Corneal pathophysiology with contact lens wear

Seeing through the strikes: a comprehensive study of ocular injuries in martial arts from 2012 to 2021

PURPOSE

Over 6 million people in the United States participate in martial arts, and the focus on striking the face poses a significant risk of injury to the eye. The objective of this study is to characterize the nature and incidence of martial arts-related eye injuries in the United States.

METHODS

A retrospective analysis of the National Electronic Injury Surveillance System (NEISS) database served as the basis of the study. The NEISS database was queried for eye injuries related to martial arts from 2012 to 2021. The entries that resulted were coded for demographic information, mechanism of injury, martial arts discipline, and diagnosis. Data were analyzed using SPSS version 28 through descriptive and χ2 analyses.

RESULTS

There were 162 reported cases of martial arts-related eye injuries in 4- to 62-years-old patients with most injuries occurring in males (84%). The most represented martial arts discipline was boxing (30.5%), and the most frequently reported mechanism of injury was punching (30.2%). The most frequently reported diagnosis resulting from eye injury was corneal abrasion (40.5%), while sight-threatening diagnoses were reported, including retinal detachment (n = 4), vitreous hemorrhage (n = 2), and open globe (n = 2).

CONCLUSIONS

Despite the significant risk of ocular injury from martial arts, corneal abrasion was the most commonly diagnosed with excellent prognosis. Sight-threatening diagnoses were also reported, indicating that martial arts athletes should be educated about the risks of eye injury and the importance of regular eye examination when practicing combat sports.

Assessment by Optical Coherence Tomography of Short-Term Changes in IOP-Related Structures Caused by Wearing Scleral Lenses

BACKGROUND

The mechanism that could increase intraocular pressure (IOP) during scleral lens (SL) wear is not fully understood, although it may be related to compression of the landing zone on structures involved in aqueous humor drainage.

METHODS

Thirty healthy subjects were fitted with two SLs of different sizes (L1 = 15.8 mm, L2 = 16.8 mm) for 2 h in the right eye and left eye as a control. Central corneal thickness (CCT), parameters of iridocorneal angle (ICA), Schlemm's canal (SC), and optic nerve head were measured before and after wearing both SLs. IOP was measured with a Perkins applanation tonometer before and after lens removal and with a transpalpebral tonometer before, during (0 h, 1 h, and 2 h), and after lens wear.

RESULTS

CCT increased after wearing L1 (8.10 ± 4.21 µm; p < 0.01) and L2 (9.17 ± 4.41 µm; p < 0.01). After L1 removal, the ICA parameters decreased significantly (p < 0.05). With L2 removal, nasal and temporal SC area and length were reduced (p < 0.05). An increased IOP with transpalpebral tonometry was observed at 2 h of wearing L1 (2.55 ± 2.04 mmHg; p < 0.01) and L2 (2.53 ± 2.22 mmHg; p < 0.01), as well as an increased IOP with Perkins applanation tonometry after wearing L1 (0.43 ± 1.07 mmHg; p = 0.02).

CONCLUSIONS

In the short term, SL resulted in a slight increase in IOP in addition to small changes in ICA and SC parameters, although it did not seem to be clinically relevant in healthy subjects.

Case report: Presumed contact lens-induced intracorneal hemorrhage in a diabetic dog

A 10-year-old castrated male miniature poodle dog with diabetes mellitus was presented for a week history of blepharospasm and epiphora in the right eye. The spontaneous chronic corneal epithelial defect (SCCED) was diagnosed, and a bandage contact lens was applied following corneal debridement with sterile cotton-tip applicators. In 1 week, SCCED was improving uneventfully, though an annular pattern of intracorneal hemorrhage was observed. The contact lens was removed and the intracorneal hemorrhage resorbed in 4 weeks. To the author's knowledge, this is the first report of presumed contact lens-induced intracorneal hemorrhage characterized by an annular pattern in a dog.

A refined model on flow and oxygen consumption in the human cornea depending on the oxygen tension at the interface cornea/post lens tear film during contact lens wear

The study of oxygen consumption rate under" in vivo" human cornea during contact lens wear has been technically a challenge and several attempts have been made in the last 20 years to model the physiology of the human cornea during contact lens wear. Unfortunately, some of these models, based on a constant corneal oxygen consumption rate, produce areas on the cornea where the oxygen tension is negative, which has no physical sense. In order to avoid such inconsistency, different researchers have developed alternative models of oxygen consumption, which predict the likely oxygen metrics available at the interface cornea/post lens tear film by determination of oxygen flux, oxygen consumption, and oxygen tension through the different layers (endothelium, stroma, and epithelium). Although oxygen deficiency produces corneal edema, corneal swelling, hypoxia, acidosis, and other abnormalities, the estimation of the oxygen distribution below the impact of a contact lens wear is interesting to know which lens transmissibility was adequate to maintain the cornea and avoid epithelial and stromal anoxia. The estimation of minimum transmissibility for a lens for extended wear applications will be very useful for both clinicians and manufacturers. The aim of this work is to present a complete discussion based on Monod kinetics model that permits give an estimation of oxygen partial pressure distribution, the profile distribution of corneal flux and oxygen consumption rate, and finally the estimation of the relaxation mechanism of the cornea depending on the oxygen tension at the interface cornea/post lens tear film. Relaxation time in this context can quantify the capability of the corneal tissue to adapt to increasing concentrations of oxygen. It is proposed this parameter as a biological meaningful indicator of the interaction between contact lens polymers and living tissues such as the corneal cellular layer.

Corneal oxygenation during contact lens wear: comparison of diffusion and EOP‐based flux models

PURPOSE

The aim of this study was to compare corneal oxygen flux values derived from an oxygen diffusion model, with estimates from a model in which equivalent oxygen percentage (EOP) values were substituted for the post-lens tear film oxygen tension in Fick's law.

METHODS

A previously described five-layer corneal oxygen diffusion model was found to artefactually allow theoretical oxygen consumption, when the predicted oxygen tension fell to zero. Consequently, an eight-layer diffusion model was constructed, with consumption set to zero at points within the cornea, where predicted oxygen tension falls to zero. Post-lens tear layer thickness was corrected to more contemporary estimates. The eight-layer and EOP-based anterior corneal oxygen flux estimates were compared across the range of commonly encountered contact lens Dk/t values.

RESULTS

The eight-layer model overcomes deficiencies in the five-layer model and provides predicted values that are remarkably similar to the EOP-based model. Open and closed eye anterior corneal oxygen flux in the absence of contact lens wear was estimated at 7.8 and 7.6 microL/cm2/hr for the open eye and 6.0 and 6.1 microL/cm2/hr for closed eye for the diffusion and EOP-based models, respectively.

CONCLUSIONS

The diffusion model supports the EOP model in that there is minimal oxygen benefit to be gained by increasing Dk/t above the Holden-Mertz criteria of 24 and 87 x 10(-9) (cm/sec)(ml0(2)/ml.mmHg) during open and closed eye wear, respectively. The eight-layer model is suitable for further definition of corneal oxygenation during contact lens wear.

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.

Antimicrobial susceptibility of isolated pathogens from patients with contact lens-related bacterial keratitis in Crete, Greece: A ten-year analysis

PURPOSE

To evaluate the bacterial spectrum and antimicrobial susceptibilities of pathogens isolated from contact lens-related bacterial keratitis cases in a large academic Greek hospital.

METHODS

All adult patients with positive corneal scrapings or contact lens culture between 2007 and 2016 at the University Hospital of Heraklion, Greece, were retrospectively identified through a local microbiology database and their medical records were reviewed.

RESULTS

A total of 240 isolates were recovered from 131 patients with culture-proven contact lens-associated bacterial keratitis. The most common microorganism identified was Serratia marcescens (17.1% of total isolates), followed by Pseudomonas aeruginosa, Klebsiella spp. and coagulase-negative staphylococci (CoNS). Rates of aztreonam-resistant P. aeruginosa and erythromycin-resistant CoNS decreased in recent years, while the decrease in oxacillin-resistant CoNS was statistically significant (p=0.009). More than 90% of the isolated organisms (S. marcescens, P. aeruginosa, Klebsiella spp. and CoNS) were susceptible to ciprofloxacin or gentamicin.

CONCLUSION

Gram-negative microorganisms are the most common causative pathogens of contact lens-related keratitis in the region of Crete. Topical antibacterials containing quinolones or gentamicin represent an effective empirical treatment for the majority of the cases. This is quite encouraging, considering that the present study was conducted in a country characterised by high antimicrobial resistance rates. However, culture-driven antimicrobial treatment is mandatory for this sight-threatening infection.

The short-term physiological impact of switching reusable silicone hydrogel wearers into a hydrogel daily disposable multifocal

Purpose: To evaluate ocular physiological responses to etafilcon A multifocal (etMF) daily disposable (DD) lenses after 4 weeks of wear, when switching from habitual silicone hydrogel (SiHy) daily wear.

Method: A single-arm, open-label, bilateral dispensing study was conducted in 39 habitual spherical SiHy wearers (14 hyperopes; 25 myopes). Clinical visits occurred with habitual SiHy (control) at baseline and after 4 weeks of etMF DD open-eye lens wear at exit. Objective limbal/bulbar hyperemia using the Oculus K5M (0–4) and subjective grading of lid wiper epitheliopathy (LWE) (0–4) were tested for non-inferiority (NI), using a margin of 1 grade. Corneal thickness along a 10 mm cord was measured using the Visante OCT and tested for NI using a 30 μm margin. Corneal staining area was graded (0–100%).

Results: The least-square mean differences (LSMD) and 95% confidence interval (95% CI) between etMF DD and habitual SiHy in central and peripheral corneal thickness (μm) were 3.64 (−2.0, 9.29) and 3.0 (−7.72, 13.72) in hyperopic, and 3.56 (−0.66, 7.78) and 6.40 (−1.62, 14.42) in myopic subjects. The LSMD (95% CI) for bulbar and limbal hyperemia were −0.08 (−0.19, 0.02) and −0.01 (−0.12, 0.09) in hyperopes, and 0.04 (−0.03, 0.12) and 0.04 (−0.04, 0.11) in myopes. The LSMD (95% CI) for LWE were 0.11 (−0.39, 0.60) and 0.30 (−0.07, 0.67) for hyperopes and myopes, respectively.

Conclusions: No clinically significant differences in a variety of physiological responses were found when habitual reusable SiHy daily wear subjects were refitted into hydrogel etMF, when the subjects were followed for 4 weeks.

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.

Endothelial alterations in 712 keratoconus patients

PURPOSE

To investigate the effect of the severity of keratoconus on the corneal endothelium using specular microscopy.

METHODS

Seven hundred and twelve eyes from the Homburg Keratoconus Center (HKC) database were included in this retrospective study. Corneal endothelium was evaluated using the Tomey EM-3000 specular microscope. Keratoconus-related topographic and tomographic data were obtained from Scheimpflug-based tomography (Oculus Pentacam^® HR). Eyes were classified into stages 0 (healthy) to 4 (severe keratoconus) according to the Topographic Keratoconus Classification (TKC). Subgroups were analysed based on contact lens (CL) type (none/rigid/soft).

RESULTS

The frequencies of keratoconus stages 0/1/2/3/4 according to TKC were 169/94/206/166/77. The endothelial cell density (ECD) for the endothelial cell area for TKC 0/1/2/3/4 was 2611/2624/2557/2487/2401 cells per mm² and the coefficient of variation (CV) was 40.9/40.0/41.6/46.2/49.0%, respectively. The more severe the keratoconus stage, the lower the endothelial cell count (p < 0.001) and the higher the CV (p < 0.001). No contact lens wearing was noted in 207 eyes (NoCL), rigid CL in 200 (RCL) and soft CL in 54 (SCL). CD for NoCL/RCL/SCL was 2523/2533/2644 per mm² and CV was 41.8/54.1/43.1%, respectively. A significant difference in CV was found between NoCL and RCL (p = 0.02), and no significant difference in CV was found between NoCL and SCL (p = 0.07). Endothelial cell density (ECD) did not differ significantly between NoCL and RCL or SCL.

CONCLUSION

Endothelial cell density (ECD) decreases and CV increases significantly with increasing tomographic severity of keratoconus. In patients with RCL compared to eyes without CL wear, we found a statistically significantly higher CV in the endothelial cell size.

Corneal Swelling with Cosmetic etafilcon A Lenses versus No Lens Wear

Purpose

To determine if the use of pigments or adding polyvinyl pyrrolidone during the fabrication of 1-DAY ACUVUE DEFINE (AD) brand contact lenses impacts open-eye corneal swelling compared with no lens wear (NLW).

Methods

A partial double-masked, randomized, bilateral crossover study was conducted in 24 Asian subjects using AD, 1-DAY ACUVUE DEFINE with Lacreon (ADL), NLW, and a control lens with no tint (1-DAY ACUVUE MOIST [AM]). Central corneal thickness was measured before insertion and immediately after removal after 8 ± 1 h of open-eye wear using an optical pachymeter in one eye. Corneal thickness along a 10-mm cord was measured in the contralateral eye using the Visante optical coherence tomographer (OCT). Corneal swelling was tested for noninferiority using a 5% margin. The endothelial bleb response was measured at baseline and 20 min after lens insertion using specular microscopy. Subjective grading of corneal staining and limbal/bulbar hyperemia were also monitored.

Results

After 8 ± 1 h of open-eye wear, central corneal swelling across the study lenses with either optical pachymeter or OCT methods was negligible. Peripheral corneal swelling least-square mean differences with OCT were −0.03% (95% confidence interval [95% CI], −0.65 to 0.58%) and −0.26% (95% CI, −0.87 to 0.36%) between AD and ADL and the control lens (AM), respectively, and 1.67% (95% CI, 1.06 to 2.29%) and 1.45% (95% CI, 0.84 to 2.06%) between AD and ADL and NLW, respectively. No endothelial blebs were observed. No clinically significant differences were distinguished between the lenses and NLW for corneal staining and limbal/bulbar hyperemia.

Conclusions

After 8 ± 1 h of open-eye wear, central and peripheral corneal swelling along the horizontal meridian with AD, ADL, AM, and NLW were equivalent. These results confirm that the addition of polyvinyl pyrrolidone or pigments to etafilcon A to obtain a limbal ring design have no impact on corneal swelling or limbal/bulbar hyperemia during normal open-eye wear.

Acute hypoxia influences collagen and matrix metalloproteinase expression by human keratoconus cells in vitro

Keratoconus (KC) is a progressive corneal ectasia linked to thinning of the central cornea. Hard contact lenses, rigid gas permeable lenses, and scleral lenses are the primary treatment modalities for early to mid- stages of KC to correct refractive error and astigmatism that develops as a result of an irregular corneal structure. These treatments are associated with significant drawbacks, including reduced availability of the tear film and oxygen to the corneal epithelium and stroma. However, it remains unknown whether hypoxia affects corneal integrity in the KC pathobiology. A number of studies have associated elevated oxidative stress with KC both in vitro and ex vivo. We hypothesized that KC-derived corneal fibroblasts are more susceptible to hypoxia-induced oxidative stress compared to healthy controls leading to exacerbation of corneal thinning in KC. This study investigated the effects of hypoxia on ECM secretion, assembly, and matrix metalloproteinase (MMP) expression in human corneal fibroblasts from healthy controls (HCFs) and KC patients (HKCs) in vitro. HCFs and HKCs were cultured in 3D constructs for 3 weeks and maintained or transferred to normoxic (21% O₂) or hypoxic (2% O₂) conditions, respectively, for 1 additional week. At the 4 week time-point, constructs were isolated and probed for Collagen I, III, and V, keratocan and MMP-1, -2, -3, -9, and -13, as well as hypoxia markers, hypoxia inducible factor-1α and lactoferrin. Conditioned media was also collected and probed for Collagen I, III, and V by Western blot. Thickness of the ECM assembled by HCFs and HKCs was measured using immunofluorescence microscopy. Results showed that hypoxia significantly reduced Collagen I secretion in HKCs, as well as upregulated the expression of MMP-1 and -2 with no significant effects on MMP-3, -9, or -13. ECM thickness was reduced in both cell types following 1 week in a low oxygen environment. Our study shows that hypoxia influences collagen and MMP expression by HKCs, which may have consequential effects on ECM structure in the context of KC.

Mechanical properties of contact lenses: The contribution of measurement techniques and clinical feedback to 50 years of materials development

PURPOSE

This review summarises the way in which mechanical property measurements combined with clinical perception have influenced the last half century of materials evolution in contact lens development.

METHODS

Literature concerning the use of in-vitro testing in assessment of the mechanical behaviour of contact lenses, and the mutual deformation of the lens material and ocular tissue was examined. Tensile measurements of historic and available hydrogel lenses have been collected, in addition to manufacturer-generated figures for the moduli of commercial silicone hydrogel lenses.

RESULTS

The three conventional modes of mechanical property testing; compression, tension and shear each represent different perspective in understanding the mutual interaction of the cornea and the contact lens. Tensile testing provides a measure of modulus, together with tensile strength and elongation to break, which all relate to handling and durability. Studies under compression also measure modulus and in particular indicate elastic response to eyelid load. Studies under shear conditions enable dynamic mechanical behaviour of the material to be assessed and the elastic and viscous components of modulus to be determined. These different methods of measurement have contributed to the interpretation of lens behaviour in the ocular environment. An amalgamated frequency distribution of tensile moduli for historic and currently available contact lens materials reveals the modal range to be 0.3-0.6MPa.

CONCLUSION

Mechanical property measurements of lens materials have enabled calibration of an important aspect of their ocular interaction. This together with clinical feedback has influenced development of new lens materials and assisted clinical rationalisation of in-eye behaviour of different lenses.

Analysis of the application of the generalized monod kinetics model to describe the human corneal oxygen-consumption rate during soft contact lens wear

This work is an analysis of the application of the generalized Monod kinetics model describing human corneal oxygen consumption during soft contact lens wear to models previously used by Chhabra et al. (J Biomed Mater Res B Appl Biomater, 2009a;90:202-209, Optom Vis Sci 2009b;86:454-466) and Larrea and Büchler (Invest Ophthalmol Vis Sci 2009;50:1076-1080). We use oxygen tension from in vivo estimations provided by Bonanno [Bonanno et al., Invest Ophthalmol Vis Sci 2002;43:371-376, and Bonanno et al 2009]. We consider four hydrogel and six silicone hydrogel lenses. The cornea is considered a single homogeneous layer, with constant oxygen permeability regardless of the type of lens worn. Our calculations yield different values for the maximum oxygen consumption rate Q_c,max , whith differents oxygen tensions (high and low p_c ) at the cornea-tears interface. Surprisingly, for both models, we observe an increase in oxygen consumption near an oxygen tension of 105 mmHg until a maximum is reached, then decreasing for higher levels of oxygen pressure. That is, when lowering the pressure of oxygen, the parameter Q_c,max initially increases depending on the intensity of the change in pressure. Which, it could be related with the variation of the pH. Furthermore, it is also noted that to greater reductions in pressure, this parameter decreases, possibly due to changes in the concentration of glucose related to the anaerobic respiration. The averaged in vivo human corneal oxygen consumption rate of 1.47 × 10^-4 cm³ of O₂ /cm³ tissue s, with Monod kinetics model, considering all the lenses studied, is smaller than the average oxygen consumption rate value obtained using the Larrea and Büchler model. The impact that these calculations have on the oxygen partial pressure available at different depths in the corneal tissue is presented and discussed, taking into consideration previous models used in this study. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2269-2281, 2017.

Severe corneal vascularization after intacs implantation and rigid contact lens use for the treatment of keratoconus

PURPOSE

To present a case of unilateral development of severe corneal vascularization in the eye of a keratoconic patient implanted with Intacs microthin prescription inserts and concurrent bilateral use of rigid gas permeable (RGP) contact lenses.

METHODS

A 20-year-old male underwent unilateral implantation of Intacs in his right eye for treatment of keratoconus and five years later began the use of RGP contact lens. In the left eye the patient continued the use of RGP contact lens.

RESULTS

Ten years after implantation the patient had developed severe central corneal neovascularization in the eye treated with Intacs implantation that recessed one month after discontinuing contact lens use. In the fellow eye he had developed only mild peripheral neovascularization.

CONCLUSIONS

Central corneal vascularization is a possible side-effect in patients treated with both Intacs implantation and RGP contact lens fitting that necessitates long term follow-up of such patients.

Achromobacter xylosoxidans keratitis after contact lens usage

To report on Achromobacter xylosoxidans keratitis in two healthy patients who had worn contact lenses foran extended period of time. A 36-year-old female and a 21-year-old female visited our hospital with ocular pain and blurred vision. Both patients had a history of wearing soft contact lenses for over fve years with occasional overnight wear. At the initial presentation, a slit lamp examination revealed corneal stromal infiltrations and epithelial defects with peripheral neovascularization in both patients. Microbiological examinations were performed from samples of corneal scrapings, contact lenses, contact lens cases, and solution. The culture resulting from the samples taken from the contact lenses, contact lens cases, and solution were all positive for Achromobacter xylosoxidans. Confrming that the direct cause of the keratitis was the contact lenses, the frst patient was prescribed ceftazidime and amikacin drops sensitive to Achromobacter xylosoxidans. The second patient was treated with 0.3% gatifoxacin and fortifed tobramycin drops. After treatment, the corneal epithelial defects were completely healed, and subepithelial corneal opacity was observed. Two cases of Achromobacter xylosoxidans keratitis were reported in healthy young females who wore soft contact lenses. Achromobacter xylosoxidans should be considered a rare but potentially harmful pathogen for lens-induced keratitis in healthy hosts.

Central corneal thickness and corneal endothelial cell changes caused by contact lens use in diabetic patients

PURPOSE

To analyze the effects of soft contact lenses on central corneal thickness and morphologic characteristics of the corneal endothelium in diabetic patients.

MATERIALS AND METHODS

Ultrasound pachymetry and noncontact specular microscopy were performed on 26 diabetic patients who regularly use soft contact lenses (group 1), 27 diabetic patients who do not use soft contact lenses (group 2) and 30 normal subjects (group 3). We compared the values in each group using the Mann-Whitney test.

RESULTS

The central cornea was found to be thicker in diabetic patients, both those who use and do not use contact lenses, than in the normal control group. The central corneal thickness was significantly higher in group 1 (564.73 ± 35.41 μm) and group 2 (555.76 ± 45.96 μm) than in the control group (534.05 ± 27.02 μm), but there was no statistically significant difference between groups 1 and 2. Endothelial cell density was significantly different between the groups, and was smallest in the group of diabetic patients using contact lenses. The coefficient of variation of cell size was significantly higher and the percentage of hexagonal cells was significantly lower in contact lens using diabetic patients than in non-contact lens using diabetic patients and in the control group.

CONCLUSION

Central corneal thickness and endothelial cell density is more affected by diabetes mellitus, and corneal endothelial cell morphology is more affected by contact lens use, when compared with normal subjects.

Modeling corneal metabolism and oxygen transport during contact lens wear

PURPOSE

A metabolic model is developed for cornea-contact-lens system to elucidate the role of glucose metabolism in oxygenation of the cornea and to gauge the role that contact lens oxygen transmissibility plays in avoiding hypoxia-induced corneal abnormalities for extended wear applications.

METHODS

Oxygen transport through the cornea and contact lens system is typically described by oxygen diffusion with reactive loss. Oxygen in the cornea, however, interacts with other metabolic species, specifically glucose, lactate ion, bicarbonate ion, hydrogen ion, and carbon dioxide via aerobic glycolysis (Krebs or tricarboxylic acid cycle) and anaerobic glycolysis. Here, corneal aerobic and anaerobic metabolic reactions are incorporated into a six-layer (endothelium, stroma, epithelium, postlens tear film, contact lens, and prelens tear film) steady-state continuum reaction-diffusion model to quantify oxygen transport. We also define a new index, the oxygen deficiency factor (ODF), for gauging corneal oxygenation. As opposed to other current gauges of hypoxia, ODF is a local and sensitive measure of both the extent and severity of corneal oxygen deprivation.

RESULTS

We calculate not only oxygenation of the cornea but also its coupled glucose, lactate, and acidosis behavior. For the first time, the metabolic shift from aerobic to anaerobic glycolysis is explicitly incorporated into the transport and consumption of oxygen in the cornea on closed-eye contact lens wear. Adoption of enzymatic Monod kinetics for the metabolic reactions permits realistic assessment of local species concentrations throughout the cornea. We find that anerobic-produced lactate transports out of the cornea into the anterior chamber, whereas buffering bicarbonate ion transports into the comea from the anterior chamber.

CONCLUSIONS

The coupling of oxygen with other reactive species in corneal metabolism provides useful insight into the transport of oxygen in cornea-contact-lens system. Specifically, we find that in addition to oxygen depletion and acidosis in the cornea, lactate concentration increases while glucose and bicarbonate concentrations decrease from the endothelium toward the epithelium. Unlike other indices of corneal oxygenation, ODF is sensitive specifically to regions of cornea with local oxygen deficiency. Accordingly, ODF is a useful physiologic index to assess the extent and severity of hypoxia in the cornea.

Corneal Oxygen Uptake Associated With Piggyback Contact Lens Systems

PURPOSE

The purpose of this study was to determine the effects of static (without blinking) and dynamic (with blinking once every 5 seconds) wear of piggyback contact lens systems on corneal oxygen uptake.

METHODS

Corneal oxygen uptake rates were measured on the right eyes of 11 human subjects by using a polarographic electrode. Measurements were made for the normal open eye and after 5 minutes of wear of 4 rigid lens materials (Dk/t 0-82.5), 4 soft lens materials (Dk/t 13-122), and 16 combinations of rigid and soft lens materials. The piggyback systems were worn under both static and dynamic conditions. Repeated-measures analysis of variance was used to compare oxygen uptake rates associated with the wear of the rigid lens components, soft lens components, piggyback systems, and static versus dynamic wearing conditions. Spearman correlation coefficients and regression analyses were used to examine relationships between corneal oxygen uptake rates.

RESULTS

Measurable differences were found among oxygen uptake rates associated with the rigid lens components, soft lens components, and piggyback systems. Blinking resulted in no reduction in corneal oxygen uptake with the piggyback systems. Corneal oxygen uptake associated with the wear of the piggyback systems could not be predicted from those associated with the rigid and soft lens components of the systems.

CONCLUSIONS

Piggyback combinations of rigid and soft lens components with the highest transmissibilities resulted in the least increase in corneal oxygen uptake beyond that of the normal open eye.

A novel method to measure oxygen permeability and transmissibility of contact lenses

PURPOSE

The present paper describes an accurate and cost-effective method requiring simple equipment to measure the oxygen permeability and transmissibility of contact lenses.

METHOD

The method involves accurate measurement of oxygen across the material by allowing the gas (oxygen) to pass through the lens material in a specially fabricated lens mould. The expelled gas is collected by dissolution in ethanol and the oxygen is measured by titration of the solvent.

RESULT

This method is suitable for the measurement of oxygen permeability and transmissibility of contact lenses of varying thickness and different radii of curvature. It can measure Dk and Dk/t of collagen bandage lenses in both wet and dry conditions.

CONCLUSION

The oxygen permeability and transmissibility values obtained by this method were compared with the oxygen permeability standard values of commercially available contact lenses and the results are summarised.

Corneal oxygen uptake: A review of polarographic techniques, applications, and variables

Factors that influence the polarographic measurement of the oxygen uptake of the cornea are reviewed. These factors include the technique, electrode assembly, oxygen reservoir, membrane material and thickness, oxygen tension of the corneal environment, duration of exposure to environmental conditions and time to application of the probe all influencing measured oxygen uptake rates. Subject factors include lid position, palpebral aperture size, blinking, corneal thickness, and corneal integrity. Contact lens wear influences corneal oxygen uptake, with lens material and design parameters influencing rates obtained both under static (without blinking) and dynamic (with blinking) conditions. Measurement of corneal oxygen uptake rates remains an excellent method to quantify the oxygen supply in contact lens systems that include the contact lens, the tears, and the cornea, in which oxygen flux is influenced by the thickness and diffusion characteristics of each component.

Rigid Gas-Permeable Contact Lens Base Curve Radius and Transmissibility Effects on Corneal Oxygen Uptake

PURPOSE

The purpose of this study is to determine the effects of rigid gas-permeable contact lens thickness, base curve radius, and material permeability on corneal hypoxic stress.

METHODS

Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode on the right eye of 10 human subjects for the normal open eye (air) and after 5 minutes of static wear of rigid contact lenses of four cornea-to-contact lens base curve fitting relationships: 0.2 mm steeper-than-K (STK), 0.1 mm STK, on K, and 0.1 mm flatter-than-K (FTK). There were also four materials (polymethylmethacrylate [Dk=0], lotifocon B [OP-2, Dk=15.9], lotifocon A [OP-3, Dk=30], lotifocon C [OP-6; Dk=60]) and three center thicknesses (0.14, 0.28, and 0.53 mm for the OP-6 lenses and 0.14 mm for all other materials) with all other parameters being constant. Each subject participated in two identical sessions. A repeated-measures analysis of variance was performed to compare the mean response across lens materials/thicknesses and the four curvature values.

RESULTS

Significant differences were found only for lens material/thickness (p<0.0001). Although OP-2 and OP-6 (0.53 mm) were manufactured to have the same Dk/t, post hoc comparisons showed that the oxygen uptake rate with OP-6 (0.53 mm) was significantly lower. The same is also true for OP-3 and OP-6 (0.28 mm), with OP-6 (0.28 mm) having a significantly lower oxygen uptake rate.

CONCLUSIONS

In contrast to past studies with PMMA, cornea-to-contact lens base curve fitting relationship, with the lens materials and designs studied here, did not affect corneal hypoxic stress, and thick, high Dk lenses resulted in less change in corneal response than did thin, lower Dk lenses of the same Dk/t. This is attributed to the lens reservoir effect that has been previously described.

A Review of the Holden–Mertz Criteria for Critical Oxygen Transmission

PURPOSE

Holden and Mertz established the foundations for the new generation of silicone hydrogel contact lenses for daily and extended wear in 1984. Absence of lens-induced corneal swelling was their premise and although this was possible with daily wear, the goal with extended wear seemed almost impossible because of the limitation of hydrogel materials. Experience with extended and daily wear of disposable contact lenses during the last 2 decades and, more recently, with silicone hydrogels has led to the reconsideration of the validity of the findings of Holden and Mertz.

RESULTS

A pivotal influence on the Holden and Mertz critical Dk/t value for closed-eye lens wear was exerted by two numbers: the no-lens wear corneal edema level (4%) and the silicone lens data point (Dk/t of 182x10, closed-eye edema level of 2.6%). Subsequent publications appear to give a more accurate assessment of the no-lens closed-eye corneal edema level, and there are also more recent measurements of silicone hydrogel lens Dk/t and corneal edema responses.

CONCLUSIONS

It now appears that the Holden and Mertz criteria for extended-wear critical Dk/t should be revised upwards to at least 125x10.

Microbial flora of tears of orthokeratology patients, and microbial contamination of contact lenses and contact lens accessories

PURPOSE

The purpose of this study is to determine if there are changes in the ocular flora of overnight orthokeratology (ortho-k) patients, and the levels of contamination of their lenses and lens accessories, and to correlate compliance with levels of contamination.

METHOD

Normal ocular flora of 41 subjects was determined twice before commencing ortho-k lens wear by culture of the lower conjunctiva. Further specimens were collected on six follow-up visits after beginning lens wear, as were samples from their lenses, cases, and suction holders. A questionnaire on lens care was administered after the fifth visit.

RESULTS

Three subjects provided conjunctival samples yielding Staphylococcus aureus on one occasion before lens wear, one being positive for this organism after beginning lens wear. Of 38 subjects yielding no growth or only normal eye flora before use, 28 remained free of ocular pathogens after beginning lens wear. Only four subjects had positive cultures on more than one occasion after lens wear. There was no significant difference in isolation levels of pathogens with lens wear (p = 0.423). Lens culture of 54% of subjects yielded no growth or normal flora only; lenses of 16 subjects yielded potential pathogens, including three subjects contaminated on more than one occasion. Lens isolates did not match the organisms transiently colonizing the eye. Lens case, the most frequently contaminated item, was associated with lens contamination (p < 0.001), the same organism being isolated from both items in 11 subjects. Lens suction holder was less frequently contaminated. Neither lens case nor suction holder contamination was associated with isolates from the eye. Reported good compliance correlated with lack of contamination in all but one subject. The most frequent breaches in the lens care protocol were failure to clean, disinfect, and replace the lens case.

CONCLUSION

Ocular flora was not altered by ortho-k lens wear over an extended period, and patients remained free of infection. Contaminants identified were generally of a transient nature. Most patients had significant contamination of at least one item, most frequently the lens case. Lens case isolates were significantly associated with those from the lens. The majority of patients reporting good compliance had low or no contamination of their lenses and accessories.

Beyond Flux: Total Corneal Oxygen Consumption as an Index of Corneal Oxygenation During Contact Lens Wear

PURPOSE

The purpose of this study is to compute total corneal oxygen consumption during contact lens wear and consider the concept as an index for describing corneal oxygenation during contact lens wear as opposed to flux, partial pressure, or Dk/t.

METHOD

Estimates of total corneal oxygen consumption were generated using a previously described eight-layer model based on oxygen diffusion equations and using contemporary estimates of tear and corneal layer thicknesses. Relative consumption, expressed as %Q (percentage of normal consumption without contact lens wear), was also calculated for daily wear and continuous wear modes, thereby providing an index of the chronic hypoxic effect of contact lens wear.

RESULTS

Corneal oxygen consumption is calculated to converge to the same value of 44.8 nL/cm/sec above a Dk/t of approximately 20 and 300 x 10(-9) (cm/sec)(mL0(2)/mL/mm Hg) for the open and closed eye scenarios, respectively. Lenses with Dk/t values of 15 and 50 x 10(-9) (cm/sec)(mL0(2)/mL/mm Hg) allow about 96% of normal long-term total oxygen consumption without a contact lens in place for daily wear and continuous wear, respectively. CONCLUSION.: Total corneal oxygen consumption is based on physical parameters, represents a direct index of corneal oxygen metabolism and thus cellular energy (ATP) production, allows direct comparison between two lenses or a lens and nonlens-wearing state, and eliminates the ambiguity of other measures, thereby providing an attractive means of describing corneal oxygenation.

The Effects of High-Dk Rigid Contact Lens Center Thickness, Material Permeability, and Blinking on the Oxygen Uptake of the Human Cornea

PURPOSE

The human corneal oxygen uptake responses associated with the static (nonblinking) and dynamic (blinking) wear of five rigid gas-permeable materials with high oxygen permeabilities were determined for three different center thicknesses and compared with the responses for the normal open eye and severe hypoxic stress (static wear of polymethylmethacrylate).

METHODS

Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode during two sessions with each of 10 human subjects. Measurements were made on the right eye for the normal open eye (air) and after 5 minutes of static and dynamic wear of polymethylmethacrylate and five rigid gas-permeable contact lens materials: Fluoroperm 92 (paflufocon A, Dk = 92), Fluoroperm 151 (paflufocon D, Dk = 151), 1992 Menicon SF-P (melafocon A, Dk = 102), 1995 Menicon SF-P (melafocon A, Dk = 159), and Menicon Z (tisilfocon A, Dk = 163-250). Lenses were manufactured in three different center thicknesses (0.12, 0.16, and 0.20 mm), with all other parameters remaining constant. Repeated-measures analysis of variance was used and included lens material (five levels), blinking condition (two levels), and lens thickness (three levels) as within-subject effects.

RESULTS

Significant differences were found in corneal oxygen responses to lens material (p < 0.001) and lens thickness (p < 0.001), with lenses of lower oxygen permeability and thicker lenses being associated with higher oxygen uptake. No statistically significant differences were noted between static or dynamic wear of the lens materials (p = 0.59).

CONCLUSIONS

For those very high Dk rigid lens materials studied here, moderate changes in lens thickness or material permeability may result in modest differences in corneal hypoxic relief, whereas blinking results in no significant improvement to corneal oxygenation.

The corneal stroma during contact lens wear

Recent technological advances have lead to novel descriptions of the microanatomy of the corneal stroma. In the first section of this review, these findings and the role they play in the maintenance of vital properties such as corneal transparency, mechanical strength, homeostasis, wound-healing response and metabolism are described. In the second part, contact lens induced stromal alterations such as acidosis, oedema, striae, thinning and opacities are reviewed as well as the more recently described phenomenon of microdot deposits and keratocyte loss with an emphasis on how lens wearing stromal effects can be minimised.

Endothelial response to overnight corneal reshaping

PURPOSE

To discuss corneal endothelial structure during different environments. Little information has been available on the structure of the corneal endothelium during overnight corneal reshaping.

METHOD

Patients were selected to represent different environments of contact lens wearing to contrast them to patients who changed their environment from contact lens use to overnight corneal reshaping.

RESULTS

There were no apparent ill effects to the corneal endothelium from overnight corneal reshaping.

CONCLUSIONS

Corneal endothelial polymegathism and pleomorphism can decrease with properly prescribed and monitored overnight corneal reshaping lenses over some standard contact lenses.

Clinical responses of the corneal endothelium

The corneal endothelium maintains corneal deturgescence and clarity by a pump-leak mechanism first described by David Maurice. This cell layer can be investigated clinically with specular microscopy, fluorophotometry, and pachymetry. We describe the clinical responses of the corneal endothelium to aging, drugs, glaucoma, contact lens wear, trauma, disease, and surgery.

Local oxygen transmissibility of disposable contact lenses

AIM

Despite global standardisation in measurement of oxygen transmissibility (Dk/t), in our clinic we continue to see limbal vasculature changes in patients wearing some disposable lenses. The use of central Dk/t as a simple designator of a contact lens oxygen performance has clearly failed, it being acknowledged as often unrelated to the oxygen performance of the lens periphery. This study investigates a new criterion for contact lens oxygen transmissibility, the local Dk/t.

METHOD

The study measured 20 spherical disposable lens brands (power range: -15.00 to +8.00 D) and 8 toric lens brands (power range: -8.00 to +4.00 D sphere, with a -1.00 D/-1.25 D x 180). An electro-mechanical gauge (Heidenhain, Germany) was used to measure lens thickness (t) at different positions on the lens. Oxygen permeability (Dk) of each lens material was calculated from the equation Dk =1.67 e (0.0397 x water content) of Morgan and Efron [Contact Lens Anterior Eye 21 (1998) 3] who used ISO/ANSI standard methodologies. The local Dk/t value was calculated from the point of maximum thickness on the lens and compared to the criterion of 12 Fatt units set by Benjamin [Int. Contact Lens Clin. 23 (1996) 188], to provide corneal oxygenation greater than or comparable to that available in the normal closed eye.

RESULTS

All lens types with a water content over 65% had a local Dk/t of 12 Fatt units or more for most or all lens powers: CIBAVision FocusDailies, Bausch & Lomb Soflens66, CooperVision Actifresh400, and CIBAVision PrecisionUV. In addition, some lenses in the 55-62% water content range had some lens powers with a local Dk/t in excess of 12: CooperVision Proclear, Johnson & Johnson Acuvue 1 and CIBAVision Focus 2 week. None of the disposable toric lenses had a minimum Dk/t of 12 or more. The silicone-hydrogels, Bausch & Lomb PureVision and CIBAVision Focus Night&Day, comfortably exceeded Dk/t of 50 for all lens powers.

CONCLUSION

Some current disposable soft lenses have local Dk/t values below physiological requirements. Contact lens manufacturers and international standards organisations should consider introducing the labelling of local Dk/t values on lens packaging so that clinicians can make informed prescribing decisions.

Preparation for the in vivo recording of neuronal responses in the visual cortex of anaesthetised marmosets (Callithrix jacchus)

The marmoset is becoming an important model for studies of primate vision, due to factors such as its small body size, lissencephalic brain, short gestational period and rapid postnatal development. For many studies of visual physiology (including single-cell recordings), it is a requirement that the animal is maintained under anaesthesia and neuromuscular block in order to ensure ocular stability. However, maintaining such a small animal (290-400 g) in good physiological condition for long periods requires expert attention. This becomes particularly important in the case of recordings from visual association cortex, where neuronal responses are known to be highly sensitive to factors such as the type and dose of anaesthetic, and the animal's physiological balance. The present protocol summarises our laboratory's experience over the last decade in developing a preparation for the study of marmoset visual cortex. It allows excellent recording from extrastriate areas for periods of at least 48 h, including the continuous study of isolated single cells for several hours.

Corneal epithelial thinning profile induced by long-term wear of hydrogel lenses

PURPOSE

To characterize the epithelial thickness profile and study the effects of long-term wear of hydrogel lenses on this profile.

METHODS

A cross-sectional study was designed. Epithelial thickness was evaluated in 15 subjects who had worn one of two types of low oxygen transmissibility (Dk) hydrogel contact lenses for an average of 10 years (range, 7-16) and compared with a group of 18 control subjects who had never worn contact lenses. Epithelial thickness was measured at the center and at four mid-peripheral and four peripheral locations in the vertical and horizontal meridians of the cornea using a modified optical pachymeter.

RESULTS

Lens wearers had significantly thinner epithelium than controls [analysis of variance (ANOVA), p < 0.001] in the central (41 +/- 7 microm versus 48 +/- 5 microm), mid-peripheral (41 +/- 7 microm versus 48 +/- 7 microm), and peripheral (42 +/- 9 microm versus 48 +/- 6 microm) cornea. The extent of this difference ranged from 8.7% to 18.4% of the total epithelial thickness, was not associated with the duration of wear (ANOVA, p = 0.87, power = 0.05), and was significantly greater for the lower Dk contact lens type (ANOVA, p < 0.001). Topographical position did not have a significant effect on epithelial thickness (ANOVA, p > 0.13, power > 0.22).

CONCLUSION

This study establishes that the epithelial thinning associated with hydrogel lens wear is topographically uniform. It also confirms that this effect is inversely related to lens oxygen transmissibility but does not appear to increase with longer duration of wear.