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(2) Tri A - Contact lenses > Week 9 - Contact lens Manufacture > Flashcards

Week 9 - Contact lens Manufacture Flashcards

1
Q

What are the 3 main cutting methods, and 2 combo methods for SCL manufacturing?

A
  1. Lathe cutting
  2. Spin-casting
  3. Cast moulding

• Moulding/lathing combo
• Spin-casting/lathing combo

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2
Q

What are the 3 SCL Lathe cutting - Terms?

A

• Anhydrous - without water
• Xerogel - a hydrogel that has not been hydrated [etymology = xero- (“dry”) gel]
• Autoclave - a pressure chamber that is used to sterilise items. Items placed inside the autoclave are exposed to high temperature steam

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3
Q

How is SCL Lathe cut?

A

• Anhydrous “button” of lens material (xerogel) is lathed in a controlled atmosphere
• The button is mounted on a spinning shaft and shaped with computer/controlled precision tools
• After front/back surfaces are shaped with cutting tool, lens is removed from lathe and hydrated to soften it
• Sealed in normal saline
• Autoclaved at 121°C for 15minutes

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4
Q

What is a Lathe?

A

Think of it like a miniature potter’s wheel that rotates at 8-12,000 pm to shape the lens. Lathes are used in metal work, glass spinning etc. to make symmetrical objects

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5
Q

What are the 6 advantages of lathe cutting?

A

• Established technology
• Easily adapted for CL manufacturing
• Few limitations on parameters that can be lathed i.e. virtually any radius or power can be made
• Reserved for the manufacture of custom or extreme range lenses that are not amenable to mass production
• Suitable for most materials
• A lathe is a finite investment

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6
Q

What are 7 disadvantages to SCL Lathe cutting?

A

• More steps and more time consuming than moulding
• Takes a few minutes to make one lens
• Higher unit cost
• Requires struct humidity control
• Cleaning/polishing required on completion
• Variable surface finish
• Lens must be soaked to remove impurities (unpolymerized monomers), prior to sterilisation and packaging

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7
Q

What is SCL Spin Casting?

A

• Liquid contact lens monomers are poured into mould
• Mould is spun in a controlled environment
• Rotational velocity, surface tension and gravity defines back curvature and therefore BVP
• Mould shape defines front surface
• UV light applied to polymerise the monomers
• Edges then buffed and polished
• Lenses are then hydrated

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8
Q

Who was Otto Wichterle?

A

• Made the first pHEMA but found that could not make pHEMA lenses by cast moulding
• Developed a spin-casting technique. Used a childrens’ mechanical construction kit!

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9
Q

What are the 3 advantages and 4 disadvantages to SCL Spin casting?

A

Advantages
• Produces a very smooth (back) surface
• Back aspheric surface matches aspheric profile of the cornea
• Least likely to produce surface defects
Disadvantages
• Slower and more complex process than moulding (more steps)
• Therefore more expensive
• Not as scalable as moulding
• Requires edge polishing

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10
Q

What is SCL Injection moulding?

A

• Soft CL monomers in liquid form
• Injected into computer designed moulds under pressure
• UV light or heat is applied to initiate polymerization, then lenses are removed from mold
• Lenses are then hydrated to soften them
• Lenses are verified, packed and sterilised in the same way as the spin cast lenses

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11
Q

What are the SCL injection moulding advantages?

A

• Very quick
• Low cost per lens
• Easy to produce many lenses at once
• No polishing required
• Most widely used method

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12
Q

SCL Injection moulding - Disadvantages

A

• Strict environmental control of temperature and humidity
• Expensive equipment, huge investment required to setup
• Number of moulds limits parameters i.e. good for stock lenses

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13
Q

What are the two combined methods?

A

• Moulding/lathing combo
- Moulding the back surface
- Lathing the front surface

• Spin-casting/lathing combo

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14
Q

What are the 7 final steps of any SCL manufacturing?

A
  1. Anhydrous lens inspected (10x mag)
  2. Lens hydrated in saline
  3. Hydrated lens inspected (10x mag)
  4. Lens inserted into blister pack (glass vial for lathe cut lenses)
  5. Blister pack sealed and labelled
  6. Blister pack and lens autoclaved
  7. Lenses dispatched
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15
Q

What are the 4 edge defects and 4 body defects that can be found?

A

• Edge defects:
- Excess material (flash)
- Roughness
- Nick
- Tear

• Body defects:
- Eccentric optic zone
- multiple pieces
- split
- blemish

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16
Q

what are the 2 main methods of RGP manufacturing?

A
  1. Lathe cutting
  2. Cast moulding
17
Q

How is RGP Lathe cutting done?

A

• Very similar to soft lenses but less margin for error
• Separately configured lathes for front/back surface
• Engraving/marking is common on RGP, e.g. R/L or the BOZR
• Edge polishing is then done
• Lens stored dry

18
Q

What are the advantages and disadvantages for RGP lathe cutting?

A

• Advantages and disadvantages are very like those for soft lenses.

19
Q

How is RGP Cast moulding done?

A

• Similar to the soft lens method in that the lens is left to solidify in the
• The mould is then broken apart to reveal the lens in its final form which doesn’t routinely require further finishing.
• Advantages and disadvantages same for soft lenses

20
Q

What are the first two toric manufacturing methods?

A
  1. Toric machining: dual-axis lathing tool on a stationary lens button to produce two radii. Computer controlled now, can produce any shape required (the most common).
  2. Crimping: lens blank is crimped across the front surface then worked into a sphere, releasing the crimping the leaves a toric surface (almost obsolete now)
21
Q

What are the 3rd and 4th method for toric manufacturing?

A
  1. Moulding - as for standard spherical lenses, just requires a separate mould for each Rx (therefore limited range)
  2. Dual-axis flying cutter: cutter is set to a particular axis, as this passes by the rotating lens button it creates the toric shape (less stable on the eye than toric machining)
22
Q

What is Prism Ballast stabilisation in toric manufacturing?

A

• Can be done on both RGP or SCL
• Balances the thickness profile, to minimise rotational effects
• 1 or 1.5 prism base down traditionally, although now can be placed peripherally (newer designs)
• This is all done by software

23
Q

What is Dynamic stabilisation in toric manufacturing?

A

• Depends on the interaction between the lids and front surface of the lens
- Dual thin zones or double slab off: Top and bottom portions of the lens are chamfered (thinner sloping edge)
too
- Truncation is now rarely used with soft lenses as difficulty in mass production moulding methods needed for disposable lenses; removes a 1-1.5mm chord from the lower edge of the lens giving excellent stability but can be uncomfortable. Edge removed with an emery board or a diamond impregnated tool and polished. Usually combined with a prism.

24
Q

What is multifocal segmented designs?

A
  • Two or three distinct portions that make up an alternating lens may be either fused (insert of higher RI) or solid portions with a range of alternative segment shapes
  • Probably lathe cut but difficult to find info as this lens type is now quite rare
  • Stabilised with prism and/or truncation
  • More commonly RGP
25
Q

What are the two types of simultaneous vision multifocal CLs?

A

• Concentric
- Distance power typically is in the center of the lens, surrounded by concentric rings of near and distance powers, moulded/etched onto the lens back surface
• Aspheric
- Distance (or near) power is in the center, with a gradual transition to other powers as you move away from the center.

26
Q

What are the hybrid CL zones?

A

• Bond zone
• RGP central zone
• Silicone hydrogel or hydrogel skirt

27
Q

How are hybrid CL’s manufactured?

A

• very similar process to lathe-cut soft contact lenses
But!
• plastic disks cut with the lathe have a rigid centre, rounded by non-hydrated soft contact lens material
• the two materials are bonded together with proprietary technology to prevent separation of the materials

28
Q

What is the different types of Slceral CL manufacturing?

A

• Lathe cutting
Production as for corneal lenses, plus cutting of a scleral portion
• Moulding
- Take mould of eye or scleral topography to measure the ocular shape
- Press warmed RGP material over model
- Or digital model of the eye is created using a 3D scanner
- Trim excess material, cut down to TD (typically around 23mm for a full-diameter scleral)
- Tidy edges
- Lathe cut & polish

29
Q

What are the 6 types of Tinting of lenses?

A

• Handling tint
• Cosmetic
• Prosthetic
• Therapeutic e.g. red for achromatopsia
• CV/dyslexia/sport
• Theatric

30
Q

What is the Dye dispersion method of tinting lenses?

A

• mainly for RGPs, dye is added to the polymer before it solidifies
• leaving an evenly dispersed colour.
• disadvantages include no ability to include a clear pupil, density is related to lens thickness and unsuitable for soft lenses because the dye is water-soluble and will leak from the polymer during hydration

31
Q

What is the Vat Dye method of tinting lenses?

A
  • Suitable for soft lenses
  • The finished lens is soaked in a water soluble dye which is then exposed to air meaning it becomes insoluble and trapped in the lens matrix
  • Used for prosthetic soft lenses
32
Q

What is the Chemical bond tinting and Printing method of tinting lenses?

A

• Chemical bond tinting
-Soaking the lens in the dye solution along with a catalyst
-Strong chemical bond formed between the dye and the lens polymer
- Results in a stable, uniform tint
• Printing
- Dye can be placed on the lens like ink is printed on paper
- This allows for complex patterns (such as an iris) and a clear pupil

33
Q

What is the dot matrix printing method of Opaque tints?

A
  • Apply a matrix of small dots to the front surface of the lens
  • Uses a chemical bond to attach the dots to the lens
  • Leaves a final appearance of a combination of dots and the natural iris behind
34
Q

What is the Lamination method of Opaque tints?

A
  • A pattern is painted on the finished lens but a second layer of clear contact lens is poured on top and left to set
  • Advantage is that the tint is within the lens and protected
  • Disadvantages include increased thickness of the lens, reduced oxygen transmissibility and a differing fit
35
Q

How are RGP lenses inspected for quality insurance? (7 points)

A
  1. Inspection with 10x mag (edge defects can usually then be polished out)
  2. Power
  3. BOZR (back optic zone radius)
  4. TD (total diameter)
  5. Optic zone diameter
  6. Lens thickness
  7. Cleaned and dispatched

If dispatched dry then the practitioner should soak the lens for 4 hours in solution to ensure optimal lens surface wettability

36
Q

How are SC lenses inspected for quality insurance? (5 points)

A
  1. Inspected dry (10x mag)
  2. Hydrated in saline
  3. Inspected wet (10x mag)
  4. Packaged either in blister pack or vial then autoclaved
  5. Dispatched

(2) Tri A - Contact lenses (16 decks)

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