In-eye Contact Lens Cleaners
Major solution manufacturers believe in-eye cleaners belong in contact lens practice, but are they effective?
By Milton Hom, OD, FAAO, and Peter Simmons, PhD, FAAO
In-eye contact lens cleaners are a relatively recent development in contact lens care. Of three recently-released rewetting drops, two of them have the added feature of in-eye cleaning.
Lens Deposit Formation
Within minutes of lens application to the ocular surface, mucins and proteins from the tear fluid form an initial layer of material on the hydrogel surface. This layer gradually thickens over time with contact lens wear. Normally, the layer is smooth and transparent. One reason that new lenses often feel more comfortable after the first few hours of wear, or better on the second day, is because of this initial surface layer. The layer helps the lens become more compatible with the eye.
Protein, lipid, minerals, cellular debris and micro-organisms can adhere to this initial layer. Dean Hart originally called this layer the pellicle, a term borrowed from oral biology (a layer that forms on the tooth surface). On the hydrogel lens surface, the pellicle has varying thickness and texture on different regions of the lens. It contributes additional material to the underlying initial layer, and can potentially cause irritating deposits. The pellicle is reversible to a certain extent, and you can remove most of it with thorough surfactant cleaning and/or soaking in enzymatic cleaners or other agents.
The overall deposition on the lens surface tends to increase with time, typically reaching a steady state by the end of the first week of wear. For some wearers, the deposits may remain fairly smooth and non-irritating, with little effect on lens comfort. For others, the proteins become denatured and less transparent (similar to the way egg white becomes opaque when cooked), and the surface of the lens becomes rougher. Comfort gradually declines until the lens needs to be thoroughly cleaned or replaced. Continued wear of a lens with a rough, deposited surface can cause giant papillary conjunctivitis (GPC) and increase the risk of microbial adherence and infection.
How Do In-eye Lens Cleaners Work?
In-eye cleaners rely on two components: the chemical action of a surfactant and the physical action of the eyelid wiping across the lens. When the drop is applied to the lens surface, the surfactant reduces the surface tension in the overlying tear fluid. The accumulated protein, lipid and debris on the lens surface mixes more easily with the tears. The lids complete the action by wiping the lens clean. The contaminants are flushed off the lens and out of the eye.
Why an In-eye Cleaner?
Contact lens manufacturers have launched new lens materials specifically designed for extended wear. Among those leading this effort are Bausch & Lomb with PureVision and CIBA Vision with Focus Night & Day. These continuous wear lenses are receiving regulatory approval for up to 30 days of consecutive wear.
Patients in extended-wear contact lenses tend to need more eye drops to maintain lens comfort than those who wear daily-wear lenses. In particular, extended-wear patients need to use drops prior to sleep and especially upon awakening to re-hydrate the lenses.
Even daily-wear patients exhibit a general trend and desire to wear lenses longer into the evening. These patients need a drop that not only can provide moistening and lubricating benefits of a standard rewetter, but also can help clean protein and debris buildup from the lens. In-eye cleaning drops are specifically formulated with a surfactant to provide cleaning action while the lens is worn, in addition to providing moisture and lubrication.
The Facts on Surfactants
The new in-eye cleaners are designed to keep lenses clean. There are currently two on the market: Allergan Blink-N-Clean and Alcon Clerz Plus. Bausch & Lomb's Preservative Free Rewetting drops falls in the class of rewetting drops.
The basic formulation of the in-the-eye cleaning drops includes active ingredients to remove or reduce deposit formation. These active ingredients are non-irritating surfactants, coupled with an appropriate buffer. For Allergan's Blink-N-Clean, the surfactant is tyloxapol, and the buffer is tromethamine. Alcon's Clerz Plus has two surfactants: PEG-11 lauryl ethyl carboxylic acid (trademarked Clens-100) and tetronic 1304, in a citrate/borate buffer.
Surfactants lower the surface tension of the lens contaminants. A surfactant essentially surrounds the surface of the contaminants with charged species. This causes the contaminants to repel each other, and they are then flushed from the lens surface.
Surfactants used in the past irritated the eye, like adding detergent to a lens solution. To make the surfactants non-irritating, manufacturers used less. However, smaller amounts of surfactants do not clean as well.
Today's technology has changed all that. Present surfactants clean better and are so non-irritating that they can clean lenses while in the eye. Hence, the evolution of lens care products into in-eye cleaners.
|Typical Method for Measuring Protein on Lenses|
The active components of Allergan's Blink-N-Clean are tyloxapol, tromethamine and hydroxypropyl methylcellulose (HPMC), preserved with polyhexamethyl biguanide (PHMB). Tyloxapol is a non-ionic surfactant that helps remove lipids and debris and lowers surface tension for better cleaning. Tromethamine is a cationic buffer (+) that ionically displaces lysozyme deposits from the surface of the lens and also helps prevent further protein deposition by occupying protein binding sites. HPMC lowers surface tension, which aids in cleaning and enhances wettability to help prevent debris from attaching to the lens surface.
Clerz Plus' PEG-11 surfactant primarily cleans debris and deposits from the contact lens. The tetronic surfactant is a wetting agent for the hydrogel surface, designed to reduce further binding of deposits.
Bausch & Lomb's new Preservative Free Lubricating & Rewetting Drops contains bendazac lysine and HPMC. Bendazac lysine reportedly has antioxidant and anti-inflammatory properties and is classified as an NSAID. In the past, it has been proposed as a prophylactic treatment to prevent progression of cataract development. Its primary function is to prevent denaturation of proteins. Other studies have suggested that bendazac lysine might reduce contact lens deposit formation, presumably by preventing denaturation of lysozyme on the lens surface.
Many different techniques can measure deposits on lenses. The two main classes are visual and chemical analysis. Clinically, the classic Rudko technique provides a rough visual assessment of the extent of lens deposition. In the laboratory, lens deposits have been examined by light and electron microscopy, and more recently with computer image analysis.
Chemical techniques most commonly used to analyze contact lens deposits are spectrophotometry, electrophoresis and HPLC (high pressure liquid chromatography) (see sidebar).
Figure 1 shows spectrophotometer cuvettes with solutions of protein standards and protein extracted from patient-worn lenses. The amount of blue color following reaction with the assay chemicals is proportional to the amount of protein, as indicated. As expected, patient samples extracted from Group I lenses have much less protein than samples from Group IV lenses.
Figure 2 shows the analysis of data from Figure 1. First, the values for absorbance and known protein amount are plotted (black dots) for the standards, and a curve (black line) is fitted to the data. For patient samples, the measured absorbance (circled values on horizontal axis) is equated to amount of protein (circled values on vertical axis) by reference to the standard curve (colored lines). Finally, due to dilution of extracted protein, the actual total amount on the entire patient lens would be twice the calculated amount for Group I lenses (green and red lines) and eight times the calculated amount for Group IV lenses (blue and violet lines).
|Figure 1. Spectrophotometer cuvettes with solutions of protein standards and protein extracted from lenses.|
Spectrophotometry can also be used to measure protein directly on the lens by taking advantage of the fact that the principal tear protein depositing on lenses is lysozyme. Lysozyme has a distinct UV absorption peak at about 280 nm. The disadvantages of this direct UV spectrophotometric method include inaccuracy in measuring deposits on highly UV-absorbing materials and accounting for the effects of lens thickness and parameters. UV spectrophotometry is more reliable in quantifying the amount of protein in recovered rinse solution; it estimates the amount of protein removed by a particular solution.
|Figure 2. Standard curve and calculation of total protein in patient samples shown in Figure 1.|
In-eye Cleaning Efficacy
Using these types of analytical methods, several studies evaluated in-eye cleaner performance. In one study, lysozyme levels on lenses from patients using Clerz Plus were lower than levels on lenses from patients using a standard non-cleaner rewetting drop.
In another study, UV spectrophotometry measured the protein removed from worn lenses following a brief exposure to either Blink-N-Clean or Clerz Plus. The lenses were briefly dipped in each of the two products. Blink-N-Clean demonstrated statistically significant better removal of protein from both Group IV lenses and silicone hydrogel lenses (n=10, p<0.02) (Figure 3). While this difference suggests that Blink-N-Clean may perform better than Clerz Plus, actual clinical performance in your patients will be determined by their individual tear chemistry, type of lens and lens wearing habits.
Figure 3. Amount of lysozyme removed from a patient-worn lens evaluates the performance of in-eye cleaners.
Contact lens solutions continue to evolve. With the return of extended wear and the movement towards silicone hydrogel lenses, in-eye cleaners are a useful new addition to the contact lens care armamentarium. We would recommend them to all of our patients for safer and more comfortable lens wear for longer periods of time.
Dr. Hom received a grant from Allergan for this article. Dr. Simmons is a scientist with Allergan.
To receive references via fax, call (800) 239-4684 and request document #84. (Have a fax number ready.)
Dr. Hom is the author of Manual of Contact Lens Prescribing and Fitting with CD-ROM Second Edition and LASIK: Clinical Co-Management. He practices in Azusa, Calif.
Dr. Simmons conducts research and development projects within the Consumer Eye Care group at Allergan, Inc., with particular interest in the area of dry eye disease and treatment. He has published in the fields of neurophysiology, ocular physiology and vision science.