Soft CLs for Astigmatism (Toric Fitting) Flashcards
1
Q
What are the lens designs in soft toric CLs?
A
- Toric back surface with a spherical front surface
- Toric front surface with a spherical back surface
- Wraps on the cornea so there is a negligible tear lens -> no induced astigmatism like w/ RGP
- Design is usually simply related to manufacturing i.e. what kind of mould is used to make them -> don’t really need to consider it
- All the ocular astigmatism is usually corrected just by incorporating cyl into lens power ordered
2
Q
What are the special design characteristics (methods of stabilisation) of toric CLs?
A
- Cyl must remain in correct place cannot be rotated otherwise px may struggle to see
- Methods of stabilisation:
o Prism-ballast:
1 to 1.5ΔD base-down
Stabilise by prism-induced CL thickness differences
Relies on lid forces (mainly upper lid forces) acting on thickness differences
Thicker inferior CL
↓CL O2 transmissibility at the particularly thick region of lens
CL always returns to bottom (gravity) so meridian should be in correct place
Discomfort w/ CL-lid interaction (on bottom lid)
o Peri-Ballast:
Similar to prism ballast – creating same base down prism effect but by thinning lens instead better than using prism ballast
Uses thickness differences as stabilising component, creates a base down prism effect
Can get discomfort w/ CL-lid interaction at thicker inferior half
↓CL O2 transmissibility in thicker regions compared to thinner regions
Thinner superiorly, thicker inferiorly
Orientation principle similar to prism-ballast
Similar overall thickness to spherical CL
o Truncation:
Usually done on a CL already w/ a prism ballast
Involves removing section of bottom of lens
Truncation aligned w/ lower lid margin can lead to some CL stabilisation
Truncation can induce some discomfort in that area
Truncation alters thickness profiles & differentials – hard to use with oblique astigmatism & an entirely new orientation can result w/ the truncation grossly misaligned w/ lower lid margin
Truncation is not always successful
More px visits can be required
Seldom used now more of a last resort if had other problems where cannot get lens to be stable
o Dynamic stabilisation:
o Toric back surface:
o Double Slab-off (DSO):
Thin zone superiorly & inferiorly
Lid forces (upper & lower) maintain orientation – if px has loose lids then results may not be as reliable
DSO CLs need to be fitted w/ larger TD to guarantee satisfactory performance in a majority of wearers
Overall thinner CL
CL is symmetrical
Can exhibit ↓rotational stability if low sph, WTR astig
o Accelerated Stabilisation Design:
Acuvue design
Thin zone superiorly & inferiorly
4 active zones of added thickness located in midperiphery of lens
Designed to be actively rotated into place upon blinking whenever it is misoriented & then held stable when lens is correctly aligned
3
Q
What are the steps in selectig a 1st soft toric CLs?
A
- Decision on modality of wear from H&S & SL exam – as for spherical
- O2 & H2O content considerations
- Decision on material – hydrogel/SiHy
- Choose a lens as a starting point
a. BOZR/TD
b. Consider range of Rx available
c. Consider cost – torics are generally more expensive than spherical - Refer to manufacturer’s guidelines for fitting guidance
4
Q
When should you use a soft toric lens?
A
- Refractive astigmatism (>0.75 but may not try straight away, especially if px is not going to wear them regularly)
- Spherical SCLs failed to mask corneal astigmatism
- Tried GP CLs but issues with residual astigmatism
- Tried GP CLs but issues with discomfort
5
Q
What are physiological considerations in soft toric lenses?
A
- Complications such as corneal oedema & corneal vascularisation more likely ↑CL thickness
- If problems occur, advise switch to SiHy CLs
- If problems persist, refit w/ GP CLs
6
Q
List some of the common soft toric CLs?
A
7
Q
What are the specialist companies for soft toric CLs?
A
- Mark Ennovy:
o Extended Rx ranges (+/- 30D & up to 8D astigmatism)
o TD of 11.50-16mm & base curve of 6.50-9.80mm available with different lenses - Cantor + Nissel:
o Base curve 6.60 to 10.40, TD 11.00 to 20.00, Power range -30.00 to +30.00D, cyls -6.00DC to -0.75DC
8
Q
What to remember when fitting soft toric CLs?
A
If cyls is not bang on – go for slightly lower one
* Not always only 1 lens for px
* May be several lenses that will be suitable – this no. may be lower w/ torics
* Make informed & safe decision based on info & then assess fit
* Trial & error element to SCL fitting
* Often 1st lens px tries will not be final lens prescribed
9
Q
How do you adjust the Rx for BVD in toric lenses?
A
K=F/(1-dF) -> for Rxs above 3/4D – needs to be done for both meridians in toric lenses (need to add the cyl and sphere to get other meridian value)
Need to then do difference between the two BVP values you get
e.g. Rx: -3.00/-2.50x90 R&L, BVD 12mm
K = F/1-df = -3.00/(1-(0.012x(-3.00)) = -2.90
K = F/1-df = -5.50/ (1-(0.012x(-5.50)) = -5.16
-2.90 -> -5.50 = -2.26
–> -2.90/-2.26x90 –> Final adjusted Rx: -3.00/-2.25x90
10
Q
What do you need to assess the fit of a soft toric CL?
A
- Centration/coverage
- Lens movement on blink/rotation – if see rotation, want to approx. how much that is & make adjustment to lens (soft lenses have marking on them to help assess this)
- Lag/sag
- Push-up test
- Comfort – ask px
- Overall tightness - % (50%=perfect, 0%= falling out, 100%=stuck) or statement (aligned, flat, steep)
11
Q
Which factors influence soft toric CL location?
A
- Lid: tension, location, action, blink force
- Vertical palpebral aperture – if particularly low lower lid, lens may drop – if low top lid then can’t see lens as well
- Gravity
- Water content
- Lens thickness
- Total diameter – if want to stabilise lens go for larger diameter
- Tailor-made for px versus stock toric
12
Q
What are the important things to note about soft toric CLs?
A
- Lens does not necessarily need to stabilise at 6, 9 or 12 o’clock – MUST NOT BE ROTATING WITH BLINK (will lead to poor vision)
- E.g lens may stabilise at 7 o’clock – this is acceptable as long as lens quickly returns to this position on movement & blink
- More important that lens is stable & VA good
- Rotating lens will most likely give variable or poor vision
13
Q
Describe the CL Rotation: Measurement in soft toric CLs?
A
- CL marks are for reference only ‒ optically, they are of no particular significance – if px’s axis is 180 it does not mean that the markings will be at 180 (not like trial frame lenses)
- Toric CLs tend to rotate nasally by ~5 to 10°
- Actual magnitude & direction of lens rotation is subject to large individual variations depending on:
o Lid Anatomy:
Lid tension (tight lids, loose lids), lid location w/ respect to cornea & palpebral aperture size
o Lens-eye Relationship:
Consider the fit of CL on eye, Large diameter versus small diameter, loose fit versus tight fit etc
o Lens Thickness Profile:
Determined by lens design & lens power – in particular axis & magnitude of astigmatic correction - Rotational influence is greatest for toric lenses with oblique axes
- Measure rotation using:
o Narrow slit-lamp beam with protractor scale
o An eyepiece with protractor-scale graticule is ideal - Estimate the magnitude of CL rotation (?° & direction) – think about it like hours on clock
- Two Mnemonics if see rotation:
o LARS (Left rotation Add to the axis, Right rotation Subtract to the axis)
o CAAS (Clockwise, Add; Anti-clockwise, Subtract) - This adjustment should be made to spec Rx & order a new Rx based on that
14
Q
What are the assessments of fit in soft toric CLs?
A
- Good fit: Full corneal coverage, good centration & movement, quick reorientation if mislocated
- Tight fit: Good centration, initially comfortable, little or no movement. Slow reorientation if mislocated
- Loose fit: Excessive movement, poor centration, uncomfortable. CL orientation unstable & inconsistent
15
Q
What is the fitting procedure in soft toric CLs?
A
- Measure Rx & vertex distance
- Select toric CL design
- Select CL power (BVP) to match both meridians
- Select BOZR &/or TD
o Err on large side when choosing CL TD as will aid CL stabilisation - Compute final Rx including axis compensation (if you have used a lens from a trial bank)
o CLOCKWISE YOU ADD
o ANTI-CLOCKWISE YOU SUBTRACT - Toric trial CL method:
o Trial CL is inserted & allowed to settle – its orientational behaviour is determined – any compensation made should be recorded - Empirical Method:
o Measure spec Rx & vertex distance & derive corneal-plane refraction (close approximation to true ocular RX)
o Measure corneal radii of curvature & HVID
o Supply data to manufacturer
o If any subsequent CLs are required, they should be based on experiences w/ 1st delivered CLs which assume the role of trial CLs
