Competency 5.1.1 Flashcards
1
Q
Factors Affecting Comfort of a Contact Lens Material
A
o Higher modulus materials are less comfortable (e.g. early SiHy)
o Higher water content lenses can dry out and cause discomfort
o Poor wettability is related to reduced comfort
2
Q
What is Oxygen Permeability?
A
o This the amount of oxygen that gets through the lens onto the cornea
o Is a combination of the oxygen transmittibility of the material and the thickness of the lens
3
Q
Oxygen Permeability of Hydrogel Lenses
A
- water content of these lenses is what determines the oxygen permeability of the lens
- to obtain high Dk/t we need a high water content and therefore we risk drying the eye out.
4
Q
Example of Hydrogel Contact Lens Material
A
Poly-HEMA
5
Q
Oxygen Permeability of Hydrogel Lenses
A
- the oxygen permeability of the lens is dependent on the amount of silicone used in the lens
- allows for high Dk/t values to be obtained without drying the eyes out towards the end of the day
- . Silicone hydrogels can produce approx 5x higher Dk/t values when compared to a hydrogel
6
Q
Advantages of Hydrogel Contact Lenses
A
- Highly flexible material
- Good initial comfort
- Thinner lenses
- Affordable
7
Q
Disadvantages of Hydrogel Contact Lenses
A
- Low oxygen permeability
- Potentially higher risk for eye infections and other hypoxia-related issues
- Not ideal for overnight wear
8
Q
Advantages of silicone hydrogel Contact Lenses
A
- High oxygen permeability
- Easier to handle (particularly for new contact lens wearers)
- Better durability
- Extended wear and overnight wear options available (longer wear time)
- Available in a variety of modalities
- Available in different designs
9
Q
Disadvantages of silicone hydrogel Contact Lenses
A
- Early silicone hydrogel lenses sometimes had wettability challenges although newer SiHy materials have similar or better wettability than hydrogel lenses
- Early silicone hydrogel lenses used stiffer materials, however, newer SiHy materials have improved flexibility similar to that of hydrogels9
- Potentially higher risk for giant papillary conjunctivitis (GPC), an allergic reaction in the inner part of the eyelid with early SiHy materials, however, this is not a concern with newer, low modulus monthly and 1 day SiHy lenses.
- Slightly higher price, although newer lenses like clariti® 1 day are close to price parity with 1 day hydrogel lenses.
10
Q
Spherical Soft Contact Lens Designs
A
- Suit patients with little to no astigmatism (upto 0.75DC)
- Go up in 0.25DS steps but this may change to 0.50DS at higher powers
- Use spherical equivalent if there is astigmatism present
11
Q
Toric Soft Contact Lens Designs
A
- Corrects astigmatic refractive errors
- Must sit in a specific orientation to provide the correction
- Toric lenses feature markings to allow for identification of where and how the lens is sitting
12
Q
Methods of Stabilising Toric SCLs
A
o Prism-ballast designs are thinner at the top and thinner at the bottom of the lens and the weight helps keep the lens orientated correctly
o Peri-ballast (similar to prism-ballast) the thicker area is closer to lens periphery.
o Thin zones at the top and bottom of lens allow the actions of the lids to maintain orientation
o Posterior torics feature a toric back surface which conforms to the cornea, allowing stability
13
Q
Types of Multifocal Soft Contact Lens Design
A
- Concentric
- Aspheric
- Segmented
14
Q
Concentric Soft Multifocal Contact Lens Design
A
o Alternating rings of distance and near prescriptions
o Can be centre-distance or centre-near
15
Q
Aspheric Soft Multifocal Contact Lens Design
A
o Features a gradual change in power from distance to near Rx
o Can be centre-distance or centre-near
16
Q
Segmented Soft Multifocal Contact Lens Design
A
o Acts more like a traditional bifocal
o Distance prescription is always located in the upper section of the lens
o Lids help move the lower near prescription up into pupil view when patient looks down to read
17
Q
Soft Contact Lens Parameters
A
- BOZR
- TD
- Water Content
- BVP
- Lens thickness
18
Q
Back Optic Zone Radius (BOZR) of a Soft Contact Lens
A
- This is the curvature of the back optical zone of the lens
- Helps to ensure good fitting of the lens over the cornea
- Obtained by taking the flattest K and multiplying by 1.1
19
Q
Total Diameter of a Soft Contact Lens
A
- Most common diameter is 14.2 but diameters fall into range of 13.8-14.3mm
- There is a change of diameter of 0.1 to 0.8mm due to reduction of ocular surface temperature compared to ambient room temperature
- TD is obtained by taking HVID and adding 2mm
- If lens is too large
- poor movement
- less tear exchange
- If lens is too small
- Excessive movement
- Poor centration
- Poorer comfort
20
Q
Water Content of a Soft Contact Lens
A
- This is the amount of water contained within a CL
- Especially in Hydrogels this is a compromise between oxygen transmissibility and amount of evaporation
- If there is a high % water content:
o Increased comfort
o Less stability
21
Q
BVP of a Soft Contact Lens
A
- Differs from that of the spectacle prescription
- Is lower in CLs for a myopic Rx
- Is higher in CLs for a hypermetropic Rx
22
Q
Lens Thickness of a Soft Contact Lens
A
- A thin lens:
o Increases oxygen transmissibility
o Reduces ease of handling
o Has a shorter life
o Creates a thinner post-lens tear film (corneal dessication)
23
Q
What Should be in a CL H&S
A
- Motivation
- Occupation
- Lifestyle
- Ocular History
- General Health
24
Q
Lens Choice Based on How Often They Would Like to Wear Them
A
- A patient who is only wishing to wear the lenses for occasion social or sporting use could benefit from daily disposable lenses
- A patient who is looking to wear the lenses FT most days of the week could benefit from monthly replacement lenses
- A patient who is looking to wear the lenses almost all waking hours could benefit from a lens which is suitable for EW
25
Lens Choice Based on Previous CL experience
- If prev non-compliance then consider daily disposables even when wear modality would be better suited to a monthly lens
26
Lens Choice Based on Occupational Demands
- VDU Use
o What type of VDU
o Working distance considerations in a presbyope
o How much time on VDU as this is known to reduce blink rate
o If lots of VDU usage then lenses with good wettability are advisable
- Driving
o Reduces blink rate
o Factors such as air conditioning can affect lenses
o Regular gaze changes can affect toric wearers
- Enviroment
o Dusty enviroments and those with toxic fumes present a contraindication
o Pilots are only allowed to wear distance only contact lenses
27
Lens Choice Based on Lifestyle/Hobbies
- Sport
o SCLs are more advisable for sports due to safety reasons
o Advise eye protection for sports such as squash
- Swimming
o Discuss the increased risk of infection
o If insistent on swimming then advise daily disposables with tight fitting goggles over the top
- Outdoor Activity
o UV protection
o Still need to encourage sunglasses wear
28
Lens Choice Based on General Health
- Diabetes
o Cornea is adversely affected in 70% of diabetics
o DM can result in slow and incomplete corneal healing
o Daily disposable CLs to minimise infection risk may be advisable
- Arthritis
o May suffer from inflammatory dry eye
o Affects manual dexterity
o Could benefit from a lens with a higher modulus
- Skin Conditions (Eczema)
o May be more vunerable to excessive deposits, lid irritation, punctate keratitis and blepharitis
- Thyroid Dysfunction
o Concern around exposure keratopathy related to TED
o TED can cause incomplete blinking and meibomian gland dropout
- Allergies
o More likely to have pre-existing papillae
o Can be more vunerable to CLAPC
o Can be more prone to reactions to SCL solutions
- Smoking
o Not commonly asked
o Associated with 3.7x increase in bacterial keratitis risk
29
Potential Ocular Health barriers to CL wear
- any acute or sub-acute inflammation of the anterior segment
- acute or chronic eye infection
- corneal dystrophies
- pterygium or pinguecula
- dry eye or blepharitis
- previous or current microbial keratitis
- Ocular conditions requiring frequent drop use such as glaucoma or allergy
30
Water Content Considerations for Lens Choice
- To maintain stability a high water content lenses absorb water from the ocular surface
- can lead to ocular dryness
- if patient has dry eyes then a lower water lens should be considered
31
Oxygen Considerations for Lens Choice
- The more time the patient would like to have the lens in their eye, the higher the oxygen transmissibility will have to be.
32
Back Optical Zone Radius Impact on Lens Choice
- calculated by flattest K multiplied by 1.1
- Most lenses will have only one BOZR option
- The idea is to fit slightly flatter than the corneal curvature to allow for tear exchange and tear movement
33
Steps of Soft Contact Lens Fitting Routine
- Lens insertion
- VA check
- Over-refraction
- Fit check
- Centration and Coverage
- Comfort
- Movement on Blink
- Lag
- Push up test
34
Over-RefractionCentration and Coverage
- Conduct a simple spherical over-refraction
- Remember not to give initial +0.25DS due to 3m room.
35
Centration and Coverage in SCL Fit
- The lens should cover the cornea, overlapping onto the bulbar conjunctiva in all positions of gaze
- of particular importance in multifocal lenses as decentration can affect the lenses aberrations.
- Use fitting cross to note centration and note that no limbal touch
36
Comfort Assessment in SCL Fit
- very subjective measure which holds little clinical value but is worth asking regardless
- ask the patient to gauge the comfort of the contact lens from 0 to 10
- Be wary of reduced corneal sensitivity in diabetics which may report a poor fitting lens as comfortable.
- worth noting that a tight fitting lens will also not be reported as uncomfortable.
37
Movement on Blink in SCL Fit
- Movement on blink with the eye elevated has been shown to be more diagnostic of overall lens movement than with the eye in primary gaze
- Normal movement is of around 1/3mm
- Use amount of lens overlap (around 1mm) to gauge movement
38
Lag in SCL Fit
- is the resistance of the lens to movement away from primary gaze
- If the lens is nice and mobile then it will tend to lag behind the movement due to interactions with anatomy e.g lids
- Lag has been shown to be more clinically useful than sag which is actually a measure of distortion of lens shape rather than lens mobility
- results can be interpreted in percentage increases in overlap:
- Ideal fit is 50-100% increase
- Loose fit is over 100% increase
- Tight fit is less than 50% increase
39
Push Up Test in SCL Fit
- very useful diagnostic test for lens fit.
- The speed of recovery is the more useful aspect of assessment compared to how easy the lens is to dislodge
- The ideal lens neither drops instantly back down nor sluggishly moves back to its position, it performs somewhere inbetween.
- clinical experience is required to make this judgement
40
Reasons not to have Limbal Touch in SCLs
- Ocular stem cells are at the limbus so if the lens was to touch this area there would be a decrease in stem cell viability
- The area of the limbus is the end of the corneal vascular area, if a lens was to touch this then neovascularisation of the cornea could occur
41
Contact Lens Specification Example
BOZR/TD/Rx
Lens name
8.4/14.0/-3.00
Acuvue Oasys
42
College Guidance for Contact Lens Specification
- Optom details
- sufficient details of any contact lens fitted to enable the lens to be replicated. This would normally include some or all of the following information:
o power
o base and peripheral curves of the lens
o total diameter
o material
o design (including details of the generic brand) of the contact lens(es)
- the date the specification expires
- any clinical information that should be taken into account by a supplier.
