Higher-order aberration correction with scleral lenses
When considering the best modality these lenses' stability are superior.
The first descriptions of scleral lenses for vision correction came independently from ophthalmologists Adolf Gaston Eugen Fick (German) and Eugene Kalt (French) in 1888.
By the early 2000s, high and hyper Dk (contact lens oxygen permeable) materials and newer designs allowed for customized fitting and advanced clinical applications.1 The designs have become commonplace in schools and colleges of optometry as well as at major university medical centers over the past decade, making scleral lenses increasingly common in the modern practice of optometry.1
Most patients who use contact lenses typically are treated by optometrists. Moreover, the awareness and understanding of the important role of scleral lenses for corneal ectasia and ocular surface disease is now better understood by both clinical optometrists and ophthalmologists.2
Historically, attention to ocular aberration in fitting scleral lenses was scarce. However, with advances in diagnostic and therapeutic methods over recent years, higher-order aberrations (HOAs) have sparked the interest of many eyecare professionals. HOAs are distortions of light passing through irregular refractive surfaces that cannot be corrected with spherical or cylindrical correction.3
There is a growing interest in corneal ectasia, particularly keratoconus, as the degradation of images (e.g., blurring, distortion, ghosting, reduced contrast sensitivity/acuity, visual fluctuation) can be highly attributed to HOAs, secondary to a significantly irregular corneal surface.4
A look at HOAs
Characteristic HOAs in keratoconus are most commonly an increase in vertical coma (reverse coma pattern with an inferior slow pattern), as well as trefoil, tetrafoil, and secondary astigmatism, but to a lesser extent.5
Measuring wavefront aberration analyzes the deviation of a wavefront originating from the optical system from the reference wavefront of an ideal optical system. The unit of aberration is microns—or fractions of wavelengths—expressed as the root mean square (RMS).5
The points spread function is a qualitative measurement of the spreading or blurring of points of light and the modulatory transfer function of an objective measurement of contrast sensitivity.6
Measurements utilizing an aberrometer are typically classified as outgoing wavefront aberrometer (Hartmann-Shack sensor), ingoing retinal imaging aberrometry (cross-cylinder aberrometer, Tscherning aberrometer, ray tracing method), or ingoing feedback aberrometer (such as optical path different method).
The set of Zernike polynomials analyzes the shape of HOAs, utilizing a combination of independent trigonometric functions that are caused by the wavefront aberrations orthogonality.5
While conventional spectacles correct for second-order aberrations—such as defocus and astigmatism—they leave the third and higher orders represented by irregular astigmatism uncorrected.
Although total HOAs can be significantly reduced with rigid gas permeable (RGP) lenses, they are still typically more prevalent compared with normal eyes with an RGP lens.5 Furthermore, RGP lenses reduce the aberrations induced by the anterior corneal surface, leaving the internal optics (posterior corneal surface and crystalline lens) unaccounted for.
It is important to note that anterior surface aberration may also be affected or skewed by the posterior corneal surface contribution. The posterior surface of the cornea can be analyzed using topographical data obtained with a slit-scanning topographer.9
It has also been shown that the anterior and posterior corneal surfaces in keratoconus show a mutually reverse pattern for trefoil, coma, tetrafoil, and secondary astigmatism. This Zernike vector reversal pattern may compensate for HOAs, secondary to the anterior corneal surface; but cannot be evaluated by simply taking the sum of the 2 surface patterns, as the incident waves of the posterior surface are deformed by the anterior corneal surface.10
Rigid lenses
Rigid lenses are known to provide improved optics, compared with soft lenses, due to their ability to mask corneal irregularities and provide an effective front optical surface. In addition to improved Snellen visual acuity, the application of rigid lenses has been shown to reduce the amount of higher-order aberrations, particularly in patients with diseased corneas.11 By correcting the aberrations induced from the irregular front surface, rigid lenses can provide patients with improved subjective vision and contrast sensitivity.
Although rigid lenses alone can be a great tool for patients with corneal ectasia, new advancements further enhance treatment of patients. Conditions such as keratoconus involve progressive thinning, which results in a significant change of the anterior and posterior aspects of the cornea.
When applying a regular rigid lens on a patient with keratoconus, there are often residual HOA that are not masked by the lens and that result from the irregular posterior cornea and the internal lenticular system.10 To correct these posterior/internal abnormalities, applied HOA correction is required.
When considering the best lens modality for HOA correction, scleral lenses are superior due to their stability. Unlike corneal RGPs, scleral lenses do not move on the eye, providing a stable surface to implement HOA correction.
It is very important that the practitioner first achieves a stable scleral lens fit prior to attempting aberration correction.
This will likely require haptic landing zone toricity to best align the edges with the sclera. Once properly aligned, an HOA measurement over the scleral lens is taken to capture the HOA data.
This information (typically coded as individual RMS values for each Zernike polynomial) is then sent to the laboratory to determine the patient’s specific HOA profile. The laboratory will then custom design a lens by applying equal and opposite aberrations onto the front surface of the lens, which will essentially cancel out the patient’s aberration profile, thus reducing visual disturbances.
Studies analyzing HOA correction of patients with keratoconus have shown a quantitative reduction in RMS values, including coma and spherical aberrations, as well as a qualitative improvement in patients’ visual function.10
Conclusion
HOA correction technology is a great tool for treating the patient who is still not fully seeing to their highest potential. When screening candidates, quantifiable aberrations should be present, and subjective visual improvement should be identified with pinhole testing.
Limitations to acquiring good quality scans—such as dense corneal opacities—may result in poor candidacy.
Overall, it is an exciting time, as increased technology allows clinicians to provide better care for their patients.
Although we are still in the early stages of implementation, HOA correction is likely to become a standard in scleral lens fitting, and we encourage all providers to utilize this effective technology.
References
1. Jedlicka J. Scleral lenses: past and present. Contact Lens Spectrum. Oct. 1, 2016. Accessed July 25, 2022. https://www.clspectrum.com/supplements/2016/october-2016/scleral-lenses-understanding-applications-and-max/scleral-lenses-past-and-present
2. Weiner G. Update on scleral lenses. American Academy of Ophthalmology. October 2018. Accessed July 25, 2022. https://www.aao.org/eyenet/article/update-on-scleral-lenses
3. Hashemi H, Khabazkhoob M, Jafarzadehpur E, et al. Higher order aberrations in a normal adult population. J Curr Ophthalmol. 2016;27(3-4):115-124. doi:10.1016/j.joco.2015.11.002
4. Kandel S, Chaudhary M, Mishra SK, et al. Evaluation of corneal topography, pachymetry and higher order aberrations for detecting subclinical keratoconus. Ophthalmic Physiol Opt. 2022;42(3):594-608. doi:10.1111/opo.12956
5. Maeda N. Clinical applications of wavefront aberrometry - a review. Clin Exp Ophthalmol. 2009;37(1):118-129. doi:10.1111/j.1442-9071.2009.02005.x
6. Pennos A, Ginis H, Arias A, Christaras D, Artal P. Performance of a differential contrast sensitivity method to measure intraocular scattering. Biomed Opt Express. 2017;8(3):1382-1389. doi:10.1364/BOE.8.001382
7. Zhao Y, Chen D, Savini G, et al. The precision and agreement of corneal thickness and keratometry measurements with SS-OCT versus Scheimpflug imaging. Eye Vis (Lond). 2020;7:32. doi:10.1186/s40662-020-00197-0
8. Nakagawa T, Maeda N, Kosaki R, et al. Higher-order aberrations due to the posterior corneal surface in patients with keratoconus. Invest Ophthalmol Vis Sci. 2009;50(6):2660-2665. doi:10.1167/iovs.08-2754
9. Atakan TG, Kurna SA, Bozkurt YB, Şahin T, Atakan M. The effect of rigid gas permeable contact lenses on visual quality and corneal aberrations in the keratoconus patients. Bosphorus Med J. 2021;8(1):21-28.
10. Marsack JD, Ravikumar A, Nguyen C, et al. Wavefront-guided scleral lens correction in keratoconus. Optom Vis Sci. 2014;91(10):1221-1230. doi:10.1097/OPX.0000000000000275
