Correcting Common Aberrations in Highly Aberrated Eyes
By Jason Marsack, PhD
The September 2012 article “Incorporating Wavefront Error Correction in Contact Lenses” described research into wavefront-guided (WFG) corrections for keratoconic eyes. WFG corrections are customized to correct the low- and high-order aberrations of a specific patient (Marsack, 2012). That article on WFG corrections summarized current work on a concept that has existed for decades.
Although the concept behind customization of contact lenses has now existed for at least 50 years (Lopez-Gill, 2002) and the technology exists to prescribe, design, manufacture, and deliver such corrections, they are still not widely available to the patients who could most benefit from them. WFG customization is challenging, and while the potential benefits for both patients and practitioners are considerable, it is clear that barriers exist to the widespread deployment of WFG corrections by clinical practitioners.
Current Clinical Limitations
Most general eyecare practices do not have access to the tools required to prescribe fully custom WFG corrections as they are commonly envisioned. More to the point, the majority of practitioners would not otherwise have a strong reason to obtain these tools, which do not provide robust utility in other aspects of their practice. At a minimum, these tools include a wavefront sensor capable of measuring the low- and high-order aberration (HOA) of the eye, as well as an apparatus to measure on-eye lens rotation and translation (critical for the correct positioning of the wavefront-correcting patch in front of the pupil).
Further, prescribing WFG contact lenses is time-consuming. Given currently available correction strategies and the cost in time and money associated with delivering a WFG correction, it would not surprise me if the successful delivery of fully custom WFG corrections is eventually concentrated in centers that have the requisite technology.
From the Lab to the Clinic
I believe that a parallel approach to WFG customization may offer a complementary method for clinicians to target HOA in highly aberrated eyes (in this article, we will focus on the keratoconus population). This approach would merge our evolving understanding of wavefront error with the well-established method for delivering soft contact lens corrections in today’s practice.
As an example, when fitting a typical soft contact lens to a myopic eye, the practitioner quantifies the level of ametropia and then typically chooses a pre-packaged contact lens of a specific power. These corrections come in discrete refractive correction steps and cover a wide range of myopic errors. But whether the correction is –10.50DS or –2.75DS, the optical error being corrected is the same: defocus. The difference in the correction between lenses depends on the magnitude of the defocus error being corrected. Not stated, but implicit in the optical design, is that leaving a myope’s cylinder and HOA uncorrected does not result in severe limitations in visual performance. So, the lenses target the common aberration present in the myopic population (defocus error) and provide clinicians with discrete correction levels for correcting that error. The key to the success of this strategy is the common ametropic error in the myopic population: defocus.
Characteristics of keratoconus include steepening of the cornea and an inferior displacement of the corneal apex. These changes in corneal morphology inevitably lead to the induction of HOA. While there is variability in the population, the ‘cone’ tends to decenter inferiorly, which typically induces negative vertical coma. Vertical coma comprises 53 percent ± 32 percent of the variance HOA and is the most dominant HOA in keratoconus (Pantanelli et al, 2007). That is to say, coma is a common refractive error in keratoconus.
If the induction of vertical coma is a common optical defect resulting from keratoconus, this could be capitalized on when designing a common correction much in the same way that defocus is corrected in myopia. Said differently, a correction could compensate for the common HOA present in the disease.
How well would corrections perform if they were designed to correct these common aberrations? Several basic, lab-based studies have reported results applicable to this potential strategy.
Correcting Common Aberrations in Keratoconus
Katsoulos et al (2009) studied correction of coma in a soft contact lens for keratoconus subjects. This study targeted patient-specific levels of vertical coma integrated with spherocylinder corrections. While the level of aberration correction was tailored to each individual eye, it did none-the-less demonstrate correction of the dominant HOA term in keratoconus, leaving the others HOAs untargeted in the optics of the lens. The study demonstrated improved visual performance of eyes affected with mild or moderate keratoconus.
Marsack et al (2013) conducted a study that defined several subcategories of aberration structure in keratoconus (referred to as templates) and mathematically corrected individual aberration structures by scaling the HOA template. This study demonstrated that the level of high-order RMS error was reduced, and in 50 percent of cases was comparable to that achieved when correcting keratoconus with GP lenses. The study did not consider low-order aberrations.
While the methods of these studies vary, they share the common theme of correcting HOA in keratoconus with a single, scalable HOA correction. As in defocus for myopia, this would permit corrections that more closely resemble those of pre-packaged contact lenses.
Such a method would have unavoidable limitations. Incomplete correction of the existing HOA would occur in every single patient because the common correction is not designed to fully correct any given individual. Misalignment of the patch with respect to the pupil would also occur, as the optics are not specifically decentered in the optical design on a patient-by-patient basis. These two factors would limit the optical performance and potential acceptability of a common correction strategy as well as increase the number of parameter combinations that would be required for clinical success. When considering the combination of sphere, cylinder, and axis to a scaled set of common HOA patterns, the number of possible combinations increases rapidly.
Further, this article has not provided any insight into how clinicians would prescribe such a lens (in particular, choosing the aberration template category or the magnitude of the aberration correction). Finally, for the technique to be successful, the underlying condition must result in an aberration structure that is common to the population of interest.
Potential Benefits of Common HOA Correction
A technique focused on common HOA structures would have several benefits for both patients and clinicians. For patients, it would provide yet another correction strategy to meet their visual needs. It seems that this mode of correction is very applicable to delivery in the form of a soft lens.
For clinicians, this technique would provide the potential to correct highly aberrated eyes using a derivative of WFG technology within an off-the-shelf contact lens prescribing paradigm already familiar to them. It is clear that clinicians and patients want the option to treat appropriate irregular corneas with soft contact lenses, as evidenced by the successful clinical deployment of at least 12 soft lenses now available for managing keratoconus (Tyler’s Quarterly, March 2013).
Current contact lens practice allows correction of individual patient ametropia with lenses designed to target refractive error common to a group. The concept described here simply proposes to extend this idea to the common aberration pattern present in highly aberrated eyes. Further, it proposes to add to the correction options intended to reduce the impact of HOA in highly aberrated eyes, such as those that have keratoconus. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #213.
Dr. Marsack completed a PhD in Physiological Optics and Vision Science at The University of Houston, College of Optometry. His research interests include optical aberration of the eye, custom and pseudo-custom correction of optical aberration, visual performance, metrics predictive of visual performance, and ocular drug delivery.