CLM - Soft Contact Lens Fitting II - Week 6 Flashcards
95% of astigmatic patients have a cyl prescription of what D or less?
-2.25D or less
What is generally considered clinically significant astigmatism? What percentage of patients have this in at least one eye?
≥ 0.75DC
~50% have this in at least one eye
List three advantages of soft toric contact lenses.
Potential to deliver enhanced VA and contrast sensitivity
Improved longer-term retention of CL wearers
Range of lens designs/modalities now available
What lens fitting characteristics are more critical with soft toric lenses (4)?
Centration
Movement
Stability
Rotation
List 2 additional considerations for soft toric contact lenses.
Lens comfort
Increased cost
In what two ways is the astigmatic correction made?
Toroidal back surface with spherical front
Sperical back surface with toroidal front
List 5 stabilisation techniques for toric soft contact lenses.
Prism ballast Dynamic stabilisation Per-ballast Truncation Back surface toricity
What is the underlying principle of a prism ballast and what does it do to the lens inferiorly? What power prism is typically used?
Balance the forces acting on the lens in order to stabilise it
Increased lens thickness inferiorly
1-1.5^BD prism used
Describe how the watermelon seen principle applies to the prism ballast.
The squeezing pressure of the upper lids causes the lens to be expelled in a direction away from the apex, pushing the prism into BD position
What structure is the dominant stabilising component?
Superior lid
What is a peri-ballast and how does it compare to a prism ballast?
Minus lens carrier with increased thicknes sinferiorly
It is a prism-free optic, and acts similar to the prism ballast
What is the most common method of soft toric lens stabilisation?
Dynamic stabilisation
Explain how dynamic stabilisation works. Note whether it uses prism in its design, whether or not it is symmetric, and how its thickness compares to other means of stabilisation. What happens to lens with this design when blinking?
The superior and inferior zones incorporate a thickness differential
It is prism-free, symmetrical, and thinner lens design
Lens moves back against the lids during blink to maintain orientation
What is truncation for toric lens stabilisation? Is it successful? What is lens comfort like?
The prism ballast lens can be truncated inferiorly to align with the lower lid margin
Has limited success and is uncomfortable
Explain 2C MAPRS for soft toric lenses.
Centration Coverage Movement Acuity Push-up Rotation Stability
What is a good toric fit in terms of coverage, movement, and rotation (3)?
1.5-2mm coverage is ideal
0.25-0.5mm with blink at primary is ideal
Good fit - quick return to axis if lens is mislocated
Tight fit - slow return to axis
Loose fit - orientation unstable
What are the three ways of determining the BVP to trial a lens with?
Empirically
Using spherical trial lenses
Using toric trial lenses
Describe the empirical method to calculating BVP for trial lenses.
Calculated according to specatcle prescription, taking into account vertex distance
Are tear lens calculations required for soft contact lenses?
No, thank fuck
How can spherical lenses be used to calculate BVP for toric lenses?
Use a spherical trial lens to determine fitting characteristics, then measure sphero-cyl over-refraction
BVP is then BVP of the trial lens + over-refraction
What is the preferred method for fitting custom toric lenses?
Using spherical trial lenses
Are toric trial lenses commonly used? What is the selection of BVP based on? What is done after to get the correct final BVP? What may need to be done to determine this?
Commonly used as they are readily available
Choose BVP based on spectacles, with vertex distance accounted for
Measure sphero-cyl over-refraction
To determine the final BVP, may need to do matrix calculations of oblique cyls
What is the effect of lens rotation on astigmatic power?
The contact lens only has to rotate a small amount to generate a large amount of cyl at a different axis
What three factors influence the extent of lens rotation? Explain each if applicable.
Lid anatomy -tighter lids have a greater effect Fitting relationship -steep lenses may not stabilise if tightly adhered to the eye Lens thickness profile -influenced by the refractive error
