|Year : 2019 | Volume
| Issue : 3 | Page : 1149-1153
Assessment of corneal biomechanics in young myopic individuals using ocular response analyzer
Hassan G Farahat1, Khalid E. S. Ahmed1, Omnia A Attia2
1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
2 Department of Ophthalmology, International Eye Hospital, Cairo, Egypt
|Date of Submission||01-Jan-2019|
|Date of Acceptance||26-Jan-2019|
|Date of Web Publication||17-Oct-2019|
Omnia A Attia
Source of Support: None, Conflict of Interest: None
The aim of this study was to assess corneal biomechanical properties using the ocular response analyzer (ORA) in myopic young individuals with normal topography.
The development in corneal refractive surgeries raises the necessity of studying corneal properties in detail. Corneal viscoelasticity, in terms of corneal biomechanics, is measured by ORA through the analysis of the corneal dynamic behavior when it is deformed by an air puff. The important parameters measured are corneal resistance factor (CRF) and corneal hysteresis (CH).
Patients and methods
This study included 116 myopic eyes, which were assessed by clinical examination, best corrected visual acuity examination, and corneal pentacam topography. CH and CRF were measured using ORA.
The mean age in our study participants was 23.6 ± 2.9 years, and the mean of the spherical equivalent was −4.56 ± 2.4 (moderate myopia). The mean CRF was 9.7 ± 1.4 mmHg, while the mean CH was 10.0 ± 1.3 mmHg. As regards corneal topography, normal participants were recruited with the following measurements: Kmax (maximum keratometry) 44.3 ± 1.4 and the mean central corneal thickness (CCT) was 529.5 ± 12.7 μm. Correlation of CRF and CH values in this moderate myopic group were negatively correlated with the spherical equivalent. Both CH and CRF had positive linear correlation with CCT and Kmax, and negative correlation with age.
The ORA is used to measure CH and CRF by which the biomechanical changes of the cornea in different degrees of myopia can be detected. It was found that it is important when evaluating the corneal biomechanical properties to take CCT and myopic status into consideration.
Keywords: corneal biomechanics, corneal topography, myopia, refractive surgical procedures, visual acuity
|How to cite this article:|
Farahat HG, Ahmed KE, Attia OA. Assessment of corneal biomechanics in young myopic individuals using ocular response analyzer. Menoufia Med J 2019;32:1149-53
|How to cite this URL:|
Farahat HG, Ahmed KE, Attia OA. Assessment of corneal biomechanics in young myopic individuals using ocular response analyzer. Menoufia Med J [serial online] 2019 [cited 2019 Nov 12];32:1149-53. Available from: http://www.mmj.eg.net/text.asp?2019/32/3/1149/268820
| Introduction|| |
Corneal biomechanics can be clearly explained by a measurement of corneal tissue properties, that is, corneal hysteresis (CH), which is the result of viscous damping in the corneal tissue . CH can be measured in vivo by the ocular response analyzer (ORA) using an applied force-displacement relationship. An air jet similar to that used in traditional air-puff tonometers generates force or pressure on the cornea . This valuable information about the CH enhances the ability to make complex decisions in a number of important participants so as to increase the confidence in the evaluation of refractive surgery candidates and improve the ability to detect keratoconus . The importance of considering the biomechanical factor in the models of refractive surgery was growing from diverse works until it became a considerable factor in ablation algorithms . It is notable that using CH measured by the ORA can be used to detect the biomechanical changes in the cornea in various corneal disorders and in different degrees of myopic states. Moreover, the evaluation of corneal biomechanical properties should take central corneal thickness (CCT) and myopic status into consideration. Previous studies reported that both CH and corneal resistance factor (CRF) were positively correlated with CCT. Lower CH but not CRF was associated with increasing degree of myopia. Evaluation of corneal biomechanical properties should take CCT and myopic status into consideration . The purpose of this study was to study the changes in CH and CRF on eyes with myopia in a young age group, and to correlate these results with other corneal properties like corneal topography and corneal thickness.
| Patients and Methods|| |
The study protocol was approved by the committee of Menoufia University. All of the included cases were informed the purpose of the study, and informed consent was obtained. A clinical observational randomized study was conducted on 58 (116 eyes) patients who were aged from 18 to 28 years old with a mean age of 23.6 ± 2.9 years. There were 32 female and 26 male patients.
Inclusion criteria were as follows: age from 18 to 28 years, myopia (from −0.50 to −10.0 D), clear cornea, and normal CCT range from 500 to 550 μm. Exclusion criteria were age out of range, evidence of corneal pathology or abnormal topography, and history of previous corneal surgery.
Baseline examination included visual acuity, slit-lamp examination, full ocular examination, and medical history.
CH and CRF were measured using ORA (2.02; Reichert Ophthalmic Instruments, Depew, New York, USA). In contrast, CCT and K readings were assessed using a scheimpflug-based topography, namely the Pentacam machine (Oculus, Wetzlar, Germany).
Statistical analysis demonstrated in the present study was conducted using SPSS (statistical package for the social sciences; SPSS Inc., Chicago, Illinois, USA) to evaluate the relations between the mentioned parameters. Each of the studied parameters (CH, CRF, Kmax, and CCT) was described as mean value, and SD. The strength of the relations between the measured parameters was evaluated using Pearson's correlation coefficient (R). A P value greater than 0.05 was considered statistically nonsignificant. A P value less than 0.05 was considered statistically significant.
| Results|| |
The patients enrolled in the study were selected from laser vision correction candidates. This was an observational randomised study.
The mean CRF in the studied individuals was 9.7 ± 1.4 mmHg (range: 6.4–15.0 mmHg). The mean CH in the studied group was 10.0 ± 1.3 mmHg (range: 7.8–14.8 mmHg) [Table 1].
|Table 1: Corneal biomechanical properties of the studied group: corneal hysteresis and corneal resistance factor|
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The mean CCT of the individuals in the study was 529.5 ± 12.7 μm (with a minimum of 503 μm and a maximum of 550 μm). The mean thinnest location corneal thickness (thnt) was 526.1 ± 13.0 μm (with a minimum of 501 μm and a maximum of 549 μm). As regards K readings, Kmax values were 44.3 ± 1.4 D (range: 41.6–47.6 D).
It was found that this group of participants randomly selected in the study had mild to severe myopia, as the mean spherical equivalent of the studied group was −4.56 ± 2.394 D (range: −10.50 to − 0.50 D).
The interactive relations between the measured parameters were studied by measuring the relation strength using Pearson's correlation coefficient. [Table 2] shows the correlation matrix of the studied parameters.
As regards the correlation of CH and CRF with age, it demonstrated a weak negative relation. The correlation coefficients (R) were − 0.164 and − 0.138 with CH and CRF, respectively.
The correlation of CH and CRF with the CCT and with the thinnest corneal thickness location (thnt) demonstrated a highly significant with moderate strength direct relation. Thus, CH and CRF values rise with the increase in CCT [Figure 1]. The correlation coefficients (r) with CCT were 0.308 and 0.222 for CH and CRF, respectively. The correlation coefficients (r) with thinnest location were 0.316 and 0.240 for CH and CRF, respectively.
|Figure 1: Scatter plots showing the correlation between corneal hysteresis and corneal resistance factor with central corneal thickness.|
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With respect to corneal topographic pattern and its relation to corneal biomechanical properties, it was found that the maximal K reading (Kmax) gives almost weak to moderate positive linear correlation with CRF and CH, being nonsignificant to CRF and significant to CH. The correlation coefficients (r) with Kmax were 0.218 and 0.181 for CH and CRF, respectively.
With reference to refractive status; the result of the analyzed data showed that the CRF is correlated with the spherical equivalent in a very weak negative nonsignificant linear correlation manner [Figure 2]. The correlation coefficients (r) were − 0.084 and − 0.008 for CH and CRF, respectively.
|Figure 2: Scatter plots showing the correlation between corneal resistance factor and corneal hysteresis with the mean spherical equivalent.|
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| Discussion|| |
Assessment of corneal biomechanical properties leads to better understanding of different corneal pathologies and gives more confidence in patient selection for refractive corneal surgeries by better expectation of postoperative complications.
Development of ectasia in cases with normal preoperative examinations highlights the need for an advanced understanding of corneal biomechanical properties .
The dynamic bidirectional applanation process used in the Reichert ORA provides a measure of corneal biomechanics called CH . With a closer inspection, it was obvious that both CH and CRF are not validated equally or in the same trend as each other. According to the manufacturer, CH is a measurement of viscous damping in the corneal tissue, whereas CRF is a measurement of the overall elastic resistance .
In our observational study, we aimed at detecting corneal biomechanics in a young age group of myopic patients who were suitable candidates for refractive laser correction procedures by their normal topographic patterns. Only one patient out of 58 patients had shown abnormal biomechanics in both eyes so the patient was excluded from undergoing refractive eye correction. Nevertheless, that was not statistically significant in the final results. A weak correlation was found between CH and CRF from one side and myopia from the other side. That is, in moderate myopia, CRF and CH show increased values, as the degree of myopia decreased.
Another study by Radhakrishnan in 2012 has indicated that the patients with mild to moderate myopia have higher corneal resistance compared with nonmyopic patients. The results of this study showed that a weak but significant negative correlation is found between CRF and spherical equivalent refractive error, with no significant correlation between CH and refractive error .
Another previous study showed that CH declined only in high myopia when compared with moderate or low myopia; subsequently, their results indicate that the mechanical stress in the anterior segment of the eye is compromised in high myopia .
Similar to our results, many studies such as those by Kotecha et al. , Terai et al. , Sullivan-Mee et al. , Hashemi et al. , Narayanaswamy et al. , and Qiu et al.  stated that both CH and CRF were positively associated with CCT in a significant manner; their results concluded significant and positive relationships between CH, CRF, and CCT. In concordance with previous reports, CCT was found to be the most prominent predictor for variations of CH and CRF.
Age-related changes of the corneal structure have been observed in old previous histological studies ,. By using ORA, the relationship between age and in-vivo corneal biomechanical properties has been investigated ,, and it was concluded that both CH and CRF were negatively associated with age, as proved in our study.
Another point to be highlighted in the literature is that the reason for the difficulty to find a universal screening tool based on corneal morphologic features is that curvature, elevation, and pachymetric changes are all secondary signs of keratoconus and postrefractive surgery ectasia and that the primary abnormality is in the biomechanical properties. Moreover, it is proposed that the biomechanical modification is focal in nature, rather than a uniform generalized weakening, and that the focal reduction in elastic modulus precipitates a cycle of biomechanical decompensation that is driven by asymmetry in the biomechanical properties. Various interventions are described in terms of how this cycle of biomechanical decompensation is interrupted, such as intrastromal corneal ring segments, which redistribute the corneal stress, and collagen crosslinking, which modifies the basic structural properties .
Future studies should be carried out on the Ocular Response Analyzer Waveform Scoring System and the Keratoconus Match Index (in topography assessment), to examine the validity of detecting abnormal cornea not suitable for laser refractive surgery.
| Conclusion|| |
We concluded from this study that CH measured by the ORA can be used to detect the biomechanical changes in the cornea in different degrees of myopic states. We also concluded that evaluation of corneal biomechanical properties should take CCT and myopic status into consideration.
Corneal biomechanics' parameters can be used as an adjunct, but not as a sole measure, with other investigations and scoring systems to detect and screen for cornea at risk of ectasia.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]