Correlations between corneal biomechanical properties measured with the ocular response analyzer and icare rebound tonometry

Purpose: To investigate the biomechanical properties of the normal cornea, and correlate them with central and peripheral corneal thickness and age. Methods: Seventy-six right eyes of volunteers were measured with Ocular Response Analyzer (ORA), ICare rebound tonometry and an ultrasound pachymeter at corneal center and at 4 mm from corneal center in the nasal and temporal directions. Results: ICare readings were significantly correlated with central and peripheral corneal thickness and corneal biomechanical properties. Corneal resistance factor was the biomechanical parameter with the higher correlation with ICare IOP values. ICare tonometry at center and Goldmann equivalent IOP obtained with ORA were significantly higher for thicker than thinner corneas (p<0.05). IOP compensated for corneal properties with the ORA was lower than the remaining IOP values measured in the study. Higher correlation was found between Goldmann equivalent IOP with ORA and ICare IOP values. Conclusions: Intra-ocular pressure values obtained with the rebound tonometer are higher in thicker corneas and are positively correlated with biomechanical corneal parameters, namely corneal resistance factor. Although corneal thickness plays a significant role in rebound tonometry, elastic and viscous properties of the cornea seem to play a significant role in the interaction of the tonometer probe with the ocular surface. However, the mechanism behind this process is presently unknown

A pilot study on the influence of biomechanical properties of the cornea on short-term response to myopic corneal refractive therapy

The authors state that they do not have any proprietary interest in the products named in this article.To study the short-term corneal response to corneal refractive therapy for myopia and correlate it with corneal biomechanical properties as measured with the ocular response analyzer. METHODS: Eight eyes from 8 young subjects were fitted with a reverse geometry contact lens, attempting a myopic correction of -4.00 D. Corneal resistance factor and corneal hysteresis (CH) were measured before contact lens fitting with the ocular response analyzer. These parameters were correlated with the degree of change in apical curvature, simulated keratometry, and central corneal thickness after 3 hours of contact lens wear (effect) and 3 hours after lens removal (recovery). RESULTS: There was a trend toward a faster effect and faster recovery of the orthokeratologic effect for corneas with less resistance in terms of biomechanical properties. Corneal resistance factor did not correlate significantly, however, with any of the topographic and pachymetric parameters. Conversely, CH was significantly correlated with changes in steep keratometry (0.758; P = 0.029) and central corneal thickness (0.755; P = 0.030) during lens wear and with changes in steep keratometry (-0.835; P = 0.010) during recovery. Overall, higher values of CH meant slower effect and recovery of the orthokeratologic effect. CONCLUSIONS: Short-term response of human cornea to corneal refractive therapy is correlated with the biomechanical properties of the cornea. Of the different theories supporting such involvement of corneal response to reverse geometry contact lenses, the most likely one seems to be the one assuming a faster response and faster recovery for corneas with lower resistance. Larger sample studies would be needed to clarify the involvement of corneal biomechanical properties on corneal response to orthokeratology.Supported in part by a grant from the Science and Technology Foundation (FCT), Ministry of Science and Superior Education (MCES), and European Social Fund (ESF) under contract 8281/2002 granted to J.M.G.-M

Symptoms in a population of contact lens and noncontact lens wearers under different environmental conditions

None of the authors has a commercial or financial interest in the instruments or materials used in the study. The authors wish to thank the assistance of AP Almeida, AP Costa, A, Fernandes, A Teixeira, C Marques, C Pinho and J Matos for their assistance in data acquisition, and Dr. Ana Costa from the program of Support to the Edition of Scientific Papers at the University of Minho for help with English editing of the manuscript.PURPOSE: To investigate ocular symptoms related to dryness in an adult population of contact lens (CL) and non contact lens wearers (n-CL) using video display terminals (VDT) for different periods of time under different indoor conditions related to air conditioning (AC) and heating units (HU) exposure. METHODS: A questionnaire was distributed to 334 people within a university population of which 258 were part of the n-CL group and 76 of the CL wearers to assess symptoms of ocular discomfort potentially related to dryness. Only soft contact lens (SCL) wearers (n = 71) were included for further statistical analysis because of the reduced number of people wearing other lens types. A 2:1 match by gender group of 142 subjects in the n-CL group was used as a control sample. RESULTS: There was a marked difference between the prevalence of symptoms and the way they are reported by CL and n-CL wearers. Red eye, itching, and scratchiness are more common among CL wearers, but the difference is statistically significant only for scratchiness (p < 0.01, chi(2)). The vast majority of subjects who reported symptoms often and at the end of the day are significantly more prevalent among CL wearers (p < 0.01, chi(2)). Gender differences were also encountered. Female CL wearers reported more scratchiness than males in the n-CL wearing group (p = 0.029, chi(2)) and in the CL group (p < 0.008, chi(2)). Females wearing CL reported symptoms of red eye (p = 0.043, chi(2)) and scratchiness (p < 0.001, chi(2)) more significantly than those in the n-CL group. Within the CL group, the prevalence of symptoms occurring sometimes or often and at the end of the day was higher among females (p < 0.001, chi(2)). The use of VDT was associated with a higher level of scratchiness among CL wearers (p < 0.05, chi(2)). The number of hours working with VDTs seemed to be associated with an increase in the prevalence of burning sensation in the CL group (p < 0.01, chi(2)), whereas symptoms like red eye and scratchiness also increased significantly among n-CL wearers. Compared to n-CL wearers, all symptoms increase in CL wearers in environments with AC and HU, except excessive tearing. However, these differences are only statistically significant for scratchiness. CONCLUSIONS: Our results show that people who wear soft CL and work with VDTs for longer periods of time are more likely to develop symptoms like eye burning and scratchiness than n-CL wearers. This risk could be higher for women than men. Scratchiness and the appearance of symptoms near the end of the day are typically associated with ocular discomfort during CL wear in this sample, and clinicians should question their patients about these symptoms to anticipate serious discomfort.This study was partially supported by a grant from the Science and Technology Foundation (FCT) - Ministry of Science and Superior Education (MCES) under contract 8281/2002 from the European Social Funding granted to JMG-M

Accuracy and repeatability of a new portable ultrasound pachymeter

To assess the accuracy and repeatability of central corneal thickness (CCT) measurements taken with a new portable ultrasound (US) pachymeter. Methods: Central thickness measurements were taken with a portable pachymeter (SP-100 Handy;Tomey, Nagoya, Japan) and a conventional US pachymeter (Nidek UP-1000; Nidek Technologies, Gamagori, Japan) from 57 right corneas of 57 young adults (19 males, 38 females) aged 18–44 years (mean ± S.D., 22.95 ± 3.92). Three repeated measures were obtained and then compared to obtain the repeatability of each instrument and the agreement between the pachymeters. The three readings taken with the portable pachymeter were compared against each other in order to evaluate intra-session repeatability and bias of each individual measurement with respect to the mean of three. Results: Mean values of CCT were 537 ± 35 micron for conventional and 534 ± 35 micron for the new portable pachymeter. A high agreement was found between the two instruments mean difference = 2.58 micron; 95% CI 1.41–3.75 micron) with only two eyes presenting differences larger than ± 8.6 micron which represents 95% CI in the Bland-Altman plots which represents 1.6% of the mean CCT. The first reading taken showed the highest agreement with the mean value for the portable pachymeter. Conclusions: The instrument tested in this study is able to take reliable measurements of corneal thickness even if a single reading is considered. Intra-session repeatability was very high, as was also the agreement between the average of three readings taken with the two US pachymeters

Topographic paracentral corneal thickness with pentacam and orbscan: effect of acoustic factor

Purpose: To evaluate the effect of an acoustic factor (AF) on the comparison of central corneal thickness (CCT) and peripheral corneal thickness (PCT) measurements with Orbscan II and Pentacam. Methods: The CCT and PCT at 1, 2, and 3 radial distances from the corneal apex were measured using Orbscan II and Pentacam in 22 right eyes of 22 healthy adults (7 men, 15 women). Three measures were obtained from each 1 of the 25 points measured and then compared to gauge the agreement between both devices at the corneal center and anular areas located at 1-, 2-, and 3-mm distances from the central measurement. Orbscan II readings were considered with and without an AF correction. Results: Pentacam provides statistically significant higher values than Orbscan II does at all the 25 locations analyzed (P,0.001). With a few exceptions, the average difference was fairly constant between 20 and 40 mm for all the corneal locations. A high correlation existed between central readings (r2=0.927; P,0.001) and average thickness at each one of the annular areas being analyzed (r2=0.897 at 1 mm, r2=0.876 at 2 mm, and r2=0.870 at 3 mm); Pentacam minus Orbscan II value averaged for all the points changed from 228610 to 22269 mm after the removal of the AF in Orbscan II. Conclusions: Central and peripheral measurements of the corneal thickness obtained with Orbscan II and Pentacam are significantly different. The removal of the AF in Orbscan II renders lower mean differences but decreases the agreement between both systems and potentially induces an overestimation of CCT and PCT by Orbsan II compared with Pentacam.Supported by national grant PI081380 from Instituto de Salud Carlos III, Spain granted to Ocular Surface and Contact Lens Research Group GI-1750