Unit 1 - Contact lens ll | 6th Semester Bachelor of Optometry

 

Soft Contact Lens (SCL) Materials & Review of Manufacturing Techniques

Introduction

Soft Contact Lenses (SCLs) have revolutionized vision correction and ocular therapeutics. Their development and clinical success largely depend on the properties of the materials used and the techniques involved in their manufacture. Choosing the appropriate material for a specific patient depends on multiple factors including oxygen permeability, water content, flexibility, surface wettability, durability, and patient comfort. Over the years, lens materials and production methods have evolved to meet the growing demands of performance, safety, and affordability.

Classification of Soft Contact Lens Materials

Soft contact lenses are primarily made from hydrogel or silicone hydrogel materials. Each has distinct properties and applications:

1. Hydrogel Lenses

These lenses are composed of hydrophilic (water-attracting) polymers, mainly poly-2-hydroxyethyl methacrylate (PHEMA). They absorb water to become soft and pliable, making them comfortable to wear. However, their oxygen permeability depends on water content, and lenses with higher water content tend to dry out quickly.

• Examples: HEMA, polyvinylpyrrolidone (PVP), N-vinyl pyrrolidone (NVP)
• Water content: Ranges from 38% to 79%
• Uses: Daily wear, cosmetic lenses, planned replacement lenses

2. Silicone Hydrogel Lenses

Silicone hydrogel materials combine silicone’s high oxygen permeability with hydrogel’s comfort. Silicone is inherently hydrophobic, so surface treatments or internal wetting agents are used to improve wettability. These lenses allow much more oxygen to pass through to the cornea than traditional hydrogel lenses.

• Examples: Lotrafilcon A, Balafilcon A, Senofilcon A
• Oxygen permeability (Dk): Up to 175 x10^-11 cm²/sec (mL O₂/mL x mmHg)
• Applications: Extended wear, therapeutic lenses

3. Water Gradient Materials

These are a newer class of materials where the lens core is silicone hydrogel for high oxygen transmission, and the surface has a gradient increasing in water content for improved comfort.

• Example: Delefilcon A
• Benefit: Combines the best of both silicone hydrogel and hydrogel lenses

4. Surface Treatments and Additives

To enhance lens performance, manufacturers use plasma treatments, internal wetting agents, UV blockers, and antimicrobial coatings (e.g., silver ions or polyquaternium-1).

Ideal Properties of SCL Materials

• High oxygen permeability
• Good mechanical strength and flexibility
• Excellent surface wettability
• Low modulus for comfort
• Biocompatibility with ocular tissues
• Deposit resistance
• Transparency and UV blocking ability (optional)

Review of Manufacturing Techniques

The method of manufacturing impacts the physical properties, durability, reproducibility, and comfort of the final lens. The most commonly used techniques include:

1. Lathe Cutting

This is a precision process in which a solid lens button is placed on a computer-controlled lathe and shaped into the desired curvature and thickness. The lens is then hydrated (if it's a hydrogel material) and polished.

• Advantages: Custom lens designs, high precision
• Disadvantages: Labor-intensive, higher cost
• Used for: Specialty lenses, toric/multifocal lenses, keratoconus lenses

2. Spin Casting

In this method, a liquid monomer is introduced into a spinning mold. The centrifugal force spreads the monomer into a thin film with a predictable shape, and the lens is then polymerized with UV or heat.

• Invented by: Otto Wichterle in the 1960s
• Advantages: Simple, fast, good for mass production
• Disadvantages: Limited to spherical shapes, less design flexibility
• Common in: Daily disposable lenses

3. Cast Molding

This is currently the most widely used method for producing soft disposable lenses. It involves injecting liquid monomer between two molds, curing it with UV or heat, then removing, hydrating, and packaging the lens.

• Advantages: Mass production, high reproducibility, low cost
• Disadvantages: Initial tooling cost is high
• Best for: Large-scale manufacturing of disposable lenses

Steps in the Contact Lens Manufacturing Process

1. Material preparation: Mixing the monomers and initiators
2. Lens forming: Lathe cutting, spin casting, or cast molding
3. Polymerization: Using heat or UV to solidify the lens
4. Hydration: For hydrogel lenses, soaking in saline to absorb water
5. Edge finishing: Polishing or plasma treatment for comfort
6. Inspection: Optical, mechanical, and cosmetic checks
7. Sterilization: Autoclaving in blister packs with saline
8. Packing and labeling: With batch number and expiry

Recent Innovations

• Antimicrobial coatings: Reduce infection risk
• Drug-eluting lenses: Deliver medications to the eye
• Smart lenses: Monitor intraocular pressure or glucose levels
• Water gradient designs: Improve comfort and oxygen flow

Conclusion

Understanding the material science and manufacturing techniques behind soft contact lenses is crucial for optometrists, ophthalmologists, and contact lens practitioners. With continuous advancements in polymer chemistry, fabrication processes, and ocular health technologies, contact lenses are becoming more versatile, comfortable, and safer for extended use. Appropriate selection of material and lens design leads to better patient compliance, reduced complications, and improved quality of life.

References

1. Phillips AJ, Speedwell L. Contact Lenses, 5th Edition. Butterworth-Heinemann, 2007.
2. Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 3rd Edition. Lippincott Williams & Wilkins, 2008.
3. Millis EAW. Medical Contact Lens Practice. Butterworth-Heinemann, 2005.
4. ICL and CLAO modules for professional optometry education.

Comparison of Rigid Gas Permeable (RGP) vs Soft Contact Lenses (SCL)

Introduction

Contact lenses have been a transformative solution for vision correction, offering alternatives to glasses and surgical interventions. Among the major types of lenses, Rigid Gas Permeable (RGP) and Soft Contact Lenses (SCL) are the most widely used. Each has distinct materials, fitting philosophies, comfort levels, advantages, and disadvantages. Understanding the comparative features of RGP and SCL is crucial for clinicians to select the appropriate lens type based on a patient’s ocular condition, lifestyle, and visual needs.

Basic Material Differences

Soft Contact Lenses (SCL)

SCLs are made of hydrophilic polymers like hydrogel or silicone hydrogel. They absorb water and become flexible, providing comfort and conformity to the corneal surface.

• Materials: HEMA, silicone hydrogel (e.g., lotrafilcon, senofilcon)
• Water content: 30% to 79%
• Oxygen permeability depends on water content or silicone content

Rigid Gas Permeable Lenses (RGP)

RGP lenses are made from durable, oxygen-permeable plastics that retain their shape. They do not absorb water and are smaller in diameter compared to soft lenses.

• Materials: Fluorosilicone acrylate, silicone acrylate
• High oxygen transmission despite no water content
• Customizable optical design

Fitting and Adaptation

Soft Contact Lenses

• Fit conformally over the cornea
• Available in standard base curves and diameters
• Minimal adaptation time – comfort is immediate
• Suitable for beginners or occasional users

Rigid Gas Permeable Lenses

• Rest on the tear film and vault over the cornea
• Require precise fitting and corneal measurements
• Adaptation period of several days to weeks due to initial discomfort
• Better for long-term, regular users

Optical Performance

SCL

• Soft lenses follow the irregularities of the cornea
• Good vision in normal corneas but not ideal for irregular corneas
• May cause slight image instability due to movement with blinking

RGP

• Retain shape over irregular cornea, neutralizing astigmatism
• Offer superior visual acuity, especially in keratoconus and post-surgical eyes
• Stable optics due to minimal movement

Oxygen Transmission

Oxygen transmission is crucial for corneal health and avoiding hypoxia.

Parameter	Soft Lenses	RGP Lenses
Dependence on water content	Yes (Hydrogel)	No
Silicone hydrogel option	Yes	Not applicable
Oxygen transmission (Dk)	20–175 depending on material	40–160 with modern materials
Overall corneal oxygenation	Lower due to larger diameter	Higher per unit area due to small lens size

Comfort and Handling

Soft Contact Lenses

• Highly comfortable due to water content and flexibility
• Easier to adapt for new users
• Easy to dislodge if the eye is rubbed
• Less durable – prone to tearing

RGP Lenses

• Less comfortable initially – require adaptation
• Smaller in size and less prone to drying
• More durable and long-lasting
• Better handling characteristics once mastered

Maintenance and Hygiene

SCL

• Available in daily, bi-weekly, and monthly disposable options
• Protein and lipid deposits can accumulate on extended wear lenses
• Risk of microbial keratitis if hygiene is poor
• Need proper disinfection and storage regimen

RGP

• Require daily cleaning with specific RGP solutions
• Less prone to deposits compared to SCLs
• Less bacterial adherence due to non-porous material
• Longer lens life (1–2 years with proper care)

Applications and Indications

SCL

• Routine refractive errors – myopia, hyperopia, astigmatism, presbyopia
• Daily wear, sports use, part-time wear
• Cosmetic lenses – colored, novelty
• Bandage lenses for epithelial healing

RGP

• High or irregular astigmatism
• Keratoconus and post-LASIK ectasia
• Myopia control and orthokeratology
• High visual demands requiring sharp acuity

Advantages and Disadvantages Summary

Feature	Soft Contact Lens	RGP Lens
Initial Comfort	Excellent	Poor to Moderate
Adaptation Time	Minimal	Several days to weeks
Visual Acuity	Good	Excellent
Oxygen Transmission	Moderate to High	High
Handling & Durability	Delicate, easy to damage	Durable, long-lasting
Cost per lens	Lower per lens, higher in long-term (due to disposables)	Higher per lens, but cost-effective long term

Recent Advances

• SCLs with water gradient materials for better comfort
• Hybrid lenses combining RGP center with SCL skirt
• Orthokeratology with reverse-geometry RGP lenses for myopia control
• Daily disposable SCLs for safer, more hygienic wear

Conclusion

Both RGP and Soft Contact Lenses have distinct roles in vision correction. While SCLs offer ease, comfort, and convenience, RGP lenses provide superior visual clarity and customization for complex corneal conditions. The choice between the two depends on individual patient needs, visual goals, ocular health, and lifestyle. As a contact lens practitioner, evaluating the pros and cons of each option is essential for achieving optimal patient outcomes and satisfaction.

References

1. Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 3rd Ed. Lippincott Williams & Wilkins, 2008.
2. Phillips AJ, Speedwell L. Contact Lenses, 5th Edition. Butterworth-Heinemann, 2007.
3. Millis EAW. Medical Contact Lens Practice. Butterworth-Heinemann, 2005.
4. ICLE Modules and CLAO educational resources.

Pre-fitting Considerations for Soft Contact Lenses (SCL)

Introduction

Before fitting a soft contact lens (SCL), a comprehensive evaluation must be performed to ensure safety, comfort, effectiveness, and long-term success. Pre-fitting considerations involve a thorough ocular and systemic history, clinical examinations, lifestyle analysis, and selection of appropriate lens parameters. Understanding the patient’s needs, refractive status, ocular surface health, and tear film condition is essential to avoid complications and enhance patient satisfaction.

1. Patient History

Taking a detailed history is the first step in the pre-fitting process. This includes:

• Ocular History: Past eye diseases, surgeries (e.g., LASIK, cataract), dryness, allergies, previous lens wear experiences
• Systemic History: Diabetes, autoimmune disorders, thyroid problems, and medications that affect the eyes (e.g., antihistamines, isotretinoin)
• Occupational and Lifestyle History: Screen time, outdoor exposure, cosmetic use, swimming habits, sports or night driving
• Motivation for Lens Wear: Cosmetic, convenience, refractive correction, sports, or therapeutic purposes

2. Refractive Status

Accurate subjective and objective refraction is critical. Consider:

• Myopia and Hyperopia: Most SCLs are available in a wide range of spherical powers.
• Astigmatism: If cylinder >0.75D, toric SCLs should be considered for better vision.
• Anisometropia: Contact lenses can provide better aniseikonia correction than spectacles.
• Presbyopia: Options include multifocal SCLs, monovision, or modified monovision.

3. Keratometry and Corneal Topography

Although soft lenses conform to the corneal shape, base curve selection depends on corneal curvature.

• Keratometry: Measures average curvature in two principal meridians (steep and flat). Helps select base curve.
• Corneal Topography: Useful in detecting corneal irregularities, asymmetry, or diseases like keratoconus which may require alternative lens types.
• Flat K (K value < 42.00D): May require flatter base curve
• Steep K (K value > 45.00D): May need steeper base curve or alternate lens material/design

4. Evaluation of Tear Film

The tear film plays a vital role in contact lens success. It provides lubrication, oxygen, and debris removal.

• Tear Break-Up Time (TBUT): Normal >10 seconds. Short TBUT indicates tear film instability.
• Schirmer’s Test: Evaluates tear volume. Less than 10 mm in 5 mins may suggest dry eye.
• Lid margin and meibomian gland assessment: Rule out meibomian gland dysfunction (MGD).
• Staining with fluorescein or lissamine green: Detects ocular surface damage or dryness

5. Eyelid and Blink Assessment

Eyelids and blinking patterns affect lens centration, stability, and tear film renewal.

• Lid Tone: Loose or tight lids may affect lens movement
• Lagophthalmos: Incomplete closure during blinking can lead to lens drying
• Blink Rate: Normal is about 12–15 blinks/min. Reduced blink rate in digital device users may cause lens discomfort

6. Corneal and Conjunctival Health

• Check for signs of neovascularization, pannus, scarring, pinguecula, pterygium
• Examine the bulbar and palpebral conjunctiva for papillae, follicles, and inflammation
• Assess for limbal hyperemia and conjunctivochalasis

7. Pupil Size

Measuring pupil size in ambient and dim light is important for multifocal and cosmetic lenses.

• Small pupils: May limit visual performance with multifocal optics
• Large pupils: May cause halos or glare, especially in low-light conditions

8. Lens Material and Modality Selection

Based on clinical findings, choose suitable lens material and modality:

• Hydrogel: For good comfort in normal eyes, limited oxygen permeability
• Silicone Hydrogel: High oxygen permeability, better for long wear or dry eyes
• Daily Disposable: Ideal for occasional wearers, allergy sufferers, and hygiene-conscious users
• Monthly/Bi-weekly: Cost-effective for full-time wearers; requires proper care

9. Vertex Distance Compensation

Especially important for high refractive errors (±4.00 D and beyond). The spectacle refraction needs to be adjusted for the corneal plane.

• Minus lenses: Decrease the power slightly when converting to contact lens power
• Plus lenses: Increase the power slightly

10. Trial Lens Selection

Trial fitting should be done with lenses closest to the calculated parameters.

• Choose appropriate base curve and diameter
• Observe lens centration, movement, and post-blink recovery
• Evaluate vision, comfort, and corneal coverage

11. Patient Education and Consent

• Educate about lens care, hygiene, insertion/removal techniques
• Explain wearing schedule and follow-up importance
• Warn about signs of complications: redness, pain, photophobia, decreased vision
• Obtain informed consent regarding benefits and risks

12. Contraindications to SCL Wear

• Active eye infection (e.g., conjunctivitis, keratitis)
• Severe dry eye syndrome
• Uncontrolled diabetes or immunosuppression
• Poor compliance or hygiene habits
• Allergy to lens materials or solution preservatives

Checklist for Pre-fitting Evaluation

1. Complete medical and ocular history
2. Refraction and visual needs analysis
3. Keratometry/topography
4. Tear film evaluation (TBUT, Schirmer)
5. Lid tone and blink analysis
6. Pupil size measurement
7. Corneal and conjunctival health check
8. Lens material and modality decision
9. Trial lens application and assessment
10. Patient instruction and counseling

Conclusion

Pre-fitting considerations for soft contact lenses form the cornerstone of a successful contact lens practice. A thorough clinical evaluation ensures that the lenses not only improve vision but also maintain ocular health and patient satisfaction. Customizing the lens choice based on individual anatomical, physiological, and lifestyle needs minimizes complications and maximizes long-term comfort and compliance. This foundational process is essential for every new wearer and for re-evaluating existing wearers experiencing discomfort or complications.

References

1. Bennett ES, Henry VA. Clinical Manual of Contact Lenses, 3rd Ed. Lippincott Williams & Wilkins, 2008.
2. Phillips AJ, Speedwell L. Contact Lenses, 5th Ed. Butterworth-Heinemann, 2007.
3. Millis EAW. Medical Contact Lens Practice. Butterworth-Heinemann, 2005.
4. ICL and CLAO Modules 1-10 for clinical guidelines