Renato Ambrósio Jr., MD, PhD Ruiz Alonso, MD; Allan Luz, MD; Daniela Jardim, MD;

Ambrósio Ambrósio et alet al

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Corneal Biomechanics, Corneal Biomechanics, Pachymetric Progression Pachymetric Progression

Profile and Corneal Volume: Profile and Corneal Volume: Indices for Detecting Ectasia Indices for Detecting Ectasia

and Screening Refractive and Screening Refractive

CandidatesCandidates Renato Ambrósio Jr., MD, PhDRenato Ambrósio Jr., MD, PhD

Ruiz Alonso, MD; Allan Luz, MD; Daniela Jardim, Ruiz Alonso, MD; Allan Luz, MD; Daniela Jardim, MD; MD;

Marcella Salomão, MD; Simone Boghossian, MD; Marcella Salomão, MD; Simone Boghossian, MD; Bruno Fontes, MDBruno Fontes, MD

Rio de Janeiro - Brazil

Ambrósio et alAmbrósio et al

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IntroductionIntroductionUltrasonic Pachymetry (US-CCT) and Ultrasonic Pachymetry (US-CCT) and

Corneal Topography are considered the Corneal Topography are considered the “gold standard”“gold standard” for screening refractive candidates

Corneal Tomography (CTm) and Corneal Tomography (CTm) and Biomechanical Measurements provide Biomechanical Measurements provide further characterization of the cornea further characterization of the cornea beyond surface maps and single point beyond surface maps and single point thicknessthickness

PurposePurposeTo describe novel parameters, based on To describe novel parameters, based on

corneal geometry or architecture corneal geometry or architecture (tomography - CTm) and biomechanical (tomography - CTm) and biomechanical properties, for diagnosing corneal ectasia properties, for diagnosing corneal ectasia and ectasia risk (susceptibility)and ectasia risk (susceptibility)

Describe examples in which these data Describe examples in which these data had enhanced the screening process for had enhanced the screening process for refractive candidatesrefractive candidates


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Pentacam exam, taking Pentacam exam, taking 25 rotating 25 rotating 360° Scheimpflug images, was used to 360° Scheimpflug images, was used to compute Corneal Tomography (CTm) compute Corneal Tomography (CTm) and extract architecture parameters and extract architecture parameters using using custom software from corneal custom software from corneal thickness and calculated corneal thickness and calculated corneal volume:volume:

Methods Methods

Thinnest Point (TP) Value & PositionThinnest Point (TP) Value & PositionCorneal Thickness Spatial Profile (CTSP) Corneal Thickness Spatial Profile (CTSP) Percentage of Increase of Thickness (PIT)Percentage of Increase of Thickness (PIT)Corneal Volume Distribution (CVD)Corneal Volume Distribution (CVD)Percentage of Increase in Volume (PIV)Percentage of Increase in Volume (PIV)


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““TRUE” Thinnest Point (TP) TRUE” Thinnest Point (TP) DataData

OD OS

OD: x is positive towards nasalOS: x is positive towards temporal


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Corneal Thickness Spatial Profile Corneal Thickness Spatial Profile (CTSP) and Percentage of (CTSP) and Percentage of

Increase of Thickness (PIT)Increase of Thickness (PIT)

Average of the thickness Average of the thickness values along twenty-two values along twenty-two imaginary circles imaginary circles centered on the thinnest centered on the thinnest point TP with increased point TP with increased diameters at 0,4mm-diameters at 0,4mm-stepssteps

Percentage of increase from the thinnest Percentage of increase from the thinnest point of each of the averages of twenty-two point of each of the averages of twenty-two imaginary circles centered on the TP using a imaginary circles centered on the TP using a simple formula:simple formula:

(CT(CT@@xx - TP) / TP - TP) / TP((x x representes the diameter of the imaginary representes the diameter of the imaginary circle)circle)


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Corneal volume was calculated with diameters Corneal volume was calculated with diameters from 1,0 to 7,0mm with 0,5mm-steps from 1,0 to 7,0mm with 0,5mm-steps

centered on the TP to create the Corneal centered on the TP to create the Corneal Volume Distribution (CVD)Volume Distribution (CVD)

The percentage of increase of volume from the The percentage of increase of volume from the 1,0mm volume (PIV) was calculated for each 1,0mm volume (PIV) was calculated for each

position using a simple formula:position using a simple formula:

(CV (CV @@ y y - CV - CV @@ 1mm 1mm) / CV ) / CV @@ 1mm1mm

where ywhere y represents the diameter (from represents the diameter (from 1,0 to 7,0mm) from which corneal volume 1,0 to 7,0mm) from which corneal volume

was calculatedwas calculated

Corneal Volume Distribution Corneal Volume Distribution (CVD) and Percentage of (CVD) and Percentage of Increase in Volume (PIV)Increase in Volume (PIV)


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Importance of the Pachymetric Importance of the Pachymetric Map to Map to detect the TRUE Thinnest detect the TRUE Thinnest

point point

34 years old patient, scheduled for LASIK34 years old patient, scheduled for LASIK(-5.75 -1.00 x 79)(-5.75 -1.00 x 79)

Normal Topography; US-CCT: 512 Normal Topography; US-CCT: 512 μμm m (Ave. of 5 measurements with St. Dev <3)(Ave. of 5 measurements with St. Dev <3)

Corneal Thickness Map (Pentacam): Thinnest Corneal Thickness Map (Pentacam): Thinnest Point located inferiorly to the geometrical Point located inferiorly to the geometrical center with 493 center with 493 μμm; careful regional US m; careful regional US

performed inferiorly and confirmed (495performed inferiorly and confirmed (495μμm)m)Decided to change for Surface AblationDecided to change for Surface Ablation

Error in determining the Thinnest Point by measuring US-CCT might be one Error in determining the Thinnest Point by measuring US-CCT might be one of the most important causes of “non explained” ectasia after LASIK of the most important causes of “non explained” ectasia after LASIK

(when no risk factors are detected)(when no risk factors are detected)

12% of normal patients 12% of normal patients have more than 10μm of have more than 10μm of

difference between central difference between central & thinnest points; & thinnest points;

Correlation between Correlation between Distance from central & Distance from central &

thinnest points & thinnest points & Difference (rDifference (r22 0.61) 0.61)


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CTSP CTSP –– Spatial Distribution of Thickness Spatial Distribution of Thickness Values for Values for NormalsNormalsand and KeratoconusKeratoconus

PIT PIT –– Percentage of Increase of Thickness from Percentage of Increase of Thickness from the Thinnest Point for the Thinnest Point for NormalsNormalsand and KeratoconusKeratoconus

Corneal Thickness Spatial Profile Corneal Thickness Spatial Profile (CTSP) and Corneal Volume (CTSP) and Corneal Volume

Distribution (CVD): Study Distribution (CVD): Study ResultsResults

Ambrósio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract

Surg. 2006 Nov;32(11):1851-9.

46 eyes with mild to moderate 46 eyes with mild to moderate keratoconus and from 364 normal eyes keratoconus and from 364 normal eyes

Statistically significant differences Statistically significant differences were observed among the groups were observed among the groups (p<0.05) for all positions of CTSP, CVD (p<0.05) for all positions of CTSP, CVD and for PIT.and for PIT.


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MethodsMethodsCorneal biomechanical properties Corneal biomechanical properties measurement was obtained using the measurement was obtained using the ORA (Ocular Response Analyzer – ORA (Ocular Response Analyzer – Reichert) which Reichert) which monitors corneal response monitors corneal response to a precise “Gaussian” air pulse, providing to a precise “Gaussian” air pulse, providing two variables:two variables:

Corneal Hysteresis (CH = p1 – p2)Corneal Hysteresis (CH = p1 – p2)Corneal Resistance Factor (CRF = p1 –Corneal Resistance Factor (CRF = p1 –[k]*p2 )[k]*p2 )


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ORA Biomechanical ORA Biomechanical Measurements: Study ResultsMeasurements: Study Results

82 eyes with forme fruste to moderate 82 eyes with forme fruste to moderate keratoconus and from 322 normal eyes keratoconus and from 322 normal eyes

CH and CRF are statistically lower in CH and CRF are statistically lower in keratoconus than normals (p<0.05).keratoconus than normals (p<0.05).

Mean – mmHg (+/-

SD) CH CRFKeratoconus 7.81 (+/-2.25) 7.2 (+/- 2.23)

Normal 10.1 (+/-1.8) 10.0 (+/- 1.9)


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CTSP, PIT and CH / CRF help explain CTSP, PIT and CH / CRF help explain “ectasia susceptibility” in “ectasia susceptibility” in mysteriousmysterious

casescases28 yo with progressive ectasia after LASIK in OS (1999) with 28 yo with progressive ectasia after LASIK in OS (1999) with no identifiable risk factorsno identifiable risk factors; c; central flap thickness: 159 µm entral flap thickness: 159 µm and residual stromal bed: 270 µm and residual stromal bed: 270 µm

OD remained stable over 7 years would still OD remained stable over 7 years would still be considered a good LASIK candidate be considered a good LASIK candidate based on topography and US-CCT.based on topography and US-CCT.

Normal Pre Op TopographyNormal Pre Op TopographyMRx: -6.00 = -1.00 x 180MRx: -6.00 = -1.00 x 180oo, 20/15 - OD , 20/15 - OD -5.75 = -1.25 x 10-5.75 = -1.25 x 10oo, 20/15 - , 20/15 - OSOSUS-CCT: 528µm OUUS-CCT: 528µm OU

These parameters have been critical for clinical decison for borderline These parameters have been critical for clinical decison for borderline LASIK candidatesLASIK candidates

OD (non operated eye)OD (non operated eye) Borderline CTSP and PIT & Borderline CTSP and PIT & low CH (9.8) and CRF (8.1) low CH (9.8) and CRF (8.1)

High “ectasia High “ectasia susceptibility”susceptibility”

Progressive Ectasia OSProgressive Ectasia OS


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Novel Parameters can also Novel Parameters can also increase specificity for LASIK increase specificity for LASIK

screening! screening! 23 yo LASIK candidate, mild asymmetry on the axial maps23 yo LASIK candidate, mild asymmetry on the axial maps

Normal CTSP and PIT and above average CRF (13.1/12.2 mmHg) and CH Normal CTSP and PIT and above average CRF (13.1/12.2 mmHg) and CH (12.1/12.0 mmHg ) help us to decide doing LASIK, with excellent and stable results (12.1/12.0 mmHg ) help us to decide doing LASIK, with excellent and stable results in both eyes (FU > 1year). in both eyes (FU > 1year).

MRx: - 9.50 -1.75 x 3º OD – MRx: - 9.50 -1.75 x 3º OD – 20/60 20/60 - 7.25 -1.25 x 179º - 7.25 -1.25 x 179º

OS – 20/20OS – 20/20

US-CCT: 565 / 545 US-CCT: 565 / 545 µm (OD / µm (OD / OS)OS)

ODODUCVA: 20/50UCVA: 20/50

OSOSUCVA: 20/20UCVA: 20/20

ODOD OSOS

Pre Op. CTSP and PIT OD / OSPre Op. CTSP and PIT OD / OS Dif. Axial Maps OD / OSDif. Axial Maps OD / OS

Pre Op. ORAPre Op. ORA normal OUnormal OU

Renato Ambrósio Jr., MD, PhD Ruiz Alonso, MD; Allan Luz, MD; Daniela Jardim, MD;

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