Basic Soft Lens Fitting
The relevance of the "k" readings following a normal pre-fitting examination is questionable in soft lens fitting. Apart from a point to refer back to in aftercare visits, its value as an indication for initial lens choice is minimal. A far greater influence on the overall comfort and fit of a given lens is the corneal diameter (HVID).
As most ECPs do not have a graticule in the eyepiece of their slit lamp, assessment using a pupillary distance (PD) rule is frequently used, with some inaccuracy. The average ECP will simply use an habitual monthly/daily lens and expect it to fit – and mostly it does!
Manufacturers of moulded hydrogel lenses with low modulus materials, have found over the years that the average cornea can be fitted with one base curve/diameter combination. Variations around these values occur mainly as a result of variations in material characteristics and thickness.
Manufacturers have found that the higher modulus of high silicone/low water-content materials were less forgiving and as a result, had to introduce two base curves . Early silicone lenses such as PureVision and Night and Day, are provided in two base curves for this reason. As newer silicone materials with higher water content and lower moduli have become available, larger manufacturers have reverted to the "one size fits all" philosophy, if the material will permit.
Studies have shown that optimal fits can be achieved in 75% of cases using these products and allows manufacturers to produce low priced lenses from stock. The cost of providing optimal fitting in all cases can be prohibitive, so the ECP must frequently jump between lens types to satisfy patients with larger or smaller corneas.
Because of this at least 25% of patients are wearing system lenses with a less-than-optimal fit. This may go some way in accounting for the high drop-out rate amongst soft lens wearers, currently estimated at 12% per annum.
So what can the ECP do about the 25% of his soft lens fits that need an off-system base curve/diameter? Well we can continue to do as we have been and accept that the number of new fits we provide with compromised comfort are tomorrow's dropouts or we can provide a custom lens that will provide the comfort that the patient deserves.
So where does this process begin?
Assessing the problem with the best fitting lens available from stock allows the ECP to decide whether to go smaller, or as is normally the case, larger. The diameter will decide the base curve. For every 0.5mm required to go larger, you must flatten the base curve by 0.3mm. Similarly for every 0.5mm required to go smaller, steepen the base curve by 0.3mm to maintain the same sag.
8.6/14.2 on eye fits well but is too small with patient awareness of the edge when looking left and right. Ideally this patient needs 0.5mm larger OD.
The required diameter is 14.7, but the lens should be flattened by 0.3mm to offset the increased diameter.
The fitter is prompted to supply 8.9/14.7 as the first lens of choice.
Most labs will offer an exchange on custom lenses and will know their respective lens design characteristics. Again a good relationship with the manufacturing laboratory will allow the ECP to meet the patient's needs and prevent them from becoming tomorrow's dropouts.