Article

CONTACT LENS DESIGN & MATERIALS

SELECTING GP CONTACT LENSES

Over the years, technological innovations have spurred new GP contact lens designs (Segal et al, 2003). Corneal GP lenses were the mainstay for decades, though the relatively recent advent of better contact lens materials has paved the way for large-diameter GP lenses (scleral lenses), which fully vault the cornea and land on the sclera (Segal et al, 2003). In fact, scleral lenses have become so popular among specialty contact lens fitters that some practitioners rarely consider fitting corneal GP lenses.

While the use of corneal GPs appears to be fading (Gill et al, 2010), they still have a place in modern eyecare practice alongside of their much larger counterparts.

Classification and Indications

Selecting a GP contact lens starts with understanding how GP lenses are defined. The Scleral Lens Education Society (SLS, 2013) has proposed the following classification guidelines for GP contact lenses based primarily on where the lens rests on the ocular surface.

Corneal GP Lenses A corneal GP lens rests entirely on the cornea (SLS, 2013). While GP lenses are historically indicated for both normal and diseased eyes, corneal GPs now tend to be fit more on patients who have normal corneas or who have mild corneal irregularities (van der Worp, 2015).

The major advantages of corneal GP lenses include improved vision compared to soft contact lenses, good tear exchange, high breathability, and often being the most economical contact lens material option (Gill et al, 2010). While initial comfort is frequently an issue for corneal GP lens wearers, the use of topical anesthetic can improve initial comfort and reduce anxiety, and the majority of patients fully adapt to corneal GP lenses (Gill et al, 2017).

Corneo-Scleral Lenses A corneo-scleral lens rests partly on the cornea and partly on the sclera (SLS, 2013). These lenses tend to be fit on normal corneas in situations in which corneal GP lenses did not fit well (Segal et al, 2003; van der Worp, 2015).

Corneo-scleral contact lenses are typically easier to handle and are less likely to get air bubbles compared to scleral contact lenses; they also move more compared to scleral lenses while at the same time avoiding scleral toricity, which can be an issue with larger contact lenses (van der Worp, 2015). Nevertheless, corneo-scleral contact lenses tend to have the most limbus interaction, which increases the risk of limbal stem cell damage (Fadel, 2017).

Scleral Lenses A mini-scleral contact lens rests entirely on the sclera, with the lens extending 6mm or less beyond the horizontal visible iris diameter, whereas a large-scleral contact lens rests entirely on the sclera, with the lens extending 6mm or more beyond the horizontal visible iris diameter (SLS, 2013). Because scleral lenses have the ability to fully vault the cornea, they tend to be indicated for highly irregular corneas and for patients who need ocular protection (Segal et al, 2003).

Compared to large-scleral lenses, mini-scleral contact lenses tend to be thinner and to have lower corneal clearance, which promotes better oxygen transmission, better acuity, and fewer bubbles (Fadel, 2017). Large-scleral lenses provide greater vault when needed to clear corneal irregularities. This may better distribute the weight of the contact lens over the sclera and provide greater coverage, which may promote less eyelid interaction and better ocular comfort; these same conditions also unfortunately decrease oxygen transmission to the cornea (Fadel, 2017).

Conclusion

The literature suggests that a small GP contact lens should be selected for relatively normal patients and that successively larger contact lenses should be selected for patients who have greater degrees of corneal irregularity/ocular surface damage (Fadel, 2017). Thus, corneal GP contact lenses still have a place in our practices, and they may even be the best option for some of our patients. CLS

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