The Role of Corneal Biomechanics in the Assessment of Ectasia Susceptibility Before Laser Vision Correction

Figures & data

Table 1 Scheimpflug Camera-Based Corneal Biomechanical Parameters with Explanation

Parameters	Abbreviations	Explanation
Corvis-derived noncontact tonometry IOP (mmHg	cIOP	Corvis-derived intraocular pressure
Corvis-derived central corneal thickness (µm)	cCCT	Corvis-derivated central corneal thickness
First Generation Parameters	Abbreviations	Explanation
Deformation Amp. Max (mm)	MaxDefoA	Corneal deformation amplitude during MaxDT, as the sum of corneal deflection amplitude and MaxWEM
A1 Time (ms)	A1T	Time from the measurement beginning to the first applanation moment
A1 Velocity (m/s)	A1V	Velocity of the corneal apex during the first applanation
A2 Time (ms)	A2T	Time from the measurement beginning to the second applanation moment
A2 Velocity (m/s)	A2V	Velocity of the corneal apex during the second applanation
HC Time (ms)	HCT	Time from the measurement beginning to the moment of reaching the highest concavity (HC)
Peak Dist. (mm)	HCPD	Distance between the corneal peaks at the HC
Radius (mm)	HCR	Radius of corneal curvature during the HC
A1 Deformation Amp. (mm)	A1DefoA	Corneal deformation amplitude during A1, as the sum of corneal deflection amplitude and MaxWEM
HC Deformation Amp. (mm)	HCDefoA	Corneal deformation amplitude during HC, as the sum of corneal deflection amplitude and MaxWEM
A2 Deformation Amp. (mm)	A2DefoA	Corneal deformation amplitude during A2, as the sum of corneal deflection amplitude and MaxWEM
A1 Deflection Length (mm)	A1DL	Horizontal length of the flattened cornea at the A1
HC Deflection Length (mm)	HCDL	Horizontal length of the flattened cornea at the HC
A2 Deflection Length (mm)	A2DL	Horizontal length of the flattened cornea at the A2
A1 Deflection Amp. (mm)	A1DA	Corneal deflection amplitude during A1, determined as the displacement of the corneal apex in relation to the initial state without the MaxWEM quantification
HC Deflection Amp. (mm)	HCDA	Corneal deflection amplitude during HC, determined as the displacement of the corneal apex in relation to the initial state without the MaxWEM quantification
A2 Deflection Amp. (mm)	A2DA	Corneal deflection amplitude during A2, determined as the displacement of the corneal apex in relation to the initial state without the MaxWEM quantification
Deflection Amp. Max (mm]	MaxDA	Corneal deformation amplitude during MaxDT, as the sum of corneal deflection amplitude and MaxWEM
Deflection Amp. Max (ms_	MaxDT	Moment of the maximum deformation, during the oscillatory phase near HC
Whole Eye Movement Max (mm)	MaxWEM	Amplitude of the maximum whole eye movement
Whole Eye Movement Max (ms)	MaxWEMT	Time at which occurs the amplitude of the maximum whole eye movement (near A2)
A1 Deflection Area (mm^2)	A1DArea	Deflection area in A1
HC Deflection Area (mm^2)	HCDArea	Deflection area in HC
A2 Deflection Area (mm^2)	A2DArea	Deflection area in A2
A1 dArc Length (mm)	A1dArcL	Delta arc length of corneal surface in A1
HC dArc Length (mm)	HCdArcL	Delta arc length of corneal surface in HC
A2 dArc Length (mm)	A2dArcL	Delta arc length of corneal surface in A2
dArcLengthMax (mm)	MaxdArcL	Delta arc length of corneal surface in MaxDT
Second Generation Parameters	Abbreviations	Explanation
Max InverseRadius (mm^^–^1)	MIR	1/HCR
DA Ratio Max (2 mm)	DARM2	Apex MaxDA/MaxDA at 2 mm from the apex
PachySlope (µm)	PqS	Peripheric (8 mm horizontal) pachymetry/apex pachymetry
DA Ratio Max (1 mm)	DARM1	Apex MaxDA/MaxDA at 1 mm from the ápex
Ambrosio Relational Thickness (horizontal 8mm)	ARTh	Ambrósio relational thickness in the horizontal 8 mm cornea of the image
Biomechanicallycorrected IOP	bIOP	IOP adjusted for biomechanical parameters
Integrated Radius (mm^^–^1)	IR	Area under the curve of the 1/HCR function
Stiffness parameter in A1	SP-A1	Air puff pressure—bIOP/A1DA
Stress Strain Index	SS-I	Finite element modeling algorithm for the estimation of the nonlinear in vivo biomechanical behavior in corneal with normal topography
Corvis biomechanical index	CBI	Exponential function score made through a logistic regression analysis of six parameters (SP-A1, DARM1, DARM2, ARTh, A1V, and MaxDefoA) and adjusted for IOP and CCT to describe ectasia risk
Tomographic and biomechanical Index	TBI	Generated by the random Forest method with leave-one-out cross-validation, including tomographic and biomechanical parameters, to detect ectasia susceptibility

Figure 1 Example of a printout from the Corvis ST^® describing the corneal shape within different timepoints and several biomechanical parameters.

Table 2 Clinical, Demographic, Tomographic and Score/Indices Analysis, All Samples

	N	Mean	SD	Ectasia High-Risk Cut-Off	Proportion of Eyes in High-Risk	Status of Sample Mean
Age	61	31.0	6.0
BCVA	61	1.0	0.1
MRSE	61	−2.01	2.27
Tomographic parameters
Pupil diameter	61	3.2	0.6
CCT	61	521.3	26.1
TPCT	61	516.5	26.0	<500	70.50%	Low risk
Kmax	61	45.7	1.5	≥45,5	54%	High risk
K1	61	43.2	1.6
K2	61	45.1	1.5
ARTmax	61	352.2	51.1	≤350	49%	Low risk
VCOMA	61	−0.022	0.218	<0	50.80%	High risk
IHD	61	0.012	0.006	≥0010	67.20%	Low risk
Scores/Indices
BAD-D	61	1.90	0.49	≥1,6	77%	High risk
AD	61	50.41	20.25
RSB	61	347.6	24.2	<300	3.20%	Low risk
ERSS	61	3.98	1.96	≥4	62.30%	Low risk
PTA	61	0.33	0.04	≥40%	3.30%	Low risk
BCRClasik	61	7.56	2.20	≥8	54%	Low risk
BCRClprk	61	2.87	3.23	≥8	16.40%	Low risk

Note: Status of sample mean: qualitative characterization of the mean value of the sample in each parameter, regarding the ectasia high risk cutoffs.

Abbreviations: BVCA, best-corrected visual acuity (decimal); MRSE, manifest refraction spherical equivalent (diopters); K1, K2 and Kmax, keratometric data (from the sagittal curvature map, diopters); CCT, central corneal thickness (µm); TPCT, corneal thickness at the thinnest point (µm); IHD, index of height descentration; ARTmax, maximum Ambrósio relational thickness; VCOMA, wavefront-based comatic aberration Zernike coefficient in the 90° meridian, (Z3-1, calculated for a pupil diameter of 6.0 mm, RMS); BAD-D, Belin–Ambrósio deviation index; AD, ablation depth, (µm); RSB, residual stromal bed, (µm); ERSS, ectasia risk score system; PTA, percent tissue altered; BCRClasik,score of the brain cornea risk calculator for LASIK; BCRCprk, score of the brain cornea risk calculator for PRK.

Table 3 Biomechanical Parameters Analysis, All Samples

Biomechanical Parameters	Abbreviation	N	Mean	SD	Ectasia Susceptibility Cutoff	Ectasia High-risk Cutoff	Proportion of Eyes Within Ectasia Susceptibility Interval	Status of Sample Mean
Corvis-derived noncontact tonometry IOP (mmHg)	cIOP	61	13.6	2.1
Corvis-derived central corneal thickness (µm)	cCCT	61	516.0	30.3
First generation parameters
Deformation Amp. Max (mm)	MaxDefoA	61	1.070	0.098
A1 Time (ms)	A1T	61	7.544	0.254	<7.46	<7	37.7%	No susceptibility
A1 Velocity (m/s)	A1V	61	0.148	0.017	>0.14	>0,19	73.8%	Ectasia susceptibility
A2 Time (ms)	A2T	61	22.101	0.385		>21		High risk
A2 Velocity (m/s)	A2V	61	−0.279	0.036	>–0.52	>–0.37	100%	High risk
HC Time (ms)	HCT	61	17.874	0.496		<17.4		No susceptibility
Peak Dist. (mm)	HCPD	61	4.951	0.280		<5.2		High risk
Radius (mm)	HCR	61	6.000	0.633	<7.52	<6.9	96.7%	High risk
A1 Deformation Amp. (mm)	A1DefoA	61	0.133	0.012
HC Deformation Amp. (mm)	HCDefoA	61	1.070	0.098		>1		High risk
A2 Deformation Amp. (mm)	A2DefoA	61	0.326	0.070
A1 Deflection Length (mm)	A1DL	61	2.104	0.305	>1.78	>2	96.7%	High risk
HC Deflection Length (mm)	HCDL	61	6.232	0.460
A2 Deflection Length (mm)	A2DL	61	2.675	0.677	>1.48	>1.8	100%	High risk
A1 Deflection Amp. (mm)	A1DA	61	0.088	0.006
HC Deflection Amp. (mm)	HCDA	61	0.906	0.099
A2 Deflection Amp. (mm)	A2DA	61	0.101	0.010
Deflection Amp. Max (mm)	MaxDA	61	0.917	0.098
Deflection Amp. Max (ms)	MaxDT	61	16.811	0.746
Whole Eye Movement Max (mm)	MaxWEM	61	0.240	0.077
Whole Eye Movement Max (ms)	MaxWEMT	61	21.678	1.407
A1 Deflection Area (mm^2)	A1DArea	61	0.151	0.055
HC Deflection Area (mm^2)	HCDArea	61	3.154	0.501
A2 Deflection Area (mm^2)	A2DArea	61	0.213	0.051
A1 dArc Length (mm)	A1dArcL	61	−0.015	0.003
HC dArc Length (mm)	HCdArcL	61	−0.107	0.027
A2 dArc Length (mm)	A2dArcL	61	−0.020	0.005
dArcLengthMax (mm)	MaxdArcL	61	−0.126	0.026
Second generation parameters
Max InverseRadius (mm^^–^1)	MIR	61	0.207	0.042		>0.2		High risk
DA Ratio Max (2 mm)	DARM2	61	4.512	0.579	>4.80	>4.86	26.2%	No susceptibility
PachySlope (µm)	PqS	61	43.491	6.761
DA Ratio Max (1 mm)	DARM1	61	1.577	0.055	>1.10	>1.63	100%	Ectasia susceptibility
Ambrosio Relational Thickness (horizontal 8 mm)	ARTh	61	481.2	81.3
Biomechanically-corrected IOP	bIOP	61	14.220	1.890
Integrated Radius (mm^^–^1)	IR	61	9.985	1.022		>9.06		High risk
Stiffness parameter in A1	SP-A1	61	93.689	16.622	<93.74	<80.8	49.2%	Ectasia susceptibility
Stress Strain Index	SS-I	61	0.960	0.153
Corvis biomechanical index	CBI	61	0.595	0.227	>0.07	>0.5	95.1%	Ectasia susceptibility
Tomographic and biomechanical index	TBI	61	0.49	0.22	>0.29	>0.79	85.3%	Ectasia susceptibility

Note: Status of sample mean: qualitative characterization of the mean value of the sample in each parameter, regarding the ectasia susceptibility cutoff and high risk cutoff.

Table 4 Comparison of Corneal Biomechanical Parameters Between Groups Divided According Clinically Relevant Selected Cutoffs (Tomographic)

TPCT	Cutoff	N	Mean	SD	p	Behavior
CBI	≥500	43	0.551	0.23	0.019
	<500	18	0.699	0.20		Softer
TBI	≥500	43	0.445	0.20	0.007
	<500	18	0.612	0.23		Softer
A1 Deflection Amp. (mm)	≥500	43	0.090	0.01	0.041
	<500	18	0.086	0.00		Stiffer
A2 Deflection Amp. (mm)	≥500	43	0.103	0.01	0.034
	<500	18	0.096	0.01		Stiffer
A1 dArc Length (mm)	≥500	43	−0.016	0.00	0.014
	<500	18	−0.014	0.00		Stiffer
A2 dArc Length ([mm)	≥500	43	−0.021	0.01	0.029
	<500	18	−0.017	0.00		Stiffer
SP A1	≥500	43	96.598	16.20	0.033
	<500	18	86.738	15.93		Softer
Kmax	Cutoff	N	Mean	SD	p	Behavior
Peak Dist. (mm)	≥45.5	33	4.870	0.23	0.016	Stiffer
	<45.5	28	5.046	0.31
A1 Deformation Amp. (mm)	≥45.5	33	0.136	0.01	0.039	Softer
	<45.5	28	0.129	0.01
HC Deflection Length (mm)	≥45.5	33	6.069	0.34	0.003	Softer
	<45.5	28	6.425	0.52
HC Deflection Area (mm^2)	≥45.5	33	3.030	0.40	0.041	Softer
	<45.5	28	3.300	0.58
PachySlope (µm)	≥45.5	33	45.065	6.40	0.047	Softer
	<45.5	28	41.636	6.82
ARTmax	Cutoff	N	Median	SD	P	Behavior
TBI	≥350	30	0.430	0.17	0.026
	<350	31	0.556	0.25		Softer
Radius (mm)	≥350	30	6.216	0.70	0.007
	<350	31	5.790	0.48		Softer
Deflection Amp. Max (ms)	≥350	30	17.019	0.74	0.032
	<350	31	16.611	0.70		Softer
IHD	Cutoff	N	Mean	SD	p	Behavior
A1 Velocity (m/s)	≥0.010	41	0.144	0.02	0.014	Stiffer
	<0.010	20	0.155	0.01
ARTh	≥0.010	41	498.872	79.11	0.014	Stiffer
	<0.010	20	445.104	75.14
VCOMA	Cutoff	N	Mean	SD	p	Behavior
CBI	≥0.000	30	0.527	0.22	0.021
	<0.000	31	0.660	0.22		Softer
Pachy (µm)	≥0.000	30	526.233	31.71	0.009
	<0.000	31	506.161	25.65		Softer
Def. Amp. Max (mm)	≥0.000	30	1.044	0.10	0.034
	<0.000	31	1.096	0.09		Softer
A1 Time (ms)	≥0.000	30	7.624	0.23	0.015
	<0.000	31	7.468	0.25		Softer
A2 Velocity (m/s)	≥0.000	30	−0.269	0.03	0.028
	<0.000	31	−0.289	0.04		Softer
Peak Dist. (mm)	≥0.000	30	4.840	0.26	0.002
	<0.000	31	5.057	0.26		Softer
HC Deformation Amp. (mm)	≥0.000	30	1.044	0.10	0.034
	<0.000	31	1.096	0.09		Softer
HC Deflection Length (mm)	≥0.000	30	6.093	0.44	0.019
	<0.000	31	6.367	0.45		Stiffer
HC Deflection Amp. (mm0	≥0.000	30	0.866	0.09	0.001
	<0.000	31	0.945	0.09		Softer
Deflection Amp. Max (mm)	≥0.000	30	0.879	0.09	0.002
	<0.000	31	0.954	0.09		Softer
HC Deflection Area (mm^2)	≥0.000	30	2.955	0.45	0.002
	<0.000	31	3.347	0.48		Softer
dArcLengthMax (mm)	≥0.000	30	−0.118	0.02	0.013
	<0.000	31	−0.134	0.03		Softer
DA Ratio Max (2 mm)	≥,000	30	4.307	0.39	0.006
	<0.000	31	4.710	0.67		Softer
DA Ratio Max (1 mm)	≥0.000	30	1.562	0.04	0.034
	<0.000	31	1.591	0.06		Softer
SP A1	≥0.000	30	101.049	15.29	0.000
	<0.000	31	86.565	14.83		Softer

Note: Behavior column: qualitative characterization of the eyes included in the groups from the same line. Wavefront-based comatic aberration Zernike coefficient in the 90° meridian (VCOMA, Z3-1, calculated for a pupil diameter of 6.0 mm, RMS). Behavior column: qualitative characterization of the eyes included in the groups from the same line.

Abbreviations: TPCT, corneal thickness at the thinnest point (µm); Kmax, maximum keratometry (diopters); ARTmax, maximum Ambrósio relational thickness; IHD, index of height descentration.

Table 5 Comparison of Corneal Biomechanical Parameters Between Groups Divided According Clinically Relevant Selected Cutoffs (Scores)

ERSS	Cutoff	N	Mean	SD	p	Behavior
CBI	≥4	38	0.635	0.25	0.051	Softer
	<4	23	0.529	0.16
TBI	≥4	38	0.572	0.22	0.000	Softer
	<4	23	0.365	0.16
A2 Time (ms)	≥4	38	22.024	0.34	0.042	Stiffer
	<4	23	22.229	0.42
dArcLengthMax (mm)	≥4	38	−0.121	0.03	0.043	Softer
	<4	23	−0.135	0.02
BCRClasik	Cutoff	N	Mean	SD	p	Behavior
TBI	≥8	33	0.545	0.24	0.050	Softer
	<8	28	0.433	0.20
A1 Deformation Amp. (mm)	≥8	33	0.136	0.01	0.043	Softer
	<8	28	0.129	0.01

Note: Behavior column: qualitative characterization of the eyes included in the groups from the same line.

Abbreviations: ERSS, Ectasia risk score system; BCRClasik, score of the brain cornea ectasia risk calculator for LASIK.