Consideration of corneal biomechanics in the diagnosis and management of keratoconus: is it important?

Keratoconus is a bilateral,non-inflammatory,degenerative corneal disease.The occurrence and development of keratoconus is associated with corneal thinning and conical protrusion,which causes irregular astigmatism.With the disruption of the collagen organization,the cornea loses its shape and function resulting in progressive visual degradation.Currently,corneal topography is the most important tool for the diagnosis of keratoconus,which may lead to false negatives among the patient population in the subclinical phase.However,it is now hypothesised that biomechanical destabilisation of the cornea may take place ahead of the topographic evidence of keratoconus,hence possibly assisting with disease diagnosis and management.This article provides a review of the definition,diagnosis,and management strategies for keratoconus based on corneal biomechanics.

Review of in-vivo characterisation of corneal biomechanics

The study of corneal biomechanics in vivo has been evolving fast in recent years.While an organised corneal structure is necessary for its transparency,resistance to occasional external insults and bearing the intraocular pressure(IOP),which several clinically relevant events can disturb.This review focuses on three techniques that are available for clinical use,namely the Ocular Response Analyzer(Reichert Ophthalmic Instruments,Buffalo,NY,USA),the Corvis ST(Oculus Optikgerate GmbH,Wetzlar,Germany)and the Brillouin Optical Scattering System(Intelon Optics Inc.,Lexington,MA,USA).The principles and the main parameters of each device are discussed along with their strategies to improve accuracy in the IOP measurement,corneal ectasia diagnosis,evaluation of corneal cross-linking procedures,and planning of corneal refractive surgeries.

Review of ex-vivo characterisation of corneal biomechanics

The evaluation of the corneal biomechanical behaviour has important clinical applications.To name a few,the accuracy of the intraocular pressure measurement,the study of corneal ectatic diseases and the assessment and optimisation of corneal surgical procedures are all highly influenced by corneal biomechanics.Over the last 45 years different ex-vivo methods were developed to study corneal biomechanical behaviour.Different tissue maintenance,support,loading systems,as well as different monitoring strategies of corneal deformations were employed.In this review,the most important and commonly used methods are outlined,including strip extensiometry,inflation,compression,indentation and tissue separation testing.Their particularities,applications,pros and cons and main applications are discussed.

Unintended changes in ocular biometric parameters during a 6-month follow-up period after FS-LASIK and SMILE

Background:Corneal refractive surgery has become reliable for correcting refractive errors,but it can induce unintended ocular changes that alter refractive outcomes.This study is to evaluate the unintended changes in ocular biometric parameters over a 6-month follow-up period after femtosecond laser-assisted laser in situ keratomileusis(FS-LASIK)and small incision lenticule extraction(SMILE).Methods:156 consecutive myopic patients scheduled for FS-LASIK and SMILE were included in this study.Central corneal thickness(CCT),mean curvature of the corneal posterior surface(K_(pm)),internal anterior chamber depth(IACD)and the length from corneal endothelium to retina(ER)were evaluated before and after surgery over a 6-month period.Results:Both the FS-LASIK and SMILE groups(closely matched at the pre-surgery stage)experienced flatter Kpm,shallower IACD and decreased ER 1 week post-surgery(P<0.01),and these changes were larger in FS-LASIK than in SMILE group.During the 1 week to 6 months follow up period,K_(pm),IACD and ER remained stable unlike CCT which increased significantly(P<0.05),more in the FS-LASIK group.Conclusions:During the follow up,the posterior corneal surface became flatter and shifted posteriorly,the anterior chamber depth and the length from the corneal endothelium to retina decreased significantly compared with the pre-surgery stage.These unintended changes in ocular biometric parameters were greater in patients undergoing FS-LASIK than SMILE.The changes present clear challenges for IOL power calculations and should be considered to avoid affecting the outcome of cataract surgery.

Accuracy and reliability of orthogonal polynomials in representing corneal topography

Fitting of corneal topography data to analytical surfaces has been necessary in many clinical and experimental applications,yet absolute superiority of fitting methods was still unclear,and their overfitting risks were not well studied.This study aimed to evaluate the accuracy and reliability of orthogonal polynomials as fitting routines to represent corneal topography.Four orthogonal polynomials,namely,Zernike polynomials(ZPs),pseudo-Zernike polynomials(PZPs),Gaussian-Hermite polynomials(GHPs)and Orthogonal Fourier-Mellin polynomials(OFMPs),were employed to fit anterior and posterior corneal topographies collected from 200 healthy and 174 keratoconic eyes using Pentacam topographer.The fitting performance of these polynomials were compared,and the potential overfitting risks were assessed through a prediction exercise.The results showed that,except for low orders,the fitting performance differed little among polynomials with orders
10 regarding surface reconstruction(RMSEs~0.3μm).Anterior surfaces of normal corneas were fitted more efficiently,followed by those of keratoconic corneas,then posterior corneal surfaces.The results,however,revealed an alarming fact that all polynomials tended to overfit the data beyond certain orders.GHPs,closely followed by ZPs,were the most robust in predicting unmeasured surface locations;while PZPs and especially OFMPs overfitted the surfaces drastically.Order 10 appeared to be optimum for corneal surfaces with 10-mm diameter,ensuring accurate reconstruction and avoiding overfitting.The optimum order however varied with topography diameters and data resolutions.The study concluded that continuing to use ZPs as fitting routine for most topography maps,or using GHPs instead,remains a good choice.Choosing polynomial orders close to the topography diameters(millimeters)is generally suggested to ensure both reconstruction accuracy and prediction reliability and avoid overfitting for both normal and complex(e.g.,keratoconic)corneal surfaces.

High intercorneal symmetry in corneal biomechanical metrics

Backgroud:To evaluate the symmetry of corneal biomechanical metrics,measured using an ocular response analyzer(ORA)and self-built corneal inflation test platform,in bilateral rabbit corneas and to investigate their relationship with physical intraocular pressure(IOPp).Methods:Twenty fresh enucleated eyes from ten rabbits were used for ex vivo whole ocular globe inflation.IOP was increased from 7.5 to 37.5 mmHg with 7.5 mmHg steps and biomechanical metrics were acquired using the ORA.At least 3 examinations were performed at each pressure stage.Two biomechanical metrics,corneal hysteresis(CH)and corneal resistance factor(CRF)were recorded and analyzed as a function of IOPp.Corneal specimens were then excised from the intact ocular globe and tested under inflation conditions up to 45.7 mmHg posterior pressure.The experimental pressure-deformation data was analyzed using an inverse modeling procedure to derive the stress-strain behavior of the cornea.Results:A comparison of corneal shape parameters showed no statistically significant difference(P>0.05)between bilateral eyes.Similarly,there were no statistically significant differences in values of CH,CRF and corneal stiffness(as measured by the tangent modulus,Et)between bilateral eyes(CH:F=0.94,P=0.54;CRF:F=4.42,P=0.35;Et:F=3.15,P=0.12)at different pressure levels.IOPp was highly correlated with CRF while the relationship with CH was less pronounced.Conclusions:An obvious interocular symmetry in biomechanical metrics is found in this research.IOP has been shown to have important influences on the value of CRF provided by ORA.