Cataract surgery post-Lasik

Intraocular lens (IOL) power calculation for cataract surgery can be inaccurate after refractive corneal laser surgery.1-3 This is partly because it becomes difficult to measure the corneal power accurately by conventional keratometry.4-6 More accurate, central averaged keratometer values can be obtained with topography. However, the index of refraction (1.3375) used to calculate the corneal power from keratometry is no longer reliable in eyes that have had photorefractive keratectomy (PRK) or laser in situ keratomileusis (Lasik). Furthermore, the nomograms appear specific to a topography machine.7 Measurement of the corneal curvature by indirect methods such as the contact lens overrefraction method8,9 is also prone to error.1 Even if the preoperative keratometry is known the calculation of intraocular lens power from the clinical history method10 can be inaccurate.1,2 This is partly because of the IOL biometry calculation formulas used, their estimation of effective lens position (ELP) from the keratometry, and the assumption that the cataract does not influence the refraction. Lately, methods have evolved (such as the double K method11) to calculate ELP from the original keratometry. Also the use of more modern formulae, which are less dependent on keratometry for calculating ELP, should also reduce this IOL power calculation error.4,5 No large studies have been conducted comparing these methods in real cataract patients who have had refractive surgery. So it remains that several methods are still used to calculate IOL power and that refractive surprises still occur in a population of patients that may have a higher expectation for good visual outcome. The preoperative biometry method (AS method) proposed here calculates the IOL power by choosing the implant which reflects the change in stable spherical equivalence achieved by the laser using the original preoperative biometry. For example, a -3.00D myope corrected to emmetropia by Lasik would have the IOL chosen from the pre-laser biometry that would leave them the difference of the spherical equivalent before and after laser; in this case -3.0 minus 0 = -3.00D. In another example, a -3D myope corrected by laser to -1D would have an implant from the pre-laser biometry that would leave them -3 minus -1 = -2D. The intraocular lens formula used would be as if the patient was undergoing a refractive lens extraction and the formula chosen would largely depend on the axial length. The advantage of the preoperative biometry method is that it eliminates the errors in calculating corneal power after refractive laser surgery, use of different formulas, and also postoperative refractive change from the cataract.