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.

#2 Optimize and reduce minus power, especially for presbyopic patients

Many prolate scleral lenses fitted on high ectasia, on irregular corneas, with a steep base curve, or to compensate for a  regular high myopia, result in high minus power. Often vision is compromised in quality with these high minus power lenses, especially in cases of mild lens decentration.  Because the base curve of an oblate scleral lens is made flatter, it generates relatively more minus power correction compared to a prolate lens, which could also be a benefit.

It is of prime importance when correcting high minus presbyopic patient. There is an immediate wow effect generated with the use of a -8D add +2.00 instead of a -14D with the same add power, while still correcting the same refractive error. 

#3 Optimize limbal clearance with smaller sclerals

When smaller scleral lenses are used, one of the main concerns is to keep an adequate clearance over the limbal area during all the wearing hours. Some designs may be adapted to increase the sagittal height under the second and the third curves of the lenses (extra limbal clearance). Another option is to increase the overall vault over the ocular surface, with the consequence of exceeding the optimal central clearance. It may be then interesting to consider the use of an oblate profile scleral lens to limit this adverse effect.

CONCLUSION

Oblate lenses can be used on prolate corneas to enhance and optimize optical and physiological outcomes. A few trial sets include oblate designs, which is helpful, but consultation with the lab or the use of calculators on their website may help to prescribe better lenses for our patients.