Background The Chang-Waring chord is provided by many ophthalmic instruments, but
proper interpretation of this chord for use in centring refractive procedures at the
cornea is not fully understood. The purpose of this study is to develop a strategy
for translating the Chang-Waring chord (position of pupil centre relative to the Purkinje
reflex PI) into angle Alpha using raytracing techniques. Methods The retrospective
analysis was based on a large dataset of 8959 measurements of 8959 eyes from 1 clinical
centre, using the Casia2 anterior segment tomographer. An optical model based on:
corneal front and back surface radius Ra and Rp, asphericities Qa and Qp, corneal
thickness CCT, anterior chamber depth ACD, and pupil centre position (X-Y position:
PupX and PupY), was defined for each measurement. Using raytracing rays with an incident
angle IX and IY the CW chord (CWX and CWY) was calculated. Using these data, a multivariable
linear model was built up in terms of a Monte-Carlo simulation for a simple translation
of incident ray angle to CW chord. Results Raytracing allows for calculation of the
CW chord CWX/CWY from biometric measures and the incident ray angle IX/IY. In our
dataset mean values of CWX = 0.32±0.30 mm and CWY = -0.10±0.26 mm were derived for
a mean incident ray angle (angle Alpha) of IX = -5.02±1.77° and IY = 0.01±1.47°. The
raytracing results could be modelled with a linear multivariable model, and the effect
sizes for the prediction model for CWX are identified as Ra, Qa, Rp, CCT, ACD, PupX,
PupY, IX, and for CWY they are Ra, Rp, PupY, and IY. Conclusion Today the CW chord
can be directly measured with any biometer, topographer or tomographer. If biometric
measures of Ra, Qa, Rp, CCT, ACD, PupX, PupY are available in addition to the CW chord
components CWX and CWY, a prediction of angle Alpha is possible using a simple matrix
operation.
