Comparison of Anterior, Posterior, and Total Corneal Astigmatism Measured Using a Single Scheimpflug Camera in Healthy and Keratoconus Eyes

High-Fluence Epithelium-off Accelerated Pulsed Corneal Cross-linking (15 mW/cm2; 7.2 J/cm2) for Pediatric Keratoconus: A 3-Year Retrospective Analysis

PURPOSE

To assess the safety and efficacy of treatment and secondarily determine the topographic changes, visual outcomes, and demarcation line depth after high-fluence pulsed light accelerated cross-linking (ACXL) in pediatric patients (younger than 18 years) with progressive keratoconus.

METHODS

This retrospective analysis included 32 eyes (25 children, aged 11 to 18 years), with progressive keratoconus treated with high-energy epithelium-off pulsed light ACXL (7.2 J/cm2, 15 mW/cm2, 12 minutes, 2 seconds on/1 second off). Corrected distance visual acuity (CDVA), Scheimpflug tomography, and anterior optical coherence tomography measurements were recorded preoperatively and 1, 2, and 3 years postoperatively.

RESULTS

A total of 32 eyes were included. Significant CDVA improvement, pachymetry, and maximum keratometry reduction were found at all follow-up visits. Mean keratometric values remained stable, and astigmatism showed a mild worsening (< 0.25 D) with statistical significance at 1 and 3 years. Total aberration showed discordant results and coma aberration had a slight improvement without statistical significance. The demarcation line depth was 265 ± 26 μm. Three patients developed mild haze without visual acuity loss. None of the patients underwent a second CXL procedure.

CONCLUSIONS

In pediatric patients, high-fluence epithelium-off pulsed light ACXL appears to be a safe and effective procedure to halt the progression of keratoconus, slightly improving the CDVA and keratometric values. [J Refract Surg. 2024;40(3):e148-e155.].

Intraocular Lens Power Calculations in Keratoconus Eyes Comparing Keratometry, Total Keratometry, and Newer Formulae

PURPOSE

To compare the utility of keratometry vs total keratometry (TK) for intraocular lens power calculations in eyes with keratoconus (KCN) using KCN and non-KCN formulae.

DESIGN

Retrospective cohort study.

METHODS

This study was conducted at 2 academic centers and included 87 eyes in 67 patients who underwent cataract surgery between 2019 and 2021. Biometry measurements were obtained using a swept-source optical coherence tomography biometer (IOL Master 700). Refractive prediction errors, including root mean square error (RMSE), were calculated for 13 formulae. These included 4 classical formulae (Haigis, Hoffer Q, Holladay 1 [H1], and SRK/T), 5 new formulae (NF) (Barrett Universal II [BU2], Cooke K6, EVO 2.0, Kane, and Pearl-DGS), 3 KCN formulae (BU2 KCN: M-PCA, BU2 KCN: P-PCA, and Kane KCN), and H1 with equivalent keratometry reading values (H1-EKR). Formulae were ranked by RMSE. Friedman analysis of variance with post hoc analysis and H-testing was used for statistical significance testing.

RESULTS

KCN formulae had the lowest RMSEs in all eyes, and BU2 KCN:M-PCA performed the best among KCN formulae in all subgroups. In eyes with severe KCN, if TK values are unavailable, the BU2 KCN: P-PCA performed better than the top-ranked non-KCN formula (SRK/T). In eyes with nonsevere KCN, if TK values are unavailable, EVO 2.0 K was statistically superior to the next competitor (Kane K). H1-EKR had the highest RMSE.

CONCLUSIONS

KCN formulae and TK are useful for intraocular lens power calculations in KCN eyes, especially in eyes with severe KCN. The BU2 KCN: M-PCA using TK values performed best for eyes with all severities of KCN. For eyes with nonsevere KCN, the EVO 2.0 TK or K can also be used.

Distribution and Characteristics of Ocular Biometric Parameters among a Chinese Population: A Hospital-Based Study

INTRODUCTION

This study aimed to describe the distribution and characteristics of ocular biometric parameters among a large Chinese population.

METHODS

This retrospective cross-sectional study included 146,748 subjects whose ocular biometric parameters were measured at the ophthalmology clinic of West China Hospital, Sichuan University, and recorded in the hospital database. Ocular biometric parameters, including axial length, anterior chamber depth, corneal keratometry, and keratometric astigmatism, were recorded. Only monocular data for each subject were analyzed to avoid bias.

RESULTS

Valid data from 85,770 subjects (43,552 females and 42,218 males) aged 3-114 years were included in this study. The mean axial length, mean anterior chamber depth, average corneal keratometry, and mean keratometric astigmatism were 24.61 mm, 3.30 mm, 43.76 D, and 1.19 D, respectively. The stratification of the ocular parameters by age and gender showed significant inter-gender and inter-age differences.

CONCLUSIONS

Analysis of a large population of subjects in western China aged 3-114 years showed that the distribution and characteristics of ocular biometric parameters, including axial length, anterior chamber depth, corneal keratometry, and keratometric astigmatism, differed by age and gender. This study is the first to describe ocular biometric parameters in subjects aged > 100 years.

Spotlight on the Corneal Back Surface Astigmatism: A Review

Recent evidence indicates that the corneal back surface astigmatism (CBSA) contributes to the refractive state of the eye in cataract surgery, especially with the implantation of toric intraocular lenses. But this has been met with some scepticism. A review of key studies performed over the past three decades shows that the mean CBSA power ranges from 0.18(±0.16)D to 1.04(±0.20)D. The clinical assessment of CBSA is problematic. There is poor agreement between the current automated systems for assessment of CBSA and it is assumed that these systems directly measure the CBSA. But CBSA cannot be measured directly in vivo. A historical review of methods used to quantify the curvature of the posterior corneal surface reveals that CBSA estimated by current systems is based on values for corneal front surface astigmatism, corneal refractive index, central corneal thickness, corneal thickness at peripheral locations and the exact distance between the corneal apex and each one of these peripheral locations. Doubts and errors in these values, coupled with the precise details of the algorithm incorporated to estimate CBSA, are the likely sources of the lack of agreement between current systems. These systematic errors cloud the assessment of CBSA. Mean CBSA may be low, but it varies from case to case. There is a clear need for a realistic, practical procedure for clinicians to independently calibrate systems for estimating CBSA. This would help to reduce uncertainty and the discrepancies between instruments designed to measure the same parameter.

The influence of routine uncomplicated phacoemulsification on the orthogonality of the cornea

Purpose:

The aim of this study was to determine the effect of routine uncomplicated phacoemulsification on the orthogonal distribution of mass within the central optical zone of the cornea.

Methods:

Astigmatism at both corneal surfaces was evaluated using Orbscan II (Bausch &and Lomb) before and up to 3 months after routine phacoemulsification (one eye/patient). The data were subjected to vector analysis to estimate the pre-and postoperative total astigmatism of the cornea (TCA).

Results:

Reporting the chief findings in minus cylinder (diopters, DC) over the central 3 mm (A) and 5 mm (B) optical zones. Mean TCA powers (±sd) at pre- and 3-months postop were A) –4.45DC (±2.00) and –5.69DC (±2.69), B) –2.91DC (±2.22) and –2.71DC (±1.60). Change in mean power was significant over 3 mm (P < 0.01, n = 49) but not over 5 mm. Inter-zonal differences were significant (P < 0.01). There was a significant linear relationship between the change in TCA power (y = preoperative-postoperative) and TCA at preoperative stage (x) where, A) y = 0.45x + 3.12 (r = 0.336, n = 49, P = 0.018), B) y = x + 2.65 (r = 0.753, n = 49, P = <0.01). Over the central 3 mm zone only, change (preoperative-postoperative) in axis (°) of TCA (y₁) was significantly associated with TCA axis at preoperative stage (x₁) where y₁ = 1.391x₁-0.008x₁²-0.701 (r = 0.635, n = 49, P < 0.01).

Conclusion:

Changes in TCA power and axis at 3 months postop, determined using Orbscan II, are indicative of orthogonal alterations in the distribution of corneal tissue. Over the central 3 mm zone, the association between y₁ and x₁ shows that a change in TCA axis is more profound when preoperative axis is near 90° i.e., against-the-rule.

Predicted and Measured Changes in Posterior Corneal Astigmatism after Uncomplicated Femtosecond Assisted LASIK (FsLASIK) and Microkeratome LASIK Correction for Myopia and Low Astigmatism

Purpose: To compare predicted and measured changes in astigmatism at the posterior corneal surface (PCS) after FsLASIK or LASIK.Methods: Astigmatism was measured at both corneal surfaces (Pentacam^TM) before and 3 months after unremarkable FFsLASIK (roup 1,n = 100) or LLASIK (roup 2,n = 100) for myopia (-7.25DS to -0.75DS) and low astigmatism (≤1.00DC). Photoablation was achieved using Schwind Amaris750S^TM laser (Aberration Free profile, centered on corneal vertex). Pre-and postop astigmatic data, according to subjective refraction and estimates for the corneal surfaces (over the central 3.2 mm zone), were subjected to vector analysis to calculate surgically induced astigmatism (SIA) by refraction (SIA_R), at the anterior (SIA_Fact) and posterior corneal surfaces (SIA_Bact). The difference vector between SIA_R and SIA_Fact was regarded as the predicted SIA at the PCS (SIA_Best).Results: Reporting key findings. Mean(±sd,95%CI) SIA_Best and SIA_Bact powers in group 1 were -0.52DC(±0.35,-0.56 to -0.45) and -0.11DC(±0.08,-0.13 to -0.10) in group 1, -0.35DC(0.20,-0.39 to -0.32) and -0.08DC(0.07,-0.09 to -0.06) in group 2. Differences between SIA_Best and SIA_Bact were significant for powers but not axes. Significant correlations(p < .01) were revealed between (I) SIA_R and SIA_Fact powers [Group 1, SIA_R = 0.370.SIA_Fact-0.292,r = 0.299. Group 2, SIA_R = 0.484.SIA_Fact-0.394,r = 0.519] but not the axes and (II) ΔC (difference between pre-[x1] and postop measured PCS astigmatic powers) and x1 [Group 1, ΔC = 0.384x1 + 0.119,r = 0.423. Group 2, ΔC = 0.135x1 + 0.047,r = 0.229,p = .022]. There was no correlation between SIA_Best and SIA_Bact powers or axes.Conclusion: The changes in posterior corneal astigmatic powers according to Pentacam measurements are small and do not account for the deficit between SIA_R and SIA_Fact after FsLASIK or LASIK.

Characterization of posterior corneal astigmatism in a population with keratoconus

Background: The curvature of the anterior corneal surface is traditionally used as a surrogate to estimate corneal astigmatism. In recent years, increasing importance has been attributed to posterior corneal astigmatism as an indicator. Our aim is to characterize the posterior corneal surface in a population with keratoconus and investigate its predictive value to keratoconus progression. Methods: Retrospective study from a tertiary care centre (Hospital de Santa Maria, Lisbon, Portugal). Eighty-five patients (85 eyes) with keratoconus were included. All patients had two tomographical examinations ≥12 months apart (Pentacam HR). Vector analysis was used to calculate anterior (ACA), posterior (PCA), and total corneal astigmatism (TCA). Multivariate logistic regression was used to assess the predictive value of PCA to keratoconus progression, adjusting for ACA, TCA and several tomographical indices. Results: Study participants had a mean age of 32 (SD = 12.5) years. Mean tomographical keratoconus classification was 2.16 (SD = 0.95), with a mean Kmax of 55.8D (SD = 7.8). Mean power of PCA, ACA and TCA was, respectively, -0.88D (SD = 0.84), 3.74D (SD = 2.36), and 3.06D (SD = 2.01) and its centroids were 0.44D x 15º, 1.65D x 112º, and 1.61D x 106º, respectively. The power of PCA was ≥0.50, 1.00 and 2.00D in 75.3%, 32.9%, and 3.5% of patients, respectively, inducing against-the-rule astigmatism in 60.0% of patients. On average, ACA overestimated TCA in 0.35D x 151º (p < .01). ACA and TCA were highly correlated but showed a lack of agreement for clinical purposes. A predictive role for PCA was excluded. Conclusions: In this population with keratoconus, PCA contributed substantially to TCA. However, PCA was not a valuable predictor for disease progression.