Changes in corneal biomechanics and intraocular pressure following Femto-LASIK using Goldman applanation tonometry and ocular response analyzer

AIM:To compare intraocular pressure(IOP)measurements before and after laser in situ keratomileusis(LASIK)with a femtosecond laser for flap creation using ocular response analyzer(ORA)and Goldmann applanation tonometry,and to identify factors that may influence the preoperative and postoperative IOP.METHODS:A prospective study conducted on myopic patients who underwent LASIK using a femtosecond laser for flap fashioning.Enrolled patients were evaluated preoperatively,6 wk and 3 mo postoperatively for manifest refraction(MR),keratometric(K)readings and central corneal thickness(CCT)using a scheimpflug-based topography.Corneal resistance factor(CRF),corneal hysteresis(CH),Goldmann correlated IOP(IOPg)and corneal compensated IOP(IOPcc)were measured using ORA besides IOP assessment by Goldman applanation tonometry(GAT).RESULTS:There was a statistically significant decrease in measures of IOPg by 3.35±0.83 mm Hg,followed by GAT which decreased by 2.2±0.44 mm Hg,and the least affected by operation was IOPcc which decreased only by 0.87±0.1 mm Hg after 6 wk.After 3 mo follow up there was a statistically significant decrease in IOPcc which decreased only by 0.76±0.4 mm Hg,followed by IOP GAT by 1.6±0.5 mm Hg,and the most affected by operation was IOPg which decreased by 2.3±0.3 mm Hg.Correspondingly,there was a statistically significant decrease in CH and CRF after 6 wk and 3 mo.At 3 mo,the preoperative MR and preoperative GAT were prominent significant predictors of the postoperative GAT changes.The prediction equation was subsumed.CONCLUSION:IOP measurements and corneal biomechanical factors reduce significantly after LASIK with a femtosecond laser for flap creation.The IOPcc values are less influenced by changes in corneal properties than IOPg and GAT,indicating that IOPcc may provide the most reliable measurement of IOP after this procedure.

Comparison of corneal biomechanical properties in normal tension glaucoma patients with different visual field progression speed

AIM：To compare the corneal biomechanical properties difference by ocular response analyzer（ORA） in normal tension glaucoma（NTG） patients with different visual field（VF） progression speed. METHODS：NTG patients with well-controlled Goldmann applanation tonometer（GAT） who routinely consulted Kitasato University Hospital Glaucoma Department between January 2010 and February 2014 were enrolled.GAT and ORA parameters including corneal compensated intraocular pressure（lOPcc）,Goldmann estimated intraocular pressure（lOPg）,corneal hysteresis（CH）,corneal resistance factor（CRF） were recorded.VF was tested by Swedish interactive threshold algorithm（SITA）-standard 30-2 fields.All patients underwent VF measurement regularly and GAT did not exceed 15 mm Hg at any time during the 3y follow up.Patients were divided into four groups according to VF change over 3y,and ORA findings were compared between the upper 25th percentile group（slow progression group） and the lower 25th percentile group（rapid progression group）.RESULTS：Eighty-two eyes of 56 patients were studied.There were 21 eyes（21 patients） each in rapid and slow progression groups respectively.GAT,lOPcc,lOPg,CH,CRF were 12.1＋1.4 mm Hg,15.8±1.8 mm Hg,12.8±2.0 mm Hg,8.4±1.1 mm Hg,7.9±1.3 mm Hg respectively in rapid progression group and 11.5±1.3 mm Hg,13.5±2.1 mm Hg,11.2±1.6 mm Hg,9.3±1.1 mm Hg,8.2±0.9 mm Hg respectively in slow progression group（P=0.214,〈0.001,0.007,0.017,0.413,respectively）.In bivariate correlation analysis,lOPcc,lOPcc-GAT and CH were significant correlated with m△MD（r =-0.292,-0.312,0.228 respectively,P =0.008,0.004,0.039 respectively）.CONCLUSION：Relatively rapid VF progression occurred in NTG patients whose lOPcc are rather high,CH are rather low and the difference between lOPcc and GAT are relatively large.Higher lOPcc and lower CH are associated with VF progression in NTG patients.This study suggests that GAT measures might underestimate the IOP in such patients.

Effect of biometric characteristics on biomechanical properties of the cornea in cataract patient

AIM： To determine the impact of biometric characteristics on the biomechanical properties of the human cornea using the ocular response analyzer （ORA） and standard comprehensive ophthalmic examinations before and after standard phacoemulsificaUon. METHODS： This study comprised 54 eyes with cataract with significant lens opacification in stages I or II that underwent phacoemulsification （2.8 mm incision）. Corneal hysteresis （CH）, corneal resistance factor （CRF）, Goldmann-correlated intraocular pressure （IOPg）, and corneal-compensated intraocular pressure （IOPcc） were measured by ORA preoperatively and at lmo postoperatively. Biometric characteristics were derived from corneal topography [TMS-5, anterior equivalent （EQTMS） and cylindric （CYLTMS） power], corneal tomography [Casia, anterior and posterior equivalent （EQaCASC, EQpCASIA） and cylindric （CYLaCASIA, CYLpCASIA） power], keratometry [IOLMaster, anterior equivalent （EQIOL） and cylindric （CYL,oL） power] and autorefractor [anterior equivalent （EQAR）]. Results from ORA were analyzed and correlated with those from all other examinations taken at the same time point. RESULTS： Preoperatively, CH correlated with EQpCASIA and CYLpCASIA only （P=0.001, P=0.002）. Postoperatively, IOPg and IOPcc correlated with all equivalent powers （EQTMS, EQIOL, EQAR, EQaCASIA, and EQpCASIA）（P=0.001, P=0.007, P=0.001, P= 0.015, P=0.03 for IOPg and P〈0.001, ,0=0.003, P〈0.001, P= 0.009, P=0.014 for IOPcc）. CH correlated postoperatively with EQaCASIA and EQpCASIA only （P=0.021, P=0.022）. CONCLUSION： Biometric characteristics may significantly affect biomechanical properties of the cornea in terms of CH, IOPcc and IOPg before, but even more after cataract surgery.

In-vivo corneal biomechanical analysis of unilateral keratoconus

AIMTo evaluate and compare corneal biomechanical findings measured by ocular response analyzer, topographic and pachymetric findings in patients with unilateral keratoconus patients and healthy controls.METHODSThis is an observational, case-control study. Patients with keratoconus in one eye and forme fruste keratoconus in the fellow eye were compared with sex and age matched with controls healthy subjects. All subjects were evaluated with rotating scheimpflug imaging system. The receiver-operating-characteristic curves were analyzed to evaluate the sensitivity and specificity of the parameters.RESULTSTwenty-seven patients with keratoconus in one eye and forme fruste keratoconus in the fellow eye were compared with 40 eyes of 40 normal subjects. Corneal hysteresis (CH) was 8.0±1.7 mm Hg in keratoconus group, 8.3±1.6 mm Hg in forme fruste keratoconus group, and 9.8±1.6 mm Hg in control groups (P=0.54 between keratoconus and forme fruste keratoconus groups, P<0.01 between control group and other groups). Corneal resistance factor (CRF) was 7.1±2.2 mm Hg in keratoconus group, 7.8±1.2 mm Hg in forme fruste keratoconus group and 9.9±1.5 mm Hg in control group (P<0.001 between control group and other groups). Using receiver-operating-characteristic analysis, the area under curve values of the parameters to distinguish forme fruste keratoconus from control subjects were: CH (0.768), CRF (0.866). Best cut-off points were 9.3 mm Hg and 8.8 mm Hg for CH and CRF respectively.CONCLUSIONOcular response analyzer parameters (CH and CRF) are found to be significantly lower in forme fruste keratoconus patients compared to normal control subjects.

Effect of fibrin glue on corneal lamellar healing and how it correlates to biomechanical properties: biomechanical wavefront analysis and confocal study

Background:To evaluate,using a rabbit model,the influence of the wound healing process at the flap edge on corneal biomechanics after sutured,glued,and non-augmented microkeratome flaps.Methods:Unilateral 160μm thick laser in situ keratomileusis(LASIK)flaps using a mechanical microkeratome were performed on the corneas of the left eyes of 36 rabbits.Animals were then divided into 3 groups of 12 rabbits each:A:the flaps were glued with human fibrin tissue adhesive(Tisseel);B:the flaps were sutured;and C:the flaps were allowed to heal without the use of sutures or glue(non-augmented).The contralateral eyes served as controls.Reichert ocular response analyzer(ORA)was used to measure corneal hysteresis(CH),corneal resistance factor(CRF),Goldmann-correlated intraocular pressure(IOPg)and cornea-compensated IOP(IOPcc)at 6 weeks and 3 months postoperatively.In vivo confocal microscopy(IVCM)was also used to study the corneal wound healing process in all groups.Results:Both mean CH and mean CRF were significantly higher in sutured and glued groups compared with the non-augmented group at 6 weeks and 3 months postoperatively(P<0.0001).No statistically significant difference in corneal biomechanics was found between controls and groups A and B at any time points.Activated keratocytes were detected at the wound edge and peripheral flap interface in sutured and glued groups.Conclusion:The healing process at the wound edge is critical for optimal corneal integrity.Fibrin glue may serve as a safe and effective substitute to sutures in enhancing the corneal flap edge healing response and in increasing its mechanical strength.

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.