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      Corneal Hysteresis and Corneal Resistance Factor in Keratoectasia: Findings Using the Reichert Ocular Response Analyzer

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          Aim: To examine corneal hysteresis (CH) and corneal resistance factor (CRF) in normal and ectatic corneas. Methods: CH and CRF were measured using the Reichert Ocular Response Analyzer in patients with clinically diagnosed keratoconus (KC), forme fruste KC (FFKC) and normal eyes. Results: 21 eyes (13 patients) with clinically diagnosed KC and 30 eyes (18 patients) with FFKC were included in the study. Mean CH and CRF in FFKC did not differ from that in pachymetry-matched normal eyes. KC eyes had significantly lower CH and CRF than normal and FFKC eyes. Conclusion: A significant overlap in CH and CRF values among the 3 groups was evident. Our findings do not indicate a role for CH and CRF measurement as a single test to aid in the detection of early ectasia. It may be of use when used in conjunction with other parameters such as aberrometry. CH and CRF values may prove to be useful in monitoring ectasia progression.

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          Most cited references 9

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          Determining in vivo biomechanical properties of the cornea with an ocular response analyzer.

           D Luce (2004)
          To study the results of an ocular response analyzer (ORA) to determine the biomechanical properties of the cornea and their relationship to intraocular pressure (IOP). Reichert Inc., Depew, New York, USA. The ORA (Reichert) makes 2 essentially instantaneous applanation measurements that permit determination of corneal and IOP effects. Measurements of several populations indicate that corneal hysteresis, a biomechanical measure, varied over a dynamic range of 1.8 to 14.6 mm Hg and was only weakly correlated with corneal thickness (r(2)=0.12); this is related to the observation that some subjects with relatively thick corneas have less-than-average corneal hysteresis. Corneal hysteresis changes diurnally, presumably as a result of hydration changes. Keratoconus, Fuchs' dystrophy, and post-LASIK patients demonstrated low corneal hysteresis. The corneal hysteresis biomechanical measure may prove valuable for qualification and predictions of outcomes of refractive surgery and in other cases in which corneal biomechanics are important.
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            The use of the Reichert ocular response analyser to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes.

            The aim of this study was to measure ocular hysteresis and corneal resistance factor (CRF), novel methods of analysing ocular rigidity/elasticity and to determine the relationship between central corneal thickness (CCT), hysteresis and CRF in normal subjects. Prospective, cross-sectional, clinical trial. The study included 207 normal eyes. Hysteresis and CRF were measured by the ocular response analyser. The CCT was measured using a hand held ultrasonic pachymeter. Ocular hysteresis and CRF in normal patients and their relationship with CCT. The mean hysteresis was 10.7+/-2.0 mmHg standard deviation (S.D.) (range 6.1-17.6 mmHg); the mean CRF was 10.3+/-2.0 (range 5.7-17.1 mmHg). The mean CCT was 545.0+/-36.4 microm (471-650 microm). The relationship between hysteresis and CCT; CRF and CCT; CRF and hysteresis were significant (p<0.0001). This study demonstrated that corneal hysteresis increased with increasing CCT, however, the correlation was moderate. It would appear that CCT, hysteresis and CRF may measure different biomechanical aspects of ocular rigidity and are likely to be useful additional measurement to CCT in the assessment of ocular rigidity when measuring intraocular pressure (IOP). This may be of particular importance when trying to correct IOP measurements for increased or decreased ocular rigidity.
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              Wavefront aberrations measured with Hartmann-Shack sensor in patients with keratoconus.

              To compare the ocular wavefront aberrations of normal and keratoconic eyes and to describe the characteristics of the higher-order aberrations in eyes with keratoconus. Prospective case control and observational study. Thirty-five keratoconic eyes and thirty-eight normal controls. Higher-order aberrations in refraction were measured with a wavefront sensor, and those aberrations resulting from the cornea were evaluated by videokeratographic data. Coma-like (S(3 + 5)), spherical-like (S(4 + 6)), and total (S(3 + 4 + 5 + 6)) higher-order aberrations in both refraction and the cornea. The mean +/- standard deviation of S(3 + 5) (1.88 +/- 1.16), S(4 + 6) (0.70 +/- 0.55), and S(3 + 4 + 5 + 6) (2.03 +/- 1.23) in refraction (6-mm diameter, root mean square, micro m) were significantly higher in the keratoconic eyes than in normal controls (0.26 +/- 0.10, 0.19 +/- 0.10, 0.34 +/- 0.11, respectively; Mann-Whitney U test, P = 0.001). Coma-like aberrations were 2.32 times larger than spherical-like aberrations in keratoconic eyes. The increase of ocular higher-order aberrations in keratoconic eyes results from an increase of corneal higher-order aberrations. Coma-like aberrations were dominant compared with spherical-like aberrations in keratoconic eyes. Wavefront sensing will enable us not only to evaluate the quality of vision but also to differentiate keratoconic eyes from normal eyes by analyzing the characteristics of the higher-order aberrations.

                Author and article information

                S. Karger AG
                September 2008
                15 July 2008
                : 222
                : 5
                : 334-337
                aMater Private Hospital, bChildren’s University Hospital, and cUniversity College Dublin, Dublin, Ireland
                145333 Ophthalmologica 2008;222:334–337
                © 2008 S. Karger AG, Basel

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                Figures: 2, References: 13, Pages: 4
                Original Paper


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