Differences in the early biomechanical effects of hyperopic and myopic laser in situ keratomileusis

Abstract

PURPOSE-To compare changes in corneal hysteresis (CH) and the corneal resistance factor (CRF) in myopic and hyperopic laser in situ keratomileusis (LASIK) and evaluate their relationship with the number of photoablative pulses delivered, a surrogate for ablation volume. SETTING-ClevelandClinic Cole Eye Institute, Cleveland, Ohio, USA. METHODS-Preoperative and 1-week postoperative OcularResponse Analyzer measurements in eyes that had femtosecond-assisted LASIK were studied retrospectively. Changes in CH and CRF were compared and tested for correlation with the number of excimer laser pulses.RESULTS-Thirteen myopic eyes and 11 hyperopic eyes were evaluated. Preoperative corneal thickness, CH, CRF, programmed correction magnitude, flap thickness, and total number of fixed spot-size photoablative pulses were similar in the 2 groups (P>.1). Decreases in CH and CRF were greater after myopic LASIK than after hyperopic LASIK (P<.005), and changes in CRF were correlated with the number of excimer laser pulses in the myopic group only (r = −0.63, P = .02). Regardless of ablation profile, changes in CH were more strongly correlated with preoperative CH values than with attempted ablation volume.CONCLUSIONS-With comparable flap thickness and attempted ablation volumes, myopic photoablation profiles were associated with greater decreases in CRF and CH than hyperopic profiles. Results indicate that preoperative corneal biomechanical status, ablation volume, and the spatial distribution of ablation are important factors that affect corneal resistance and viscous dissipative properties differently. Preferential tissue removal in the natively thicker paracentral cornea in hyperopia may partially account for the rarity of ectasia after hyperopic LASIK.During laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), phototherapeutic keratectomy, and other photoablative corneal procedures, immediate Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. circumferential severing of anterior corneal lamellae occurs in the ablation zone. The resulting structural alteration of the cornea has been proposed to decrease resistance to stromal swelling pressure peripheral to the ablation, shift tension to the intact lamellae deep to the interrupted lamellae, and generate centripetal stresses that favor central corneal flattening and a shift toward farsightedness.1 , 2 Because intraocular pressure (IOP) also manifests as a force against the posterior corneal surface, deeper ablations concentrated in the cornea's thinnest regions can lead to offsetting corneal steepening effects.3 , 4 T...

Examination of ocular biomechanics with a new Scheimpflug technology after corneal refractive surgery

“…glaucoma, keratoconus and postrefractive eyes). A significant change in the biomechanical properties of the cornea is found after PRK, LASIK and epi-LASIK, measured with ORA [4][5][6]9,[16][17][18][19][20][21][22]. The decrease in the biomechanical values was significantly larger after LASIK than after PRK, three months postoperatively, with the same preoperative data [20].…”

Corneal biomechanical characteristics in patients with diabetes mellitus

“…As DCT IOP measurements were slightly lower in eyes of type 1 diabetic patients, the type 1 cohort may have had slightly lower IOPs than their type 2 counterparts. Corneal biomechanical properties have been shown to alter following corneal refractive 31,42,43 and cataract 44,45 surgery procedures, and it has been suggested that there may be a role for ORA metrics to better predict refractive outcomes of these procedures. 19 Our data show that compared with eyes of nondiabetic subjects, eyes of diabetic patients have altered corneal biomechanics that may be related to blood glucose concentration.…”

Identification of Biomechanical Properties of the Cornea: The Ocular Response Analyzer

“…[70][71][72][73][74] Some studies showed a reduction of CH and CRF after LASIK -from 10.4 ± 1.7 mmHg to 9.3 ± 1.9 mmHg and from 10.1 ± 1.9 mmHg to 8.1 ± 1.8 mmHg, respectively. 59,71 For these parameters, however, no further changes occurred during a 6-months follow-up period, in contrast to progression of keratoconus.…”