ELLIPSOIDAL FITTING OF CORNEAL TOPOGRAPHY DATA AFTER ARCUATE KERATOTOMIES WITH COMPRESSION SUTURES

BACKGROUND AND OBJECTIVE: After paired arcuate keratotomies and compre ssion sutures (AK) for treatment of high postkeratoplasty astigmatism, corneal topography tends to be irregular The purpose of this study wa s to demonstrate a mathematical method for approximation of discrete c orneal topography power data with an ellipsoid for better appreciation of the clinical outcome after AK. PATIENTS AND METHODS: Thirty-one ey es of 28 consecutive patients who underwent AK for excessive postkerat oplasty astigmatism were studied. Regular keratometry, corneal topogra phy (TMS-I), subjective refraction, and best-corrected visual acuity ( VA) were assessed preoperatively and at 1 week and 1 rear postoperativ ely. A simplex algorithm was applied for fitting an ellipsoidal surfac e to raw corneal topography power data. A set of parameters (meridiona l power, axis, and asphericity) were calculated. The cylinder of subje ctive refraction was correlated with the keratometric readings, the si mulated keratometry (SimK) of the topography system, and the respectiv e parameters of the model surface. RESULTS: Keratometric astigmatism a nd the cylinder of the model surface decreased from 8.1 +/- 3.2 and 7. 9 +/- 2.9 D preoperatively to 4.5 +/- 2.1 and 5.3 +/- 2.0 D after I ye ar, respectively The asphericity in both meridional cross sections cha nged from a prolate ellipse preoperatively to an ablate ellipse at the early postoperative follow-up stage. Regarding the cylinder axis, the re was a significant correlation of the model surface with the refract ive cylinder at all examinations (P<.05), whereas there was no signifi cant correlation of the SimK axis and the refractive cylinder axis. CO NCLUSION: The approximation of corneal topography power data with an e llipsoidal model surface renders reconstruction of clinically relevant corneal topography parameters, including corneal asphericity with a m arked data compression. Even in markedly irregular corneal surfaces, s uch as after AK, the correlation of amount/axis of refractive cylinder with the model surface parameters is more accurate than it is with re spective SimK values of corneal topography analysis.