REFRACTIVE SURGERY Factors affecting corneal hysteresis in normal eyes Kazutaka Kamiya & Mana Hagishima & Fusako Fujimura & Kimiya Shimizu Received: 11 March 2008 /Revised: 1 May 2008 /Accepted: 2 May 2008 / Published online: 11 June 2008 # Springer-Verlag 2008 Abstract Background To evaluate factors affecting corneal hysteresis (CH) in normal eyes. Methods We examined 86 normal eyes of 43 healthy volunteers (age, 39.1��14.5 years (mean �� standard devia- tion) range, 19 to 68 years gender, 26 men, 60 women manifest refraction, -2.25��2.89 diopters (D) range, -9.13 to 3.88 D). We quantitatively assessed the value of CH using an Ocular Response Analyzer��� (Reichert Ophthal- mic Instruments). We carried out this measurement three times, and the average value was used for statistical analysis. Multiple regression analysis was used to assess the relevant factors of the CH. Results The mean CH was 10.2��1.3 mmHg. Explanatory variables relevant to the CH were, in order of magnitude of influence, the central corneal thickness (CCT) (partial regression coefficient B=0.022, p0.0001), and the intra- ocular pressure (IOP) (B=-0.119, p=0.04). No significant correlation was seen with other clinical factors such as age, gender, manifest refraction, or mean keratometric readings. Conclusions Eyes with thinner CCT and eyes with higher IOP are more predisposed to have lower CH. Refractive surgeons should, from a biomechanical viewpoint, take not only CCT but also IOP into consideration before performing keratorefractive surgery. Keywords Corneal biomechanics . Corneal hysteresis . Corneal thickness . Intraocular pressure Introduction There have been numerous reports of iatrogenic ectasia developing after excimer laser surgery, possibly resulting from biomechanical weakening of the cornea. Moreover, the biomechanical properties of the cornea may affect not only the refractive outcomes after keratorefractive surgery such as laser in situ keratomileusis (LASIK), leading to unpredictability for this surgical technique [1���5], but also the measurement of the intraocular pressure (IOP) especial- ly in eyes undergoing LASIK [6���9]. Thus, it is of clinical importance to assess the biomechanical properties of the cornea, but the methodology for the in vivo assessment of corneal biomechanics has not long been established. Recently, the Ocular Response Analyzer��� (Reichert Ophthalmic Instruments, Depew, NY, USA) has enabled us quantitatively to assess the biomechanical properties of the cornea such as corneal hysteresis (CH) [10], but there have been no studies on the clinical factors behind changes in CH. The purpose of the current study is to investigate the factors that affect the CH in healthy eyes. Material and methods Eighty-six eyes of 43 healthy volunteers (26 of men and 60 of women) who had no ophthalmic diseases other than refractive errors, were enrolled in this observational study. The age of the volunteers was 39.1��14.5 years (mean age �� standard deviation range, 19 to 68 years). The manifest refraction (spherical equivalent) was -2.25��2.89 diopters Graefes Arch Clin Exp Ophthalmol (2008) 246:1491���1494 DOI 10.1007/s00417-008-0864-x K. Kamiya (*) : M. Hagishima : K. Shimizu Department of Ophthalmology, University of Kitasato School of Medicine, 1���15���1 Kitasato, Sagamihara, Kanagawa 228���8555, Japan e-mail: kamiyak-tky@umin.ac.jp F. Fujimura School of Allied Health Sciences, University of Kitasato, Kanagawa, Japan

(D) (range: from -9.13 to 3.88 D). Informed consent was obtained from all volunteers. The study adhered to the tenets of the Declaration of Helsinki. Institutional review board approval was not required for the study. We measured the biomechanical properties of the cornea characterized by corneal hysteresis (CH) using an Ocular Response Analyzer���. This device utilizes a sudden air impulse to deform the cornea, and the shape changes are monitored by an electro-optical system. The puff of air induces two corneal applanations, inward and outward, of the cornea. The air stream deforms the cornea through an initial applanation event (peak 1), then beyond it into a concavity, which then subsides, allowing the cornea to rebound through a second applanation (peak 2). The main outcome measure was the value of CH, which was calculated as the difference between pressure peaks 1 and 2 (mmHg). We carried out this measurement three times, and the average value was used for statistical analysis. Stepwise multiple regression analysis was performed to investigate the factors that significantly affected the value of CH. The dependent variable was CH. The explanatory variables included age, gender, manifest refraction (spher- ical equivalent), mean keratometric readings, central corne- al thickness (CCT), and intraocular pressure (IOP). The mean keratometric readings were measured using an autorefractometer (ARK-700A, Nidek, Gamagori, Japan). The CCT was measured using an ultrasound pachymeter (DGH-500, DGH Technologies, Exton, PA, USA). The IOPs were measured using a Goldmann applanation tonometer (Zeiss, Oberkochen, Germany). Each measure- ment was repeated three times, and the average value was used for analysis. On account of the known effect of tonometry in lowering IOP, we selected at random the tonometer type and the first eye to be measured. Topical anesthetic was placed in each eye before the ultrasonic pachymetry and Goldmann tonometry. All measurements were performed at the same time of day to decrease the effect of the diurnal curve. All statistical analyses were performed using SPSS (SPSS Inc, Chicago, IL, USA). The results are expressed as mean �� standard deviation, and a value of p0.05 was considered statistically significant. Results Demographic data of healthy volunteers are listed in Table 1. The mean keratometric readings were 43.6��1.6 D (range: from 40.0 to 47.4 D). The mean CCT was 540.4��31.0 ��m (range: from 476 to 598 ��m). The mean IOP was 14.4�� 2.0 mmHg (range: from 10 to 21 mmHg). The mean CH was 10.2��1.3 mmHg (range: from 6.7 to 13.3 mmHg). The results of multiple regression analysis are shown in Table 2. The explanatory variables relevant to the CH were the CCT (partial regression coefficient B=0.022, p0.0001), and the IOP (B=-0.119, p=0.04). The multiple regression equation was expressed as follows: CH (mmHg) = (0.022��CCT) + (-0.119��IOP) - 0.141. No significant correlation was seen with other clinical factors such as age, gender, manifest refraction, or mean keratometric readings. We calculated the standardized partial regression coefficient in order to determine the magnitude of each variable���s influence. The CCT was the most relevant variable, and the IOP was the second, as Table 2 shows. The relationships of the CH with the CCT and with the IOP are shown in Figs. 1 and 2 respectively. With a thinner CCT, a higher IOP, or both, the CH became significantly lower in healthy eyes. Discussion In the current study, we have demonstrated that CH decreases in eyes with thinner CCT or higher IOP, or both. Although the CCT and the IOP alone cannot provide sufficient explanation, as evidenced by the small R2 value Table 1 Demographic data Item Measurement Gender (% of women) 69.8% Manifest refraction (D) -2.25��2.89 D (range, -9.13 to 3.88 D) Mean keratometric readings (D) 43.6��1.6 D (range, 40.0 to 47.4 D) Central cornea thickness (��m) 540.4��31.0 ��m (range, 476 to 598 ��m) Intraocular pressure (mmHg) 14.4��2.0 mmHg (range, 10 to 21 mmHg) Corneal hysteresis (mmHg) 10.2��1.3 mmHg (range, 6.7 to 13.3 mmHg) Table 2 Results of stepwise multiple regression analysis to select variables relevant to the corneal hysteresis (CH) Variables Partial regression coefficient Standardized partial regression coefficient P value Central corneal thickness (��m) 0.022 0.564 0.0001 Intraocular pressure (mmHg) -0.119 -0.198 0.04 Constant -0.141 Adjusted R=0.309 1492 Graefes Arch Clin Exp Ophthalmol (2008) 246:1491���1494