SCL Fitting 1 Flashcards
On what basis are RGPs replaced?
Replacement every 1-3 years or earlier if scratched
On what basis are soft contact lenses replaced?
Depending on the type they are:
– Daily disposables (DW)
– Bi-weekly or Monthly disposables (EW)
[EW- extended wear]
What is a daily disposible lens?
FDA definition: a lens replaced after each use (i.e. remove and discard)
What are examples of daily soft contact lenses?
- Acuvue 1-Day Moist
- Dailies Aqua Comfort Plus
- Biotrue ONEday
- Proclear 1 Day
What are examples of Bi-weekly replacement soft contact lenses?
- Acuvue 2
- Acuvue Oasys
- Avaira
What are examples of monthly soft contact lenses?
- Air Optix Aqua
- Proclear, Frequency 55
- Biofinity
- Air Optix Night & Day
- Purevision 2
In the UK, which is the most popular type of contact lens, SCL or RGP?
SCL
What factors are driving the growth of the SCL market?
Growing rate of myopia
Growing Geriatric population
Increasing preference fo contact lenses
What factors are hindering the growth of the SCL market?
Unregulated online sales
Regulatory barriers concerning contact lens sale
What was the first soft hydrogel contact lens material?
HEMA (in 1970).
[It continues to be the monomer most often utilised]
Is HEMA very stable or unstable of a material?
Very stable - variations in temperature, ph, etc have vewry little effect on it
Is HEMA a comfortable lens material?
Yes
[It contains water and transports O2]
True or false- HEMA has good wetability
True
True or False- Silicone hydrogels allow for less oxygen transmission than HEMA based contact lenses
False - SiH CLs allow for more oxygen transmission
Which are more expensive HEMA (hydrogel) Lenses or SiH (Silicone hydrogel) lenses?
Silicone Hydrogel lenses
How does wetability affect comfort?
Wetability aides the closure of the lid over the lens thereby improving comfort and preventing changes to the papillary surface (of inner eyelid).
A very wetable surface creates an even stable tearfilm optimsing comfort, visual acuity and resistance against deposits.
What does it mean to have no net increase in the amount of contact lens wearers?
It means that for every new contact lens wearer there is an old contact lens wearer that discontinues use
What is a material’s modulus?
A value that expresses a material’s ability to keep its shape against stress, and resistance to deformation.
What are the properties of materials with a higher modulus?
They are stiffer, resist deformation, hold their shape better, are easier to handle and may provide better VA.
[Stiffer lenses though may have adverse effects as they cause an edge lift and can cause a SEAL, they can cause mucin balls or CLPC (Contact Lens Papillary Conjunctivitis)]
What is the relationship between sleeping in contact lenses and eye infections?
Sleeping in contact lenses makes you 8 times as likely to develop an eye infection
What are the strict rules regarding contact lenses and water?
Don’t ever bath, shower or swim in contact lenses!
List some indications for fitting Soft Contact Lenses
What are the main contra-indications for use of Soft Contact lenses?
Highlighted in red + pregnancy
What are the two possible care regimes for soft contact lenses?
and what are the advantages of both?
You can use Multipurpose solutions or Hydrogen Peroxide
Multipurpose solutions:
- These are Simple and convenient
- And can be used for Cleaning, disinfecting, and storing
- Method of use is to Clean, rub, rinse and store in (for min 4-6 hrs)
Hydrogen peroxide:
- These are a Metallic disc in case or tablet to neutralise the disinfectant before wear
- Px cannot have adverse reaction to preservatives
- Gold standard?
What are consequences of non-complaince of a Soft Contact Lens care regime?
- Lens/case contamination
- Increased lens deposits
- Decreased comfort
- Vision affected
- Lens wear ‘drop out’
- Serious complications
What are the top reasons for contact lens px ceasing wear?
Too much effort to clean
Too expensive
Forget to order new supply in time
Not comfortable
Only worn for certain occasions
Prefer spectacles
What may the base curve (BC) of SCL also be referred to as?
BOZR or BOR
[Back Optic Zone Radius or Back Optic Radius]
What soft contact lens specifications are stated on the box?
What BOZR do we choose for a soft contact lens?
On average 0.8mm flatter than the flattest K
The range is 0.6mm to 1.00mm flatter than the flattest K
What does the BOZR we choose for a soft contact lens depend on?
Water Content and Diameter.
Contact lenses with a low water content need to be fitted a little bit flatter so 1.00mm flatter than the flattest k (KF+ 1.00m)
Contact lenses with a high water content need to be feeded 0.6mm flatter than the flattest K
- Order flatter BOZR for smaller HVIDs (than average 11.8mm)
- Order steeper BOZR for larger HVIDs
How do we choose the total diameter for a Soft contact lenses?
~2mm larger than HVID
What are british standards for the total diameter?
That it must be recorded to 2 dp
What range of total diameters are available for soft contact lenses?
⊘ 13.80
⊘ 14.00
⊘ 14.20
⊘ 14.40
⊘ 14.50
Why do we order the total diameter of a sofft contact lens to be 2mm larger than the HVID?
The lens should cover the cornea in primary gaze and in all positions of gaze – To avoid desiccation of exposed cornea
[Ideally the the lens should cover 0.5mm of the conjunctiva in all directions of gaze]
How do you determine the BVP of a SCL?
Spectacle Rx = ocular (or contact lens) Rx
Unlesss power id >4.00D – In which case Use Vertex Distance Table,
or – K=Fsp/(1-(d*Fsp)) equation
, where • K=ocular refraction • Fsp=spectacle power • D=BVD in m
• You could also choose Mean spherical equivalent (sphere + ½ * cyl) if the cyl is less than -0.75 DC
Greater than -0.75DC onwards do not use MSE instead consider prescribing toric SCL
After having taken history and symptoms and measurements what is the routine for fitting contact lenses on a px?
- Establish most optimum BOZR/TD BVP
- Pick trial lenses – Try different design/ manufacturer in each eye
- Assess fit and establish which lens is most comfortable for px
- Establish final choice – Same design/ manufacturer for both eyes
- Prescribe trial lenses for 5-7 days, book teaching session for insertion and removal, book follow up appointment to observe fit after 5 days
What do you assess when judging the fit of a soft contact lens?
Physical fit:
- Check it is Centred on-eye
- Check Full coverage with versions (i.e. that it covers the cornea in all 8 positions of gaze)
- Sufficient movement (this is needed to remove metabolic debris from the cornea) - (should not increase lens movement if you view signs of hypxia - you actually want to increase Dk/t of the material)
- Check it Follows cornea and conjunctival curvature
Performance:
• Check PxComfort • Check for Stable vision
What does a tightly fitted soft contact lens cause?
It won’t cause hypoxia.
(Steep RGP will though)
Can cause identationand press on conjunctival vessels,
If tears are stagnated in the limbal area , this will cause oxygen problems in that region.
What is ‘Fluting’ and why is it bad?
When the edge of a soft contact lens stand off.
This is uncomfortable for the px
What do we refer to a flat fit in SCL as?
A loose fit
How can we change the base curve in order to adjust a SCL fit that is too loose?
Steepening the BC radius (8.60 to 8.30 mm) tightens the lens fit
[Unfortunately only one manufacturer, johnsons and johnsons, provides lenses in two base curves]
How can we change the total diameter in order to adjust a SCL fit that is too loose?
Increasing the lens diameter (13.0 to 14.0 mm) tightens the lens fit , (reducing lens movement)
[This isn’t always possible though bc manufacturers don’t always make lenses in a variety of total diameters]
What is more important for the comfort of a SCL px, the BC or the sag height?
Sag height is more important than BC for comfort
What can the sagital height of the cornea/eye be predicted according to?
– the central corneal radius (keratometry)
– overall corneal diameter (most influential)
– corneal asphericity (important; e-value)
– the radius and asphericity of the para-limbal sclera
A study in 2002 found that patients who had HVIDs between 11.6mm and 12.0mm had a high percentage of success using a traditional fitting approach (Kf +0.8mm)
• However, eyes that have a smaller HVID may need a _______ diameter or ______ base curve
However, eyes that have a smaller HVID may need a smaller diameter or flatter base curve
[The image shows 4 lenses that have the same BOZR - and how changing the total diameter affects the sag height and the fit]
A study in 2002 found that patients who had HVIDs between 11.6mm and 12.0mm had a high percentage of success using a traditional fitting approach (Kf +0.8mm)
Eyes that have a larger HVID may require a ___________ diameter or ___________ base curve
Eyes that have a larger HVID may require a larger total diameter or steeper base curve
To summarise what are the fitting rules for Soft Contact lenses?
