Changes In Posterior Corneal Surface Research Articles

Changes in posterior corneal elevation after small incision lenticule extraction for different myopic diopters.

To study the changes and effect factors of posterior corneal surface after small incision lenticule extraction (SMILE) with different myopic diopters. Ninety eyes of 90 patients who underwent SMILE were included in this retrospective study. Patients were allocated into three groups based on the preoperative spherical equivalent (SE): low myopia (SE≥-3.00 D), moderate myopia (-3.00 D>SE>-6.00 D) and high myopia (SE≤-6.00 D). Posterior corneal surfaces were measured by a Scheimpflug camera preoperatively and different postoperative times (1wk, 1, 3, 6mo, and 1y). Posterior mean elevation (PME) at 25 predetermined points of 3 concentric circles (2-, 4-, and 6-mm diameter) above the best fit sphere was analyzed. All surgeries were completed uneventfully and no ectasia was found through the observation. The difference of myopia group was significant at the 2-mm ring at 1 and 3mo postoperatively (1mo: P=0.017; 3mo: P=0.018). The effect of time on ΔPME was statistically significant (2-mm ring: P=0.001; 4-mm ring: P<0.001; 6-mm ring: P<0.001). The effect of different corneal locations on ΔPME was significant except 1wk postoperatively (1mo: P=0.000; 3mo: P=0.000; 6mo: P=0.001; 1y: P=0.001). Posterior corneal stability was linearly correlated with SE, central corneal thickness, ablation depth, residual bed thickness, percent ablation depth and percent stromal bed thickness. The posterior corneal surface changes dynamically after SMILE. No protrusion is observed on the posterior corneal surface in patients with different degrees of myopia within one year after surgery. SMILE has good stability, accuracy, safety and predictability.

Factors associated with changes in posterior corneal surface following laser-assisted in situ keratomileusis.

To identify factors associated with changes in the posterior corneal curvature following laser-assisted in situ keratomileusis (LASIK). This retrospective study included myopic astigmatic eyes that underwent LASIK between January and December 2013 at Care-Vision Laser Center, Tel-Aviv, Israel. The average posterior keratometry was measured with the Sirius device at a radius of 3mm from the center. The correlations between the surgically induced change in average posterior keratometry and preoperative parameters such as preoperative sphere, cylinder, spherical equivalent, central corneal thickness (CCT), refraction, Baiocchi Calossi Versaci (BCV) index, ablation depth, percent of tissue altered (PTA), and residual stromal bed (RSB) are reported. A total of 115 eyes with a mean age of 32.5 ± 8.3years (range 22-56years) were included. Central corneal thickness (p < 0.005), preoperative sphere (p < 0.001), spherical equivalent (p < 0.005), and preoperative posterior inferior/superior ratio (p < 0.05) were all significantly correlated with the percentage of change in the mean posterior K. According to ranked stepwise multiple regression analysis, 22% of the variance of change in posterior K could be explained by the examined factors. The factors that remained significant were the percentage of change in posterior inferior/superior ratio, preoperative subjective sphere, and preoperative mean posterior K (for all, p < 0.001). The percentage of change in posterior inferior/superior ratio, subjective sphere, and preoperative mean posterior K are all correlated with change in the mean posterior K after LASIK. Understanding of the variables that can influence posterior corneal changes following refractive surgery may play a role in the prevention of iatrogenic keratectasia.

Changes in the posterior corneal surface after femtosecond laser-assisted lenticule intrastromal keratoplasty (LIKE) performed into a pocket (SMI-LIKE) or under a flap (FS-LIKE)

BackgroundTo compare the changes in posterior corneal surface after small-incision lenticule intrastromal keratoplasty (SMI-LIKE) and femtosecond laser-assisted lenticule intrastromal keratoplasty (FS-LIKE) for hyperopia correction.MethodsIn this prospective comparative randomized study, 23 eyes with hyperopia were recruited. Eyes were categorized into two groups—SMI-LIKE group (11 eyes) and FS-LIKE group (12 eyes). Lenticules from myopia small incision lenticule extraction were implanted into a pocket (SMI-LIKE group) or at a depth of 100 µm under a flap (FS-LIKE group). Posterior corneal elevations in the center, mid-periphery, and periphery, as well as mean keratometry of the posterior corneal surface (Kmb) were measured using a Pentacam over a three-month follow-up.ResultsAll surgeries were completed successfully and no complications occurred. At one day postoperatively, there was a slight backward change with SMI-LIKE and a forward change with FS-LIKE in the central region of the posterior corneal elevation. Conversely, the peripheral area showed forward displacement in SMI-LIKE and an apparent backward change in FS-LIKE. The mid-peripheral regions manifested a backward change after the procedure throughout the entire follow-up in both groups. Kmb exhibited flattening at one month postoperatively and subsequently returned to its original level at three months after SMI-LIKE while in FS-LIKE, Kmb steepened after lenticule implantation with a significant change noted at one day postoperatively (P = 0.001).ConclusionsPosterior corneal surface after SMI-LIKE and FS-LIKE exhibited different change patterns in various corneal regions, with the most prominent change occurring at one day postoperatively during the three-month follow-up.Trial registration: Chinese Clinical Trial Registry: ChiCTR-ONC-16008300. Registered on Apr 18th, 2016. http://www.chictr.org.cn/edit.aspx?pid=14090&htm=4

Posterior corneal elevation changes during 12 month of overnight orthokeratology

Aimsthe aim of this study to investigate the elevation changes in posterior corneal surface after 12 months of orthokeratology (ortho-k) treatment. MethodsIn this retrospective chart review, medical records of 37 Chinese children who wore ortho-k lenses over 12 months were reviewed. The data of only right eye were analyzed. Variables including the flat and steep keratometry of anterior and posterior corneal principal meridians, central corneal thickness (CCT), posterior thinnest elevation of cornea (PTE), posterior central elevation of cornea (PCE) and posterior mean elevation of cornea (PME) were measured by Pentacam. Variables including anterior chamber depth (ACD), lens thickness (CLT) and ocular axis length (AL) were measured by optical biometry. All variables differences between baseline and 12 months after ortho-k treatment were assessed by statistical analyses. ResultsThe average age of all subjects was 10.70 ± 1.75 years (range 8–15 years old). The baseline spherical equivalent (SE) was −3.26 ± 1.52 D (−0.50D to −5.00D). Both flat and steep keratometry of anterior corneal surface and CCT were significantly decreased after 12 month follow up during ortho-k treatment (both P < 0.000). Both flat and steep keratometry of posterior corneal surface were not significantly different after 12 month follow up compared with that of baseline (P = 0.426, 0.134 respectively). PCE, PTE and PME were not significantly changed over 12 months of ortho-k treatment (P = 0.051, 0.952 and 0.197 respectively). The ACD was significantly decreased in 12 month follow up during ortho-k treatment (P = 0.001). The CLT and the AL were significantly increased during this period (both P < 0.000). ConclusionAlthough the anterior corneal surface was significantly changed by ortho-k lens, the posterior corneal surface did not show any changes during 12 months follow up. Simultaneously, The ACD, CLT and AL were significantly changed during this period.

Effect of conventional and orthokeratological hard contact lenses on anatomical and functional characteristics of the cornea

This study evaluated the potential effects of conventional hard contact lenses (HCL) and orthokeratological contact lenses (OKL) on anatomical and functional condition of the cornea in myopia correction. The study included 80 patients (159 eyes) with low, moderate and high myopia. Group 1 consisted of 40 patients (79 eyes) who used conventional HCL; group 2 included 40 patients (80 eyes) who used OKL. All patients underwent topographic keratometry, corneal pachymetry mapping (Pentacam, Oculus, Germany; Visante OCT, Carl Zeiss, Germany), high-order aberrometry (OPD Scan, Nidek, Japan), confocal microscopy (Confoscan 4, Nidek, Japan), evaluation of corneal biomechanical properties (ORA, Reichert Technologies, USA) and corneal light scattering (Pentacam, Oculus, Germany). Examinations were taken before the treatment started and after 8-12 months of contact lens wear. Group 1 showed corneal thickness increase in all zones (by 0.9-1.1%), no significant changes in corneal refraction, and increase in corneal hysteresis and corneal resistance factor. OKL wear led to a significant flattening of anterior surface of the cornea and steepening of its paracentral area, decrease in central corneal thickness (by 2.8%) and increase in mid-peripheral thickness (by 2.2%). No significant changes in posterior corneal surface were observed in the study patients. More significant increase in higher-order aberrations and the degree of light scattering was observed in group 2, while changes in subepithelial nerve plexus and stromal structure were more significant in group 1. Long-term OKL wear shows more significant morphological and functional corneal changes compared to RGP lens.

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.].

Two-year stability of posterior corneal surface after transepithelial photorefractive keratectomy with a residual stromal thickness less than 350μm.

This study aimed to investigate the stability of posterior corneal surface 2years after transepithelial photorefractive keratectomy (TPRK) in patients with a residual stromal thickness less than 350μm. In total, 408 eyes of 212 patients (160 women, 52 men) who underwent TPRK were enrolled in this retrospective study. All surgeries were performed in the Amaris 750S excimer laser platform with smart pulse technology. The posterior corneal elevation, anterior chamber depth, Q value, and curvature were measured using Pentacam preoperatively and postoperatively. All patients were followed up for 2years. The relationship between percent tissue altered (PTA), age, and changes in posterior corneal surface was analyzed. The mean preoperative spherical equivalent was - 6.80 ± 1.18 D (range: - 9.00 to - 2.63 D). The mean residual stromal thickness was 336.46 ± 7.25μm (range: 310-348μm). The mean PTA was 30.93 ± 2.03% (range: 24.29-35.28%). At 2years after surgery, the elevation of six points in the central area decreased by 1.91 ± 2.97μm, 2.98 ± 3.23μm, 1.17 ± 3.85μm, 1.70 ± 2.88μm, 1.36 ± 3.19μm, and 1.65 ± 3.18μm, compared with the preoperative value (P < 0.05). The elevation of three points in the peripheral area increased by 1.87 ± 6.34μm, 0.68 ± 6.00μm, and 0.95 ± 5.50μm (P < 0.05). There was no significant linear relationship between PTA, age, and changes in posterior corneal surface, anterior chamber depth, and K2 (all P > 0.05). Within 2years after TPRK, the posterior corneal surface remained stable in patients with a residual stromal thickness between 310 and 350μm. There was no sign of iatrogenic ectasia during the follow-up period.

Keratoconus: Diagnosis and Staging.

The advent of refractive surgery and corneal cross-linking for ectatic disease has further highlighted the need to recognize early or subclinical ectatic disease. Historical systems depend on anterior corneal changes that occur late in the disease course and are commonly associated with visual loss. Tomographic imaging allows for the acquisition of posterior corneal surface data and corneal thickness distribution. This led to the development of modalities to diagnose early or subclinical keratoconus and to screen patients for refractive surgery.This article reviews the modern methods for assessing changes in posterior corneal surface and pachymetric distribution, now accepted by the major cornea societies to be the hallmarks of ectatic disease. Screening tools utilized by the commonly used tomographic imaging devices are discussed, and the difference between screening for ectasia and diagnosing keratoconus is highlighted. The Belin ABCD staging and classification system and the ABCD Progression Display are reviewed as a new grading and monitoring system that can be used for earlier intervention and prevention of visual loss in keratoconus.

Factors associated with changes in posterior corneal surface following photorefractive keratectomy.

To identify factors associated with changes in the posterior cornea curvature following laser refractive surgery. This retrospective study included myopic astigmatic eyes that underwent PRK between January 2013 and December 2013 at Care-Vision Laser Centers, Tel-Aviv, Israel. The average posterior K was measured with the Sirius device at a radius of 3mm from the center. The correlations between the surgical induced change in average posterior k and preoperative parameters such as central corneal thickness (CCT), refraction, Baiocchi Calossi Versaci index (BCV), ablation depth, percent tissue altered (PTA), and residual stroma bed (RSB) were analyzed. A total of 280 eyes with a mean age of 24.9 ± 6.1years (range, 18-47years were included in this study. The mean PTA was 14.8 ± 6.0%. A greater change in posterior K was found in females (p = 0.01), smaller treatment zones of 6.0mm (p = 0.02) and PTA > 20% (p < 0.001). A lower CCT (r = - 0.24, p < 0.001), higher myopia (r = - 0.34, p < 0.001), higher astigmatism (r = - 0.17, p < 0.001), higher total BCV (r = 0.13, p = 0.03), lower back BCV (r = - 0.12, p = 0.05), higher front BCV (r = 0.16, p = 0.01), higher posterior I-S ratio (r = 0.16, p = 0.01), and a lower RSB (r = - 0.42, p < 0.001) were all significantly correlated with percentage of change in mean posterior K. In ranked stepwise multiple regression analysis, 26.2% of the variance of change in posterior K could be explained by the examined factors. The factors that remained significant were PTA (p < 0.001), CCT (p = 0.001), and posterior I-S ratio (p = 0.001). PTA alone accounted for 15% of the variance in posterior K changes in multivariate analysis. Understanding of factors affecting a change in posterior cornea after refractive surgery may have an important practical value for the prevention of iatrogenic keratectasia. Preoperative CCT, posterior I-S ratio, and PTA were significantly associated with changes in posterior K after PRK. PTA was the strongest predictor of posterior corneal changes (p < 0.001).

Correction of Moderate to High Hyperopia With Implantation of an Allogeneic Refractive Lenticule.

To describe the visual outcomes of correcting moderate to high hyperopia after two different surgical procedures of lenticular implantation. This prospective small case series study used epikeratophakia combined with transepithelial phototherapeutic keratectomy (PTK-EP) and femtosecond laser-assisted lenticule intrastromal keratoplasty (LIKE) using myopic small incision lenticule extraction (SMILE)-derived refractive lenticule for correcting hyperopia ranging from +3.00 to +10.00 diopters (D). The implanted refractive lenticule was matched with the recipient's manifest refraction. The visual, refractive, corneal tomographic, and epithelial thickness outcomes were analyzed after surgery. Four eyes underwent PTK-EP and 6 eyes underwent LIKE. At the last postoperative visit, 6 of 10 eyes had uncorrected distance visual acuity equal to or better than the preoperative corrected distance visual acuity (CDVA). No eyes lost one line or more of CDVA postoperatively. Postoperative spherical equivalent was within ±0.50 D of the target for 9 of 10 eyes. Posterior corneal curvature slightly steepened for PTK-EP and LIKE. Postoperative epithelial thickness demonstrated the expected doughnut pattern in both groups characterized by epithelium in the central zone (5 mm) thinner than that in the peripheral zone (5 to 7 mm). Visual outcomes, refraction, posterior corneal surface changes, and epithelial remodeling after PTKEP or LIKE show a potentially useful modality for correcting moderate to high hyperopia. [J Refract Surg. 2020;36(11):772-779.].

Comparison between Posterior Corneal Surface Changes after LASIK Measured by Sirius Pentacam and Orbscan IIz

AbstractPurpose: To investigate the reliability of Sirius Pentacam and OrbscanIIz in evaluating posterior corneal surface changes after LASIK.Methods: This prospective observational case series study was conducted on thirty eyes of 15 patients underwent LASIK for myopia with or without myopic astigmatism at both the Ophthalmology Department of Tanta Universal Teaching Hospital and Nour El-Ein Hospital. OrbscanIIz and Sirius Pentacam examination (CSO, Firenze, Italy) were performed after LASIK to study their reliability in evaluating posterior corneal surface changes after LASIK.Results: The mean elevation of the highest points on posterior corneal elevation maps after LASIK by Orbscan and Sirius was 35.47±11.6mm (range from 14mm to 49mm) and 9±2.9mm (range from 5mm to 16mm) respectively with no statistically significant difference between the values measured by the two methods (p-value 0.06). Regarding the maximal highest point on posterior corneal elevation maps by Orbscan and Sirius was 50mm and 17mm respectively, the difference of means by Orbscan and Sirius was 14.53 and 8 respectively, the limits of agreement from –8.2 to 37.3 by Orbscan and 2.4 to 13.6 by Sirius respectively. The coefficient of repeatability was 36.20% by Orbscan and 16.60% Sirius, hence Orbscan is better than Sirius.Conclusions: There was no statistically significant differ-ence between the values measured by Orbscan IIz and Sirius (p-value 0.06). The coefficient of repeatability was 36.20% by Orbscan and 16.60% Sirius, hence Orbscan is better than Sirius. Surface irregularity has benefit to detect abnormal corneal topography with normal value about ±2 within 3mm zone or ±2.5 within 5mm zone. It is not present in Sirius but only Orbscan and this is one of advantages of Orbscan.

Biomechanical Modeling of Femtosecond Laser Keyhole Endokeratophakia Surgery

To apply a finite element model to endokeratophakia and evaluate anterior and posterior corneal surface changes. Spatial elevation data (Pentacam HR; Oculus, Wetzlar, Germany) were obtained for the front and back corneal surfaces of an eye prior to undergoing an endokeratophakia procedure. These were used to warp a spherical template finite element model of the cornea to create a patient-specific finite element mesh and the initial stress distribution was computed with an iterative approach. The finite element model (Optimeyes; Integrated Scientific Services, Biel, Switzerland) included non-linear elastic characteristics of the stroma. The endokeratophakia procedure was recreated in the model: a donor lenticule (-10.50 diopters [D], 5.75-mm zone, 127-µm thick) was inserted into a lamellar pocket (180-µm deep, 6.25-mm diameter) and two 2-mm small incisions were made at 150° and 330°. Anterior and posterior surfaces, computed by the finite element model, were compared to clinical data to assess accuracy and reliability of finite element modeling. The postoperative axial curvature produced by the model closely resembled the patient data; average curvature was 48.01 D clinically and 48.23 D in the simulation, and corneal astigmatism was 3.01 D clinically and 2.88 D in the simulation. The posterior best-fit sphere elevation map also matched the patient data, replicating inward bulging of the posterior surface by approximately 40 µm. Stress distribution modeling predicted a stress increase by 159.94% ± 73% in the cap and a stress decrease by 32.41% ± 21% in the stromal bed. Finite element modeling of the cornea reproduced the clinically observed anterior and posterior corneal surface changes following an endokeratophakia procedure. This case sets the stage for further study to refine and yield predictive finite element modeling for the evaluation of corneal refractive surgical procedures.

Assessment of the posterior corneal surface changes after LASIK treatment of myopia using Pentacam

Aim of the workThe aim of this work is to study the changes that occur in the posterior corneal surface after laser-assisted in-situ keratomileusis (LASIK) for correction of myopia using Pentacam.Patients and methodsIn a prospective interventional case-series study, 50 myopic eyes of 27 patients were subjected to the LASIK procedure. Preoperative, 1 week, and 3 months postoperatively, Pentacam examination was performed at the Elminia Roaa Refractive Center between January and October 2011. Pentacam was used for the assessment of preoperative and postoperative pachymetry, anterior and posterior corneal curvatures, anterior and posterior corneal elevations, and asphericity of the posterior corneal surface.ResultsThis study included 50 eyes of 27 patients, 19 women (70.3%) and eight men (29.7%). Patients' age ranged from 20 to 44 years, mean 27.3 ± 7.03 years. No statistically significant changes were found in the posterior horizontal corneal curvature (Rh). A statistically significant change in the posterior vertical corneal curvature (RV) from 6.1 ± 0.3 to 6.06 ± 0.2 mm 1 week after LASIK was found, but increased after 3 months to 6.2 ± 0.3S mm. No statistically significant changes in the posterior mean corneal curvature (Rm) were observed either at 1 week or 3 months after LASIK. A significant change in the Q-value (asphericity) was found both at 1 week and 3 months postoperatively. There were significant changes in the posterior corneal elevation both at 4 and at 7 mm from the center both at 1 week and 3 months after LASIK.ConclusionPost-LASIK changes of the posterior corneal surface affect mainly the vertical curvature, asphericity, and posterior corneal elevation. Fortunately, no case of post-LASIK ectasia was observed during the follow-up period of this study. Further studies are needed to document posterior corneal surface changes in patients with suspected post-LASIK ectasia.

Minor Influence of Sub-Bowman Keratomileusis on the Posterior Corneal Surface at Early Stage

Purpose: To evaluate the changes in posterior corneal surface (Q value and elevation) measured with Pentacam after femtosecond laser assisted Sub-Bowman Keratomileusis (SBK).Methods: The Q values and elevation of posterior corneal surface were determined in 166 myopic/myopic astigmatism eyes of 88 patients using Scheimpflug imaging (Pentacam; Oculus, Wetzlar, Germany), which estimates asphericity and elevation for several areas of cornea analysed (Q value: 6, 7, 8 and 9 mm; elevation: 2, 4, 6 and 8 mm) before 1 and 3 months after SBK. The correlations between the changes of Q value or elevation and the mean preoperative spherical equivalent (SE), central corneal thickness (CCT), central ablation depth (AD), estimated residual bed thickness (RBT) and RBT/CCT ratio were investigated.Results: Meridian and area differences in Q of posterior surface have been showed. Two major meridians (horizontal/vertical) present the change of significant negative direction before surgery (all p < 0.001), but the alteration of significant positive direction for all post-SBK follow-up visits (all p < 0.05). There was no statistically significant difference in posterior elevation at 2 mm zone (horizontal meridian: p = 0.439; vertical meridian: p = 0.233). Compared with preoperation, minor but significant forward displacements were found in posterior elevation at 4 and 6 mm areas of cornea analyzed (horizontal meridian: all p < 0.001; vertical meridian: p < 0.001, p = 0.024, respectively). However, posterior elevation in two meridians (horizontal/vertical) at 8 mm region was displayed significant backward shift (p = 0.001, p < 0.001, respectively). The Pearson correlation test showed no significant correlation between the changes in the Q values and elevation data in vast majority of areas of cornea analyzed and the SE, CCT, AD, RBT, and RBT/CCT ratio at 1 and 3 months after surgery (all p > 0.05).Conclusions: The posterior corneal surface showed central flattening and peripheral steepening at early stage post-SBK. To combine the asphericity with the elevation of the posterior corneal surface can overall and accurately understand the posterior corneal shape and its variations after refractive surgery.

The shape of the anterior and posterior surface of the aging human cornea

Purpose To determine the shape and astigmatism of the posterior corneal surface in a healthy population with age, using Scheimpflug photography corrected for distortion due to the geometry of the Scheimpflug imaging system and the refraction of the anterior corneal surface. Methods Scheimpflug imaging was used to measure in six meridians the cornea of the right eye of 114 subjects, ranging in age from 18 to 65 years. Results The average radius of the anterior corneal surface was 7.79 ± 0.27 (SD) mm and the average radius of the posterior corneal surface was 6.53 ± 0.25 (SD) mm. Both surfaces were found to be flatter horizontally than vertically. The cylindrical component of the posterior surface of 0.33 mm is twice that of the anterior surface (0.16 mm). The asphericity of both the anterior and the posterior surface was independent of the radius of curvature at the vertex, refractive error and gender. In contrast with that of the anterior corneal surface, the asphericity of the posterior corneal surface varied significantly between meridians. With age, the asphericity of both the anterior and the posterior corneal surface changes significantly, which results in a slight peripheral thinning of the cornea. Conclusion On average, the astigmatism of the posterior corneal surface (−0.305 D) compensates the astigmatism of the anterior corneal surface (0.99 D) with 31%. The results show that the effective refractive index is 1.329, which is lower than values commonly used. There is no correlation between the asphericity of the anterior and the posterior corneal surface. As a result, the shape of the anterior corneal surface provides no definitive basis for knowing the asphericity of the posterior surface.

Posterior corneal surface changes after hyperopic laser in situ keratomileusis

To evaluate posterior corneal surface topographic changes after hyperopic laser in situ keratomileusis (H-LASIK) using Orbscan I (Orbtek, Inc.). Department of Ophthalmology, Nara Medical University, Nara, Japan. In 25 eyes of 15 patients who had H-LASIK, the posterior corneal surface was measured with slit-scanning corneal topography (Orbscan I) preoperatively and 1 year postoperatively. The center as a fit zone and calculated posterior corneal surface changes were taken at 4 points: nasal, temporal, superior, and inferior sides in the 5.0 mm diameter. The posterior corneal topographic changes were analyzed using an analysis of variance. The postoperative:preoperative magnification ratio of the posterior corneal surface was calculated in a theoretical eye model. When a "+" reading was defined as the forward displacement and "-" was defined as the backward displacement, the mean posterior corneal topographic changes were -2.8 microm +/- 27.9 (SD) at the nasal side, -4.5 +/- 27.8 microm at the temporal side, -3.9 +/- 20.1 microm at the superior side, and -2.3 +/- 20.1 microm at the inferior side. The posterior corneal surface between any 2 examined points showed no significant difference after H-LASIK. In addition, the hypothetical change in the posterior cornea was -8.3 microm after +3.0 diopter H-LASIK, which was approximately closer to the study results. In each side, the amount of the attempted correction was significantly correlated with the posterior corneal topographic change. Clinical measurement of the posterior corneal displacement after H-LASIK with Orbscan revealed a backward shift. This change corresponded to the hypothetical artifactual changes with Orbscan; that is, changes in the magnification ratio.

Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography.

To obtain the shape of the posterior corneal surface in a healthy population, using Scheimpflug photography corrected for distortion due to the geometry of the Scheimpflug imaging system and the refraction of the anterior corneal surface. The posterior corneas of 83 subjects, ranging in age from 16 to 62 years, were measured in the vertical meridian using corrected Scheimpflug photography. The aspherical shape of the anterior Corneal surface was also determined in conjunction with the correction of Scheimpflug images. The average radius of the anterior corneal surface was 7.87 +/- 0.27 mm (SD), while the average radius of the posterior corneal surface was 6.40 +/- 0.28 mm (SD). The ratio between the posterior and the anterior radius of curvature was 0.81 +/- 0.02. The asphericity of the anterior and the posterior corneal surfaces was 0.82 +/- 0.18 and 0.62 +/- 0.27, respectively. The asphericity of the posterior corneal surface decreased significantly with age. The posterior/anterior asphericity ratio is also dependent on age and was 0.98 +/- 0.17 at 16 years of age and 0.53 +/- 0.30 at 62 years of age. Corrected Scheimpflug photography is an appropriate technique for measuring the radius and asphericity of the posterior corneal surface. The asphericity of the posterior corneal surface changes with age.