5. CL V - Soft CL Fitting & Evaluation Flashcards
What is the acronym for soft spherical lens fitting evaluations?
2C MAP
Centration, Coverage, Movement, Acuity, Push-up
Why do we check lens centration?
To ensure the lens isn’t rubbing against the ... to prevent ..., ..., ..., ..., and ....
Why do we check lens centration?
To ensure the lens isn’t rubbing against the limbus to prevent neovascularisation, staining, injection, corneal nerve exposure, and general discomfort.
What do the following lag movements mean when checking centration?
No lag: ... and no ..., it’s basically ...
Too much lag: ...
Sliding around: ...
What do the following lag movements mean when checking centration?
No lag: too tight and no tear flow, it’s basically suction cupped to the eye
Too much lag: too tight
Sliding around: too loose
How is lens centration assessed?
- Primary gaze
Ensure...and...overlap onto the...in all meridia - Peripheral gazes
Look at...and...gazes. Need to assess the...which is.... Can be assessed with...,...,..., or.... Does not need to be...in each gaze.
Insufficient: ...mm
Acceptable: ...-...mm
Excessive: ...mm
How is lens centration assessed?
- Primary gaze
Ensureuniformandsymmetricaloverlap onto thesclerain all meridia - Peripheral gazes
Look atlateralandverticalgazes. Need to assess thelens lagwhich isthe amount the lens trails the eye movements. Can be assessed withTD,HVID,blood vessels, orslit lamp beam. Does not need to beequalin each gaze.
Insufficient: <0.3mm
Acceptable: 0.3-0.7mm
Excessive: >0.7mm
What are the factors to consider with poor lens centration?
BOZR selection -> ...
Comfort -> ...
Visual acuity -> ... or ...
Lens diameter -> ...
What are the factors to consider with poor lens centration?
BOZR selection -> too flat?
Comfort -> enhanced awareness
Visual acuity -> suboptimal or fluctuating
Lens diameter -> too small?
How is lens coverage assessed?
The lens needs to ... the cornea at ..., in each ....
Unacceptable: ...mm as it may ...
Acceptable: ...-...mm
Ideal: ...mm
How is lens coverage assessed?
The lens needs to completely cover the cornea at all times, in each position of gaze.
Unacceptable: <0.5mm as it may rub against limbus
Acceptable: 0.5-1mm
Ideal: >1mm
Why is complete corneal coverage important?
Mechanical: ..., ..., and ... trauma is possible
Physiological: minimises ..., ..., and ...
Comfort
Why is complete corneal coverage important?
Mechanical: corneal, limbal, and conjunctival trauma is possible
Physiological: minimises corneal exposure, dessication, and tear film disturbance
Comfort
What are the factors to consider with poor lens coverage?
TD -> ...
BOZR selection -> ...
Could go for a ... as they have larger ....
What are the factors to consider with poor lens coverage?
TD -> too small?
BOZR selection -> too flat?
Could go for a toric lens as they have larger TDs.
How do you assess lens movement?
Looking at the ... change in lens position ....
First in ..., then in ..., and then in ... if necessary.
Insufficient: ...mm
Ideal: ...-...mm
Generous: ...-...mm
Excessive: ...mm
How do you assess lens movement?
Looking at the vertical change in lens position before and after a normal blink.
First in primary gaze, then in upward gaze, and then in lateral gazes if necessary.
Insufficient: <0.2mm
Ideal: 0.2-0.5mm
Generous: 0.5-1.0mm
Excessive: >1.0mm
How is the push-up test conducted?
Examiner pushes the lens ... by .... The lens must return to normal position within ... second.
The lens is too tight if it ... on return or .... The lens is too loose if it ... on return.
If the lens does not move, ... as it is too tight.
How is the push-up test conducted?
Examiner pushes the lens superiorly by pushing the lens up via the inferior lid. The lens must return to normal position within 1 second.
The lens is too tight if it undershoots on return or moves up easily. The lens is too loose if it overshoots on return.
If the lens does not move, DO NOT PRESCRIBE as it is too tight.
Explain lens settling.
It is a ... where the tear film is ... but is then ... which results in ... and ....
Explain lens settling.
It is a post-application equilibration process where the tear film is initially trapped behind the contact lens but is then squeezed out by initial blinking which results in settling and reduced lens movement.
Name the 8 International Standards Organisation (ISO) terms for contact lens parameters.
- Back Optic Zone Radius (BOZR)
- Total diameter
- Back Vertex Power (BVP)
- Water content
- Dk
- Material
- Design
- Thickenss
- BOZR (
...) =... - TD (
...) =... -
...(...) = power of the lens - WC (
...) =... - Dk =
...(barrer) =... -
...= oxygen transmissibility = the amount of oxygen passing through a contact lens of...over a set amount of... and ....
- BOZR (
mm) =radius of curvature of the back surface of the lens - TD (
mm) =full (overall) lens diameter -
BVP(D) = power of the lens - WC (
%) =percentage of water in the contact lens material - Dk =
oxygen permeability(barrier) =The amount of oxygen passing through a contact lens material over a set amount of time and pressure difference -
Dk/t= oxygen transmissibility = the amount of oxygen passing through a contact lens ofspecified thicknessover a set amount oftime and pressure difference.
What is the standard format of specifying a lens?
Manufacturer, Lens Brand - BOZR/ TD/ BVP
What is the relationship between radius of curvature and the curvature?
The longer the radius, the flatter the curve; The shorter the radius, the steeper the curve; The lower the curvature, the flatter the curve; the higher the curvature, the steeper the curve.
How do you convert radius in mm to curvature in D, and vice versa?
Divide 337.5 by either radius or curvature e.g. 337.5/9mm = 37.5D
How do you select BOZR initially for your patient?
Select a BOZR 10% flatter than a flat sim K, which equate to having 0.6 to 0.8mm flatter than average sim K reading. However, most disposables only come in one or two BOZR choices. We should expect that BOZR is 8.6 to 8.7mm will fit most corneas.
What are the consequences of having a flat or steep BOZR?
Flat BOZR may result in poor lens centration. Steep BOZR may result in reduced lens movement.
Does changing the BOZR of disposable soft CLs changes lens movement?
No, change in BOZR doen’t greatly change lens movement. However, different lens designs in the same BOZR can have very different movement profiles. Lens fit depends on various factors: BOZR, lens design, material, thickness and total diameter.
How should you select BOZR for silicon hydrogel?
We should choose the steepest BOZR in order to minimise mechanical issues with SiHy. This is because of the higher modulus of SiHy. The steeper the lens, the tighter the lens, therefore gives less lid interaction and lens movement.
How do you select the total diameter (TD) of SCLs?
2.00mm larger than horizontal visible iris diameter (HVID). Larger TD reduces lens movement and may impede tear exchange; Small TD gives excess movement, poor centration and less comfort. Often TD is restricted as they are only made in one diameter for a given brand.
How is BVP of SCL determined?
BVP is calculated from spectacle refraction or from over-refraction with a trial contact lens. For spherical lenses, ‘spherical equivalent’ power may be used: SE(D) = sphere + 0.5 (cylinder). For torics, BVP needs to be calculated separately in each meridian. The vertex distance need to be adjusted for spectacle Rx ≥ ± 4.00D: Foc = Fs/ (1-dFs)
What are the general difference between speactacles and CLs prescriptions for myopes and hyperopes?
CL Rx for myopes tend to be lower than their spectacles Rx; CLs Rx for hyperopes tend to be higher than their spectacle Rx
Outline the choice of CL material overtime. What is the preferred material nowadays?
Historically, a low water content material was preferred due to the durability and ease of manufacture. Mid-water and high-water content hydrogel materials were preferred due to the improved Dk/t. Nowadays, silicon hydrogel material are the material of choice.
What are the 4 factors that influence the choice of CL materials?
- Desired Dk/t
- Deposition profile
- Wettability/ Dry eye issue
- Modality
Why are hydrogel lenses made as thin as possible? But what are the disadvantages?
In order to maximise oxygen transmissibility. Thinner lenses are less easy to be handled and has a lower lifespan as they dry out quicker. Thin lenses give thinner post-lens tear film, therefore there is less movements with blinks and may give desiccation staining of the cornea.
Which type of CLs are relatively thicker?
Plus lenses are thicker in the centre; Toric lenses are thicker in the periphery. Hydrogel lenses are thicker to higher water content. Silicon hydrogel lenses have a higher modulus and can be thicker.
What are the eye factors that can influence the fit of a CLs?
Topography of the anterior corneal surface; Eyelid position/ tension/ blink strength/ surface regularity; HVID/ corneal size
What are the lens factors that can influence the fit of a CLs?
- Lens prescription & thickness profile;
- Lens material (e.g. modulus/ rigidity);
- Design (peripheral curves, spherical/ aspheric, edge profile);
- ‘Sag’ properties of the lens
What are the 6 components needed for SCL selection?
- BOZR (0.6-0.8mm flatter than Kaverage)
- TD
- BVP (consider lens effectivity for Rx ≥ 4.00D/ astigmatic Rx)
- Material (SiHy»Hy)
- Design
- Modality
