1. Corneal Biomechanics, Pachymetric Progression Profile and Corneal Volume: Indices for Detecting Ectasia and Screening Refractive Candidates
Renato Ambrósio Jr., MD, PhD
Ruiz Alonso, MD; Allan Luz, MD; Daniela Jardim, MD;
Marcella Salomão, MD; Simone Boghossian, MD; Bruno Fontes, MD
Rio de Janeiro - Brazil

2. Introduction
Ultrasonic Pachymetry (US-CCT) and Corneal Topography are considered the “gold standard” for screening refractive candidates
Corneal Tomography (CTm) and Biomechanical Measurements provide further characterization of the cornea beyond surface maps and single point thickness
Purpose
To describe novel parameters, based on corneal geometry or architecture (tomography - CTm) and biomechanical properties, for diagnosing corneal ectasia and ectasia risk (susceptibility)
Describe examples in which these data had enhanced the screening process for refractive candidates

3. Methods
Pentacam exam, taking 25 rotating 360° Scheimpflug images, was used to compute Corneal Tomography (CTm) and extract architecture parameters using custom software from corneal thickness and calculated corneal volume:
Thinnest Point (TP) Value & Position
Corneal Thickness Spatial Profile (CTSP)
Percentage of Increase of Thickness (PIT)
Corneal Volume Distribution (CVD)
Percentage of Increase in Volume (PIV)

4. “TRUE” Thinnest Point (TP) DataOD OS
OD: x is positive towards nasal
OS: x is positive towards temporal

5. Corneal Thickness Spatial Profile (CTSP) and Percentage of Increase of Thickness (PIT)
Average of the thickness values along twenty-two imaginary circles centered on the thinnest point TP with increased diameters at 0,4mm-steps
Percentage of increase from the thinnest point of each of the averages of twenty-two imaginary circles centered on the TP using a simple formula:
- TP) / TP
(x representes the diameter of the imaginary circle)

6. Corneal Volume Distribution (CVD) and Percentage of Increase in Volume (PIV)
Corneal volume was calculated with diameters from 1,0 to 7,0mm with 0,5mm-steps centered on the TP to create the Corneal Volume Distribution (CVD)
The percentage of increase of volume from the 1,0mm volume (PIV) was calculated for each position using a simple formula:
y - 1mm) / 1mm
where y represents the diameter (from 1,0 to 7,0mm) from which corneal volume was calculated

7. Importance of the Pachymetric Map to detect the TRUE Thinnest point
12% of normal patients have more than 10μm of difference between central & thinnest points;
Correlation between Distance from central & thinnest points & Difference (r2 0.61)
34 years old patient, scheduled for LASIK
( x 79)
Normal Topography; US-CCT: 512 μm
(Ave. of 5 measurements with St. Dev <3)
Corneal Thickness Map (Pentacam): Thinnest Point located inferiorly to the geometrical center with 493 μm; careful regional US performed inferiorly and confirmed (495μm)
Decided to change for Surface Ablation
Error in determining the Thinnest Point by measuring US-CCT might be one of the most important causes of “non explained” ectasia after LASIK
(when no risk factors are detected)

8. 46 eyes with mild to moderate keratoconus and from 364 normal eyes
Corneal Thickness Spatial Profile (CTSP) and Corneal Volume Distribution (CVD): Study Results
46 eyes with mild to moderate keratoconus and from 364 normal eyes
Statistically significant differences were observed among the groups (p<0.05) for all positions of CTSP, CVD and for PIT.
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 Nov;32(11):

9. Methods
Corneal biomechanical properties measurement was obtained using the ORA (Ocular Response Analyzer – Reichert) which monitors corneal response to a precise “Gaussian” air pulse, providing two variables:
Corneal Hysteresis (CH = p1 – p2)
Corneal Resistance Factor (CRF = p1 –[k]*p2 )

10. ORA Biomechanical Measurements: Study Results
82 eyes with forme fruste to moderate keratoconus and from 322 normal eyes
CH and CRF are statistically lower in keratoconus than normals (p<0.05).
 Mean – mmHg (+/- SD) CH
CRF
Keratoconus
7.81 (+/-2.25)
7.2 (+/- 2.23)
Normal
10.1 (+/-1.8)
10.0 (+/- 1.9)

11. High “ectasia susceptibility”
CTSP, PIT and CH / CRF help explain “ectasia susceptibility” in mysterious cases
28 yo with progressive ectasia after LASIK in OS (1999) with no identifiable risk factors; central flap thickness: 159 µm and residual stromal bed: 270 µm
Normal Pre Op Topography
MRx: = x 180o, 20/15 - OD
-5.75 = x 10o, 20/15 - OS
US-CCT: 528µm OU
Progressive Ectasia OS
OD remained stable over 7 years would still be considered a good LASIK candidate based on topography and US-CCT.
OD (non operated eye) Borderline CTSP and PIT & low CH (9.8) and CRF (8.1)
High “ectasia susceptibility”
These parameters have been critical for clinical decison for borderline LASIK candidates

12. Novel Parameters can also increase specificity for LASIK screening!
23 yo LASIK candidate, mild asymmetry on the axial maps
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 in both eyes (FU > 1year).
MRx: x 3º OD – 20/ x 179º OS – 20/20
US-CCT: 565 / 545 µm (OD / OS) OD OS
Pre Op. ORA
normal OU OD
UCVA: 20/50 OS
UCVA: 20/20
Pre Op. CTSP and PIT OD / OS
Dif. Axial Maps OD / OS