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