Abstract
The purpose of this study is to develop a straightforward mathematical concept for determination of object to image magnification in both phakic and pseudophakic eyes, based on biometric measures, refractometry and data from an anterior segment optical coherence tomography (OCT). We have developed a strategy for calculating ocular magnification based on axial length measurement, phakic anterior chamber and lens thickness, keratometry and crystalline lens front and back surface curvatures for the phakic eye, and axial length measurement, anterior chamber and lens thickness, keratometry and intraocular lens power, refractive index and shape factor for the pseudophakic eye. Comparing the magnification of both eyes of one individual yields aniseikonia, while comparing the preoperative and postoperative situation of one eye provides the gain or loss in ocular magnification. The applicability of this strategy is shown using a clinical example and a small case series in 78 eyes of 39 patients before and after cataract surgery. For the phakic eye, the refractive index of the crystalline lens was adjusted to balance the optical system. The pseudophakic eye is fully determined and we proposed three strategies for considering a potential mismatch of the data: (A) with spherical equivalent refraction, (B) with intraocular lens power and (C) with the shape factor of the lens. Magnification in the phakic eye was -0.00319±0.00014 and with (A) was -0.00327±0.00013, with (B) was -0.00323±0.00014 and with (C) was -0.00326±0.00013. With A/B/C, the magnification of the pseudophakic eye was on average 2.52±2.83%/1.31±2.84%/2.14±2.80% larger compared with the phakic eye. Magnification changes were within a range of ±10%. On average, ocular magnification does not change greatly after cataract surgery with implantation of an artificial lens, but in some cases, the change could be up to ±10%. If the changes are not consistent between the left and right eyes, then this could lead to post-cataract aniseikonia.
Highlights
Over the last two decades, anterior segment optical coherence tomography (OCT) instruments have become established in clinical practice, increasingly replacing corneal topographers
The newest generation of OCTs, with a measurement field of more than 10 mm in depth and a lateral scan field exceeding 10 mm, have the capability to measure the shape of the crystalline lens before cataract surgery, as well as the position of the artificial intraocular lens post-operatively.[1,2]
Once the postoperative model was fully determined, we considered three different scenarios for a potential mismatch of the optical system: in A, we assumed that the equivalent power of the intraocular lens power (IOLP), the shape factor q and the refractive index of the intraocular lens nIOL are correct, and we re-adjusted the postoperative spherical equivalent refraction at the spectacle plane (SEQpostadj) to focus an object sharply onto the image plane and documented the difference between SEQpost and SEQpostadj for proof of concept
Summary
Over the last two decades, anterior segment optical coherence tomography (OCT) instruments have become established in clinical practice, increasingly replacing corneal topographers. These OCTs are not restricted to measuring the shape of the corneal front and back surfaces, but derive data from the anterior chamber of the eye. In contrast to ultrasound measurements using an ultrasound biomicroscopy (UBM) or Scheimpflug cameras,[1] OCT offers a much higher resolution and a non-contact measurement of the entire anterior eye segment within seconds. The decentration (magnitude and orientation) and degree of tilt (tilt angle and orientation) of the crystalline lens in a phakic eye, or artificial lens in a pseudophakic eye, can be assessed alongside the axial position and thickness
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