By their very nature, soft lenses are soft and pliable, and will therefore conform to the shape of the eye. A range of eyes may comfortably be fitted with a limited number of base curves.
Soft lenses are designed to be larger than the visible iris diameter and to overlap onto the sclera (white) of the eye by approximately 1.5-2.0mm. This is to allow the lens to move slightly when the patient blinks and to allow the edge of the lens to remain on the white of the eye. If the edge encroaches onto the cornea when blinking or during extreme eye movement, the patient will become aware of the edge.
Most soft lenses fitted these days are moulded lenses made in a limited number of parameters, (mostly, just one). Nevertheless, they provide safe and comfortable wear for most users. For those patients with a slightly larger or smaller cornea than average however, or whose cornea falls outside the normal range of curvature, these mass-produced lenses are much less comfortable, or effective.
Practitioners should therefore acquaint themselves with the techniques associated with fitting a custom soft lens. It is estimated¹ that up to 23% of eyes fall outside the ‘average’ range of corneal diameters, and would therefore benefit from a custom lens.
The starting point for a custom soft lens fit is normally the horizontal visible iris diameter (HVID). This can be measured with a HVID rule, a slit lamp reticule or a topographer, although in reality the practitioner will have already tried a moulded lens and found it to be unsuitably large or small. An estimate of the required adjustment in diameter can then be made, or the slit lamp beam can be adjusted to the minimum (0.3mm) to give a comparison if a reticule is not fitted.
Manufacturers will normally provide a guide for initial lens choice using ‘K’ readings and HVID.
The lens should move slightly when the patient blinks or gazes upward, but not enough for the edge to transgress across the limbus. Too much movement can be corrected by a steeper radius (normally in 0.30mm steps) or by increasing the diameter (0.50mm steps). (As a general rule, 0.50mm in diameter is equivalent to 0.30 in base curve).
If the lens coverage is good, then a ‘push-up’ test will reveal the lens moving freely across the surface of the cornea. Any resistance to moving the lens with the lower lid indicates a tight fitting. Flattening the base curve in 0.30mm increments will remedy this, as will reducing the diameter in 0.50mm steps, if required.
A more subtle indication of a tight fit is an otherwise comfortable patient complaining of blurring.
If vision is better immediately following an eye blink, but then fades post-blink, then the lens may be tightening over the optic and distorting vision. The eye blink flattens the optics to give momentary relief, which fades as the elasticity of the material takes over and the optics distort once more.
In general, a patient with a smaller cornea may tolerate a soft lens that is marginally larger than optimal. On the other hand, a patient with a large cornea will rarely be able to tolerate a lens that is too small.