
The study investigated the effect of prism order on near vertical fusional vergence ranges (FVR) using the smooth and step methods. Twenty-seven normal participants were tested with Base-Up (BU) and Base-Down (BD) prisms, either in smooth or step methods. After crossover testing, the results showed that the order of prism presentation had a significant effect on the break values of both methods, but no significant effect on recovery values. However, the mean differences in values were clinically insignificant, being below 0.50Δ. The study concluded that, unlike horizontal FVR, prism order does not significantly affect near vertical FVR, simplifying clinical assessments and supporting the concept of treating horizontal and vertical fusion systems as separate entities.
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