Background and purpose Aniseikonia as one of the major risk factors for asthenopic
problems is mostly overlooked in modern cataract surgery. The purpose of this study
was to develop a simple calculation scheme for clinicians to predict the object to
image magnification in a pseudophakic eye with biometric data. Methods The calculation
scheme for object to image magnification in the pseudophakic eye is based on a vergence
calculation of the lens power with theoretical optical formulae. From the biometric
data, which are typically derived from both eyes during lens power calculation, the
vergences in front of and behind the 3 or 4 refractive surfaces of the pseudophakic
eye model are used to predict the magnification for objects at infinity or objects
located at a finite measurement distance (e.g. 5 m). Results With a formula-based
lens power calculation a pseudophakic eye model is set up with 3 or 4 refractive surfaces
(postoperative spectacle refraction; thick cornea described by anterior surface or
thick cornea characterized by anterior and posterior surfaces; intraocular lens).
The vergence in front of and behind each refractive surface is derived by means of
linear Gaussian optics. The quotient of the product of all vergences in front of the
surfaces and the product of all vergences behind the respective surfaces describes
the object to image magnification of the eye. A comparison of the object to image
magnification of both eyes yields the retinal image size disparity or aniseikonia.
This calculation strategy is shown in a step-by-step approach exemplarily for the
Haigis and Hoffer-Q formulae (3 surfaces) and the Castrop formula (4 surfaces). Conclusion
If during planning and lens power calculation biometry is performed for both eyes,
ocular magnification of both eyes can be easily derived with this calculation scheme
and aniseikonia can be extracted from a comparison of magnification of both eyes.
Such a simple prediction should be established as a standard for precataract biometry
and lens power calculation for early detection and avoidance of asthenopic complaints
after cataract surgery.
