High Water Content Lenses Research Articles

Rabbit Models of Contact Lens–Associated Corneal Hypoxia: A Review of the Literature

To provide a review of the literature on rabbit models of contact lens-associated corneal hypoxia. The literature was searched for articles describing corneal hypoxia in the rabbit coincident with contact lens wear. The articles were identified by key word searches for rabbit models, contact lens, hypoxia, and cornea. Designing a rabbit model of contact lens-associated corneal hypoxia requires identification of the key determinants of the physiologic response. The retention of soft contact lenses, particularly high-water content lenses, is problematic in the rabbit eye, in which blink frequency is on the order of minutes. An intact nictitating membrane may be beneficial in the hydration and, therefore, retention of soft contact lenses. Although tarsorrhaphy improves lens retention, lid closure also enhances the hypoxic response and may obscure the benefits of the greater oxygen transmissibility of soft contact lenses. The duration of lens wear in most reviewed studies was 24 hours or less, with only a few studies evaluating the hypoxic effect of lens wear of 1 week or more. Methods commonly used to characterize corneal hypoxia in rabbits include pachymetry, tear film lactate dehydrogenase measurements, clinical observations of limbal hyperemia, and more invasive methods of assessing cell proliferation and apoptosis. In summary, the ideal rabbit model of contact lens-associated corneal hypoxia depends on the question to be answered. This review tabulates the known studies, highlights key considerations in study design, and describes useful methods in characterization of the hypoxic response.

Evaluation of soft contact lens fitting characteristics.

The assessment of soft contact lens fit by observation of the lens behavior on the eye is one of the most common procedures undertaken in contact lens practice. Nevertheless, the guidelines for doing so have been derived empirically with little or no research into the usefulness of these observations. Over 2000 evaluations of soft contact lens fit were analyzed to determine the predictive value of various indicators of contact lens fit, namely: subjective comfort, lens centration, postblink movement (PBM), version and upgaze lag, and tightness assessed by the push-up test. The receiver-operator characteristic (ROC) curves were prepared for each variable by plotting sensitivity values against false positive rates (FPRs) for various cut-off points. Sensitivity was defined as the proportion of "bad" fits correctly identified by a given variable; FPR was defined as the proportion of "good" fits incorrectly identified as bad fits. With each fitting variable, the optimum cut-off point and its corresponding sensitivity and FPR were determined from the ROC curves. Subjective comfort was found to be of little value in identifying tight fitting lenses but was of some limited value in identifying loose fitting lenses, particularly high water content (HWC) lenses. The push-up test was the most accurate single test of lens fit acceptability having a high sensitivity for both tight (92%) and loose (80%) lens fits. Assessment of PBM was a sensitive indicator of tight fitting lenses (95%), but was a poor indicator of loose fitting lenses, having a low sensitivity (51%). Assessment of version lag showed better sensitivity than upgaze lag in assessing loose fits, whereas upgaze lag showed better sensitivity for tight lens fits. Lens centration was of no predictive value in identifying tight fitting lenses but, as expected, was of some value in identifying loose fitting lenses. The results of this study enable a critical reassessment of the procedures that practitioners follow when fitting soft contact lenses.

Performance of low versus high water content plus lenses for presbyopes

This study compared the clinical performance of thin low water content (LWC) and standard thickness high water content (HWC) low plus hydrogel lenses. In two separate 3-month trials, 10 presbyopic subjects wore monovision correction, consisting of an LWC distance lens and an HWC near lens (Trial 1) and then HWC lenses for both distance and near (Trial 2). The corneal edema response was assessed, using striae and folds counts, after 1 week, 1 month, and 3 months of daily lens wear. Corneal staining, subjective comfort, and lens preference were also recorded. Overall, a significantly greater striae and folds response was found with the thin LWC lenses than with the thicker HWC lenses, after an average of 5 hours of open-eye lens wear. The severity of the edema response did not correlate with levels of corneal staining or with comfort and preference responses. We conclude that HWC lenses are preferable to LWC lenses in low plus powers for presbyopic patients.

Soft contact lens water content and five common post‐fitting complications. Are there relationships?

A retrospective analysis was made of soft lens wearers fitted in clinical practice. First fit patients wearing 38 per cent 0.06 poly‐HEMA, 50–55 per cent, and 67 per cent water content materials were assessed for the incidence of five common soft contact lens post‐fit complications over the first two years of wear. Low water content lenses were more commonly associated with superior keratitis; whereas medium and high water content lenses were associated with infiltrative keratitis. High water content lenses were associated with snowflake epithelial defect.

Quantity of Protein Deposited on Hydrogel Contact Lenses and Its Relation to Visible Protein Deposits

The purposes of this study were to determine if the quantity of protein deposited (QPD) upon hydrogel lenses was affected by enzymatic cleaning and to test the potential relation between QPD and visible protein deposition (VPD) and change. Seventy-four contact lens patients classified as "heavy depositors" wore new lenses for an average of 80 (SD = 32) days. Cleaning and disinfection solutions varied. One lens was cleaned weekly by a papain enzymatic treatment. The distribution of QPD measurements was bimodal and was related to the FDA material for nonionic, low water content lenses (FDA Materials Group no. 1). The mean deposition was 45 micrograms/cm2 (N = 112) compared with that of ionic, high water content lenses (FDA Materials Group no. 4), which was 1010 micrograms/cm2 (N = 30). VPD distributions were the same for the FDA Group no. 1 and no. 4 lenses. Enzymatic treatment did not significantly reduce QPD; however, enzymatic treatment did reduce VPD. Thus QPD and VPD are independent phenomena and possible reasons for this are given.

Effect of Hydrogel Lens Wear on Tear Film Stability

Noninvasive break-up time (NIBUT) of the tears was measured in a controlled, randomized, double-masked study to assess: (1) the stability of the prelens tear film during wear of new high and low water content lenses and (2) the stability of the precorneal tear film following lens removal after 1 h of wear. The prelens tear film NIBUT of 6 subjects was found to be relatively constant over a 1-h wearing period, averaging 6.1 +/- 1.1 s (mean +/- SEM). These values were significantly (Scheffe's S test, p less than 0.05) lower than those recorded for the precorneal tear film before lens insertion (33.5 +/- 10.6; mean +/- SEM), although 85% of prelens tear film NIBUT's were greater than the 3-s average interblink period reported previously for soft lens wearers. After lens removal, precorneal tear film NIBUT was reduced significantly compared to prewear levels (Scheffe's S test, p less than 0.05) for up to 15 min. Application of the monomolecular growth model to the NIBUT recovery data revealed a half-time for recovery of 6.0 min, with recovery 95% complete 25.8 min after lens removal. Lens type was not a significant factor in tear film stability, either during wear or after lens removal. The basis for reduced precorneal tear film NIBUT after lens removal is unknown; however, a disruption of the mucin layer coating the corneal epithelium is the most likely mechanism. Indeed, the technique of measuring precorneal tear film NIBUT after lens removal may be a useful determinant of the extent to which contact lens wear disrupts the precorneal mucin layer, providing an indication of the susceptibility of the cornea to a variety of complications.

Acanthamoeba adherence to contact lenses and removal by cleaning agents

The quantitative adherence of a keratitis isolate of Acanthamoeba polyphaga to low- and high-water content non-ionic soft contact lenses and one type of rigid gas-permeable lens was investigated. Adherence of trophozoite and cyst forms of the organism was observed in vitro, and adherent amoebae counted by a plaque assay method following detachment. Trophozoites adhered to all lens types with adherence being statistically significantly greater to high water content soft lenses. Cyst attachment occurred only to the soft lenses but not to gas-permeable lenses, and was significantly higher for the high water content lenses. Attachment of cysts was significantly lower than that of trophozoites to each lens tested. Recommended cleaning procedures using two commercial solutions removed all adherent trophozoites and cysts from lenses. These studies demonstrate (i) that lenses may act as a vector in Acanthamoeba keratitis, particularly for high-water content lenses and trophozoite amoebae, (ii) that lens cleaning agents may prevent keratitis by removing adherent Acanthamoeba.

Aetiology of arcuate epithelial lesions induced by hydrogels

One hundred and fifty hyperopic and myopicpresbyopes were fitted with daily wear high water content lenses. Within one month, twenty patients (14 %) developed arcuate punctate staining of the superficial cornea either unilaterally or bilaterally, within I mm of the superior or inferior limbus (the area normally covered by the eyelid). Of these patients, eight developed a full-depth epithelial arcuate lesion “epithelial splitting” within two months of wear. Our study investigated patient factors (lid coverage of cornea, lid tightness, angle of limbal sulcus), lens factors (age, peripheral design and material) and care regimen (disinfection system and wearing time). Results indicated that one or more patient factors predisposed wearers to the condition. Fifty percent of arcuate splits were resolvable by altering lens factors and/or care regimen. These results will be discussed.

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.

Determinants of the Initial Comfort of Hydrogel Contact Lenses

A double-masked, randomized study was conducted to determine the effects of power and water content on the initial comfort of hydrogel contact lenses in 10 unadapted subjects. Three lens powers (-0.50, -5.00, and -10.00 D) were used in each of three water contents (38, 55, and 70%). A significant negative correlation (p less than 0.05) was found between lens comfort and lens water content; that is, lower water content lenses of lesser bulk were more comfortable than higher water content lenses. These data will allow practitioners to predict patient awareness to various lens types. When fitting hydrogel lenses to an apprehensive patient who has not worn contact lenses previously, it may be advisable to insert a thin, low water content lens initially, thereby maximizing lens comfort.

DEFINITIONS FOR HYDRATION CHANGES OF HYDROGEL LENSES

Authors of studies on the hydration characteristics of hydrogel lenses have used a variety of definitions to describe the changes observed with exposure of the lenses to different environments. This has resulted in difficulties in the interpretation of published data. In particular, the single term "percentage dehydration" has been used to denote different mathematical expressions for alterations to the lens composition. A detailed theoretical analysis of hydrogel hydration levels is presented to examine definitions for lens hydration changes, compare results obtained under different definitions and propose appropriate usage for the alternative definitions according to the aspect of lens performance under consideration. The interrelationships between the commonly used definitions of dehydration are shown to be independent of initial lens mass but dependent on initial water content. Typically, higher water content lenses undergo considerably larger mass changes than lower water content lenses, an effect that may be masked if these changes are presented as changes of water content. There are a number of clinical consequences of lens dehydration, the importance of which will vary depending on the initial lens water content. It is therefore essential when comparing different water content lenses on the basis of the extent of dehydration to do so with respect to a specific clinical consequence rather than in general terms.

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.

Feasibility of extended wear lens use in chlorinated swimming pools.

A group of 14 active swimmers was studied to evaluate corneal and conjunctival health immediately after extended wear contact lenses were worn in a chlorinated swimming pool. Two different lenses, a high (70%) and a low (38%) water content, were used for each subject in order to compare the corneal response to the high and low water content lenses. No significant difference in corneal and conjunctival response was found between the two lenses. Statistically significant differences were seen in corneal thickness and halo size between swimming with and without contact lenses. Corneal superficial erosion was of greater concern under both the control and test conditions, but this is believed to be due to unforeseen experimental design errors. The frequency of contact lens loss in this study was 3.57%. Hence, the conclusion that the rate of loss of hydrophilic lenses in a swimming situation is not the only concern in regard to lens wear during swimming, but there are other important factors such as lens adherence to the cornea and bacterial contamination of the lenses that need to be studied further.

Low complication rate in extended wear of contact lenses

The study was designed with the intention of reducing the risk of complications in extended wear. 214 eyes were fitted with Scanlens 75 high water content lenses for extended wear and followed for one year. The first 100 eyes to pass the 1-year check-up successfully were then followed for another year. The lenses used are high in oxygen transmissibility and at the same time rigid enough to allow good movements, which are considered very important. Thorough patient information, flat fits, frequent check-ups, and frequent lens replacements were principles applied to prevent discomfort and complications. Visual acuity and comfort were very good. Discomfort and unfavourable findings were low in frequency and healed without any sequelae whatsoever. The success rate was as good as 94.9% at 6 months, 92.1% at 1 year, and 88.4% at 2 years. The favourable results are considered to be due to the type of lens used as well as to the principles employed.

Studies on corneal staining with thin hydrogel contact lenses

Very thin high water content contact lenses can provide sufficient oxygen transmission to the cornea during extended wear to reduce hypoxia-related clinical findings such as overnight corneal oedema, epithelial microcysts, and endothelial polymegathism. However, we have found that such lenses can produce significant corneal staining. We have shown that there are patient factors, lens factors, and environmental factors that influence the occurrence of the corneal staining with very thin high water content lenses.

The Effect of Therapeutic Soft Contact Lenses on Antibiotic Delivery to the Cornea

The effects of high (71%) and low (38.6%) water content lenses on antibiotic delivery to the cornea were studied in rabbits by measuring corneal tobramycin concentrations 1, 2, 4 and 6 hours after topical application at intervals of 15 minutes. In the presence of the low water content lens corneal drug levels were higher than in control corneas for every time point assayed. This difference was only significant at 4 hours (P < 0.05). In eyes wearing the high water content contact lens corneal drug concentration was also higher at every time point tested except one hour. The difference was significant only at 4 hours (P < 0.01). The data suggest that in the normal, noninflammed eye, the presence of a therapeutic soft contact lens will not compromise aminoglycoside delivery to the cornea.

Alloplastic Implants for the Correction of Refractive Errors

Convex-shaped, hydrophilic lenticles of various water content were implanted into microkeratome-dissected corneal pockets in a baboon model. Freshly trephined, cryolathed, and factory-lathed hydrogels were evaluated for periods exceeding 15 months, documenting clinical response, corneal curvature shifts, and morphologic features. Previously implanted hydrogels were subsequently removed, documenting the reversibility of the procedure while allowing the corneas to return to their preoperative condition. Various alloplastic materials were then reimplanted into the same intrastromal pockets. Medium and high water content lenses produced good results without affecting the recipient cornea, whereas thick or low water content materials extruded or produced severe anterior segment inflammation. The use of medium and high water content, convex-shaped lenticles for the correction of aphakia is possible using current techniques.

Use of hyper-thin, low water content hydrophilic lenses for extended wear.

A study was conducted to determine the efficacy of low water content, hyper-thin lenses for extended wear by myopic patients. Patients selected had from 1 to 7 D of simple myopia or myopia with up to 2 D of astigmatism. Patients were either first time wearers or previously successful soft lens wearers. All patients were fitted with Bausch & Lomb O3 or O4 series lenses and were evaluated for 6 months. Either heat or a chemical disinfection system was used. The study shows that Bausch & Lomb O Series lenses may be used successfully for up to 1 week of continuous wear. Visual acuity, patient comfort, lens life, and physiological tolerance compared favorably with those achieved when the currently available high water content lenses were used for extended wear.

Curvature changes in dehydrating soft lenses (Part I)

Introduction Most practitioners have seen soft lenses tightening on an eye from an initially good fitting. Some contact lens materials may be susceptible to variations in pH and tonicity and may produce parameter changes when placed on the eye. If the storage solution pH in particular is markedly different from the eye pH this phenomenon may be encountered. However, the most probable cause of lens tightening is that of water loss from the hydrogel. This may be caused by evaporation from the lens surface and/or a heating up of the lens when placed on the eye. The problems of dehydrating lenses or lens surfaces is obviously greater in higher water content lenses where there is potentially more water to he lost. Many patients' eyes react adversely when wearing soft lenses in dry and warm environments where the lens dehydration is accelerated. The lenses may not feel comfortable, vision may be poorer (Fatt and Chaston, 1981) and less stable; there may also be some physiological embarrassment to the cornea. Coupled with a patient 's blinking pattern, crystallization of tear components on to the lens front surface may be accelerated by increased evaporation from the lens front surface (Hathaway and Lowther, 1978). This is particularly so if the individual's tear film by its nature is going to encourage evaporation. The tear film over the lens front surface may be quite different from the tear film over the wearer's naked cornea. Hamano et al, 1981 found that the rate of evaporation from a low water content gel lens could be up to 2.5X that of the naked eye. The changes in soft lens properties and parameters when the lens starts drying out on the eye have received scant attention by researchers and the literature is bereft of data on the subject. For this reason the author embarked on the present investigation.

Wet cell measurement of soft lens power

The view has been expressed by Chaston 1 dimension of power is the most critical of all soft lens dimensions because such small differences can give what the patient may think of as a large reduction in visual acuity. Sarver, Harris, Mandell and Weissman 2 described how the power of a soft lens can be measured by blotting the surfaces with a lint-free tissue to remove excess moisture, then air drying for 15 to 30 seconds before positioning on a focimeter. This method is probably the one in most common use at the present time both by laboratories and practitioners. Kaplan 3 adapted a vertical contact lens modification unit to function as a centrifuge so a soft lens, supported by an afocal hard lens, could be spun for a few seconds in order to dry it prior to power measurement. Sarver and others 2 established in the case of polyHEMA lenses, power readings should be completed within about five minutes after removal of the lens from a vial or storage case to prevent degradation of the focimeter image as a result of dehydration. Whilst spherical, single vision lenses can be measured within this period, speed and additional dexterity may be necessary in the case of ultra-thin, toric, or bifocal lenses. Special care is required when handling thin high water content lenses since their fragility may result in breakage during blotting. The essential advantage of measurement of power in air is it can be applied to lenses of unknown refractive index. Chaston 4 mentioned the principal disadvantage of this method is that the indistinct image from the blotted lens gives a spread of readings. A further disadvantage is the ambient air temperature and humidity may be uncontrolled. Poster 5 argued the use of a saline-filled cell is more convenient and reliable as well as determining the power in the condition the lens will be worn (hydrated state). When a soft lens is immersed in saline, the measured power is significantly less than measured in air. It is essential, therefore, measurements are made using a focimeter of very high accuracy. Mechanical modification of a conventional focimeter to increase its accuracy was described by Dormon-Brailsford 6 7 and Hampson 8. A dial guage was attached to the instrument to determine back vertex focal length in hundredths of a millimetre and this value was converted to dioptres. Slides shown to illustrate the manufacture of the Softens R have revealed Bausch and Lomb replaced the standard, graduated power drum of a focimeter with one of much larger diameter in order to increase precision of power readings when using a wet cell. It is surprising Nakajima, Shibata, Magatani, Hirano and Terao 9 used a projection focimeter with a scale marked only in intervals of 0.25 D. for wet cell power measurement. The major advantage of measuring the power of a soft lens immersed in a saline-filled cell is the environment can be controlled. According to Holden lo, only way of checking soft lenses is to evaluate the finished product in the hydrated state, i.e. in a wet cell. Ideally, all parameters should be measured in this way because of the susceptibility of lenses to the environmental conditions. Another advantage of wet cell power measurement which seems to have been neglected is it permits verification of very high powered lenses which would otherwise be beyond the range of the focimeter. This advantage is worthy of consideration in the case of both soft and hard lenses. A disadvantage of wet cell power measurement is the refractive index of the lens must be known. Hampson 8 has pointed out surface aberrations of the lens may be masked as a consequence of its