Lecture 17 Orthokeratology Flashcards
What is orthokeratology?
*A technique for correcting refractive errors in vision by changing the shape of the cornea with the temporary use of progressively flatter hard contact lenses
*A form of refractive correction
What happens when a ortho-k lens is fitted?
*We put a flatter than standard lens RGP cl on a cornea
*Worn overnight
*Once is removed, you recheck the cornea
*The longer you wear the lens, the more reshaping.
*Re moulds soft corneal tissue overnight.
*Takes 2-3 weeks for full correction.
What are other terms attributed to orthokeratology?
*Corneal bending
*Remodelling of the anterior corneal layers
*Hydraulic theory
*Central corneal epithelial thinning
*Mid peripheral thickening of the stroma and epithelium
Explain the NaFl pattern seen in orthokeratology.
*There is positive compression centrally on the cornea
*Causes negative tension in the periphery of the cornea
*Forces cells out into reverse curve. This is shown by band of fluorescein. This is where cells get pushed to.
*There should be NO touch with cornea, always central clearance.
*We use positive compression and negative tension to reshape cornea. This happens beneath the lens.
Why can it looks like there is touch between lens and cornea in NaFl pattern?
*Fluorescein doesn’t fluoresce at less than 30 microns.
*Ideal central clearance is 10-13 microns.
*Not enough fluorescein beneath centre of lens to fluoresce.
What changes do you do to the structure of cornea after wearing ortho-k lenses?
Central thinning of epithelial cells
Thickening of cells and stroma in periphery
What equipment must you have to fit ortho-k lenses?
corneal topographer
What amount can ortho-k lenses correct?
The higher the correction, the smaller the treatment zone.
- If px has a large pupil and treatment zone is small over pupil, you can cause visual disturbance especially in low lighting.
- myope correction is max 5.00D and 2.50D WTR astg.
What patients won’t be suitable for orthokeratology?
*Occupation- e.g., shift workers, hard to get consistent 8 hours wear.
*Compliance
*Hygiene- not disposable, will have them for at least 6 months
*Motivation
What are the features of topography?
*High degree of accuracy and repeatability
*Statistical analysis of repeated readings of apical radius and eccentricity or elevation
*Axial, tangential, and refractive power and curvature maps
*A difference or subtractive map function. Not all do this.
*Pupil recognition from centration pov.
*A large area of corneal coverage with minimal interpolation. We get 9-14 mm of anterior surface of cornea. Minimum of 10 mm needs to be measured.
What does the tangential graph show?
What does the axial/refractive graph show?
-localised changes
-best indicator of corneal shape and centration after ortho k
-monitoring change and showing central island post ortho k
What does the tangential subtractive map show?
What does the refractive subtractive map show?
What does the axial subtractive map show?
- centration
the size of treatment zone and change in refractive power
change in axial corneal surface power
What is required to do topography on a px?
*Px needs to keep eyes wide open
*Tilt head, if necessary, as eyelashes/nose can obstruct good image
*Normal blink rate required
*Good tear quality (can use artificial tears if necessary)
*Take a minimum of four images to compare
*Delete poor images
*Keep 3-4 good images as base line
What indicates a poor image?
-missing ares, px not opening eyes wide enough
-wavy mires, unstable tear film
What are some of the types of ortho-k lenses?
*Paragon CRT: fitting set lens. based on eccentricity and other values from topographer
*Eye dream by no 7: software-based lens
*Euclid, Aiko, emerald
*Menicon Z night using easy fit software
What does the optimum fluorescein pattern look for an ortho-k fitting?
clearance: 0.4mm
central zone: 3-5mm
reverse zone: 0.5-1mm
What is the advantages of topography in ortho-k fittings?
Pre-treatment
*Topography sets a baseline
*Helps to select lens
*Screening of abnormal cornea, poor corneal shape, decentred apexes
*Allows to fit with more caution in people with tilted cornea, abnormal lid architecture, limbus to limbus astigmatism
Post treatment
*Determine centration
*Do we have a good treatment zone size
*Refractive change
What topographical responses can you get?
- Bulls eye
- Smiley face
- frowny face
- true central island
- smiley face with false central island
- lateral displacement
- central divot
What is the ideal topography pattern you expect to see after ortho-k?
Bull’s eye
A well-centered area of corneal flattening
A circle of mid-peripheral corneal steepening
Little or no peripheral corneal change
What symptoms can a small treatment zone cause?
flare
poor vision
What causes a smiley face topography pattern?
What are the consequences?
What is the treatment?
Causes by flat fitting lens
*Px will have poor vision or variable vision
*Negative over refraction and unwanted cyl or increase in astigmatism
*Increased WTR astigmatism
*Px will experience ghosting, glare and flare
*Need to increase lens sag, steepen the lens centrally or peripherally in landing zone, steepen RC/BOZR
*Need to increase RZA, LZA or BOZR
*Decrease cone angle
What causes a frowny face topography pattern?
What are the consequences?
What is the treatment?
Caused by Flat fitting
*Causes some visual disturbances such as ghosting, glare and flare
*Negative over refraction
*Need to increase lens sag, steepen AC,RC/BOZR, RZD, LZA
*Need to decrease cone angle
What causes a true central island topography pattern?
What are the consequences?
What is the treatment?
steep fitting lens
*Over refraction: no end point
*Poor BCVA
*Too much lens sag and eccentricity
*If central island is small (less than 1D), may self-resolve in a week
*If island doesn’t resolve, refit with reduced sag
What are the risks/disadvantages of ortho-k?
*Requires more time to fit
*Topographer is essential
*Aftercare is more frequent to ensure efficacy and safety
*Noncompliance or poor compliance due to lens care or handling
*Full correction may not be achievable in all px
*Top up contact lens may be needed during the treatment process. 1/3 of px prescription may be corrected after first night. Need top up lenses during first initial weeks.
What are the benefits of orthokeratology?
*Vision correction
-Successful overnight wear results in good vision that can be maintained throughout the day post lens removal
-Average reduction of myopia of -4.00 D gives the highest rate of success
-Completely reversible: if lens wear is stopped, cornea can revert to its pre-treatment state. Depends on how well fitted lenses were and length of time of wear.
*Lifestyle
-Convenience for those who do sports, some occupations and those who don’t want to wear conventional contact lenses or spectacles during the day.
-Complications associated with dryness, dust, allergies with conventional lens types avoided as only wearing them overnight.
What can corneal staining indicate about the fitting?
How can you prevent lens binding?
-sub optimal lens fit (not enough clearance)
-solution sensitivity
-will expect some superficial staining in all px to begin with
-px should wait 15 minute to make sure tears are circulating rather than taking them out first thing.
What types of staining can you get and what is the cause?
central staining- flat fit
inferior conjunctival staining- steep fit
superior conjunctival staining- flat fit
dimple veil- steep fit (too much clearance) or optimum fit but trapped tears
lens binding- poor fit
How does myopic progression work?
*We need to stop axial elongation
*Myopes have longer axial length
*We believe there is a trigger for axial elongation
*Putting a negative lens Infront of the cornea, we cause hyperopic defocus in peripheral retina
*Eye stretches in response to this defocus. Increases axial elongation.
*We need to create myopic defocus in peripheral retina to stop this trigger for axial elongation
How does orthokeratology slow myopic progression?
*When using an ortho k lens, we change the shape of the cornea. This creates myopic defocus on peripheral retina.
*Due to increased thickness in peripheral cells, we are effectively creating an ADD on the cornea
