Gas permeable (GP) lenses are fitted almost in alignment with the cornea. The centre of the lens normally clears the apex of the cornea by ±20 microns, and the lens aligns with the periphery of the cornea to distribute the weight.
The edge of the lens lifts off (edge lift) to provide tear exchange and to ensure that it moves freely when the patient blinks and does not adhere through suction. Lenses sit within the limbal margin, and therefore diameters are typically 1-1.5mm smaller than the horizontal visible iris diameter (HVID). Lenses should move by 1-1.5mm with each blink of the eye, and remain centred over the visual axis most of the time to provide stable vision without peripheral blurring or ‘haloes’ that can occur if the edge of the optic zone of the lens can pass over the pupil.
Spherical lenses are fitted slightly flatter than flattest ‘K’ readings to allow for the fact that the cornea flattens towards the periphery. Today, most lenses have an aspheric back surface to match the natural flattening of the corneal curvature, and therefore the lenses are fitted to align centrally with the flattest ‘K’.
Very few corneas are spherical, and practitioners should not be put off by small to moderate amounts of astigmatism. For ‘with-the-rule’ astigmatism, where the minus cylinder power lies along 180 degrees or near, up to -2.00 to -2.50 dioptres of astigmatism can be successfully fitted with a spherical GP lens. In fact, a small degree of astigmatism such as this does not provide sufficient difference in shape for a toric back surface to ‘lock’ onto. Over 2.50 dioptres of astigmatism is best corrected by using a back surface toric design.
Against-the-rule astigmatism, where the minus cylinder axis lies along 90 degrees, poses a greater problem, as the flattest meridian of the cornea lies along the vertical. Aligning the lens with the flattest meridian means that the lens will naturally tend to want to rock from side to side as the lid tries to move the lens up and down. The resultant rotational movement tends to result in reduced comfort and variable vision. For against-the-rule astigmatism, it may be necessary to reach for a toric back surface for smaller degrees of astigmatism and to use prism ballast to help stabilise the lens.
For even experienced practitioners, fitting a toric GP lens can appear to be a complex procedure. That is why it is important to use your laboratory consultant to discuss lens selection. Take all necessary measurements: spectacle Rx, keratometry, HVID, and pupil size and topography, if possible, and speak to your chosen laboratory. They know their designs and can advise what, in their experience, will work best for a given situation. Even for experienced fitters, it is often reassuring to discuss your findings with a second party prior to ordering the final lens.