Lecture 17 Orthokeratology Flashcards
(29 cards)
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.
