As contact lens materials and solution formulations become ever more complex, the compatibility of these products with the eye and with each other is an area of growing research interest.
A paper in the January issue of Eye & Contact Lens by researchers at the University of Waterloo in Canada looked at the interaction of benzalkonium chloride (BAK) with silicone hydrogel and conventional hydrogel lenses.1
Four lens types - Focus Monthly (vifilcon A), Focus Night & Day (lotrafilcon A), Acuvue Advance (galyfilcon A) and Surevue (etafilcon A) - were soaked for 24 hours in various concentrations of BAK. After 24 hours, the lenses were washed and then incubated in a balanced salt solution for seven days.
BAK released into the solution was used as the test chemical for three assay methods, including a bovine lens assay used with confocal scanning microscopy. The amount of BAK extracted from the various contact lenses was measured using an Abbe refractometer. Controls consisted of lenses subjected to the same conditions as the treated contact lenses, but without BAK.
At 0.1 per cent BAK extraction, all lens extracts showed increased levels of back vertex distance variability of the cultured bovine lens. Extracts from soaking vifilcon A lenses in BAK caused the most damage to the epithelium and mitochondrial metabolism. The authors conclude that these unexpected findings indicate unknown chemical agents may be leached from contact lens polymers.
Staining studies
Corneal staining remains one of the principal signs used by practitioners to determine the compatibility of contact lenses and care products with the eye, although opinions are divided as to its clinical significance.
US researcher Dr Gary Andrasko has developed the Andrasko Corneal Staining Grid,2 as a reference tool for the level of biocompatibility of various multipurpose contact lens solutions (MPS) in soft lens wearers. As new MPS designed specifically for use with silicone hydrogels emerge, the grid contains useful information for practitioners to consider when deciding which lens/solution combination to prescribe.
Ten MPS are listed, most of which are also marketed in the UK, and seven soft lens materials including all the currently available silicone hydrogels. The grid gives the percentage of average corneal staining area two hours after insertion for each lens/solution combination. Each cell in the staining grid represents a study with a sample size of 30 and the methodology is outlined.
A second goal of these studies was to determine if a relationship exists between corneal staining and lens wearing comfort. Plotting average staining area against the average comfort rating of each lens/solution combination tested indicates that comfort tends to decrease as staining increases. Also, as wearing time increases, comfort tends to decrease if significant staining area exists.
Other researchers are taking different approaches to reducing solution-related staining. A study by Paugh and co-workers3 at the Southern California College of Optometry published in the January issue of Optometry and Vision Science investigated whether a pre-application drop containing the viscosity agent carboxymethylcellulose (CMC) could reduce corneal staining induced by polyhexanide-containing MPS.
The cumulative fluorescein staining scores showed a significant increase over time. This effect was expected and likely to be driven by the MPS. However, increases in the mean cumulative staining scores for CMC and povidone were significantly different, with a greater increase in staining for the povidone lubricant. There were no significant differences in symptom scores from zero to four weeks by regimen or between drops. The authors conclude that a CMC-containing pre-application drop can reduce corneal staining resulting from disinfection with a polyhexanide MPS. CMC appears to neutralise cationic disinfectants and may offer clinicians another means to reduce this type of staining.
Identifying potential drop-outs
For patients, comfort is the key attribute of contact lens wear and discomfort is the principal reason for dropping out. A new study by Richdale et al4 at the Ohio State University, published in the February issue of Cornea, looks at factors associated with contact lens dissatisfaction and discontinuation.
A total of 730 subjects were surveyed using a self-administered questionnaire. The survey collected information about age, sex, history of contact lens wear, types of lenses worn, age at starting wear, current wearing schedule, contact lens satisfaction and lens-related problems.
Dissatisfied contact lens wearers had reduced wearing times compared with satisfied contact lens wearers. The primary reason given for both contact lens dissatisfaction and discontinuation was ocular symptoms (dryness and discomfort), followed by preference for another corrective modality.
Previous lens wearers were more likely than current lens wearers to be men, older (by 9.5 years), have started contact lens wear at a later age (four to five years later), and have tried either rigid or both soft and rigid lenses. The study therefore offers some help to practitioners in identifying contact lens wearers who were most likely to drop out.
References
1 Dracopoulos A, Dixon, D G, Jones L W et al. In vitro assessment of medical device toxicity: interactions of benzalkonium chloride with silicone-containing and p-HEMA-containing hydrogel contact lens materials. Eye & Contact Lens, 2007 33:1 26-37.
2 Andrasko Corneal Staining Grid. www.staininggrid.com.
3 Paugh J R, Marsden H J, Edrington T B et al. A pre-application drop containing carboxymethylcellulose can reduce multipurpose solution-induced corneal staining. Optom Vis Sci, 200784:1 65-71.
4 Richdale K, Sinnott L T, Skadahl E et al. Frequency of and factors associated with contact lens dissatisfaction and discontinuation. Cornea, 200726:2 168-174.