Introduction to RGP lenses Flashcards
What are RGP lenses?
a contact lens designed using durable plastic that transmits oxygen. It’s diameter is less than the visible iris diameter.
What are the advantages of RGPs?
- vision (better optics of the lens and VA due to retained shape).
- superior tear pump
- high Dk/t
extremely durable (lens lasts longer) - ease of handling
- reduced contamination and deposition of lenses due to no water content therefore reduced infection and pathology.
- heavy upfront cost but overall less expensive than SCLs
Disadvantages of RGPs?
- comfort (require initial adaption, intermittent wear harder to do)
- lens ejection more common (due to small diameter)
- dirt/FB easy to get under lens because lens designed to move on blink
- corneal staining more frequent
- flare due to small optical zone
- greater care requirements due to longer replacement schedules
- greater fitting time and skill necessary
- no options for cosmetic or therapeutic uses
Indication for selecting px’s for RGP lenses?
- myopia control
- irregular astigmatism or corneal irregularities
- high rx
- existing wearers
- px at higher risk of infection
- when soft cl’s fail (vision, handling)
Contraindications for RGP lenses?
- intermittent lens wear
- contact sports
- dusty environment
- young children
- patients with highly sensitive eyes
- low corneal, high residual cylinder
Manufactures of RGPs?
Scotlens
Boston (Bausch and Lomb)
No7 contact lenses
Factors affecting lens fit?
- corneal topography
- lid force and position
- surface tension
- frictional forces
- lens centre of gravity
What is corneal topography?
What is lid force and position?
The lids help the lens from falling from the eye.
Relationship between lens size and IPA/corneal diameter is not straightforward.
A higher upper lid = larger diameter lens often required.
lower lid high on limbus = smaller diameter lens
What is surface tension and friction?
Lens is held in place by capillary forces in the post-lens tear film and surface tensional force in the tear meniscus at the lens edge.
The force is greater in spherical corneas vs astigmatic corneas.
Surface tension forces act as lens edge where the tear meniscus is not covered by eyelid.
What is the lens centre of gravity?
The gravitational force of the lens and the pre-lens tear film cause the lens to drop. The lens is less stable the further forward the lens lies. The centre of gravity is further forward in plus lenses vs minus lenses.
Rigid lens design consists of..
BOZR/BOZD
diameter
thickness
edge lift/edge clearance
tear lens
spherical vs aspherical designs
materials
manufacture
Front surface enables..
lens power
clean wettable surface for stable tear film
interacts with lids to aid with comfort, movement and centration
Back surface enables..
post-lens tear film and vision (the thicker the tear film the bigger the refractive impact on vision)
edge bevel which aids lens removal and maintains the tear pump
What is BOZR?
AKA base curve. The central curve of the lens that is matched to corneal curvature to optimise vision.
Depends on flattest K reading and corneal toxicity
BC selection critical for effective tear pump
ranges from 0.05mm flatter or 0.05mm steeper than flattest k
