Rigid contact lenses – what’s in a name?
Rigid contact lenses are available in many forms. The classification of rigid lenses is usually based on differences in total diameter, although the terminology can vary. I favour the rigid lens nomenclature whereby corneal lenses have a diameter of 7.0 to 12.0 mm, corneal-scleral lenses have a diameter of 12.0 to 15.0 mm, mini-scleral lenses have a diameter of 15.0 to 18.0 mm and scleral lenses have a diameter of 18.0 to 25.0 mm. Up until recently, corneal lenses have been the major form of rigid lens generally prescribed, however over the last few years there has been a dramatic increase in the usage of the larger diameter lenses (especially the mini-scleral lenses).
The original rigid contact lenses that were developed over 50 years ago were also called hard lenses and these were made from a material known as polymethyl methacrylate (PMMA) that was not gas permeable. Consequently, the wearing of these lenses resulted in corneal hypoxia and other associated complications as oxygen could not get through the contact lens to the eye. Thankfully, there was subsequently the development of rigid gas permeable materials which solved this problem, so the term ‘rigid gas permeable’ (RGP) lens was introduced to distinguish between rigid lenses which were made from PMMA and those which were made from gas-permeable materials. All rigid lenses today are made of gas permeable materials (PMMA is now rarely used) so the adjective ‘gas-permeable’ is effectively redundant. In this regard, ‘rigid lenses’ now refers to all lenses made from rigid gas-permeable materials; the term ‘RGP lens’ is no longer necessary and hence should not be used.
The other problem with rigid lens nomenclature relates to the fact that for the last 40 years, over 90% of rigid contact lenses prescribed have been the smaller diameter corneal lenses. Hence, the term ‘rigid lens’ or ‘RGP lens’ has become synonymous with corneal lenses. With the rapidly increasing use of the other (larger diameter) rigid lenses, it is no longer going to be appropriate to simply refer to corneal lenses as rigid lenses. I suspect that in due course we will revert back to calling rigid lenses what they really are – corneal, corneal-scleral, mini-scleral or scleral lenses.