Week 3 - Manufacturing Lenses and Fitting Guidelines Flashcards
What is the base curve (BC) on a contact lens also know as?
central posterior curve
What is the optic zone/ posterior zone also known as? 2
-central zone
-apical zone
What is the posterior apical radius of the contact lens?
radius of curvature of the back surface of a lens at the apex
What does central thickness refer to in a contact lens?
distance between anterior and posterior of the lens at its centre (+ thicker/ - thinner)
What is the back vertex power used for?
to measure the power of lenses (back vertex power to focal point)
What are the 3 main methods of manufacturing contact lenses?
- Lathe cutting with manual or
automated lathes - Molding methods
- Spin casting(injection molding)
Which method is faster and more cost-effective? Lathe cutting or injection molding (spin casting).
injection molding process
Explain roughly how contact lenses are made during the lathe cut process.
1) A button is used (12 to 15mm)
2) Diameter, base curve, periphery curve are all carved into the button
3) Polished (still a semi finished blank)
4) Anterior surface is cut using lathe
5) Lens hydrated
6) Quality Check
* Surface defects
* Base curve
* Power
What are 2 advantages of fitting lathe cut lenses?
- thickness = easier to handle (good for new wearers or px with bad dexterity)
- good centration (stable on eye, consistent vision correction)
What are 3 disadvantages of fitting lathe cut lenses?
-Time consuming
-Expensive (custom)
-Hard to reproduce
Lathe cutting allows for highly customisable lenses great for High Rx, RGPS, keratoconas cornea etc. T/F
TRUE
Explain roughly how to cast molding contact lenses.
- Lens is pressed into place
- Anterior mold is injected with liquid monomer
- Posterior mold used to close on top
- Mold taken through series of UV lights –
eventually lens hardens - Finished lens produced (good optics and smooth surfaces
Cast molding is good for mass productions. T/F
TRUE
What are the most common ways to make contact lenses now? 2
lathe cutting
Molding methods
Explain roughly the spin casting (injection mold) method of using contact lenses.
1) Liquid polymer is injected into the spinning mold
2) Spins lenses into shape
3) Aspheric base curve is formed by the spinning force
4) UV light is applied to solidify or cure the lens
5) Periphery of lens is buffed and polished
6) Lens then is hydrated
What process is usually done to create RGP lenses?
Lathe cut lenses
Avoid polishing compounds with water in when polishing contact lenses. T/F
TRUE
What are 3 advantages of cast molding?
-precision
-consistency
-efficient method
What is the most commonly used process for making soft contact lenses?
Cast molding
What are 5 advantages to spin casting (injection mold)?
- Smooth surfaces
-Enhanced comfort during wear
-Back aspherical surface matches with the aspheric profile of
the cornea
-Minimal lens movement on the eye with acceptable tear
film
-One base curve makes it easy to fit
Spin cast lenses are easy to replicate. T/F
True
Which process starts out as a rod or a button of material?
Lathe cutting
What should we consider when determining a px will be good candidate for soft cls? 6
- lifestyle
-motivation
-physical state - dexterity
-eye conditions
-willingness to comply
-General environment of wear - lens hydration
What is part of the pre-fitting examination? 3
-assesing if they are good candidates
- ocular measurements for inital fit
-collect baseline clinical information to compare with later
Review the questions to ask during history taking (pre-fitting soft lenses).
Turn over card.
- Have you ever worn contact lenses in the past?
- If yes, how many years have you worn contact lenses?
- If yes, what is the brand of lenses you are currently using?
- If yes, have you used other brands?
- If yes, when? Why did you switch?
- If yes, what time do you insert your lenses and what time do you remove them?
- If yes, how many times a week do you wear your lenses?
- If yes, how old are the lenses you are currently wearing?
- Are your lenses comfortable from the time you wear them
to the end of the day? - How do you clean your lenses?
- What solution do you use?
- Are you able to wear the contact lenses comfortably all day?
- How is your vision when wearing the lenses?
- Not all questions are listed based on patient answers
will determine the direction of you next set of questioning.
What do we do if a patient arrives wearing contact lenses to their contact lens fit in regards to the Visual Acuity? 3 steps
- Assess VA with
contact lenses on - Next assess CL under slit lamp
- Then have the Px remove lens
and do unaided VA
What do we do if a patient arrives not wearing contact lenses to their contact lens fit in regards to the Visual Acuity? 2 steps
- Assess unaided VA’s
- Next assess VA’s with eyeglasses worn
What are the 7 other physical measurements and checks we can do before fitting contact lenses?
-HVID
-PA / PF
-K’ readings
-Lid eversion
-Tear Break up
-Tear Prism
-get eyeglass prescription
Do not test unaided VAs on someone who is +/-4.00D as it is unnecessary information (can not see anything) T/F
TRUE
