Change In Corneal Power Research Articles

Orbscan Evaluation of Corneal Power after Myopic PRK: Placido Disc-Based vs Scanning Slit Topography

Purpose To compare Placido disc-based and scanning slit topography in the evaluation of corneal power changes after PRK. Methods The changes in corneal power, evaluated with an Orbscan II utilizing both Placido disc-based and Scanning Slit topography, before and 1, 3 and 6 months after PRK were compared with refractive changes in 109 individuals (149 eyes) who underwent photorefractive keratectomy (PRK) with a sphero equivalent refraction, calculated at the corneal apex, ranging from -0.25 to -12 D (mean -4.58±2.54 D). Results 1,3 and 6 months refractive changes showed a good correlation with changes in BFS (r 2=0.64, r 2= 0.53, r 2= 0.62, respectively); in Sim K's (r 2= 0.77, r 2= 0.8, r 2= 0.78, respectively) in Mean Power 3 mm (r 2= 0.81, r 2= 0.88, r 2= 0.87, respectively) in Mean Power 5 mm (r 2= 0.68, r 2= 0.78, r 2= 0.78, respectively) in Anterior Mean Power 3 mm (r 2= 0.88, r 2= 0.85, r 2= 0.85, respectively) in Anterior Mean Power 5 mm (r 2= 0.75, r 2= 0.7, r 2= 0.75, respectively) in Total Mean Power 3 mm (r 2= 0.75, r 2= 0.71, r 2= 0.79, respectively) in Total Mean Power 5 mm (r 2= 0.75, r 2= 0.71, r 2= 0.78, respectively). However, all differences were statistically significant ( P<0.001). Conclusion The data obtained with the scanning slit system seem to be more reliable, but need correcting factors to improve their precision.

Long-term Keratometric Changes After Penetrating Keratoplasty for Keratoconus and Fuchs Endothelial Dystrophy

To compare long-term keratometric changes after penetrating keratoplasty (PK) for keratoconus and Fuchs endothelial dystrophy. Retrospective, comparative case series. We retrospectively analyzed 168 corneas after PK for keratoconus (85 eyes of 63 subjects) and Fuchs dystrophy (83 eyes of 60 subjects). Patients were examined after final suture removal at 12 months after PK to 30 years after surgery. Operations were performed by one surgeon (W.M.B.) using the same suturing technique in all cases. Eyes were excluded from further analysis after regrafting or after relaxing incisions. Mean keratometric corneal power and astigmatism were measured by manual keratometry. Data were assessed by using generalized estimating equation models to determine change over time. Mean keratometric corneal power and astigmatism increased through 30 years after PK for keratoconus (P < .001 and P < .001), but did not change through 20 years after PK for Fuchs dystrophy (P = .55 and P = .55) The change in keratometric corneal power and astigmatism after PK in keratoconus patients only differed from the change in Fuchs dystrophy patients 10 or more years after PK (P = .002 and P = .003). Corneal curvature and regular astigmatism increase progressively after PK for keratoconus, but remain stable after PK for Fuchs dystrophy. Our data suggest that keratometric instability after PK for keratoconus is attributable to delayed, progressive ectasia in the host corneal rim.

Effect of Vitrectomy on IOL Calculation for Cataract Surgery : Study of Vitrectomized Eyes

Purpose: To evaluate the effect of vitrectomy in IOL power calculation for cataract surgery in the eyes that previously performed vitrectomy. Methods: This study comprised 41 eyes of 37consecutive patients who underwent cataract surgery after previous vitrectomy. IOL power calculation with the measurement of axial length and corneal power was conducted before vitrectomy and before cataract surgery. Target refractions at each surgery, assuming that the same power of IOL was implanted, were compared to actual manifest refraction after cataract surgery. Results: There was no statistically significant change in both axial length and corneal power after vitrectomy (p>0.05). Before vitrectomy, target refraction was -0.22±0.61D compared to -0.37±0.52D before cataract surgery. The actual refraction after cataract surgery was -0.47±0.94D, and there were no significant differences among the three refractions (p>0.05). Conclusions: In the eyes that underwent vitrectomy, vitrectomy itself does not have an influence on calculating IOL power for cataract surgery.

Topographic Changes After Hyperopic LASIK With the SCHWIND ESIRIS Laser Platform

To analyze and compare refractive power changes with topographic changes after hyperopic LASIK using the ESIRIS laser platform. Sixty-six consecutive eyes of 37 patients were evaluated retrospectively. Outcomes were evaluated at 3 months. In all cases, standard examinations including pre- and postoperative topographic and corneal wavefront analysis with a Keratron topographer were performed. Preoperative mean spherical equivalent was +/- 2.74 diopters (D) with a mean cylinder of 0.67 D. The ESIRIS excimer laser system was used to perform ablations. Topographic changes of Maloney index, simulated keratometry (sim-K), and K-reading at 5 and 7 mm were evaluated. Ninety-two percent of eyes were within +/- 0.50 D manifest refraction at 3 months postoperatively. Changes in refraction, Maloney indices, and sim-K indices after surgery were strongly correlated with the intended correction. Induction of negative corneal spherical aberrations and increased prolate asphericity was correlated with the achieved defocus correction. No other Zernike mode was significantly correlated. Topographically, an overcorrection within the central 3 mm and a gradual peripheral undercorrection also was observed. Analyzing the topographic maps, the corneal power change of the Maloney indices correlated with the intended correction. After hyperopic LASIK, an overcorrection was observed in the 3-mm central zone, and progressive undercorrections were observed in the 5- and 7-mm zones.

The Influence of Chronic Diabetes Mellitus on the Thickness and the Shape of the Anterior and Posterior Surface of the Cornea

To determine the influence of diabetes mellitus (DM) type 1 and type 2 on the thickness, radius of curvature, power, and asphericity of the cornea. In this observational cross-sectional study, 102 patients with DM type 1, 101 patients with DM type 2, and 69 healthy subjects were measured by means of Scheimpflug imaging to determine central corneal thickness and the radius and asphericity of the anterior and posterior corneal surfaces. Corneal power was calculated from these parameters. Several systemic parameters (eg, duration of diabetes, glycated hemoglobin, blood glucose levels, and type of medication) and ocular comorbidity (eg, stage of retinopathy) were recorded. Patients with DM type 1 and 2 had significantly smaller posterior corneal radii (P < 0.05) than those of healthy subjects (men: 6.49/6.48/6.64 mm; women: 6.36/6.30/6.49 mm). As a result, the optical power of the posterior corneal surface of the patients with diabetes differed from that of the healthy subjects (P < 0.01; men: DM, -6.2 D; healthy, -6.0 D; women: DM, -6.3 D; healthy, -6.2 D). However, corneal thickness, anterior radius and asphericity, and overall corneal power did not differ significantly between the groups. Furthermore, none of the systemic factors or ocular comorbidity had any influence on the corneal thickness or shape. DM affects the posterior corneal radius, resulting in a small change in posterior corneal power. However, chronic DM does not seem to significantly influence the overall corneal power.

Nearwork-Induced Changes in Topography, Aberrations, and Thickness of the Human Cornea After Interrupted Reading

Recent experiments have suggested transient changes in corneal optics after short durations of reading. This study investigated an interrupted 1-hour period of reading on corneal topography in a sample of young myopic and emmetropic adults. Ten myopic and 9 emmetropic young adult subjects were recruited for the study. They read adult-level text binocularly at 40 cm with approximately 30 degree downward gaze for 1 hour, with distance correction in place. Orbscan II corneal topography was performed at baseline and 5, 15, 30, and 60 minutes after reading during 1 test session to assess for changes in corneal power, aberrations, and thickness. Myopes exhibited greater corneal power and spherical aberration than emmetropes both before and after reading. Increased corneal higher-order aberrations (at baseline only) and thickness (superior position only) were observed in myopes versus emmetropes. Several corneal parameters differed in the myopes after reading as compared with emmetropes. Differences in corneal parameters were smaller in this study than found in earlier studies, perhaps because of the interrupted nature of the task in the former versus the continuous nature in the latter.

Descemet membrane endothelial keratoplasty using descemetorhexis and organ‐cultured donor corneal tissue

Abstract Purpose: To report results of two surgical procedures of endothelial keratoplasty Methods: Prospective, noncomparative study. Thirty height eyes of 38 patients with 21 corneal edema Fuchs dystrophy, 11 pseudophakia dystrophy, 4 decompensation of penetrating graft and 2 corneal edema after herpes endothelitisIn all eyes, the recipient Descemet membrane was excised using descemetorhexis through a 3.2 mm corneal incision. In 17 cases endothelial transplant was harvested by lamellar manual dissection. In 21 cases separation of endothelium was made performed with viscoelastic dissection.A temporary gas bubble was used for donor tissue adherence.Preoperative and postoperative best spectacle‐corrected visual acuity (BCVA), manifest refraction, astigmatism, pachymetry and endothelial cell density (ECD) were evaluated 2 and 6 months after surgery. Results: At the end of follow‐up 37 grafts remained transparent and grafts were healed in good position. One eye required conversion to standard penetrating keratoplasty.At 2 months, BSCVA in the group of Fuchs dystrophy is 20/40 and 20/25 at 6 months. In the group of pseudophakia keratopathy BSCVA is 20 /100 and 20/63 at 2 and 6 months respectively.The average MR astigmatism was 1.3 D. The mean pachymetry is stable after 1 month at 540 mm .The average ECD at 2 months was 1500 cells/mm2 and 1300 cells/mm2 at 6 months. Conclusions: This procedure, with its absence of corneal surface incisions and sutures, is a safe procedure that preserves the normal corneal topography, minimizes astigmatism and corneal power changes. This technique offers considerable advantages over penetrating keratoplasty in the treatment of endothelial dysfunction.

Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography

To measure total corneal power using optical coherence tomography (OCT). Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA. Thirty-two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high-speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography. The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK-induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (-18.8%) (P<.001). Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK-induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post-LASIK eyes.

Posterior Corneal Curvature Changes after Undersurface Ablation of the Flap and In-the-Bed LASIK Retreatment

To analyze LASIK retreatment-induced changes in the posterior corneal curvature (PCC) with undersurface ablation of the flap (UAF) and in-the-bed techniques. Nonrandomized, comparative, interventional study. Forty-six eyes with a residual spherical equivalent refraction between -0.37 and -2.75 diopters (D) and astigmatism between 0.0 and -1.25 D were included prospectively. In 23 eyes, the calculated postenhancement flap thickness exceeded 150 microm using micropachymetric optical coherence tomography, whereas with further ablation of the bed, the residual bed thickness (RBT) would have been <250 microm, or <55% of the pre-LASIK central pachymetry. In another 23 eyes, RBT allowed the planned ablation for a calculated post-retreatment RBT exceeding 250 microm, >55% of the pre-LASIK central pachymetry. Eyes with insufficient RBT for further ablation underwent UAF retreatment, whereas those with adequate RBT received conventional in-the-bed LASIK retreatment. Examinations were performed before retreatment and 3 and 6 months postoperatively. No eye was lost to follow-up. Micropachymetry, Orbscan II scanning-slit PCC data, and visual acuity (VA). The groups did not differ in age, intraocular pressure, or retreatment ablation depth, but the UAF eyes had a lower mean pre-retreatment RBT (270.7+/-25.4 microm) than conventional enhancement eyes (353.0+/-41.5 microm) (P = 0.001). Eyes undergoing UAF had no significant change in PCC, whereas eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone (P = 0.008) 3 months after retreatment. No significant changes occurred thereafter. The amount of change in posterior corneal power within the 3-mm central zone from before to after retreatment differed significantly between the groups (mean difference, 0.135 D; 95% confidence interval, 0.022-0.248 D; P = 0.02). No keratectasia developed clinically, and no retreated eye lost or gained > or =2 lines of best-corrected VA. Six months after retreatment, the efficacy and safety indices for the UAF procedure were 0.96 and 1.01, respectively, and 1 and 1.06 for conventional LASIK enhancement. Undersurface ablation of the flap retreatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia.

Axial Growth and Changes in Lenticular and Corneal Power during Emmetropization in Infants

Purpose: To evaluate the contribution made by the ocular components to the emmetropization of spherical equivalent refractive error in human infants between 3 and 9 months of age. Methods: Keratophakometry in two meridians was performed on 222 normal birth-weight infant subjects at 3 and 9 months of age. The spherical equivalent refractive error was measured by cycloplegic retinoscopy (cyclopentolate 1%). Anterior chamber depth, lens thickness, and vitreous chamber depth were measured by A-scan ultrasonography over the closed eyelid. Results: Both the mean and the SD for spherical equivalent refractive error decreased between 3 and 9 months of age (+2.16 ± 1.30 D at 3 months; +1.36 ± 1.06 D at 9 months; P < 0.0001, for the change in both mean and SD). Average ocular component change was characterized by increases in axial length, thinning, and flattening of the crystalline lens, increases in lens equivalent refractive index, and decreases in lens and corneal power. Initial refractive error was associated in a nonlinear manner with the change in refractive error (R2 = 0.41; P < 0.0001) and with axial growth (R2 = 0.082; P = 0.0005). Reduction in hyperopia correlated significantly with increases in axial length (R2 = 0.16; P < 0.0001), but not with changes in corneal and lenticular power. Decreases in lenticular and corneal power were associated with axial elongation (R2 = 0.40, R2 = 0.12, respectively; both P < 0.0001). Conclusions: Modulation in the amount of axial growth in relation to initial refractive error appeared to be the most influential factor in emmetropization of spherical equivalent refractive error. The associations between initial refractive error, subsequent axial growth, and change in refractive error were consistent with a visual basis for emmetropization. The cornea and crystalline lens lost substantial amounts of dioptric power in this phase of growth, but neither appeared to play a significant role in emmetropization.

Changes in Corneal Power and Refraction Due to Sequential Suture Removal Following Nonmechanical Penetrating Keratoplasty in Eyes With Keratoconus

To assess the changes in corneal power and refraction due to sequential suture removal after penetrating keratoplasty (PK). Retrospective consecutive case series. setting: Clinical practice. study population: We studied 67 phakic keratoconus eyes (central excimer laser trephination, primary keratoplasty, graft/recipient diameter 8.1/8.0 mm; double running suture) in this longitudinal study. main outcome measures: Zeiss keratometry (equivalent power (KEQ), astigmatism (KAST)), corneal topography (equivalent power (TEQ), astigmatism (TAST)) and subjective refractometry (spherical equivalent (SEQ), refractive cylinder (RAST)) were assessed with sutures in place (interval 1), with one suture out (interval 2), and with all sutures out (interval 3). observation procedure: Corneal power and refraction was decomposed into vector components and the changes were derived between time stages. The mean follow-up period was 3.9+/-1.7 years. At interval 1, the axes of KAST/TAST/RAST were almost randomly distributed. At interval 2, the with/against the rule component of KAST/TAST/RAST decreased slightly and the oblique component increased significantly, so that the axes tended to have a preferred oblique direction. At interval 3, the with/against the rule component of KAST/TAST/RAST increased slightly and the oblique component decreased significantly, so that the with/against the rule component exceeded the oblique component by approximately 23%/28%/25%. Median KEQ/TEQ/SEQ changed by 0.64/0.62/-1.11 diopters (interval 1 to interval 2) and by -0.85/-0.90/1.56 diopters (interval 2 to interval 3). As a result of removal of the first running suture, corneal astigmatism as well as the refractive cylinder tend to oblique axes. As a result of removal of the second running suture, the final corneal astigmatism and refractive cylinder tend to orientation axes with/against the rule.

Corneal Topographic Changes After Transconjunctival (25-gauge) Sutureless Vitrectomy

To evaluate the topographic changes in the cornea after pars plana vitrectomy (PPV) with 25-gauge transconjunctival sutureless vitrectomy (TSV) system. Prospective, interventional case series. In this prospective study, we evaluated the topographic changes of the cornea in 32 eyes of 32 patients who underwent PPV with the 25-gauge TSV system. The topographic parameters that were analyzed statistically were the average corneal power, corneal surface cylinder, surface asymmetry index, and surface regularity index. Mean induced astigmatism was estimated by vector analysis software. Wilcoxon test was used for statistical analysis. There was no significant change in average corneal power, corneal surface cylinder, surface asymmetry index, and surface regularity index parameters at first day, first week, and first month after the operation. Mean induced astigmatism was 0.38 diopters at 15 degrees. Corneal surface and astigmatic changes were observed to be insignificant in the early postoperative period after PPV with the 25-gauge TSV system.

Axial Growth and Changes in Lenticular and Corneal Power during Emmetropization in Infants

To evaluate the contribution made by the ocular components to the emmetropization of spherical equivalent refractive error in human infants between 3 and 9 months of age. Keratophakometry in two meridians was performed on 222 normal-birthweight infant subjects at 3 and 9 months of age. The spherical equivalent refractive error was measured by cycloplegic retinoscopy (cyclopentolate 1%). Anterior chamber depth, lens thickness, and vitreous chamber depth were measured by A-scan ultrasonography over the closed eyelid. Both the mean and SD for spherical equivalent refractive error decreased between 3 and 9 months of age (+2.16 +/- 1.30 D at 3 months; +1.36 +/- 1.06 D at 9 months; P < 0.0001, for the change in both mean and SD). Average ocular component change was characterized by increases in axial length, thinning, and flattening of the crystalline lens, increases in lens equivalent refractive index, and decreases in lens and corneal power. Initial refractive error was associated in a nonlinear manner with the change in refractive error (R(2) = 0.41; P < 0.0001) and with axial growth (R(2) = 0.082; P = 0.0005). Reduction in hyperopia correlated significantly with increases in axial length (R(2) = 0.16; P < 0.0001), but not with changes in corneal and lenticular power. Decreases in lenticular and corneal power were associated with axial elongation (R(2) = 0.40, R(2) = 0.12, respectively; both P < 0.0001). Modulation in the amount of axial growth in relation to initial refractive error appeared to be the most influential factor in emmetropization of spherical equivalent refractive error. The associations between initial refractive error, subsequent axial growth, and change in refractive error were consistent with a visual basis for emmetropization. The cornea and crystalline lens lost substantial amounts of dioptric power in this phase of growth, but neither appeared to play a significant role in emmetropization.

Spontaneous Long-Term Changes of Corneal Power and Astigmatism After Suture Removal After Penetrating Keratoplasty Using a Regression Model

To assess the diagnosis-based spontaneous long-term changes in corneal power and refraction with a regression model in the all-sutures-out time period following non-mechanical penetrating keratoplasty (PK). Retrospective non-randomized clinical trial. setting: Clinical practice. study population: 147 eyes [47 Fuchs dystrophy (FD); 100 keratoconus (KC)] were studied after suture removal in this retrospective longitudinal study. main outcome measures: Zeiss keratometry [equivalent power (KEQ) and astigmatism (KAST)], corneal topography analysis [equivalent power (TEQ) and astigmatism (TAST)], and subjective refractometry [spherical equivalent (SEQ) and refractive cylinder (RAST)] were assessed in at least three up to 16 ophthalmologic examinations in the all-sutures-out time period. observation procedure: The time course of each target variable was analyzed in a longitudinal manner (time interval > or = 12 months) separately for each patient with a linear regression model. Post-keratoplasty follow-up ranged from 31 months to 10.3 years. In the linear regression model, the annual change in FD/KC showed an increase/a decrease in KEQ (0.29 +/- 0.50/-0.63 +/- 0.46 diopters, P = .02) and an increase/a decrease in TEQ (0.37 +/- 0.54/-0.69 +/- 0.49 diopters, P = .04) corresponding to a decrease/an increase in SEQ (-0.31 +/- 0.47/0.63 +/- 0.43 diopters, P = .02). KAST/TAST/RAST showed a minimal annual decrease (-0.06 +/- 0.41/-0.05 +/- 0.45/-0.06 +/- 0.41 diopters) in FD but an increase in KC (0.46 +/- 0.41/0.51 +/- 0.43/0.46 +/- 0.38 diopters) (P = .05/0.06/0.12). In the follow-up after post-keratoplasty suture removal, patients with FD/KC tend to develop a spontaneous myopic shift (steepening of the cornea)/hyperopic shift (flattening of the cornea). In contrast with those with FD, patients with KC should be counseled on the fact that astigmatism may increase again over time after suture removal.

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To report the 6- and 12-month results of the first United States clinical series of deep lamellar endothelial keratoplasty (DLEK) in the treatment of endothelial dysfunction.Prospective, noncomparative, interventional case series.Eight eyes of eight patients with corneal edema from Fuchs’ dystrophy and pseudophakia.A 9.0-mm limbal, scleral, partial-depth incision provided access for a deep lamellar corneal pocket dissection. A 7.5- to 8.0-mm posterior lamellar disc of recipient tissue was then excised and replaced through the pocket with a same size donor disc containing healthy endothelium. A temporary air bubble in the anterior chamber was used for donor tissue adherence, and no surface corneal incisions or sutures were necessary.Preoperative and postoperative best spectacle-corrected visual acuity (BSCVA), manifest refraction astigmatism, TMS-1 topography, ultrasonic pachymetry, Orbscan topography, and endothelial cell density were evaluated. Intraoperative and postoperative complications are reported.At 6 and 12 months after surgery, all eight corneas were clear and the grafts were healed in good position. At 6 months, the BSCVA varied between 20/30 and 20/70, the average change in astigmatism from before surgery was +1.13 diopters (D; ±1.50 D), the average change in corneal power was −0.4 D (±1.7 D), the average pachymetry was 648 μm (±134 μm), and the average endothelial cell count was 2290 cells/mm2 (±372 cells/mm2). At 12 months, three of the four eyes reaching this time gate were 20/40 or better, with a change in astigmatism from before surgery of only +0.81 D (± 0.55 D), a corneal power change of −1.3 D (± 0.4 D), and an endothelial density of 2409 cells/mm2 (± 154 cells/mm2). One of the original nine eyes entered into this study required conversion to standard penetrating keratoplasty as a result of a microperforation during recipient pocket dissection and has experienced no ill effects.The DLEK procedure, with its absence of corneal surface incisions and sutures, is a safe procedure that preserves the normal corneal topography, minimizes astigmatism and corneal power changes, and provides a healthy donor endothelial cell count and function. If interface optical clarity can be maintained, then this technique offers considerable advantages over penetrating keratoplasty in the treatment of endothelial dysfunction.

Corneal power, thickness, and stiffness: Results of a prospective randomized controlled trial of PRK and LASIK for myopia

To compare the short-, medium-, and long-term changes in corneal optical power and corneal aberrations, central corneal thickness, and corneal "stiffness" assessed by pneumotonometry readings in patients having laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) for myopia. Department of Ophthalmology, Arhus University Hospital, Arhus, Denmark. One eye of each of 45 patients with myopia ranging from -6.00 to -8.00 diopters (D) (spherical equivalent spectacle refraction [SER]) was randomized to LASIK (n=25; mean SER -7.12 D +/- 0.57 [SD]) or PRK (n=20; mean SER -6.91 +/- 0.57 D). Data were collected prospectively before and 1, 3, 6, 12, and 36 months after surgery. Measurements included corneal topography (TMS-1, Tomey), corneal thickness (ultrasound pachymetry), and apparent intraocular pressure (IOP) (pneumotonometry). Retreatments were not performed during the first year, and retreated eyes were excluded from the 3-year follow-up. Changes in corneal power and aberrations, thickness, and apparent IOP were calculated in a pair-wise manner for 3 time periods: short term (preoperative to 1 month after surgery), medium term (1 to 12 months after surgery), and long term (1 to 3 years after surgery). In the short term, corneal power decreased equally in LASIK and PRK eyes. Spherical aberrations and coma-like aberrations increased equally, while corneal thickness decreased significantly less in LASIK eyes than in PRK eyes. The apparent IOP decreased more in LASIK eyes than in PRK eyes. In the medium term, corneal power increased significantly in both groups. Spherical aberrations decreased significantly in PRK eyes but not in LASIK eyes. From 1 to 12 months, corneal thickness increased more in PRK eyes than in LASIK eyes. During this period, the apparent IOP increased significantly in LASIK eyes. In the long term, corneal power and corneal aberrations did not change significantly in either group. Corneal thickness increased slightly but significantly in both groups. The apparent IOP increased significantly more in PRK eyes. Differences between LASIK and PRK related to time-dependent events affecting corneal shape and structural integrity were present. Peripheral changes in flap hydration in LASIK eyes and epithelial and/or stromal thickening in PRK eyes appeared to be the most important factors in optical power changes in the first year after treatment. The changes in apparent IOP suggest that some interlamellar healing occurred during the first year after LASIK. After LASIK and PRK, corneal bending stiffness seemed permanently decreased, although some restiffening may occur in PRK eyes in the long term.

The Longitudinal Orthokeratology Research in Children (LORIC) in Hong Kong: A Pilot Study on Refractive Changes and Myopic Control

Purpose: Myopia is a common ocular disorder, and progressionof myopia in children is of increasing concern. Modern overnight orthokeratology(ortho-k) is effective for myopic reduction and has been claimed to beeffective in slowing the progression of myopia (myopic control) in children,although scientific evidence for this has been lacking. This 2 year pilot studywas conducted to determine whether ortho-k can effectively reduce and controlmyopia in children. Methods: We monitored the growth of axial length (AL)and vitreous chamber depth (VCD) in 35 children (7–12 years of age), undergoingortho-k treatment and compared the rates of change with 35 childrenwearing single-vision spectacles from an earlier study (control). For the ortho-ksubjects, we also determined the changes in corneal curvature and the relationshipswith changes of refractive errors, AL and VCD. Results: The baselinespherical equivalent refractive errors (SER), the AL, and VCD of the ortho-kand control subjects were not statistically different. All the ortho-k subjectsfound post-ortho-k unaided vision acceptable in the daytime. The residual SERat the end of the study was –0.18 ± 0.69 D (dioptre) and the reduction (lessmyopic) in SER was 2.09 ± 1.34 D (all values are mean ± SD). At the endof 24 months, the increases in AL were 0.29 ± 0.27 mm and 0.54 ± 0.27 mmfor the ortho-k and control groups, respectively (unpaired t test; p = 0.012);the increases in VCD were 0.23 ± 0.25 mm and 0.48 ± 0.26 mm for theortho-k and control groups, respectively (p = 0.005). There was significantinitial corneal flattening in the ortho-k group but no significant relationshipswere found between changes in corneal power and changes in AL and VCD.Conclusion: Ortho-k can have both a corrective and preventive/control effect inchildhood myopia. However, there are substantial variations in changes in eyelength among children and there is no way to predict the effect for individualsubjects.

Topographic and biomechanical differences between hyperopic and myopic laser in situ keratomileusis

To evaluate the size, shape, and uniformity of the videokeratographic functional optical zone (FOZ) after laser in situ keratomileusis (LASIK) in 2 cohorts of patients with equivalent amounts of preoperative myopic or hyperopic astigmatism. Pepose Vision Institute, St. Louis, Missouri, USA. Eyes with myopic or hyperopic astigmatism (n=27 in each group) that had LASIK with the Visx Star S3 laser were retrospectively selected to match for level of preoperative refractive error. Slit-scanning videokeratography was performed preoperatively and 6 months postoperatively and analyzed using custom software. The FOZ was calculated by analyzing refractive power maps using a region-growing algorithm. Difference maps were generated from slit images and compared for interval change in corneal elevation, tangential curvature, and refractive power. The difference maps were also averaged (mean difference maps) for each target population. A Zernike decomposition of corneal first-surface elevation was performed to compare postoperative values with baseline parameters. The mean postoperative refractive sphere at 6 months was -0.17 diopter (D) +/- 0.66 (SD) and +0.25 +/- 0.85 D in the myopia group and hyperopia group, respectively, and the mean postoperative astigmatism, -0.49 +/- 0.32 D and -0.65 +/- 0.52 D, respectively (P=.11). Based on the refractive power maps, the mean preoperative and postoperative myopic FOZ was 33.09 +/- 7.30 mm(2) and 30.94 +/- 5.43 mm(2), respectively, and the mean hyperopic FOZ, 33.19 +/- 7.96 mm(2) and 37.99 +/- 6.88 mm(2), respectively. After LASIK, there was an increase in magnitude of negative anterior corneal surface spherical-like Zernike values in the myopia group (P<.0001) and an increase in magnitude of positive spherical-like Zernike values in the hyperopia group. Postoperatively, significant induction of corneal surface horizontal coma was noted in hyperopic eyes (P<.0001). Hyperopic eyes, on average, had larger topographic FOZs after LASIK, but with less uniformity of curvature and power change than myopic eyes. Hyperopic LASIK, which involves more transition points along the ablation diameter, produced a less uniform topographic FOZ than typical myopic treatments. Less predictable biomechanical changes from the circumferential release of tension on collagen bundles after midperipheral hyperopic ablation and greater variation in beam centration and the angle of incidence may contribute to the greater variability in corneal curvature and power in hyperopic LASIK than in myopic LASIK.

Changes in keratometric corneal power and refractive error after laser thermal keratoplasty

Purpose: To evaluate the effect of laser thermal keratoplasty (LTK) on corneal power and refractive error to develop a logical approach to calculating accurate intraocular lens (IOL) power for cataract surgery. Setting: Department of Ophthalmology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea. Methods: Laser thermal keratoplasty was performed in 27 eyes of 23 patients. Preoperatively and postoperatively, the refractive error was measured and the corneal power obtained by manual keratometry and topography. The changes in keratometric corneal power and refractive error after LTK were compared. Results: The mean age of the 15 women and 8 men was 45.0 years ± 4.6 (SD) (range 43 to 61 years). The mean preoperative refractive error was +1.43 ± 0.97 diopters (D) (range 0 to +3.63 D) at the spectacle plane and +1.46 ± 1.01 D (range 0 to +3.79 D) at the corneal plane. The mean postoperative refractive error was −0.44 ± 1.07 D (range −2.24 to +2.18 D) at the spectacle plane and −0.44 ± 1.08 D (range −2.18 to +2.23 D) at the corneal plane. After surgery, corneal powers measured by manual keratometry were significantly smaller than those measured by topography ( P<.001) and refractive error changes were significantly smaller than keratometric changes ( P<.001). Conclusions: After LTK, corneal power measured by manual keratometry was smaller than that measured by corneal topography and changes in corneal power measured by conventional keratometric instruments were greater than changes in refractive error. This difference should be considered in calculating IOL power in post-LTK eyes to prevent undesirable hyperopia after cataract surgery.

Correcting the cornea power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis

In reference to the article by Shammas and associates (Am J Ophthalmol 136:426–432, September 2003), given the same definition as used by Shammas and associates, we present the following alternative equations that relate the preoperative and postoperative K readings (Kpre and Kpost) and the true post-LASIK cornea power (Dpost). The cornea power is given by D = 377/r1 − 41/r2, where r1 and r2 are the cornea radius of curvature for the anterior and posterior surfaces, respectively. It is clear that the posterior surface contributes to approximately 11% of the cornea total power; however, it does not affect the corneal power change in the LASIK procedure, which is only governed by the change of the anterior curvature radius (dr) and its initial value (r1), shown as: dD = −377(dr) / (r1)2, in which r2 plays no role at all. (Equation 1)Dpost=1.117 Kpost−41/r2(Equation 2)Dpost=1.117 Kpre+dD−41/r2(Equation 3)Kpre=Kpost+0.895dD Depending on whether either postoperative or preoperative K readings is known, one may easily calculate the corneal post-LASIK power by Eq. 1 or 2. In addition, Kpre may be calculated by Eq. 3 if one knows the refractive power correction amount after LASIK (the dD term). The standard intraocular lens power calculation using the post-LASIK K reading, therefore, will have errors of about 6.0 to 6.5 diopters for dD = −1.0 to −5.0 diopters, as shown by the above equations.