RGPs Flashcards
How are RGPs specified?
Always specified as radius followed by diameter
Tricurve corneal lens BOZR:BOZD
Centration of RGPs
The lens should remain centred over the pupil in primary gaze and maintain reasonable centration with each blink. The goal of RGP lens centration is to ensure that the visual axis remains within the back optic zone diameter (BOZD) for as long as possible to optimise visual acuity and avoid flare. The lens should also remain on the cornea during all positions of gaze to minimise conjunctival staining from the periphery of
the lens onto the limbal conjunctiva.
How should the corneal coverage be for RGPs?
Unlike soft lenses, RGP lenses should be smaller than the corneal diameter. They should have a total diameter of at least 1.4mm less than the horizontal visible iris diameter (HVID) to facilitate tear exchange under the lens and help optimise the alignment of the lens fit.
What is the dynamic fit of an RGP?
As well as allowing metabolic and tear debris to be removed from underneath the lens, the RGP lens must move to enable oxygen exchange due to the tear pump. Unlike soft lens fitting, there is a significant exchange of oxygen underneath an RGP lens during the blinking cycle.
Lens movement is one of the key characteristics of an ideal RGP fit. The lens should move around 1 to 1.5mm with each blink.
The movement should be smooth and unobstructed
in the a vertical plane, indicating a near alignment fit.
Lens movement occurs either as a response to the eyelid force or by upper lid attachment.
An immobile lens causes tears to stagnate beneath its surface, leading to corneal staining and distortion, while a lens with excessive movement causes patient discomfort, inconsistent vision and may also be
associated with conjunctival staining.
What should the alignment be for a RGP?
The ideal RGP fit should show alignment of the back surface of the lens with the cornea over most of the surface. A narrow band of edge clearance at the periphery is required to enable adequate tear
exchange and facilitate lens removal. The alignment
of the back surface with the cornea allows the force of the lens to be distributed across the maximum bearing surface of the cornea. However, slight apical clearance and an area of light corneal touch in the mid-periphery will enhance lens centration. Excessive touch can lead to tear stagnation, staining and/or distortion, while points of excessive clearance lead to an unstable lens fit in terms of centration, comfort and vision.
What does the edge lift do?
Provides reservoir of oxygenated tears at lens edge
When selecting intial lenses what do you need to decide?
TD BOZR BVP Lens design Lens material
What should the total diameter be?
Dependent on HVID
Usually HVID – 2mm
Large enough for back optic zone to cover pupil in low light
Small enough to allow corneal oxygenation (less important with modern high Dk materials?)
What should the BOZR be?
Optimum likely to be similar to flattest K
What should the BOZD be?
Often set by manufacturer
Approx 0.7mm larger than max pupil diameter
What should the peripheral curve be?
Again, often pre-determined
First peripheral curve usually about 0.7mm flatter than BOZR
Tricurve lens – final peripheral curve approx 10.50mm for with of 0.5-1.00mm
-Flatter – less corneal irritation
-Steeper – less lid irritation
Too little peripheral clearance causes poor tears exchange
Too much gives unstable fit
What should the BVP be?
Use sphere power of spectacle Rx in negative cyl form
Ignore cyl, consider flat meridian only
So if spec Rx -3.25/-0.75x5, choose BVP of -3.25
What should the lens design be?
Designs often commercially sensitive Can design own multicurve Generally use manufacturer’s set design eg B&L Maxim Two diameters – 9.30 and 9.80mm
What should the lens material be?
Again, limited by availability B&L Maxim available in: ML92 – 92 Dk ML 210 – 210 Dk All other Boston materials Tend to use ML92 good compromise between wettability, resistance to warpage and oxygen permeability
Checking fit?
Eye probably lacrimating, need to allow lens to settle before meaningful assessment possible
White light assessment -Dynamic fit -Slit lamp --Medium magnification --Broad beam Look at: *Centration *Movement -On blink -On version Want lens that: -Centres well -Moves freely on blinking -Does not cross limbus
Fluorescein assessment Slit lamp: -Broad beam -Cobalt blue filter ---Burton lamp as alternative Instil fluorescein into lower fornix Assess fluorescein pattern: -steep -alignment -or flat fit Note that lens must be centred on cornea – move if necessary Also assess lens total diameter relative to: -iris diameter -pupil size
