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CONTACT LENSES

By Mine TÜRKTAŞ

Outline

1.                 Introduction

·        History of the contact lenses

·        Properties of an appropriate contact lens

·        Types of contact lenses

2.                 Rigid contact lenses

3.                 Soft contact lenses

4.                 Gas permeable contact lenses

5.                 Disinfection of contact lenses

6.  Hollow bandage contact lenses

          Contact lenses are thin, finely crafted plastic discs designed to fit over the cornea of the eye, usually to correct vision problems caused by refractive error. In addition to their obvious cosmetic advantages over eyeglasses, contact lenses provide better peripheral vision and virtually eliminate the image distortion sometimes caused by eyeglasses. Contact lenses are now available for the correction of most vision problems caused by refractive defects of the eye including nearsightedness, farsightedness and astigmatism.

The history of contact lens can be listed as shown in table below:

  In the late 17^th through the 19^th century, lenses were made of glass. For many reasons, glass contact lenses proved to be impractical. Before they could be fit the lenses had to be blown into a plaster mold taken from the patient’s eye. These lenses were also usually quite large and covered the entire eye. By the 1950’s new materials had come into wide use.

   Compact lenses are quite useful. They have many advantages when you compare it to eyeglasses:

Eyeglasses

Contact Lenses

The distance between your eye and the lens sometimes creates distortion.	Worn right on the eye, for more natural vision.
Poor peripheral (side) vision.	Your entire field of view is in focus. This is especially important in sports and in driving, where you need to see as much around you as possible.
Constant awareness of frame and lens edge, as well as reflections off the backside of the lens.	With contacts, no annoying obstructions or reflections are in view.
Uncomfortable weight on your face and ears. Periodic need for tightening or other adjustment.	No weight and resulting discomfort. No frame constantly slipping down your nose.
Glasses fog up with changes in temperature.	Contacts don't fog up.

There’re some properties that you should give importance when getting lens:

1.          Lens of choice is available in the power the patient require

2.                 Diameter

          -larger lenses staying under lids are more comfortable (13.8      to14.5mm)

          -if a centration problem use a larger lens                                                                                                                                                

3.          Thickness

         -ultrathin usually good choice for daily wear

         -good oxygen, relative durable, handling not a major problem

         -if handling a problem use standard

         -if dry eye problem use thicker lens

         -if edema a problem use thinner lens

         -superthins for low water content extended wear

4.         Water content

         -low or medium water best for daily wear

         -durable, less deposits

         -if edema problems use higher water content

         -with present lenses Dk related to water

         -practically no oxygen reaches cornea due to tear exchange

         -extended wear: medium or high water

          5.       Lens centration

         -lens must be center well enough that there is not corneal exposure and lens optics stays in front of pupil.     

         -bigger lenses center better

         -steeper lenses center better than flatter lenses

          6.       Lens movement

-must move to flush material from under lens

-0.25 to 1.0 mm movement desired (more for extended wear than daily wear)

-larger diameters move less

-larger optical zones move less

-thinner lenses move less

-peripheral thickness (lenticulars) important

-spin cast and molded lenses move less than lathe cut

-low water lenses move more than high water lenses (if all other dimensions the same)

7.       Base curve selection

          -middle of range provided in stock fitting sets (usually 8.6 to 8.9 mm range)

          -if very flat or steep cornea vary appropriately

          -final selection based on lens evaluation on eye                                                                                                                          

          -total back sagittal depth concept is important

          -lens sag vs sag of eye

          -usually can not specify optical zone or peripheral curves

After placing the contact lens you should wait for a while. Because:

1.                tearing will subside with decrease in movement

2.                 lens dehydrates with time and movement decreases

3.                 high water content lenses loss more water

4.                 thin lenses loss more water

5.                 humidity and wind have an effect

6.                 if fit doesn’t look reasonable in 5 minutes change lens to one that does

7.                 with a good fit wait 15+ minutes for final judgement

 Materials, which are used for contact lenses, have some properties:

1.                 Ability to coat cellular linings

2.                 Biologically inert and non-toxic

3.                 Zero shear rate viscosity

4.                 Low resistance to aspiration

5.                 Low surface tension

6.                 Rapid clearance from eye

7.                 Sterilizable and pyrogen-free

8.                 Hyhrophilic

The tear film of the eye is made up of 3 layers. The layer closest to the cornea is known as the mucin layer. This mucin layer contains biomaterials that enhance the wettability of the surface of the cornea. The next and thickest layer is the aqueous layer. Third layer is the lipid layer whose purpose is to retard the evaporation of the aqueous layer. A properly functioning tear film will remain spread across the corneal surface for a period of time longer than the time between a patient’s blink. When a contact lens is placed into this system it is important that the contact lens surface be hydrophilic. This hydrophilicity will promote the spread and maintenance of the tear film across the lens surface. Failure to maintain an adequate tear film can lead to visual as well as mechanical stress on the eye. An unstable tear film can also promote the deposit of materials that exist in the tears     ( proteins, lipids, salts ). This accumulation leads to failure and replacement of the lens.                                                                                                                         

Long term overnight contact lens wear causes profound changes in corneal epithelial metabolism and  physiology, eg. epithelial thinning and reduced oxygen uptake. Short term high oxygen permeable contact lens wear has reduced clinical signs of hypoxia.

The polymers which have been used are:

·        cellulose acetate butyrate

·        poly methyl methacrylate (PMMA)

·        poly 2 hydroxyethyl methacrylate ( poly HEMA)

·        siloxan methacrylates

·        silicones

Types of Contact Lenses

Contact lenses are classified according to their modulus of elasticity. There are 3 types of contact lenses:

     1.  Rigid-Non Permeable

          2.  Soft Permeable 

          3.  Rigid Permeable

All contact lenses have advantages and disadvantages as shown in table:

Type	Advantage	Disadvantage
Soft Permeable	High oxygen permeability	Deposition of lipomucoprotein and bacterial growth
Rigid Permeable	High oxygen permeability	Accumulation of surface deposits
Rigid Non-Permeable	Optical properties	Low oxygen Permeability

                       Lippman ,1990

      www.incvisioncare/allabout.html                                                                           

1. RIGID CONTACT LENSES

The first rigid contact lenses were manufactured by Kevin Tuohey in 1948. In rigid contact lenses the material used is polymethyl methacrylate (PMMA).

PMMA has an amorphous structure. Its water absorbtion rate is 0.3% and elongation is 48%. In the organic PMMA materials, the side groups tend to be highly polar and somewhat bulky compared to methyl groups. Both the polarity and the bulk enhance the stiffness of the polymer structure due to the carbon backbone close packing. Because of this highly organized molecular structure, the organic PMMA materials are low permeable. PMMA has good optical properties like light in weight, durable.

  PMMA materials are hydrophilic. Also PMMA lenses induce a foreign body-type encapsulation. They release free monomer. The most important problem arising from the use of methacrylate polymers is their potential chemical toxicity associated to the presence of C=C reactive centers due to residual monomers or also to unsaturated end groups of oligomer or polymer chains, which may form during the polymerization reaction, as found for PMMA. They also require long adaptation period. 

It is suitable for disposable contact lenses. Because 55-70% of water is an appropriate amount for increased transport of oxygen to cornea as well as for drawing off metabolites as lactic acid and carbon dioxide.

Its cleaning form absorbed proteins is both mechanically and enzymatically quite difficult, so these lenses aren’t suitable for long-term wearing. Rigid contact lenses are the original type of contact lenses but they haven’t been used anymore.

2. SOFT CONTACT LENSES

The first soft contact lens was developed by Otto Wichterle in 1961. They are also known as hydrogels. The population of soft contact lens weavers are 20 million. Soft contact lenses vary depending on either the refractive correction that they are prescribed for or the design and type of material. Major types include :

2.1.  Spherical

Single prescription power 360 degrees around the lens.

2.2. Toric

Contains both a  spherical and cylinder component to correct prescriptions which have astigmatism.  Lenses may be thicker in one meridian or have modified thickness profiles to enable the lens to maintain correct orientation on the eye.

2.3  Disposable

Spherical or toric contact lenses which are designed to be worn for a certain time period, eg, weekly, two- weekly, monthly. Lenses are generally sold in a "blister" combination pack, eg. 3 months supply with intention that lenses are "disposed" of at the end of the time period. 

2.4  Extended Wear

Lens manufactured from a high-water content material and/or with a very thin center thickness to enable maximum oxygen transmission. Lenses can be worn overnight, or for a number of days without removal. It has some disadvantages: hypersensitive reactions, hypoxia, infections, toxicity.

2.5 Bifocal Lenses

There are a variety of designs in bifocals, essentially all trying to provide a transition or reading zone for use at near.  The different designs include;

·                    aspheric multifocal

·                    simultaneous vision concentric  has either a central near or

·                    distance zone with surrounding zone of opposite type to center (eg.  center near, distance surround)

·                    diffraction/holographic - based on diffraction grating principles

Soft lenses are subdivided into low, medium and high water content. It is theoretically possible to link this subdivision to daily, extended and permanent wear. The oxygen permeability of contact lenses is determined by 2 factors: The thickness of the lens and the water content of the lens material.

                                    Biomaterials 2001, Volume 22, Ocular Biomaterials and Implants                      

CHARACTERISTICS OF SOFT HYDROPHILIC LENS MATERIALS

Since the introduction of soft lenses two important alternatives have been developed in order to improve the original material poly-2-hydroxyethyl- methacrylate (HEMA). The most important alternative consists of adding polyvinyl pyrolidone (PVP) to the original material. By adding PVP it is possible to manipulate the water content which is absorbed by the hydrogel. The initial applications of this alternative showed discoloring and cracks. Nowadays the combination of HEMA/PVP can be applied fairly successfully as long as the water content is not too high (±55 %). With the other alternative, non-HEMA hydrogels are applied to obtain higher water content which in turn gives a better gas-permeability. By doing so the water content can amount to around 70 % while the lens, provided that it is not too thin, remains strong enough. 
 

Water content	Low (40%)	Medium (±55%)	High (±70%)
Material	HEMA	HEMA/PVP	MMA/PVP/GMA
Application	Daily wear	Daily wear Extended wear	Daily wear Extended wear Permanent wear (on medical indication)
Replacement system	when necessary or 2-6 times/year	when necessary or 2-6 times/year or weekly	when necessary

www.incvisioncare/allabout.html

HEMA’s cleaning from absorbed proteins is both mechanically and enzymatically quite difficult so these lenses are not suitable for long-term wearing. PVP lenses have elastic properties closer to those of the cornea itself than those of the HEMA. PVP lenses show turnover of water between the lens and the surrounding biological environment. This turnover plays probably an important role in the water loss of the lens when in contact with ocular tissue, since it could increase surface reactivity and change the elastic properties of the lens.

HEMA, DEGMA and MAA crosslinked with EGDMA polymers have been found suitable for disposable soft contact lens applications.

Diameters of soft lenses are:

Overall diameter: 12.5 mm to 16.0 mm (13.8 mm to 14.5 mm most common).                          

Base curve radii: 8.0 mm to 9.5 mm range depending on lens diameter, flexibility and other design factors.

Thickness: Three main ranges

1. 0.03 to 0.04 mm - hyperthin

   -high oxygen transmission

   -hard to handle

   -fragile

2. 0.05-0.07 mm range- ultrathin

3. 0.10-0.15 mm- standard thickness

   -low oxygen transmission

   -sturdy

   -loses less water than thin lenses

3. GAS PERMEABLE CONTACT LENSES (RGP)

Around 1975 and through the experience with soft lenses, more knowledge was gained about the physiology of the cornea area while wearing lenses. Specialists became increasingly convinced of the fact that large, thick, and rigid lenses made of PMMA were less suitable. The most logical step was to use small, thin PMMA lenses where possible. These lenses were called thin or ultra-thin after their inventor. The name flexible was also used for this thin PMMA lens. The introduction of gas-permeable material, however, increased the confusion because in the explanation to customers this type of lens was often referred to as half-rigid or half-soft. These lenses offer: 

·        easy care

·        deposit resistance

·        easy handling

·        excellent eye health

·          good value

The first gas-permeable stable material that was applied on a sizeable scale is cellulose acetate butyrate (CAB). This material is not much applied anymore because of the low gas-permeability, the instability of the lens parameters, and the increased risk of deposits.

With the second generation of materials silicone was added in order to achieve the highest possible gas-permeability (expressed in Dk-value). This group is known as silicone-acrylates, however silicone is flexible and hydrophobic. They are as much as 6-10 times as permeable to oxygen as hydrogel materials. They are also biologically inert. Lenses made from this material are therefore instable, and they easily show deposits and scratches. As with soft hydrophilic contact lenses, the complex and expensive daily care formed a drawback. Many people were disappointed with the shorter life of the lens when compared to the old rigid lens.

The most noticeable characteristic of the third generation of materials is that it included other chemical groups next to silicone. In the development stage the prime focus was on improving surface characteristics.

The keyword was to develop a material with the lowest possible contact angle, ie., a material that easily accepts tear moisture. An extremely low contact angle, however, shows an increased distortion of the lens. Next to the low contact angle attempts were made to develop a more durable surface with less sensitivity to deposits.                                                                                                                                

The drawback was a brittle, easily breakable material. In general this concerns the fluoro- silicone- acrylates. The oxygen permeability of this material and the corneal swelling observed after sleeping was unchanged.

Increasing the level of oxygen available to the cornea during RGP extended wear significantly increases the chances of maintaining successful extended contact lens wear for 12 months (decreased oxygen transmissibility causes increased corneal swelling, binding of Pseudomonas to epithelial cells, lowered corneal pH). RGP extended wear shows lower incidence and severity of contact lens-associated complications compared with soft contact lens wear. Actually both lenses approximately have same oxygen level, so why RGP is better in decreasing complications? Perhaps the answer is related to differences in lens performance. Eg. when eyes are closed, there is no tear exchange and debris builds up between the cornea and lens for both rigid and soft lenses. However, as soon as the eyes are open, the trapped debris under the rigid lens is removed within a few blinks because of the good tear exchange; unfortunately this is not the case for soft lenses, which have very poor tear mixing. 

The first RGP contact lenses were not very oxygen permeable (Dk 10-30 Barrers). But the latest ones have great permeability (Dk 600 Barrers). 

DIFFERENCES BETWEEN RGP AND SOFT LENSES

Oxygen Permeable Rigid Lenses	Soft Lenses
Made of special polymers that allow your eyes to breathe.	Many don't allow enough oxygen to get through to your eyes. This can lead to corneal problems.
Have superior optics. Since they're firm, they retain their shape better when you blink, so your eyes don't have to refocus as much. And they are superb for astigmatism or bifocal needs.	When you blink, soft lenses are more likely to distort; your eyes must then refocus, which can be annoying if you're reading, or detrimental to your performance if you're driving or participating in sports.
Require a short adaptation period.	Comfortable from just about the moment you put them on.
Made of a firm plastic, so they don't scratch or tear. And they stay clear over time.	Made of a gel-like plastic, so easy to tear and protein deposit buildup clouds the lenses over time.
Their smooth finish and lack of water retention mean they harbour fewer protein deposits from your tear film. This is healthier and more comfortable for your eyes.	Since soft lenses absorb more of your tears, they are more likely to contain protein deposits from your tears and harbour bacteria. More deposits are scratchy, too.
Oxygen permeables are much less expensive to maintain; also, they last longer so you don't have to spend as much on replacements.	Soft lenses require significant spending on cleaning supplies; and they don't last as long, so you buy new lenses more often.

DISINFECTION OF CONTACT LENSES

Ineffective disinfectant solutions or inadequate disinfection times can enable patient contaminant organism survival which, if the lens cases are not cleaned and air dried properly, could subsequently proliferate during contact lens storage.

Three systems have been used to disinfect contact lenses: chemical, hydrogene peroxide and heat. Disinfection by heat and hydrogene peroxide are not used for rigid contact lenses. Chemical disinfection is the only system recommended for use with rigid contact lenses. To simplify contact lens hygiene and promote better patient compliance, multipurpose solutions (MPS) have been developed. These represent a single solution for the cleaning disinfection, and storage of contact lenses and hence offer continued antimicrobial protection during lens storage.

Adenovirus is the etiologic agent of epidemic keratoconjunctivitis. Adenovirus is an extremely hardy virus that can be recovered from hands, nonporous surfaces and ophthalmic solutions up to 49 days after inoculation. Although chemical and hydrogen peroxide disinfections significantly reduced the level of adenovirus contamination, Adenovirus nonetheless survived. Heat is effective in completely eliminating adenovirus from soft contact lenses.

Another microorganism which contact lens weavers are mostly subjected to is Acanthamoeba .It is a common soil and water amoeba characterized by a feeding and replicating trophozoid and dormant cyst stage.

The organism causes a severe and potentially blinding keratitis. Opti-Free expres contact lens solution is effective against Acanthamoeba cysts. Also Staphlococcus epidermidis and S. aureus are effective.                                                                                                                       

Pseudomonas aeruginosa is also responsible for many corneal infections. The production of biofilm by microorganisms is responsible for deposits on lenses(shown in figure). It acts as a scaffold for bacterial adhesion and proliferation and creates a coat on bacterial cells, thus protecting them from the hosts’ defense mechanisms and drugs. The use of non-steroidal anti-inflammatory  drugs (NSAID) decreases bacterial adherence and particularly affects the biofilm forming. NSAID can be also used as solution.        www.uchc.edu/biomaterials                                                                      

          HOLLOW BANDAGE CONTACT LENSES

Beside correct vision and cosmetic applications, contact lenses are used as bandage. One of the most important early postoperative complications of trabeculectomy or combined cataract extraction and trabeculectomy is an aqueous leak through the conjunctival surgical wound. This complication has been reported in some studies to exceed 20% of the operations. Aqueous leak through the conjunctiva may not close spontaneously and should be repaired to prevent endophtalmitis cataract.

Treatment modalities for leaking blebs may be divided into surgical and non-surgical. The surgical methods involve the advancement of or suturing the conjunctiva and sometimes revision of the conjunctival flap.

Among non-surgical ways, one can find the pressure patch and hypotensive medications that inhibit aqueous inflow, thus reducing aqueous humor flow. Also soft bandage contact lens (BCL) can be used. One of the drawbacks of the use of soft BCL for leaking blebs is the inability to measure the intraocular pressure unless the lens is temporarily removed. However the frequent, sometimes daily removal, washout and reinsertion of the BCL may increase the risk of its contamination and possibly interrupt the continuity of the conjunctival epithelial closure. Therefore researchers have evaluated the efficacy of a new soft hollow BCL for the treatment of the leaking bleb.                                                                                                                

The hollow BCL consists of vinyl pyrolidone/copolymer with water content of 78% and an oxygen permeability of 170 Dk. Its external diameter is 16.5 mm with a central round hollow of 6.0 mm. By using hollow BCL it is possible to measure intraocular pressure without removal of the lens. On the other hand, the drawback of the current hollow BCL may be related to its size, 16.5 mm in diameter, which probably would not cover a leaking site that is too far from the limbus. Other drawback may be related to its potential of becoming infectious. Another worrisome point is the possibility that the patients might be unaware if the lens had fallen from their eyes.

References:

1.  Fusaro R.E, Polse K, Graham A, Gan C, Rivera R,Lin M,Sanders T, McNamara N,Chan J.The Berkeley Contact Lens Extended Wear Study: Part I Ophtalmology, 2001;108: 1381-1388

2.  Fusaro R.E, Polse K, Graham A, Gan C, Rivera R,Lin M,Sanders T, McNamara N,Chan J.The Berkeley Contact Lens Extended Wear Study: Part II Ophtalmology, 2001;108: 1389-1399

3.  Silver F. Biomaterials, Materials Used in Ophthalmology, Medical Devices and Tissue Engineering 1994;120-152

4.  Kowalski R, Sundar-Raj C.V,Romanowski E,Gordon Y. The Disinfection of Contact Lenses Contaminated With Adenovirus, American Journal of Ophthalmology 2001;132: 777-779

5.  Kilvington S,Anger C.A Comparison of Cyst Age And Assay method of The Efficacy of Contact Lens Disinfectants Against Acanthamoeba, British Journal of Ophthalmology 2001; 85: 336-340

6.  Fitzgerald J.Silicone Contact Lenses, Advances in Biomaterials 1987:209-216

Monti P, Simoni R, Caramazza R,Bertoluzza A. Applications of Raman Spectroscopy to Ophthalmology: Spectroscopic Characterization of Disposable Soft Contact Lenses, Biospectroscopy 1998; 4:413-419

7.  Krakovsky I, Jelinkova M, Vacik J. Elastic And Swelling Behaviour of 2-Hydroxyethyl Methacrylate, Diethylene Glycol Methacrylate and Methacrylic Acid Copolymers, Journal of  Application of Polymer Sciences 1997; 64: 2141-2148

8.  Perilli R, Marziano M, Formisano G, Caiazza S, Scorcia G, Baldassarri L. Alteration of Organized Structure of Biofilm Formed By Staphylococcus epidermidis on Soft Contact Lenses, Journal of Biomedical Materials Research 2000; 49: 53-57

9.  Stapleton F, Kasses S, Bolis S, Keay L. Short Term Wear of High Dk Soft Contact Lenses Does Not Alter Corneal Epithelial Cell Size Or Viability, British Journal Of Ophthalmology 2001; 85: 143-146

10.  Porges Y, Ophir A. Hollow Bandage Contact Lenses, Ophthalmic Surgery and Lasers 2001; 32: 124-128

11. Llyod A, Faragher R, Denyer S. Ocular Biomaterials and Implants, Biomaterials 2001; 22: 769-785

http://www.contactlenses.org/home2/htm

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http://www.opt.indiana.edu/v633/lectures/hydrogel.html

http://www.polymerweb.com/_datash/pmma.html

http://www.incvisioncare/allabout.html