25 February 2016
Irregular-cornea contact lens fitting is both frustrating and satisfying. No two keratoconic eyes are the same; no two corneal grafts are the same
As Ken Pullum, principal optometrist of the scleral lens service at Moorfields Eye Hospital, put it at 100% Optical 2015: fitting corneal lenses to a keratoconic cornea often “offends my sense of symmetry.”
How do you fit a square peg in a round hole? I’ve been trying to do this for years. My conclusion? Stop trying and get a mini-scleral fitting set instead.
Why not bridge over the whole cornea and forget about trying to ‘fit’ something that doesn’t fit. Resting the lens on the sclera (or bulbar conjunctiva, I suppose) means no mechanical stress on the cornea, which is a great advantage when you have a diseased cornea and this is why mini-sclerals have become my troubleshooting lenses.
New-found popularity
Mini-sclerals appear big and scary, but the response when the patient puts one in is one of pleasant surprise. They really are more comfy than the corneal lenses they have been battling with. I used to think patients with anything but a large vertical palpebral aperture would struggle with them but actually, at 15–18mm, mini-sclerals are similar in diameter to a soft lens and don’t drape and stick to the finger. However, insertion does require the patient to face down and fill the lens with saline, which takes practice.
For me, chair time tends to be less than for smaller diameter lens fitting as you don’t waste time on the cornea. I had to get my head around using sagittal depth rather than base curve. Essentially you’re asking yourself ‘how bulgy is this eye?’ as opposed to ‘how curved is this cornea?,’ so keratometry readings are of little value. Better to use the topographer and, less scientifically, stand at the patient’s side and look at the cornea in profile.
The resurgence in popularity of mini-sclerals appears to be driven by improvements in manufacturing accuracy and reproducibility, improvements in lens materials and better instrumentation for measuring the eye, for example extended topography and optical coherence tomography (OCT).
Manufacturers are happy to help you learn how to use these lenses and provide efficient support during the fitting process.
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