1. Relationship between Pachymetry and Ocular Response Analyzer Measurements Alejandro Rivera MD

2. Private Practice. Mexico City. dralexrg@prodigy.net.mxdralexrg@prodigy. Dr. Rivera does not have a financial or propietary interest in any material or method mentioned.

3. The ultrasonic pachymeter is one of the most widely used devices to evaluate the ocular rigidity by means of the measurement of the central corneal thickness (CCT). The Ocular Response Analyzer (ORA) is a device developed by David A. Luce, PhD and Reichert Inc. to determine in vivo biomechanical properties of the cornea (1). It has been used to estimate refractive surgery outcomes and to study intraocular pressure measurement interference.

4. The ORA produces two measurements of corneal biomechanical properties. The corneal hysteresis (CH) refers to the energy lost during the stress-strain cycle and is a measurement of the viscoelastic behavior of the corneal tissue. The corneal resistance factor (CRF) is a measurement of the elastic properties of the cornea (2).

5. Low values of CCT, CH and CRF have been found in keratoconic eyes (3,4). Previous studies have shown the statistical correlation of the CH, the CRF and the CCT (5). Can the CCT alone give us an evaluation of the biomechanical propeties of the cornea? This study was performed to evaluate the clinical relationship between these measurements.

6. MATERIALS AND METHODS. A total of 815 eyes of 408 patients were studied. The data were taken from files of patients evaluated for refractive surgery. Eleven patients with clinical and topographic diagnosis of keratoconus were excluded. The Ocular Response Analyzer (Reichert Corporation, Depew, USA) was used to determine the CH and the CRF. An ultrasonic pachymeter (CompuScan P Model UPC 1000 Storz Instrumens Co. St Louis USA) was used to measure the CCT. The statistical analysis was performed using the Analise-it for Microsoft Excel program (Version 2.07, Analise-it Software, Ltd. Leeds, UK).

7. RESULTS. Relationship between CCT and CRF The relationship was significant (p < 0.0001) Correlation coefficient r = 0.52

8. RESULTS. Relationship between CCT and CH The relationship was significant (p < 0.0001) Correlation coefficient r = 0.44

9. RESULTS. Bland-Almand plot. CR and CH are not measures of the same parameter.

10. DISCUSSION Elasticity refers to how a material deforms in response to stress. The cornea has a non-linear elastic response. Viscous materials do not regain their original shape immediately when the stress is removed. Hysteresis refers to the energy lost when the stress is removed from a viscous material (6). As with most biological materials, collagen is viscoelastic and therefore shows hysteresis. The results of this study demonstrated that the correlation of the CCT measurements with the CRF was stronger than the correlation between CCT measurements and the CH. This means that the CCT is more correlated with the elastic properties of the cornea than with the viscoelastic behavior of the corneal tissue.

11. The ultrasonic pachymeter is one of the most widely used devices to estimate the central corneal thickness in the clinical setting, but it may not be the best device to evaluate the biomechanical properties of the cornea. This could explain why some patients with an average preoperative corneal thickness develop corneal ectasia. Although it has been said that the CH as measured by the Ocular Response Analyzer is not the best predictor of keratoconus (7) it is my opinion that the analysis of all the information we can gather from our patients will give us the best results in our practice.

12. REFERENCES 1.Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005; 31:156-162. 2.Kotecha A. What biomechanical properties of the cornea are relevant for the clinician? Surv Ophthalmol 2007; 52: Suppl 2 S109-S114. 3.Kerautret J, Colin J, Touboul D, Roberts C. Biomechanical characteristics of the ectatic cornea. J Cataract Refract Surg 2008; 34: 510-513. 4.Ortiz D, Piñero D, Shabayek MH et al. Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes. J Cataract Refract Surg 2007; 33:1371-1375. 5. Shah S, Laiquzzaman M, Cunliffe I, Mantry S. The use of the Reichert ocular response analyzer to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes. Contact Lens Anterior Eye 2006; 29: 257-262. 6. Dupps WJ. Hysteresis: new mechanospeak for the ophthalmologist. J Cataract Refract Surg 2007; 33: 1499-1501. 7. Shah S, Laiquzzaman M, Bhojwani R, et al. Assessment of the biomechanical properties of the cornea with the Ocular Response Analyzer in normal and keratoconic eyes. Invest Ophthalmol Vis Sci 2007; 48: 3026-3031.