Contact Lens Materials and Evaluation Techniques

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Questions and Answers

What is the average blink rate in a healthy individual?

24 blinks per minute (correct)

34 blinks per minute

20 blinks per minute

10 blinks per minute

Which of the following is a non-invasive technique for assessing the tear layer?

Break-Up-Time (BUT)

Schirmer test

Lissamine Green staining

Tear prism height (correct)

Which condition is NOT considered when assessing general health in relation to refraction?

Pregnancy

Hobbies (correct)

Systemic medications

Ocular conditions

Which invasive assessment technique is specifically used to evaluate tear break-up time?

Break-Up-Time (BUT) Signup and view all the answers

What should be considered as part of the final evaluation for trial fitting of contact lenses?

All of the above Signup and view all the answers

What is a primary feature of silicone acrylate materials used in lenses?

Rigid gas permeability Signup and view all the answers

Which property of Fluoro-Siloxane Acrylates improves their performance over regular silicone acrylate?

Lower surface charge Signup and view all the answers

What characteristic do soft contact lens materials primarily share?

Hydrophilic properties Signup and view all the answers

How are synthetic elastomers distinct from other contact lens materials?

They exhibit rubber-like behavior Signup and view all the answers

What is the significance of having a high Dk value in contact lens materials?

Enhances oxygen permeability Signup and view all the answers

Which of the following materials does NOT belong to the soft contact lens materials category?

PMMA Signup and view all the answers

What type of polymer base is used for thermoplastics in contact lens material?

Flexible polymer Signup and view all the answers

What is a notable benefit of high water content lenses?

Excellent oxygen permeability Signup and view all the answers

What characterizes mid water contact lenses?

They have a good combination of both high and low water materials Signup and view all the answers

Which manufacturing process uses liquid polymer injected into a spinning mold?

Spincasting Signup and view all the answers

What is the average corneal thickness as noted in clinical studies?

0.50 – 0.65 mm Signup and view all the answers

What does the refractive index calibration for the cornea typically use?

1.3375 Signup and view all the answers

What is a primary disadvantage of using photokeratoscopes?

Limited field of view due to nose or orbit anatomy Signup and view all the answers

What is the formula to compute the power of the cornea based on the radius?

$D = 337.5 / r$ Signup and view all the answers

Which method is used to assess the peripheral curvature deviation in corneal asphericity?

Limbal topography observation Signup and view all the answers

What is a characteristic of the cornea's apical area?

It has the shortest radius Signup and view all the answers

What is the purpose of the slit-lamp examination during the preliminary examination?

To assess the anterior segment of the eye Signup and view all the answers

Which of the following is NOT part of the ocular dimensions measurement?

Lens fit evaluation Signup and view all the answers

How is lid tension graded according to Swarbrick & Holden?

From +3 (very tight) to -3 (very loose) Signup and view all the answers

What does HVID and VVID measure in ocular dimensions?

Horizontal and vertical corneal diameters Signup and view all the answers

Which grading system is NOT mentioned for ocular health assessment?

Johnson Grading System Signup and view all the answers

Which measurement is important for evaluating corneal curvature?

Degree of internal astigmatism Signup and view all the answers

What is typically assessed during biomicroscopy in contact lens fitting?

Lens fit and anterior eye changes Signup and view all the answers

Which structure is NOT included in the preliminary examination of the anterior segment?

Retina Signup and view all the answers

Why is basal measurement important before contact lens fitting?

To establish an eye health baseline Signup and view all the answers

In terms of lid positions and shape, how do the measurements differ for Asians?

Lid aperture is smaller with a more acute angle Signup and view all the answers

What is the primary characteristic of aphakia?

It is regarded as a refractive condition. Signup and view all the answers

What does the term 'CLM' represent in relation to retinal image sizes?

Comparison of contact lens correction versus spectacle lens correction. Signup and view all the answers

Which statement is true regarding the impact of switching from spectacles to contact lenses for myopes?

It may precipitate the need for a near correction. Signup and view all the answers

What is a significant optical advantage of contact lenses compared to spectacles?

They offer a wider field of view. Signup and view all the answers

What defines the concept of convergence in relation to hyperopia?

It influences the amount of near correction needed. Signup and view all the answers

What is a common disadvantage of using contact lenses?

Moving lenses may cause disturbances of vision. Signup and view all the answers

Which condition would cause meridional aniseikonia when using spectacle lenses?

High corneal astigmatism. Signup and view all the answers

What consequence does the prismatic effect have when the line of sight does not align with the optical center of a lens?

It can result in visual discomfort and double vision. Signup and view all the answers

What is the result of lens decentration in contact lenses?

It produces a phenomenon known as 'ghosting'. Signup and view all the answers

What does Prentice's Rule help to calculate?

The prismatic effect caused by lens decentration. Signup and view all the answers

Study Notes

Contact Lenses

Defined as a thin medical device placed directly on the eye's surface, used to correct vision.

History of Contact Lenses

Leonardo da Vinci (1508): First concept of a lens in contact with the eye.
John Herschel (1827): Described a glass contact lens shaped to match the eye.
F.E. Muller (1887): Fitted a blown glass lens to an eye removed surgically.
A.E. Fick (1888): Described the first contact lens intended for vision correction.
Muller and Obrig (1938): Created the first plastic scleral contact lens.
E. Kalt (1940s): Designed and fitted glass corneal contact lenses. All available contact lenses at that time were scleral-type.
Kevin Touhy (1947): Created the first plastic corneal lens from PMMA.
Otto Wichterle: Developed HEMA.

Classification of Contact Lenses

By Material

Hard: Non-gas permeable, Gas permeable
Soft:
- Conventional
- Disposable

By Design

Monocurve
Bicurve
Tricurve
Aspheric

By Size

Corneal
Semi-corneal
Scleral

By Mode of Use

Daily Wear:
- Disposable
- Planned replacement
Extended:
- Continuous wear
- Flexible wear

By Purpose or Use

Optical:
- Spherical
- Toric
- Presbyopic
Therapeutic
Cosmetic

Spherical Contact Lens

Used to correct hyperopia, myopia, aphakia, and a small amount of astigmatism.
Available in a wide range of corrections (-20.00D to +20.00D).

Toric Contact Lens

Used to correct significant astigmatism.

Presbyopic Correction

Monovision/Monofit: One eye fitted for distance, the other for near vision.
Bifocal soft contact lens: Alternating vision for distance and near through different portions of the lens.
Multifocal: Patient views through both distance and near portions simultaneously.

Tinted Contact Lenses

Visibility tints (effective on lighter colored eyes).
Makes handling easier.
Opaque colored lenses: Change the apparent color of the eye.

Therapeutic (Bandage) Lenses

Used to promote healing of different corneal disease conditions.

Modern Contact Lenses

Ortho-K contact lenses: Gas-permeable lenses that temporarily reshape the cornea to correct refractive errors (myopia, hyperopia, astigmatism).
Hybrid contact lenses: Large-diameter lenses with a rigid gas-permeable central zone, surrounded by a peripheral zone made of soft or silicone hydrogel material.
Wink-activated telescope contact lens: 1.55mm thick, extremely thin reflective telescope, activated by eye blinks (magnifies 2.8x).
Google contact lens (prototype): Monitors glucose levels in tears.
Triggerfish contact lens (SENSIMED): Embeds a micro-sensor that measures intraocular pressure.
iOptik contact lens: Enhances vision by allowing light from the display to pass through the center of the pupil, while light from the surrounding environment passes through the outer portion of the pupil.
Terminator Eyes contact lens: Shows images like road directions and text messages from smartphones.

Contact Lens Parameters

Base Curve (BC)

Radius of curvature of the posterior zone.

Optic Zone (OZ)

Posterior central optic portion of the lens, indicating the usable portion.
SCL: 7-12 mm
RGP: 7-9 mm

Lens Diameter (LD)

Maximum external dimension of the lens.
- SCL: 13-16 mm
- RGP: 8-20 mm

Posterior Peripheral Curve Radius (PPCR)

Radius of the peripheral curve.

Peripheral Curve Width (PCW)

Width of the posterior peripheral curve portion of the lens.

Sagittal Value (Sag)

Distance from the line between the outer edges of the lens to the center of the back surface.

Optic Cap (OC)

Central anterior optic portion of a lens.

Optic Cap Radius (OCR)

Radius of the central anterior optic portion of a lens.

Anterior Peripheral Curve Zone (APCZ)

Anterior peripheral or lenticular portion of the lens.

Lens Thickness (CT)

Central Thickness (CT)
- SCL: 0.035-0.2 mm
- RGP: 0.1–0.2 mm
Edge Thickness (ET)
- SCL: 0.01-0.05 mm
- RGP: 0.08 -0.12 mm

Properties of Contact Lens Materials

Oxygen Performance

Lens acts as a barrier to atmospheric oxygen.
Corneal physiological processes compromised without enough oxygen.
Short-term problems: Central Corneal Clouding, Vertical striae, Microcysts, Epithelial and stomal thinning.
Long-term problems: Corneal distortion, Disruption of endothelium, Changes in refractive status of the eye.

Dk (Oxygen Permeability)

Quality of material allowing oxygen to travel through it.
- D = diffusion coefficient, k = solubility coefficient
Higher water content = higher the Dk
Low Dk = less than 30
Medium Dk = 30-60
High Dk = higher than 60

Dk/t (Oxygen Transmissibility)

States how much oxygen passes through the lens.
Thinner lens = more oxygen passing through.
25 for daily wear, 87 and above for overnight wear

EOP (Equivalent Oxygen Permeability)

Describes oxygen flux through contact lens like corneal response.
Measurement of the cornea's oxygen thirst following lens wear.
Assesses various lens thicknesses to compare performances.

Corneal Swelling

Most valid indicator of real-life clinical performance. Measures the percent increase in corneal thickness.

Tensile Properties

Measures the physical tensile properties of a material.
- Tensile strength: Force applied before breakage.
- Modulus of elasticity: Flexibility/stretchability.
- Coefficient of elongation: Stretching percentage before breakage.
- Tear strength: Force causing a tear in material.

Water Content

Higher water content lenses provide more oxygen to the cornea.
High-water-content materials lose more water.

Refractive Index

Defines how light rays are affected passing through a material.
Important for achieving the proper optical effect on the eye.
Higher water content = lower refractive index.
Requires thicker lens design for lower refractive indices.

Biocompatibility

Contact lenses interact with corneal tissue.
Importance of assessing physiological balance.
Several different tests are used for evaluation:
- 21-Day Rabbit Test
- Resistance to microbial growth
- Compatibility of the lens material with care systems

Protein Deposits

During development, lens materials are evaluated for protein uptake.
High water content lenses absorb more protein.

Dimensional Stability

Assessed by measuring water content, sagittal value, power, thickness, and base curve.
Ensures consistent lens performance.

Hydrolytic Stability

Since contact lenses are placed in the tear film (primarily water), the hydrolytic stability of the material is assessed.
Only materials stable in water are suitable for contact lenses.

Lens Handling

Handling eases with more rigid lens materials.

Wettability

Characteristic of lens performance important to RGP materials.
Soft materials are inherently hydrophilic.
RGP materials (silicone) can act as water repellents.
Wettability is tested by measuring the wetting angle.
Lower wetting angle (closer to 0) indicates higher wettability, while higher wetting angles indicate lower wettability.

Rigid Contact Lens Materials

Material primarily used for rigid lenses: PMMA.
Stable
Resistant to warpage; wets well and clean easily.
Lack of permeability to oxygen and tear exchange phenomena.

Gas Permeable Lens Materials

Cellulose acetate butyrate (CAB): Low oxygen permeability.
Silicone acrylate: Combined with PMMA; high Dk values; Introduced in 1970.
Fluor-siloxane acrylates (FSAs): Higher Dk values with enhanced wettability, but risk of scratching.
Perfluoroethers: High Dk.

Soft Contact Lens Materials

PHEMA is similar to PMMA with added hydroxyl groups, making it hydrophilic.
Water content is ~38%
Other variants exist to further improve PHEMA.

Ideal Contact Lens Material Properties

Needs to meet corneal oxygen requirements.
Needs to be physiologically inert.
Needs optimal in vivo wetting properties.
Needs to be dimensionally stable.
Needs to be durable, optically transparent, and require minimal patient care; Needs to be easily machineable.

Important Material Properties

Oxygen permeability
Wettability
Scratch resistance
Rigidity (for rigid gas-permeable lenses)
Flexibility (for soft contact lenses)
Durability
Deposit resistance

Optical Properties

Refractive index
Spectral transmission
Dispersion

Corneal Topography: Measurement and Significance

Normal Corneal Dimensions and Topography:
- HVID (horizontal visual diameter): 10-14 mm, average 11.7 mm
- VVID (vertical visual diameter): 10.6 mm, smaller than HVID (0.5-1.0 mm)
Corneal Thickness:
- Clinical studies: 0.50-0.65 mm
- Gullstrand's Eye No. 1: 0.50 mm
Variations in Central & Peripheral Thickness
Corneal Measurement: Pachometers (ultrasonic, beam-splitting device on slit-lamp)
Corneal Curvature: Characteristics of the central corneal region, Peripheral portion (limbus) and Limbal topography influencing SCL fitting.
Corneal Topography Asphericity: Cornea is aspheric, apex area has shortest radius, Deviation of peripheral curvature from apical curvature called asphericity, Prolate (normal), Oblate (laser surgery or ortho-k)
Uses: Estimation of refractive errors, Assessment of pathology, Contact lens fitting, effects of contact lenses and refractive surgery.
Instrumentation: Optical (photokeratoscope, placido disc, keratometer), Computer assisted tomography, Contact methods (casting and molding, ultrasound, trial contact lenses)
Photokeratoscope Images:
- Patterns depicting astigmatism, scars, irregularities, and keratoconus.

Disadvantages of Photokeratoscopy

Corneal astigmatism is not quantified.
Absence of central-mire reflection may limit field size.

Keratometers

Measure the radius of curvature of the optic cap (3-4 mm).
Measures the total power of the cornea.
Types: 2-position, 1-position.

Contact Lens Nomenclature

Contact lens materials are broadly classified as hydrophilic and hydrophobic.
Materials with high water percentage are categorized with "-filcon" name.

Contact Lens Material Categorization

Based on ionic or non-ionic nature, as well as water content: Materials with high water percentage are "-filcon" materials, while "-focon" are those with low-wat content.

Ionic vs Non-Ionic Contact Lens

Ionic:

interact with polar molecules (water)
increase lens hydration
more sensitive to changes in PH and osmolality
Greater interaction with ocular environment and increase in protein uptake. Non-ionic:
interact with water without a formal charge
do not greatly influence lens hydration

Low Water Content Lenses

Provide excellent physiological response for patients with refractive errors.
Compatible with most lens care systems.
Lower protein uptake tendencies which leads to longer lens life. Higher tensile strength.
Exhibit good material stability.
Don't absorb preservatives (no discoloration).

High Water Content Lenses

High oxygen permeability; excellent for thicker high plus/minus lenses.
Lower tensile strength.
Not compatible with all lens care systems.
Higher incidence of acute red eye and lens discoloration. Usually produced by lathe cut or cast molding process.

Mid Water Content Lenses

Typically ionic or non-ionic materials.
Attempt to combine best qualities of high & low water materials.
Good physiology; thin, comfortable designs.
Increased protein uptake is a characteristic.
Usually not compatible with thermal disinfection techniques.

Manufacturing Process

Spincasting
Lathe Cutting
Cast Molding

Optics of Contact Lenses (Correction for Ametropia)

Establishing the appropriate power of contact lens based on manifest refraction result, sphere amount, cylinder amount, sphere-cylinder ratio and vertex distance of refraction.
Spherical equivalent of the contact lens prescription.
Correcting for vertex distance: Formula for contact lens prescription power (DCL) adjusted for vertex distance.

Contact Lens Magnification

Relative size of image seen through contact lenses in comparison to spectacles.
Hyperopes tend to experience a smaller image size in contact lenses than with spectacles. Myopes experience the opposite; a larger image in contact lenses versus spectacles.

Relative Magnification

Helpful specifically for anisometropia.
Aetiology of the ametropia may be unknown in some cases.
K readings may mirror ametropia, suggesting potential refractive issue.

Specialized Considerations

Axial ametropia, refractive ametropia
Aphakia (where IOLs are not implanted- contact lenses are preferred).
High corneal astigmatism (considered refractive ametropia). Spectacle lenses can cause significant meridional aniseikonia.
Incipient Presbyopia: How contact lens use can affect near vision capabilities.
Prismatic effect: Induced if line of sight does not pass through the optical center of the lens.

Convergence in Hyperopia and Myopia

Contact lenses increase convergence difficulty for hyperopes and decrease for myopes.

Optical Advantages of Contact Lenses

Eliminates astigmatism in oblique pencils.
No image distortion.
No chromatic aberration.
No limitations on the field of view.
No spectacle frame diplopia.

Optical Disadvantages of Contact Lenses

Lens decentration can produce "ghosting."
Toric lenses rotating may result in refraction and decreased vision.
Moving lenses can cause vision disturbances.

Routine Preliminary Examination for Contact Lenses

Slit-lamp examination of anterior segment.
Measurement of ocular dimensions.
Assessment of tears (tear flow, volume, break-up time, osmolality, pH).
Spectacle refraction.
Preliminary examination of anterior structures: Eyelids, Conjunctiva, Tears, Cornea, Anterior Chamber, Iris, and Lens, using Ocular Health Grading System.
Biomicroscopy before fitting, during, and after fitting to assess lens fit and identify anterior eye changes.
Other considerations:
- Corneal curvature (central and peripheral keratometry, shape regularity, corneal vs internal astigmatism).
- Corneal measurements (HVID and VVID).
- Palpebral aperture (lids' size and shape).
- Lid tension.
- Blink rate.

Studying That Suits You

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Description

Test your knowledge on contact lenses with this quiz focusing on materials and evaluation methods. Explore the average blink rate, non-invasive assessment techniques, and the distinctions among various contact lens materials. Perfect for students and professionals in optometry or ophthalmology fields.