Effect of atigmatic keratotomy on pherical equivalent: reult of the atigmatim reduction clinical trial Original Reearch Article

PURPOE: To determine the effect of atigmatic keratotomy on pherical equivalent, a meaured by the coupling ratio and a new quantity, coupling contant. METHOD: In a propective multicenter tudy, ubject underwent arcuate keratotomy at a 7-mm optical zone by mean of the Lindtrom nomogram for correction of atigmatim. One hundred fifty-even eye of 95 patient who had a follow-up examination 1 month potoperatively were tudied. Mean preoperative refractive cylinder ± EM wa 2.82 ± 1.17 diopter. Coupling ratio wa defined a the ratio of the flattening of the incied meridian to the teepening of the oppoite meridian. Coupling contant wa defined a the ratio of the change in pherical equivalent to the magnitude of the vector change in atigmatim. Coupling ratio, coupling contant, and change in pherical equivalent were calculated on the bai of change in refraction and keratometry. REULT: On the bai of change in refraction, coupling ratio wa 0.95 ± 0.10 (mean ± EM) and coupling contant wa −0.01 ± 0.03, conitent with a minor hift in the pherical equivalent of −0.03 ± 0.07 diopter. On the bai of change in keratometry, coupling ratio wa 0.84 ± 0.05 and coupling contant wa −0.04 ± 0.02, conitent with minor potoperative keratometric teepening of −0.10 ± 0.04 diopter. Coupling ratio baed on change in refraction wa not tatitically different from the coupling ratio predicted by the Gau’ law for inelatic dome (P = .370). Inciion length and number, amount of achieved cylinder correction, age, and ex had no tatitically ignificant effect on coupling ratio, coupling contant, and change in pherical equivalent. CONCLUION: Cornea behaved a an inelatic urface in repone to arcuate keratotomy performed with the Atigmatim Reduction Clinical Trial tudy nomogram. On average, atigmatic keratotomy had a minimal effect on pherical equivalent refraction. There wa variability, however, in coupling ratio, coupling contant, and change in pherical equivalent from eye to eye after atigmatic keratotomy. Caution i therefore advied when imultaneou correction of cylinder and pherical equivalent i planned.