6. CL VI - Toric CL Fitting & Evaluation Flashcards
Astigmatism is a ... in which the ... of one or more of the refractive components of the eye, usually the ..., prevents light rays from ..., resulting in ... There are approximately ...% spectacle wearers who have clinically significant astigmatism (≥ ...DC)) in at least one eye. There are also around ...% of the population needed astigmatic CL correction. ...% of astigmatic patients have cylinder Rx of -...DC or less. ...% of astigmatic patients have cylinder axes at or near ...º or ...º ± 20º.
Astigmatism is a visual defect in which the unequal curvature of one or more of the refractive components of the eye, usually the cornea, prevents light rays from focussing to a single point focus on the retina, resulting in blurred vision. There are approximately 45% spectacle wearers who have clinically significant astigmatism (≥ 0.75DC)) in at least one eye. There are also around 47% of the population needed astigmatic CL correction. 94% of astigmatic patients have cylinder Rx of -2.25DC or less. 80% of astigmatic patients have cylinder axes at or near 90º or 180º ± 20º.
What is the trend in toric CL prescribing in Australia?
There are increasing levels of toric CLs prescribing in Aus, which is nearing the threshold of 45% with clinically significant astigmatism.
What are the advantages of toric soft CLs?
- Can deliver enhanced VA and contrast sensitivity
- Improved longer term retention of CL wearers
- Range of lens designs/ modality options are now available, including SiHy, daily disposables
What are the important considerations of soft toric CLs?
- Fitting of toric lenses are more specific: 2C MAP (centration, coverage, movement, acuity, push-up, rotation)
- Lens comfort
- Increased cost to px
- Oxygen transmissibility
- Mechanical insult to cornea
What are the factors that need to be considered when prescribing soft toric CLs?
- Degree of astigmatism
- Intended wearing schedule
- Ocular dominance
- Viability of the alternatives
- Assessment of visual needs
- Design of toric CLs (surface optics, stabilisation techniques)
In terms of surface optics, what are the 4 ways we can create astigmatic correction in RGPs? What about for soft CLs?
- Spherical back surface with spherical front surface
- Toroidal back surface with spherical front surface
- Spherical back surface with toroidal front surface
- Toroidal back surface with toroidal front surface (bitoric);
Soft CLs alaways have bitoric design due to soft material wrapping onto the eye
What are the 5 stabilisation techniques of toric CLs?
- Prism ballast
- Peri-ballast
- Dynamic stabilisation (double slab off)
- Truncation
- Back surface toricity
How do eyelids and blinking influence movement of CLs?
Our superior eyelid moves CLs downwards during blinking; Inferior eyelid moves CLs with a slight nasal rotation.
Prism ballast technique ... in order to balance .... It creates around ...Δ ... prism. This uses the “watermelon seed principle” where the ... from the ... will cause the lens to be expelled in a direction ..., therefore pushing the lens into a BD position. It is the ... interaction with the ... of the lens (superiorly) that keep the CLs stable.
Prism ballast technique increases lens thickness inferiorly in order to balance the forces acting on the lens to stabilise it. It creates around 1.0-1.5Δ BD prism. This uses the “watermelon seed principle” where the squeezing pressure from the upper lids will cause the lens to be expelled in a direction away from the apex, therefore pushing the lens into a BD position. It is the superior lid interaction with the thinnest meridian of the lens (superiorly) that keep the CLs stable.
Peri-ballast is a ..., with .... The prismatic thickness profile changes is confined to the .... Similar to ..., the mechanics of the ... help stabilise the orientation of the CL.
Peri-ballast is a minus lens carrier, with incrased thickness inferiorly. The prismatic thickness profile changes is confined to the lens carrier. Similar to prism ballast, the mechanics of the superior eyelid help stabilise the orientation of the CL.
Dynamic stabilisation is the .... The ... from the dynamic stabilisation portions, which incorporate a .... This generates a ..., ..., ... lens design. During blinking, the lens ... to maintain stabilisation.
Dynamic stabilisation is the most common method of stabilisation. The superior and inferior zones from the dynamic stabilisation portions, which incorporate a thickness differential. This generates a prism-free, symmetric, thinner lens design. During blinking, the lens moves back against the lids to maintain stabilisation.
Truncation is a ... lens that is truncated ... to align with the ... along a 0.5-1.5mm chord. This is not commonly used due to ... and ....
Truncation is a prism ballast lens that is truncated inferiorly to align with the lower eyelid margin along a 0.5-1.5mm chord. This is not commonly used due to limited success and uncomfortable.
What are the 3 things we need to ensure in order provide constantly clear and comfortable vision?
- CLs needs to be aligned over the central cornea
- CLs needs to provide correct refractive power when on the eye
- CLs need to be stabilised so that the refractive powers are aligned with the intended meridia
What does 2C MAPRS stands for?
Centration Coverage Movement Acuity Push-up Rotation Stability
Outline what defines a good fit for toric CLs.
(Similar to soft spherical CLs)
* Good centration
* 1.5-2mm coverage
* 0.25-0.50mm movement with blink in primary gaze
* Quick return to axis if the lens is mislocated
How is BVP determined for a trial toric CLs?
- Empirically
- Using spherical trial lenses
- Using toric trial lenses
Empirical determination of BVP is calculated according to the ... and also .... Any ... should also be accounted for when determining astigmatic axis. ... are required for soft CLs.
Empirical determination of BVP is calculated according to the spectacle Rx and also taking vertex distance into account. Any estimated degree of len rotation should also be accounted for when determining astigmatic axis. No tear lens calculations are required for soft CLs.
Using a spherical trial lens, ... can be determined and then ... can be measured to find the required BVP. The required BVP = ... + .... We should also account for ... of the lens when ordering .... This is the preferred method for fitting of ....
Using a spherical trial lens, fitting characteristics can be determined and then Sphero-cylindrical overrefraction can be measured to find the required BVP. The required BVP = BVP of trial lens + ORx. We should also account for estimated rotation of the lens when ordering cyl axis. This is the preferred method for fitting of custom tonic lenses.
Topical trial lenses are commonly used to determine BVP due to .... BVP is chosen based on .... Then with toric trial lens in px’s eye, measure ... or ... :. Then determine final BVP, ... may be needed .
Topical trial lenses are commonly used to determine BVP due to ready available disposable toric trial lens sets. BVP is chosen based on spectacle Rx adjusted for vertex distance. Then with toric trial lens in px’s eye, measure spherical or Sphero-cyl O-Rx. Then determine final BVP, matrix calculations of oblique cylinders may be needed .
What is the effect of lens rotation on astigmatic power?
The larger the degree of rotation, the larger change in refractive error. This increases with increased astigmatic power to start with.
What 3 factors influence the extent of lens rotation?
3 factors
- Lid anatomy - tighter lids have greater lens mislocation effects (physiological nasal rotation with blinking)
- Fitting relationship (e.g. steep lens may not stabilise well if too tightly fitted onto the cornea)
- Lens thickness profile
What are the assumptions of CL rotation on the eye?
5-10 degrees of nasal rotation
How is the orientation of soft toric CLs determined?
There are reference marks that help orientate the lens on the eye, which also helps estimate the direction, stability, and degree of lens roattaion. These may be applied to the lens with dye or are laser inscribed. Different manufacturers will use different orientation markers. The direction of rotation is taken with respect to the inferior axis of the lens.
What are the rules for calculating eye rotation? (Which direction is positive/ negative)
LARS = Left Add, Right Subtract;
CAAS = Clockwise Add, Anti-clockwise Subtract;
ASS = Anti-clockwise Subtract Swing
Original spectacle cylinder axis = 90º; Ordered CL axis = 90; CL is found to rotate 15º to the right. What is the new CL axis that should be ordered?
90º-15º=75º. This assumes the same degree of rotation will occur with the new CLs, and the CLs will rotate to the true axis, which is at 90º
What can be used to estimate the degree of rotation of CLs under the slit lamp?
Think of CL as a clock, for each clock hour, there are 30 degrees of rotation. Rotation can also be measured using the slit lamp’s vertical beam.
With how much degree of rotation must you compensate for aka order new CLs to compensate for rotation? What are the other indications that compensation is needed?
≥ 10 degrees = must compensate for rotation. Perform a Sphero-cyl refraction over the lens, if VA improves, Rx may be adjusted or change degree of rotation of CLs accordingly.
The degree of ... of a toric soft CL on the eye can be calculated by using either ... or .... ... are used to facilitate this calculation. It is important to beware that online calculators .... There are large variations depending on ..., ..., ... etc. We should base the initial CL order for axis upon ..., then compensate by .... Beware that once you have adjusted for lens rotation, ..., however this is expected. If the ..., it indicates that the lens is stable on the eye.
The degree of misalignment of a toric soft CL on the eye can be calculated by using either px's ocular refraction or refraction over the mislocaing lens. Matrix optics are used to facilitate this calculation. It is important to beware that online calculators does not accurately predict the required adjustments. There are large variations depending on lens-eye relationship, lid anatomy, thickness profile of lens etc. We should base the initial CL order for axis upon the axis obtained in the spectacle refraction, then compensate by looking at the rotation on the eye itself. Beware that once you have adjusted for lens rotation, the lens orientation markers will still appear to be rotated, however this is expected. If the degree of rotation is constant between blinks, it indicates that the lens is stable on the eye.
What does it indicate if the degree of rotation varies between blinks with a toric CLs?
3 things
- Stabilisation methods are not working
- BOZR is too flat
- Diameter is too small
What signifies an ideal fit for soft toric CLs?
3 things
- Full corneal coverage
- Appropriate centration & movement
- Predictable rotation
What signifies a tight fit for soft toric CLs?
3 signifiers
- Full corneal coverage
- Appropriate centration but little movement
- Slow return to axis if mislocated
What signifies a loose fit for soft toric CLs?
5 signifiers
- Poor centration
- Excessive movement
- Uncomfortable
- Poor stability
- Fluctuating VA
Toric soft CLs are more difficult with ...; oblique cylinders due to ...; ...; and .... Toric CLs tend to be ..., therefore can result in reduced ... and increased ....
Toric soft CLs are more difficult with low spherical components; oblique cylinders due to poor stability; large cylinders; and irregular astigmatism. Toric CLs tend to be thicker, therefore can result in reduced Dk/t and increased mechanical irritation.
