Relationship among Corneal Biomechanics, Anterior Segment Parameters, and Geometric Corneal Parameters

Purpose. To investigate the relationship between corneal biomechanical parameters, anterior segment parameters, and geometric corneal parameters in a healthy Caucasian group. Methods. This retrospective study included the healthy eyes with best corrected visual acuity of at least 20/40 of 122 Caucasian subjects. The anterior segment parameters and geometric corneal parameters such as corneal volume, central corneal thickness, horizontal and vertical corneal radii, anterior and posterior steep, and flat keratometric values were measured with a Scheimpflug camera. The biomechanical properties were measured with Ocular Response Analyzer. Results. One hundred and twenty-two healthy Caucasian subjects (67 males, 55 females) with a mean age of 45.32 ± 20.23 were enrolled. Both corneal hysteresis and corneal resistance factor were positively correlated with CCT (r = 0.529, p < 0.001; r = 0.638, p < 0.001) and CV (r = 0.635, p < 0.001; r = 0.579, p < 0.001) and negatively correlated with age (r = −0.373, p < 0.001; r = −0.249, p < 0.001). Both in age-gender and multivariate models, CH and CRF had statistically significant negative association with the posterior steep K value. Conclusions. CH and CRF are negatively correlated with posterior steep and average posterior K values.


Introduction
Many factors affect corneal biomechanical properties in healthy eyes [1][2][3]. Anterior segment parameters such as central corneal thickness (CCT) are among these factors that have been intensively studied. Corneal hysteresis (CH) and corneal resistance factor (CRF) are strongly correlated with CCT [4][5][6]. A relation between refractive error and corneal biomechanical properties has also been reported [7]. The relationship between other anterior segment parameters such as corneal curvature (CC), corneal astigmatism (CA), corneal volume (CV), mean keratometric ( ) value, and corneal biomechanical properties has been also investigated and the results are controversial [8][9][10][11]. Several studies in children reported that flatter corneal curvature was related to lower CH and CRF values [12,13]. In this study, we aimed to investigate the relationship between corneal biomechanical parameters, anterior segment parameters, and geometric corneal parameters, including horizontal and vertical corneal radii and anterior and posterior steep and flat values with 3 mm distance from the apex in a healthy Caucasian group.

Results
One hundred and twenty-two healthy Caucasian subjects (67 males, 55 females) with a mean age of 45.32 ± 20.23 were included in the study. The biomechanical properties, anterior segment parameters, and refraction values are given in Table 1. Table 2 Table 3. Correlations were found between CH and mean anterior value, anterior flat value, mean posterior value, posterior steep value, and vertical radius of CC. CRF was also correlated with mean posterior value and posterior steep value. No correlation was detected between CH, CRF, and mean value (Table 4). In multivariate linear regression analysis, the associations between the corneal biomechanical parameters with values of anterior and posterior surfaces and anterior segment parameters were investigated (Tables  5 and 6). Both in age-gender and multivariate models, CH and CRF had statistically significant negative association with the posterior step value (Table 5). When IOP, ACD, ACV, CV, CCT, and SE were included in multivariate analysis, no association was detected between corneal biomechanical properties and values. Age and IOP were negatively ( = −0.275, = 0.016; = −0.174, = 0.048, resp.) and CCT was positively associated with CH ( = 0.477, = 0.043). In addition, there were positive associations between IOP, ACD, and CCT with CRF ( = 0.323, < 0.001; = 0.350, = 0.028; = 0.741, = 0.001, resp.) ( Table 6).

Discussion
Previous studies showed that there is a correlation between corneal biomechanical properties and age. Though there  are some controversial reports, most of the studies found a negative correlation between biomechanical properties and age [3,10,[13][14][15][16].
In the present study, it is found that CH and CRF are negatively correlated with age in Caucasian population. Kotecha et al. also reported that CH and CRF are negatively correlated with age in a group of patients which included patients of different ethnic origin [10]. In a recent study, Jóhannesson et al. compared 50 Swedish young subjects with 43 elderly. They reported that CH was significantly lower in elderly group, but there was no difference in CRF between the groups [15]. Disaccordingly, in a study from Japan, no correlation was found between age and CH within 86 eyes of 43 healthy subjects with an age range of 19-64 years [3]. Ortiz et al. reported significant difference in CH between the youngest age group (9 to 14 years) and oldest age group (60 to 80 years) only [16].
Central corneal thickness is one of the important geometric corneal parameters that influence IOP measurement [17]. The biomechanical properties of cornea as CH and CRF are strongly correlated with CCT [4][5][6]18]. The results of this study were in accordance with the literature. Besides CCT, the relationship of corneal curvature with corneal biomechanics was also studied. In children CC was found to be negatively correlated with CH and CRF [12,13,19]. In adults, the findings of the studies that investigated CC relation with corneal biomechanics are controversial. Franco and Lira found no relationship between CH, CRF, and CC in adults (20-63 years of age range) in 63 eyes [8]. Kamiya et al. reported similar result; they did not find any correlation between mean value and CH and CRF in 86 eyes of 43 patients with a mean age of 39 years [3]. Wong and Lam found no correlation between mean and CH and CRF in Chinese adults [9]. However, 2 different studies from Far East reported  [10]. In other studies only CH was found to be associated with mean [11,21]. Diversity of the results in these studies may be related to the racial variety of the patients and age groups. Another reason for these results may be due to the different devices used for measurement for corneal parameters.
Apart from the other studies, this study investigated both steep and flat values of anterior and posterior surface of the cornea. Although no correlation was found between mean value and CH and CRF, it is noted that there are correlations with posterior steep value and vertical radius of CC. CRF was also correlated with mean posterior value and posterior steep value. According to a multivariate regression model in the present study, posterior values were found to be associated with CH and CRF. This finding may suggest that posterior surface of the cornea may affect corneal biomechanical properties more compared to anterior surface in normal eyes. In a recent study, it has been shown that CH and CRF were negatively associated with max value in keratoconic eyes, but this correlation did not exist for normal eyes [22].
CV and its relation with corneal biomechanical properties have also been investigated in normal and keratoconic eyes in several studies [23,24]. Hwang et al. found that corneal volume was positively correlated with CH, but not CRF [20]. Wong and Lam reported positive correlation both with CH and CRF [9]. In a recent study both CH and CRF were found to be correlated positively with CC and CV for the normal, keratoconic, and crosslinked eyes [22]. In this study, there are also positive correlations noted between CH, CRF, and CV.
In conclusion, CH and CRF are positively correlated with CCT and CV and negatively correlated with age in a healthy Caucasian population. It is also shown with this study that CH and CRF are negatively correlated with posterior steep and average posterior values. Anterior and posterior values account for 16.6% and 9.5% of variation in CH and CRF, respectively. Journal of Ophthalmology 5   Journal of Ophthalmology