Basic Gas Permeable Lens Fitting
After a full history, as would be the norm for all potential wearers, the first thing the text books tell us is to refer to the physical dimensions of the cornea. This presumes that the ECP is going to design a lens from scratch, which for the inexperienced fitter, may be quite daunting.
Measurements such as HVID and Palpebral Aperture, are useful (if they are accurate), but for most ECPs working from a fitting set or system lens, the initial diameter of the first lens is often determined by the fitting set available.
The average cornea has always been presumed to be 11.8 mm in diameter, although recent studies have shown this figure may be an underestimate, so most system lenses now start with an initial lens of around 9.8mm, which allows for a movement of 1-2mm with blink and is a good starting point.
The next decision is the base curve, this is where accurate information is important. Assuming that the luxury of a topographer is not available, the Keratometer is the instrument which provides an initial idea of the curvature of the cornea. Bear in mind that the readings from this instrument refer only to the central 3-4mm of the corneal curvature. Nevertheless it gives a good starting point and is a reference should the fitter suspect that changes in curvature may occur in the future.
First Trial Lens
The main difficulty in fitting rigid lenses is that we are trying to align a rigid piece of plastic to a non-spherical surface. The cornea flattens as we move from the central zone and so if the initial lens is chosen from the central "k" readings alone, the lens may not fully align over its entire diameter, and could appear tight. The manufacturers will therefore suggest that the first lens is chosen to align with flattest "k" or some other calculation based upon a particular design.
With aspheric lenses, the manufacture allows for the corneal flattening and therefore the initial lens will normally be a steeper curve than for a traditional spherical design.
Essentially, the initial lens should be selected in accordance to the manufacturer's instructions.
Use of Anaesthetic
For a new wearer, the use of a topical anaesthetic makes the initial experience a little more pleasant and allows the ECP to assess the fit more clearly as the fluorescein pattern will be more easily discerned. Some fitters may prefer to forego the use of this fitting tool, but many ECPs find that application may provide a better experience for both the ECP and the patient.
Assessment of Trial Lens
The assessment of fluorescein patterns is something that cannot be easily taught from a text. Obviously steep and flat lenses can readily be detected, but observation over many fits will hone the ECP's skills to be able to detect very subtle differences over time. Other clues are more easily detectable and can give the new fitter additional data as to the performance of the lens on the eye.
Tight fit – note central pool
Loose fit – central touch and pooling at edge
After a "push-up" with the lower lid, a well-fitted lens should drop gently over the apex of the cornea, whereas a flat-fitting lens will prefer to slide around the apex with a more rapid movement. A tight lens will move much more slowly and prefer to remain over the apex rather that drop towards the lower limbus.
If alignment fit is achieved then the over-refraction should agree with the spectacle Rx, transposed to minus cylinder form (allowing for vertex distance). If more minus is required, then it suggests a plus tear lens under the trial lens and therefore the lens might be steep. Conversely less minus than anticipated may indicate a flat lens. Basically, 0.25 Dioptre of tear lens is equivalent to 0.05mm in curvature, so if the fitter finds -0.50D more minus on over-refraction, the result could be 0.10mm steep on the base curve.
If apical clearance is required, then expect to need a little more minus (0.25D) to allow for a slight positive tear lens.
Fashions have changed with rigid lens fitting, before gas permeable materials were available the only way to get oxygen to the cornea was around the lens. Smaller lenses with good movement were the norm, with diameters as low as 8.8 and 8.6 being used. With the advent of materials now transmitting sufficient oxygen, the patient can anticipate minimal movement, in fact just enough to allow metabolic by-products to escape.
Larger lenses tend to be more comfortable and more stable; some of the newer designs (Intra Limbal and Semi–Scleral) are increasingly being used in more complex cases with very successful results.
Adjusting the Fit
If the lens proves to be steep or tight, then the process is fairly straightforward. Many fitters advise that there is no point in "tweaking" a base curve by 0.05mm. If a lens is obviously tight or loose, such a small adjustment will make no difference. The fitter is advised to make gross adjustments of at least 0.10mm to achieve any noticeable change. If visual acuity is otherwise good, a suggested allowance for the new tear lens to be created should be made.
For the inexperienced ECP, altering the Optic Zone (OZ) should be avoided if possible. If the lenses need to be tightened then use the base curve to achieve this result. Making the OZ larger will steepen the lens, but will also alter the bearing surfaces around the lens. Speak to your laboratory for advice before ordering the lens.
Generally if you need a larger OZ, a minimum of 0.30mm increase is required, this will be equivalent to a 0.05mm steepening of the base curve as far as the fitting is concerned. This will not alter the power of the tear lens, so no power adjustment is needed.