Advanced GP Lens Fitting

The major advantage that GP lenses offer over silicone or hydrogel lenses is the quality of vision provided. Normally these lenses are utilised on patients with astigmatism, who find the acuity with flexible lenses unacceptable. For small amounts of astigmatism (< 1.75D), a spherical/aspheric lens should mask corneal astigmatism and the regular lens can still satisfy the requirements of comfort, stability and acuity that determine an acceptable fit. Once the corneal cylinder goes over 2.00D, it is worth considering a back-surface toric design. Many ECPs shy away from these and persist with spherical lenses, and many experienced fitters will claim to have fitted higher cylinders with spherical lenses. This may be a questionable fit - as soon as the "K2" readings are 0.50mm different between the flattest and steepest meridians – which equates approximately to a 2.25D cylinder – a back surface toric design is strongly recommended.

Fit the flattest curve to align with the flattest meridian and specify the steeper curve 0.50mm steeper – if the steeper corneal curve is slightly more than this, the tear lens will compensate for any remaining cylinder. The result is a lens that stabilises well and does not rock over the corneal shape. Usually a simple spherical over refraction will provide sufficient acuity, without the need for more complex front-surface geometries.

Smaller differentials have often been attempted – say 0.30mm – for smaller amounts of astigmatism, but very often the lack of toricity means that the lens does not locate properly and vision fluctuates as the induced cylinder rotates.

On occasion the over-refraction will require a cylindrical component and this should be combined on the front surface, provided the rest of the fit is satisfactory. This situation is quite rare and should not deter the new fitter from trying these lenses. 

Standard lens on astigmatic cornea. Note alignment along 20Ëš. Excessive edge lift along 110Ëš 


Toric lens on same cornea. Alignment fit over majority of cornea, much reduced edge lift along steeper meridian.
 

Front-surface torics

Occasionally the cornea may be nearly spherical and yet a cylindrical correction is found on refraction. This is rare, but can still be corrected with RGP lenses. Without the toricity of the corneal surface to lock the back surface of the lens into place, it is necessary to provide some other method to maintain the lens on axis. After establishing the best fitting lens, incorporate the sphero-cylindical over-refraction onto the front surface with a base down prism of about 1.5 prism dioptres. This normally stabilises the lens, but as with soft toric lenses the lid pressure over the front surface may result in the final lens stabilising off the anticipated 6 o'clock position. In this case, allowing for this swing should improve final acuity. These fits are rare and it is not always apparent at the beginning of the process what is going to be necessary. The process is fairly straightforward however and should not deter the fitter from attempting this course of action.