Combination lenses are a rigid lens fitted on top of a soft contact lens, often called a piggy back lens. Originally these lenses were solely P M M A lenses on top of Hema lenses. My first early experiments with this were about twenty-one years ago when the original Czechoslovakian lenses came out and we started using them. They were extremely small, uncomfortable lenses with very poor visual acuity and I was curious to know whether in fact low visual acuity was because there was something wrong with the material or whether it was just an irregular surface or whatever so I took K readings on top of these lenses and found the K cylinder to be very similar to that of the naked cornea, obviously this was the cause of the trouble. Out of curiosity we put some hard lenses on top of these lenses and immediately the visual acuity improved dramatically but they were extremely uncomfortable. When in the Sixties British companies also began to make soft lenses we made exactly the same experiments with those and some patients actually wore these combination lenses, albeit for a few hours, in the Sixties but with rather poor results in general. About 1967 a lot of us began to fit cases of corneal pathology with soft lenses for various reasons. These lenses were frequently extremely comfortable but often gave very bad visual acuity. In Zimbabwe there is extremely good co-ordination between ophthamology and optometry which of course makes it very much easier for those of us in optometry to be involved in pathology work which is interesting and very rewarding. The idea of a combination developed from treating these pathology cases and we now use this frequently in my practice mainly in abnormal and irregular eyes. I think we have to concede that despite the excellent skills in Great Britain in scleral lens fitting they are by no means always successful although some cases work extremely well with scleral or corneal lenses and you do not need combination lenses even in fairly gross cases, such as a very bad graft when a CAB lens can work or a case of 1.4 difference in radii in different meridians on a traumatic case where a corneal lens can work well. You do not always need to use these dramatic combination lenses but the first case we actually fitted for constant use was a woman in 1970 with keratoconus. She had scleral and corneal lenses before with a brief history with all of them, she could only manage a few hours a day with one or two hours rest in the middle. She had staining, scarring, neo vascularisation up to the pupil margin and this was the first case we actually fitted for the continuing use of lenses. It was a very uncomfortable looking eye in general but this was the only way that this woman could see anything. The fitting technique then, as now, is to initially take a K reading on the cornea as a guide only and from then on there is a lot of experience involved. Basically one is trying to fit a large soft lens to get a conventional looking soft lens on that eye. It often means changing your soft lens fitting set but having got that soft lens fitting in the way you want it you then take what I call an over K, a K reading on top of the soft contact lens. If you are using a minus lens then this K reading is a lot flatter and that is very advantageous, especially if you have not got a very big fitting set, because it now brings the radii you are dealing with very much nearer a fitting set lens you probably have in your fitting set and you can adjust the fit accordingly. If the readings are still a lot too steep for your particular fitting set then you can use a --6.00D instead of a 3 . 0 0 D , or whatever, and you can modify those over K readings by using different lenses. The results by juggling these figures are often extremely good.
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Oct 10, 2021
Dexing Zhu + 3
Open Access
