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Calculating Contact Lens Power In Aphakia
- Primarily used with Silsoft Contact lenses
Methods
Trivedi-Wilson (TW) Regression Formula
$P = 84.4 -(3.2 x Al)$
where P= Contact Lens Power, AL = Axial Length in millimeters
Go to the Calculator
The TW formula was derived from 50 eyes of infants who underwent cataract surgery before 1 year of age (mean age 2.4 months) using SilSoft silicone elastomer lenses. The R² was 0.82
https://www.aao.org/education/editors-choice/guidelines-selecting-contact-lens-power-in-aphakic
SRK/T formula
SRK/T formula Input the following:
- A-constant = 112.176
- Keratometry readings
- Axial Eye Length
SRK/T Calculators
In the IATS cohort, using the SRK/T formula yielded a mean prediction error of −2.0 D and mean absolute PE of 2.9 D.
Aphakic Contact Lens (aCL) Formula
$$ Contact lens power = (1334/Axial Length mm) - 0.6685 Mean Keratometry D -15.25 + Desired Target Refraction $$ Go to the Calculator
Christoph Lwowski et al developed the aCL formula, and showed median absolute prediction error of only 0.53 D, comparable to retinoscopy (0.69 D) and substantially outperforming the TW formula (median AE 3.25 D) and the default +32 D lens (median AE 2.63 D). [3]