Theoretical Relationship Among Effective Lens Position, Predicted Refraction, and Corneal and Intraocular Lens Power in a Pseudophakic Eye Model

Abstract

Purpose To ascertain the theoretical impact of anatomical variations in the effective lens position (ELP) of the intraocular lens (IOL) in a thick lens eye model. The impact of optimization of IOL power formulas based on a single lens constant was also simulated. Methods A schematic eye model was designed and manipulated to reflect changes in the ELP while keeping the optical design of the IOL unchanged. Corresponding relationships among variations in ELP, postoperative spherical equivalent refraction, and required IOL power adjustment to attain target refractions were computed for differing corneal powers (38 diopters [D], 43 D, and 48 D) with IOL power ranging from 1 to 35 D. Results The change in ELP required to compensate for various systematic biases increased dramatically with low-power IOLs (less than 10 D) and was proportional to the magnitude of the change in refraction. The theoretical impact of the variation in ELP on postoperative refraction was nonlinear and highly dependent on the optical power of the IOL. The concomitant variations in IOL power and refraction at the spectacle plane, induced by varying the ELP, were linearly related. The influence of the corneal power was minimal. Conclusions The consequences of variations in the lens constant mainly concern eyes receiving high-power IOLs. The compensation of a systematic bias by a constant increment of the ELP may induce a nonsystematic modification of the predicted IOL power, according to the biometric characteristics of the eyes studied. Translational Relevance Optimizing IOL power formulas by altering the ELP may induce nonsystematic modification of the predicted IOL power.

Evaluation of corneal power from an AS-OCT thick lens model and ray tracing: reliability of the keratometer index

“…3,7 However, the "true" corneal power value is relevant in situations where we use the corneal power value to predict, for example, the appropriate intraocular lens power or to estimate the ocular magnification, and we then have to take care that the conversion from corneal front surface radius to corneal power matches the assumptions of the lens power formula. 23 Using current instruments, the geometry of the cornea can be measured including front and back surface curvature and corneal thickness. 7,24 However, we have to be aware that these measure optical path lengths and a conversion of the measurement data to CCT or Rp require the refractive index of the cornea.…”

“…However, this systematic backward shift of the intraocular lens away from the geometric lens position in the pseudophakic eye affects short eyes requiring high powered lens implants much more than long eyes with low powered implants. 23,24 As a consequence, short eyes with high powered lenses could end up with myopia, whereas long eyes with low powered lenses end up with hyperopia after cataract surgery.…”

An update on intraocular lens power calculations in eyes with previous laser refractive surgery

“…Studies have indicated that the magnitude of errors in IOL power prediction from ELP tend to be most significant for the SRK/T formula and least for the Hoffer Q formula [9]. This observation might explain why formulas that do not rely on corneal power in their ELP estimation, such as Haigis-L and Shammas, yield better results in LRS eyes [10,11].…”

Positive dysphotopsia after implantation of an EDOF intraocular lens

“…The 0.14 mm difference in AL may have resulted in a 1.03 D difference in spherical equivalent, which in combination with the effect of the effective lens position (0.20 D difference) neutralizes the myopic target deviation. 2 Tilt and decentration analyses (Casia2, Tomey Corp.) showed a tilt of 6.7 degrees at 191 degrees and a decentration of 0.29 mm at 164 degrees for the right eye. The IOL calculation of the left eye was adjusted for this myopic shift.…”