Depending on when you completed your formal education in eye care, your exposure to the topic of optical aberrations in visual, adaptive, or physiologic optics may be limited. And, as the old adage goes: If you don’t use it, you’ll lose it! Nevertheless, incorporating wavefront correction into specialty contact lenses—into scleral lenses in particular—continues to spark exciting discussions in industry publications and contact lens forums on social media.1-3
We believe that wavefront correction has the potential to be the “next big thing” in specialty contact lenses for patients who have highly aberrated eyes. But, where does the rubber meet the road for practitioners who are not working in research laboratories? This article explores practical applications of this evolving science.
IMPACT OF HIGHER-ORDER ABERRATIONS
We all have met patients who report seemingly unexplainable blur or night-vision disturbances—or who are simply not happy with their vision—despite a well-fitting contact lens or an accurate spectacle refraction. No eye is optically perfect in its ability to refract a propagating wavefront of light onto the retina. The sphero-cylindrical components of conventional contact lenses and spectacles correct the most common sources of defocus (myopia, hyperopia, and astigmatism) or lower-order aberrations on the retina. Higher-order aberrations (HOAs) represent the resultant imperfections that cannot be corrected with conventional optical aids. Some types of HOAs are associated with certain eye diseases and conditions4 (see sidebar below).
Unique HOA wavefronts can be described with mathematical functions called Zernike polynomials.5 The wavefront is the summation of individual Zernike terms and coefficients and is commonly measured clinically by Shack-Hartmann wavefront sensors. This technology uses a lenslet array that forms a focal spot pattern containing wavefront information on a charge-coupled device (CCD) camera.6 When the spot patterns deviate from a reference point, the resulting slopes can be reconstructed into wavefront data using specialized software.
Certain eye diseases are associated with characteristic aberration patterns that will affect patients’ vision. For example:
- Coma and secondary astigmatism are common in keratoconic eyes.
- Exaggerated amounts of trefoil and spherical aberration are observed in eyes that have undergone corneal transplantation or radial keratotomy.
- Positive spherical aberration can be observed in eyes that have a history of laser refractive surgery.
INSIGHTS ON ADAPTIVE OPTICS
As understanding of HOAs has advanced through wavefront imaging, so have applications to benefit patients using laser refractive surgery and adaptive optics (wavefront spectacles and contact lenses). The surface of a wavefront-guided contact lens must be the inverse of a patient’s measured wavefront error, and lens characteristics such as refractive index will further scale the magnitude of the wavefront to ensure propagation of a perfect wavefront of light to the retina. A laboratory that has a computer-numerical-control (CNC) lathe can then manufacture the desired wavefront-guided lens based on this information.3,7-10
Each wavefront-correction modality has specific advantages and limitations. For wavefront correction to provide meaningful vision improvement, precise alignment of the correcting lens and the visual axis must be maintained or vision will be degraded—similar to axis misalignment of a toric contact lens.
While laser refractive surgery may appear to be the obvious best choice, treatments such as laser-assisted in situ keratomileusis (LASIK) are contraindicated for highly aberrated eyes resulting from ocular diseases such as keratoconus. These eyes may benefit from wavefront correction in spectacles or contact lenses. Spectacles do not move with visual fixation, and the position of a soft contact lens will change based on the dynamics of globe movement and blink, resulting in inconsistent vision. Scleral lenses may provide more stable vision with limited movement as compared to soft contact lenses and spectacles.
Some practitioners and laboratories may use corneal topographical data for what they call wavefront correction in contact lenses, but others do not consider this to be wavefront correction. Metrics such as asphericity and eccentricity modifications, while useful in improving vision quality, do not take into account all HOAs of the entire visual system measured by wavefront aberrometers, and they may not always produce optimal outcomes. Research has shown that, in some cases, correcting the anterior cornea, the posterior cornea, and the internal optics together produces better results compared to correcting each individual component.11,12 When vision is corrected with a conventional scleral or corneal GP lens, most aberrations resulting from the anterior cornea are compensated through the refractive index difference between the cornea and the fluid reservoir created under the lens. Nevertheless, residual aberrations may remain, suggesting that additional aberrations arise from the posterior cornea and intraocular components.11,12 The anterior cornea is only one component of the entire visual system, which also includes the back surface of the cornea, the aqueous and vitreous humors, and the crystalline lens.
True wavefront correction of the visual system is achieved only by adding the front and back surfaces of a contact lens along with a fluid reservoir (if applicable to the modality used) to the aforementioned ocular media. Light must refract through this system undeterred to provide an optimal image on the retina.
Some topographers and similar devices may provide eccentricity and wavefront data for the anterior surface of the cornea to aid practitioners and laboratories in correcting aberrations that affect vision, specifically spherical aberration. This is a function of the natural eccentricity of a cornea in relation to the spherical back surface of a lens. Some laboratories may offer algorithms for lathes to control for scleral lens-induced aberrations based on parameters and fitting characteristics. Other parameters that contribute to patient-perceived aberrations are angle alpha, which is the separation of the visual axis from the optical center of the cornea, and angle kappa, which is the separation between the visual axis and the center of the pupil. Aberrometers and Placido-based topographers, used over the natural eye or while the patient is wearing a scleral lens, can help practitioners identify these metrics. Large variations in these optical misalignments may induce coma and trefoil, which patients may describe as blur, prompting seemingly ever-increasing cylinder adjustments in over-refraction that produce no meaningful improvements. Certain lens designs can correct for this variation by independently adjusting the back surface of the lens to best fit the ocular surface and the front optic zone for visual axis alignment.
If you have a topographer or aberrometer in your office, you may be asking how these devices aid in designing and offering wavefront correction in scleral lenses. Aberrometers that measure the entire visual system are of no value to vision improvement if they cannot be applied to contact lens manufacturing. Contact lens information regarding HOA spots and reference-axis/off-axis locations, combined with lens orientation and tilt, make sharing this information with laboratories time- and labor-intensive. A practitioner must verify compatibility of the device information used with the laboratory, obtain accurate data, and coordinate manufacturing; otherwise, visual quality for a patient may be even worse than with conventional optics.9
Research reports and ongoing testing by contact lens manufacturers have shown that fabricating wavefront scleral lenses is possible and complex.7 The process entails measuring and obtaining wavefront data between trial lenses and spherical equivalent lenses to produce final wavefront-guided scleral lenses correcting second- to fifth-order Zernike polynomials. Empirically applying aberrometry data to a contact lens manufacturer’s lens design without customizing the optics based on the lens fit and ocular condition may not produce an acceptable final lens design.
CANDIDATES FOR WAVEFRONT CORRECTION
Even an emmetropic human eye has imperfections that cause aberrations to some degree; wavefront correction can be an option for patients seeking enhanced vision every day or for specific sports or hobby-related activities. Pathologies, such as keratoconus and other forms of corneal ectasia; postsurgical eyes, including those that have undergone penetrating keratoplasty or refractive surgery; ocular trauma; or high ametropia can leave some patients with poor visual quality of life, even with well-fitting specialty contact lenses. Patient selection is of paramount importance to ensure efficient use of your chair time and to instill realistic expectations for outcomes.
The Catch-22 when managing conditions that may cause high aberrations is the risk of associated sequelae, such as ocular surface disease, corneal scarring, early cataracts, improperly positioned intraocular lenses, or other ocular abnormalities, that can complicate an otherwise straightforward wavefront-guided scleral lens fitting. Research on wavefront-guided scleral lenses often excludes such pathology from the inclusion criteria, which makes it difficult to know how wavefront correction may or may not perform in these difficult cases.3,7 Without question, media scattering by a poor tear film, contact lens wetting issues, corneal scarring, and cataracts will induce haze that cannot be described mathematically or compensated through wavefront analysis; however, technologies continue to emerge to provide wavefront correction in scleral lens designs. Clinicians can add to our understanding of these technologies by sharing case reports of patients who have complicated sequelae and by detailing their attempts to push the limits and get creative for patients who have no other options. With difficult cases, you’ll never know whether something works unless you try.
As scleral lenses emerge as an effective modality for diseased eyes, our understanding of scleral shape becomes even more important when fitting these lenses for wavefront correction. A poorly fitting scleral lens that does not have wavefront correction may induce aberrations if lens characteristics such as refractive index, rotation or tilt, and centration are not considered and corrected. To help make fitting more precise, many laboratories incorporate additional modifiable parameters in the lens midperiphery and haptics in specific quadrants. Ocular surface mapping and impression-based lens designs can help improve fitting accuracy, especially in cases of difficult topography arising from pathological conditions such as pterygia, tube shunts, and blebs.
Pupil size is another important component to consider for the final wavefront design, as a patient’s pupil diameter will vary during wearing time. The dynamic nature of lens wear combined with centration, rotation, and pupil size will make or break a patient’s visual experience.
Finally, research on wavefront correction for patients who have experienced chronic blur related to their underlying conditions reveals that they may struggle to achieve visual acuity similar to that of an otherwise normal eye, even when their aberrations are significantly reduced with wavefront-corrected optics; this phenomenon may be related to reduced sensitivity to fine spatial detail over time, analogous to meridional amblyopia in high astigmatism, thereby limiting visual performance.9 It is possible that neural plasticity could enable the visual system to re-adapt to the new optics through natural daily experience or through vision training to improve visual quality of life over time.13 During lens habituation, practitioners may need to educate these patients similarly to those who are neuro-adapting to multifocal lenses for the first time.
A 29-year-old woman who has keratoconus wears scleral contact lenses as her habitual correction. She recently failed an occupational vision screening for her work duties in medical manufacturing. The patient reported ongoing suboptimal vision in the right eye despite a well-fitting lens. Over time, visual acuity in the right eye has decreased from 20/30 to 20/70. This reduction seems to be exacerbated by an oval cone observed directly over the visual axis on topography (Figure 1). The cornea has no striae or scarring, and no other media opacities are present. Aberrometry performed over the habitual scleral lens reveals 1.61 microns of higher-order root mean square (HO RMS). Wavefront data based on a stable, well-fitting scleral lens was sent to the laboratory for lens fabrication.
After dispensing and lens habituation, visual acuity improved to 20/25 in the right eye and remained stable between visits. The HO RMS was reduced to 0.73 microns, with a large reduction in coma (Z7 and Z8), which is common in keratoconic eyes (Figures 2, 3, and 4). The patient reported improved overall visual quality of life.
Wavefront-guided scleral lenses are transitioning from theory and research to a real-world option for practitioners to incorporate into their arsenal of specialty contact lenses. Over time, as laboratories and developers continue to improve wavefront technology in tandem with enhancements to scleral lens fitting characteristics, patients who have highly aberrated visual conditions will have access to powerful solutions for meaningful improvements in their quality of life. CLS
Acknowledgements: Troy Miller, AccuLens; Michael Johnson, Art Optical; Christine Sindt, OD, EyePrint Prosthetics; Felix Kim, Ovitz Corporation; Nick Brown, Ovitz Corporation; George Mera, TruForm Optics; CharlieRae Edmunds, Valley Contax.
- Barnett M. Scleral Lens Correction for the Highly Aberrated Eye. Scleral Lens Monthly. 2018 Jan. Available at https://www.clspectrum.com/newsletters/scleral-lens-monthly/january-2018 . Accessed June 15, 2021.
- Marsack JD. Incorporating Wavefront Error Correction in Contact Lenses. Contact Lens Spectrum. 2012 Sep;27:26-29. Available at https://www.clspectrum.com/issues/2012/september-2012/incorporating-wavefront-error-correction-in-contac . Accessed June 15, 2021.
- Hastings GD, Applegate RA, Nguyen LC, Kauffman MJ, Hemmati RT, Marsack JD. Comparison of Wavefront-guided and Best Conventional Scleral Lenses after Habituation in Eyes with Corneal Ectasia. Optom Vis Sci. 2019 Apr;96:238-247.
- Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor. Ophthalmology. 2007 Nov;114:2013-2021.
- Thibos LN, Applegate RA, Schwiegerling JT, Webb R; VSIA Standards Taskforce Members. Standards for reporting the optical aberrations of eyes. J Refract Surg. 2002 Sep-Oct;18:S652-S660.
- Liang J, Grimm B, Goelz S, Bille JF. Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor. J Opt Soc Am A Opt Image Sci Vis. 1994 Jul;11:1949-1957.
- Marsack JD, Ravikumar A, Nguyen C, et al. Wavefront-guided scleral lens correction in keratoconus. Optom Vis Sci. 2014 Oct;91:1221-1230.
- Sabesan R, Jeong TM, Carvalho L, Cox IG, Williams DR, Yoon G. Vision improvement by correcting higher-order aberrations with customized soft contact lenses in keratoconic eyes. Opt Lett. 2007 Apr;32:1000-1002.
- Sabesan R, Johns L, Tomashevskaya O, Jacobs DS, Rosenthal P, Yoon G. Wavefront-guided scleral lens prosthetic device for keratoconus. Optom Vis Sci. 2013 Apr;90:314-323.
- Yoon G, Jeong TM, Cox IG, Williams DR. Vision improvement by correcting higher-order aberrations with phase plates in normal eyes. J Refract Surg. 2004 Sep-Oct;20:S523-S527.
- Artal P, Guirao A, Berrio E, Williams DR. Compensation of corneal aberrations by the internal optics in the human eye. J Vis. 2001 Jan;1:1-8.
- Chen M, Yoon G. Posterior corneal aberrations and their compensation effects on anterior corneal aberrations in keratoconic eyes. Invest Ophthalmol Vis Sci. 2008 Dec;49:5645-5652.
- Sabesan R, Barbot A, Yoon G. Enhanced neural function in highly aberrated eyes following perceptual learning with adaptive optics. Vision Res. 2017 Mar;132:78-84.