Contact Lens-Induced Corneal Swelling and Surface Changes Measured with the Orbscan II Corneal Topographer

Applications of high‐speed videokeratoscopy

High-speed videokeratoscopy is an emerging technology that has the potential to provide new information on dynamic changes of corneal topography and tear film behaviour. We have developed a high-speed videokeratoscope that has the ability to acquire data at the rate of 50 Hz. Two major applications of the technology are considered in this paper. First, the analysis of tear film stability in the inter-blink interval is evaluated and techniques for estimating the tear film build-up and break-up times are considered. The second application involves the study of the dynamic response of the corneal anterior surface to mechanical forces exerted by the eyelids during horizontal eye movements in downward gaze. The limitations and potential opportunities for the use of this new technology are discussed.

CLEAR - Effect of contact lens materials and designs on the anatomy and physiology of the eye

This paper outlines changes to the ocular surface caused by contact lenses and their degree of clinical significance. Substantial research and development to improve oxygen permeability of rigid and soft contact lenses has meant that in many countries the issues caused by hypoxia to the ocular surface have largely been negated. The ability of contact lenses to change the axial growth characteristics of the globe is being utilised to help reduce the myopia pandemic and several studies and meta-analyses have shown that wearing orthokeratology lenses or soft multifocal contact lenses can reduce axial length growth (and hence myopia). However, effects on blinking, ptosis, the function of Meibomian glands, fluorescein and lissamine green staining of the conjunctiva and cornea, production of lid-parallel conjunctival folds and lid wiper epitheliopathy have received less research attention. Contact lens wear produces a subclinical inflammatory response manifested by increases in the number of dendritiform cells in the conjunctiva, cornea and limbus. Papillary conjunctivitis is also a complication of all types of contact lenses. Changes to wear schedule (daily disposable from overnight wear) or lens materials (hydrogel from SiHy) can reduce papillary conjunctivitis, but the effect of such changes on dendritic cell migration needs further study. These changes may be associated with decreased comfort but confirmatory studies are needed. Contact lenses can affect the sensitivity of the ocular surface to mechanical stimulation, but whether these changes affect comfort requires further investigation. In conclusion, there have been changes to lens materials, design and wear schedules over the past 20+ years that have improved their safety and seen the development of lenses that can reduce the myopia development. However, several changes to the ocular surface still occur and warrant further research effort in order to optimise the lens wearing experience.

Effect of Scleral Lenses on Corneal Topography in Keratoconus: A Case Series of Cross-Linked Versus Non-Cross-Linked Eyes

PURPOSE

To evaluate the changes in anterior corneal topography induced by short-time wear of scleral contact lenses (SLs) in keratoconic subjects with and without a history of corneal cross-linking (CXL).

METHODS

Nine keratoconic patients (14 eyes) were fitted with 18.5 mm SLs for optical rehabilitation. Subjects were divided into 2 groups: 7 eyes without a history of CXL (Non-CXL group) and 7 with a history of CXL (CXL group). Corneal topography was performed at baseline and after 2 and 5 hours of lens wear. The differences for simulated flat (Kflat), steep (Ksteep) and maximal (Kmax) corneal curvatures, central corneal astigmatism (CCA), and central cornea thickness were evaluated.

RESULTS

No statistically significant difference was detected between Non-CXL and CXL groups in any of these measures. Statistically significant flattening was detected in Ksteep Repeated measures analysis of variance ([RM-ANOVA), F (2,24) = 11.32, P < 0.0001], CCA [RM-ANOVA, F (2,24) = 15.34, P < 0.0001], and Kmax [RM-ANOVA, F (2,24) = 19.10, P < 0.0001). From baseline to 5 hours of SL wear, Ksteep decreased on average from 53.1 to 52.4 D, Kmax decreased from 56.7 to 55.8 D, and CCA decreased from 7.2 to 6.3 D. Kmax showed a trend toward more flattening in the Non-CXL group. Central cornea thickness showed significant thickening over time from baseline (451 μm) to 5 hours (458 μm) of SL wear [RM-ANOVA, F (1,12) = 319.3, P < 0.0001].

CONCLUSIONS

Short-term scleral lens wear in keratoconic patients may cause flattening of the anterior cornea. A history of CXL treatment does not guarantee corneal shape stability after scleral lens wear. Practitioners should be aware of these changes because scleral lens wear may mask the signs of keratoconus progression.

Mixed Model Analysis of Between-Subject Variability in Overnight Corneal Swelling and Deswelling With Silicone Hydrogel Lenses

Purpose

To model between subject variability of corneal swelling (CS) and deswelling after overnight wear of silicone hydrogel (SiHy) contact lenses.

Methods

A total of 29 neophyte subjects wore 12 SiHy lenses with central transmissibility range of 31 to 211 Dk/t units on separate nights, in random order, and on one eye only. The contralateral eye served as the control. Central corneal thickness was measured using digital optical pachymetry before lens insertion, immediately after lens removal on waking, then 20, 40 minutes, 1, 2, and 3 hours later. Mixed modelling was conducted for simultaneous analysis of group and between-subject effects of CS and deswelling.

Results

The best model for overnight CS versus Dk/t was linear with a random intercept showing constant between-subject differences in CS for different Dk/t values. The best fit for corneal deswelling versus time was a curvilinear random intercept and random slope model. About 90% of the total between-subject deswelling variance in either lens or control eyes was due to the intercept variability with much less (∼10%) being due to the variability of the individual deswelling rate (slope). Subject age, sex, and ametropia were not predictors of individual corneal swelling in the swelling versus Dk/t analysis. Age, however, was a significant (inverse) predictor of the rate of corneal deswelling, only in lens-wearing eyes.

Conclusions

A large proportion of variability in corneal swelling is because of subject-specific differences in corneal response to hypoxia. This shows that "low swellers" and "high swellers" actually do exist.

Ocular Straylight with Different Multifocal Contact Lenses

PURPOSE

Multifocal contact lenses have been growing in popularity as a modality to correct presbyopic eyes, although visual side effects such as disability glare have been reported. The objective of this study was to investigate the effect of multifocal contact lenses on disability glare by means of ocular straylight.

METHODS

A prospective randomized, comparative study was performed that included 16 subjects free of ocular pathology. Straylight was measured using a commercial straylight meter with the natural and dilated pupil. Participants were fitted with Proclear Multifocal (Distance/Near), ACUVUE Oasys for Presbyopia, and Air Optix Aqua Multifocal randomized to the left or right eye. Straylight measurements were repeated with the contact lens in situ after the pupil dilation. Results obtained with the dilated pupil without contact lens acted as a control.

RESULTS

Diameter of the natural and dilated pupil was 2.87 ± 0.40 mm and 7.45 ± 0.86 mm, respectively (P < .001). After pupil dilation, straylight increased from 0.92 ± 0.13 log(s) to 1.04 ± 0.11 log(s) (P < .001). Of the four studied lenses, a significant difference was only found between Air Optix and the control group (P = .006). The latter showed also slightly increased light scatter.

CONCLUSIONS

A difference in measured straylight was found between the studied multifocal lenses. The observed variability and the straylight-pupil size dependency should be taken into account to avoid elevated straylight in multifocal contact lens wearers. The reason for the observed differences in straylight must be the subject of future studies.

Assessment of modifications in thickness, curvatures, and volume upon the cornea caused by disposable soft contact lens wear

PURPOSE

To assess the effect of different disposable soft contact lenses on several corneal parameters-thickness, anterior and posterior curvature, and volume-by means of a Scheimpflug imaging-based device (Pentacam HR). Diurnal variations of these parameters were taken into account.

METHODS

Twenty-one young, healthy subjects wore 4 different types of daily disposable soft contact lenses on 4 different days: Dailies AquaComfort Plus, SofLens, Dailies Total1, and Acuvue TruEye. The lenses had different material and water content. Pachymetry and curvature maps and corneal volume values were obtained using the Pentacam HR twice a day: one before putting the lens on and one after an 8-hour period of contact lens wear. Measurements were also taken without any contact lenses being worn.

RESULTS

Regarding corneal thickness, the lens with the most similar behavior to the naked eye scenario was the Dailies Total1, causing a thickening of 0.2 ± 0.1% in the central zone and 0.6 ± 0.2% in the periphery. All 4 lenses caused a slight but not significant flattening in the anterior corneal curvature, whereas the posterior corneal curvature only experienced a significant but small steepening with the SofLens. The use of these lenses increased corneal volume slightly.

CONCLUSIONS

Variations in corneal parameters seem to depend on the type of contact lens used (material, oxygen transmissibility, water content). However, the magnitude of the changes introduced by the use of soft contact lenses over the 8-hour period was small and probably not large enough to influence either visual acuity or comfort.

Assessment of corneal morphological changes induced by the use of daily disposable contact lenses

PURPOSE

To assess the effect of different disposable soft contact lenses upon corneal thickness, and upon anterior and posterior corneal curvatures using a dual-Scheimpflug imaging based device.

METHODS

Twenty-eight young, healthy subjects wore four different types of daily disposable soft contact lenses on four different days: Dailies Total1, Proclear 1 Day, Clariti 1-Day and 1-Day Acuvue Moist. The lenses had different material and water content. Pachymetry maps and keratometry values were obtained using the Galilei G4 twice a day: one before putting the lens on and one after an eight-hour period of contact-lens wear. Measurements were also recorded without any contact lenses being worn during a day.

RESULTS

Clariti 1-Day lens caused the greatest thickening in the central (8.9±2.8 μm; p<0.01) and in the peripheral cornea (10.1±4.6 μm; p<0.01), whereas Dailies Total1 was the lens that had the most similar behaviour to the non-contact lens scenario. All the lenses caused a slight flattening in the anterior corneal curvature, except Clariti 1-Day, which induced a very slight steepening. The four lenses caused a steepening of different magnitude in the posterior corneal curvature.

CONCLUSIONS

The magnitude of the changes introduced by the use of soft contact lenses over the eight-hour wearing period was rather small. Thus it is probable it will not influence the vision nor the comfort of the subject. Also, variations on corneal parameters seem to depend on the type of contact lens used.

Corneal swelling caused by conventional and new-design low-Dk soft contact lenses following a 10-day daily wear trial regime

PURPOSE

To investigate the efficacy and safety of a fenestrated and channelled soft contact lens (F-SCL) compared to a standard and non-fenestrated soft contact lens (S-SCL) in experienced soft contact lens (SCL) wearers.

METHODS

This was a randomised, crossover, single-blinded (subject), and multicentre clinical trial. Sixteen experienced SCL wearers were randomly divided into two groups (FS and SF). The FS group first wore F-SCLs followed by S-SCLs, each for 10 days, separated by a 1-week washout period, whereas the SF group wore the S-SCLs first and crossed over to F-SCLs in the same manner. The F-SCLs were designed with three equally spaced, symmetrical fenestrations and a partial-thickness, connecting, circumferential channel on the back surface of the mid-periphery of the lens. Measurement of central corneal thickness using ultrasonic pachymetry was performed on the day of screening, after the 1-week washout period, and after 10 days of wearing each kind of lens, based on which central corneal swelling was calculated and compared. One eye in each subject was chosen at random for analysis.

RESULTS

Central corneal swelling was 1.92±1.73% vs. 5.26±2.14% in F-SCLs vs. S-SCLs wearers, which was statistically significant (P<0.001). There was no significant difference between the groups in terms of SCL-corrected visual acuity or SCL-related adverse events.

CONCLUSION

The use of F-SCLs led to reduced corneal swelling compared to S-SCLs. The newly incorporated features appear to improve tear mixing and thereby the oxygen supply to the cornea, which results in reduced corneal oedema.

Evaluation of a recently developed noncontact specular microscope in comparison with conventional pachymetry devices

PURPOSE

The study was conducted to assess the central corneal thickness (CCT) of the healthy cornea with a recently developed noncontact specular microscope (EM-3000; Tomey) and compare the results with those measured with a contact specular microscope and an ultrasound pachymeter. Agreement between measurements taken by 2 investigators was also studied.

METHODS

The right eyes of 41 healthy individuals who had negative history of contact lens wear, ophthalmic disease, or ocular surgery were examined. The CCT was determined sequentially with a noncontact specular microscope, a contact specular microscope (EM-1000; Tomey), and an ultrasound pachymeter (AL-2000; Tomey). Each evaluation with the specular microscopes was performed by 2 independent operators.

RESULTS

A significant difference was detected in pachymetry measurements among the 3 instruments (p=0.01; analysis of variance). The mean CCT values were lower measured with the ultrasound pachymeter (537+/-30 microm) than the contact endothelial microscope (543+/-37 microm, p=0.17, Student t-test) and the noncontact microscope (549+/-33 microm, p<0.0001) (operator 1). There was no statistically significant difference in CCT measurements between the 2 endothelial microscopes (p=0.19). We found significant correlations (p<0.0001) in thickness measurements between each pair of instruments (r=0.91, noncontact microscopy and ultrasound pachymetry; r=0.74, noncontact and contact microscopy; r=0.72, contact microscopy and ultrasound pachymetry; Spearman rank correlation).

CONCLUSIONS

The strong correlations among the 3 pachymetry devices suggest that the tested instruments provide reliable measurements; however, they cannot be used interchangeably.

Investigation of posterior corneal curvature in CL-induced corneal swelling

PURPOSE

To evaluate Orbscan II measurements without contact lens (CL) removal to monitor in vivo posterior corneal curvature in CL (hypoxia)-induced corneal swelling during extended wear (EW).

METHODS

Orbscan II corneal topographer was used to measure posterior corneal curvature 1 week before CL wearing and during 1 week of EW. Central corneal thickness (CCT) was also measured during the 2-week study with OCT to determine the corneal swelling percentage. High Dk (lotrafilcon A) and low Dk (etafilcon A) soft CL were randomly fitted in EW in the right and left eyes of 20 subjects with normal ocular health. Orbscan and OCT were also performed without CL removal after 3 and 7 days of wear.

RESULTS

Low Dk lenses induced significantly higher corneal swelling (2.41+/-2.65%), as compared to the high Dk lenses (0.17+/-2.28%; P<0.05 repeated measures ANOVA (Re-ANOVA and paired t-test) measures with OCT. The low Dk lens induced a significantly posterior corneal flattening compared to the high Dk lens (0.036+/-0.06mm versus 0.006+/-0.04mm; P<0.05 Re-ANOVA and paired t-test). High repeatability for Orbscan-measured posterior best fit sphere (BFS) radii with (r(2)=0.15, 95% limits of agreement (LoA)=0.10 to -0.06) and without (r(2)=0.00, 95% LoA=0.08 to -0.08) CL removal was also observed and suggest that posterior corneal changes was not due to Orbscan variability.

CONCLUSIONS

Orbscan is a non-invasive and reproducible technique to evaluate and monitoring posterior corneal curvature in vivo in CL (hypoxia)-induced corneal swelling during EW.

Regional variation in the refractive-index of the bovine and human cornea

PURPOSE

Given the refractive importance of the human cornea, surprisingly little attention has been directed to the study of local variation in corneal refractive-index. This in vitro and in vivo study measures the refractive-index of different portions of the bovine and human cornea.

METHODS

Fifty fresh bovine corneas (obtained from an abattoir) and 10 human subjects were used for the study. The refractive-index of the central, nasal, and temporal corneal epithelium was measured with a bench-top Abbe refractometer in the case of bovine corneas and with a hand-held refractometer with humans.

RESULTS

The mean (+/-standard deviation) refractive-indices of the central, nasal, and temporal bovine corneal epithelium were 1.3760 (+/-0.003), 1.3757 (+/-0.002), and 1.3746 (+/-0.002), respectively. Refractive-indices of the anterior and posterior bovine corneal stroma were 1.3731 (+/-0.002) and 1.3708 (+/-0.004), respectively. The mean (+/-standard deviation) refractive-index in the central, nasal, and temporal periphery of the human cornea epithelium were 1.3970 (+/-0.001), 1.3946 (+/-0.001), and 1.3940 (+/-0.001), respectively.

CONCLUSION

There are small local differences in the refractive-index of the bovine and human corneal epithelium and the refractive-index of the epithelium is higher than that of the anterior and posterior stroma of the bovine cornea.

Contact lens-induced corneal peripheral swelling differences with extended wear

PURPOSE

To compare central and peripheral corneal swelling with high-Dk (lotrafilcon A) and low-Dk (etafilcon A) contact lenses (CLs) during extended wear (EW).

METHODS

Corneal thickness was measured in 5 corneal locations (central, superior, inferior, nasal, and temporal) with Orbscan II pachymetry 1 week before the wearing of CLs and after 1 week of EW. High-Dk (lotrafilcon A) and low-Dk (etafilcon A) soft CLs were randomly fitted for EW in the right and left eyes of 20 subjects with normal ocular health. Orbscan was also performed before CL removal after 3 and 7 days of EW.

RESULTS

Lotrafilcon A induced a lower percentage of corneal swelling in all 5 corneal locations than etafilcon A (P < 0.05, paired t test). Lotrafilcon A swelling percentages were as follows: central (1.67% +/- 2.22%), superior (2.04% +/- 3.25%), inferior (1.38% +/- 1.74%), nasal (1.50% +/- 2.00%), and temporal (1.27% +/- 1.47%). Etafilcon A swelling percentages were as follows: central (4.98% +/- 3.07%), superior (6.62% +/- 8.39%), inferior (5.24% +/- 5.54%), nasal (3.41% +/- 3.14%), and temporal (4.46% +/- 3.32%). Differences between the central and peripheral corneal swelling during EW of both types of lenses were not significant (P > 0.05, repeated-measures analysis of variance).

CONCLUSIONS

Central and peripheral corneal thicknesses increase more with etafilcon A than with lotrafilcon A during EW, as measured by Orbscan pachymetry.

Identifying relationships between tomography-derived corneal thickness, curvature, and diameter and in vivo confocal microscopic assessment of the endothelium in healthy corneas of young adults

PURPOSE

Despite the importance of a healthy endothelial layer in anterior segment surgery, the possible influence of corneal parameters on endothelial cell density (ECD) has largely been ignored in the young adult eye. This study investigated the possible associations between corneal tomographic parameters and ECD values in young adults.

METHODS

Subjects aged 21-30 years, with normal corneas were recruited. Mean ECD, mean cell area (MCA), coefficient of variation for cell area (COVA), and proportion of hexagonal cells were derived by in vivo confocal microscopy. The Orbscan II system was used to measure corneal parameters including: thickness, horizontal corneal diameter, corneal curvature, anterior and posterior elevation, and eccentricity.

RESULTS

Sixty-two subjects (42 female, 20 male) were included (mean age 25+/-3 years). Overall mean ECD was 3169+/-309 cells/mm(2) with no differences between genders. Mean percentage of hexagonality was 53+/-5%, male subjects (55%) had a higher percentage of hexagonal cells than female subjects (51%) (P=0.02). Central corneal thickness (CCT) was 529+/-43 microm. Central ECD was significantly correlated with CCT (Pearson's r=0.26, P=0.04). However, horizontal corneal diameter (r=0.19, P=0.14), anterior corneal curvature (r=-0.07, P=0.6), and posterior corneal curvature (r=-0.07, P=0.6) were not correlated with ECD or percentage of hexagonality. There was no statistically significant association between anterior chamber depth (3.6+/-0.3 mm) and ECD (r=0.15, P=0.3).

CONCLUSION

Corneal thickness is related to ECD in normal young adult eye, with lower ECD values identified in thinner corneas; however, corneal diameter and corneal curvature do not have a significant correlation with ECD.

Central corneal thickness measurement with Pentacam, Orbscan II, and ultrasound devices before and after laser refractive surgery for myopia

PURPOSE

To determine the agreement in central corneal thickness (CCT) measurements between the gold standard method of ultrasound (US) pachymetry (UP-1000, Nidek) and 2 noncontact systems based on Scheimpflug imaging (Pentacam, Oculus) and scanning-slit topography (Orbscan II, Bausch & Lomb) in myopic eyes before and after laser refractive surgery.

SETTING

Noor Vision Correction Center, Tehran, Iran.

METHODS

In this prospective study, 30 consecutive patients having refractive surgery for myopia were enrolled. All 60 eyes were examined with the 3 devices preoperatively and 6 weeks after surgery; the US measurements were performed last. The Pentacam and Orbscan II CCT readings were compared with the US readings. Both the original and corrected Orbscan II readings were used in the analyses.

RESULTS

The mean CCT readings with US, Pentacam, and Orbscan II were, respectively, 555 microm, 548 microm, and 580 microm before surgery and 478 microm, 468 microm, and 474 microm after surgery. Preoperatively, the 95% limits of agreement (LoA) with US were -31 microm and +19 microm for the Pentacam device and -5 microm and +57 microm for the Orbscan II device. Postoperatively, the LoA were -40 microm and +19 microm and -51 microm and +50 microm, respectively. Corrected Orbscan II measurements gave 95% LoA of -48 microm and +6 microm before surgery and -85 microm and +5 microm after surgery.

CONCLUSIONS

Refractive surgery had a modest effect on the agreement between Pentacam readings and US measurements. With Orbscan II, the 95% LoA width nearly doubled after surgery. Although the Pentacam seems to show better agreement than Orbscan II, especially after refractive surgery, it is not advisable to use the 3 devices interchangeably in every clinical situation.

Oxygen Demands with Hybrid Contact Lenses

PURPOSE

The purpose of this study was to assess the corneal response, as measured by corneal oxygen uptake, of keratoconic corneas to SoftPerm and SynergEyes hybrid contact lenses at the central cornea, 2.0 and 4.5 mm temporal to the central cornea, and 1 mm temporal to the limbus.

METHODS

Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode on the right eyes of 14 subjects and the left eye of 1 subject, all with keratoconus. Measurements were made at the central cornea, 2.0 and 4.5 mm temporal to the central cornea, and 1 mm temporal to the limbus. They were made for the open eye condition, as well as following 300 s of SoftPerm and SynergEyes hybrid contact lens wear. A one-way analysis of variance (ANOVA) was used to determine the effect of measurement location on oxygen uptake rates under uncovered eye conditions. To determine the difference among oxygen uptake rates relative to those of the uncovered eye at each measurement location for each hybrid lens, a two-way repeated measures ANOVA was used. Multiple comparisons with Tukey-Kramer adjustment were used post hoc to determine which locations were significantly different.

RESULTS

For the uncovered keratoconic cornea, there was no significant difference among the oxygen uptake rates associated with the three corneal locations; however, the oxygen uptake rates measured 1 mm temporal to the limbus were significantly higher than those measured at the three corneal locations. Comparison of oxygen uptake rates measured with the SoftPerm and SynergEyes lenses relative to those of the uncovered eye at each location revealed significantly higher rates at the peripheral cornea than at the central cornea. At all locations, the relative oxygen uptake rates obtained with the SynergEyes lenses were lower than those obtained with the SoftPerm lenses.

CONCLUSIONS

The SynergEyes lens allows significantly more oxygen to reach the cornea during wear than the SoftPerm lens at the central cornea, as well as 2.0 mm and 4.5 mm temporal to the central cornea.

Corneal Refractive Therapy with Different Lens Materials, Part 2: Effect of Oxygen Transmissibility on Corneal Shape and Optical Characteristics

PURPOSE

To compare the effects of two different oxygen transmissible (Dk/t) lenses on corneal shape and optical performance after one night of corneal refractive therapy (CRT(R)) for myopia.

METHODS

Twenty myopic subjects were fit with Menicon Z (MZ) (Dk/t = 90.6, Paragon CRT(R) lenses) on one eye and an Equalens II (EII) CRT lens (Dk/t = 47.2) on the contralateral eye (eye randomized). Corneal topography, refractive error and aberrations were measured before lens insertion (baseline), and the following day after overnight lens wear, on lens removal and 1, 3, 6, 12 h later. Root mean square wavefront errors were measured using 4.5 mm pupils.

RESULTS

Averaged over position and time, the horizontal corneal curvature was statistically different between the MZ and EII lens-wearing eyes (p = 0.011). The central cornea flattened similarly (p = 0.886) and the mid-periphery steepened in both eyes (p = 0.061) from baseline. The EII lens-wearing eyes were steeper in the mid-periphery than the MZ eyes immediately after lens removal and at the 1-h visit (p < or = 0.032). Central corneal flattening and mid-peripheral corneal steepening regressed over time (all p < 0.001) but did not recover to baseline by 12 h (all p < 0.004). Myopia was reduced equally by 0.84 +/- 0.83 D for the MZ-lens wearing eyes and 0.84 +/- 0.87 D for the EII eyes (p = 0.969). Coma increased from baseline 1.85X (0.056 +/- 0.081 microm) for the MZ-lens wearing eyes and 1.72X (0.048 +/- 0.084 microm) for the EII eyes (both p < 0.001). Spherical aberration increased from baseline 4.55X (0.101 +/- 0.077 microm) for the MZ-lens wearing eyes and 4.31X (0.085 +/- 0.076 microm) for the EII eyes (both p < 0.001), but there were no differences between the MZ and EII eyes (all p > or = 0.308). Coma and spherical aberration did not return to baseline by 12 h (both p < or = 0.007).

CONCLUSIONS

After one night of CRT lens wear, changes in corneal shape were slightly different, with more mid-peripheral steepening in the EII eyes compared to the MZ eyes. Change in central corneal curvature and optical performance were similar in both eyes.

Corneal Refractive Therapy with Different Lens Materials, Part 1: Corneal, Stromal, and Epithelial Thickness Changes

PURPOSE

To assess the corneal swelling response to two myopic correction corneal refractive therapy (CRT) lenses of varying Dk/t values, worn for a single night. Change in thickness of the total cornea, stroma, and epithelium was measured across the horizontal meridian using optical coherence tomography (OCT).

METHODS

In this double-masked, randomized study, twenty subjects wore a CRT design lens in each eye, manufactured from Menicon Z (MenZ; Dk/t = 91) and Equalens II (EqII; Dk/t = 47) materials. Baseline corneal thickness was measured centrally and at four points either side of the central cornea using OCT, the night before sleeping at the Centre for Contact Lens Research. The next morning, lenses were removed, and thickness measurements were repeated 1, 3, 6, and 12 h after removal.

RESULTS

On lens removal, the MenZ eye had central and paracentral corneal swelling (mean +/- SD) of 4.1 +/- 2.0% and 5.6 +/- 2.4%, and the EqII eye had 5.8 +/- 2.6% and 7.0 +/- 2.6%. These values were significantly different from baseline (ReANOVA; p < 0.001) and were different between lens materials (p < 0.001). The central epithelium thinned by 10.0 +/- 4.5% in the MenZ eye and by 10.2 +/- 8.5% in the EqII eye, with the mid-peripheral epithelium thickening by 13.4 +/- 7.9% in the MenZ eye and 18.3 +/- 9.8% in the EqII eye (all changes different from baseline p < 0.001). These epithelial values were not statistically different between materials (p > 0.05). Stromal swelling values on lens removal were 5.7 +/- 2.2% centrally and 5.5 +/- 3.0% mid-peripherally (MenZ) and 7.7 +/- 3.1% centrally and 6.6 +/- 2.9% mid-peripherally (EqII) (all p < 0.001 from baseline). Central stromal swelling was different between eyes at lens removal (p < 0.001). Stromal thickness in both eyes returned to baseline values within 3 h.

CONCLUSION

The higher-Dk/t MenZ material caused significantly less overnight corneal and stromal swelling than the Eqll material, which reinforces the need to prescribe lenses with high Dk/t for overnight wear. Neither central epithelial thinning nor paracentral thickening are significantly affected by Dk/t.

Repeatability and Reproducibility of Central Corneal Thickness Measurement With Pentacam, Orbscan, and Ultrasound

PURPOSE

The purpose of this study was to compare central corneal thickness (CCT) measurements obtained with a novel rotating Scheimpflug camera (Pentacam; Oculus) with scanning slit topography (Orbscan; Bausch & Lomb), and with ultrasound pachymetry (SP-2000; Tomey).

METHODS

CCT in 30 healthy eyes was measured twice with each modality by 2 independent observers in random order. The results from scanning slit topography are given both with and without multiplication with the "acoustic correction factor" of 0.92. In addition, the displayed images from the rotating Scheimpflug camera and scanning slit topography were used to calculate the signal difference-to-noise ratios (SD/N) between cornea and background signal.

RESULTS

The mean CCT values as determined with the different modalities (+/-standard deviation) were: 542+/-29 microm, 576+/-37 microm, 530+/-34 microm, and 552+/-32 microm for rotating Scheimpflug imaging, for uncorrected and for corrected scanning slit pachymetry, and for ultrasound, respectively. The differences between modalities (+/-95% limits of agreement) were -9.8+/-31 microm between rotating Scheimpflug and ultrasound, 24+/-31.2 microm between scanning slit and ultrasound, and 33+/-27 microm between scanning slit and rotating Scheimpflug imaging. The limits of agreement for within and between observer effects were within 4.2% of the absolute CCT values for scanning slit and ultrasound and within 2.2% for the rotating Scheimpflug imaging. The rotating Scheimpflug camera showed similar SD/N ratios but steeper edges of the corneal surfaces in the intensity profile plots.

CONCLUSION

In the assessment of normal corneas, the Pentacam measured CCT values closer to ultrasound pachymetry and with less variability compared with Orbscan. The (interobserver) reproducibility with the Pentacam was highest of all 3 modalities.

Short-Term Adaptation of the Human Corneal Endothelium to Continuous Wear of Silicone Hydrogel (Lotrafilcon A) Contact Lenses After Daily Hydrogel Lens Wear

PURPOSE

The purpose of this study is to assess whether improved oxygen availability to the cornea resulted in changes in the corneal endothelium.

METHODS

Eighteen adult (average age 25.3 +/- 5.1 years) hydrogel lens wearers (average of 5.5 years prior daily lens wear, range 3-9 years) were refitted with silicone hydrogel lenses (Focus Night and Day) for continuous wear over 30 days and nights. They were assessed in detail immediately before the refit (baseline measures) and again after 6 months of wear with lens replacement every 30 days. Assessments included slit biomicroscopy (for grading of limbal and bulbar redness), corneal staining with fluorescein, and then by noncontact specular microscopy for central corneal thickness (CCT) and endothelial cell layer images. The endothelial images exported as JPEG files and printed at 7000x magnification. The cell-cell borders were marked and then the areas of an average of 255 cells/image measured with a digitizer pad in stream mode. The number of cell sides was also counted.

RESULTS

After 6 months of silicone hydrogel lens wear, most subjects showed an improvement in mean bulbar (1.1 to 0.9) and limbal redness (1.0 to 0.6, p < 0.001) and epithelial fluorescein staining (0.5 to 0.3) grades, and the mean CCT values decreased slightly in most subjects (0.527 to 0.520 mm), although the decrease was not statistically significant (p = 0.565). The mean endothelial cell area also increased slightly (358 to 363 microm2; p nonsignificant [NS] = 0.701), whereas the mean coefficient of variation (COV) decreased slightly (30.2 to 29.1%, p NS = 0.357). The calculated mean endothelial cell density (ECD; area/1000000 microm2) also decreased slightly from 2821 to 2774 cells/mm, but this change also was not statistically significant (p = 0.620). However, the changes in ECD showed a very substantial relationship to the changes in CCT, i.e., as CCT decreased, so the apparent ECD decreased; this change was highly significant (r = 0.747, p < 0.001). This relationship was slightly stronger still when possible image magnification differences were corrected for. The percentage of six-sided ("hexagonal") cells increased slightly (58.3 to 60.1%).

CONCLUSION

The results indicate that, in silicone hydrogel lens wearers exhibiting positive external eye signs thought to be associated with improved oxygen availability, subtle morphologic changes (marginally decreased polymegethism and pleomorphism) in the central region of the corneal endothelium can occur. It remains to be established, however, whether these changes can be directly attributed to oxygen effects (reduced hypoxia and hypercapnia altering the endothelial cells) or to a mechanical effect (in which changes in corneal thickness result in a reorganization of the corneal endothelium).

Orbscan computerized topography: Attributes, applications, and limitations

An extensive electronic search was undertaken in January 2004 to identify all relevant peer-reviewed publications on Orbscan slit-scanning/Placido computerized topography. Ninety-one publications were identified. These address elevation topography and best-fit sphere, accuracy and repeatability of anterior and posterior corneal elevation and keratometric maps, comparison of Orbscan-acquired data and Placido-based computerized videokeratography instruments, pachymetry measurement and correlation with ultrasound, screening eye-bank corneas, detection of keratoconus, identifying corneal ectasia after refractive surgery, and miscellaneous applications. Studies were analyzed and critically compared in relation to attributes, applications, and limitations of Orbscan corneal topography. The review highlights advantages of this technique in assessing the cornea in health and disease and after surgery and identifies specific aspects that require further investigation and clarification.