K J Hoffer1. 1. Jules Stein Eye Institute, University of California, Los Angeles, USA.
Abstract
BACKGROUND: Calculating the intraocular lens (IOL) power for an eye that has previously had refractive keratotomy is a problem because of the difficulty of accurately measuring the central power of the cornea using standard keratometers. METHODS: Three methods are proposed to better estimate this parameter. The clinical history method involves subtracting the change in myopia induced by the refractive keratotomy from the average corneal power measured before the keratotomy. The contact lens method determines the difference between the manifest refraction with and without a plano hard contact lens of known base curve and subtracts this difference from that base curve. Videokeratography measures the central corneal power inside the approximately 3-mm zone measured by keratometry, and therefore gives a more accurate power to use in IOL calculation formulas, especially with newer software applications becoming available. RESULTS: Published reports have demonstrated that standard keratometers do not accurately measure corneal power after refractive keratotomy and that regression formulas are less accurate than modern third-generation theoretic formulas for eyes that have flatter corneas from refractive surgery. CONCLUSION: For eyes that have had refractive surgery, the corneal power derived from clinical history, contact lens refraction, or videokeratography should be used in a third-generation theoretic formula (Hoffer Q, Holladay, SRK/T) to calculate the intraocular lens power used during cataract surgery.
BACKGROUND: Calculating the intraocular lens (IOL) power for an eye that has previously had refractive keratotomy is a problem because of the difficulty of accurately measuring the central power of the cornea using standard keratometers. METHODS: Three methods are proposed to better estimate this parameter. The clinical history method involves subtracting the change in myopia induced by the refractive keratotomy from the average corneal power measured before the keratotomy. The contact lens method determines the difference between the manifest refraction with and without a plano hard contact lens of known base curve and subtracts this difference from that base curve. Videokeratography measures the central corneal power inside the approximately 3-mm zone measured by keratometry, and therefore gives a more accurate power to use in IOL calculation formulas, especially with newer software applications becoming available. RESULTS: Published reports have demonstrated that standard keratometers do not accurately measure corneal power after refractive keratotomy and that regression formulas are less accurate than modern third-generation theoretic formulas for eyes that have flatter corneas from refractive surgery. CONCLUSION: For eyes that have had refractive surgery, the corneal power derived from clinical history, contact lens refraction, or videokeratography should be used in a third-generation theoretic formula (Hoffer Q, Holladay, SRK/T) to calculate the intraocular lens power used during cataract surgery.