RGP lens fitting and assessment Flashcards
Ideal lens fit..
centration
corneal coverage
dynamic fit
alignment
patient response
centration
lens centred over pupil in primary gaze, maintained with blink
corneal coverage
smaller than corneal diameter
dynamic fit
must have reasonable movement to facilitate tear pump. should move 1-2mm vertically with each blink
alignment
alignment of back surface with cornea, with a narrow band of edge clearance
patient response
stable vision and comfort after a period of adaption
Why use topical anaesthetic on first insertion of RGPs?
to improve initial comfort
px more likely to continue with lens wear
px will graduallt experience lens awareness while it wears off
give px an overall better experience when trying to adapt to the lens
Why not use topical anaesthetic on first insertion of RGPs?
softens epithelium which may result in greater incidence of staining
misleading px’s as to true comfort of GP lens
How to work out astigmatism?
the difference between the flattest k and steepest k reading
difference of 1mm = 5D
difference of 0.1mm = 0.50D
difference of 0.05mm = 0.25D
What happens when toricity is more than 1.50D?
fit becomes unstable
lens decentres
corneal moulding
physiological impact
fluctuating vision
lens flexure
lens discomfort
What is diagnostic fitting?
Diagnostic lenses used in an initial fitting appointment and final lens order determined from results obtained.
ADV: potentially fewer re-orders, greater px satisfaction and better compliance
DISADV: px unable to leave with lenses, no satisfactory vision obtained with first lens, lens must be carefully cleaned, disinfected and stored
What is empirical fitting?
Specifically designing lenses without using diagnostic trial lens sets. Only spherical lenses — tricurve (C3) and quadcurve (C4).
ADV: less initial chair time, good initial vision, minimise transfer of diagnostic lens contaminants
allows use of topography software.
DISADV: unless using corneal topography it is impossible to predict how lens will interact with cornea
What is topography-based fitting?
topography provides information about apex location, rate of flattening and
irregularity of the cornea. Software allows for lens selection, modification of parameters and virtual simulation of NaFl patterns.
ADV: ability to design lenses with a specific clearance
What is lid attachment fit?
a desirable characteristic when achievable. GP lens will tuck up under the upper lid. Generally a larger and flatter lens is required to achieve this fit
What is interpalpebral fit?
where lid attachment is not feasible, IP fitting is. Use of smaller, steeper lens to achieve centration between the upper and lower lids. Lens initially selected is 0.05mm steeper than flattest k.
Diameter ranges between 8.8 to 9.4mm.
Optic zone diameter will range from 7.4 to 7.8mm
