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SRK/T (Sanders-Retzlaff-Kraff/Theoretical) Formula

The SRK/T is a third-generation IOL power calculation formula combining theoretical optics with empirical regression optimization, published by Retzlaff, Sanders, and Kraff in 1990.

Input Variables

  • K = average keratometry (diopters)
  • AL = axial length (mm)
  • A = IOL A-constant

Step 1: Corneal Radius of Curvature

Calculate the corneal radius of curvature $$ r = 337.5 / K $$

r= corneal radius
where 337.5 is the keratometric index converting K (diopters) to corneal radius (mm).

Step 2: Corrected Axial Length (LCOR)

If AL ≤ 24.2:

LCOR = AL

If AL > 24.2:

$$ LCOR = -3.446 + 1.716 × AL - 0.0237 × AL² $$

This regression-based correction adjusts for the relationship between measured and optical axial length, incorporating a retinal thickness factor.

Step 3: Computed Corneal Width (Cw)

Corneal Width is derived from the corneal radius $$ Cw = -5.40948 + 0.58412 × LCOR + 0.098 × K $$

Step 4: Corneal Height (H)

Using the Pythagorean theorem applied to corneal dome geometry:

$$ H = r - √(r² - (Cw / 2)²) $$

Step 5: Estimated Postoperative ACD

The A-constant is converted to an ACD estimate and combined with corneal height:

ACDconst = 0.62467 × A - 68.747

Offset = ACDconst - 3.336

ACDest = H + Offset

Thus:

$$ ACDest = H + ACDconst -3.336 $$

Step 6: Retinal Thickness and Optical Axial Length

Retinal thickness (Rethick) and optical axial length (LOPT) are calculated:

  1. $Rethick = 0.65696 - 0.02029 × AL$
  2. $LOPT = AL + Rethick$

Step 7: IOL Emmetropic Power

The IOL power for emmetropia is calculated using a modified vergence formula:

$$ IOLemme = (1000 × na × (nr × r - nc × LOPT)) / ((LOPT - ACDest) × (na × r - nc × ACDest)) $$

where:

  • na = refractive index of aqueous/vitreous (1.336)
  • nc = corneal refractive index (optimized, typically 1.333)
  • nr = retinal refractive index

Step 8: Refraction Correction

To target a specific postoperative refraction (REF) rather than emmetropia:

$$ IOLpower = IOLemme - (REF / (1 - 0.012 × REF)) × (1000 × na) / ((LOPT - ACDest)² × ((na × r) / (na × r - nc × ACDest))⁻¹) $$

Key Design Features

  • Corneal refractive index, retinal thickness correction, and ACD prediction were interactively optimized using regression on 1,677 posterior chamber lens cases
  • Can use either the SRK A-constant or a direct ACD estimate as input
  • The T2 modification (Sheard et al., 2010) later addressed nonphysiologic behavior in the corneal height calculation step