CONTACT LENS – I
UNIT 2
Topic 1: Introduction to Contact Lens Materials – Monomers and Polymers
Introduction
Contact lens materials are the substances used for manufacturing contact lenses. The quality, comfort, oxygen permeability, flexibility, durability, and safety of contact lenses depend greatly on the material from which the lens is made.
Over the years, contact lens materials have evolved from rigid glass materials to advanced silicone hydrogel materials with high oxygen transmission and improved comfort.
Understanding contact lens materials is essential because different materials have different physical, chemical, physiological, and optical properties. These properties influence lens performance, patient comfort, and ocular health.
The basic components used in contact lens materials are monomers and polymers.
Contact Lens Materials
A contact lens material should possess the following characteristics:
- Good oxygen permeability
- Optical transparency
- Biocompatibility
- Durability
- Flexibility
- Wettability
- Resistance to deposits
- Ease of manufacturing
Modern contact lens materials are mainly divided into:
- Rigid materials
- Soft hydrogel materials
- Silicone hydrogel materials
Monomers
Definition of Monomer
A monomer is a small simple molecule capable of chemically combining with other similar molecules to form a polymer.
Monomers act as the basic building blocks of contact lens materials. During the manufacturing process, monomers join together through chemical reactions called polymerization to form large molecules known as polymers.
Characteristics of Monomers
- Small molecular size
- Chemically reactive
- Capable of polymerization
- Form repeating units in polymers
Polymerization
Polymerization is the chemical process in which many monomer molecules combine together to form a larger molecule called a polymer.
This process is extremely important in contact lens manufacturing because the final lens material is produced after polymerization of monomers.
Types of Polymerization
1. Addition Polymerization
In addition polymerization, monomers combine without the loss of any small molecules. This is the most common method used in contact lens material production.
2. Condensation Polymerization
In condensation polymerization, monomers combine with the elimination of small molecules such as water or alcohol.
Examples of Important Monomers Used in Contact Lens Materials
1. Methyl Methacrylate (MMA)
Methyl methacrylate is one of the earliest and most important monomers used in contact lens manufacturing.
Polymerization of MMA forms polymethyl methacrylate (PMMA), which was widely used in rigid contact lenses.
Characteristics of MMA
- Forms rigid material
- Excellent optical clarity
- Good durability
- Poor oxygen permeability
Although PMMA lenses provided good vision, they caused corneal hypoxia because oxygen could not pass through the material.
2. Hydroxyethyl Methacrylate (HEMA)
HEMA is the most important monomer used in soft contact lens materials.
Polymerization of HEMA produces hydrogel materials capable of absorbing water. These materials become soft and flexible after hydration.
Characteristics of HEMA
- Hydrophilic nature
- Water absorption capacity
- Soft and flexible
- Good patient comfort
HEMA revolutionized contact lens practice because it led to the development of soft contact lenses.
3. Silicone Monomers
Silicone-containing monomers are used in silicone hydrogel lenses and RGP lenses.
These monomers improve oxygen permeability significantly.
Advantages of Silicone Monomers
- High oxygen transmission
- Improved corneal health
- Suitable for extended wear
Disadvantages
- Hydrophobic nature
- May attract deposits
4. N-Vinyl Pyrrolidone (NVP)
NVP is a hydrophilic monomer used to increase water content in soft contact lenses.
Functions of NVP
- Improves water absorption
- Enhances softness
- Improves comfort
5. Methacrylic Acid (MAA)
Methacrylic acid is added to contact lens materials to improve wettability and ionic properties.
It helps increase surface wetting and improves tear film interaction.
Polymers
Definition of Polymer
A polymer is a large molecule formed by the repeated joining of many monomer molecules through polymerization.
Polymers form the actual material from which contact lenses are manufactured.
Characteristics of Polymers
- Large molecular structure
- High stability
- Variable flexibility
- Can be rigid or soft
- Can absorb water depending on composition
Types of Polymers Used in Contact Lenses
1. PMMA (Polymethyl Methacrylate)
PMMA is formed by polymerization of methyl methacrylate monomers.
It was the first widely successful plastic material used in contact lenses.
Properties of PMMA
- Rigid material
- Excellent optical quality
- Good durability
- No oxygen permeability
Due to poor oxygen transmission, PMMA lenses are rarely used today.
2. Hydrogel Polymers
Hydrogel polymers are soft materials capable of absorbing water and becoming flexible.
These materials are mainly formed from HEMA and related hydrophilic monomers.
Properties of Hydrogel Materials
- Soft and comfortable
- Water containing
- Good biocompatibility
- Lower oxygen permeability compared to silicone hydrogel
Hydrogel lenses became highly popular because of their comfort and easy adaptation.
3. Silicone Hydrogel Polymers
Silicone hydrogel materials combine hydrogel softness with silicone oxygen permeability.
These are among the most advanced soft lens materials used today.
Properties of Silicone Hydrogel Materials
- Very high oxygen permeability
- Improved corneal physiology
- Suitable for extended wear
- Good comfort
Silicone hydrogel lenses reduce complications associated with corneal hypoxia.
4. Rigid Gas Permeable (RGP) Polymers
RGP materials are rigid polymers that allow oxygen transmission through the lens material.
These materials contain silicone and fluorine components to improve oxygen permeability.
Properties of RGP Materials
- Rigid structure
- Excellent optical quality
- Good oxygen permeability
- Durable material
RGP lenses are especially useful in keratoconus and astigmatism.
Hydrophilic and Hydrophobic Materials
Hydrophilic Materials
Hydrophilic materials attract water and become soft after hydration.
Example:
- HEMA-based hydrogels
Advantages
- Good comfort
- Good wettability
- Easy adaptation
Hydrophobic Materials
Hydrophobic materials repel water and usually have lower wettability.
Example:
- Silicone-containing materials
Advantages
- High oxygen permeability
- Durability
Disadvantages
- Poor wettability
- More deposit formation
Cross-Linking in Contact Lens Materials
Cross-linking refers to the formation of chemical bonds between polymer chains.
Cross-linking agents improve:
- Lens strength
- Shape stability
- Durability
However, excessive cross-linking may reduce flexibility and comfort.
Importance of Contact Lens Materials
The choice of contact lens material affects:
- Lens comfort
- Oxygen supply to cornea
- Lens wettability
- Deposit resistance
- Visual quality
- Lens durability
- Patient tolerance
Selection of appropriate material is essential for successful contact lens wear and prevention of complications.
Modern Trends in Contact Lens Materials
Modern research focuses on developing materials with:
- Higher oxygen permeability
- Better wettability
- Lower deposit formation
- Improved comfort
- Antimicrobial properties
- Drug delivery capability
Advanced materials continue to improve safety and patient satisfaction in contact lens practice.
Classification of Contact Lens Materials
Contact lens materials are broadly classified into rigid lens materials and soft lens materials. These materials differ in their rigidity, oxygen permeability, water content, flexibility, and clinical applications.
The following classification is commonly used in contact lens practice:
1. Rigid Lens Materials
Rigid contact lenses maintain their shape on the eye and provide excellent optical quality. They are mainly divided into non-gas permeable and gas permeable materials.
A. Non-Gas Permeable Materials
Hard Lenses (PMMA)
The earliest rigid contact lenses were manufactured from polymethyl methacrylate (PMMA). PMMA is a transparent plastic material with excellent optical quality and durability.
However, PMMA does not allow oxygen to pass through the lens material. As a result, prolonged wear may lead to corneal hypoxia and related complications.
Properties of PMMA
- Rigid material
- Excellent optical clarity
- Highly durable
- Poor oxygen permeability
- Good shape stability
Advantages of PMMA Lenses
- Excellent vision quality
- Long-lasting material
- Easy handling
Disadvantages of PMMA Lenses
- No oxygen transmission
- Corneal hypoxia
- Poor comfort
- Long adaptation period
Due to poor oxygen permeability, PMMA lenses are rarely used today.
B. Gas Permeable Materials
Gas permeable materials were developed to overcome the disadvantages of PMMA lenses. These materials allow oxygen to pass through the lens and improve corneal physiology.
Gas permeable lenses are mainly divided into RGP materials and elastomeric materials.
Rigid Gas Permeable (RGP) Materials
Rigid gas permeable lenses combine the optical advantages of rigid lenses with improved oxygen transmission.
Different materials are added to improve oxygen permeability, wettability, and durability.
1. CAB (Cellulose Acetate Butyrate)
CAB was one of the earliest gas permeable materials used in rigid contact lenses.
Characteristics of CAB
- Better oxygen permeability than PMMA
- Lightweight material
- Good flexibility
- Moderate durability
Although CAB improved oxygen transmission, its oxygen permeability was lower compared to modern RGP materials.
2. Siloxane Materials
Siloxane-containing materials were introduced to improve oxygen permeability in rigid lenses.
Silicone components increase oxygen transmission through the lens material.
Advantages of Siloxane Materials
- High oxygen permeability
- Improved corneal health
- Better physiological response
Disadvantages
- Hydrophobic surface
- Poor wettability
- Higher deposit formation
3. Fluoro-Siloxane Methacrylate
Fluoro-siloxane methacrylate materials are advanced RGP materials containing both fluorine and silicone components.
These materials provide excellent oxygen permeability along with improved wettability and deposit resistance.
Advantages
- Very high oxygen transmission
- Good wettability
- Reduced deposits
- Improved comfort
These materials are widely used in modern RGP contact lenses.
4. Alkylstyrene Materials
Alkylstyrene-containing materials are used to improve the physical properties of RGP lenses.
Functions of Alkylstyrene
- Improves rigidity
- Enhances durability
- Maintains lens shape stability
Elastomeric Lens Materials
Elastomeric materials are flexible gas permeable materials that combine properties of rubber and plastic materials.
These materials are mainly divided into silicone rubber and acrylic rubber materials.
1. Silicone Rubber
Silicone rubber materials possess extremely high oxygen permeability because oxygen passes easily through silicone.
Advantages of Silicone Rubber
- Very high oxygen transmission
- Flexible material
- Good corneal physiology
Disadvantages
- Poor wettability
- Surface deposits
- Fragile material
2. Acrylic Rubber
Acrylic rubber materials combine flexibility with improved mechanical strength.
These materials are more stable and durable than pure silicone rubber materials.
2. Soft Lens Materials (Hydrogels)
HEMA (Hydroxyethyl Methacrylate)
Soft contact lenses are mainly manufactured from hydrogel materials. The most important hydrogel monomer is hydroxyethyl methacrylate (HEMA).
HEMA-based materials absorb water and become soft and flexible after hydration.
Properties of HEMA Materials
- Hydrophilic nature
- High water absorption
- Soft and flexible
- Comfortable lens wear
Advantages of Hydrogel Lenses
- Excellent comfort
- Quick adaptation
- Good lens stability
- Suitable for daily wear
Disadvantages of Hydrogel Lenses
- Lower oxygen permeability than silicone hydrogel
- Higher risk of deposits
- May dehydrate during wear
Modern Development of Contact Lens Materials
Modern contact lens materials are designed to improve:
- Oxygen permeability
- Comfort
- Wettability
- Durability
- Deposit resistance
- Corneal physiology
Advanced silicone hydrogel materials are now widely used because they provide both high oxygen transmission and excellent comfort.
Oxygen Permeability (Dk) and Oxygen Transmissibility (Dk/t)
Introduction
Oxygen supply to the cornea is extremely important during contact lens wear because the cornea is an avascular structure and receives oxygen mainly from the atmosphere through the tear film.
Contact lens materials differ in their ability to allow oxygen passage. Therefore, two important terms used in contact lens materials are:
- Dk (Oxygen Permeability)
- Dk/t (Oxygen Transmissibility)
Dk (Oxygen Permeability)
Dk refers to the ability of the contact lens material itself to allow oxygen to pass through it.
It depends upon:
- Material composition
- Water content
- Silicone content
Higher Dk value means better oxygen permeability.
Dk/t (Oxygen Transmissibility)
Dk/t refers to the amount of oxygen transmitted through the lens considering both the material permeability and lens thickness.
It is affected by:
- Dk value of material
- Thickness of lens
Higher Dk/t value means better oxygen transmission to the cornea.
Table: Contact Lens Materials with Dk and Dk/t Values
| Material | Type of Lens | Approximate Dk Value | Approximate Dk/t Value | Important Features |
|---|---|---|---|---|
| PMMA (Polymethyl Methacrylate) | Hard Lens | 0 | 0 | No oxygen permeability, excellent optical clarity |
| CAB (Cellulose Acetate Butyrate) | RGP Lens | 4 – 8 | Low | First gas permeable material, improved comfort over PMMA |
| Siloxane Acrylate | RGP Lens | 10 – 30 | Moderate | Good oxygen permeability but hydrophobic surface |
| Fluoro-Silicone Acrylate | RGP Lens | 30 – 100+ | High | Excellent oxygen transmission and wettability |
| Silicone Rubber | Elastomeric Lens | 80 – 150 | Very High | Very high oxygen permeability, poor wettability |
| Acrylic Rubber | Elastomeric Lens | 40 – 80 | High | Flexible and durable material |
| HEMA Hydrogel | Soft Lens | 8 – 20 | Low to Moderate | Soft, hydrophilic, comfortable |
| High Water Hydrogel | Soft Lens | 20 – 40 | Moderate | Increased water content improves oxygen permeability |
| Silicone Hydrogel | Soft Lens | 60 – 175 | Very High | Modern material with excellent oxygen transmission |
Topic 2: Properties of Contact Lens Materials
Introduction
The success of contact lens wear depends greatly on the properties of the lens material. Different contact lens materials possess different physical, physiological, and optical characteristics that influence comfort, oxygen supply, visual performance, durability, and ocular health.
An ideal contact lens material should provide good vision, sufficient oxygen transmission, proper wettability, durability, flexibility, and patient comfort without causing damage to ocular tissues.
The properties of contact lens materials are mainly divided into:
- Physiological properties
- Physical properties
- Optical properties
PHYSIOLOGICAL PROPERTIES
Physiological properties determine how the contact lens interacts with the ocular tissues, especially the cornea and tear film.
1. Oxygen Permeability (Dk)
Definition
Oxygen permeability refers to the ability of the contact lens material to allow oxygen to pass through it.
It is represented by the symbol Dk.
The cornea is avascular and receives oxygen mainly from the atmosphere through the tear film. Therefore, adequate oxygen transmission through contact lenses is necessary to maintain corneal health.
Components of Dk
Dk consists of:
- D = Diffusion coefficient of oxygen
- k = Solubility coefficient of oxygen
Higher Dk value means better oxygen permeability.
Importance of Oxygen Permeability
- Maintains normal corneal metabolism
- Prevents corneal hypoxia
- Reduces corneal edema
- Prevents neovascularization
- Improves comfort during lens wear
Factors Affecting Dk
- Lens material
- Water content
- Silicone content
- Temperature
Clinical Effects of Low Dk
Low oxygen permeability may cause:
- Corneal hypoxia
- Corneal edema
- Blurred vision
- Neovascularization
- Corneal staining
2. Oxygen Transmissibility (Dk/t)
Definition
Oxygen transmissibility refers to the amount of oxygen transmitted through the contact lens considering both material permeability and lens thickness.
It is represented by Dk/t.
Where:
- Dk = Oxygen permeability
- t = Thickness of lens
Importance of Dk/t
Dk/t is more clinically important than Dk because actual oxygen reaching the cornea depends upon both the material and the thickness of the lens.
Factors Affecting Dk/t
- Dk value of material
- Lens thickness
- Lens design
Importance in Contact Lens Practice
High Dk/t lenses are preferred for:
- Extended wear lenses
- Continuous wear lenses
- Patients with dry eye tendency
- Long duration lens wear
3. Water Content
Definition
Water content refers to the percentage of water present within the contact lens material.
It is mainly important in soft hydrogel lenses.
Classification According to Water Content
| Type | Water Content |
|---|---|
| Low Water Content | Less than 40% |
| Medium Water Content | 40–60% |
| High Water Content | More than 60% |
Advantages of High Water Content
- Improves oxygen permeability
- Provides better comfort
- Soft flexible material
Disadvantages of High Water Content
- Lens dehydration
- Fragility
- Higher deposit formation
- Reduced durability
Importance of Water Content
Water content affects:
- Lens comfort
- Oxygen transmission
- Lens flexibility
- Dehydration tendency
4. Ionicity
Definition
Ionicity refers to the electrical charge present on the contact lens material.
Lens materials may be:
- Ionic
- Non-ionic
Ionic Materials
Ionic materials possess electrical charge and attract proteins from tears.
Characteristics
- More protein deposits
- Good wettability
- Higher dehydration tendency
Non-Ionic Materials
Non-ionic materials have minimal electrical charge and therefore attract fewer deposits.
Characteristics
- Less deposit formation
- Better lens cleanliness
- Improved durability
Importance of Ionicity
Ionicity affects:
- Deposit formation
- Lens comfort
- Lens cleaning requirements
- Lens wettability
PHYSICAL PROPERTIES
Physical properties determine the mechanical behavior and durability of the contact lens material.
1. Elasticity
Definition
Elasticity is the ability of a material to return to its original shape after deformation.
Importance of Elasticity
- Maintains lens shape
- Provides flexibility
- Improves handling
- Enhances comfort
Soft lenses have greater elasticity than rigid lenses.
2. Tensile Strength
Definition
Tensile strength refers to the ability of a material to resist breaking when stretched.
Importance of Tensile Strength
- Prevents lens tearing
- Improves durability
- Important during handling and cleaning
Materials with low tensile strength are more likely to tear during lens handling.
3. Rigidity
Definition
Rigidity refers to the resistance of the material to bending or deformation.
Importance of Rigidity
- Maintains optical shape
- Provides stable vision
- Useful in astigmatism correction
Rigid lenses provide better optical quality because they maintain their shape on the eye.
Advantages of High Rigidity
- Excellent optical performance
- Better correction of irregular astigmatism
- Stable lens shape
Disadvantages of Excessive Rigidity
- Reduced comfort
- Long adaptation period
- Increased foreign body sensation
OPTICAL PROPERTIES
Optical properties determine the visual performance of the contact lens.
1. Light Transmission
Definition
Light transmission refers to the ability of the lens material to allow visible light to pass through it.
Importance
- Provides clear vision
- Maintains optical transparency
- Improves visual quality
An ideal contact lens material should be highly transparent.
2. Refractive Index
Definition
Refractive index is the ratio of the speed of light in air to the speed of light in the lens material.
Importance of Refractive Index
- Determines lens thickness
- Affects optical power
- Influences lens design
High Refractive Index Materials
Materials with higher refractive index can produce thinner lenses for the same power.
Low Refractive Index Materials
Materials with lower refractive index require thicker lenses to produce the same power.
Wettability
Definition
Wettability refers to the ability of tears to spread evenly over the lens surface.
Importance of Wettability
- Improves comfort
- Provides stable tear film
- Reduces dryness
- Improves optical quality
Poor wettability may cause dryness, blurred vision, and discomfort.
Deposit Resistance
Definition
Deposit resistance refers to the ability of the lens material to resist accumulation of protein, lipid, and debris deposits.
Importance
- Maintains comfort
- Improves lens hygiene
- Reduces infection risk
- Improves lens life
Biocompatibility
Definition
Biocompatibility refers to the ability of the contact lens material to function without causing toxic or harmful effects to ocular tissues.
Importance
- Maintains ocular health
- Prevents allergic reactions
- Improves long-term lens tolerance
Topic 3: Indications and Contraindications of Contact Lenses
Introduction
Contact lenses are widely used in optometry for correction of refractive errors, therapeutic management, cosmetic improvement, and special clinical conditions. However, not every patient is suitable for contact lens wear.
Before prescribing contact lenses, the optometrist must carefully evaluate the patient’s ocular condition, systemic health, occupation, motivation, hygiene, and ability to handle lenses properly.
Conditions in which contact lenses are beneficial are called indications, while conditions in which contact lens wear should be avoided are called contraindications.
INDICATIONS OF CONTACT LENSES
Definition
Indications are the conditions or situations in which contact lenses are recommended because they provide better visual, cosmetic, or therapeutic benefits.
Types of Indications
Indications of contact lenses are mainly divided into:
- Optical indications
- Medical or therapeutic indications
- Cosmetic indications
- Occupational and sports indications
1. OPTICAL INDICATIONS
Optical indications refer to refractive or visual conditions where contact lenses provide superior optical performance compared to spectacles.
1. Myopia
Contact lenses are highly useful in myopia, especially high myopia.
In high myopia, spectacles produce:
- Image minification
- Peripheral distortion
- Ring scotoma
- Restricted visual field
Contact lenses reduce these problems because they are placed directly on the cornea.
Advantages in Myopia
- Reduced image minification
- Wider visual field
- Better peripheral vision
- Improved cosmetic appearance
2. Hypermetropia
Contact lenses are useful in hypermetropia because they reduce image magnification caused by plus spectacle lenses.
Advantages in Hypermetropia
- Reduced magnification
- Better cosmetic appearance
- Wider field of vision
3. Astigmatism
Rigid gas permeable lenses provide excellent correction of corneal astigmatism by creating a smooth refractive surface over the cornea.
Toric soft lenses are also used for regular astigmatism.
Advantages
- Better visual acuity
- Reduced distortion
- Stable vision
4. Anisometropia
Anisometropia is a condition in which the refractive power differs significantly between the two eyes.
Spectacles may produce unequal retinal image sizes leading to aniseikonia and binocular vision problems.
Contact lenses minimize image size difference because they reduce spectacle magnification effects.
Advantages
- Reduced aniseikonia
- Improved binocular vision
- Better fusion
5. Aphakia
Aphakia refers to absence of the crystalline lens.
High plus spectacle lenses used in aphakia produce:
- Excessive magnification
- Ring scotoma
- Image distortion
- Restricted visual field
Contact lenses greatly reduce these problems and provide better visual rehabilitation.
6. Keratoconus
Keratoconus is a progressive thinning and cone-shaped protrusion of the cornea.
Rigid gas permeable lenses improve vision by masking corneal irregularities and creating a smooth optical surface.
Advantages
- Improved visual acuity
- Correction of irregular astigmatism
- Better optical quality
7. Irregular Cornea
Contact lenses are useful in corneal irregularities caused by:
- Corneal scars
- Post-corneal surgery
- Trauma
- Corneal dystrophies
RGP and scleral lenses help provide a regular refractive surface.
2. MEDICAL OR THERAPEUTIC INDICATIONS
Therapeutic contact lenses are used to protect the ocular surface and promote healing.
1. Corneal Ulcers and Epithelial Defects
Bandage contact lenses protect the cornea from friction caused by blinking and promote epithelial healing.
2. Bullous Keratopathy
Therapeutic soft lenses reduce pain caused by ruptured epithelial bullae.
3. Recurrent Corneal Erosion
Bandage contact lenses stabilize the corneal epithelium and reduce recurrent epithelial breakdown.
4. Dry Eye Management
Special scleral lenses create a fluid reservoir over the cornea and help manage severe dry eye conditions.
5. Post-Surgical Protection
Contact lenses are used after refractive surgery or corneal surgery for protection and healing.
3. COSMETIC INDICATIONS
1. Cosmetic Enhancement
Colored contact lenses are used to change or enhance eye color for cosmetic purposes.
2. Corneal Opacity
Prosthetic contact lenses help mask corneal scars and improve appearance.
3. Aniridia and Coloboma
Special prosthetic lenses improve cosmetic appearance and reduce photophobia in these conditions.
4. OCCUPATIONAL AND SPORTS INDICATIONS
1. Sports Activities
Athletes prefer contact lenses because they provide:
- Wider visual field
- Better peripheral vision
- No fogging
- Better stability during movement
2. Occupational Needs
Contact lenses are useful in occupations where spectacles may interfere with work, such as:
- Police personnel
- Military personnel
- Actors
- Drivers
- Athletes
CONTRAINDICATIONS OF CONTACT LENSES
Definition
Contraindications are conditions in which contact lens wear is unsuitable or may be harmful to the patient.
Types of Contraindications
- Ocular contraindications
- Systemic contraindications
- Environmental contraindications
- Patient-related contraindications
1. OCULAR CONTRAINDICATIONS
1. Active Ocular Infection
Contact lenses should not be prescribed in active infections such as:
- Conjunctivitis
- Keratitis
- Blepharitis
Lens wear may worsen infection and delay healing.
2. Severe Dry Eye
Patients with severe tear deficiency may experience:
- Poor lens tolerance
- Discomfort
- Corneal staining
- Blurred vision
3. Corneal Hypoesthesia
Reduced corneal sensitivity increases the risk of unnoticed corneal injury and infection.
4. Allergic Eye Disease
Conditions such as allergic conjunctivitis and giant papillary conjunctivitis may worsen during contact lens wear.
5. Severe Blepharitis and Meibomian Gland Dysfunction
These conditions increase deposit formation and lens intolerance.
6. Poor Lid Function
Abnormal blinking or lid abnormalities interfere with lens stability and tear distribution.
7. Recurrent Corneal Erosion
Improper lens wear may aggravate epithelial instability unless therapeutic lenses are specifically indicated.
2. SYSTEMIC CONTRAINDICATIONS
1. Diabetes Mellitus
Poorly controlled diabetes may reduce corneal sensitivity and delay healing, increasing complication risk.
2. Autoimmune Diseases
Diseases such as rheumatoid arthritis and Sjögren syndrome may produce severe dry eye and poor lens tolerance.
3. Immunocompromised Conditions
Patients with reduced immunity are at increased risk of ocular infection.
4. Certain Medications
Some medications reduce tear production and increase dryness.
Examples:
- Antihistamines
- Antidepressants
- Oral contraceptives
3. ENVIRONMENTAL CONTRAINDICATIONS
1. Dusty Environment
Dust particles may accumulate beneath the lens causing irritation and infection.
2. Chemical Exposure
Industrial chemicals and fumes may interact with contact lenses and damage ocular tissues.
3. Dry Climate
Low humidity environments increase lens dehydration and discomfort.
4. PATIENT-RELATED CONTRAINDICATIONS
1. Poor Hygiene
Improper hygiene significantly increases the risk of microbial keratitis and other complications.
2. Lack of Motivation
Successful contact lens wear requires proper care and patient cooperation.
3. Poor Compliance
Patients who fail to follow lens care instructions are at greater risk of complications.
4. Inability to Handle Lenses
Some patients may have difficulty inserting, removing, or cleaning lenses properly.
Relative Contraindications
Some conditions do not completely prevent contact lens wear but require careful monitoring.
Examples include:
- Mild dry eye
- Pregnancy
- Seasonal allergies
- Controlled diabetes
Importance of Proper Patient Selection
Proper patient selection is essential for successful contact lens practice.
Before prescribing lenses, the optometrist should evaluate:
- Ocular health
- Tear film quality
- Corneal condition
- Patient motivation
- Occupational needs
- Ability to maintain hygiene
Topic 4: Parameters, Designs and Terminology of Contact Lenses
Introduction
Contact lenses are manufactured in different sizes, shapes, curves, thicknesses, and designs according to the patient’s ocular condition and refractive error. To understand contact lens fitting properly, it is important to know the various lens parameters, lens designs, and commonly used terminology.
Contact lens parameters help determine how the lens fits on the eye, moves during blinking, transmits oxygen, and provides visual correction. Proper understanding of these terms is essential for successful contact lens practice.
CONTACT LENS PARAMETERS
Definition
Contact lens parameters are the measurable specifications of a contact lens that determine its fitting characteristics and optical performance.
1. Base Curve (BC)
Definition
Base curve is the curvature of the back central surface of the contact lens.
It is usually expressed in millimeters (mm).
Importance of Base Curve
- Determines lens fitting relationship with cornea
- Affects lens movement
- Influences comfort and tear exchange
Steep Base Curve
A steep base curve fits tightly on the cornea and reduces lens movement.
Effects
- Reduced tear exchange
- Tight lens fit
- Possible corneal edema
Flat Base Curve
A flat base curve produces loose fitting and excessive movement.
Effects
- Lens decentration
- Excessive movement
- Discomfort
2. Diameter (DIA)
Definition
Diameter is the total width of the contact lens measured from one edge to the opposite edge.
It is expressed in millimeters.
Importance of Diameter
- Determines lens coverage
- Affects lens centration
- Influences comfort and movement
Typical Diameter Values
| Lens Type | Approximate Diameter |
|---|---|
| RGP Lens | 8.5 – 10.5 mm |
| Soft Lens | 13.5 – 15 mm |
| Scleral Lens | 15 – 24 mm |
3. Optic Zone Diameter (OZD)
Definition
Optic zone diameter is the central portion of the lens that provides optical correction.
Importance
- Determines visual quality
- Must adequately cover pupil area
- Influences lens fitting
4. Peripheral Curves
Definition
Peripheral curves are flatter curves surrounding the optic zone of the contact lens.
Functions of Peripheral Curves
- Improve tear exchange
- Enhance comfort
- Reduce edge pressure
- Improve lens movement
5. Center Thickness (CT)
Definition
Center thickness refers to the thickness of the lens at its center.
Importance of Center Thickness
- Affects oxygen transmissibility
- Influences lens flexibility
- Affects handling characteristics
Thick Lenses
- More durable
- Easier handling
- Reduced oxygen transmission
Thin Lenses
- Better oxygen transmission
- Improved comfort
- More fragile
6. Lens Power
Definition
Lens power refers to the refractive correction provided by the contact lens.
It is measured in diopters (D).
Types of Lens Power
- Plus power lenses
- Minus power lenses
- Cylindrical power lenses
7. Edge Design
Definition
Edge design refers to the shape and contour of the lens edge.
Importance of Edge Design
- Affects comfort
- Influences lid interaction
- Controls tear exchange
- Determines lens movement
CONTACT LENS DESIGNS
Definition
Contact lens design refers to the shape and optical structure of the lens used for different refractive and ocular conditions.
1. Spherical Lenses
Definition
Spherical contact lenses have the same curvature and power in all meridians.
Uses
- Myopia
- Hypermetropia
Advantages
- Simple design
- Easy fitting
- Stable vision
2. Toric Lenses
Definition
Toric lenses contain different powers in different meridians for correction of astigmatism.
Types of Toric Lenses
- Soft toric lenses
- Back surface toric lenses
- Front surface toric lenses
- Bitoric lenses
Advantages
- Corrects astigmatism
- Improves visual acuity
- Provides stable vision
3. Multifocal Lenses
Definition
Multifocal lenses contain multiple optical powers for distance, intermediate, and near vision.
Uses
- Presbyopia
Advantages
- Simultaneous distance and near vision
- Reduced dependence on spectacles
4. Bifocal Lenses
Bifocal contact lenses contain separate optical zones for distance and near correction.
5. Aspheric Lenses
Definition
Aspheric lenses have gradually changing curvature from center to periphery.
Advantages
- Reduced spherical aberration
- Improved visual quality
- Better centration
6. Scleral Lenses
Definition
Scleral lenses are large diameter lenses that rest on the sclera and vault over the cornea.
Uses
- Keratoconus
- Irregular cornea
- Severe dry eye
7. Hybrid Lenses
Definition
Hybrid lenses combine a rigid central zone with a soft peripheral skirt.
Advantages
- Excellent vision quality
- Improved comfort
8. Orthokeratology Lenses
Definition
Orthokeratology lenses are specially designed rigid lenses used to temporarily reshape the cornea.
Uses
- Myopia control
- Temporary refractive correction
IMPORTANT CONTACT LENS TERMINOLOGY
1. Wettability
Wettability refers to the ability of tears to spread evenly over the lens surface.
2. Tear Exchange
Tear exchange is the movement of tears beneath the contact lens during blinking.
3. Lens Movement
Lens movement refers to the displacement of the lens during blinking.
Adequate movement is necessary for:
- Oxygen supply
- Tear exchange
- Removal of debris
4. Centration
Centration refers to the alignment of the contact lens with the center of the cornea.
Good centration improves comfort and visual quality.
5. Sagittal Depth
Sagittal depth refers to the depth of the lens from the back surface to an imaginary chord across the lens diameter.
It influences lens fitting characteristics.
6. Edge Lift
Edge lift refers to the space between the lens edge and cornea.
Proper edge lift allows tear exchange and improves comfort.
7. Fluorescein Pattern
Fluorescein pattern refers to the distribution of fluorescein dye beneath a rigid contact lens used to assess lens fitting.
8. Lens Flexure
Lens flexure refers to bending of the contact lens on the eye due to corneal shape or eyelid pressure.
9. Apical Clearance
Apical clearance refers to the space between the central cornea and contact lens.
10. Apical Bearing
Apical bearing occurs when the contact lens touches the apex of the cornea.
11. Tear Lens
The tear lens is the layer of tears trapped between the cornea and the back surface of the contact lens.
It contributes to the final optical power of the contact lens system.
12. Lens Decentration
Lens decentration refers to displacement of the lens away from the corneal center.
It may cause blurred vision and discomfort.
Importance of Understanding Lens Parameters and Terminology
Knowledge of lens parameters and terminology is essential for:
- Accurate lens fitting
- Assessment of lens movement
- Selection of appropriate design
- Troubleshooting fitting problems
- Improving patient comfort
Topic 5: Rigid Gas Permeable (RGP) Contact Lens Materials
Introduction
Rigid Gas Permeable (RGP) contact lenses are rigid lenses that allow oxygen to pass through the lens material to the cornea. These lenses were developed to overcome the disadvantages of PMMA hard lenses, which had excellent optical quality but did not permit oxygen transmission.
RGP lenses combine the optical advantages of rigid lenses with improved corneal physiology. They provide excellent visual acuity, better correction of astigmatism, and improved oxygen supply to the cornea.
Modern RGP materials are manufactured by combining various polymers such as silicone, fluorine, and methacrylate compounds to improve oxygen permeability, wettability, durability, and patient comfort.
Definition of RGP Lens Materials
Rigid Gas Permeable materials are rigid polymeric materials that maintain their shape on the eye while allowing oxygen transmission through the lens.
Need for Development of RGP Materials
Earlier PMMA lenses had several disadvantages due to lack of oxygen permeability.
Problems with PMMA Lenses
- Corneal hypoxia
- Corneal edema
- Neovascularization
- Poor comfort
- Reduced wearing time
To overcome these complications, gas permeable materials were introduced.
Characteristics of Ideal RGP Material
An ideal RGP material should possess:
- High oxygen permeability
- Good wettability
- Excellent optical quality
- Good durability
- Deposit resistance
- Dimensional stability
- Flexibility with adequate rigidity
- Biocompatibility
Composition of RGP Materials
Modern RGP materials are produced by combining different chemical components to improve lens performance.
The main components include:
- Silicone
- Fluorine
- Methacrylate polymers
Silicone Component
Silicone is added to improve oxygen permeability because oxygen passes easily through silicone-containing materials.
Advantages of Silicone
- High oxygen transmission
- Improved corneal physiology
- Reduced hypoxia
Disadvantages of Silicone
- Hydrophobic surface
- Poor wettability
- Increased lipid deposits
Fluorine Component
Fluorine is added to improve wettability and reduce deposit formation.
Advantages of Fluorine
- Improves surface wettability
- Reduces lipid deposits
- Enhances tear film stability
Methacrylate Component
Methacrylate polymers provide structural stability and rigidity to the lens material.
Functions of Methacrylate
- Maintains lens shape
- Provides durability
- Improves optical quality
Types of RGP Materials
RGP materials have evolved through different generations.
1. CAB (Cellulose Acetate Butyrate)
CAB was one of the earliest gas permeable materials used after PMMA.
Properties of CAB
- Moderate oxygen permeability
- Better comfort than PMMA
- Flexible material
- Moderate durability
Advantages
- Improved corneal physiology compared to PMMA
- Lighter material
Disadvantages
- Lower oxygen permeability than modern RGP materials
- Poor long-term stability
2. Silicone Acrylate Materials
Silicone acrylate materials were developed to increase oxygen permeability significantly.
These materials contain silicone molecules combined with methacrylate polymers.
Advantages
- High oxygen permeability
- Excellent optical performance
- Improved corneal health
Disadvantages
- Poor wettability
- Hydrophobic surface
- Higher deposit formation
3. Fluoro-Silicone Acrylate Materials
Fluoro-silicone acrylate materials are advanced RGP materials containing both silicone and fluorine.
These are among the most commonly used RGP materials today.
Advantages
- Very high oxygen permeability
- Good wettability
- Reduced deposits
- Improved comfort
- Excellent durability
Disadvantages
- More expensive
- Fragility in high Dk materials
Properties of RGP Lens Materials
1. Oxygen Permeability (Dk)
RGP lenses possess higher oxygen permeability than PMMA lenses.
High Dk materials improve corneal physiology and reduce hypoxic complications.
2. Wettability
Wettability refers to the ability of tears to spread evenly over the lens surface.
Good wettability improves:
- Comfort
- Stable vision
- Tear film quality
3. Rigidity
RGP materials maintain their shape on the eye and therefore provide stable optical performance.
This rigidity helps neutralize corneal astigmatism.
4. Durability
RGP lenses are more durable than soft lenses and resist tearing.
5. Deposit Resistance
Modern fluorinated materials resist protein and lipid deposits better than older materials.
6. Surface Wettability
Surface wettability is essential for comfortable lens wear and proper tear film stability.
Advantages of RGP Lenses
1. Excellent Visual Acuity
RGP lenses maintain their shape and provide excellent optical quality.
2. Better Correction of Astigmatism
RGP lenses neutralize corneal astigmatism using the tear lens formed beneath the lens.
3. High Oxygen Transmission
Modern RGP materials provide adequate oxygen supply to the cornea.
4. Reduced Deposit Formation
RGP lenses accumulate fewer deposits compared to soft lenses.
5. Durability
RGP lenses have longer life and better resistance to damage.
6. Lower Infection Risk
Because of lower water content and reduced deposits, RGP lenses generally have lower infection risk than soft lenses.
Disadvantages of RGP Lenses
1. Initial Discomfort
RGP lenses require an adaptation period because of their rigid nature.
2. Lens Awareness
Patients may initially feel the lens during blinking.
3. Lens Displacement
Smaller diameter RGP lenses may dislodge during sports or eye rubbing.
4. Adaptation Required
Regular wear is necessary to maintain comfort and adaptation.
Clinical Uses of RGP Lenses
1. Astigmatism
RGP lenses provide superior correction of corneal astigmatism.
2. Keratoconus
RGP lenses improve vision by masking corneal irregularities.
3. Irregular Cornea
Used in:
- Post-LASIK ectasia
- Corneal scars
- Post-corneal transplant
4. High Refractive Errors
Provide excellent visual quality in high refractive errors.
Comparison Between PMMA and RGP Lenses
| Feature | PMMA Lens | RGP Lens |
|---|---|---|
| Oxygen Permeability | Absent | Present |
| Comfort | Less | Better |
| Corneal Health | Poor | Better |
| Visual Acuity | Excellent | Excellent |
| Hypoxia Risk | High | Low |
| Deposit Formation | Less | Less |
Factors Affecting Selection of RGP Materials
- Corneal physiology
- Required oxygen permeability
- Lens durability
- Deposit resistance
- Patient comfort
- Wearing schedule
Modern Advances in RGP Materials
Modern RGP materials focus on:
- Ultra-high Dk materials
- Improved surface coatings
- Better wettability
- Reduced deposits
- Enhanced comfort
Advanced manufacturing techniques have improved lens precision and clinical outcomes.
