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Ophthalmology > Day 5 (3): Contact Lens Fundamentals > Flashcards

Day 5 (3): Contact Lens Fundamentals Flashcards

1
Q

What are the basic contact lens parameters?

A
  1. Type/Material (Rigid Gas Permeable Lens vs Soft Contact Lens)
    - oxygen permeability (RGP > SCL)
    - wettability
    - flexure (RGP > SCL)
    - durability (RGP > SCL)
  2. Base Curve (mm converted to dioptric power)
    - curvature of the central posterior surface of the lens
    - obtained using keratometry
  3. Lens Diameter
    - maximum edge-to-edge width
    - RGP: 9 - 10 mm
    - Soft: 13 - 15 mm
  4. Lens Power (PLUS or MINUS)
  5. Dk value
    - measure of oxygen permeability of material
    - D: diffusion coefficient for oxygen movement in the material
    - K: solubility constant of oxygen
  6. Center thickness (L)
    - distance from anterior to posterior surface of central part of lens
  7. Dk/L
    - measure of oxygen transmissibility
    - the higher the diffusion coefficient and the solubility constant or the thinner the lens, the lens becomes more oxygen transmissible
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2
Q

Advantages of RGP lenses.

A
  • More durable
  • Higher oxygen permeability
  • Resistance to deposits
  • Suited for pts with DED or tear film irregularities
  • Able to correct moderate amounts of corneal astigmatism due to “tear lens” or the tear reservoir which forms in between the posterior surface of the contact lens and the anterior surface of the cornea
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3
Q

What are the steps when fitting for SCL?

A
  1. Base curve
  2. Lens diameter
  3. Lens power
  4. Check the fit
    - special tests: keratometry (mires), retinoscopy
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4
Q

What are the steps when fitting for RGP lenses?

A
  1. Base curve
  2. Diameters
    - lens diameter
    - optical zone diameter
    - palpebral fissure: for lens positioning
  3. Lens power
  4. Do over-refraction using manifest/subjective refraction
  5. Check the fit
    - using diagnostic trial lens set
    - special test: fluorescein pattern under slip-lamp examination
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5
Q

How is the base curve obtained?

A

Formula:

Lens Power/K (D) = (N2 - N1)/Radius of Curvature or Base Curve

  • N1: refractive index of 1st medium (always air) = 1.000
  • N2: refractive index of 2nd medium (always cornea) = 1.3375
  • denotes the power of the refracting surface
  • relates radius of curvature of a lens using the (flattest/lower) K reading and the refractive index difference between the surfaces
  • imagine the arc/curve as part of a circle: a more curved arc means it’s part of a circle thats smaller while a flatter arc means it’s part of a larger circle.
  • the LARGER the radius of curvature, the LOWER the power, the FLATTER the curve,

SCL:
Final Base Curve = FLATTEST Base Curve + 0.7 mm
- fit FLATTER than the FLATTEST K or HIGHEST base curve

RGP Lenses:
Final Base Curve = FLATTEST Base Curve
- fit ON the FLATTEST K or HIGHEST base curve
- NO need to add 0.7 mm

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6
Q

How is the lens diameter determined?

A

Horizontal Visible Iris Diameter (HVID)

  • approximates size of the corneal diameter
  • lenses are not fitted exactly the same size as the cornea to allow lens movement without limbal impingement

Formula:
SCL diameter = HVID + 2 mm
- paralimbal (larger than cornea)

RGP lens diameter = HVID - 2 mm

  • intracorneal (smaller than cornea)
  • should contain Optical Zone Diameter + Peripheral Curves
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7
Q

What is the optical zone diameter?

A
  • Measured only if using RGP lenses
  • Use base curve (mm) as estimate
  • If OZD too small: (+) flare or haloes around the lens in the periphery
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8
Q

How is the lens power obtained?

A
  1. Express prescription in MINUS cylinder form
    - if cyl < 1/4 of sph: disregard cyl power (negligible effect)
  2. Separate spherocylindrical prescription into the two component cylindrical prescriptions (CRAC)
    - C (1st): Copy the sph power
    - R (1st): Reverse cyl axis
    - A (2nd): Add cyl to sph power while following the signs
    - C (2nd): Copy cyl axis
  3. Correct the component cyl prescriptions for the vertex distance if necessary (CAP FAM)
    - if high myope/hyperope (> 4.00 D): correct
    + Closer (Spectacle –> Contact Lens): Add PLUS power
    + Farther (Contact Lens –> Spectacle): Add MINUS power
    - if < 4.00 D: no need to correct (negligible effect)
  4. Combine the corrected component cyl prescriptions into the final spherocylindrical contact lens prescription (CSC)
    - C: Copy/use the more plus power (or less minus) as sph
    - S: Subtract sph power from other power while following signs to yield the cyl power
    - C: Copy/use axis of the other power

If using RGP lenses:
5. Adjust final sph power for base curve compensation
- If power ~ K: no adjustment required
- If power > K (steeper): creates a PLUS tear reservoir (apical/central pooling)
- If power < K (flatter): creates MINUS tear reservoir
(peripheral pooling)

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9
Q

What is the spectacle vertex distance and how is it corrected in contact lens prescriptions?

A
  • Distance of anterior surface of cornea from the posterior surface of the spectacle lens
  • N: 10 - 12 mm
  • Vergence = 1/Distance means that the distance between the spectacle and cornea (or the lack thereof between contact lens and cornea) has implications in the overall power of the system

Formula: Power adjustment = D^2/1000 x Vertex Distance (mm)

  • only done if power > 4.00 D
  • if power < 4.00 D: no need to correct because effect is negligible
  • adjustments are made in increments or decrements of 0.25 D
  • computed value rounded up or down to nearest multiple of 25
  • CAP FAM: Closer Add Plus, Farther Add Minus

Spectacle –> Contact Lens

  • moved CLOSER to cornea (vertex distance is eliminated because CL is adjacent to cornea)
  • convergence of rays is moved POSTERIOR to the retina –> hyperopic
  • add PLUS power (or make LESS MINUS) to focus light forward back into the retina

Contact Lens –> Spectacle

  • moved FARTHER from cornea
  • need to account for the vertex distance
  • convergence of rays moved ANTERIOR to retina –> myopic
  • add MINUS power (or make LESS PLUS) to focus light posteriorly into the retina
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10
Q

Indications of a well-fitted contact lens.

A
  1. Constant, clear and stable vision
  2. Comfortable even with prolonged use
  3. Slit Lamp
  • Sufficient movement on blinking (1 - 2 mm movement)
    + important for tear exchange and oxygen supply to the cornea
    + > 2 mm: loose (too flat)
    + 0 - 1 mm: tight (too steep)
  • Proper centration on blinking
    + avoids prismatic effect
    + NOT centered: loose (too flat)
  • If using RGP lenses: Fluorescein pattern
    + Minimal apical clearance: good fit
    + Peripheral dye pooling: flat/loose (lens power < K)
    + Central/apical dye pooling: steep/tight (lens power > K)
    + Dumbbell-shaped: (+) astigmatism

If using SCL:

  1. Retinoscopy
    - Sharp red reflex with NO distortion
    - (+) central distortion: tight
  2. Keratometry
    - Clear mires
    - (+) Blurred on blinking: loose
  3. Shape: BOWL-like
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11
Q

Signs and symptoms of LOOSE-fitting contact lens.

A
  • Too much movement (> 2 mm)
  • Uncentered with blinking
  • Variable/fluctuating vision
  • Bothersome awareness of the lens
  • (+) Bubbles in lens edge
  • SCL: Keratometry mires that blur on blinking then clear up after
  • RGP: Peripheral fluorescein dye pooling on slit lamp
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12
Q

Signs and symptoms of TIGHT-fitting contact lens.

A
  • No or restricted movement (0 - 1 mm)
  • Variable/fluctuating vision
  • Discomfort with prolonged use
  • Burning sensation
  • Ciliary injection
  • SCL: Central distortion in red reflex using retinoscope streak
  • RGP: Central/apical fluorescein dye pooling on slit lamp
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13
Q

How to adjust ill-fitting contact lens?

A

IDTS: Increase Diameter to Tighten or Steepen

  • cornea is prolate: steeper centrally and flatter peripherally
  • lens power/K reading is inversely proportional to base curve and radius of curvature
  • increasing CL diameter: more of the cornea is covered = tighter fit
  • decreasing CL diameter: less of the cornea is covered = looser fit

If loose/too flat:

  1. Increase diameter OR
  2. Decrease base curve/radius of curvature to steepen

If tight/too steep:

  1. Decrease diameter OR
  2. Increase base curve/radius of curvature to flatten
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14
Q

Summary of differences between SCL and RGP lens.

A

SCL

  • Fit flatter than flattest K (BC + 0.70 mm)
  • Lens is paralimbal (Diameter = HVID + 2 mm)
  • Choose a high Dk value (less oxygen permeable vs RGP)

RGP

  • Fit on flattest K (DON’T add 0.70 mm)
  • Lens is intracorneal (Diameter = HVID - 2 mm)
  • More durable
  • Higher oxygen permeability (naturally higher Dk value)
  • Need to do over-refraction
  • Need to check fit using diagnostic trial lens set
  • Need to evaluate fluorescein pattern under slit-lamp
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Ophthalmology (44 decks)

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