contact lenses Flashcards
give the 2 different types of hard contact lenses
-AKA rigid corneal lenses
-rigid gas permeable (RGP)
give 2 types of soft CLs
AKA hydrogels, HEMA lenses
what are the three sizes of hard contact lenses?
-corneal
-semi scleral
-scleral
give an example of a therapeutic CL
a bandage lens
what are the soft lens modalities?
-daily disposables
-reusable lenses that consist of 2-weekly, monthly and planned replacement
what are the advantages to contact lenses?
-contacts stay in place unlike glasses which can slip down and fog up
-give you a wider fov than spectacles
-comfortable to wear without the weight of glasses
-good for an active lifestyle
what are the cons of CLs?
-can be expensive
-need to be removed before sleeping
-cant be worn when showering or swimming
-take time to adjust at the start
-can cause dry eye
-can increase risk of ocular infections
what are the stages of the CL fitting patient journey?
-initial fitting appointment
-teach
-CL trial
-aftercare of (CL check, check-up and follow up)
what do you do as the practitioner in the initial contact lens fitting
-ask about details of the medical and ocular history
-ask details about needs and expectations of CL wear
-look at their spectacle prescription
-examine the anterior eye and tear film
what is the point of the initial CL fitting appointment?
-discuss the contact lens options w the patient to make informed choice
-advise if they are unsuitable for CLs
-record this info on the patient record
What happens in the patient teach?
-show patient how to apply and remove their lenses
-explain how to wear and care for lenses
-explain the wearing and replacement schedule
-advise the patient what to do if they have a problem
-record the advice given on the patient record
-arrane the follow-up appointment
-give the patient a written copy of the advice
what happens in the CL trial?
the patient takes the CLs to trial them for a short period of time. allows patients to ensure the lenses are meeting their needs and expectations
what happens in CL aftercare check?
-discuss how the lenses performed compared to the patients expectations
-ask about any problems experienced
-check the patients compliance
-check the VA and lens fit
-examine the anterior eye after removing the lenses
-record this info on the patient record
in contact lens aftercare, what do you if the lenses are not appropriate
a different lens could be fitted
in CL aftercare, how do you proceed of the lenses are appropriate?
the patient is issued with the CL spec and can buy lenses from the practise, another practise or online up to the expiry date
what happens after the intial aftercare?
the patient attends regular aftercare and can purchase lenses until the CL spec expires
What are the most common contact lens complications?
discomfort, vision problems and redness and these just increase risk of contact lens dropout
what is a serious infection that CLs increase the risk of?
microbial keratitis (MK)
how many people get MK for
-daily soft wearers
-overnight soft lens wearers
-2-4 per 10,000 wearers per year for daily soft lens wearers
-20 per 10,000 wearers per year for overnight soft lens wearers
what are the main risk factors for MK?
-poor hygiene
-overnight wear
what is MK?
inflammation of cornea through direct infection from the microbial agent being bacteria, virus, fungus, protozoa
what % of acanthamoeba keratitis (AK) occurs in CL wearers?
85%
what is AK and what are the risk factors?
a rare but very serious form of MK
-swimming, showering in lenses
-contamination from tap water
what are the steps to advise applying lenses?
- clean dry hands
- remove lens from packaging/CL case and inspect
- lid positioning
- place lens directly onto cornea
What are the steps to advise for lens removal
- clean, dry hands
- lid postitioning
- drag lens away from cornea and pinch off
- discard the lens if its a daily disposable
5 clean the lens with solution NOT WATER, if it is reusable
what do you tell a patient when explaining how to wear and care for lenses?
-reinforce the importance of hygiene and compliance so no water contamination and no sleeping in the lenses
-prevent lens contamination so makeup, water and eye drops
-only use prescribed lenses and recommended care solutions
-attend regular appointments as problems can occur without symptoms
what do you tell a patient in case they have a problem with their CLs?
-have a daily checklist, do your eyes feel well, see well and look well, if in doubt take them out
-remove lenses immediately if problems occur and contact the practitioner
-do not resume lens wear until checked
-provide practise phone number and advise what to do out of hours
how long after the trial appt of the CLs should you arrange a follow up?
1-2 weeks after the teach
what should you tell the patient to bring to the follow up appointment?
-the patient should come in wearing their CLs
-bring their glasses
-bring their CL case is the lenses are reusable
what are the three methods used to make CLs?
-lathing
-spin casting
-moulding
what are the names used to describe soft contact lenses?
-HEMA
-PHEMA
-poly HEMA
what two materials start off contact lens manufacture?
-polymer
-liquid monomer
how is the material of polymer prepped for CL manufacture?
- anhydrous material in rod form is sliced into buttons
- button is shaped then hydrated to form the lens
how is liquid monomer material prepped for soft CL manufacture?
- injected into a mould
- polymerisation
- hydration
how are soft CLs made through lathing?
- polymer button mounted onto brass tools
- lathed with diamond tipped tool
- back (concave) surface is lathed, diameter is cut down to the required size
- button is transferred and front convex surface is lathed
- lens is polished to remove lathe marks
- lens is inspected and hydrated
- inspected, packaged, autoclaved and labelled
What are the advantages to lathing?
-relatively simple
-wide range or parameters
-relatively inexpensive to start production
what are the disadvantages to lathing?
-complex designs are difficult
-labour intensive
-high cost per lens
-variable surface finish
-relatively slow
-hard to produce in large volumes
-reproducibility is hard
What are the steps used to produce soft CLs with spin casting?
- convex stainless steel tool (male) used to create a disposable plastic concave (female) mould
- liquid monomer is introduced to the rotating concave female
- centrifugal forces distribute the monomer across the mould
- polymerisation via UV and or heat cure occurs at the same time to produce the lens
- the lens edge is polished if necessary
- lens is inspected and hydrated
- inspection, blister package and autoclaved
what are the advantages of spin casting?
-higher volume manufacture compared to lathing
-automated production
-low cost per lens
-rapid
-volume production is relatively easy
what are the disadvantages of spin casting?
-expensive to start production
-expense limits parameter range
-lens edge may need further finishing
-poorer reproducibility than moulding
-lower volume than moulding gets produced
what are the steps in moulding to make soft CLs?
- stainless steel male and female tool used to create disposable plastic moulds
- liquid monomer introduced between moulds and cured (UV/ heat)
- inspected and hydrated
- inspected, blister packaged and autoclaved
what are the positives to using moulding to manufacture hydrogels?
-high volume manufacture
-automated production
-low cost per lens
-volume production is easy
-good surface quality
-good reproducibility
what are the disadvantages to moulding?
-expensive to start production
-expense limits parameter range
what should you explain to patients to make them aware of the environmental impact of CLs?
-the environmental impact of lens manufacture
-annualised waste of different wearing modalities
-recycling options
In what ways can the polymer chain be modified to change the physical characteristics of the plastic that will make the CL?
-long chain structure creates toughness and elasticity
-interaction of polymer chains gives resultant characteristic physical properties
-increased stability is achieved by cross linking polymer chains
What are the three types of materials used to make soft CLs?
-standard hydrogel
-silicone hydrogel (high oxygen permeability)
-water gradient materials (in between standard hydrogel and silicone hydrogel)
why are co polymers added to pHEMA CLs?
-to increase their water content and so increase the oxygen permeability
-to alter their wettability
-to alter their modulus
-to alter their ionicity
give some common copolymers
-PVA: polyvinyl alcohol
-PVP: polyvinyl pyrrolidone
-MMA: methyl methacrylate
what methods do manufacturers use to enhance lens comfort?
-packaging additives such as surfactants
-lubricants and wetting agents bound to lens materials
-lubricants and wetting agents released from lens material
for packaging additives,
-give examples
-how they advance CL comfort
-HMPC and PVP
-have a cushion effect upon insertion
for lubricants and wetting agents bound to lens material:
-give examples
-how they advance CL comfort
-pvp and PEG
-maintain wettability and reduce dehyrdation
for lubricants and wetting agents released from lens material:
-give examples
-how they advance CL comfort
-PVA
-‘time release’ / ‘blink activated’
what are the advantages of having hydrogels with high water content?
-have greater oxygen permeability
-less hypoxia
-hydrophillic surface
-softer so lower modulus and less stiff
what are the disadvantages of having hydrogels with a high water content?
-greater tendancy to dehydrate
-greater deposition
-difficult handling
-fragile and easy to tear
-difficult to manufacture
-must be made thicker to compensate
what is silicone?
an organic compound made of silicon and oxygen
why use siicone hydrogels?
as they offer greater oxygen performance compared to hydrogels as oxygen is more soluble in silicone than in water and so oxygen passage is no longer dependent on the water content
why do silicone hydrogels first need to be modified?
As silicone is inherently hydrophobic and so lenses need to be modified to make the surface hydrophillic and wettable
what are the three types of material modification that determine the generation of the silicone hydrogel?
-surface treatment
-internal wetting agent
-polymer
what is the definition of a biomaterial?
a natural or synthetic material that is suitable for introduction into living tissues, especially as part of a medical device
what are the bulk properties of CLs?
water content, oxygen permeability, density, dimensional stability, uv blocking, tensile strength, tear strength and modulus of rigidity
what are the surface properties of CLs?
wettability, coefficient of friction/ lubricity, ionicity, deposition and microbial adherence
what are the optical properties of CLs?
refractive index, optical transmittance
what is the range of WC (water content) range in soft CLs?
24-79%
what is the formula for oxygen permeability? what are the units for oxygen permeability?
Dk where
-D is diffusivity in in Barrer Fatt units - 10-11(cm2/sec)(mlO2/ml x mmHg)
-k is solubility in ISO units hPa
how does Dk relate to WC in contact lenses?
they are proportional to each other
when does Dk not depend on the WC of the lens material?
in silicone hydrogels as oxygen is more soluble in silicone than in water
how is density of hydrogel dependent on WC?
density of hydrogel decreases as WC increases
what does dimensional stability of a material mean?
its the ability of the material to maintain the original dimensions under various conditions
what is the dimensional stability of a CL affected by?
-temperature
-pH
-tonicity
what percentage of UVA and UVB does class I UV blocking polymers block?
-UVA 90%
-UVB 99%
what percentage of UVA and UVB does class II UV-blocking polymers block?
-UVA 70%
-UVB 95%
what are the drawbacks to a lens having a higher modulus?
they can cause mechanical complications and reduced comfort so this has to be balanced with the better durability and handling
what is the modulus range of most soft lenses?
0.2-1.0 MPa
what is the coefficient of friction? (CoF)?
the force required to move an object divided by its mass sometimes described as lens lubricity so can affect tear film quality
what does it mean if the lens surface is ionic?
it can hold a negative charge
what is the good side and bad side to ionic surfaced CLs?
they have a greater wettability but can also lead to increased deposits
what is the good side and bad side to non ionic CLs?
they tend to resist deposits better but may not wet as well as ionic CLs
Give two examples that show how microbial adhesion rates are related to silicone hydrogels
-in vitro studies have shown greater bacterial adhesion to unworn silicone hydrogels compared to hydrogels
-studies show 2x greater risk of corneal infiltrates with silicone hydrogels compared to hydrogels
what are the two essential optical properties of CL and what determines them?
-optical transmittance
-refractive index
polymer composition
give two first gen silicone hydrogels and how they have been treated? what is an advantage for them? what is a disadvantage against them?
-B&L PureVision
-Alcon Air Optix Night & Day
-adv: high Dk so low WC and high slicone content
-disadv: reduced wettability and high modulus due to low WC so increases mechanical issues and discomfort
how do second generation silicone hydrogels make the lens surface hydrophillic?
They rely on internal wetting agents to make the lens surface hydrophillic
give 2 examples of second gen silicone hydrogels?
-J&J Acuvue Oasys
-CooperVision Clariti
how do third gen silicone hydrogels make sure their surface is hydrophillic?
they have no internal wetting agents or surface treatment and instead rely on novel polymers and aquaform technology so can maintain a high Dk despite the reduced SiH content
give 2 examples of third gen silicone hydrogels
-CooperVision Biofinity
-CooperVision Avaira
Give three problems with Dk
-Dk is a property of the lens material so only allows comparison of lenses with the same thickness
-High WC so high Dk lenses have a low modulus and have to me made thicker to improve handling, manufacture and reduce the dehydration tendency of the lenses
-low WC so lower Dk lebses can be made thinner due to the increased modulus of the material
why may the Dk of a high WC material be the same as that of a low WC material?
because a high WC material is thick so less O2 would reach the cornea anyway
how to calculate transmissibility, give the units. what does it say about the thickness of the CL?
transmissibility = Dk/t
- where t is the lens thickness and this is typically the centre thickness of a 3.00D CL
-units barrer / cm = 10^-9 (cm2/sec)(mlO2/ml x mmHg)
-means a thick high WC lens has about the same transmissibility as a thinner low WC lens
how much transmissibility safe for CL wear when its
-daily (open eye)
-extended (when the eyes are closed)
-35
-125
how do you examine a stained cornea for observation pre CL?
-direct illumination
-2mm wide beam, angle 45, max illumination
-use colbalt blue and wratten filter
-start at 16x and then increase for greater detail
-examine the cornea in the 3 positions of gaze
-assess the depth using the optic section
how should the slit lamp be set up for lid aversion?
-10-16x mag
-medium brightness
-parallelepiped
what is the order of examining the tear film?
- tear meniscus height and tear film quality
- white light assessment of anterior eye
- instill fluorescein
- fluoroscein tear break-up time
- ocular surface staining
- lid aversion
what’s the order you should write down the CL specification?
- manufacturer and lens name
- BC
- TD
- Power
what is the the BC of a CL?
the base curve which is the radius of curvature of the back surface of the lens
TD of a CL?
the total diameter which is the overall size of the lens
what three things should you check to inspect the lens before application
-check the lens profile
-do the crease test
-check the lens markings read the right way round
how should you dispose of CLs in clinic?
put them in the yellow bin
what questions should you ask a patient when working out their reason for visit at a CL fitting
- motivation
- needs and expectations of CL wear
- has the patient tried CL before?
how young can patients be to wear CLs?
-there’s no lower age limit for CL fitting so its based on the practitioners judgement of their maturity
how does the risk compare in different ages among young people?
young patients (8-15yrs) are associated with a lower risk of CL complications compared to teens and young adults (15-25yrs)
how does CL fitting change with age of patient?
-older people are more likely to have dry eye and so this needs to be taken into consideration more than younger people when prescribing CLs
-ages 40+ may start having reduced accommodation and so they may need specific near vision correction
what CL properties can a higher prescription have an impact on?
oxygen transmissibility (Dk/t) due to the increased thickness that comes with higher prescription
why do you need to ask a patient wether they smoke or vape in a CL fitting?
smoking is associated with a 3.7x increased risk of moderate and severe microbial keratitis (stapleton, 2012)
why is it important to ask a patient to bring a cop[y of their prescription or list of medications as well as asking them about topical ocular medications or over the counter meds in a CL fitting?
because many systemic medications can affect the tear film and in turn the success of CL wear and
What is normal for tear meniscus height?
it should be between 0.2 and 0.3mm high any less than 0.18mm means dry eye
what should the beam height be on the slit lamp for tear meniscus height?
1mm high and on a high mag
how long should the tear break up time be?
more than 10 seconds as less indicates dry eye
is fluorescein break up time test invasive or not, explain your answer
it is invasive because fluoroscein destabilises the tear film
when describing corneal staining, what does diffuse mean?
a vast array of closely separated punctates
when describing corneal staining, what does coalescent/patch mean?
its a confluent pattern of diffuse staining
when describing corneal staining, how do you refer to small dots of staining?
punctuates so you can get either micropunctates or macropunctates
what is optimal lag for CLs?
0.5-1.0mm
what is optimal movement on blink for CLs?
0.25-0.5mm
what is optimal movement in the push up test?
2-4mm
why examine the anterior eye and tear film for CLs?
-to assess sutiability for CL wear
-to potentially identify conta-indications for contact lens wear
-to help inform sutiability of different lens type, material or modality
-to record baseline values before CL fitting and monitor any changes from CL wear
-to monitor the ocular health of established lens wearers
what change in grading is considered statistically insignificant?
a change in grading of >1 unit is typically considered clinically significant
why use a grading scale?
-helps objectively decide the level of normality or severity
-improves accuracy and consistency of record keeping
-record baseline values for new lens fit
-monitor progression of ocular complications
give 5 common ocular conditions that can affect the suitability for CL wear
-corneal staining
-papillary conjunctivitis
-blepharitis
-meibomian gland dysfunction
-dry eye
generally what lens modality is most suitable for patients with blepharitis?
daily disposables
what can meibomian glad dysfunction affect?
-tear film stability
-contact lens wetting
-comfort
why may there be cases where conventional hydrogel (HEMA) lenses are more suitable than silicone hydrogels for patients with MGD that want contacts
as slilicone hydrogels tend to attract more lipid so HEMA lenses may be more suitable of there is problems with deposits
what lenes may be best for someone with dry eye?
silicone hydrogels or RGPs
what is HVID? what is its average?
-its the horizontal visible iris diameter and can be used to estimate the ideal CL diameter however little evidence about the relevance of this measurement for selecting the ideal CL has been published
-average is 11.8mm
what is VPA
vertical palperable aperture but there’s little evidence to suggest its relevant to the CL fit
give some ocular surface measurements that ‘maybe’ could be used to determine the ideal CL parameters?
-HVID
-VPA
-pupil size
-corneal radius of curvature
what is pupil size measurement useful for?
corneal RGP lens fitting
give two ways corneal radius of curvature can be measured
-a keratometer which measures the radius
-a corneal topographer which gives a detailed map of the ocular surface curvature
what are the advantages of soft contact lenses compared to corneal RGP?
-good initial comfort
-straight forward to fit
-higher replacement frequency so less lens deposits and its easier to change prescription
-good for both full time or occasional wear
what are the disadvantages of soft contact lenses compared to corneal RGP?
-lead to a higher risk of infection and complication compared to corneal RGP
-they are not customisable
what are the disadvantages of RGP compared to soft CLs?
-the fitting process is longer as lenses need to be ordered individually and manufactured
-adaptation is required to reduce corneal sensitivity and allow for increased wearing time which takes about 2 weeks
-initial lens cost is high so lens replacement is expensive
-need careful cleaning
-not for contact sports of dusty environment
what are the advantages of RGP compared to soft CLs?
-rigid lens surface offers better quality of vision
-reduced risk of infection so less complications
-can be tailor made and fully customisable with a large range of parameters
-lenses are typically replaced every year so are a good value for money
-what are the advantages of soft daily disposables?
-cost effective when worn less than 3 days per week
-ideal for flexible wear
-minimal compliance needed
-no solution needed
-increased lens replacement associated with reduced risk of complications and infections compared to reusable lenses
what are the disadvantages of soft daily disposables?
-expensive on a full time basis
-they have a higher environmental impact
-they have a limited parameter range compared to other modalities
what is the advantage of soft fortnightly lenses?
increased lens replacement compared to monthlies
what are the disadvantages of soft fortnightly CLs?
-more expensive than monthlies
-Patients often poorly complaint and wear lenses for a moth
-hassle of cleaning the lenses
-less parameters than monthlies
what are the advantages of soft monthly lenses?
-cheaper than daily disposables if worn more than 4 times a week
-larger range of parameters
-lower environmental impact compared to dailies
what are the disadvantages of soft monthly lenses
-greater build up of lens deposits
-hassle of cleaning lenses
-higher risk of complications and infection compared to dailies
give the advantage and disadvantage of soft extended wear CLs
adv
-convenient
dis
-increased risk of severe corneal infection
what are the advantages of soft planned replacement CLs?
-have huge range of paraemeters including hand painted cosmetic lenses
what are the disadvantages of planned replacement CLs?
-lens deposits and complications
-need careful cleaning
-expensive to replace
what are the advantages of silicone hydrogel compared to HEMA?
-they have a much greater o2 permeability so suitable for longer WTs
-they have a lower water content and so less on eye dehydration
-can be better for px with dry eye
-new gen materials have a lower modulus so more flexible and comfortable
what are the disadvantages of silicone hydrogels compared to HEMA?
-more expensive than HEMA
-have a higher risk of corneal inflammatory events (CIEs)
-increased lipid deposits
-higher modulus mainly in first gen leads to mechanical complications and poorer initial comfort
what are the advantages of HEMA compared to SiH?
-lower deposition rate
-lower modulus so better comfort
-cheaper
-lower risk of CIEs
what are the disadvantages of HEMA compared to SiHs?
-lower o2 permeability so greater dehydration tendancy especially in think lenses
-not suitable for EW
-complications from insufficient corneal o2 supply
what are the advantages of water gradient lenses?
-combine the best of both SiH and HEMA lenses so comfort and wettability from HEMA and high DKk from siH core
-low coeffiecient of friction
what are the disadvantages of water gradient lenses?
-expensive
-some patients struggle with removal due to slippery lens surface
-less choice as its only made by alcon so 2 dailies and 1 monthly disposable w DC powers only options available
how do you explain the options for CLs?
-use lay language
-relay pros and cons
-explain why an option may not be suitiable
-relate the options to their needs and expectations
-give approx costs
-reassure the patient that this is for the trial and they can always try something else
what are the two drawbacks to poor lens fit?
-adversely affects comfort and linked to drop out (young et al, 2002)
-poor lens fit can adversely affect ocular surface physiology
-contribute to complications
what is saggital heigh and whay does saggital height tell you
the saggital height is determined by the BC and the TD of the CL helps determine the lens fit relative to the saggital height of the cornea
give 8 ocular variables that affect CL fit
-saggita; height of cornea (sag)
-corneal apex
-corneal diameter and rate of flattening
-palperbral aperture
-upper lid angle
-lid pressure
-tear film
give 8 contact lens variables affecting the lens fit
-saggital height
-lens design and edge thickness
-lens material
-WC
-modulus
-wettability/ coefficient of friction
-centre thickness
what kind of comfort rating will a tight lens give
initially it will be good but then decreases the longer the lens is in
what is ideal lens adaptation time
10 minutes
when are loose lenses most uncomfortable
when blinking
what are the steps to a soft lens fit assessment?
-lens adaptation time
-assessment of px comfort
-assessment of VA and over-refraction (OR)
slit lamp examination
-lens surface quality
-centration and coverage
-edge alignment
-movement on excursions
-movement on blink
-push up test
what other variables aside from lens fit can affect the initial lens comfort?
-practitioner technique
-small foreign body trapped under lens
-lens thickness
-lens modulus
-lacrimation
when assessing CL fit, what does it mean if the vision is not stable once the CL is in?
-if the vision improves after blinking it could indicate a tight fit
-if vision worsens after blinking it could indicate a loose fit
how do you amend a tight fit
-try a different manufacturer with the same/ similar BC
-increase BC is lens avaliable in 2 BCs (this is only avaliable for J&J)
how do you amend a loose fitting CL?
-try manufacturer with same/similar BC
-reduce BC if lens is available in 2 BCs
how do you assess vision in the soft lens fit assessment?
-assess the distance and near VA and then record
-optimise the prescription with over -refraction OR
-do spherical lens refraction over CL
