Scleral lenses have become a popular go-to among contact lens fitters to maximize visual potential in patients who suffer from corneal irregularities. We diligently inspect the fit to ensure that appropriate vaulting over the cornea is achieved during the fitting process.
Hybrid lenses, which are also a good option for abnormal corneas, require the same level of inspection by fitters; however, vaulting considerations for these applications are slightly different.
Hybrid Lens Vault Characteristics
- The lenses are not designed to vault over the entire cornea. Currently, the only hybrid lenses that have U.S. Food and Drug Administration clearance to manage irregular corneas have an 8.5mm wide GP center. We know that the average horizontal visible iris diameter (HVID) of the human eye hovers around 11.5mm. Naturally, the GP portion will have landing or weight-bearing zones on the cornea, while the soft skirt will rest on the both the cornea and sclera. The goal is to have the GP portion of the lens vault over the entire corneal irregularity during the fitting visit (Figure 1).
- Hybrid lenses do settle over time. A study carried out on the settling behavior of hybrid lenses fit on keratoconic eyes revealed that hybrid lenses settled approximately 50% during the first hour of wear (Montani, 2017). This is similar to what has been reported in the literature for scleral contact lenses. However, scleral lenses settle mainly onto the bulbar conjunctiva. In my experience, I have found that a 0.5:1 ratio (or slightly greater) of tear clearance over the corneal apex to hybrid GP lens thickness is a good rule of thumb to strive for during the initial hybrid contact lens fit (Figure 2).
- Excessive vault often leads to a “crash” of the soft skirt. Figure 3 shows that a higher vault of a hybrid contact lens can result in reduced clearance under the lens. If you are unsure whether you have too much clearance, check the fit again after 30 to 60 minutes before finalizing an order. This action can save you a refit during a future office visit. CLS
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