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:
$$ H = r - \sqrt{r^2 - \left( \frac{Cw}{2} \right)^2}* $$
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:
- $$ Rethick = 0.65696 - 0.02029 × AL $$
- $$ LOPT = AL + Rethick $$
Step 7: IOL Emmetropic Power
The IOL power for emmetropia is calculated using a modified vergence formula:
$$ \text{IOL}{\text{emme}} = \frac{1000 \times na \left( na \times r - nc \times L{OPT} \right)}{\left( L{OPT} - ACD{est} \right) \times \left( na \times r - nc \times ACD{est} \right)} $$
where:
- na = refractive index of aqueous/vitreous (1.336)
- nc = corneal refractive index (1.333)
Step 8: Refraction Correction
To target a specific postoperative refraction (REF) rather than emmetropia:
$$ \text{IOL}{\text{power}} = \text{IOL}{\text{emme}} - \frac{\text{REF}}{1 - 0.012 \times \text{REF}} \times \frac{1000 \times na}{\left( L{OPT} - ACD{est} \right)^2 \times \left( \frac{na \times r}{na \times r - nc \times ACD{est}} \right)^{-1}} $$
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