CONTACT LENS – I
UNIT 3
Topic 1: Manufacturing of Rigid and Soft Contact Lenses – Various Methods
Introduction
Contact lens manufacturing is the process of converting raw contact lens materials into finished lenses with specific optical powers, base curves, diameters, and designs. Modern manufacturing techniques are highly precise and use advanced technology to produce comfortable, durable, and optically accurate lenses.
Different manufacturing methods are used for rigid contact lenses and soft contact lenses because of differences in material properties and lens design.
The choice of manufacturing technique affects:
- Lens accuracy
- Surface quality
- Comfort
- Oxygen transmission
- Durability
- Production cost
Objectives of Contact Lens Manufacturing
- To produce accurate optical power
- To provide proper lens curvature
- To ensure smooth surface finish
- To maintain lens transparency
- To improve comfort and fitting
- To produce reproducible lens parameters
MANUFACTURING OF RIGID CONTACT LENSES
Introduction
Rigid contact lenses are mainly manufactured from RGP materials such as silicone acrylate and fluoro-silicone acrylate polymers.
Rigid lenses require precise shaping and polishing because even small surface irregularities can affect visual quality and comfort.
Methods of Manufacturing Rigid Contact Lenses
The main methods include:
- Lathing or Lathe-Cut Method
- Molding Method
- Spin-Casting Method (limited use)
- Computer-Controlled Manufacturing (CNC)
1. Lathe-Cut Method
Introduction
The lathe-cut method is the most commonly used technique for manufacturing rigid contact lenses.
In this method, a solid dry polymer button is mounted on a precision lathe machine and shaped into a contact lens using cutting tools.
Steps in Lathe-Cut Manufacturing
1. Preparation of Lens Button
A small cylindrical button of RGP material is prepared. The material is hard and dry before machining.
2. Mounting on Lathe
The lens button is fixed securely on a computer-controlled lathe machine.
3. Cutting of Back Surface
The posterior surface of the lens is cut first to produce the required base curve and peripheral curves.
4. Cutting of Front Surface
The anterior surface is then cut to produce the required optical power and lens thickness.
5. Edge Formation
The lens edges are shaped carefully to improve comfort and tear exchange.
6. Polishing
The lens surface is polished to produce a smooth optical finish and improve wettability.
7. Hydration and Inspection
The lens is cleaned, inspected, and verified for parameters such as:
- Base curve
- Diameter
- Power
- Surface quality
Advantages of Lathe-Cut Method
- High accuracy
- Customized lens production
- Suitable for specialty lenses
- Excellent surface quality
Disadvantages
- Time consuming
- Higher manufacturing cost
- Material wastage during cutting
2. Molding Method
Introduction
In the molding method, the contact lens material is shaped using pre-designed molds.
This method is mainly used for mass production.
Procedure
Liquid polymer material is placed into a mold having the desired lens shape. Polymerization occurs inside the mold, producing the final lens shape.
Advantages
- Fast production
- Suitable for large-scale manufacturing
- Less material wastage
Disadvantages
- Less customization
- Lower precision compared to lathe-cut method
3. Spin-Casting Method
Introduction
Spin-casting is a manufacturing technique in which liquid polymer is rotated at high speed inside a mold.
Centrifugal force shapes the lens during polymerization.
Characteristics
- Used mainly for soft lenses
- Produces smooth surfaces
- Suitable for mass production
4. Computer Numerical Control (CNC) Manufacturing
Introduction
Modern contact lenses are commonly manufactured using computer-controlled precision lathes known as CNC machines.
These machines provide highly accurate and reproducible lens parameters.
Advantages of CNC Technology
- High precision
- Reduced human error
- Excellent reproducibility
- Suitable for specialty designs
MANUFACTURING OF SOFT CONTACT LENSES
Introduction
Soft contact lenses are manufactured mainly from hydrogel or silicone hydrogel materials.
These materials absorb water and become soft after hydration. Manufacturing soft lenses requires special techniques because the material changes size and shape after hydration.
Methods of Manufacturing Soft Contact Lenses
- Spin-Casting Method
- Cast-Molding Method
- Lathe-Cut Method
1. Spin-Casting Method
Introduction
Spin-casting is one of the earliest methods used for manufacturing soft contact lenses.
In this method, liquid monomer is poured into a rotating mold.
During rotation, centrifugal force shapes the lens while polymerization occurs.
Procedure
1. Placement of Monomer
Liquid hydrogel monomer is placed inside the mold.
2. Rotation
The mold rotates at high speed.
3. Polymerization
Heat or ultraviolet light initiates polymerization and lens formation.
4. Hydration
The dry lens is hydrated to achieve final soft lens shape and size.
Advantages
- Smooth lens surface
- Suitable for mass production
- Good reproducibility
Disadvantages
- Limited customization
- Less flexibility in lens design
2. Cast-Molding Method
Introduction
Cast-molding is the most commonly used method for modern disposable soft contact lenses.
Procedure
1. Preparation of Molds
Two molds are prepared corresponding to the front and back surface of the lens.
2. Injection of Monomer
Liquid monomer is injected between the molds.
3. Polymerization
Ultraviolet light or heat polymerizes the material.
4. Hydration
The lens is hydrated to obtain final softness and dimensions.
Advantages
- Low production cost
- High-speed manufacturing
- Excellent reproducibility
- Ideal for disposable lenses
Disadvantages
- Less customization
- Limited specialty lens production
3. Lathe-Cut Method for Soft Lenses
Introduction
Soft lenses may also be manufactured using lathe-cut techniques.
A dry polymer button is cut on a precision lathe machine similar to rigid lens manufacturing.
Procedure
- Cutting of dry polymer button
- Formation of front and back surfaces
- Edge finishing
- Hydration of lens
Advantages
- Suitable for custom lenses
- Useful for specialty designs
- Accurate parameters
Disadvantages
- Higher cost
- Time consuming
Hydration Process
Definition
Hydration is the process by which dry hydrogel lenses absorb water and become soft and flexible.
Importance of Hydration
- Determines final lens dimensions
- Provides flexibility
- Improves comfort
Polishing and Finishing
After manufacturing, lenses are polished and finished to:
- Improve surface smoothness
- Enhance comfort
- Reduce deposits
- Improve wettability
Quality Control in Contact Lens Manufacturing
Finished lenses are checked carefully for:
- Lens power
- Base curve
- Diameter
- Center thickness
- Surface defects
- Edge quality
- Transparency
Sterilization and Packaging
After manufacturing and inspection, lenses are sterilized and packaged in sterile containers with storage solution.
Sterilization prevents microbial contamination and ensures patient safety.
Modern Advances in Manufacturing
Modern contact lens manufacturing uses:
- Computer-controlled lathes
- Laser precision technology
- Advanced molding systems
- Automated quality control
These technologies improve lens accuracy, comfort, and production efficiency.
Comparison Between Rigid and Soft Lens Manufacturing
| Feature | Rigid Lens | Soft Lens |
|---|---|---|
| Main Material | RGP Polymers | Hydrogel / Silicone Hydrogel |
| Common Method | Lathe-Cut | Cast-Molding |
| Hydration Needed | No | Yes |
| Customization | High | Moderate |
| Mass Production | Less Common | Very Common |
Topic 2: Pre-Fitting Examination of Contact Lens Patient
Introduction
Pre-fitting examination is a detailed ocular and systemic evaluation performed before prescribing contact lenses. The purpose of this examination is to determine whether the patient is suitable for contact lens wear and to select the most appropriate lens type, material, and design.
A proper pre-fitting examination is essential for successful contact lens practice because it helps prevent complications, improves patient comfort, and increases the success rate of contact lens wear.
The examination includes assessment of:
- Patient history
- Visual status
- Refractive error
- Corneal condition
- Tear film
- Eyelids and conjunctiva
- Corneal curvature
- Binocular vision
Objectives of Pre-Fitting Examination
- To determine suitability for contact lens wear
- To identify contraindications
- To select appropriate lens type and material
- To establish baseline ocular findings
- To improve lens fitting and comfort
- To reduce complications
STEPS OF PRE-FITTING EXAMINATION
1. Patient History
Introduction
History taking is the first and one of the most important steps in contact lens evaluation.
A detailed history helps identify ocular, systemic, occupational, and environmental factors affecting contact lens wear.
A. Personal History
The following details should be recorded:
- Name
- Age
- Occupation
- Address
- Lifestyle
B. Chief Complaint
The patient’s reason for seeking contact lenses should be determined.
Common reasons include:
- Cosmetic appearance
- Sports activities
- Blurred vision with spectacles
- High refractive error
- Occupational needs
C. Ocular History
Information regarding previous ocular conditions should be obtained.
Important questions include:
- Previous contact lens wear
- History of allergy
- Dry eye symptoms
- Eye surgery
- Trauma
- Redness or irritation
D. Systemic History
Certain systemic diseases affect contact lens tolerance and ocular health.
Conditions to evaluate include:
- Diabetes mellitus
- Thyroid disease
- Autoimmune disorders
- Skin diseases
- Allergies
E. Drug History
Certain medications may reduce tear secretion and affect lens comfort.
Examples include:
- Antihistamines
- Oral contraceptives
- Antidepressants
- Diuretics
F. Occupational and Environmental History
Occupation and environment may influence lens selection.
Dusty or dry environments may reduce contact lens tolerance.
2. Visual Acuity Assessment
Introduction
Visual acuity assessment is performed to determine the patient’s present visual status.
Types of Visual Acuity Recorded
- Unaided visual acuity
- Spectacle corrected visual acuity
- Pinhole visual acuity
Importance
- Provides baseline vision
- Evaluates refractive correction
- Assesses visual potential
3. Refraction
Introduction
Accurate refraction is essential for determining the required contact lens power.
Types of Refraction
- Objective refraction
- Subjective refraction
- Cycloplegic refraction (when necessary)
Importance of Refraction
- Determines refractive error
- Helps calculate contact lens power
- Assesses astigmatism
4. Keratometry
Definition
Keratometry is the measurement of corneal curvature.
Importance of Keratometry
- Determines corneal curvature
- Helps select base curve
- Detects corneal astigmatism
- Assesses corneal regularity
Clinical Importance
Steep or irregular corneas may require special contact lens designs such as RGP or scleral lenses.
5. Slit Lamp Examination
Introduction
Slit lamp examination is essential for evaluating the health of anterior ocular structures.
Structures Examined
- Eyelids
- Conjunctiva
- Cornea
- Tear film
- Limbus
- Anterior chamber
Importance
- Detects ocular abnormalities
- Identifies contraindications
- Provides baseline findings
6. Tear Film Evaluation
Introduction
Adequate tear quantity and quality are necessary for successful contact lens wear.
Tests for Tear Film Assessment
- Tear Break-Up Time (TBUT)
- Schirmer test
- Tear meniscus evaluation
A. Tear Break-Up Time (TBUT)
Definition
TBUT measures tear film stability.
Procedure
Fluorescein dye is instilled and the time taken for dry spots to appear after blinking is measured.
Normal Value
More than 10 seconds.
B. Schirmer Test
Definition
Schirmer test measures tear secretion.
Normal Value
More than 10 mm wetting in 5 minutes.
Importance of Tear Evaluation
- Detects dry eye
- Helps select lens material
- Predicts lens tolerance
7. Corneal Evaluation
Introduction
The cornea should be carefully evaluated before contact lens fitting.
Points Evaluated
- Corneal transparency
- Corneal curvature
- Corneal scars
- Neovascularization
- Corneal staining
- Corneal sensitivity
Importance
Corneal abnormalities may affect:
- Lens fitting
- Comfort
- Visual quality
8. Conjunctival Evaluation
The conjunctiva is examined for:
- Hyperemia
- Allergy
- Papillae
- Follicles
- Inflammation
9. Lid Examination
Introduction
Normal lid anatomy and blinking are essential for proper lens positioning and tear distribution.
Points Evaluated
- Lid position
- Blink rate
- Blink completeness
- Meibomian gland function
- Blepharitis
10. Pupil Examination
Importance
Pupil size affects:
- Optic zone diameter selection
- Night vision quality
- Lens centration requirements
11. Corneal Diameter Measurement
Definition
Horizontal visible iris diameter (HVID) is measured to help determine lens diameter.
Importance
- Selection of lens diameter
- Improved lens fitting
12. Evaluation of Binocular Vision
Importance
Binocular vision assessment is important because contact lenses may alter accommodative and convergence demands.
Tests Include
- Near point of convergence
- Phorias
- Stereoacuity
- Accommodation assessment
13. Intraocular Pressure Measurement
Baseline intraocular pressure may be recorded, especially in long-term contact lens wearers.
14. Trial Lens Fitting
Introduction
Trial lenses are placed on the eye to assess fitting characteristics.
Parameters Evaluated
- Centration
- Movement
- Coverage
- Comfort
- Visual acuity
15. Fluorescein Evaluation (For RGP Lenses)
Fluorescein dye is used to evaluate tear distribution beneath RGP lenses.
Importance
- Determines lens fitting relationship
- Detects apical clearance or bearing
- Evaluates edge lift
Recording of Findings
All examination findings should be recorded carefully.
Information Recorded
- Refraction
- K readings
- Tear assessment results
- Lens parameters
- Ocular findings
- Visual acuity
Importance of Recording
- Provides baseline documentation
- Useful during follow-up visits
- Helps monitor complications
Importance of Pre-Fitting Examination
- Improves fitting success
- Reduces complications
- Improves patient comfort
- Helps select appropriate lens type
- Ensures ocular safety
Common Reasons for Contact Lens Failure Detected During Pre-Fitting
- Dry eye
- Poor hygiene
- Blepharitis
- Allergic conjunctivitis
- Poor motivation
- Unrealistic expectations
Topic 3: Correction of Astigmatism with Rigid Gas Permeable (RGP) Contact Lenses
Introduction
Astigmatism is a refractive condition in which the refractive power of the eye differs in different meridians, resulting in blurred or distorted vision. Contact lenses, especially rigid gas permeable (RGP) lenses, are highly effective in correcting astigmatism because they create a smooth refractive surface over the cornea.
RGP lenses provide better visual acuity than spectacles and soft toric lenses in many astigmatic patients, particularly in corneal and irregular astigmatism.
Understanding the correction of astigmatism with RGP lenses is essential for successful contact lens fitting and visual rehabilitation.
Definition of Astigmatism
Astigmatism is a refractive error in which light rays entering the eye fail to focus at a single point because different meridians of the cornea or lens possess different refractive powers.
Types of Astigmatism
1. Regular Astigmatism
In regular astigmatism, the principal meridians are perpendicular to each other.
Types of Regular Astigmatism
- With-the-rule astigmatism
- Against-the-rule astigmatism
- Oblique astigmatism
2. Irregular Astigmatism
In irregular astigmatism, the principal meridians are not perpendicular and corneal curvature is irregular.
It commonly occurs in:
- Keratoconus
- Corneal scars
- Post-surgical corneas
- Corneal trauma
Sources of Astigmatism
- Corneal astigmatism
- Lenticular astigmatism
- Internal astigmatism
Why RGP Lenses are Effective in Astigmatism
RGP lenses maintain their rigid shape on the eye and do not conform to the corneal surface like soft lenses.
A tear layer forms between the back surface of the lens and the anterior corneal surface. This tear layer is called the tear lens.
The tear lens neutralizes corneal irregularities and corrects much of the corneal astigmatism.
Tear Lens
Definition
The tear lens is the layer of tears trapped between the posterior surface of the RGP lens and the anterior corneal surface.
Functions of Tear Lens
- Neutralizes corneal astigmatism
- Improves optical quality
- Creates smooth refractive surface
- Enhances visual acuity
Principle of Astigmatism Correction with RGP Lens
The anterior surface of the RGP lens remains optically regular even when placed on an astigmatic cornea.
The tear lens formed beneath the contact lens fills the gap between the cornea and lens surface, thereby masking corneal toricity.
As a result:
- Irregular corneal surface becomes optically smooth
- Corneal astigmatism is neutralized
- Better visual acuity is achieved
Correction of Corneal Astigmatism
Corneal astigmatism is effectively corrected by spherical RGP lenses because the tear lens compensates for corneal toricity.
Example
Suppose a patient has:
- K readings: 43.00 D / 45.00 D
- Corneal astigmatism: 2.00 D
When a spherical RGP lens is fitted, the tear lens neutralizes most of the corneal astigmatism, producing clear vision.
Residual Astigmatism
Definition
Residual astigmatism is the astigmatism that remains after fitting a spherical RGP lens.
Causes of Residual Astigmatism
- Lenticular astigmatism
- Internal astigmatism
- Incomplete corneal neutralization
- Lens flexure
Management of Residual Astigmatism
- Front toric RGP lenses
- Bitoric lenses
- Soft toric lenses
- Spectacle over-correction
Types of RGP Lenses Used in Astigmatism
- Spherical RGP lenses
- Front surface toric lenses
- Back surface toric lenses
- Bitoric lenses
1. Spherical RGP Lenses
Introduction
Spherical RGP lenses have spherical front and back surfaces.
They are commonly used for corneal astigmatism less than approximately 2.50 D.
Advantages
- Simple fitting
- Excellent vision
- Good centration
- Lower cost
Disadvantages
- May not fully correct high residual astigmatism
- May show excessive lens movement on highly toric corneas
2. Front Surface Toric RGP Lenses
Introduction
Front surface toric lenses have a spherical back surface and toric front surface.
These lenses are used mainly when residual astigmatism remains after fitting a spherical RGP lens.
Indications
- Residual astigmatism
- Lenticular astigmatism
Advantages
- Corrects residual cylinder
- Maintains simple back surface fitting
Disadvantages
- Lens rotation may affect vision
- More complex fitting
3. Back Surface Toric RGP Lenses
Introduction
Back surface toric lenses have toric posterior surfaces and spherical front surfaces.
These lenses align better with toric corneas.
Indications
- High corneal astigmatism
- Toric corneal surfaces
Advantages
- Better lens stability
- Improved centration
- Reduced lens rocking
Disadvantages
- More complicated fitting
- More expensive
4. Bitoric RGP Lenses
Introduction
Bitoric lenses possess toricity on both front and back surfaces.
They are used in patients with high corneal and residual astigmatism.
Advantages
- Excellent stability
- Better visual acuity
- Correction of high astigmatism
Disadvantages
- Complex fitting
- Higher cost
Fitting Considerations in Astigmatism
1. Keratometry
Keratometry is essential to determine corneal toricity and select appropriate lens design.
2. Corneal Topography
Corneal topography helps evaluate irregular corneal surfaces and detect keratoconus.
3. Lens Centration
Proper centration is necessary for stable vision and comfort.
4. Lens Movement
Adequate movement allows tear exchange and prevents corneal hypoxia.
5. Fluorescein Evaluation
Fluorescein pattern helps assess lens fitting relationship.
Fluorescein Patterns in Astigmatic Cornea
1. Alignment Fit
Even fluorescein distribution with good lens positioning.
2. Steep Fit
Excessive central pooling with reduced tear exchange.
3. Flat Fit
Central bearing with excessive edge clearance.
Advantages of RGP Lenses in Astigmatism
- Excellent visual acuity
- Correction of irregular astigmatism
- Reduced distortion
- Stable optical surface
- Better contrast sensitivity
Disadvantages of RGP Lenses in Astigmatism
- Initial discomfort
- Adaptation period required
- Lens awareness
- Possible lens displacement
Indications for RGP Lenses in Astigmatism
- Moderate to high corneal astigmatism
- Irregular astigmatism
- Keratoconus
- Poor vision with spectacles
- Failure with soft toric lenses
Complications During Astigmatic Lens Fitting
- Lens decentration
- Lens flexure
- Residual astigmatism
- Corneal staining
- Discomfort
Management of Fitting Problems
- Modify base curve
- Change lens diameter
- Use toric designs
- Adjust edge lift
- Improve lens material selection
Topic 4: Types of Fit – Steep, Flat and Optimum Fit on Spherical Cornea with Spherical Lenses (Soft and RGP)
Introduction
Contact lens fitting is the process of selecting and positioning a lens on the eye so that it provides good vision, comfort, adequate movement, and proper corneal physiology.
An ideal contact lens fit should maintain a proper relationship between the lens and the cornea. If the lens is too steep or too flat compared to the corneal curvature, fitting problems and complications may occur.
The fitting relationship is commonly described as:
- Steep fit
- Flat fit
- Optimum or alignment fit
These fitting relationships are evaluated differently in RGP and soft contact lenses.
Definition of Contact Lens Fit
Contact lens fit refers to the relationship between the posterior surface of the contact lens and the anterior surface of the cornea.
Importance of Proper Lens Fit
- Provides good comfort
- Maintains corneal health
- Ensures stable vision
- Allows tear exchange
- Prevents corneal complications
FACTORS AFFECTING CONTACT LENS FIT
- Base curve of lens
- Lens diameter
- Corneal curvature
- Lens material
- Lens thickness
- Eyelid tension
- Tear film quality
FITTING OF RGP LENSES ON SPHERICAL CORNEA
Introduction
Rigid Gas Permeable (RGP) lenses maintain their shape on the eye and do not drape over the cornea like soft lenses.
RGP fitting is mainly assessed using:
- Lens centration
- Lens movement
- Fluorescein pattern
- Patient comfort
- Visual acuity
1. STEEP FIT IN RGP LENSES
Definition
A steep fit occurs when the base curve of the lens is steeper than the curvature of the cornea.
The lens fits too tightly on the cornea.
Characteristics of Steep Fit
- Minimal or absent lens movement
- Tight lens fitting
- Excessive central clearance
- Poor tear exchange
- Lens adherence
Fluorescein Pattern in Steep Fit
Fluorescein examination shows:
- Bright central pooling
- Mid-peripheral bearing
- Reduced edge clearance
Clinical Signs of Steep Fit
- Reduced lens movement
- Blurred vision after blinking
- Corneal edema
- Conjunctival redness
- Lens sticking
Advantages of Slightly Steep Fit
- Better centration
- Reduced lens awareness
Disadvantages of Excessive Steep Fit
- Poor tear exchange
- Hypoxia
- Corneal edema
- Discomfort
- Debris accumulation
Management of Steep Fit
- Flatten the base curve
- Reduce lens diameter
- Increase edge lift
2. FLAT FIT IN RGP LENSES
Definition
A flat fit occurs when the base curve of the lens is flatter than the corneal curvature.
The lens rests excessively on the corneal apex.
Characteristics of Flat Fit
- Excessive lens movement
- Poor centration
- Apical bearing
- Lens decentration
- Foreign body sensation
Fluorescein Pattern in Flat Fit
Fluorescein examination shows:
- Central dark bearing area
- Excessive peripheral pooling
- Excessive edge clearance
Clinical Signs of Flat Fit
- Lens decentration
- Lens discomfort
- Excessive movement
- Corneal staining
- Variable vision
Advantages of Slightly Flat Fit
- Good tear exchange
- Better oxygen supply
Disadvantages of Excessive Flat Fit
- Corneal abrasion
- Poor comfort
- Lens instability
- Poor visual quality
Management of Flat Fit
- Steepen the base curve
- Increase lens diameter
- Reduce edge lift
3. OPTIMUM OR ALIGNMENT FIT IN RGP LENSES
Definition
An optimum fit occurs when the posterior lens surface aligns properly with the anterior corneal surface.
The lens provides adequate movement, centration, tear exchange, and comfort.
Characteristics of Optimum Fit
- Good centration
- Adequate lens movement
- Stable vision
- Good comfort
- Proper tear exchange
Fluorescein Pattern in Optimum Fit
Fluorescein pattern shows:
- Light central touch or alignment
- Even fluorescein distribution
- Adequate peripheral clearance
Ideal Lens Movement
The ideal movement of an RGP lens is approximately:
0with each blink.
Advantages of Optimum Fit
- Good visual acuity
- Excellent comfort
- Healthy corneal physiology
- Stable lens position
- Reduced complications
SOFT CONTACT LENS FITTING ON SPHERICAL CORNEA
Introduction
Soft contact lenses drape over the cornea and conform closely to corneal shape.
Soft lens fitting is assessed mainly by:
- Centration
- Coverage
- Movement
- Push-up test
- Comfort
1. STEEP FIT IN SOFT LENSES
Definition
A steep soft lens fit occurs when the lens fits too tightly on the cornea.
Characteristics of Steep Soft Lens Fit
- Minimal movement
- Tight adherence
- Reduced tear exchange
- Difficulty in lens movement
Clinical Signs
- Conjunctival indentation
- Blurred vision
- Corneal edema
- Lens adherence
- Reduced comfort
Push-Up Test in Steep Fit
During push-up test:
- Lens moves with difficulty
- Lens returns slowly to center
Complications of Steep Soft Lens Fit
- Corneal hypoxia
- Corneal edema
- Reduced tear exchange
- Inflammation
Management of Steep Soft Lens Fit
- Flatten base curve
- Reduce diameter
- Select thinner lens
2. FLAT FIT IN SOFT LENSES
Definition
A flat soft lens fit occurs when the lens is flatter than the corneal curvature.
Characteristics of Flat Soft Lens Fit
- Excessive movement
- Poor centration
- Lens decentration
- Edge fluting
Clinical Signs
- Foreign body sensation
- Variable vision
- Lens displacement
- Excessive movement during blinking
Push-Up Test in Flat Fit
During push-up test:
- Lens moves excessively
- Lens returns rapidly to center
Complications of Flat Soft Lens Fit
- Lens instability
- Discomfort
- Mechanical irritation
- Lens dislocation
Management of Flat Soft Lens Fit
- Steepen base curve
- Increase lens diameter
- Select appropriate lens design
3. OPTIMUM FIT IN SOFT LENSES
Definition
An optimum soft lens fit provides proper centration, adequate movement, full corneal coverage, and good comfort.
Characteristics of Optimum Soft Lens Fit
- Good centration
- Full corneal coverage
- Adequate movement
- Good comfort
- Stable vision
Ideal Movement of Soft Lens
The ideal movement of a soft lens is approximately:
1with blinking.
Push-Up Test in Optimum Fit
During push-up test:
- Lens moves easily with gentle pressure
- Lens returns smoothly to central position
Advantages of Optimum Soft Lens Fit
- Excellent comfort
- Stable vision
- Good tear exchange
- Healthy corneal physiology
- Reduced complications
COMPARISON OF STEEP, FLAT AND OPTIMUM FIT
| Feature | Steep Fit | Flat Fit | Optimum Fit |
|---|---|---|---|
| Lens Movement | Reduced | Excessive | Adequate |
| Centration | Good initially | Poor | Good |
| Tear Exchange | Poor | Excessive | Normal |
| Comfort | Reduced | Reduced | Good |
| Corneal Health | Hypoxia risk | Mechanical irritation | Healthy |
| Vision Stability | Variable | Variable | Stable |
Importance of Proper Fit
A proper contact lens fit is necessary to:
- Maintain corneal health
- Provide clear and stable vision
- Ensure patient comfort
- Reduce complications
- Improve lens tolerance
Complications Due to Improper Fit
- Corneal edema
- Corneal staining
- Lens intolerance
- Hypoxia
- Conjunctival redness
- Mechanical irritation
Topic 5: Types of Fit – Steep, Flat and Optimum Fit on Toric Cornea with Spherical Lenses (Soft and RGP)
Introduction
A toric cornea is a cornea in which different meridians possess different curvatures. Usually, one meridian is steeper while the other is flatter. This difference in curvature produces corneal astigmatism.
Fitting spherical contact lenses on a toric cornea is more challenging than fitting them on a spherical cornea because the lens interacts differently with the steep and flat meridians of the cornea.
Both soft and RGP spherical lenses may show steep fit, flat fit, or optimum fit on toric corneas depending upon the relationship between lens curvature and corneal toricity.
Proper fitting is essential to maintain:
- Good visual acuity
- Lens stability
- Corneal health
- Patient comfort
- Adequate tear exchange
Toric Cornea
Definition
A toric cornea is a cornea having different curvatures in different meridians.
Typically:
- One meridian is steeper
- One meridian is flatter
Effects of Toric Cornea on Contact Lens Fitting
A toric cornea affects:
- Lens centration
- Lens movement
- Lens rotation
- Tear distribution
- Visual stability
Challenges of Fitting Spherical Lenses on Toric Cornea
- Unequal lens alignment in different meridians
- Lens rocking
- Lens decentration
- Variable vision
- Excessive movement
FITTING OF SPHERICAL RGP LENSES ON TORIC CORNEA
Introduction
RGP lenses maintain their rigid shape and do not drape over the toric cornea. A tear lens forms beneath the lens, helping neutralize corneal astigmatism.
However, because the cornea has unequal curvature in different meridians, the lens may fit differently along the steep and flat meridians.
1. STEEP FIT OF SPHERICAL RGP LENS ON TORIC CORNEA
Definition
A steep fit occurs when the base curve of the spherical RGP lens is steeper than the flatter corneal meridian.
The lens fits tightly, especially along the steeper meridian.
Characteristics of Steep Fit
- Reduced lens movement
- Central pooling
- Poor tear exchange
- Lens adherence
- Lens binding
Fluorescein Pattern in Steep Fit
Fluorescein evaluation shows:
- Excessive central pooling
- Mid-peripheral bearing
- Reduced edge clearance
Because of corneal toricity, fluorescein distribution may appear uneven between the two principal meridians.
Clinical Signs
- Blurred vision after blinking
- Reduced comfort
- Conjunctival redness
- Corneal edema
- Minimal movement
Complications of Steep Fit
- Hypoxia
- Corneal edema
- Lens adherence
- Debris accumulation
Management of Steep Fit
- Flatten base curve
- Reduce lens diameter
- Increase edge lift
2. FLAT FIT OF SPHERICAL RGP LENS ON TORIC CORNEA
Definition
A flat fit occurs when the spherical RGP lens is flatter than the steep corneal meridian.
The lens tends to rest excessively on the corneal apex and shows instability.
Characteristics of Flat Fit
- Excessive movement
- Lens decentration
- Lens rocking
- Apical bearing
- Variable vision
Fluorescein Pattern in Flat Fit
Fluorescein examination shows:
- Central bearing
- Excessive peripheral pooling
- Excessive edge clearance
On a toric cornea, the lens may align differently in each meridian producing uneven fluorescein appearance.
Clinical Signs
- Foreign body sensation
- Lens instability
- Excessive movement
- Lens displacement
- Corneal staining
Complications of Flat Fit
- Mechanical irritation
- Corneal abrasion
- Lens intolerance
- Poor visual stability
Management of Flat Fit
- Steepen base curve
- Increase lens diameter
- Use toric RGP design if required
3. OPTIMUM FIT OF SPHERICAL RGP LENS ON TORIC CORNEA
Definition
An optimum fit occurs when the spherical RGP lens provides acceptable alignment with the toric cornea while maintaining adequate movement and centration.
Characteristics of Optimum Fit
- Good centration
- Adequate movement
- Stable vision
- Good tear exchange
- Acceptable fluorescein pattern
Fluorescein Pattern in Optimum Fit
Fluorescein pattern shows:
- Mild central alignment
- Even tear distribution
- Adequate peripheral clearance
Slight variation may occur between the principal meridians due to corneal toricity.
Advantages of Optimum Fit
- Excellent visual acuity
- Good comfort
- Reduced complications
- Stable lens performance
Problems of Spherical RGP Lens on Highly Toric Cornea
When corneal toricity is high:
- Lens rocking increases
- Centring becomes poor
- Movement becomes excessive
- Residual astigmatism may remain
In such cases, toric or bitoric RGP lenses may be required.
FITTING OF SPHERICAL SOFT LENSES ON TORIC CORNEA
Introduction
Soft lenses drape over the corneal surface and closely conform to corneal toricity.
Unlike RGP lenses, spherical soft lenses do not neutralize corneal astigmatism effectively because they conform to the toric shape of the cornea.
1. STEEP FIT OF SPHERICAL SOFT LENS ON TORIC CORNEA
Definition
A steep soft lens fit occurs when the lens fits too tightly over the toric cornea.
Characteristics of Steep Fit
- Reduced movement
- Tight adherence
- Poor tear exchange
- Reduced lens mobility
Clinical Signs
- Blurred vision
- Conjunctival indentation
- Corneal edema
- Lens adherence
- Reduced comfort
Push-Up Test
During push-up test:
- Lens moves with difficulty
- Returns slowly to center
Complications
- Hypoxia
- Corneal swelling
- Reduced tear exchange
Management
- Flatten base curve
- Reduce diameter
- Select thinner lens
2. FLAT FIT OF SPHERICAL SOFT LENS ON TORIC CORNEA
Definition
A flat fit occurs when the spherical soft lens is flatter than the toric corneal shape.
Characteristics of Flat Fit
- Excessive movement
- Lens decentration
- Edge fluting
- Poor stability
Clinical Signs
- Variable vision
- Foreign body sensation
- Lens displacement
- Excessive blinking awareness
Push-Up Test
During push-up test:
- Lens moves excessively
- Returns rapidly to center
Complications
- Mechanical irritation
- Lens instability
- Poor comfort
Management
- Steepen base curve
- Increase lens diameter
- Use toric soft lens if required
3. OPTIMUM FIT OF SPHERICAL SOFT LENS ON TORIC CORNEA
Definition
An optimum fit occurs when the spherical soft lens provides good centration, adequate movement, and acceptable comfort on the toric cornea.
Characteristics of Optimum Fit
- Good centration
- Adequate movement
- Full corneal coverage
- Good comfort
- Stable lens position
Ideal Lens Movement
Ideal movement of a soft lens is approximately:
0Advantages of Optimum Fit
- Good comfort
- Stable lens positioning
- Reduced complications
- Better tear exchange
Limitations of Spherical Soft Lenses on Toric Cornea
Spherical soft lenses may not fully correct moderate or high astigmatism because the lens conforms to the corneal toricity.
Patients with higher astigmatism may experience:
- Blurred vision
- Residual astigmatism
- Reduced visual quality
Such patients may require toric soft lenses or RGP lenses.
COMPARISON OF FITTING TYPES ON TORIC CORNEA
| Feature | Steep Fit | Flat Fit | Optimum Fit |
|---|---|---|---|
| Lens Movement | Reduced | Excessive | Adequate |
| Centration | Good initially | Poor | Good |
| Tear Exchange | Poor | Excessive | Normal |
| Comfort | Reduced | Reduced | Good |
| Vision Stability | Variable | Variable | Stable |
| Corneal Health | Hypoxia risk | Mechanical irritation | Healthy |
Importance of Proper Fit on Toric Cornea
- Improves visual acuity
- Maintains corneal health
- Provides stable vision
- Reduces complications
- Improves patient comfort
Topic 6: Calculation and Finalizing Contact Lens Parameters
Introduction
Calculation and finalization of contact lens parameters is one of the most important steps in contact lens practice. After completing the pre-fitting examination and trial lens evaluation, the optometrist must determine the final lens specifications that will provide good vision, comfort, proper fitting, and healthy corneal physiology.
Contact lens parameters include:
- Lens power
- Base curve
- Diameter
- Optic zone diameter
- Peripheral curves
- Center thickness
- Lens material
Proper calculation and selection of these parameters are essential for successful contact lens fitting.
Objectives of Finalizing Contact Lens Parameters
- To provide accurate refractive correction
- To achieve optimum fitting relationship
- To maintain corneal health
- To provide stable visual acuity
- To improve patient comfort
- To minimize complications
IMPORTANT PARAMETERS IN CONTACT LENS FITTING
1. Lens Power
Definition
Lens power refers to the refractive correction provided by the contact lens.
It is measured in diopters (D).
Calculation of Contact Lens Power
The contact lens power is determined from:
- Subjective refraction
- Vertex distance compensation
- Over-refraction findings
Vertex Distance Compensation
Introduction
For high refractive errors, the spectacle prescription cannot be used directly as contact lens power because contact lenses sit directly on the cornea.
Vertex compensation becomes important usually for powers greater than:
0Formula for Vertex Conversion
1Where:
- Fc = Contact lens power
- Fs = Spectacle lens power
- d = Vertex distance in meters
Example of Minus Lens Conversion
Suppose:
- Spectacle power = –8.00 D
- Vertex distance = 12 mm
After vertex compensation, the contact lens power becomes approximately:
2Example of Plus Lens Conversion
Suppose:
- Spectacle power = +10.00 D
- Vertex distance = 12 mm
After vertex compensation, the contact lens power becomes approximately:
3Over-Refraction
Definition
Over-refraction is the procedure of determining additional refractive correction over the trial contact lens.
Purpose of Over-Refraction
- To refine final lens power
- To improve visual acuity
- To detect residual astigmatism
2. BASE CURVE SELECTION
Definition
Base curve is the curvature of the posterior central surface of the contact lens.
Importance of Base Curve
- Determines lens fitting relationship
- Affects centration
- Influences movement and comfort
- Controls tear exchange
Selection of Base Curve in RGP Lenses
RGP base curve selection mainly depends upon keratometry readings.
General Guidelines
- Flat K may be selected initially
- Steeper fitting may be required in some cases
- Final decision depends on fluorescein pattern
Relationship Between Base Curve and Fit
| Base Curve Relationship | Resulting Fit |
|---|---|
| Too Steep | Steep Fit |
| Too Flat | Flat Fit |
| Proper Alignment | Optimum Fit |
Selection of Base Curve in Soft Lenses
Soft lens fitting depends more on:
- Lens diameter
- Lens material
- Corneal sagittal depth
Manufacturers usually provide standard base curve options.
3. DIAMETER SELECTION
Definition
Diameter is the total width of the contact lens measured from one edge to the opposite edge.
Importance of Diameter
- Affects lens coverage
- Influences centration
- Controls movement
- Improves comfort
Diameter Selection in RGP Lenses
RGP lens diameter is selected according to:
- Horizontal visible iris diameter (HVID)
- Palpebral aperture
- Corneal curvature
Typical RGP Diameter
4Diameter Selection in Soft Lenses
Soft lenses are generally larger than the cornea to provide complete corneal coverage.
Typical Soft Lens Diameter
54. OPTIC ZONE DIAMETER (OZD)
Definition
Optic zone diameter is the central optical portion of the lens responsible for refractive correction.
Importance of OZD
- Provides clear vision
- Must adequately cover pupil area
- Reduces glare and halos
Factors Affecting OZD Selection
- Pupil size
- Lighting conditions
- Lens design
5. PERIPHERAL CURVES
Definition
Peripheral curves are flatter curves surrounding the optic zone of the contact lens.
Functions of Peripheral Curves
- Improve tear exchange
- Reduce edge pressure
- Improve comfort
- Assist lens movement
6. CENTER THICKNESS (CT)
Definition
Center thickness is the thickness of the lens at its central portion.
Importance of Center Thickness
- Affects oxygen transmissibility
- Influences lens flexibility
- Affects durability
- Influences handling
Thin Lenses
- Better oxygen transmission
- Better comfort
- More fragile
Thick Lenses
- Easier handling
- More durable
- Reduced oxygen transmission
7. SELECTION OF LENS MATERIAL
Importance
Lens material selection affects:
- Oxygen permeability
- Comfort
- Deposit resistance
- Lens durability
- Wettability
Factors Affecting Material Selection
- Tear quality
- Wearing schedule
- Corneal physiology
- Patient lifestyle
- Refractive error
FINALIZATION OF RGP CONTACT LENS PARAMETERS
Steps in Finalizing RGP Lens
1. Evaluate Fluorescein Pattern
Fluorescein pattern is used to assess:
- Central alignment
- Apical bearing
- Peripheral clearance
- Edge lift
2. Assess Lens Movement
Ideal RGP lens movement is approximately:
63. Evaluate Centration
The lens should center properly over the cornea.
4. Perform Over-Refraction
Residual refractive error is corrected by over-refraction.
5. Assess Patient Comfort
Patient adaptation and comfort are essential before finalizing parameters.
FINALIZATION OF SOFT CONTACT LENS PARAMETERS
Steps in Finalizing Soft Lens
1. Assess Lens Coverage
The lens should completely cover the cornea.
2. Assess Lens Movement
Ideal soft lens movement is approximately:
73. Push-Up Test
The lens should move easily and return smoothly to center.
4. Evaluate Vision
Stable visual acuity should be achieved after blinking.
5. Assess Comfort
The patient should feel comfortable without irritation or excessive awareness.
Factors Influencing Final Lens Selection
- Corneal curvature
- Tear film quality
- Refractive error
- Occupation
- Patient motivation
- Ocular health
Common Problems During Finalization
- Lens decentration
- Excessive movement
- Tight fit
- Residual astigmatism
- Variable vision
- Poor comfort
Modification of Parameters
If fitting problems occur, the following modifications may be required:
- Change base curve
- Alter diameter
- Modify edge design
- Change material
- Adjust lens power
Importance of Proper Finalization
- Improves lens tolerance
- Maintains ocular health
- Reduces complications
- Provides stable vision
- Increases patient satisfaction
