There has been a recent surge of interest in assessing corneal biomechanical properties due to potential clinical applications, particularly in the early detection of keratoconus (KC). This review discusses the effects of keratoconus on the biomechanical properties of the cornea and the current techniques used to detect these changes both in the laboratory and clinical setting. Specific structural changes occurring in the corneal stroma as part of the disease process can be linked to alterations in the viscous and elastic properties of the cornea in keratoconus. Although there are extensive ex vivo studies using techniques such as extensometry and inflation testing to analyse the biomechanical properties of the normal cornea, few have investigated the keratoconic cornea using the same methods. There are a number of ex vivo studies that confirm the effectiveness of collagen cross‐linking in increasing Young's modulus in healthy corneas. Recently, research has focussed on measuring corneal biomechanical parameters in vivo using two commercially available instruments: the Ocular Response Analyser (ORA) and the CorVis ST (CST). Both instruments analyse the dynamic behaviour of the cornea, when temporarily deformed by an air puff; however, the outputs of these instruments are not directly comparable due to differences in the characteristics of the air puff and output parameters. Studies using these instruments have reported significant differences between keratoconic and healthy corneas; however, neither instrument can currently be used in isolation to reliably diagnose keratoconus. Further research analysing the outputs of these instruments may enhance their diagnostic capabilities.

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Citation: Vellara HR, Ali NQ, Gokul A, Turuwhenua J, Patel DV, McGhee CNJ. Quantitative analysis of corneal energy dissipation and corneal and orbital deformation in response to an airpulse in healthy eyes. Invest Ophthalmol Vis Sci. 2015;56:6941-6947. DOI:10.1167/iovs.15-17396 PURPOSE. To examine and evaluate ocular biomechanical metrics and additionally derived corneal and orbital components using a noncontact Scheimpflug-based tonometer (CorVis ST) in a population of healthy eyes. METHODS.A total of 152 eyes of 152 participants were examined by slit-lamp biomicroscopy, corneal tomography, and the CorVis ST (CST). This determined the distribution of outputs from the CST, such as deformation amplitude (DA), and additionally derived parameters, including maximum corneal deformation (MCD), maximum orbital deformation (MOD), and corneal energy dissipation (CED).RESULTS. The mean age of participants was 35.88 6 13.8 years. Deformation amplitude significantly correlated with age (r ¼ 0.24, P ¼ 0.002) but not sex or ethnicity (P > 0.05). Multiple linear regression analysis revealed significant correlations between DA and age (r ¼ 0.19, P ¼ 0.006) and DA and IOP (r ¼ À0.59, P < 0.001). Age correlated with MCD (r ¼ 0.20, P ¼ 0.01), MOD (r ¼ 0.18, P ¼ 0.03), and CED (r ¼ 0.39, P < 0.001). Males had a lower MOD than females (0.24 vs. 0.26 mm, respectively, P ¼ 0.01); however, there were no differences in MCD or CED between sexes (P > 0.05). There were no significant differences between ethnicities for MCD, MOD, and CED (P > 0.05). Multiple linear regression analysis revealed significant correlations between MCD and IOP (r ¼ À0.65, P < 0.001), CED and age (r ¼ 0.41, P < 0.001), CED and IOP (r ¼ 0.28, P ¼ 0.001), and between CED and central corneal thickness (CCT) (r ¼ À0.36, P < 0.001).CONCLUSIONS. The isolation of the corneal component (MCD) should be used when analyzing deformation characteristics in diseases that only affect the cornea. This study establishes a baseline for a population of healthy eyes. Future publications will identify differences in MCD, MOD, and CED between healthy and diseased populations.

Advances in anterior segment imaging have enhanced our ability to detect keratoconus in its early stages and characterize the pathologic changes that occur. Computerized corneal tomography has elucidated the alterations in shape of the anterior and posterior corneal surfaces and alterations in thickness as the disease progresses. Automated screening indices such as the keratoconus screening index were developed to assist in detecting keratoconus in suspicious cases. In vivo assessment of keratoconic corneas has revealed that compromised corneal biomechanics can now be measured clinically. Optical coherence tomography has demonstrated alterations in corneal epithelial thickness and distribution in keratoconus, has a role in assessing Descemet's membrane detachment in acute corneal hydrops (ACH) and the depth of the demarcation line following corneal collagen cross-linking. In vivo confocal microscopy has exhibited cellular changes that occur in keratoconus and provided insight into cellular events that may be related to the development of neovascularization in ACH.

Purpose: Keratoconus progression should be treated with corneal cross-linking (CXL) in a timely manner. This study aimed to investigate patient factors associated with keratoconus progression between time of listing and at time of CXL. Methods: Prospective observational study at a tertiary center. Ninety-six eyes of 96 patients with keratoconus. Demographic, clinical, and tomographic parameters were analyzed to determine the risk factors for keratoconus progression. Analyzed tomographic indices included steepest keratometry, average keratometry, cornea thinnest point, index of surface variance, index of vertical asymmetry, keratoconus index, center keratoconus index, index of height asymmetry, and index of height decentration. Results: A total of 38 eyes (39.6%) were found to have keratoconus progression during an average waiting time of 153 ± 101 days. There were significant differences in preoperative tomographic parameters such as index of surface variance (111.3 ± 36.6 vs. 88.3 ± 31.8; P = 0.002), index of vertical asymmetry (1.1 ± 0.4 vs. 0.9 ± 0.4; P = 0.005), keratoconus index (1.31 ± 0.12 vs. 1.22 ± 0.11; P < 0.001), and index of height decentration (0.16 ± 0.07 vs. 0.11 ± 0.06; P = 0.015) between eyes that progressed and those that remained stable. There were no significant differences in steepest keratometry, average keratometry, cornea thinnest point, and center keratoconus index. Multivariate analysis did not reveal age, presence of atopy/atopic keratoconjunctivitis, eye rubbing, or waiting time to be a significant risk factor for progression; however, Maori ethnicity was a risk factor (odds ratio = 3.89; P = 0.02). Conclusions: A significant proportion of eyes were found to be progressing while waiting for CXL. A risk stratification score for patients awaiting CXL may reduce the risk of progression.

Importance Keratoconus is a debilitating condition with a disproportionately high impact on health resources and vision‐specific quality of life. Background This study aimed to compare 2‐year outcomes of epithelium‐off pulsed (p‐ACXL) and epithelium‐off continuous (c‐ACXL) accelerated corneal crosslinking in progressive keratoconus. Design Prospective, interventional case series. Participants Eighty eyes of 80 patients were included. Methods The visual, refractive and tomographic results of the two crosslinking protocols were compared. Main Outcome Measures Uncorrected distance visual acuity, corrected distance visual acuity (CDVA), manifest refraction spherical equivalent (MRSE) and maximum keratometry (KMAX) on corneal tomography assessment. Results The mean patient age was 22.51 ± 6.12 years (SD) and 22.08 ± 5.72 years in the p‐ACXL and c‐ACXL groups, respectively. The mean CDVA significantly improved from 0.30 ± 0.16 logMAR at baseline to 0.23 ± 0.17 logMAR at 24 months (P = .04) in the p‐ACXL group and from 0.36 ± 0.22 logMAR to 0.26 ± 0.27 logMAR (P = .02) in the c‐ACXL group. The mean induced change in MRSE (+1.79 ± 2.30 D vs +0.27 ± 3.19 D, P = .04) and KMAX (−1.75 ± 1.80 D vs −0.39 ± 1.95 D, P = .04) were superior in the c‐ACXL group compared to the p‐ACXL group at 24 months. No complications were encountered. Conclusions and Relevance In this prospective study, both p‐ACXL and c‐ACXL treatments were safe methods to halt the progression of keratoconus within a follow‐up period of 24 months. c‐ACXL appeared to offer superior refractive and tomographic outcomes when compared to p‐ACXL but this did not translate into better visual outcomes.

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Importance Keratoplasty is a surgical procedure to create a more regular optical surface following biomechanical weakening of the cornea in keratoconus. The ideal keratoplasty procedure should also restore corneal biomechanics to that of the healthy cornea. Background This study aimed to evaluate and compare the biomechanical properties of corneas following penetrating keratoplasty (PKP) and predescematic deep anterior lamellar keratoplasty (DALK) to those of healthy eyes. Design Prospective cross‐sectional study. Participants Two cohorts of post‐keratoplasty eyes (42 eyes with PKP and 27 eyes with DALK) with each other, and with a cohort of 152 healthy eyes. Methods All eyes were examined by slit‐lamp biomicroscopy, tomography, anterior segment‐OCT and non‐contact tonometry CorVis ST (CST). Main Outcome Measures CST biomechanical parameters, maximum corneal deformation (MCD) and corneal energy dissipation were compared between keratoplasty techniques, and with healthy eyes. Results The mean age of participants with PKP and DALK were 35 ± 13.7 and 36.1 ± 12.6 years, respectively. None of the CST parameters were significantly different between PKP and DALK eyes. However, when compared to healthy corneas, numerous parameters were significantly different for both keratoplasty techniques. Of note, MCD was significantly higher in PKP compared to DALK and healthy corneas, after controlling for co‐factors. Conclusions and Relevance Neither type of keratoplasty technique utilized in keratoconus completely restored corneal biomechanical properties to that of healthy corneas. However, PKP resulted in a greater number of parameters significantly different to healthy corneas, compared to DALK.