Contact lenses: Contact lenses are usually made from polymethyl- methacrylate, a hard hydrophobic plastic, but some softer hydrophilic lenses have been developed and are coming to use. Types of contact lenses: 1- Hard contact lenses 2- Soft contact lenses
1.HARD CONTACT LENSES: Wearers of hard contact lenses generally use two solutions.
- A wetting solution, which is used primarily for treating the lenses before insertion.
- A storage solution, which is used for overnight cleansing, soaking and storage.
Wetting solutions: because of its hydrophobic nature, polymethyl methacrylate is poorly wetted by the lachrymal fluid and requires moistening with a wetting agent to render the surface hydrophilic and make insertion easy and comfortable. This treatment also prevents smudging when the lens is held, on a well –scrubbed finger tip, for insertion. Unlike most eye-drops, contact lens solutions are used daily for years and, therefore, they should be exceptionally bland.
They may contain:
A wetting agent : polyvinyl alcohols, which are strongly hydrophilic, and polysorbate 80 , a non-ionic surfactant in which hydrophilic groups predominate, are examples. Polyvinyl alcohols are obtained from polyvinyl acetate by hydrolysis and may contain traces of the parent compound from which irritating amounts of acetic acid may be liberated at neutral or alkaline pHs. Hence, solutions are often adjusted to a pH between 5 and 6. Polysorbate 80, at concentrations of 0.5 percent and above, inactivates Benzalkonium chloride and chlorhexidine acetate and at lower concentrations may increase or decrease their activity according to the conditions.
An antimicrobial agent: prolonged use of solutions containing the concentrations of antimicrobial agents recommended for eye-drops may cause irritation or allergic reactions. Consequently, lower levels are used in lens solutions; e.g. Benzalkonium chloride 0.004 percent, chlorhexidine acetate 0.005 to 0.006 % and chlorbutol 0.3 to 0.4 percent. Using more than one antimicrobial agent is a way of reducing the concentration of each and , at the same time, maintain an acceptable level of preservative activity. For example, disodium edentate(0.1 to 0.2 %) may be combined with Benzalkonium chloride, Benzalkonium chloride and chlorbutol or Benzalkonium chloride and chlorhexidine acetate.
A substance for adjusting the osmotic pressure: it is customary to adjust wetting solutions to approximately the same osmotic pressure as lachrymal secretion by adding sodium chloride alone or with potassium chloride. Wearing contact lenses seems, by interfering with corneal metabolism, to increase the sensitivity of the eye to solutions that are not iso-osmotic with tears.
A buffering agent: Hind and Goyan found that solutions of pH 8.0 to 8.8 were best tolerated by the eye while more alkaline or acid preparations caused smarting or fogging. Hypromellose Eye Drops B.P.C., which is sometimes used as a contact lens solutions, is buffered to pH 8.4 to 8.6 with boric acid and borax. Sodium bicarbonate 2 per cent , an early contact lens solution, has a pH of about 8.2 and is well tolerated.
A thickening agent: Comfort is significantly increased if the solution contains a substance that raises the viscosity, e.g. polyvinyl alcohols or the cellulose derivatives. The thickener must be compatible with the preservative(s) and produce a clear solution that does not interfere with vision. Generally, the vehicle for lens solutions is properly stored purified water; tap water is unsuitable because it contains salts, that may cause deposits, and chlorine, which may be irritating.
Storage solutions: Hydrophobic lenses attract hydrophobic substances from the eyelids, fingers, etc. and, therefore, require cleaning after use. After removal they are wetted with the wetting solution and rubbed between well-scrubbed fingers until they appear clear after rinsing with purified water. Then they are stored in a soaking solution intended to continue the cleaning process and prevent dehydration. Polymethyl methacrylate, although hydrophobic, does absorb a little water and it is undesirable for this to be taken from the eye.
Storage solutions usually contain
- A non-ionic surface active agent – to aid cleaning.
- A mixture of antimicrobial agents.
The concentrations can be greater than in wetting solutions because storage solutions are not used in the eye. The solution is changed every few days because the preservatives may be partially inactivated by organic debris.
2.SOFT CONTACT LENSES: These are soft flexible type of lenses. Certain medicaments from wetting and storage solutions are strongly absorbed by the soft contact lenses. Due to this reason patients wearing soft lenses should be advised to remove them before instilling eye-drops.
For cleaning soft contact lenses are heated in 0.9% sodium chloride solution. The wetting and storage solutions formulated for hard lenses must never be used. Special proprietary storage solutions are available. Wetting is not a problem because of the hydrophilic nature of lens. The wetting of soft contact lenses is not a problem because of hydrophilic nature of the lens. Contact lens solutions should be sterile.
Multiple choice questions:
1.Contact lenses are usually made from polymethyl- methacrylate, a hard hydrophobic plastic.
2.Types of contact lenses are
a)Hard contact lenses
b)Soft contact lenses
c)Both of these
d)None of these
3.Wearers of hard contact lenses generally use which solutions?
d)a and b
4.Which solution is used primarily for treating the lenses before insertion?
d)all of these
5.Which solution is used for overnight cleansing, soaking and storage?
d)all of these
6.Wetting solution contains
d)all of these
7.Which of the following wetting agents are used in wetting solution?
d)All of these
8.Benzalkonium chloride is used in ____ concentration in wetting solutions?
9.Sodium bicarbonate 2 per cent , an early contact lens solution, has a pH of about
10.Storage solutions usually contain
a)non-ionic surface active agent
b)mixture of antimicrobial agents
c)both of these
d)none of these
11.What is the significance of non-ionic surface active agent in storage solution?
12.Contact lens solutions should be
d)all of these
13.For cleaning soft contact lenses are heated in
b)0.9% sodium chloride solution
d)all of these
14.Polymethyl methacrylate, although hydrophobic, does absorb a little water and it is undesirable for this to be taken from the eye.
15.It is customary to adjust wetting solutions to approximately the same osmotic pressure as lachrymal secretion by adding
- c)Both of these
- d)a and b
- a)wetting solution
- b)storage solution
- d)all of these
- a)Polyvinyl alcohols
- a)0.004 percent
- c)both of these
- a)aid cleaning
- b)0.9% sodium chloride solution
- a)sodium chloride
- Remington Essential of Pharmaceutics, 1st edition 2013, page no. 559-561.