Reliability and validity analysis.

METHODS: Using an analysis involving geometric optics, the effective prism power (EPP), measured in prism diopters (PD), was calculated for glass and acrylic prisms and was the main outcome measure. These results were also validated using optical bench measurements. No patients were involved.

RESULTS: Plotting the calculated effective prism power as a function of rotational angle produced curves demonstrating that the frontal plane position lies along the flatter portions of the curves, while the Prentice position lies along the steeper portions of the curves. Calculated values of prism power for the standard clinical positions as well as the percentage errors from accepted standards were compared. Acrylic prisms can be held in the frontal plane position with acceptable amounts of error (<5%) for prisms up to 50 PD. Glass prisms are capable of producing significant errors for measurements much greater than 10 PD.

CONCLUSIONS: The use of glass prisms as currently calibrated should be abandoned or at least limited to small measurements. Acrylic prisms produce acceptable errors if careful attention is given to their position.