Abstract Background The angles alpha and kappa are widely discussed for centring refractive
procedures, but they cannot be determined with ophthalmic instruments. The purpose
of this study is to investigate the Chang-Waring chord (position of the Purkinje reflex
PI relative to the corneal centre) derived from an optical biometer before and after
cataract surgery and to study the changes resulting from cataract surgery. Methods
The analysis was based on a large dataset of 1587 complete sets of preoperative and
postoperative IOMaster 700 biometry measurements from two clinical centres, each containing:
valid data for pupil and corneal centre position, the position of the Purkinje reflex
PI originated from a coaxial fixation target, keratometry (K), axial length (AL),
anterior chamber depth (ACD), lens thickness (LT), central corneal thickness CCT,
and horizontal corneal diameter W2W. The Chang-Waring chord CW was derived from pupil
centre and Purkinje reflex PI analysed preoperatively and postoperatively, and a multilinear
regression model together with a feedforward neural network algorithm was set up to
predict postoperative CW chord from preoperative CW chord, K and biometric distances
of the eye. Results The Y component of CW chord shows a slight shift in the inferior
direction in both left and right eyes, before and after cataract surgery. The X component
shows some shift in the temporal direction, which is more pronounced preoperatively
and slightly reduced postoperatively but with a larger variation. The change in CW
chord from preoperative to postoperative shows a slight shift in the superior and
nasal directions. Our algorithms for prediction of postoperative CW chord using preoperative
CW chord, keratometry and biometry as input data performed with a multilinear regression
and a feedforward neural network approach were able to reduce the variance, but could
not properly predict the postoperative CW chord X and Y components. Conclusion The
CW chord as the position of the Purkinje reflex PI with respect to the pupil centre
can be directly measured with any biometer, topographer or tomographer with a coaxial
fixation light. The mean Y component does not differ between right and left eyes or
preoperatively and postoperatively, but the mean temporal shift of the X component
preoperatively is slightly reduced postoperatively, but with a larger scatter of the
values.
