Water Content Hydrogel Lenses Research Articles

The influence of water content of hydrogel contact lenses when fitting patients with ‘tear film deficiency’

THe fitting of patients presenting a deficient tear film with hydrogel contact lenses offers a challenge to the professional who must choose between materials having either a low or a high water content. Conflicting conclusions from previous studies concerning the better option led the authors to design a single-centre, double masked, clinical study. Twenty-six subjects deemed to have ‘tear film deficiency’ were fitted with a low water content hydrogel lens on one eye and a high water content one on the other eye. Each subject wore the lenses on a daily wear basis (about 10 hours a day) for 30 days. A detailed questionnaire about subjective considerations, such as comfort and ocular appearance, was completed by the subjects at the end of every day of contact lens wear. The data obtained from these questionnaires and those recorded at three follow-up visits led to the final conclusions. THe questionnaires showed no statistically significant difference between the two eyes in dryness sensation, redness, light sensitivity, blurred vision or overall comfort. During the follow-up visits, no observed sign showed a clinically significant difference between the two eyes. Accordingly, the authors conclude that the water content of hydrogel lens is not a key factor when fitting hydrogel lenses to patients with ‘tear film deficiency’.

A three-month comparative daily-wear study of two high-water-content soft lenses

Proclear ™ (Omafilcon A) is a novel biomimetic, 59% water content hydrogel lens. The material is termed “biomimetic” because it incorporates synthetic phosphorylcholine (PC), which is the predominant headgroup found on the outer surface of human cell membranes. PC has been shown to be the primary cell component responsible for human cell compatibility. The purpose of the study was to further investigate an earlier finding that Proclear lenses were more comfortable than Acuvue lenses. This was a 100-subject, 3-month, daily-wear study undertaken to assess the comparative clinical performance of Proclear (Biocompatibles) and Acuvue (Vistakon) lenses. The study was conducted at 10 optometric sites in the United States. Before the study, all of the subjects were successful Acuvue wearers. Subjects wore a Proclear lens in one eye and an Acuvue lens in the other eye. The Acuvue lenses were worn on a 14-day replacement schedule, whereas Proclear lenses were only replaced as necessary. The Proclear lenses achieved higher mean comfort scores at each of the follow-up visits, and this difference was significant ( p = 0.03). More subjects expressed a preference for the Proclear lens at all visits, and this number was statistically significant at the 1-week, 1-month, and 3-month visits ( p < 0.01). The reasons for lens preference tended to be because of comfort. Significant differences were noted in lens fit at all visits. More Proclear lenses showed minimal decentration, postblink movement, and optimal overall fit than did the Acuvue lenses. Significantly greater dehydration was noted with the Acuvue lens compared with Proclear ( p < 0.001). The study confirmed that the Proclear lenses were preferred to the Acuvue lenses. Preference was primarily based on subjective comfort.

Absorption of Ammonia by High Water Content Hydrogel Lenses: An Inexpensive Method of Analysis

Few studies have been done on the absorption and release of chemical vapors by high water content hydrogel lenses. In this study we investigated the absorption of ammonia vapors by this type of contact lens (CL). Ten high water content hydrogel lenses were exposed to vapor generated in a cuvette by 1 drop of aqueous ammonia. The contact lens was transferred to a 0.9% saline solution containing a colorometric indicator for ammonia and the amount of ammonia released into the saline was measured. We found that significant quantities of ammonia were absorbed into the lenses and the relation between average concentration of ammonia vapor and amount absorbed per contact lens was nearly linear from 50 parts per million (ppm) to 250 ppm ammonia vapor. For concentrations greater than 250 ppm there appears to be a saturation effect. Our findings indicate that high water contact lenses will absorb ammonia and release it into a solution similar to tears.

Osmotic determinants of postlens tear film morphology and hydrogel lens movement

The determinants of postlens tear film (PTF) composition in hydrogel lens wear are poorly understood, although this layer has important roles in lens movement and corneal integrity. We investigated the hypothesis that the PTF could be depleted by instillation of hypotonic saline, using a randomized, double masked, placebo controlled study design. Solutions of 0.90, 0.60 and 0.45% NaCl were instilled into the eyes of 12 subjects wearing ionic and nonionic high water content hydrogel lenses. Postlens tear film appearances in specular reflection were categorized as amorphous, faint coloured or coloured, where the coloured patterns represent a progressive thinning of the PTF. With instillation of the hypotonic solutions (0.60 and 0.45% NaCl), the appearance of the PTF in specular reflection changed to a faint coloured or coloured pattern in at least 67% of subjects for each lens type (Friedman ANOVA, P < 0.002). For the 0.45% NaCl solution, median lens movement decreased from 0.50 to 0.10mm (Friedman ANOVA, P = 0.02); however, there were no significant changes in measured lens parameters and no difference between lens types. Postlens tear film depletion due to a hypotonic shift in tear osmolality, as demonstrated here, may explain the clinically observed phenomenon of lens binding.

Osmotic determinants of postlens tear film morphology and hydrogel lens movement

SummaryThe determinants of postlens tear film (PTF) composition in hydrogel lens wear are poorly understood, although this layer has important roles in lens movement and corneal integrity. We investigated the hypothesis that the PTF could be depleted by instillation of hypotonic saline, using a randomized, double masked, placebo controlled study design. Solutions of 0.90, 0.60 and 0.45% NaCl were instilled into the eyes of 12 subjects wearing ionic and non‐ionic high water content hydrogel lenses. Postlens tear film appearances in specular reflection were categorized as amorphous, faint coloured or coloured, where the coloured patterns represent a progressive thinning of the PTF. With instillation of the hypotonic solutions (0.60 and 0.45% NaCl), the appearance of the PTF in specular reflection changed to a faint coloured or coloured pattern in at least 67% of subjects for each lens type (Friedman ANOVA, P &lt; 0.002). For the 0.45% NaCl solution, median lens movement decreased from 0.50 to 0.10mm (Friedman ANOVA, P = 0.02); however, there were no significant changes in measured lens parameters and no difference between lens types. Postlens tear film depletion due to a hypotonic shift in tear osmolality, as demonstrated here, may explain the clinically observed phenomenon of lens binding.

Adaptation to monovision

Many patients who begin wearing monovision contact lenses report that it requires days or weeks for them to become adjusted to the anisometropia; however, it is not clear if there are measurable improvements in their vision during this period. We therefore studied both the objective and subjective characteristics of adaptation to monovision over a period of 8 weeks. Sixty presbyopic subjects wore high water content hydrogel lenses in the study. During the 8 weeks of monovision wear, there were no significant changes in the subjects' visual acuities (distance, intermediate, or near) in high- and low-luminance conditions, near stereopsis, or interocular blur suppression characteristics. Nevertheless, the subjects reported significant improvement in various aspects of visual performance during the same period. Forty-eight percent of the subjects reported complete tolerance to the vision with their correction within the first week of monovision wear and a total of 78% of subjects were completely satisfied with the monovision correction at the completion of 8 weeks wear. The improved tolerance to monovision may be related to changes in aspects of vision other than those measured in this study or may reflect a psychological adjustment to the anisometropia.

Daytime Edema Levels with Plus Powered Low and High Water Content Hydrogel Contact Lenses

Eleven unadapted contact lens wearers wore a high (74%) water content hydrogel lens (Permaflex, CooperVision) of oxygen transmissibility Dk/Lav 14 x 10(-9) in one eye and a low (43%) water content hydrogel lens (Aquaflex Superthin) of Dk/Lav 4 x 10(-9) in the other eye under open-eye conditions for 8 h. After 8 h, average corneal edema for the lower water content lens was 7.9 +/- 2.6%, which was significantly more than that for the higher water content lens, 1.7 +/- 1.6%. Significantly fewer corneal striae and folds were also seen in the eyes wearing the higher water content lens. Subjective ratings of lens comfort were significantly better for the higher water content lens. Low water content positive power hydrogel lenses of the thicknesses used in this study place unacceptable hypoxic stress on the cornea and therefore should not be used for all-day wear.

Surface hydration of hydrogel contact lenses

A hand refractometer was used to measure the surface hydration of soft contact lenses of three different water contents, in a randomized, double masked study conducted on nine subjects. No statistically significant difference was found after 30 minutes of lens wear between the hydration levels at the front and back surfaces of Permathin (42.5 % nominal water content) and Snoflex 50 (50% nominal water content) hydrogel lenses; however, there was a significant difference for Gelfiex 60 (60% nominal water content) lenses, the back surface being more hydrated than the front surface. These findings suggest that there may be a water gradient between the front and back surfaces of higher water content hydrogel lenses during wear. Whilst the magnitude of the difference between surface hydration levels is small and of little consequence in the routine clinical measurement of water content using a hand refractometer, this phenomenon could be of aetiological significance with respect to the occurrence of multiple corneal erosions observed in patients wearing very thin, high water content lenses.

Epithelial erosions caused by thin high water content lenses

The occurrence of discrete epithelial erosions with very thin high water content hydrogel lenses is discussed. These multiple, small, coalescing, epithelial erosions develop centrally and superiorly after a few hours of lens wear. It is postulated that the erosions result from on-eye lens dehydration, which causes epithelial desiccation.

Deswelling of the cornea after hypoxia.

Central corneal thickness changes were monitored in nine subjects who wore low water content hydrogel lenses of varying thickness for 3 h in the open and closed eye conditions. At the end of 3 h of lens wear, corneal deswelling to prelens thickness for the different levels of hypoxia was determined by pachometry performed every 5 min for the first 30 min, every 10 min for the next 40 min, and every 15 min thereafter. Examination of the individual deswelling profiles indicates (1) a biphasic response in those with corneal edema greater than 14%, and (2) a large portion of the deswelling occurred during the first 30 min, followed by a more gradual rate until baseline was reached. Small amounts of corneal edema (less than 5%) required up to 25 min to return to baseline, whereas 6 to 15% required 30 to 70 min, and 20% required 95 min. The time taken for the cornea to deswell to baseline was linearly related to the amount of corneal swelling (r = 0.88; p less than 0.0001).