Aim: To determine the degree of error induced when measuring the prism fusion range by using the Clement Clarke plastic prism bar in the frontal position rather than its calibrated position of use, the Prentice position, and to compare this with a previous model of induced error and to assess the level of agreement with the Gulden prism bar.
Method: The base-out blur, break and recovery- points of the near and distance prism fusion range were recorded in 20 normal participants (aged 18-28 years), using the Clement Clarke plastic prism bar and Gulden prism bar in the frontal position.
Results: At near, each measure was shown to be larger when using the Clement Clarke plastic prism bar (p < 0.05). The median blur point was 25Δ base-out (range 14Δ-75Δ) with the Clement Clarke bar and 18Δ base-out (range 12Δ-65Δ) with the Gulden bar. Median break point was 62Δ base-out (range 25Δ-85Δ) with the Clement Clarke bar and 43Δ base-out (range 16Δ-80Δ) with the Gulden bar. The median recovery was 54Δ base-out (range 16Δ-80Δ) with the Clement Clarke bar and 38.5Δ base-out (range 14Δ-70Δ) with the Gulden bar. At distance there was no statistically significant difference between blur or break point with cither bar but the median recovery point was significantly overestimated (p < 0.05), measuring 20Δ base-out (range 8Δ–85Δ) with the Clement Clarke bar and 18Δ base-out (range 8Δ-70Δ) with the Gulden bar. However, after application of a correction formula to the Clement Clarke measurements, the limits of agreement between these and the Gulden measurements showed, with a few exceptions, results lying within ±2 standard deviations.
Conclusions: Using the Clement Clarke plastic prism bar in the frontal position rather than the calibrated Prentice position overestimates the measurement of the prism fusion range at near. Practical measures are similar to those that may be calculated mathematically for the error induced due to the use of the bar in a position for which it is not calibrated.
How to Cite:
Bath, J.K. and Firth, A.Y., 2007. Prism fusion range: Gulden versus Clement Clarke plastic prism bar. British and Irish Orthoptic Journal, 4, pp.58–64. DOI: http://doi.org/10.22599/bioj.214