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After Corneal Refractive Surgery

Corneal refraction is the most important quantitative factor in IOL power calculation. Presently, it is impossible to exactly measure the corneal refraction that was subjected to corneal refractive surgery (e.g. by RK, PRK, LTK, Lasik or Lasek). For this reason, a different method of determining corneal refraction must be adopted for the IOL power calculation. Three methods are currently available:

• Refractive History Method
• Contact Lens Method
• Haigis L Method (should the preLasik or corresponding contact lens values not be available)

• clicking on IOL or pressing the <I> button.
• Select the Prior Refractive Surgery tab .

WARNING: This step is necessary ONLY with corneas pretreated by refractive surgery. With untreated corneas, IOL calculation starts instantly upon selection of the biometric formula (see: IOL Calculation).

Refractive History Method

The following values must be known for the refractive history method:

• Preoperative corneal refraction (i.e., before corneal refractive surgery
• Preoperative refraction
• Stable postoperative refraction
• Corneal vertex distance.

As the change in refraction was achieved by variation of the corneal refraction, the current effective corneal refraction directly results from the difference between preoperative and postoperative refraction, corrected by the corneal vertex distance (vertex correction). The computational method is described in the technical literature. If the corresponding data of the patient are available, the refractive history method delivers the most accurate results.

Contact Lens Method

The contact lens method (contact lens over-refraction) attempts to determine the currently effective corneal refraction on the basis of two refraction measurements, once with and once without a hard "plane" contact lens.

The following parameters are needed:

• Refraction with contact lens
• Refraction without contact lens
• Refractive power of the (plane or almost plane) hard contact lens - refractive power of contact lens back surface and
• Corneal vertex distance.

In the ideal case, the refractive power of the contact lens back surface is equal to the unknown corneal refraction. For this purpose, several hard plane contact lenses with refractions of the back surface between 30 and 45 D should be available. For the calculation of the corneal refraction, enter the appropriate patient data into the display mask. The values will now be calculated.

IOL calculation window "Prior Refractive Surgery"

WARNING: The calculated refractive power/radii values may NOT be edited in the IOL Calculation window for the selected formula.

The corneal K's transferred to the IOL calculation are marked in the printout of the lens calculation with (**) and the calculation method.

Haigis L Method

In contrast to the above-described methods of determining corneal refraction, the Haigis formula allows for surgical changes to the cornea and permits the calculation of the IOL from the measured values Axial Length, Corneal K's and Optical ACD.

WARNING: This method may ONLY be used for eyes with myopic LASIK, myopic PRK and myopic LASEK. Do not use after RK or hyperopic treatments! Lenses by hyperopic LASIK/LASEK/PRK or myopic/hyperopic RK should never be calculated. The corneal radii and axis lengths as measured by the IOLMaster are required for the formula. The measured values cannot be edited here.

For further information:

• IOL Power Calculations
• 4-Formula Calculation
• Phakic Lens Implants
• Lens Constants Optimization