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Ana Laura C. Canedo, MD

Comparative Study of Corneal Biomechanical Properties Based on Waveform–Derived Parameters and Tomographic Thickness in Normal and Keratoconic Eyes. Renato Ambrósio Jr, MD, PhD; Ricardo Lousada, MD; Marcella Salomão, MD; Bruno Valbon, MD; Frederico P. Guerra, MD; Michael W. Belin, MD, FACS.

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Ana Laura C. Canedo, MD

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  1. Comparative Study of Corneal Biomechanical Properties Based on Waveform–Derived Parameters and Tomographic Thickness in Normal and Keratoconic Eyes Renato Ambrósio Jr, MD, PhD; Ricardo Lousada, MD; Marcella Salomão, MD; Bruno Valbon, MD; Frederico P. Guerra, MD; Michael W. Belin, MD, FACS Ana Laura C. Canedo, MD

  2. Introduction • Diagnosis of keratoconus and ectatic conditions is a critical part for screening refractive candidates to prevent ectasia. • Biomechanical assessments are promising for assessing ocular rigidity and ectasia susceptibility. • The Reichert ORA (Ocular Response Analyzer) is the first clinically available instrument. • Classical metrics are Corneal Hysteresis (CH), Corneal Resistance Factor (CRF) and gold standard (Goldmann)-correlated intraocular pressure (IOPg) and corneal-compensated intraocular pressure (IOPcc). • CH and CRF are statistically different among keratoconus and normals but there is significant overlap. • Other metrics derived from the waveform provide more detail beyond CH and CRF about corneal biomechanics

  3. Ocular ResponseAnalyzer (ORA, Reichert) Measurement • Corneal response to a collimetric air pulse is monitored by the infrared light reflection (applanation => peak) • Detects two applanation events correlated with the air pulse pressure (INWARD - p1 and OUTWARD - p2) • The delay of p2 is caused by corneal viscous damping • [CH = p1 – p2] and [CRF = p1 - (K * p2)] • Normal Values: CH: 10.17 ± 1.82 (3.23 to 14.58) (Fontes et. Al, JRS 2008) CRF: 10.14 ± 1.8 (5.45 to 15.1) • Ectasia leads to lower CH and CRF and altered signals • CH or CRF < 8.8mmHg is considered a relative contra indication for LASIK based on normal population values • New parameters and waveform score (WS) derive from the ORA signal. ORA Signal Purpose • To CH, CRF IOPcc, IOPg and the novel ORA waveform–derived parameters in normal and keratoconic eyes. • Settings: Instituto de Olhos Renato Ambrósio; Rio de Janeiro Corneal Tomography and Biomechanics Study Group

  4. 226 normal corneas from 113 patients and 88 keratoconic eyes from 44 pa tients. • Eyes were diagnosed as keratoconus based on clinical examination, including corneal topography (Placido) and tomography (rotating Scheimpflug). • CH, CRF and 38 new parameters derived from the ORA waveform signal were extracted from the 2.0 ORA software. • The best waveform signal was chosen from the exam of each eye. • Statistical analysis were accomplished by the BioEstat 5.0 and MedCalc 11.2 • Using unpaired Ttest and Mann Whitney test were used to evaluate statistical significance between groups • Receiver operating characteristic (ROC) curves were used to determine the test’s overall predictive accuracy (area under the curve) and to identify optimal cutoff points to maximize sensitivity and specificity in discriminating keratoconus from normals. • Comparison of ROC Curves were accomplished to evaluate the superiority of the best waveform-derived parameters than CH and CRF. Methods

  5. Statistical significant differences between keratoconus and normals were found in all but 6 parameters: IOPcc; dslope2; W2; dslope21; w1; w21. • The parameters correlated to the area under the applanation signals and first applanation signal height had the best performances to separate the groups. • CRF and CH had best cut off values of 8.3 and 9.1mmHg respectively. • The sensitivity and specificity of CRF were 84,1% and 82,7% and for CH, 81.8 and 78.3%. CRF ranked as the 8th and CH, as 16th parameter on the AUROC. P1area had sensitivity and specificity of 84.1% and 92% and P2-area1, 87.5% and 87.2% respectively. Results

  6. Corneal Hysteresis (CH) p1 area and p2 area and height derived parameters outperformed CH to diagnose keratoconus P-value for ROC Comparisons

  7. Corneal Resistance Factor (CRF) P-value for ROC Comparisons

  8. CH and CCT are not enough. Case examples: A-CCT: 500µm; B-CCT: 531µm; CH is 9.1 mmHg in both. Thickness Profile, CRF and Waveform signal provided critical information for correct diagnostic interpretation! A - TopographyNormal Thin Cornea B - Keratoconus CCT: 536 µm A - Normal Thin CorneaCCT: 500µm B – Topography: Keratoconus

  9. There were significantly higher ORA metrics in normals than in keratoconic eyes. • IOPcc was not significantly different among normals and keratoconus eyes. • Novel metrics derived from the ORA waveform signal provided better performance to identify keratoconus than CH and CRF. • A combination of waveform parameters and tomographic parameters is likely to improve diagnostic performance and provides great potential for artificial intelligence methods for detecting ectasia and its susceptibility. Conclusions

  10. alccanedo@gmail.com

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