Ascrs Post Refractive Calculator

Post-Refractive IOL Power Calculator

Clinical Tool for Post-LASIK/PRK/RK Calculations

Post-Myopic LASIK/PRK Post-Hyperopic LASIK/PRK Post-RK (Radial Keratotomy)

Calculation Results

Estimated IOL Power (Emmetropia): –

Corrected Keratometry (K_c): –

*Note: This calculation uses the Shammas-PL (No-History) formula methodology. Clinical judgment is required for final lens selection.

Understanding IOL Calculations Post-Refractive Surgery

Calculating the correct Intraocular Lens (IOL) power after a patient has undergone corneal refractive surgery (like LASIK, PRK, or RK) is one of the most significant challenges in modern cataract surgery. Standard formulas often result in "hyperopic surprise" because they rely on assumptions that are no longer valid for altered corneas.

The Three Main Sources of Error

1. The Instrument Error: Standard keratometers measure the front surface of the cornea and estimate total power based on a standard refractive index. Post-LASIK, the relationship between the front and back of the cornea is permanently changed.
2. The Index Error: The standardized refractive index (usually 1.3375) is incorrect for eyes that have had tissue removed by an excimer laser.
3. The Formula Error: Many standard formulas (like SRK/T) use the measured K-value to predict the Effective Lens Position (ELP). In a post-LASIK eye, a flat cornea leads the formula to predict a shallow anterior chamber, which is anatomically incorrect.

The Shammas-PL Methodology

This calculator utilizes a regression-based approach similar to the Shammas-PL formula, which is a "no-history" method. It does not require the patient's pre-LASIK data, making it highly practical for clinical use. The formula adjusts the measured keratometry to account for the refractive surgery before applying a modified IOL power calculation.

Example Calculation

Consider a patient with a history of myopic LASIK:

• Axial Length: 25.10 mm
• Flat K: 38.50 D
• Steep K: 39.00 D
• A-Constant: 118.4

In this scenario, the average K is 38.75 D. However, the Shammas correction would adjust this value significantly lower to compensate for the surgical flattening. Without this correction, a surgeon might choose a lens that is 2.0 to 3.0 Diopters too weak, leaving the patient with blurred vision.