Low Water Content Lenses Research Articles

Trends in Australian contact lens prescribing during the first decade of the 21st Century (2000–2009)

Purpose: The aim was to document contact lens prescribing trends in Australia between 2000 and 2009.Methods: A survey of contact lens prescribing trends was conducted each year between 2000 and 2009. Australian optometrists were asked to provide information relating to 10 consecutive contact lens fittings between January and March each year.Results: Over the 10‐year survey period, 1,462 practitioners returned survey forms representing a total of 13,721 contact lens fittings. The mean age (± SD) of lens wearers was 33.2 ± 13.6-years and 65 per cent were female. Between 2006 and 2009, rigid lens new fittings decreased from 18 to one per cent. Low water content lenses reduced from 11.5 to 3.2 per cent of soft lens fittings between 2000 and 2008. Between 2005 and 2009, toric lenses and multifocal lenses represented 26 and eight per cent, respectively, of all soft lenses fitted. Daily disposable, one‐ to two‐week replacement and monthly replacement lenses accounted for 11.6, 30.0 and 46.5 per cent of all soft lens fittings over the survey period, respectively. The proportion of new soft fittings and refittings prescribed as extended wear has generally declined throughout the past decade. Multi‐purpose lens care solutions dominate the market. Rigid lenses and monthly replacement soft lenses are predominantly worn on a full‐time basis, whereas daily disposable soft lenses are mainly worn part‐time.Conclusions: This survey indicates that technological advances, such as the development of new lens materials, manufacturing methods and lens designs, and the availability of various lens replacement options, have had a significant impact on the contact lens market during the first decade of the 21st Century.

A decade of contact lens prescribing trends in the United Kingdom (1996–2005)

Purpose To document contact lens prescribing trends in the United Kingdom over the past decade (between 1996 and 2005). Methods An annual survey of contact lens prescribing trends was conducted each year by asking a random selection of optometrists and dispensing opticians to provide information relating to 10 consecutive contact lens fits between January and March. Results Over the 10-year survey period, 1251 practitioners returned survey forms representing a total of 12,050 contact lens fits. The mean (±standard deviation) age of lens wearers was 33 ± 13 years of which 64% were female. Over the survey period, rigid lens new fits decreased from 22% to 4%. Low water content lenses reduced from 20% to 2% of new soft lens fits. Soft toric lenses gradually increased to the point where they accounted for the expected proportion of patients with astigmatism ≥1.00DC (i.e. >20% of soft lens fits). Monthly and daily disposable lenses accounted for >90% of all soft lens fits between 2003 and 2005. The proportion of new soft fits and refits prescribed as extended wear has gradually increased, stabilizing at about 5% of new fits and 12% of refits. Multi-purpose lens care solutions increased from 60% of all care systems in 1997 to 91% in 2005. Rigid lenses and monthly replacement soft lenses are predominantly worn on a full time basis, whereas daily disposable soft lenses are mainly worn part time. Conclusions This survey indicates that technological advancements such as the development of new lens materials, manufacturing methods and lens designs have had a significant impact on the contact lens market over a relatively short period of time.

Contact lens practice in Hong Kong in the new millennium

Purpose: We evaluated the contact lens services provided by practitioners in Hong Kong, the fitting habits of the contact lens practitioners and their attitudes regarding the use of orthokeratology (ortho‐k) and silicone hydrogel (SH) lenses for overnight wear.Methods: Questionnaires were sent to all Hong Kong optometrists licensed to prescribe contact lenses.Results: Two hundred and seventy‐five questionnaires (21 per cent) were returned. Most of die respondents (96 per cent) were employed in optical shops and worked in practices that provided contact lens services. The number of new/refit cases per practice per month was about 55, with die percentages of new and refit cases being 48 per cent and 52 per cent respectively. Eighty‐eight per cent of the patients were fitted widi soft lenses and 10 per cent were fitted widi rigid gas permeable (RGP) lenses. Fifty‐nine per cent of soft lens patients were fitted widi planned replacement (frequent replacement and disposable) soft lenses. Fifty per cent of soft lenses were low water content lenses and 85 per cent of RGP lenses were in low DK (≤ 40) material. Multipurpose solutions and soaking solutions with separate cleaner were die most commonly prescribed care regimens for soft and rigid lenses, respectively. Thirteen per cent of die practitioners provided ordio‐k services and die mean number of ortho‐k cases per mondi per ortho‐k practitioner was six, die majority of patients being children. DreimLens was die lens design most commonly used and Boston XO die most used material. However, 96 per cent of die responding practitioners had reservations about die practice of ortho‐k and die use of SH lenses for overnight wear.Conclusions: The contact lens market in HK is still dominated by soft lenses, particularly planned replacement lenses, whereas die percentage of RGP fits has changed little in the past decade. Few respondents used overnight ortho‐k and SH lenses in view of concerns about their efficacy and long‐term safety.

The therapeutic use of disposable contact lenses

Introduction: Disposable contact lenses (generally used as part of planned replacement schemes) have become a major part of the general contact lens armamentarium. Because of this widespread acceptance in general optometric practice, it was considered useful to assess their use as therapeutic devices. Several previous studies have considered low water content lenses; the present study reports on the use of a high water contact lens.Materials and methods: A 74% water content cast‐moulded lens (8.70/14.40 plano) was fitted to 66 eyes of 60 patients over an eight‐month trial period. Indications for lens fitting were wide ranging and the lens was used as a first choice unless there was a definite contraindication to its use. Periods of wear ranged from two days to permanent wear, with exchange every few months.Results: The lens was found to fit satisfactorily in 56 eyes (85%). With regard to ocular abnormality, there was no consistent reason for poor fitting in the remaining 10 eyes. Lens wear was discontinued in three eyes due to lens‐related complications. The success rate from the use of this lens was, therefore, calculated to be 53 out of 66 eyes (80%).Conclusion: The relatively successful use of a high water content disposable lens for therapeutic purposes was considered very encouraging and confirms the results of other studies using lower water content lenses. The cost benefits and ready availability of such lenses make them a very useful product for hospital contact lens practice. In the Nottingham Unit they have become the lens of first choice unless there is a specific indication for a different type of lens.Further details of indications, fitting problems and complications will be given in the poster.

A Piggyback Contact Lens for the Correction of Irregular Astigmatism in Keratoconus

Although the combination of a hard contact lens "piggybacked" on a soft lens base for the correction of severe astigmatism in keratoconus has existed for two decades, little is known about its effect on the corneal epithelium and endothelium. This study involves two parts: the measure of oxygen pressure in rabbits, and the long-term effects on patients with keratoconus wearing these hybrid lenses. A polarographic sensor was used to measure the oxygen pressure on rabbit corneas under two types of piggyback lenses: oxygen-permeable hard lenses on high water-content soft lenses, and polymethylmethacrylate (PMMA) lenses on low water-content soft lenses. In the clinical evaluation, 11 patients with keratoconus (8 men, 3 women; 25.1 +/- 4.9 years of age) who could not wear hard contact lenses due to constant pain or inadequate lens fitting were given oxygen-permeable piggyback contact lenses and were observed for at least 9 months. In the animal study, the oxygen pressure under piggybacked oxygen-permeable hard contact lenses was 95 +/- 14 mmHg after 5 minutes wear, but it was only 34 +/- 14 mmHg when PMMA and low water-content lenses were used. Ten of 11 patients were successfully treated using this technique, whereas one patient had persistent contact lens loss, requiring penetrating keratoplasty. Specular microscopic observation of the corneal epithelium and endothelium did not show any changes during the follow-up period. These results suggest the efficacy and safety of the piggyback combination of oxygen-permeable hard and soft contact lenses for the correction of astigmatism in patients with keratoconus.

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.

Interactions of Benzalkonium Chloride With Soft and Hard Contact Lenses

We measured the uptake and washout of benzalkonium chloride, using radioactive tracer, by representative hard and soft contact lenses. Uptake by soft contact lenses after 7 days of continuous exposure is high (30 to 56 micrograms/mg of lens weight), with a low percentage of washout in 24 hours (between 0.2% and 1.5% of total uptake). High-water content lenses absorb greater quantities of benzalkonium than do low-water content lenses. Hard lenses take up a much smaller quantity of benzalkonium but release between 30% and 60% of total uptake during washout for 24 hours. Fluorosilicone-acrylate polymer lenses adsorb and release the most preservative, while polymethylmethacrylate lenses (Paragon Optical Inc, Mesa, Ariz) adsorb and release the least. The released benzalkonium from either soft or hard lenses is of a sufficient concentration to be at or above the upper limits of safety.

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.

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.

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.