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Contact Lens I Part 2 > Contact Lens 1-2: Toric SCL Fitting > Flashcards

Contact Lens 1-2: Toric SCL Fitting Flashcards

1
Q

What is a Toric Lens?

A
  1. a Lens w/Different Optical Power and Focal Length in 2 orientations PERPENDICULAR to each other
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2
Q

Patient Selection

  1. Toric SCL: Purpose is to correct what?
  2. As a GENERAL RULE: when do you use a Toric SCL?
  3. What % of Pts wear Astigmatic spectacle Corrections of 0.75 DC or Greater?
  4. Best for what kind of Refractive Error correction?
A
  1. REGULAR Astigmatism
  2. When Spec Rx has equal to or greater than 0.75 DC
  3. 45%
  4. Correct Entire Refractive Error
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3
Q

Rule of Four

  1. When do you use Toric SCL?
A
  1. If cyl power is >25% of the Sphere Power (in glasses), use a TORIC SCL
  2. If cyl power is <25% of the sphere power (in glasses), use a spherical SCL
    * Be aware of the Rule of Four: but for exam purposes…use the General Rule unless specified otherwise
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4
Q

Diagnostic Fitting of Toric SCL

  1. We have to choose a brand first based on what 3 things?
  2. What do we do after that?
A
  1. a. Intended wearing schedule
    b. Parameter Availability
    c. Colored or Clear
  2. Given brand –> One Diameter; Choose BC
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5
Q

Keratometry Readings

  1. Average?
  2. What is considered RELATIVELY FLAT?
  3. What is considered RELATIVELY STEEP?
A
  1. 41.00-45.00 D

2. 45.00 D

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6
Q

Determining CL Power

  1. Put Rx on what?
  2. Vertex any meridian > or equal to what?
  3. Difference b/w 2 meridians?
A
  1. on a Power Cross
  2. +/- 4.00D
  3. Toric CL if > or equal to 0.75 DC
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7
Q

Compensated Power

  1. F CL = ?
A
  1. Fglasses/(1-dF)

(+) d when lens is moved towards the EYE

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8
Q

Spherocylindrical Rx

  1. After Putting power on power cross, vertexing, and determining new Rx for CL, what do you do if Cyl is now equal to or less than 0.50 DC?
A
  1. Take the Spherical Equivalent
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9
Q

Choosing a Diagnostic Lens

  1. 3 things…Prioritized how?
A
  1. AXIS > Cyl Power > Spherical Power
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10
Q
  1. Degree increments that’s common for off-the-shelf Toric Lenses?
A
  1. 10 Degree increments
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11
Q

What if you are at the Midpoint of 2 axes? Which one do you pick?

A
  1. Choose the Axis CLOSEST to a MAJOR MERIDIAN!
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12
Q

Cyl Power

  1. What DC increment is common for off-the-shelf toric lenses?
A
  1. 0.50 DC
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13
Q

Increments of Cyl Power in Toric SCL

  1. Can you over-correct Cyl Power?
  2. If necessary to under-correct by > or equal to 0.50 DC, you will need to maintain what?
A
  1. NO! DONT DO IT!!

2. the Spherical Equivalent

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14
Q

Why is Spherical Power the Least Important parameter to match up to the Spec Rx?

A
  1. It’s the EASIEST to perform a SPHERICAL OVER-REFRACTION!

* Can order Diagnostic lens w/Spherical over-refraction incorporated very easily

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15
Q

Assessing Fit of a Toric SCL

  1. What 3 things are Assessed in ALL SCLs?
  2. What 2 are only assessed in Toric SCLs only?
A
  1. Coverage, Centration, Movement

2. Rotation and Stability

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16
Q

Rotation

  1. It’s a description of what?
  2. Rotation is judged by examining the location of what?
A
  1. of the Lens position in PRIMARY GAZE AFTER it’s settled post-blink
  2. by Examining the location of markings on the lens (“Scribe marks”)
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17
Q

Toric SCL Markings

  1. Do they represent the Cyl Axis of the Lens?
  2. What are they then?
  3. Do all lenses of a particular brand have the same markings regardless of power/axis?
  4. When assessing Rotation, is it from patient perspective?
A
  1. NO. THEY DO NOT!
  2. Reference points for assessing Lens Rotation
  3. YES.
    * Same lens, Different Cyl Axis, Same Markings
  4. NO! It’s from Doctor’s Perspective!
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18
Q

Assessing Toric SCL rotation

  1. When do you assess it?
A
  1. AFTER it has SETTLED Post-Blink to determine whether its position has rotated to the LEFT or RIGHT from your perspective
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19
Q

Why does a lens rotate?

  1. What 3 things?
A
  1. Torsional forces of lids on the lens during a blink (TENDENCY for NASAL UP-ROTATION)
  2. Lens design, Prescription, and fitting parameters influence Lens Stability
  3. ROTATION LEADING to AXIS MISLOCATION is the MAJOR CAUSE of DECREASED VISION in TORIC SCL!!
    * Mislocation = Consistent lens rotation to some resting position other than what is desired
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20
Q

How to calculate how much the lens is rotated?

  1. 6 things to do
A
  1. Brighten the Light
  2. Let the Lens settle (takes some time)
  3. Determine Rotation behind the Slit Lamp
  4. Narrow beam to Optic Section
  5. Read off the angle on Protractor
  6. Rotate beam to be superimposed w/the Scribe Mark
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21
Q

What if the Lens rotates?

  1. Rule for Compensating for rotation?
A
  1. Cyl not delivered at INTENDED AXIS!!

2. LARS (Left Add; Right Subtract) (Do this to the SPECTACLE Rx)

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22
Q

LARS Adjusting and Lens Rotation

  1. Does adjusting for LARS CHANGE the LENS ROTATION?
A
  1. NO! A given lens brand will rotate the SAME way on the SAME Eye no matter what axis is put on.
23
Q

Quantifying Rotation

  1. What 5 things are necessary?
A
  1. Optic Section
  2. Bright Light
  3. Look 360 for the Toric Marker
  4. Rotate your beam so it’s parallel to the Toric marker
  5. Assess Direction BEFORE Looking outside of the slit lamp.
    * Beam should be parallel to Scribe Mark and Rotation is relative to 090 or 180
24
Q

Assessing Rotation Direction

  1. When assessing the DIRECTION of rotation, do so BEHIND the Slit lamp when looking thru the oculars at the actual contact lens.

DO NO do what?

  1. What about Quantifying?
A

DO NOT Assess direction by looking at the outside of the slit lamp.

  1. Quantify outside of slit lamp after determining Rotation BEHIND the Slit Lamp
25
Q

ESTIMATING Rotation

  1. When quantifying rotation what should you ALWAYS do?
  2. In practice, if you choose to estimate rotation instead of quantifying it, what should you remember?
A
  1. ROTATE the BEAM! (Must be done in Clinic/Practical)

2. Each clock hour is equivalent to 30 degrees rotation

26
Q

Stability

  1. When assessing rotation, what are we checking for?
  2. If a lens consistently settles w/the same amt of rotation, we compensate for it using what?
  3. If the lens rotation varies from 1 time point to another, is it stable?
A
  1. CONSISTENCY
  2. using LARS
  3. NO! It won’t work for the patient
27
Q

2 Ways a Toric SCL Stabilizes: What are they?

A
  1. Lens Fit: (Relationship b/w Cornea, Lens and Lid)

2. Stabilization System

28
Q

Lens Fit and Toric Lens rotation/stability

  1. Tight fitting lens will demonstrate what kind of lens rotation?
  2. Loose fitting lens?
A
  1. STABLE lens rotation; Little deviation w/blink and a SLOW return to its original position
  2. demonstrate unstable and inconsistent Rotation
29
Q

Toric SCL Stabilization Techniques

  1. 6 things
A
  1. Prism Ballasting
  2. Periballasting
  3. Thin zone
  4. Truncation
  5. Back Surface Toricity
30
Q

Prism Ballasting

  1. How much prism is added?
  2. What could it do?
  3. Based on 2 concepts
  4. What does it do to INFERIOR LENS THICKNESS?
  5. What does it do to Dk/t in that same area?
A
  1. 0.75-2.00 D of BD prism
  2. Could induce prismatic effect if Unilateral and within Optic Zone
  3. a. Gravity
    b. Watermelon Seed Principle
  4. Increased
  5. Decreased
31
Q

Watermelon Seed Principle

  1. Squeeze Watermelon seed b/w thumb and index finger: What happens?
  2. What does the Upper Eyelid do?
  3. If a Wedge is squeezed, what will happen?
A
  1. Rounded (thicker) portion tends to precede to the APEX as it leaves your fingers
  2. Squeezes the Thinnest portion of the lens, pushing the THICK portion downward.
  3. It will be EXPELLED AWAY from the LENS APEX
32
Q

Periballasting

  1. What is it?
  2. What is the OPTIC ZONE?
  3. What is the PERIPHERAL ZONE?
A
  1. Prism Ballasting Limited to the Lens Periphery (outside of the Optic Zone)
  2. it’s the Optically usable portion, found centrally
  3. Found outside of the Central Optic Zone
33
Q

Truncation

  1. What is it?
    a. Some designs were truncated where?
  2. Truncation is paralleled to what?
  3. Stabilized by aligning with what?
A
  1. 0.5-1.5 mm REMOVED from the lower lens edge
    a. at the upper and lower edge
  2. to the lid margin
  3. with the lid margin
34
Q

Thin Zone

  1. AKA?
  2. Thinning of what?
  3. How does it work?
A
  1. Dynamic Stabilization or Double Slab Off
  2. Thinning of BOTH Upper and Lower Edge of Lens outside the Optic Zone
  3. by the “watermelon seed principle” (Upper and lower lid forces push thickest portion of lens away)
    * Upper lid FORCES LENS DOWN; LOWER lid forces lens Up
35
Q

Back Surface Toricity

  1. Where can it be applied?
  2. Can it be used by itself?
  3. Where will the Lens align?
    a. Like what analogy?
    b. Flatter BC to where?
    c. Steeper BC to where?
  4. Where are Toric Curves confined to?
A
  1. on the Back of a Contact Lens
  2. No. Insufficient to be used alone
  3. to the Least Resistance
    a. like a SADDLE FITTING the HORSE

b. to Flatter Corneal Meridian
c. to Steeper Corneal Meridian
4. to the Optic Zone

36
Q

Back Surface Toricity Concept

  1. Back surface toricity stabilization is based on the PRINCIPLE of what?
A
  1. of Lens and Cornea Aligning like a LOCK and KEY
37
Q

How much stable rotation is acceptable?

  1. If lens rotates in a stable manner, use what?
  2. If it doesn’t, or if rotation is >30 degrees after lens equilibration, what do you do?
A
  1. use LARS
  2. Change Lens Parameter
    a. BC/Diameter –> IMPROVE FIT
    b. Different Brand of Toric SCL –> Different Stabilization System
38
Q

Troubleshooting

  1. Axis Mislocation: Consistent Lens Rotation to what?
    a. Stable –>
    b. Unstable –>
A
  1. to some resting position other than that desired
    a. LARS
    b. Change fit: Stabilization System (Brand), BC/Diameter
39
Q

Toric Lenses form 4 Major Manufacturers

  1. Alcon
  2. Bausch + Lomb
  3. Coopervision
  4. Vistakon
A
  1. Air Optix for Astigmatism
  2. Purevision 2 for Astigmatism
  3. Biofinity Toric
  4. Acuvue Oasys for Astigmatism
40
Q

Alcon: Air Optix for Astigmatism

  1. 2 methods of stabilization: What are they?
  2. Markings?
A
  1. Periballasting (Precision 8/4 design) and Back Surface Toricity
  2. 3 Markings: 3,6 and 9 o’clock
41
Q

Bausch + Lomb: Purevision 2 for Astigmatism

  1. Method for stabilization?
  2. Markings?
A
  1. Hybrid Prism/Periballast

2. Single Marking: 6 o’clock

42
Q

Coopervision: Biofinity Toric

  1. Method for Stabilization?
  2. Markings?
A
  1. Prism Ballast

2. Single Marking (6 o’clock)

43
Q

Vistakon: Acuvue Oasys for Astigmatism

  1. Method for Stabilization?
  2. Markings?
A
  1. Thin Zones (Accelerated Stabilization design)

2. 2 markings (6 and 12 o’clock)

44
Q

Assessing Toric SCL

  1. Let Toric Lenses settle: How long?
  2. First thing to do after this?
  3. Then assess what things?
  4. Compensate using what?
A
  1. 5-20 minutes
  2. Spherical Over-Refraction (SOR) First (even if patient is 20/20)
  3. C/C/M then Rotation/Stability
  4. using LARS and SOR if rotation is stable and not excessive; otherwise, change BC or brand
45
Q

When LARS Fails

  1. Usually, if vision is reduced in toric SCL, it’s due to what?
  2. Compensate with what?
A
  1. to Axis Mislocation (“Rotation”)

2. with LARS if stable

46
Q

SCOR

  1. What is usually sufficient?
  2. If vision still not adequate after using SOR and compensating with LARS, then perform what?
  3. SCOR: Fish for cyl where?
  4. If cyl is accepted, what do you do?
A
  1. SOR
  2. Sphero-Cylindrical Over-Refraction (SCOR)
  3. in the 4 major meridians
  4. Fine tune axis, then refine power
    * Power-Axis-Power
47
Q

Why SCOR?

  1. What 4 reasons?
A
  1. Errors in refraction
  2. Vertex Calculating Errors
  3. Possible Tear lens formation in thicker/stiffer lenses, affecting power patient is getting
  4. Errors in assessing rotation or applying LARS
48
Q

Need Accuracy in all components: What 6 components?

A
  1. Refraction
  2. Vertex Distance
  3. Lens Power
  4. Lens Markings
  5. Assessment of Rotation direction
  6. Quantification of Rotation
49
Q

Assessing Candidacy for Tolerance to Toric SCL

  1. Becherer Twist
    a. Put in Patient’s what in the phoropter?
    b. What do you do next?

c. Quantify what?

A
  1. a. Best Subjective Refraction
    b. Cylinder knob is twisted until patient reports blur
    c. twisting in both directions away from cylinder axis.
    * > 20 degrees in each direction: First lens success will be achieved > 90% of the TIME

15 degrees: 90% success w/2 lenses

10 degrees: success: 70% with 3 lenses

5 degress: any success will depend on how tolerant the patient is to decreased contrast

50
Q

Setting Expectations

  1. Toric SCLs, even when fit optimally, MAY result in what?
A
  1. Inferior visual quality and acuity as compared to spectacles…set realistic pt expectations, esp in higher cyl powers or certain personality types.

GP CLs may provide better optics

51
Q

Determining Whether to correct Cyl in SCL:

  1. Factor
    a. Sphere Power
    b. Astigmatism Axis
    c. Previous Correction
    d. Usage
    e. Visual Tasks
    f. Eye Dominance
A
  1. Toric SCL Less important when

a. Higher Sphere
b. WTR
c. Spherical w/o issues
d. Part-time
e. Undemanding
f. Astigmatism Lowest in Dominant Eye

52
Q

Custom SCL

  1. Customize Diameter
  2. Customize BC
  3. Customize Cyl Power and Axis
  4. Order how?
A
  1. Based on HVID
  2. Good for pts outside the norms
  3. Axis Steps down to 1 degree
  4. Order EMPIRICALLY
53
Q

Summary

  1. Toric SCLs correct what?
  2. Put Rx on what?
  3. Use toric SCL if VERTEXED spec Rx has what?
  4. Better to under/over correct cyl power?
  5. Assess what?
  6. What do you do next?
  7. When do you use SCOR?
A
  1. REGULAR astigmatism
  2. on Power cross and vertex any meridian > or equal to 4.00 when determining Toric SCL Power
  3. More than or equal to 0.75 DC
  4. UNDER correct cyl power
  5. C/C/M and Rotation and Stability
  6. Quantify Rotation and Compensate with LARS
  7. ONLY in cases where you can’t make sense of what’s going on after compensating with LARS and SOR

Contact Lens I Part 2 (5 decks)

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