principles of rigid lens fitting III Flashcards
what are aspheric lenses based on ?
- based on elliptical form of the cornea
- radius flattens towards the periphery of cornea
- variation in shape of an ellipse is called the eccentricity (e)
what is an aspheric lens classified as?
- when both either one or both surfaces are non spherical
what do aspheric lenses utilise in ?
- utilise in non spherical surface astigmatism to neutralise oblique astigmatism
- produced by off-centre refraction
what do aspheric lenses provide ?
- they provide both the visual advantages of best form lenses and cosmetic advantages of thinner , flatter , lighter lenses
what is the multi curve design ?
- BOZR - back surface of the CL
- you have multiple back peripheral curve to make up remaining peripheral zone of lens up to TD(
what is the difference between multi curve and aspheric design ?
- in aspheric design lens flattens towards the periphery ( BOD) covers wide area
- in multi curve you can see peripheral curve
- BOZR in aspheric design is not consistent
what do elliptical spheric lenses need ?
- elliptical aspheric lenses generally need a flatter fitting than spherical lenses
. to provide alignment across the corneal surface - FLUROSCENE PATTERN
. lenses must be 0.05 mm flatter than flattest K
what is different about polynomial and variable eccentricity aspheric designs?
- polynomial and variable eccentricity aspheric designs can be fitted in very much the same way as spherical lenses
- on flattest K
what are the designs of aspherical RGP?
- fully aspheric
- mainly aspheric/part sphere
- spherical optic/aspheric periphery
what are fully aspheric lenses ?
. complete aspheric back surface . fits perfectly along cornea . may be uncomfortable ( lens edge presses into cornea) . e.g. Quasar ( No 7 laboratory ) . must look at fitting
what are mainly aspheric / part sphere lenses ?
. mainly aspheric back surface with spherical peripheral curve
. good edge clearance
. prevents edge pressing into cornea
. assists in tear exchange
. however, edge lift is sometimes insufficient
. e.g. persecon (92) E (CIBA vision)
. should be fitted 0.05 mm flatter than flattest K
what is spherical optic/aspheric periphery ?
. central spherical portion (BOZ) with aspheric peripheral zone area
- aka polynomial
- fitted 0.05 mm flatter than flattest K
- e.g. Quantum (Bausch and Lomb)
- refer to fitting guide
what happens if TD is too small in aspheric lenses ?
. in aspheric lenses, if the total diameter TD is considered too small and there is insufficient coverage of the pupil or poor centration, the large diameter should be ordered with no change to the radius or power
what happens if aspheric lens is too large ?
. if the lens is considered too large and there is insufficient vertical movement with the blink, smaller TD should be ordered
. no change to radius or power
what are the advantages of aspherics ?
. achieves true alignment . fit is more closely to corneal topography . more comfortable . pressure more evenly distributed over cornea . less lid sensation - poor comfort . useful in higher astigmatism . more gradual FP ( centre to periphery) . less critical fitting . fewer parameters
what are the disadvantages of aspherics ?
. flat fittings
. larger lenses are needed to help centration
. the lens edge may press into peripheral cornea
. bi-aspheric lenses can induce residual astigmatism and aberrations
. variation in design - harder to customise lens if px is uncomfortable
. manufacturer’s info should be read
. less option to modify design
why are multi curve (MC) preferred over (AS)?
- more control over individual lens areas
e.g.
. BOZD
. flatter/steeper periphery
. adjust edge width / clearance
e.g. pupil too large: px complaints of halo’s with AS: BOZD needs to be increased
why are AS preferred over MC ?
. often successful first fit
. easy choice of 3 parameters ( BOZR, TD, BVP)
. uniform FP
. no blended junctions; more comfort for px
e.g. edge clearance too large with MC; changes to AS
what are other aspheric lens features ?
- centre and edge thickness
- edge carrier
- edge profile
- tint
what is centre and edge thickness ?
. dependent on BVP ( back vertex power ), design and material
. depends on the back vertex power when the lens had BOZR of 7.80 and TD 9.60
. positive lenses are thicker in centre and thinner in periphery compared to negative lenses
. if BVP greater than -6.00D and +4.00D the lenses should be lenticulated
what does lenticulated lens mean?
- FOZD is made smaller which reduces tc for plus and reduces te for minus lenses
what is edge carrier ?
- lenticulation of a lens creates a carrier portion
- can be plano, negative or positive in shape
- negative carrier for positive lenses and positive carrier for negative lenses to improve centration
- a carrier will minimise centre thickness of low minus powered nd plus powered lenses while making overall thickness more uniform
what does plano carrier do ?
maintains even thickness
what does positive carrier do ?
reduces edge thickness
what does negative carrier do ?
increase lens thickness and lift a lens up
why is the correct anterior lenticular construction essential ?
- minus lenses show natural lid attachment causing high riding
- plus lenses do not - possibly low riding , the superior lens can’t hold edge of lens- sit low -as plus lens is thin
where is centre of gravity in negative lenses ?
- centre of gravity ( C ) is further back in negative lenses - high riding- sits high
where is centre of gravity in steep lenses ?
- C is further back in steep lenses - better centration
where is centre of gravity in larger TD lenses?
- C is further back in larger TD lenses - better centration
what is edge profile ?
- most important for comfort
- must be smooth and well finished
- can help lens removal
- aspheric tend to have well rounded edges
what is handling tint ?
- usually given to RGP
- usually grey or blue
- used to see lens if dropped and in lens case
- grey - for RE
- blue - for LE
- they enhance iris colour
why does it more commonly occur in plus lens ?
because centre of gravity of plus lens sits much more anteriorly than a negative lens
