Image analysis was performed by one examiner (ALS) using the inbuilt Visante system software (ver. 2.0), which utilizes edge-detection algorithms to locate corneal surfaces and assign appropriate refractive indices to each portion of the image and adjust its dimensions. An index of 1.000 (air) is applied to the region anterior to the cornea, 1.338 (cornea) for the area within the corneal boundaries, and 1.343 (aqueous humor) for structures posterior to the cornea.
50 A refractive index of 1.000 was applied to the whole image, using the Edit Surfaces option, before measurements were taken. After this adjustment, the Visante calipers were used to measure a range of ciliary muscle biometric characteristics. The software allows up to seven calipers to be positioned simultaneously on each image, with the option of hiding from view those not required—for example, if they obscure the region required for another measure. Overall visible ciliary muscle length was defined as the anteroposterior distance from the scleral spur, representing the anterior insertion, to the posterior tip of the ciliary muscle (
Fig. 2). Because intraindividual variability in the relative visibility of these landmarks on different images, each subject's images for one stimulus vergence level were examined before measurements were taken, with adjustment of brightness and contrast settings where necessary, to facilitate localization of these points. In addition, the software allows the operator to magnify the image and more accurately place the calipers. Anterior length was measured from the point of maximum width of the ciliary muscle to the scleral spur. To obtain this measurement, a caliper was first placed along the widest portion of the ciliary muscle, and a second caliper, perpendicular to the first, was used to determine the distance from the widest region to the scleral spur (
Fig. 2). In addition to lengths, a range of width measurements were obtained, judged from the ciliary muscle–sclera boundary to the pigmented ciliary epithelium. Width measurements (including determination of maximum width) were always obtained with a caliper positioned perpendicular to the ciliary muscle–sclera boundary. Three key width measures were acquired, selected because of the known anterior shift of the ciliary muscle with contraction.
29,32 Using knowledge of the predetermined overall length, the width of the muscle was determined at the point that fell 25% of the total length posterior to the scleral spur (CM25;
Fig. 3). Similar measures were obtained at locations 50% and 75% of the overall ciliary muscle length posterior to the scleral spur (CM50 and CM75, respectively;
Fig. 3). In addition, the ciliary muscle thickness at a set location 2 mm posterior to the scleral spur was determined (CM2). Bailey et al.
35 included this width measurement in image analysis and found that it correlated negatively with refractive error in children.