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srk [2026/07/22 15:43] – [Step 8: Refraction Correction] Scott Larsonsrk [2026/07/28 12:31] (current) – Scott Larson
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 ====== SRK/T (Sanders-Retzlaff-Kraff/Theoretical) Formula ====== ====== SRK/T (Sanders-Retzlaff-Kraff/Theoretical) Formula ======
-FIXME+
 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. 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.
  
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 Using the Pythagorean theorem applied to corneal dome geometry: Using the Pythagorean theorem applied to corneal dome geometry:
  
-$$ H = r - √(r² - (Cw / 2)²) $$+$$ H = r - \sqrt{r^2 - \left( \frac{Cw}{2} \right)^2}* $$
  
 ---- ----
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 The IOL power for emmetropia is calculated using a modified vergence formula: The IOL power for emmetropia is calculated using a modified vergence formula:
  
-$$ \text{IOL}{\text{emme}} = \frac{1000 \times na \left( nr \times r - nc \times L{OPT} \right)}{\left( L{OPT} - ACD{est} \right) \times \left( na \times r - nc \times ACD{est} \right)} $$+$$ \text{IOL}{\text{emme}} = \frac{1000 \times na \left( na \times r - nc \times L{OPT} \right)}{\left( L{OPT} - ACD{est} \right) \times \left( na \times r - nc \times ACD{est} \right)} $$
  
  
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   * **na** = refractive index of aqueous/vitreous (1.336)   * **na** = refractive index of aqueous/vitreous (1.336)
-  * **nc** = corneal refractive index (optimized, typically 1.333) +  * **nc** = corneal refractive index (1.333)
-  * **nr** = retinal refractive index +
- +
-----+
  
 ===== Step 8: Refraction Correction ===== ===== Step 8: Refraction Correction =====
  
 To target a specific postoperative refraction (**REF**) rather than emmetropia: To target a specific postoperative refraction (**REF**) rather than emmetropia:
- 
-$$ IOLpower = IOLemme - (REF / (1 - 0.012 × REF)) × (1000 × na) / ((LOPT - ACDest)² × ((na × r) / (na × r - nc × ACDest))⁻¹) $$ 
  
 $$ \text{IOL}{\text{power}} = \text{IOL}{\text{emme}} - \frac{\text{REF}}{1 - 0.012 \times \text{REF}} \times \frac{1000 \times na}{\left( L{OPT} - ACD{est} \right)^2 \times \left( \frac{na \times r}{na \times r - nc \times ACD{est}} \right)^{-1}} $$ $$ \text{IOL}{\text{power}} = \text{IOL}{\text{emme}} - \frac{\text{REF}}{1 - 0.012 \times \text{REF}} \times \frac{1000 \times na}{\left( L{OPT} - ACD{est} \right)^2 \times \left( \frac{na \times r}{na \times r - nc \times ACD{est}} \right)^{-1}} $$
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   * The **T2 modification** (Sheard et al., 2010) later addressed nonphysiologic behavior in the corneal height calculation step   * The **T2 modification** (Sheard et al., 2010) later addressed nonphysiologic behavior in the corneal height calculation step
  
 +=====Reference=====
 +[[https://docs.eyefixer.org/Cataract/IOL%20Calculations/Development%20of%20the%20SRK_T%20intraocular%20lens%20implant%20power%20calculation%20formula.pdf|Development of the SRK/T intraocular lens implant power calculation formula. Retzlaff et al. J Cataract Refract Surg. 1990;16:333-339]]
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 +{{tag>cataract}}