Calculating Contact Lens Power In Aphakia
- Primarily used with Silsoft Contact lenses
Methods
Trivedi-Wilson (TW) Regression Formula
$P = 84.4 - (3.2 \times AL)$
- P= Contact Lens Power
- AL = Axial Length in millimeters
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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
Aphakic Contact Lens (aCL) Formula
$P = \frac{1334}{Al} - 0.6685 \cdot K - 15.25 + Tr$
- P = contact lens power
- Al = axial eye length (mm)
- K = mean keratometry (D)
- Tr = desired target refraction (D)
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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]
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.