Introduction
This chapter reviews the care systems used with rigid contact lenses. Of course, many of the general principles of contact lens care, such as the rationale for lens cleaning and disinfection, regulatory control of the contact lens care industry and various approaches to comparing the efficacy of different solutions, have already been discussed in Chapter 10 and will not be repeated here.
Disinfection and Wetting Solutions
Traditionally, rigid lens products were preserved with benzalkonium chloride, thiomersal and chlorhexidine. However, there is some evidence that sufficient levels of chlorhexidine or benzalkonium chloride can bind to the surface of a rigid lens, leading to a toxic reaction at the ocular surface after lens insertion ( ). More recent products have seen a move away from these preservative agents or, as in the case of the Boston Advance product, a reduction in chlorhexidine concentration compared with previous care solutions. Also, polyhexanide (more traditionally part of soft lens disinfectant products) has been introduced as a second preservative in rigid lens solutions ( Table 18.1 ). For example, Boston Simplus multiaction solution (Bausch & Lomb) contains polyhexanide as its disinfectant ( Fig. 17.1 ).
| Company | Product | Preservative (ppm) | Surfactant/Conditioner/Viscosity Agents |
|---|---|---|---|
| Advanced Eyecare Research | RGP contact lens solution | Oxychlorite |
|
| Bausch & Lomb | Boston conditioning solution |
|
|
| Bausch & Lomb | Boston Simplus |
|
|
| Johnson & Johnson Vision | Total Care | Polyhexanide (5) | Hydroxyethylcellulose |
Multipurpose solutions for cleaning and disinfecting rigid gas-permeable lenses have replaced single-purpose solutions, but there are few reports of the efficacy of these multipurpose solutions, or of the effects of storage conditions on their disinfecting capacities. showed that multipurpose solutions for rigid lenses lose activity over the 3 months’ recommended time of use but remain satisfactory for use. Disinfecting capacity was reduced more quickly when the solution was stored in the refrigerator. Rigid lens disinfecting solutions are not able to kill Acanthamoeba species during manufacturer-recommended disinfection times ( ).
In addition to their role in lens disinfection, most rigid lens storage solutions also act to wet or to condition the lens. This role is principally to act as a lubricant, affording a degree of protection to the cornea and lid margins when the lens is inserted. The cushioning effect minimizes discomfort at insertion. The secondary effects of successful lens wetting are that the lens surface is, first, rendered hydrophilic to aid a stable prelens tear film and, second, made more biocompatible, which might reduce protein deposition.
Various agents are incorporated into rigid lens solutions to aid surface conditioning. Polyvinyl alcohol is a positively charged polymer that is attracted to the negatively charged surface of lenses containing methacrylic acid to provide a more wettable lens ( ). Another agent used to increase wettability is the viscosity agent hydroxyethylcellulose. In addition to preservative and conditioning/wetting agents, rigid lens care solutions contain buffering agents to maintain a stable pH, and chelating agents to increase antimicrobial action and assist in lens cleaning.
Cleaning Solutions
Some rigid lenses are cleaned with a separate solution to the disinfectant and wetting product, whereas others follow many of the soft lens care systems and are multipurpose products. Separate rigid lens-cleaning solutions can be more intensive than their soft lens equivalents because there is less opportunity for the solution to enter the lens material, with the subsequent possibility of toxic reaction. For example, Boston Advance Cleaner (Bausch & Lomb) contains a silica suspension of microscopic beads, which act like a gentle polish on the lens; this is beneficial with deposits such as denatured proteins, which can otherwise be difficult to remove. This cleaner also contains an alcohol base, which assists in removing lipid-type spoilation.
Protein Removal Solutions
Protein removal is arguably more important with rigid lenses than with soft lenses, in view of the fact that most soft lenses prescribed today are replaced more regularly than rigid lenses. Where available, protein removal systems that were originally designed for use with soft lenses can also be used with rigid lenses. The frequency with which patients should be advised to use protein removal systems, and how such systems should be applied to the lenses, will vary depending on the lens material and the strength of the active ingredient in the protein removal system. Advice on these issues should be obtained from the manufacturer.
Individual patient factors will also have an impact on the way protein removal systems should be applied. Patients who display a propensity for depositing protein on lenses, and who wear their lenses more frequently and for longer periods of time, may need to treat their lenses more regularly. Typical frequencies of usage of protein removal systems vary from weekly to monthly.
Disinfection of Trial Lens Sets
Proper application of the standard lens care protocols described in this chapter will be efficacious at killing most bacteria, viruses, fungi and protozoa, especially those known to cause infection in the eye. However, certain infectious agents have more recently been identified that are apparently resistant to current soft and rigid lens care regimens. Of particular concern is a proteinaceous vector known as a prion – a chameleon-like infectious agent that exists in different strains that have distinct biological properties and can alter when the disease for which it is responsible crosses the species barrier. It has been suggested by health authorities in the United Kingdom that there is a remote theoretical risk of transmission of variant Creutzfeldt–Jacob disease (vCJD) between humans, via transfer of bodily tissues and fluids such as tears.
An extension of the above argument leads to the conclusion that vCJD could theoretically be transmitted from an infected individual to another person via a trial contact lens contaminated with the offending prion. Although such transmission is theoretically possible, it remains highly improbable ( ). In the light of this, the College of Optometrists recommends that single-use trial lenses should be used whenever possible to limit any chance of disease transmission. Indeed, excellent success rates in fitting rigid lenses empirically (i.e. the lens is ordered based on measures of refraction and ocular dimensions) have been demonstrated. However, it is recognized that in certain cases, particularly where there is disease or abnormality of the lid, cornea or ocular surface, special complex diagnostic contact lenses may be necessary for a successful clinical outcome. These lenses may need to be reused. An obvious case is keratoconus, as a practitioner who fits patients with this condition may have access to a number of trial fitting sets, each representing a different design philosophy.
Where empirical fitting is impracticable then suitable items should be decontaminated using a recognized method. found that Menicon Progent and Menicon MeniLAB (0.4% and 0.5% sodium hypochlorite, respectively) decreased the infectivity of prions retained on the surface of experimentally contaminated lenses by a factor of at least 10 million. Current advice, however, is to use a readily available sodium hypochlorite 1% solution (10,000 ppm available chlorine), such as is sold for household use, allowing the lens to be safely reused. concluded that sodium hypochlorite does not appear to distort rigid lenses; however, it may not be used on soft lenses.
Conclusion
Multipurpose care systems are becoming the norm for the maintenance of rigid contact lenses. Because rigid lenses typically have a life span in excess of 6 months, occasional use of protein removal systems may be required. The reuse of rigid trial lenses to determine the best lens fit should be minimized because of the remote theoretical risk of transmission of diseases by agents such as prions, which are resistant to conventional antimicrobial methods. Sodium hypochlorite solution is an effective rigid lens disinfecting solution with antiprion activity.
Acknowledgement
The author acknowledges the assistance of Sarah Smith for photography in Fig. 18.1 .
