Scleral Lens Correction for the Highly Aberrated Eye
It is well established that both lower and higher order aberrations are present in the human eye. Lower order aberrations include defocus and astigmatism, or sphero-cylindrical refractive errors that can be corrected by conventional spectacles, contact lenses or refractive surgery. Higher order aberrations including trefoil, coma, and quadrafoil refer to irregular astigmatism that cannot be remedied with conventional optical correction.
The first reference to wavefront was in 1961. Advancements in ocular wavefront sensing technology for more than two decades, such as Shack-Hartmann aberrometry, have helped to quantify the ocular aberrations more objectively and rapidly. These advancements have transformed the way we understand the optics of the eye and how it affects vision. Several publications have demonstrated that the visual performance of normal eyes with optimal conventional correction may still be limited by uncorrected higher order aberrations. Visual performance can be improved by correcting these aberrations, especially when the eye’s pupil is relatively large in dim light conditions. In highly aberrated eyes, such as those with keratoconus, the visual benefit of correcting higher order aberrations is even more significant.
As the ability to measure and quantify scleral contour and higher order optical aberrations expands, scleral lenses may become an ideal delivery platform for accurate and stable correction of higher order aberrations. In eyes with corneal irregularities, both corneal and scleral rigid gas permeable lenses can neutralize irregularities of the front surface of the cornea. However, aberrations from disparities in the corneal stroma or posterior corneal surfaces are often not addressed in lenses with standard optics. Scleral lenses are rotationally and translationally stable, and they are an ideal platform for the correction of higher order aberrations.
Several publications have evaluated the use of scleral lenses to achieve improved visual performance. Gemoules and Morris reported a reduction of 65% in higher order aberrations with corneoscleral lenses in patients with severe post-surgical irregularity.1 Sabesan et al. reported successful correction of higher order aberrations in 6 patients (11 eyes) with advanced keratoconus using a custom scleral lens with wavefront-guided optics. Mean visual acuity improved by an average of 1.9 lines with these lenses. Contrast sensitivity was also significantly improved. Although HOAs were reduced, vision was substantially worse than that of a normal eye.2 Gumus et al. reported a greater reduction in spherical aberration in lenses which incorporated front surface optical eccentricity (86%) compared to lenses without an eccentricity adjustment (66%).3 Marsack et al. reported the use of wavefront-guided scleral lenses in 7 subjects with moderate to severe keratoconus. An average gain of 1.5 lines of high-contrast visual acuity was reported, and higher order root mean square was reduced to within 1 SD or less than age-matched normals. However, visual acuity in keratoconic patients with wavefront-guided scleral lens correction remained lower than that of normal eyes.4
Additional research in this area will hopefully lead to additional scleral lens designs with higher order aberration correction.
1. Gemoules G, Morris KM. Rigid gas-permeable contact lenses and severe higher-order aberrations in postsurgical corneas. Eye Contact Lens 2007; 33(6 Pt 1): 304-7. [http://dx.doi.org/10.1097/ICL.0b013e318033edde] [PMID: 17993826]
2. Sabesan R, Johns L, Tomashevskaya O, et al. Wavefront-guided scleral lens prosthetic device for keratoconus. Optometry and vision science: official publication of the American Academy of Optometry 2013; 90(4): 314-23.
3. Gumus K, Gire A, Pflugfelder SC. The impact of the Boston ocular surface prosthesis on wavefront higher-order aberrations. Am J Ophthalmol 2011; 151(4): 682-90 e2. [http://dx.doi.org/10.1016/j.ajo.2010.10.027]
4. Marsack JD, Ravikumar A, Nguyen C, et al. Wavefront-guided scleral lens correction in keratoconus. Optometry and vision science: official publication of the American Academy of Optometry 2014.
Dr. Melissa Barnett is a Principal Optometrist at the UC Davis Eye Center in Sacramento, specializing in anterior segment disease and specialty contact lenses. Dr. Barnett is the Immediate Past President of The Scleral Lens Education Society. She lectures and publishes extensively on topics including dry eye, anterior segment disease, contact lenses and creating a healthy balance between work and home life for women in optometry.