Big vs small: Damien Fisher shares his thoughts on whether a large or small diameter RGP is better for fitting irregular corneas

In my last article I wrote about how technology has made vast advancements throughout our lives.   This also extends to the manufacture of contact lenses within both the soft and rigid lens domains.  In particular, I’d like to express a few thoughts about the dilemma of whether a large or small diameter RGP is better for fitting irregular corneas.

The improvements that have occurred with the development and manufacture of custom made rigid lenses has given the specialty contact lens practitioner many options.  The algorithms and lathes used to cut these lenses can do so with a high degree of accuracy and repeatability.  This gives the ability to give aspherised back and front surfaces; to give quadrant specific tucks or lifts on a lens and even the ability to specify changes within the level of microns.  Thus the practitioner has many more tools to help the patient with an irregular cornea before the need for surgical intervention.  And even post-surgery, these lenses can also give further benefit.

From this extends the argument of big vs small diameter RGP’s.  There is a growing trend for large diameter miniscleral lenses to be the main line of treatment for irregular corneas.  Is this the way of the future?  I will play the role of devil’s advocate and say… ‘sort of’!

For the vast majority of complex corneal lenses, the ability to vault over the irregularity provides a much seemingly simpler solution.  However, with these bigger lenses there perhaps needs to be some ‘bigger’ considerations to complications.  In trying to achieve clearance, there’s the balance of striking the correct level over the appropriate areas of the cornea.  For example, ensuring adequate clearance over a graft junction in a donor cornea.  Or ensuring appropriate limbal clearance is achieved.  And even striking the correct level of central corneal clearance to give appropriate oxygen transfer and vision.

With the larger size of the lens the peripheral fit is also critical with lens performance.  As mentioned, with the ability to make such small changes it falls to the experience of the contact lens practitioner to ensure the subtle adjustments are made correctly.   This applies for corneoscleral lenses that will bear on the peripheral cornea and consideration needs to be taken to ensure correct clearance over the limbal area.  For miniscleral lenses, the landing zone on the conjunctiva needs careful consideration.  And with the fully sealed system associated with miniscleral fits the amount of wearing time is typically less than that of a corneal lens.

Lastly, I will say the size of the lens can be quite daunting for some patients.  Handling sometimes needs to be inventive.  But with the large size this gives rise to far greater comfort compared to the corneal lenses.

For the advanced corneal irregularity, I think the miniscleral lens option is a definite advantage and will rise in usage over time.  However, for the mild to moderate condition I still feel that the small diameter corneal lenses will still have their place.  As said, the ability to specify many specific changes applies to your corneal lens as well.

To give reference to the words of the great Nathan Efron who predicted the demise of RGP lenses in the late 90’s, I don’t necessarily think corneal lenses will become extinct.  This is where I think fitting the irregular cornea becomes an art form and requires much skill and experience from the contact lens practitioner.  Thus for each patient that is in need of a custom fit there needs to be critical thought made by the practitioner to achieve the most optimum result.