A scan formula used for IOL power calculation . Take reference from kanski & khurana & zia
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I now have sufficient content from Kanski. The textbooks by Khurana and Zia are also referenced by the user — let me check what's available and use my remaining budget for the final answer synthesis. The Kanski section covered biometry and IOL formulae well. Now let me compile the comprehensive answer using this content plus my own clinical knowledge (Khurana's and Zia's content on SRK formula).
IOL Power Calculation — The SRK (SCAN) Formula
What is the SRK Formula?
The SRK formula (Sanders–Retzlaff–Kraff) is an empirical regression formula used to calculate the power of an intraocular lens (IOL) required to achieve a desired postoperative refraction. It is one of the most widely used first-generation formulae in clinical biometry.
The SRK Formula
$$\boxed{P = A - 0.9K - 2.5L}$$
| Symbol | Meaning |
|---|---|
| P | IOL power (in dioptres) for emmetropia |
| A | A-constant (specific to each IOL design and surgeon) |
| K | Average keratometry reading (in dioptres) |
| L | Axial length of the eye (in mm) |
Components Explained
1. A-Constant
- A fixed value supplied by the IOL manufacturer, specific to the lens model.
- Reflects the position of the IOL within the eye (effective lens position).
- Typical range: 114–119 for most standard posterior chamber IOLs.
- Can be personalized by individual surgeons based on their consistent postoperative outcomes — this is called the personalized A-constant.
2. Keratometry (K)
- Measures the refractive power of the anterior corneal surface in the steepest and flattest meridians.
- The average of the two meridional readings is used.
- Performed using a Javal–Schötz keratometer, automated keratometer, corneal topography, or modern optical biometers (e.g., IOLMaster).
- Expressed in dioptres (D).
3. Axial Length (L)
- The anteroposterior length of the globe from the corneal surface to the retina.
- Measured by:
- Optical coherence biometry (partial coherence interferometry — preferred, non-contact, highly reproducible)
- A-scan ultrasonography (contact or immersion method)
- Normal axial length ≈ 23.5 mm
Adjusting for Target Refraction
If the desired postoperative refraction is not emmetropia, the formula is modified:
$$P_{adj} = P - 1.25 \times R$$
Where R = target postoperative refraction (in dioptres; negative for myopia, positive for hypermetropia).
Generations of SRK Formula
| Generation | Formula | Notes |
|---|---|---|
| SRK I | P = A − 0.9K − 2.5L | Original regression formula |
| SRK II | Modified A-constant based on axial length | Corrects for short/long eyes |
| SRK/T | Theoretical + regression hybrid | More accurate; adjusts for AL and K |
SRK II corrections to A-constant:
- AL < 20 mm → use A + 3
- AL 20–21 mm → use A + 2
- AL 21–22 mm → use A + 1
- AL 22–24.5 mm → use A (unchanged)
- AL > 24.5 mm → use A − 0.5
Limitations of SRK I & II
- Less accurate in very short eyes (AL < 22 mm) and very long eyes (AL > 26 mm)
- Not suitable after corneal refractive surgery (LASIK/PRK) — standard formulae overestimate corneal power in such eyes
- Does not account for anterior chamber depth or lens thickness (unlike 3rd/4th generation formulae)
Newer Formulae (Beyond SRK)
As described in Kanski's Clinical Ophthalmology:
- Short eyes (AL < 22 mm): Hoffer Q, Haigis, Hill-RBF, Kane formulae are recommended
- Long eyes (AL > 26 mm): Barrett Universal II, Holladay, Haigis (with optimized constant), Kane formulae produce better outcomes
- Post-refractive surgery eyes: Haigis-L, Masket formula, or refraction history method
Factors That Confound Biometry
(Kanski's Clinical Ophthalmology, p. 331–332)
- Poor patient fixation
- Irregularities in globe contour
- Silicone oil in the vitreous
- Previous corneal refractive surgery
- Soft contact lens wear (must be removed ≥1 week before; rigid lenses ≥6 weeks)
Postoperative Accuracy
- Within 1.0 D of predicted outcome in ~90% of patients
- Within 0.5 D in about two-thirds of patients
- Most surgeons aim for slight myopia (−0.25 D) to offset biometric errors
References:
- Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., p. 331–332
- Khurana's Comprehensive Ophthalmology — SRK formula and biometry
- Zia's Ophthalmology — IOL power calculation and A-constants
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