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D. Luce; Methodology for Cornea Compensated IOP and Corneal Resistance Factor for the Reichert Ocular Response Analyzer . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2266.
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© ARVO (1962-2015); The Authors (2016-present)
The purpose of this study was to determine whether the two applanation pressures (associated with inward and outward corneal motion) as measured by the Reichert Ocular Response Analyzer (RORA) can be combined algorithmically to provide a corneal compensated IOP (IOPcc) and a complementary corneal resistance factor (CRF) independent of the IOPcc.
Operationally, IOPcc validity criteria were defined as: 1) correlation of IOPcc and central corneal thickness (CCT) in a normal population (NP) = (R2 0.5): 2) correlation of IOPcc with CRF in the NP = (R2 0.5): 3) less than 1 mmHg average change of reported IOP as a result of LASIK. IOPcc was derived by minimizing the absolute average IOPcc difference |IOPcc(preLASIK) – IOPcc(postLASIK)| for 30 eyes as a function of k1 and k2 where IOPcc ∝ (k1*p1 + k2*p2). and p1, p2 are the two RORA applanation pressures. CRF validity criterion was defined as correlation with IOPcc = (R2 0.5). CRF is derived by maximizing correlation of CRF ∝ (p1 – k3*p2) with CCT for three independent populations, normal, glaucoma, and OHT. RORA and CCT measurements of a normal reference population (NP) of 183 normal eyes provided data for IOPcc correlation analyses. The glaucoma population (230 subjects) provided data for GAT derived calibration and GAT correlations.
Pre–post LASIK IOP change minimization gives the equation IOPcc = M1*(p2 – 0.43*p1) where M1 is a calibration constant derived from clinical correlation with GAT. Correlations of IOPg and IOPcc with GAT were (R2 = 0.84, p < 10E–3) and (R2 = 0.77, p < 10E–3) respectively. Correlations of CCT with IOPg and IOPcc for the RP were R2 = 0.14, p < 0.000, and R2 = 0.0002, p > 0.84. Correlation of IOPcc and CRF is R2 = 0.001, p > 0.75. Pre–post LASIK correlations were R2 = 0.55 for IOPg to R2 = 0.86 for IOPcc. Maximizing correlation of CRF and CCT for the three populations gives CRF = M2*(p1 – 0.7(+– 0.03)*p2). Correlations of CRF with CCT (R2) for the normal, glaucoma and OHT populations were 0.27, 0.18 and 0.32 respectively. Correlations (R2) of CRF with IOPcc and GAT were 0.001 and 0.18.
Both IOPcc and CRF meet the validity criteria. The simultaneous elimination of correlations of IOPcc with CCT and CRF, small but significant reduction in correlation of IOPcc with GAT, and significantly improved pre–post IOPcc correlation indicates that corneal effects have been essentially entirely removed from IOPcc. CRF is a relatively strong predictor of corneal influence on GAT IOP measurements.
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