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.
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).
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.
Corneal Width is derived from the corneal radius $$ Cw = -5.40948 + 0.58412 × LCOR + 0.098 × K $$
Using the Pythagorean theorem applied to corneal dome geometry:
$$ H = r - \sqrt{r^2 - \left( \frac{Cw}{2} \right)^2}* $$
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:
$$ ACDest = H + ACDconst -3.336 $$
Retinal thickness (Rethick) and optical axial length (LOPT) are calculated:
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:
To target a specific postoperative refraction (REF) rather than emmetropia:
$$ \text{IOL}{\text{power}} = \text{IOL}{\text{emme}} - \frac{\text{REF}}{1 - 0.012 \times \text{REF}} \times \frac{1000 \times na}{\left( L{OPT} - ACD{est} \right)^2 \times \left( \frac{na \times r}{na \times r - nc \times ACD{est}} \right)^{-1}} $$