Comparative Study of Corneal Biomechanical Properties between Myopes and Hyperopes

Research Article

Austin J Clin Ophthalmol. 2014;1(1): 1002.

Comparative Study of Corneal Biomechanical Properties between Myopes and Hyperopes

May Ibrahim AlKhudair1, Rania Medhat Fahmy1 and Ahmed A Al-saleh2,*

1Optometry Department, King Saud University, KSA

2AlHokama Eye Specialist Center, KSA

*Corresponding author: Ahmed A Al-saleh, Assistant Medical Director, AlHokama Eye Specialist Center, P.O.Box 27629, Riyadh 11427, KSA

Received: January 02, 2013; Accepted: January 17, 2014; Published: January 20, 2014

Abstract

Purpose: The aim of this study is to compare the biomechanical corneal properties between myopes and hyperopes among Saudi Arabian adults using an Ocular Response Analyzer.

Methods: The study involves 50 healthy Saudi Arabian adults: 18 males (35.35 %) and32 females (64.65%), with a mean age 25.66 ± 5.8 years (range 15 – 44). The subjects were divided into two groups: Group 1 includes 78 eyes of 39 myopic subjects with spherical equivalents ranging from −1.25 DS to −10.00 DS; Group 2 includes 21 eyes of 11 hyperopic subjects with spherical equivalents ranging from +1.00 DS to +5.50 DS. An Ocular Response Analyzer [Reichert Ophthalmic Instruments, Buffalo, USA] was used to measure the corneal biomechanical properties such ascorneal hysteresis (CH), corneal resistance factor (CRF), IOPcc and IOPg. The Pentacam [Oculus, Wetzler, Germany] was used to measure the central corneal thickness (CCT).

Results: This study found that CH has a highly significant correlation with spherical equivalent in hyperopes (r=0.684; P<0.001) compared with n insignificant correlation with spherical equivalent in myopes (r = − 0.009; P=0.939).Also, it was found that a highly significant difference between myopes and hyperopesin CH values exists (P<0.001). Our study did not show any significant correlation between CRF and spherical equivalent with myopes (r=0.071; P=0.536) or with hyperopes (r=0.357; P=0.112). A significant correlation was found between CCT and the amount of spherical equivalent in hyperopes (r = 0.510; P<0.001). In myopes the CCT had an insignificant correlation with the amount of spherical equivalent (r = − 0.013; P = 0.913). The amount of CCT in this study is significantly correlated with corneal hysteresis in myopia (r = 0.504;P< 0.0001) and in hyperopia (r = 0.651, P<0.001).In themyopia group of our study, it was found that CH had a significant correlation with IOPcc (r = − 0.640; P<0.0001) and was more significant than in hyperopic group (r = −0.457; P=0.037).

Conclusion: In conclusion, this study found that hyperopic refractive errors have higher and significant values of corneal hysteresis and other biomechanical properties.

Keywords: Corneal biomechanical properties; Corneal hysteresis; Ocular response analyzer.

Introduction

The cornea is composed of a pressurized, thick–walled, partially woven, uni–directional, fibril–reinforced laminate bio–composite; it represents an excellent compromise between stiffness, strength and extensibility. It is able to withstand internal and external forces that may stress it, distort its shape or threaten its integrity [1].

The cornea is a complex anisotropic composite with non–linear elastic and viscoelastic properties. It is a composite because its properties are determined by the interaction of disparate materials like collagen and polyanionic ground substance, and is anisotropic because its properties are not directionally uniform [2].

A new measure of corneal biomechanics was identified by David Luce called corneal hysteresis (CH) which equates to the difference in the inward and outward pressure values obtained during dynamic bidirectional application employed by the Ocular Response Analyzer (ORA), as a result of viscous damping in the cornea. The corneal biomechanical properties, corneal hysteresis and corneal resistance factor (CRF) are subjects of recent research interest [3].Corneal hysteresis is thought to predominantly reflect the viscous properties of the cornea and CRF is an empirically derived measurement of thecornea’s elastic properties [4]. These parameters are derived from the complex interaction between the cornea’s collagen composition, thickness, hydration, age, and other physiological factors [5].

Until now, studies to correlate corneal findings with refractive errors among different age groups have produced conflicting results [6].The aim of the current study is to correlate corneal biomechanical properties between myopes and hyperopes among Saudi Arabian adults using an Ocular Response Analyser.

Subjects and Methods

This study involves 50 healthy Saudi Arabian human subjects of both genders. It was conducted from the 23rd of February 2013 until the 10th of May 2013, at two locations: King Saud University, College of Applied Medical Sciences (female campus, Oleisha), and at the Alhokama Eye Specialist Center. The mean age was 28 years old and the range was between 15 to 44 years old. The subjects were divided into two groups: Group 1: included 78 eyes of 39 myopic subjects with spherical equivalents ranging from −1.25 DS to −10.00 DS; Group 2: includes 21 eyes of 11 hyperopic subjects with spherical equivalents ranging from +1.00 DS to +5.50 DS. Patients with keratoconus, Fuch’s dystrophy, glaucoma, anterior segment inflammation or any systemic diseases with ocular complications were excluded. All subjects underwent full ophthalmological examination including a visual acuity test, subjective refraction and slit lamp examination. The automated refraction was done to determine refractive errors, the slit lamp examination was used to evaluate the anterior segment, and the central corneal thickness was measured using Pentacam[Oculus, Wetzler, Germany] [7].

The key point of this study is to measure corneal biomechanical properties using the Ocular Response Analyzer [Reichert Ophthalmic Instruments, Buffalo, USA]. This is a non–contact tonometer that allows measurement of corneal biomechanical properties by way of the corneal hysteresis measurement. The CH value provides a basis for understanding two additional parameters: Corneal–Compensated Intraocular Pressure (IOPcc) and corneal resistance factor (CRF) [8].

Statistical Analysis

The collected data were entered into SPSS 17.0 (SPSS Inc.) for statistical analysis. Datasets for myopic and hyperopic refractive errors groups were compared using an analysis of variance. The Pearson correlation coefficient was used to assess the relationship between spherical equivalent power and the corneal biomechanical properties. This was then compared to the relationship between myopia and hyperopia using the student t–test, unpaired.

Results

A total of 99 eyes of 50 patients were analyzed. The demographic factors revealed 18 males (35.35 %) and 32 females (64.65%), with a mean age of 25.66 ± 5.8 years (range 15 – 44). Group 1: included 78 eyes of 39 myopic subjects with a spherical equivalent mean and standard deviation of 3.73±1.64, ranging from −1.25 DS to −10.00 DS. Group 2: included 21 eyes of 11 hyperopic subjects with spherical equivalent mean and standard deviation of 2.84±1.42, ranging from +1.00 DS to +5.50 DS.

In myopic subjects we did not find any significant correlation between spherical equivalent (SE) and corneal biomechanical parameters as shown in table 1 and Figure 1.