Correlations between the deformation parameters measured by Corvis ST and corneal morphology parameters in myopic patients

Background

The novel dynamic biomechanical device Corvis ST can measure a variety of corneal deformation parameters and record the dynamic process of cornea deformation.Understanding various corneal deformation parameters is of important clinical significance for morphological and functional prediction for myopic eyes in corneal refractive surgery.However, the range and meaning of corneal deformation parameters in myopic eyes are still unknown.

Objective

This study was to establish the range of corneal defamation parameters by Corvis ST and explore its clinical significance in myopic eyes.

Methods

A descriptive study was designed under the approval of Ethic Committee of Tianjin Eye Hospital and informed consent of patients.This study protocol complied with Helsinki Declaration.Four hundred and seventy-seven eyes of 257 myopic patients who were going to receive corneal refractive surgery were included in Tianjin Eye Hospital from June 2014 to January 2015.The corneal deformation parameters including the first applanation time (1^st A-time), first applanation lengh (1^st A-length), first applanation velocity (1^st A-velocity), second applanation time (2^nd A-time), second applanation length(2^nd A-length), second applanation velocity (2^nd A-velocity), highest concavity time (HC-time), highest concavity deformation amplitude (HC-DA), highest concavity peak distance (HC-PD), highest concavity radius (HC-R), non-contact intraocular pressure (IOPnct) and the central corneal thickness (CCTst) were measured by Corvis ST.Corneal structural parameters including CCT and corneal curvature were measured by 3-D anterior segment analysis system( Pentacam), and corneal biomechanical parameters including corneal resistance factor (CRF), corneal hysteresis (CH), intraocular pressure mimic Goldmann (IOPg) and intraocular pressure of corneal compensation (IOPcc) were measured by Ocular Response Analyzer (ORA). The distributions of the data were tested with the Shapiro-Wilk test and 95% confidence intervals (CI) were calculated.The correlations of the variables were evaluated by Pearson correlation coefficient and Spearman rank correlation coefficient.Multiple linear regression was used to analyze the changes of 1^st A-time, 2^nd A-time and HC-DA with corneal morphology and biomechanical parameters.

Results

Only 1^st A-time, 2^ndA-time, HC-time, HC-DA, CCTst and IOP_nct showed the normality distribution in corneal deformation parameters.The positive correlations were found between 1^st A-time, 1^st A-length, 2^nd A-length, 2^nd A-velocity, HC-R with CCT (r=0.338, r_s=0.129, r_s=0.282, r_s=0.374, r=0.306, all at P<0.01), while 1^st A-velocity, 2^nd A-time, HC-D and HC-PD showed the negative correlations with CCT (r_s=-0.235, r=-0.130, r=-0.259, r=-0.226, all at P<0.01). CRF and CH showed positive correlations with 1^st A-time, 2^nd A-length, 2^nd A-velocity, HC-time, HC-R and negative correlations with HC-PD, △A-length (all at P<0.05).1^st A-time, 2^nd A-time and HC-DA changed with CRF and IOPcc with the regression formations of 1^st A-time =6.185+ 0.066CRF+ 0.034IOPcc (F=300.123, P=0.000), 2^nd A-time=23.397-0.074IOPcc-0.044CRF (F=227.979, P=0.000) and HC-DA= 1.523-0.017IOPcc-0.017CRF (F=152.662, P=0.000). The CCTst, IOPst and IOP_nct values by Corvis ST were significantly lower than CCT by Pentacam ([548.23±26.31]μm, IOPg ([15.02±2.72]mmHg and ([16.02±2.56]mmHg) by ORA respectively (t=11.00, 2.919, 6.815, all at P<0.01).

Conclusions

The corneal deformation parameters by Corvis ST can quantitatively describe corneal biomechanical properties in myopic eyes.Both IOP and the corneal steep curvature of the 3 mm center area (K2) affect the deformation response of cornea.The reliability of Corvis ST in the measurement of CCT and IOP remains to be further studied.