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Rigid lens - materials and designs - Coggle Diagram
Rigid lens - materials and designs
Two form of Rigid Lens
Scleral and corneal.
Underlying principles explained in this apply for both
RLs usage
Netherlands - 33pc
Other countries - 15pc
Uk - 5pc
It has unique properties so only used to give reasonable vision to pxs. I.e., to pxs with kerataconus (curvature of the cornea is quite different in different parts)
Different curvatures mean there are different powers required.
Bad vision will be a result of the curvature but also a large difference in refractive index because the cornea is in air - and RI difference is about 0.336
RLs are used but not to alter the shape of the cornea. But it reduces refractive variation as the cornea is no longer in air. It is instead next to the tear film
Good vision can only be reached in this case where all parts of the cornea have same powers
This reduces the refractive index difference to 0.003
Other indications for RLs
1 - WIth high refractive errors, there is a little that can done with soft lenses to increase o2 permeability as the thickness of the lens increases.
With RLs there is extra lens movement with each blink. this allows some exchange of tear film behind the lens (fresh tears with dissolved o2 move behind the lens and old tear film is reduced)
2-Corneal astigmatism
A simple spherical rigid lens is able to correct corneal astigmatism.
With higher levels if astigmatism, a spherical RLs can be problematic due to the difference in shape. therefor a a back surface toric or a lens which is toric on both back and front is required.
3- special lens such as orthokeratology lenses which are designed to be slept in.
This aids in correcting myopia as it flattens the central cornea.
4- adverse effects or handling problems with SLs
Also allows for greater exchange of bacteria and microorganisms . reducing the chance of an infection or inflammatory response.
This is also explains the reduced adverse effects in RLs wearers vs Sys wearers.
Lens design
1- Spherical lens: Is used for both spherical lens or with mild.moderate levels of anterior corneal astigmatism.
2-High levels of corneal astigmatism or lenticular astigmatism.
3-used for presbyopia.
4-Ortho-k. Used to flatten the central region of the cornea.
The lens chosen have the correct degree of flattening when worn overnight. On wakening myopia is corrected. The cornea takes a day or two to return to its original shape.
Lens modality
Can be used as a daily wear or extended wear. However, they are typically used for daily wear to minimise infections.
Ortho-k are slept in (extended wear)
Lens replacement
Half of the pxs supplied with lenses are to be replaced every 6 months.
The remaining half are prescribed unplanned replacement lenses which are only replaced when their performance diminishes to a level which is noticed by the px
Material properties
O2 permeable
Wettability (aids comfort and vision)
deposit resistance(aids comfort and vision)
mechanical stability(breakage)
scratch resistance (vision)
Cellulose acetate butyrate (CAB)
Wettability and resistance is quite good
o2 permeability is poor (DK=6)
prone to lipid deposits
scratches can distort vision is lathed
Silicone acrylates
better o2 performance
may get protein deposition
Fluorosilicone acrylates
Good o2 performance, wettable DK>50
little protein deposition
can get lipid deposition
More information of RIGID LENSES
Axial edge thickness
-Describes the distance from the edge of the lens to the cornea. It relates to the lens on the eye and is estimated by the fluouroscein pattern
E.g., most lenses provide 0.08-0.12mm AEL
larger corneas and pupil point towards larger lenses
Optic zone diameter
BOZR should be equal to or larger than the pupil
Often pre-determined for many lens desgin
lenses can be custom ordered if necessary
Lens design - detail
Cornea is not circular. Its aspheric. However making a aspheric lens can be too good of a fit and can create potential problems such as sticking onto the cornea
Solution is to use
tricurve
contact lens. Centre curve is used to to see and the other two are for mechanical use.
Radius of curvature of the curves at the periphery are larger as the peripheral lens is much more flatter than the centre
Astigmatism
Match the BOZR to the flattest K (highest value)measured with the keratometer or Topograph. The tear film will correct the steepest meridian.
Astigmatism won't be corrected if the refractive cyl is not from the cornea