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Modification Procedures
The Importance of the Practitioner Being Able to Carry Out Modifications
Nowadays, when modifications can be carried out relatively quickly and cheaply by laboratories, it is often difficult to impress on students, and even qualified practitioners, the following reasons why contact lens practitioners should be able to carry out their own modifications:
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There is no interruption to the patient's wearing schedule or necessity for extra visits.
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Often only the practitioner knows exactly what is required, and it may be difficult to describe this to the laboratory (particularly so with scleral lenses).
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Modifications can be done in increasing stages, with the effect on the fit of the lens noted at each stage.
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Subsequent modifications needed may become apparent and can be done immediately.
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Practitioners who carry out modifications are better able to evaluate lenses that have been made or modified by a laboratory.
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Patients are more satisfied because they do not have to be without their lenses.
Against these advantages must be set the cost of the equipment involved. However, much of this cost can be redeemed, not only from tax relief but also from the savings of doing the modification oneself. Further, most of the equipment, especially the more expensive items, will last a lifetime (see ‘Points to Note When Modifying RGP Lenses’).
Modification Equipment for Corneal Lenses
Corneal lens holders
Glass Tube and Double-Sided Adhesive Tape
A glass tube of approximately 6 mm in diameter with rounded ends has a small strip of double-sided adhesive tape stretched over one end, onto which the lens is lightly pressed. An inverted glass tube from a dropper bottle is ideal.
Modified Scleral Lens Holder
An ordinary scleral lens suction holder is cut down and thinned on a rotating rough grinding stone to an internal diameter of approximately 7.5 mm and an edge thickness of approximately 0.5 mm ( Figs 33.1 and 33.2 ). A corneal lens holder may also be used with a hollow plastic tube in the stem to give support ( ). These holders are best used filled with water and rotated manually to prevent oval optics.
Modified Corneal Suction Holder
The end of a corneal lens suction holder is placed over a short nylon or Perspex rod cut (see Figs 33.1b and 33.2c ). Again, best used with the open end full of water, this type gives good control and spins easily with one finger resting lightly on the upper end.
Spinner
A more efficient version of the previous holder, this consists of a rotatable spindle with a corneal lens suction holder at the end, mounted in a handle (see Fig. 33.2b and e ). The lens can rotate freely on the end of the spindle while the handle is held still.
Double-Ended Suction Holder
This has one end for attachment to convex surfaces and one end for concave surfaces (see Fig. 33.1c ). As this is the only type of lens holder, apart from those using adhesive tape, which will attach to the back surface of a corneal lens, it finds particular application in the alteration of lens power and removal of front surface abrasions (see below). Suction holders have the disadvantage that the lens may flex, particularly with thinner lenses, and the curves may be produced slightly steeper than desired, or slightly distorted. In addition, care must be taken when removing the lens from the holder.
Vertical spindle or Office Modification Unit ( Fig. 33.3 )
A speed of between 500 and 1500 rev/min is required – ideally, variable over this range, especially in the slower region.
The unit should be mounted in a bench or unit with the part shown situated above the level of the bench ( Fig. 33.3 ). This enables the operator to rest with the elbows on the tabletop and steady the hands, and to roll the cutting blade and smoothing tools around the edge of the lens when shaping the edge or reducing the total diameter (TD).
Spindle lens chuck or edging chuck ( Fig. 33.4 )
This is used for edge and TD modifications and should be carefully chosen so that the taper fits the practitioner's own vertical spindle (e.g. standard or 0-morse tapers). It is preferable to have a chuck, approximately 6 mm wide and 2 cm long, held by a stem that is not too short or thick.
The same chuck may also be used for holding lenses for peripheral curve modification of the back surface. The lens is stuck onto the chuck with wax or double-sided adhesive tape and held rigid on the tool so that no flexing occurs during polishing; however, if the stem is very narrow, the periphery of a large corneal lens may still flex.
Adhesive wax and bunsen burner
Adhesive wax, obtainable in stick form, is one way of attaching the lens to the spindle lens chuck. A Bunsen or compressed butane burner is used to melt the wax. More sophisticated electric heaters are also available to heat the metal chucks. Low-melting-point wax should be used for rigid gas permeable (RGP) materials.
Centring devices
These are necessary when double-sided adhesive tape is used on the spindle lens chuck instead of beeswax, and where the lens must be accurately centred on the chuck.
The lens is placed in the well of the device. The centring part is carefully lowered onto the lens and slowly rotated to centre the lens within the well. It is then withdrawn.
These devices are not usually as accurate as centring by hand (see below).
Wax polishing tools
The practitioner makes these tools by pouring molten wax into moulds and allowing this to set. They are used for accurate polishing to a specific radius (see Fig. 33.4 ). Instructions for their manufacture are given by the laboratory supplying them; alternatively, they may be made up by the laboratory. Normally, only convex curves are cut on these tools.
Female gauges
These cut wax tools to their required radius (see Fig. 33.4 ). Stainless steel gauges are the most accurate and long-lasting. A razor blade should be used to cut the wax cylinder to approximately the correct radius, and the female gauge used only for final ‘truing-up’ to prevent excessive wear.
Metal or plastic tools and adhesive tape
As an alternative to the wax polishing tools, adhesive cloth zinc oxide ‘sticking plaster’ is stretched over a suitable tool and used for polishing lens peripheries, an allowance of approximately 0.2 mm (depending on the tape used) being made for the thickness of the tape. When using this type of tool, check with the manufacturing laboratory whether an allowance has been made for an average tape thickness when constructing the tool.
This method has the advantage over the wax tools that it is not normally necessary to put on a separate transitional curve. However, the accuracy of curves polished by ‘cloth polishing’ cannot be guaranteed.
Tools may also be covered in muslin, stretched taut and held in place with a rubber band.
Polishing liquids
The polishing liquid used most frequently for polymethylmethacrylate (PMMA) lenses is the metal polish Silvo. Allow the container to stand for 2 or 3 days; then pour off most of the excess clear fluid and use the slightly thicker polishing fluid.
The ammonia in Silvo acts as a solvent for most RGP materials and should not therefore be used on these types of lenses, nor should solvents or polishing compounds containing alcohol, esters, chlorinated hydrocarbons, ammonia, etc. Suitable polishes are X-Pal, Linde A, SPI, Hyprez, Boston, Silo2 Care polish, etc., but ideally consult the individual material manufacturers.
For soft lenses, a good standard polish that will give an excellent surface finish is CCPI polishing compound mixed with silicone oil.
Paraffin oil
Ordinary paraffin oil (kerosene) is used to remove any wax that remains attached to a lens after removal from a chuck. It may also be used as a lubricant with diamond-impregnated tools when putting on the peripheral curve of a corneal lens prior to final polishing, or grinding out the back optic surface of a scleral lens. Being more viscous than water, it prevents such deep scratches (although grinding is slower), enabling the final polishing to be done more quickly.
Razor and scalpel blades
Single-edge (‘safety’) razor blades and scalpel blades (straight and curved) are useful for certain modifications (see below). Alternatively, a thicker square-edged cutter may be used.
Carborundum tool for total diameter reduction
This is an internally tapering cone, held in a Jacob's chuck ( Fig. 33.5 ). The tool is soaked in water, and the lens must be placed centrally to remove peripheral material. If it is not central, material will be removed unequally ( ). With lenses incorporating a prism, more material may be removed from the thinner lens edge.
Assorted carborundum or diamond-impregnated burrs
These ( Fig. 33.6 ) are used for final edge shaping. Very fine emery paper may also be used.
Polishing cloth
This very soft cloth (e.g. ‘Selvyt’ cloth) is used for adding small amounts of power to lenses. Suitable cloth may be obtained from any contact lens laboratory at little cost; alternatively, ordinary velveteen material may be purchased.
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