I am an educator, so I have a tendency to think about the minute devilish details, how those details fit into the big picture and—most importantly—how I’m going to teach them. Recently, as I was reviewing contact lens solutions with my wide-eyed, mortified looking residents, I realized I had lost sight of the big picture.
I had drifted so far away from the basics that I had lost the message altogether.
The basics of contact lens solutions that I’ve slowly moved away from are actually some of the most important aspects of lens care to understand: how they clean, disinfect and wet the lens surface to provide comfortable and safe contact lens wear.
So, let’s take some time to step back and review some of the basics of contact lens solutions. These products contain two major ingredients: surfactants and disinfectants. This month’s column will focus on the role each of these ingredients play in the finished product.
A surfactant, or surface-active agent, is used to both clean and wet the contact lens. Surfactants are composed of both hydrophilic and hydrophobic ends in the molecule structure.
In the case of surfactant cleaning products, the hydrophobic ends cluster around debris to form micelles. The free hydrophilic ends are then able to react with water, and the micelle can then be whisked off the lens surface. This creates a process that is very similar to what happens when we wash our hands with soap.
In the case of a surfactant wetting agent, the hydrophobic end interacts with the dry hydrophobic lens surface, allowing the lens surface to regain hydrophilicity, as well as lubricity. The size of the surfactant molecule affects how tightly it will bind to the lens surface, and subsequently how long it will stay on the lens surface.
Newer generation contact lens solution surfactants are custom designed specifically to interact with hydrophobic silicone to create a more comfortable and long lasting wettable silicone hydrogel experience.
There are three major chemical disinfectants currently on the market: biguanides (primarily poly-hexyl-methyl-biguanide, aka PHMB), polyquaterium-1 (Polyquad) and peroxide.
PHMB is a cationic disinfectant that has efficacy against both gram-negative and gram-positive bacteria. PHMB also has some effect against Acanthamoeba. Its efficacy is attributable to the electrostatic interaction of cationic sites of PHMB with anionic sites of the bacterial cell membrane. These interactions result in the disturbance of the membrane structure and the leakage of intracellular components.
Polyquad is a cationic surfactant that is larger in size than PHMB. Polyquad also affects the phospholipid membrane but, because of its size, it has a reduced uptake and release pattern in contact lenses and cases.
PHMB and Polyquad can be used together or with other antimicrobial agents to broaden anti amoeba/antifungal disinfection.
Hydrogen peroxide is a potent disinfectant because it is lipid soluble. It produces hydroxyl free radicals, which attack and penetrate lipid-containing cell membranes, and subsequently destroy DNA, mitochondria and other cell components.
Peroxide is active against amoebae, protozoa, viruses, bacteria and fungi, but its efficacy depends on contact time and concentration. It’s worth noting that perfect disinfection occurs at four hours of straight 3% peroxide, but no commercially available products are capable of achieving this.
These “basics” may seem simple enough, but things can become complicated rather quickly. Adding chelating agents, other wetting agents (such as hyaluronate or methylcellulose), buffering agents, compliance issues, the vast chemistry of lens materials, real-world bugs with varying virulence, contact lens cases and, of course, the human eye, can make keeping the patient “comfortable and safe” seem nearly insurmountable.
Of course, no task is truly insurmountable if proper attention to detail is paid—and as I’ve often heard, “the devil’s in the details.”