RGP, Toric, Multifocal, Hybrid CL, Scleral CL Manufacture & Tinting of Lenses Flashcards
What are the 2 main methods of RGP manufacture?
- Lathe cutting
- Cast moulding
Describe RGP manufacture - lathe cutting?
- Process similar as for soft lenses but with even less margin for error as this is the final lens (won’t be swollen w/ water (which can cover multitude of sins on the xerogel)) – however these lenses come off the lathe is how they’ll be on eye
- Separately configured lathes for front and back surface
- Engraving/marking is common on RGP lenses e.g. R/L or the BOZR
- Edge polishing is then done – for comfort
- Lenses stored dry – rather than blister packs/vials
- Specialist lenses – much wider range of power available
o Lathe can cut it to any parameter want
What are the advantages of RGP lathe cutting?
- 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 manufacture of custom or extreme range lenses that are not amenable to mass production
- Suitable for most materials
- A lathe is a finite investment
What are the disadvantages of RGP lathe cutting?
- More steps & more time consuming than moulding
- Takes a few mins to make one lens
- Higher unit cost
- Requires strict humidity control
- Cleaning / polishing required on completion
- Variable surface finish
- Lenses must be soaked to remove impurities (unpolymerized monomers) prior to sterilization and packaging
Describe RGP manufacture - cast moulding?
- Similar to the soft lens method in that the lens is left to solidify in the mould
- The mould is then broken apart to reveal the lens in its final form which doesn’t routinely require further finishing
What are the advantages of RGP cast moulding?
- Very quick
- Low cost per lens
- Easy to produce many lenses at once
- No polishing required
- Most widely used method
What are the disadvantages of RGP cast moulding?
- 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
What are the 7 quality assurances of RGP manufacture? What should the practitioner do if the lens is sent dry?
- Inspection w/ 10x mag (edge defects can usually then be polished out)
- Power measured
- BOZR (back optic zone radius) measured
- TD (total diameter) measured
- Optic zone diameter measured
- Lens thickness measured
- Cleaned and dispatched
If dispatched dry, then practitioner should soak lens for 4 hours in solution to ensure optimal lens surface wettability
What are the 4 types of toric lens manufacturing?
- Toric machining: dual-axis lathing tool on a stationary lens button to produce 2 radii. Computer controlled now, can produce any shape required (most common)
- 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)
- Moulding – as for standard spherical lenses, requires a separate mould for each Rx (therefore limited range) -> only good for stock
- 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)
Describe prism ballast stabilisation in toric CL manufacturing?
o 1 or 1.5 prism base down traditionally, although now can be placed peripherally (newer designs)
o Balances thickness profile, to minimise rotational effects
o RGP & SCL
o Produced by software (no longer manual)
Describe dynamic stabilisation in toric CL manufacturing?
o Depends on interaction between lids & front surface of lens
Dual thin zones or double slab off: top & bottom portions of lens are chamfered (thinner sloping edge) to
o Truncation is now rarely used w/ soft lenses as it does not lend itself to mass production moulding methods required by disposable lenses
Removing a 1-1.5mm chord from the lower edge of lens
In general, they give excellent stability but can be uncomfortable
Edge removed w/ an emery board or a diamond impregnated tool & polished
Usually combined with a prism
Describe segmented designs in multifocal CL manufacturing?
o 2 (or 3) 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
o Probably lathe cut but difficult to find info as this lens type is now quite rare
o Stabilised with prism and/or truncation – to stop rotation & keep in right position
o More commonly RGP
Describe simultaneous designs in multifocal CL manufacture?
- Concentric:
o Distance power typically is in centre of lens, surrounded by concentric rings of near & distance powers - Aspheric:
o Distance (or near) power is in centre, w/ gradual transition to other powers as move away from centre – intermediate zone into near in periphery - Concentric rings or diffractive pattern are moulded/etched onto lens back surface
Describe hybrid CL manufacture?
- Rigid central part of lens – made out of RGP material Bonded to hydrogel/SiHy skirt
- V similar process to lathe-cut soft CLs
- BUT:
o The plastic disks cut w/ the lathe have a rigid centre, surrounded by non-hydrated soft CL material
o The 2 materials are bonded together w/ proprietary technology to prevent separation of the materials
Describe scleral CL manufacture (lathe cutting and moulding)?
Lathe cutting:
* Production as for corneal lenses, plus cutting of a scleral portion (much larger)
Moulding:
* Take mould of eye or scleral topography to measure the ocular shape
* Press warmed RGP material over model
o or digital model of 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
