Contact Lens Materials and Evaluation Techniques

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.

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.