Microbial contamination of contact lens storage cases is common. The aggregation of microorganisms inside the lens case may form a biofilm, which is more resistant to disinfection, and consequently poses a risk for contact lens-related infection and inflammation. Unlike hygiene guidelines for contact lenses which have, by and large, been well-established, guidelines for optimal care regimens for lens cases are yet to be fully determined. Some contact lens practitioners advise patients to follow manufacturers’ guidelines, which commonly involve rinsing the case with multipurpose solutions (MPS) and air-drying. Others suggest the incorporation of a rubbing step. Clinical evidence to support these recommendations has not been readily available. In addition, lens cases from different manufacturers vary in material, size, shape (ridged versus smooth non-ridged, flat versus barrel) and in the incorporation of added antimicrobial features (silver nitrate lining to retard biofilm formation). Diverse antimicrobial strategies of these cases might require specific lens cleaning regimens. A group of researchers in Australia has investigated these ideas using an evidence-based approach. Their aim was to evaluate and compare the effectiveness of several lens case care regimens in removing contaminating biofilms from inoculated contact lens cases.
Wu YT, Zhu H, Willcox M, Stapleton F. The effectiveness of various cleaning regimens and current guidelines in contact lens case biofilm removal. Invest Ophthalmol Vis Sci 2011, 52(8):5287-5292.
This in vitro study tested two commercially available MPS (Opti-free Replenish; Alcon Ltd and COMPLETE EasyRub; Abbott Medical Optics Inc.) and their accompanying lens case for the ability to eradicate or reduce biofilm contamination. One type of lens case tested had ridges in the surface wall and the other was smooth (non-ridged). The challenge bacteria strains were Staphylococcus aureus and Pseudomonas aeruginosa, both of which are associated with contact lens-related microbial keratitis and were able to form adequate biofilms on the test cases. Each lens case was challenged with each bacteria strain individually and was subjected to one of four cleaning regimens:
1. Manufacturers’ guidelines (rinse and air-dry);
2. Rub, rinse and air-dry;
4. Rub, rinse, tissue-wipe and air-dry.
The extent of residual biofilm was calculated and the effectiveness of each regimen was expressed as log colony-forming units (CFU) reduction, compared to a control (no cleaning).
The results showed that irrespective of the MPS and type of lens case, the most effective cleaning regimen was rubbing and rinsing with MPS followed by tissue-wiping and air-drying face-down on a clean tissue. This regimen removed 4 to 6 log CFUs of bacteria, which was significantly more than that removed by the rinsing and air-drying regimen under manufacturers’ guidelines (0.8 to 2.2 log CFU).
In summary, this study has shown that rinsing and rubbing lens cases with MPS followed by tissue wiping and air-drying face down is more effective than manufacturers’ guideline (rinsing and air-drying alone).
Use of friction or mechanical force
The usefulness of friction (rubbing and tissue-wiping) to remove biofilm was highlighted in the study, and was particularly evident with the smooth lens case. Tissue wiping alone led to a significant reduction of biofilm on the smooth cases, even without the rinsing step.
Lens case design
Another merit of the study was to demonstrate the impact of the interior design of a lens case on the ease of biofilm removal, which may have been overlooked in the past. The level of contamination was affected by the design of the lens case “well”: tissue wiping removed biofilm more successfully from the smooth case than from ridged case. The authors suggest that the grooves in the ridged lens case were more difficult to reach during wiping, and could harbour bacteria.
Applicability to other types of lens cases and care regimens
Special lens case designs may also require a different approach. For example, silver impregnated lens cases require capping between uses, to allow the silver ions to react with the fresh MPS residue, which continues to kill pathogenic microbes.
This regimen may not be applicable with hydrogen peroxide systems, due to differences in case design.
Overall, this in vitro study has provided groundwork for future verification of the proposed contact lens case care regimen in vivo, which may lead to better clinical guidelines for practitioners and wearers.