CORNEAL POWER CORRECTION FACTOR FOR PHOTOREFRACTIVE KERATECTOMY

BACKGROUND: Studies of corneal power changes resulting from photorefra ctive keratectomy generally rely on keratometer or videokeratograph me asurements. These instruments convert corneal radius of curvatures val ues to optical powers by means of the single refracting surface formul a, which incorporates an index of refraction value of 1.3375. This ind ex approximates that of the tears but not the 1.376 index of the corne al epithelium or stroma. A hypothetical optical model was used to dete rmine the most appropriate index to be chosen with respect to corneal power calculations relative to photorefractive keratectomy. METHODS: T he contribution of each refractive element in the tear lens-corneal su rface to the total power of the eye was calculated in order to identif y which index of refraction was most appropriate for the corneal power calculation. RESULTS: The outer tear surface has significant optical power but the tear layer as a whole has nearly zero power due to the o ffsetting negative power of the posterior tear surface. There is no si gnificant difference in the effective power of light leaving the corne al anterior surface when considered with or without the tear layer. Ph otorefractive keratectomy changes the epithelium and anterior surface of the corneal stroma, but does not affect the posterior stroma or oth er ocular media. Hence the refractive index for the corneal epithelium or stroma of 1.376 should be used in converting radius to optical pow er values. The error in assuming a corneal index of 1.3375 is a consta nt proportion equal to 11.4% of the corneal power reading. CONCLUSIONS : Photorefractive keratectomy presents a situation in which the actual corneal refractive index of 1.376 should be used for correct corneal radius to power conversions. This may be accomplished by changing the index value in the instrument algorithm for keratometry and videokerat ography to 1.376 or by adding a correction factor of either 11.4% of t he regular reading to its value or multiplying by the factor 1.114. In other applications of keratometry or videokeratography, the index 1.3 375 may be more appropriate.