Dr Spors - Ortho-K Study Guide

Front 1

What are some important characteristics of BC of DRG lens?

Back 1

BC is the 1st curve or Treatment Curve
BC is usually 6.0 – 6.2 mm in diameter
BC is not aligned with the cornea --> BC radius is always FLATTER than the Flat-K reading!!!

Front 2

What are the main functions of the BC of DRG lens?

Back 2

1) To provide structural template to which the cornea can adjust
2) To apply positive pressure forces via the tear film to the cornea
3) To provide the optical treatment effect

Front 3

What is Double Reverse Geometry (DRG)?

Back 3

Reverse = steeper than the base curve (BC)
Double Reverse = two zones are steeper than BC

1st curve: returns to corneal level
2nd curve: follows shape of cornea in mid-periphery

Front 4

What are the lens curves found in a DRG lens?

Back 4

1. Base curve (BC)
2. Reverse Curve (RC)
3. Alignment Curve (AC)
4. Peripheral Curve (PC)

Front 5

What is the average size of a DRG lens?

Back 5

Ortho-K lenses are pretty large!
Range: 10.0 mm – 11.2 mm (mode OAD = 10.6 mm)

Front 6

How do you calculate the BC of DRG lens?

Back 6

BC = (Flat K) + (sphere) + (RR)

Flat K = the central flat corneal radius

Sphere = the spherical component of the Rx

RR = Regression Reserve can vary between -0.50 D to -0.75 D depending on the lens manufacturer

Front 7

What are some important characteristics of Reverse Curve (RC) of DRG lens?

Back 7

- It’s the 2nd curve (aka Return Curve)
- About 0.6 mm wide
- RC is much STEEPER than the BC by 2.0 – 2.6 times the amount which the BC is FLATTER than the Flat-K reading
- Range from 6.00 D – 12.00 D steeper than the BC

Front 8

What is the main function of Reverse Curve (RC) of DRG lens?

Back 8

Connects the first curve (BC) with the 3rd curve and allows corneal tissue to move in the created space

Front 9

What are some important characteristics of Alignment Curve (AC) of DRG lens?

Back 9

- It’s the 3rd curve (aka Landing Curve)
- About 1.2 mm wide
- Fit in alignment with the mid-peripheral cornea

Front 10

What is the main function of Alignment Curve (AC) of DRG lens?

Back 10

Centration of the lens

Front 11

How do you determine the Alignment Curve Radius?

Back 11

Central K-reading and temporal K-reading, or information from topography can be used.

Front 12

What are the 2 major methods for determining the AC radius?

Back 12

1) Eccentricity value is obtained by topography
2) Trial lens set

Front 13

What are some important characteristics of Peripheral Curve (PC) of DRG lens?

Back 13

- The 4th curve (aka Bevel)
- It’s the OUTERMOST curve of the lens
- It’s very similar to the PERIPHERAL curve of standard RGP lenses
- most commonly 0.4 mm wide
- has a radius of curvature of 10.50 mm (rare) to 12.50 mm (more common)

Front 14

What is the main function of Peripheral Curve (PC) of DRG lens?

Back 14

Tear exchange and removal of debris

Front 15

Orthokeratology lens fit is based on what 3 principles?

Back 15

1) Baseline keratometry values (Flat-K)
2) Spherical component of Rx (in minus cylinder form)
3) Peripheral shape data (Eccentricity, e-value, p, sf, Q)

Front 16

Why is the lens BC is flatter than the flat-K reading?

Back 16

Lens induces central corneal flattening, the myopic refractive error is reduced or eliminated

Front 17

How do you stabilize the ortho-K lens?

Back 17

To stabilize the lens, STEEPER curves than BC are needed in the corneal mid-periphery --> that’s the alignment curve (AC)

Front 18

What are the roles of the different curves in DRG?

Back 18

BC radius = is purely based on optical calculation

RC radius = is chosen to connect BC and AC

AC radius = is purely based on corneal shape

PC radius = assures exchange of tears

Front 19

Given: Flat-K: 44.00 D; Rx: -4.00 D; RR = -0.75 D. How much FLATTER do we have to make the BC?

Back 19

BC = (Flat K) + (sphere) + (RR)
= (44.00 D) + (-4.00 D) + (-0.75)
= 39.25 D

The new BC is 39.25 D, which is 44.00 – 39.25 = 4.75 D flatter

Front 20

As a general rule for all orthokeratology lenses, how is the BC affected?

Back 20

- The BC is NOT adjusted to change the fit
- BC changes are solely done for treatment adjustment

Front 21

With respect to safety of ortho-K, what is the most common visual side effect, and how can it be treated?

Back 21

Most common visual side effect is the experience of glare or halos around lights in dim conditions.

This side effect can be reduced by increasing the size of the central treatment zone.

Front 22

Who is a GOOD candidate for ortho-K?

Back 22

- Moderate to low level myopes: -1.00 to -5.00 D
- Corneal astigmatism < 1.50 D
- E-values of 0.5 and higher
- Corneal diameters > 11.00 mm
- Pupil sizes < 6 mm (in dim illumination)
- Apical radius (Ro) from 8.44 mm (40.00 D) to 7.34 mm (46.00 D)
- Soft lens wearers & spectacle wearers
- Astigmatism should NOT be higher than ½ of sphere power

Front 23

For the following Rx’s which ones are good candidates for ortho-K?
a) -4.00 – 1.50 x 180
b) -1.00 – 1.00 x 180

Back 23

a) sphere = -4.00 /2 = -2.00 > -1.50 (cylinder power) --> GOOD candidate!

b) sphere = -1.00/2 = -0.50 &lt; -1.00 (cylinder power) --> poor candidate

Front 24

What’s the FDA approval for Paragon CRT lenses?

Back 24

- up to -6.00 D of myopia
- with or without up to -1.75 D of astigmatism
- HDS 100 (Dk 100 Barrer)
- NO age restrictions

Front 25

What’s the FDA approval for B&L VST lenses?

Back 25

- Up to -5.00 D of myopia
- With or without up to -1.50 D of astigmatism
- Boston Equalens II (Dk 85 Barrer)
- NO age restrictions

Front 26

Which topography maps do you need to evaluate in an ortho-K case?

Back 26

1) Take an axial map reading:

- Use Sim-K readings (here Flat-K) and Eccentricity to determine the BEST fitting lens
- Check the ring images quality before using this map (no dry spots, no debris, etc)
- Even better: select the apical radius (Ro) and Eccentricity (or the sagittal depth)

Front 27

What 3 types of topographical patterns can be seen after lens wear?

Back 27

1) Bull’s Eye pattern = ideal fit
2) Central island pattern = lens too TIGHT fit
3) Smiley face pattern = lens too FLAT fit

Front 28

What are the 3 different topographical maps? Which map is the most important concerning ortho-K?

Back 28

1) Refractive power map = most important
2) Axial
3) Tangential

NOTE: If the topographical maps look acceptable, and the patient is happy, then DON’T change the fit!

Front 29

What are some specific care regimens for ortho-K lenses?

Back 29

Boston Simplus

Clear Care

Front 30

What are some COMPLICATIONS with ortho-K?

Back 30

1) Decentration
- Biggest concern!
- Lens look too FLAT, too steep, too small
- Look up topography and/or NaFl pattern
2) Vaulting
- Reduced treatment effect
- Sagittal depth too large
3) Induced Astigmatism
- Decentration, lenticular astigmatism, lens warpage
4) Adherence
- Tight fit, deposits
5) Over responding
- Lens BC too flat (i.e. miscalculation)
6) Under responding
- Vaulting, Lens deformation over time
7) Ghosting
- Treatment zone too small for pupil size
- Normal at the beginning
8) Corneal staining
- Allergies, dry eyes, too tight fit (metabolism decreases)

Front 31

How is WTR astigmatism affected in ortho-K patients?

Back 31

WTR astigmatism is good up to 1.50DC (sometimes a little bit more, depending on the manufacturer’s claim)

Front 32

How is ATR astigmatism affected in ortho-K patients?

Back 32

ATR astigmatism is critical, especially when more than 0.75DC

Front 33

How is central astigmatism affected in ortho-K patients?

Back 33

Central astigmatism is okay, but limbus-to-limbus astigmatism is more critical

Front 34

Which initial centration is better? Tangential AC or 4 Curve DRG?

Back 34

4 Curve DRG:

Initial CENTRATION is likely to be BETTER than with a Tangential AC design, since the AC, as a REAL curve, follows the corneal shape over an annulus which is usually 1.2 mm in width (the width of the AC).

Front 35

For the 4 Curve DRG lens, which radius controls the primary centration?

Back 35

Primary centration control is the AC radius

(meaning if the lens is too flat or too steep, this radius is being changed)

Front 36

CRT lens = ?

Emerald lens = ?

Back 36

CRT lens = Tangential AC lens

Emerald lens = 4 Curve DRG

Front 37

How many curves are found in the AC for 4 Curve DRG lens?

Back 37

AC may split into 2 curves with the second curve being a little FLATTER to allow corneal tissue to migrate in (which is rare, but may happen in the corneal mid-periphery).

Front 38

How is centration adjusted in CRT lenses?

Back 38

To adjust centration, usually 2 steps can be done, a primary step & secondary step:

1. The primary centration control is Return Zone Depth (RZD), which is expressed in sagittal height (i.e. 550 microns).

2. The secondary centration control (less important) is via the LZA, which is usually done for BETTER peripheral tear exchange, and is expressed in degrees (i.e. 33°)

Front 39

For CRT lenses, which zone controls the primary centration?

Back 39

Return Zone Depth (RZD)

The primary centration control is Return Zone Depth (RZD), which is expressed in sagittal height (i.e. 550 microns).

Front 40

What are the 2 major zones found in CRT lenses?

Back 40

AC = Landing Zone

RC = Return Zone

Front 41

If the lens is too FLAT in the CRT lenses, then how do you fix it?

Back 41

If the lens is too FLAT, then RZD has to get LARGER (i.e. from 525 to 550 microns) to STEEPEN the lens.

Front 42

How is the Landing Zone Angle (LZA) changed to fix a CRT lens that is too FLAT?

Back 42

1. If increasing the RZD (i.e. from 525 to 550 microns) to STEEPEN the lens does NOT work, then you'll need to change the LZA too.

2. the LZA also has to get LARGER (i.e. from 33° to 34°)

Front 43

If the lens is too STEEP in the CRT lens, then how do you fix it?

Back 43

1. RZD has to get SMALLER (from 550 to 525 microns)

2. If that's NOT enough, then:
LZA is reduced from 34° to 33°

Front 44

How is the Landing Zone affected in Tangential AC lens/CRT lens?

Back 44

Landing Zone:
- is NOT following the corneal shape over a wide annulus, since its radius is FLAT (infinitely LARGE, a tangent line)
- it has a annular contact to the mid-peripheral cornea, but that’s a much SMALLER area than in a DRG lens, hence, initial CENTRATION is a little bit MORE critical!

Front 45

How does the AC radius appear on all CRT lenses?

Back 45

Since the AC radius remains FLAT in ALL CRT lenses, its corneal alignment can be solely adjusted by the Landing Zone Angle (LZA).

Front 46

Describe the alignment curve (AC) in 4 Curve DRG/Emerald lens.

Back 46

AC is a REAL curve which follows the corneal shape in MID-PERIPHERY.