Correlation of central and peripheral corneal thickness in healthy corneas

Purpose

To study the thickness profile of the normal cornea in order to establish any correlation between central and peripheral points.

Methods

Sixty-seven eyes of 40 patients were subjected to central corneal thickness measurement (CCT) with an ultrasound pachymeter (UP) and corneal thickness mapping with the Oculus Pentacam. The corneal apex thickness (CAT), pupil centre thickness (recorded as CCT and corresponded to CCT of UP) and thickness at the thinnest location (CTL) were obtained and compared with each other. Corneal thickness data at 3 mm and 7 mm temporally, nasally, superiorly and inferiorly from the corneal apex were obtained. The mean corneal thickness values along the 2, 4, 6, 8 and 10 mm diameter concentric circles, with the CTL as the centre, were also obtained. The above data at different points were statistically correlated.

Results

There was no significant difference between CCT readings measured by UP and Pentacam (P = 0.721). There was high positive correlation between the CAT values and the thickness at 3 mm (R ≥ 0.845, P < 0.001) and at 7 mm points (R ≥ 0.654, P < 0.001). A gradual increase in thickness was noted from the centre to the periphery with a high positive correlation between the CTL values and the mean thickness at the circles of 2, 4, 6, 8 and 10 mm (R ≥ 0.635, P < 0.001).

Conclusion

The results suggest that central corneal thickness can serve as a good guide for predicting peripheral thickness. For surgical procedures specifically undertaken at mid-peripheral and peripheral zones, the actual measurements at the site of surgery may confer some advantage.     

Corneal structure,transparency, thickness and optical density (densitometry), especially as relevant to contact lens wear—a review

Clinical instruments using Scheimpflug image-based methods to obtain optical sectional images of the cornea have been introduced in recent years along with proposals that it should be possible to routinely and reliably measure the optical density (referred to as the densitometry) of the human cornea in situ. Such a concept is reviewed from the perspective of what might be considered as the basic principles underlying the understanding of corneal transparency (from the 1950’s) and the progressive changes in these ideas from subjective slitlamp-based clinical observations from the late 1960’s, especially in contact lens wearers. Much more has been learned about the overall macrostructure (including corneal thickness) and the ultrastructure of the cornea from contemporary studies in the 1990’s, and these aspects of the cornea will be reviewed alongside consideration of the methods of assessing the optical characteristics of the cornea in the living eye. From these perspectives, in this review systematic consideration will be given to what objective (quantitative) output one of these Scheimpflug-based systems provides and how this information might be actually related to corneal transparency characteristics that might be observed clinically, particularly after long-term contact lens wear.     

The temporal dynamics of miniscleral contact lenses: Central corneal clearance and centration

Purpose

To examine the time course of the reduction in central corneal clearance and horizontal and vertical lens translation (decentration) during miniscleral contact lens wear and the theoretical influence upon the optics of the post-lens tear layer.

Efficacy of corneal tomography parameters and biomechanical characteristic in keratoconus detection

AimTo determine the efficacy of corneal thickness parameters and corneal biomechanical properties (CBPs) in discriminating between normal and keratoconus eyes.MethodAfter performing a comprehensive ophthalmic examination, 50 mild to moderate keratoconus and 50 age and sex matched myopic astigmatism eyes were prospectively included in the study. The corneal topographic maps and CBP were obtained by Pentacam and Ocular response analyser, respectively. Central corneal thickness (CCT), thinnest corneal thickness (TCT), corneal thickness (CT) and percentage thickness increase (PTI) at 1, 3 and 5 mm from the thinnest point and corneal volume (CV) at 3, 5, 7 and 10 centred on thinnest point, corneal hysteresis (CH) and corneal resistance factor (CRF) were recorded. Independent t-test and receiver operating characteristic (ROC) were done with SPSS software (version 15.0, SPSS, Inc.).ResultsCCT, TCT, CT at 1, 3 and 5, CV at 3, 5, 7 and 10 mm, CH and CRF were significantly lower in keratoconus eyes compared to controls (p < 0.001). In addition, PTI at 1, 3 and 5 mm from the thinnest point showed significantly higher values in keratoconus group. ROC analysis demonstrated good predictive accuracy for cut-off point values. However, the centrally located indices had higher predictive accuracy compared to the peripherally located indices.ConclusionAlthough good sensitivity and specificity were found for the mentioned parameters, the centrally located indices had higher predictive accuracy compared to peripherally located indices. It is suggested to use a combination of corneal pachymetry together with CBP for more accurate detection of keratoconus.     

Corneal topography with an aberrometry-topography system

Purpose

To investigate the agreement between the central corneal radii and corneal eccentricity measurements generated by the new Wave Analyzer 700 Medica (WAV) compared to the Keratograph 4 (KER) and to test the repeatability of the instruments.

Patient attitudes and behavior regarding hygiene and replacement of soft contact lenses and storage cases

Purpose

To review important aspects of contact lens compliance and to survey contact lens replacement frequency, steps in lens care and hygiene and replacement of the lens storage case via online surveys.

Methods

Random US samples (n = 645 and 787) of frequent replacement contact lens wearers answered questions on contact lens wear in online, sponsor-masked surveys.

Results

Wearers of lenses prescribed by their practitioner for 2 weekly replacement reported that they replaced lenses as follows: within 2 weeks 45%, within 3 weeks (minor stretching) 68%, within 4 weeks 89%, ≥8 weeks (extreme stretching) 4%, compared to Monthly wearers who reported: within 4 weeks 37%, within 5 weeks (minor stretching) 57%, ≥8 weeks 23% (≥8 weeks extreme stretching, Chi-square p = 0.001). Median frequency for cleaning lens storage case was 2–3 times per week, while 33% reported cleaning monthly or less often. Median lens storage case replacement was every 4–6 months, while 48% reported annual replacement or less often. Most patients cleaned their lens case with hot or cold tap water (72%).

Conclusions

These internet surveys revealed significant non-compliance with lens care steps, case hygiene and lens replacement schedule. More than half of the contact lens wearers surveyed were not compliant with their 2-weekly or monthly schedule. Lenses prescribed for longer replacement intervals can lead to more extreme stretching of lens life. To encourage better patient habits, eye care providers should reinforce their recommended frequency of replacement of lenses and storage cases and should reinforce care and hygiene steps at each patient visit.     

Corneal stability after discontinued soft contact lens wear

PURPOSE: To determine the time needed to reach corneal stability after discontinued daily wear of soft contact lenses and to identify corneal topographic and pachymetric changes during this period. METHODS: We prospectively studied the eyes of 21 consecutive soft contact lens wearers evaluated for keratorefractive surgery. Each eye was examined once immediately after lens removal, 3 and 7 days later, and weekly thereafter. At each visit, manifest refraction, keratometry, corneal topography (EyeSys Corneal Analysis system, EyeSys Laboratories, Houston, TX) and pachymetry were performed. Incidence of associated corneal warpage and changes in corneal curvature and thickness during corneal stability time were determined. RESULTS: Of the 42 eyes examined, 26 corneas showed no significant change after the initial evaluation (stable group). The minimum stability time was 3 days in the remaining 16 eyes (unstable group), 7 of which had significant evidence of abnormal topography. Statistical analysis showed no significant differences between two groups regarding age, sex, duration of contact lens wear, and refractive and topographic measures. The mean central corneal thickness on the final visit was significantly different between two groups (P<0.05). CONCLUSION: Contact lens induced corneal warpage may occur in a considerable proportion of soft contact lens wearers. A 2-week contact lens free period seems to be adequate for the cornea to stabilize; however, one cannot predict the minimum time needed for each individual patient. Repeating examinations to document corneal stability before refractive surgery may be a safer alternative.