Instrument-induced measurement errors during strabismus surgery

Purpose:The purpose of this study was to determine the clinical setting where errors in measurements of muscle position during strabismus surgery made by the Scott curved ruler or by calipers become important and to characterize the magnitude of those errors. Methods: Geometric analysis was used to determine the measurement error between true are lengths of 3.0 to 20.0 mm versus Scott curved ruler measurements and caliper measurements for axial lengths ranging from 18 to 30 mm. Results: For measurements less than 9.0 mm, neither the Scott curved ruler nor calipers had any clinically important measurement error for any axial length. For axial lengths substantially smaller than 21 mm or larger than 24 mm the Scott curved ruler, although more accurate than calipers, caused clinically important measurement errors with are length measurements as small as 12 mm in very small eyes and 14 mm in large eyes. For axial lengths of 30 mm or more, both calipers and the Scott curved ruler had similar accuracy for measuring long are lengths. Conclusions: Both the Scott curved ruler and calipers are accurate in measuring are lengths 9.0 mm or less, For longer are length measurements, accuracy becomes dependent on axial length. The Scott curved ruler, although substantially more accurate than calipers for most common axial lengths, can introduce clinically important measurement errors when measuring are lengths as small as 12 mm. Axial length should be considered when measuring muscle position during strabismus surgery.