The effect of corneal anterior surface eccentricity on astigmatism after cataract surgery

Three-year change in corneal asphericity in children at the age of emmetropisation

PURPOSE

To determine the 3-year changes in anterior and posterior values of corneal asphericity (Q) in 6- to 12-year-old children.

METHODS

The first and second phases of the study were conducted in 2015 and 2018, respectively. The target population in the first phase was 6- to 12-year-old students in Shahroud, Iran. Multistage cluster sampling was performed on urban students. Additionally, all rural students in Shahroud county were invited to participate. Corneal imaging and ocular biometry were performed using the Pentacam-HR and Allegro Biograph, respectively.

RESULTS

A total of 4961 right eyes were analysed in this report. The mean (95% confidence interval) anterior and posterior Q values for an 8-mm chord diameter were -0.38 ± 0.11 (-0.39 to -0.38) and -0.32 ± 0.12 (-0.33 to -0.32) in the first phase of the study, respectively. The mean 3-year changes in anterior and posterior Q values were 0.00 ± 0.08 (95% CI: 0.00-0.00) and -0.01 ± 0.06 (95% CI: -0.01 to -0.01), respectively. Based on a multiple regression model, advancing age (β = -0.002; [-0.003 to -0.001]), 3-year increase in mean keratometry (β = -0.04; [-0.06 to -0.02]), central corneal thickness (β = -0.001; [-0.001 to -0.000]), lens thickness (-0.05; [-0.11 to -0.003]) and anterior chamber depth (-0.09; [-0.15 to -0.03]) showed a statistically significant association with increased anterior Q prolation. Increased axial length was associated with a decrease in Q (β = 0.02; [0.004 to 0.03]). In addition, female gender (β = -0.004; [-0.007 to -0.001]) was significantly associated with a more prolate posterior Q value, while rural residence (β = 0.005; [0.002 to 0.009]) was significantly associated with a more oblate posterior Q value.

CONCLUSION

Three-year changes in anterior and posterior Q values were very small. The cornea tends to become more prolate with increasing age.

The normal distribution of corneal eccentricity and its determinants in two rural areas of north and south of Iran

Purpose

The aim of this study was to determine the distribution of corneal eccentricity (E-value) in a normal population and to examine related factors.

Methods

In this cross-sectional study, two villages were selected in Iran using multistage cluster sampling. Selected persons were invited to have a comprehensive eye examination. Examinations in each village were performed at a specific location under standard conditions. After testing for vision and refraction and conducting the slit-lamp exam, E-value was measured with Pentacam.

Results

Of the 3851 selected individuals, 3314 participated in the study. After applying the exclusion criteria, data from 2610 subjects was used in the analysis for this report. Mean E-value was 0.53 [95% confidence interval (CI): 0.52 to 0.54]. E-value was not significantly different between males and females. Mean E-value reduced with age from 0.60 in subjects aged 6–20 years to 0.47 in subjects older than 70 years. The hyperopic group of participants had significantly lower E-value than myopic and emmetropic ones (P < 0.001). The relationship of E-value with age, gender, and other anterior segment variables and spherical equivalent was examined in a multiple linear regression model. In multiple linear regression model, age (coef = −0.003), spherical equivalent refraction (coef = −0.005), pupil diameter (coef = 0.018), anterior chamber volume (coef = −0.001), and anterior chamber angle (coef = 0.003) significantly correlated with E-value.

Conclusions

The results of this study showed that the cornea in normal populations is prolate, and the degree of prolateness varies by age, such that older age is associated with a less prolate cornea. This study showed that factors such as age and refractive errors and anterior chamber indices influence the E-value.

Factors Influencing Efficacy of Peripheral Corneal Relaxing Incisions during Cataract Surgery

Purpose. To evaluate influencing factors on the residual astigmatism after performing peripheral corneal relaxing incisions (PCRIs) during cataract surgery. Methods. This prospective study included patients who were scheduled for cataract surgery with PCRIs. Optical biometry (IOLMaster 500, Carl Zeiss Meditec AG, Germany) was taken preoperatively, 1 week, 4 months, and 1 year postoperatively. Additionally, corneal topography (Atlas model 9000, Carl Zeiss Meditec AG, Germany), ORA (Ocular Response Analyzer, Reichert Ophthalmic Instruments, USA), and autorefraction (Autorefractometer RM 8800 Topcon) were performed postoperatively. Results. Mean age of the study population (n = 74) was 73.5 years (±9.3; range: 53 to 90) and mean corneal astigmatism preoperatively was -1.82 D (±0.59; 1.00 to 4.50). Mean corneal astigmatism was reduced to 1.14 D (±0.67; 0.11 to 3.89) 4 months postoperatively. A partial least squares regression showed that a high eccentricity of the cornea, a large deviation between keratometry and topography, and a high preoperative astigmatism resulted in a larger postoperative error concerning astigmatism. Conclusions. PCRI causes a reduction of preoperative astigmatism, though the prediction is difficult but several factors were found to be a relevant source of error.

Predicting postoperative astigmatism using Scheimpflug keratometry (Pentacam) and automated keratometry (IOLMaster)

PURPOSE

To compare keratometry measurements obtained using an automated keratometer (AK, IOLMaster) and a Scheimpflug keratometer (Pentacam) in predicting residual astigmatism after cataract surgery.

METHODS

Preoperative corneal astigmatism was calculated using preoperative refraction, an AK and a Scheimpflug keratometer (anterior corneal power [ACP] and true net power [TNP]) in 155 eyes of 107 Asian subjects. Phacoemulsification cataract removal and nontoric intraocular lens insertion (Akreos(®)MI-60™, Bausch & Lomb, Rochester, NY, USA) were performed through a 2.8 mm temporal clear corneal incision. Six months later, postoperative astigmatism (postA) was calculated using manifest refraction. Error angle (EA) and error magnitude (EM) of above keratometries (AK, ACP, and TNP) in prediction of postA was calculated. The correlation between preoperative astigmatism and postA was analyzed using power vectors (J(0) and J(45)).

RESULTS

AK resulted in the lowest EM and ACP resulted in the lowest EA. Preoperative astigmatism (preA) measured using AK, ACP, and TNP showed significant correlation with postA in both J(0) and J(45) components. (AK: rJ(0) = 0.554, rJ(45) = 0.559, ACP: rJ(0) = 0.346, rJ(45) = 0.281, TNP: rJ(0) = 0.409, rJ(45) = 0.231). Preoperative refractive astigmatism showed no significant correlation in any components.

CONCLUSIONS

PreA determined using AK showed superior performance in prediction of postA than ACP and TNP.