Hi Lexa
There are various types of contact lens solutions. In general, you can divide them into those for soft contact lenses, and those for rigid/hard contact lenses.
In terms of solutions for soft contact lenses, which are the most commonly used ones, currently you have multipurpose '3-in-1' solutions and hydrogen peroxide based disinfecting systems such as AOSept.
3-in-1 solutions mean they can be used to 1) clean/disinfect, 2)store/soak and 3) rinse lenses. In the past, these functions were usually done with separate solutions ie special cleaners, soaking solutions, and then rinsing with saline. These multipurpose solutions are very convenient to use. The different brands of multipurpose solutions are very similar in their function and effectiveness.
AOSept and other hydrogen peroxide systems can also perform these functions, but the lens must be stored in a special case and soaked in the AOSept for at least 6 hours, so that the hydrogen peroxide gets neutralised to water and oxygen. The solution cannot be used to rinse the lens just before wearing, as any un-neutralised hydrogen peroxide stings a great deal.
There are pros and cons with each of these 2 main types of solutions. 3-in-1 solutions are the most convenient of all, but some people develop allergies to the ingredients, which can cause persistent redness and itching. Hydrogen peroxide systems are impossible to become allergic to, but demand discipline in soaking for at least 6 hours in a particular container before use.
Expired contact lens solutions should never be used. In the first instance, such old bottles could be colonised by biofilms of various micro-organisms/germs. Even if this wasn't the case, the disinfectants could be degraded and not work properly. As such, using such solutions puts the eye at higher risk of getting corneal ulcers, which could permanently affect vision.
