While intuitively it may appear logical to select an oblate scleral lens profile to be fitted on an oblate-shaped ocular surface, other clinical indications may optimize the scleral lens fitting with the use of an oblate design.
#1 – Vaulting over high ectasia
To vault over these very steep surfaces, it can be a challenge to adapt a scleral lens when the ectasia is located very low on the cornea or very steep in height. It implies that another area of the cornea, habitually at the opposite, will be overvaulted if a conventional spherical design is used. Overvault means that the clearance over the superior area of the cornea will exceed 300 um, sometimes reaching 400 to 500 um. Not only this has a significant impact on oxygen delivery to the cornea, but the visual acuity may also be altered when such high clearance is kept over the pupil area. The tear film profile will also be prismatic in shape, generating residual astigmatism. Finally, a lens fitted with too much fluid under it is less stable and tends to decenter.
One of the ways to deal with such a situation is to use an oblate design. In this case, the central base curve is flatter than its periphery. This modification on the back surface of the scleral lens helps to reduce the clearance over the central part of the cornea while keeping an optimal vault over the ectasia. The lens is more stable, visual acuity is improved, and oxygen delivery is less penalized.
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