Changes In Posterior Corneal Curvature Research Articles

Intraocular lens power calculation after corneal refractive surgery

Challenges in IOL power calculation in eyes after corneal refractive surgery are caused by difficulties in accurately determining corneal refractive power and in predicting the effective lens position. Three factors contribute to inaccurate corneal refractive power estimation ‐ inaccurate measurement of anterior corneal curvature by standard keratometry or corneal topography, inaccurate calculation of total corneal power from the anterior corneal curvature can result from using the standardized value for the refractive index of cornea (1.3375) and finally changes in posterior corneal curvature. There are three categories of methods proposed based on the use of historical data acquired prior to the corneal refractive surgery (formulas using only historical data, no historical data or a combination of historical data and current data). The most frequently used methods of IOL power calculation in eyes after corneal refractive surgery are discussed and their accuracy are compared. The latest studies and meta‐analyses in this area are presented. Despite availability of new measurement technologies and development of new IOL calculation formulas, further advances are needed to improve outcomes of cataract surgery in eyes that have undergone corneal refractive surgery.

Posterior keratometry changes after steep axis phacoemulsification: a prospective study.

Purpose: To measure changes in posterior corneal curvature after steep axis phacoemulsification and investigate the possibility of its effect on SIA. Methods: This was a prospective longitudinal study on 60 consecutive eyes of age-related cataract with regular astigmatism and absence of co-morbidities undergoing uneventful cataract surgeries with main incision at steep meridian. Preoperative and 4 weeks postoperative measurements of anterior and posterior corneal curvatures using Scheimpflug based corneal topographer were performed. Posterior corneal curvature was assessed at 3, 5- and 7-mm diameters. Results: The study found a statistically significant change in posterior corneal K1, K2 and mean astigmatism in all zones (3, 5 and 7 mm) at 4 weeks postoperative, when compared to preoperative readings. Conclusion: Posterior keratometry is likely to be an important determinant of Surgically Induced Astigmatism (SIA) and should be factored in for refractive cataract surgery.

Regional Changes in Posterior Corneal Surface During a 6-Month Follow-up Period After tPRK, FS-LASIK, and SMILE.

To investigate changes in corneal curvature in different zones of the posterior corneal surface during a 6-month follow-up period after transepithelial photorefractive keratectomy (tPRK), femtosecond laser-assisted excimer laser in situ keratomileusis (FS-LASIK), and small incision lenticule extraction (SMILE). The study included a total of 202 eyes, including 65, 77, and 60 that underwent tPRK, FS-LASIK, and SMILE, respectively. Elevation data for the posterior surface were obtained preoperatively (pre), as well as 1 week (pos1w), 1 month (pos1m), 3 months (pos3m), and 6 months (pos6m) postoperatively. Changes in posterior corneal curvature (M) were analyzed in the central (diameter: 0 to 3 mm), paracentral (diameter: 3 to 6 mm), and peripheral (diameter: 6 to 9 mm) regions. Over all follow-up periods, the central region of the posterior surface in all patients became flatter (P < .05), with FS-LASIK showing the largest change, whereas the paracentral and peripheral regions became steeper. The posterior curvature changes between pre and pos6m, determined before and after correction for ablated stromal depth, tended to follow similar trends in the three regions and after the three surgeries. There was also no significant correlation (P > .05) between the changes in the mean curvature (M, recorded between pre and pos6m) and each of the refractive error corrections, the changes in spherical aberration postoperatively, the optical zone diameter, ablated stromal depth, and residual stromal bed thickness in the central and peripheral regions, but the correlation was significant in the paracentral region. The postoperative changes in posterior corneal shape followed different trends in the central, paracentral, and peripheral regions. The FS-LASIK group exhibited the most notable changes in posterior corneal curvature, especially in the central region. These changes were statistically correlated with variations in spherical aberration, and ablated and residential stromal thickness in the paracentral region. [J Refract Surg. 2022;38(11):708-715.].

Corneal Tomography Changes and Refractive Outcomes After Descemet Stripping Without Endothelial Keratoplasty.

To investigate corneal tomography changes and refractive outcomes after Descemetorhexis without endothelial keratoplasty (DWEK). Retrospective chart review of 25 eyes from 16 patients with Fuchs corneal endothelial dystrophy that underwent successful DWEK combined with cataract surgery. There was no significant change in anterior corneal curvature on tomography maps. However, all tomography maps demonstrated an increase in central posterior float and all but one demonstrated a decrease in pachymetry after DWEK. Only 7 of 25 eyes had increased irregular astigmatism on tomography after DWEK, of which 57% had preexisting irregular astigmatism before the procedure. The mean difference between the targeted and outcome refraction was +0.65 diopters (D) at the time of corneal clearance, but this improved to +0.38 D 1 month after corneal clearance. Postoperative refraction was within 1 D of target refraction for 92% of cases (23 eyes) but within 0.5 D of target refraction for only 48% of cases (12 eyes). The differences between expected and actual refractive outcomes were correlated with change in posterior corneal curvature and pachymetry. DWEK induces an increased central posterior float localized to the site of Descemet membrane stripping, confirming the need for centralized stripping. Irregular astigmatism can occur after DWEK but is typically minimal and occurs more commonly in the setting of preoperative irregular astigmatism. DWEK induces about a 0.5 D hyperopic shift, which should be considered when determining intraocular lens power with simultaneous surgery.

Effect of Cycloplegia on Corneal Biometrics and Refractive State.

Purpose:To determine changes in refractive state and corneal parameters after cycloplegia with cyclopentolate hydrochloride 1% using a dual Scheimpflug imaging system.Methods:In this prospective cross-sectional study patients aged 10 to 40 years who were referred for optometric evaluation enrolled and underwent autorefraction and corneal imaging with the Galilei dual Scheimpflug system before and 30 minutes after twice instillation of medication. Changes in refraction and astigmatism were investigated. Corneal biometrics including anterior and posterior corneal curvatures, total corneal power and corneal pachymetry were compared before and after cycloplegia.Results:Two hundred and twelve eyes of 106 subjects with mean age of 28 ± 5 years including 201 myopic and 11 hyperopic eyes were evaluated. Mean spherical equivalent refractive error before cycloplegia was -3.4 ± 2.6 D. A mean hyperopic shift of 0.4 ± 0.5 D occurred after cycloplegia (P < 0.001). The astigmatism power did not significantly change (P = 0.8), however, 26.8% of eyes with significant astigmatism experienced a change of more than 5 degrees in the axis of astigmatism. Changes in posterior corneal curvature were scant but statistically significant (P = 0.001). Moreover, corneal thickness was slightly increased in the central and paracentral regions (P < 0.001 and P < 0.001, respectively).Conclusion:Cycloplegia causes a hyperopic shift and astigmatism axis changes, along with an increase in central and paracentral corneal thickness and change in posterior corneal curvature. The effects of cycloplegia on refraction and corneal biometrics should be considered before cataract and refractive surgeries.

Posterior Corneal Curvature Assessment after Epi-LASIK for Myopia: Comparison of Orbscan II and Pentacam Imaging

PurposeTo compare the changes in posterior corneal curvature using scanning slit topography (Orbscan II) and Scheimpflug imaging (Pentacam) before and after Epi-laser in situ keratomileusis (LASIK) for myopia.MethodsIn a prospective observational case-series study, 20 myopic patients having undergone Epi-LASIK were examined serially with two different devices, Orbscan II and Pentacam, preoperatively and one month postoperatively. Posterior central elevation (PCE) and posterior maximal elevation (PME) were compared between the two devices, and the changes in parameters after Epi-LASIK were analyzed using a difference map.ResultsAll parameters (preoperative and postoperative PCE and preoperative and postoperative PME) that were measured using the Orbscan II were significantly greater compared to those of the Pentacam (for all p < 0.001). PCE and PME were significantly increased one month postoperatively in the Orbscan II measurements (p < 0.05) but were not significantly increased in the Pentacam measurements. Also, ΔPCE and ΔPME, in the difference map obtained by each serial scanning, were significantly greater in the Orbscan II measurements than with the Pentacam (p = 0.012, p = 0.016).ConclusionsThe Pentacam measurements displayed significantly reduced values in all parameters related to posterior corneal elevation compared to those of the Orbscan II. The Pentacam showed no significant change in posterior corneal curvature after Epi-LASIK, based on the difference map.

Stability of the Posterior Corneal Surface After Laser Surface Ablation for Myopia

The purpose of this study was to evaluate changes in the posterior corneal surface before and after laser surface ablation using the Pentacam. Thirty consecutive myopic eyes having laser surface ablation (14 photorefractive keratectomies, 16 laser-assisted subepithelial keratectomies) were evaluated preoperatively and postoperatively with the Pentacam. Changes in the posterior corneal curvature, posterior elevation, and anterior chamber depth were measured. The 30 eyes had a mean correction of -4.50 D (range, -2.12 to -8.0) and a mean ablation depth of 58.4 microm (range, 21-106 microm). The change in mean posterior corneal curvature was -0.02 +/- 0.07 mm in the central 4-mm zone (P = 0.106) and the mean posterior displacement was 0.47 +/- 4.23 microm (P = 0.551). The mean change in the anterior chamber depth was -0.06 +/- 0.08 mm (P < 0.001). The amount of forward shift of the posterior corneal surface had no correlation with the residual corneal thickness and ablation depth. There was no significant change in the posterior corneal surface after laser surface ablation.

Pentacam Assessment of Posterior Lamellar Grafts to Explain Hyperopization after Descemet's Stripping Automated Endothelial Keratoplasty

To evaluate changes in posterior corneal curvature as a possible cause of the hyperopic refractive shift observed after Descemet's stripping automated endothelial keratoplasty (DSAEK). Prospective, noncomparative, interventional case series. Thirty-four eyes of 29 patients with Fuchs' endothelial dystrophy or pseudophakic bullous keratopathy. A standard DSAEK procedure was performed in 34 eyes using the pull-through technique for graft delivery. When cataract was present (n = 7), phacoemulsification with posterior chamber intraocular lens implantation was combined. Each eye underwent Pentacam (Oculus, Wetzlar, Germany) evaluation 1, 3, and 12 months after surgery. Corneal graft thickness was calculated on Scheimpflug scans at 9 locations (1 central, 4 peripheral, and 4 mid peripheral). The mean radius of posterior corneal curvature (Rm) was recorded. At each postoperative examination time, manifest refraction was determined and compared with pre-DSAEK values in simple procedures or with intended postoperative refraction, if cataract surgery had been performed. Manifest refraction, Rm, and corneal graft thickness at 1, 3, and 12 months after surgery. The mean+/-standard deviation posterior corneal curvature was 6.5+/-0.56 mm before surgery and varied from 5.52+/-0.39 mm 1 month after surgery to 5.83+/-0.37 mm at 3 months after surgery and 5.92+/-0.35 mm at 12 months after surgery. The grafts were significantly thicker in the periphery and mid periphery than in the center at all examination times. Thickening diminished significantly over time at all locations. The average reduction of corneal thickness was higher at the edges (91.5 microm) than in the mid periphery (38.3 microm) or in the center (24.2 microm). The average postoperative spherical equivalent+/-standard deviation changed from -0.31+/-2.35 diopters (D) before surgery to 1.03+/-2.21 D 1 month after surgery, 0.61+/-2.07 D 3 months after surgery, and +0.31+/-2.03 D 12 months after surgery. The difference in thickness between center and periphery of the DSAEK graft induces a change in posterior corneal curvature, resulting in a hyperopic shift that decreases with time and is negligible for spectacle correction. However, when performing a triple procedure, intraocular lens selection should take into account the refractive change induced by DSAEK. Proprietary or commercial disclosure may be found after the references.

Keratometry in Epikeratophakia

ABSTRACT: In keratomileusis and radial keratotomy, keratometry does not accurately reflect the change in refraction produced by the surgery. The authors report that this also occurs in patients undergoing epikeratophakia. The keratometer underestimates the clinically observed changes in refractive error, and changes in keratometry correlate poorly with changes in refraction. The reason for this lies partly in the method by which the keratometer estimates corneal refractive power. The authors attempted to improve the correlation between keratometry and refractive error by applying mathematical corrections to the keratometer measurements. The accuracy of these corrections was examined in three groups of patients who underwent epikeratophakia procedures: aphakic children, aphakic adults, and myopic adults. One correction, ΔD 1 , improved the estimation of change in refractive error in all three groups, while another, ΔDc, did so for two of the three groups. Neither correction significantly improved the correlation between the change in keratometry and the change in refractive error. The persistently poor correlation may be related to the irregularity of the corneal surface, the limited area of measurement by the keratometer, changes in refractive index, and changes in posterior corneal curvature. In addition, errors may occur in adjustment for vertex distance, or performing and recording measurements.