Contact Lenses and Wavefront Aberrometry
Contact Lenses and Wavefront Aberrometry
A review of current instrumentation and contact lens options designed for the pursuit of superior vision.
By Kenneth A. Lebow, OD, FAAO
Dr. Lebow is in private practice in Virginia Beach, VA specializing in the fitting of advanced contact lens designs, primary care and cataract co-management. He has served as past President of the Virginia Optometric Association and was a charter member and past Chair of the American Optometric Association Contact Lens Section. Formerly, he held the rank of an Associate Professor of Optometry, Clinical Research Consultant, for the New England College of Optometry, Boston, MA as well as serving as faculty at Vistakon's The Vision Care Institute.
Wavefront technology has dramatically changed the way we look at refractive analysis and contact lens fitting. Our original refractive concerns were limited to simply computing the spherical, cylindrical and axis requirements of a patient's prescription. While these components still remain critical elements of the eye examination, they are now designated as defocus (either myopic or hyperopic) or tilt (astigmatism) and together are referred to as lower-order aberrations (LOAs). LOAs represent the majority of the wavefront error in the human eye.
Of particular interest with wavefront analysis are new terms that describe various aberrations of the visual system such as spherical aberration, coma, trefoil, quadrafoil and others that collectively describe higher-order aberrations (HOAs).
The obvious goal of wavefront technology is to correct both LOAs and HOAs in an attempt to achieve an ideal correction that produces ‘super vision.’ To this end, wavefront-guided refractive surgery, aberration-correcting intraocular lenses, adaptive spectacle lens optics and aberration-controlling contact lenses all represent attempts to provide patients with higher quality vision.
Various devices have been developed to measure HOAs. One of the more common methods of aberration measurement uses Shack-Hartmann wavefront technology, which was originally designed for astronomical studies and was later applied to measuring ocular aberrations. Other types of aberration measurement such as ray tracing, scanning slit refractometry, dynamic skiometry and other more subjective methods have improved our ability to measure, detect and understand the influence and effect that HOAs have on the human ocular system. Following are several types of aberrometers and their description technology:
• Shack-Hartmann Aberrometer – reflection aberrometry in which light exiting the eye is collected at the entrance pupil and the wavefront error is reconstructed after passing through a lenslet array. This may be of limited value in patients who have abnormally high magnitudes of aberrations because of distorted patterns and the large displacement between the spot image and the corresponding lenslet axis.
• Tscherning Aberrometer – ray tracing device that measures the wavefront aberration in the image plane and uses retinal imaging to photograph the patient's actual retinal image, incorporating the visual distortion to facilitate diagnosis of conditions. The Tracey Aberrometer (Tracey Technologies, Corp.) is another example of a ray tracing device.
• Nidek OPD-Scan Aberrometer – this device uses retinoscopic double pass aberrometry and differs from other aberrometers in that it measures the time that light takes to reach photo detectors after reflecting off of the retina and passing through the optical system. Other methods attempt to determine the position of points passing through the optical system and are projected onto a lenslet array (or a disc of tiny lenses). By using a time-based analysis (versus a position-based analysis), this instrument has a broader dynamic range of measurement, higher resolution and increased accuracy. Clinically this is similar to dynamic retinoscopy.
• Emory Vision InterWave Aberrometer – spatially resolved refractometry based on the Scheiner principle, which allows patients to interact during the testing process.
Aberration Descriptions and Definitions
The shape of a wavefront is typically described by a complex series of mathematical functions called Zernike polynomials (Figure 1). These polynomials have varying radial orders and meridional frequencies and can be demonstrated as different wavefront error maps. Another way to describe the wavefront shape uses a single number to describe the amount the wavefront deviates from a plane wave. The amount of the deviation is called the root mean square (RMS) error. Mathematically it represents the standard deviation of the wavefront from a plane wave and is often used to describe the overall optical quality of the eye. As the RMS approaches zero, a perfect optical system would exist.
Figure 1. Zernike polynomials.
Two other important measurements used in wavefront aberrometry are modulation transfer function (MTF) and point spread function (PSF), which both indicate the overall quality of the retinal image. The MTF describes the ratio of image contrast to object contrast as a function of the spatial frequency of a sinusoidal grating across a patient's range of visual performance. It can also enable individualized comparisons to patients who demonstrate higher quality vision. The PSF defines how a single object is imaged by the optical system and provides a means to display how various aberrations affect a point of light, and therefore demonstrates the fundamental quality of a patient's vision. Although both Zernike polynomials and RMS are effective mathematical systems to describe optical aberrations, other factors such as perception, retinal image quality and human subjective interpretation of sight influence the quantity and quality of what a patient actually sees.
When evaluating routine refractive errors with aberrometry, uncorrected LOAs typically mask the presence of HOAs, which are relatively small in magnitude. An important point to remember when you apply a contact lens to the eye is that as the lens already either partially or completely corrects LOAs, the HOAs become more readily apparent. Usually this effect is limited to just myopic or hyperopic defocus depending upon the amount of residual astigmatism that remains. Certainly, uncorrected astigmatism through a contact lens would appear as a residual LOA in a wavefront error map. Therefore, taking wavefront measurements through a contact lens on the eye provides an excellent method to evaluate HOAs of the eye-contact lens optical system.
Spherical aberration is a rotationally symmetrical aberration that typically contributes considerably to higher-order wavefront errors and, depending upon lens power, can induce either positive or negative aberration. Positive spherical aberration occurs when the peripheral rays of light are refracted more than the central rays (plus lenses) while negative spherical aberration describes when peripheral rays are refracted less (either less positive or more negative) than the central rays (minus lenses). We know, for example, that the cornea induces approximately +0.27 units of spherical aberration to the wavefront error, which is fairly constant throughout life unless any refractive surgical interventions occur. Other factors which influence spherical aberration include pupil size (larger pupils increase spherical aberration), accommodation (spherical aberration becomes negative with active accommodation) and the application of different contact lenses (induces negative spherical aberration).
Clinically, with contact lenses and spectacles alike we need to differentiate between a ‘generalized’ and an ‘individualized’ aberration correction. In addition to correcting patients' ametropia, one goal in designing contact lenses is to adjust the curvature of the lens using aspheric optics to minimize ocular aberrations. If a contact lens applied to an eye induces –0.27 units of spherical aberration, the net result would eliminate spherical aberration as a component of the refraction and subsequently improve vision. Different hydrogel lens manufacturers using mass-produced contact lenses have incorporated this generalized approach to reduce spherical aberration with varying degrees of success. For example, PureVision (Bausch & Lomb) aberration-controlled contact lenses incorporate aspheric optics in an attempt to compensate for these inherent corneal aberrations.
One study reports that the effects of fitting contact lenses on HOAs are similar to what often happens after LASIK — namely, decreased total aberrations but an increased percentage of HOAs. Generally, aberration-controlled contact lenses overcorrect spherical aberration, but wavefront aberrations in the eyes with different soft contact lenses vary from one individual lens type to another.
For example, the Night & Day lens (CIBA Vision) vehicle harbors positive spherical aberration and coma, independently of the lens power. The negative power of contact lenses induces negative spherical aberration which, at large values, compensates for the lens vehicle positive spherical aberration to produce a net negative spherical aberration.
Comparing Aberration-Control to Non-Aberration-Control Lens Designs
Clinically, contact lens manufacturers and practitioners claim that aspheric lens designs mask residual astigmatism. It's helpful to use wavefront technology to evaluate the effectiveness of this approach and the extent to which aspheric designs correct astigmatism. Clinically, the question is whether to use aspheric or toric soft lens geometries to correct small amounts of astigmatism. Kollbaum and Bradley (2005) concluded that soft aspheric contact lenses generally fail to mask astigmatism via an optical correction and that a toric contact lens proved to be more effective in correcting astigmatism. Possibly the acuity improvement reported when using aspheric lenses occurs from a reduction in spherical aberration. However, one aspheric lens, the Biomedics Premier (CooperVision), appeared to effectively correct for the spherical aberration that the lens power induces and the spherical aberration of the average human eye.
Efron et al (2008) report as it pertains to the brand of lenses tested, the fitting of aspheric design soft contact lenses does not result in superior visual acuity, aberration control or subjective appreciation compared with equivalent spherical design soft contact lenses. To demonstrate this point, I fitted one patient with a variety of spherical and aspheric contact lenses. Each lens had identical spherical power and, after settling for 15 minutes, a wavefront scan was performed. Figure 2 compares an Acuvue Oasys (Vistakon) spherical contact lens with no aberration control to a Frequency 55 Aspheric (CooperVision) aberration-controlled contact lens. In this situation, visual acuity, PSF and MTF (change from 0.344 to 0.803) all indicate that the aspheric aberration-controlled design actually reduces vision. Comparing the aspheric aberration-controlled design to the identical spherical lens without aspheric optics (Frequency 55, CooperVision) demonstrates that the spherical lens provides better visual acuity, PSF and MTF (change from 0.344 to 0.565) than does the aberration-controlled lens, although not as well as the original Acuvue Oasys spherical contact lens (Figure 3).
Figure 2. A comparison of a spherical contact lens with aberration control (Frequency 55 aspheric: left four photos) with a non-aberration-control spherical design (Acuvue Oasys: right four photos).
Figure 3. A comparison of a spherical contact lens with aberration control (Frequency 55 aspheric: left four photos) with a similar design without aberration-control optics (Frequency 55: right four photos).
With regard to high- and low-contrast visual acuity for large pupils, toric soft lenses provide approximately one-half line or more improvement of vision compared to aspheric contact lenses. It's important to determine the effect of incorporating a spherical equivalent refraction compared to using a toric soft lens to correct astigmatism. One patient refracted with a –4.00 –1.00 ×180, which corrected vision to 20/20. Figure 4 shows that changing from a –3.75D Acuvue Advance (Vistakon) sphere to a –4.25D Acuvue Advance sphere (spherical equivalent) actually reduced visual performance and is not a satisfactory solution to this patient's vision problem. The MTF actually decreased from 0.684 to 0.453, demonstrating a reduction in retinal image quality. Using an Acuvue Advance for Astigmatism (Vistakon) toric soft lens with no aberration control (Figure 5) significantly improved this patient's visual acuity, PSF and MTF, which improved from 0.684 to 0.998, representing almost ‘perfect’ vision. Further, Figure 6 shows that when we compare a toric lens (O2Optix Toric, CIBA Vision) to a spherical lens with aspheric optics (PureVision), visual acuity, PSF and MTF (change from 0.398 to 0.651) were superior with the toric lens.
Figure 4. Comparing the performance of a spherical power (Acuvue Advance: left four photos) to the spherical equivalent power (also Acuvue Advance: right four photos).
Figure 5. Comparing the performance of a spherical power (Acuvue Advance: left four photos) to the same patient with an astigmatism-correcting lens (Acuvue Advance for Astigmatism: right four photos).
Figure 6. Comparing the performance of a spherical lens with aberration-control optics (PureVision: left four photos) to the same patient with an astigmatism-correcting lens (O2Optix Toric: right four photos).
While aberration-controlled soft lens designs typically produce variable clinical results at best, primarily due to the flexible nature of the material, rigid lens materials appear to have more success in correcting HOAs. Certainly this is attributable to the more rigid nature of the material. Conforma Laboratories has recently introduced an aberration-controlled line of GP lenses called HD Optics. This design incorporates aspheric curvatures on the front lens surface, which are designed to reduce inherent aberrations and improve vision. The front-surface curvature varies depending on the lens power, and in that sense HD Optics represents a more customized (at least for power) ‘corrected curve’ design.
In an attempt to evaluate the performance of HD Optics, I had two identical lenses prepared except that one lens incorporated a front-surface corrected curve (HD Optics) while the other did not. I placed each lens on the patient's eye and, after settling, performed a wavefront scan through the lens. Figure 7 shows the comparison between a Menicon spherical contact lens and the Conforma HD Optics contact lens. While visual acuity was subjectively acceptable for both lenses, PSF and MTF (change from 0.636 to 0.741) charts demonstrated an overall improvement in retinal imaging with the aberration-controlled design. Figure 8 shows similar improvements in visual acuity, PSF and MTF (change from 0.559 to 0.734) for the aberration-controlled spherical lens manufactured by Conforma Laboratories compared to a spherical GP lens manufactured in Boston XO (Bausch & Lomb).
Figure 7. Comparing the performance of a spherical GP lens (Menicon Z: left four photos) to a an aberration-control GP lens design (HD Optics: right four photos).
Figure 8. Comparing the performance of a spherical GP lens (Boston XO: left four photos) to a an aberration-control GP lens design (HD Optics: right four photos).
Another attempt to improve vision when residual astigmatism is present incorporates the use of two uniquely different lens designs: Panofocal (Conforma Laboratories) and RAU (Advanced Corneal Engineering). While the approaches are similar in that a spherical equivalent refraction is attempted, the Panofocal design adds plus power to the periphery and the RAU design adds minus power to the periphery. To compensate for a residual refractive correction of Pl –1.00 ×114, I incorporated a Panofocal front surface design with a spherical GP contact lens. In this example, aberrometry obtained through the Panofocal contact lens demonstrated improvement in visual acuity, PSF and MTF (change from 0.476 to 0.579) compared to the spherical contact lens (Figure 9). Moreover, the patient appreciated the improvement in his vision.
Figure 9. Comparing the performance of a spherical GP lens (left three photos) to a an aberration-control GP lens design correcting for residual astigmatism (Panofocal: right three photos).
Wearing multifocal contact lenses inherently induces increased HOAs. Center-near vision multifocal contact lenses appear to induce large amounts of negative spherical aberrations, while center-distance vision contact lenses appear to induce increased positive spherical aberrations. Relative decentration of the lens to the pupil may also explain the increase in odd HOAs. These results might be useful to understand the visual complaints of patients fitted with multifocal contact lenses. The VFL3 (Conforma Laboratories) is an aspheric, center-near multifocal contact lens. Figure 10 demonstrates improvement in visual acuity, PSF and MTF (0.283 to 0.441) when HD Optics is applied to the VFL3 aspheric multifocal lens.
Figure 10. Comparing the performance of a center-near GP multifocal lens without aberration-control optics (VFL3: left four photos) to this same lens with aberration-control optics added (HD Optics: right four photos).
Customized aberration-corrected lenses are currently being investigated by WaveTouch Technologies, which hopes to introduce this technology to the marketplace soon. The challenges facing customized aberration-corrected contact lenses include maintaining lens centration, alignment and rotation to ensure the proper registration of the prescription. Unlike applying a wavefront correction directly to the corneal surface, which is relatively fixed and stable, a contact lens represents a dynamic optical system that moves vertically and rotationally on the eye in relation to the blink. Companies designing such lenses will need to use prism-ballasted lens designs for both soft and rigid lenses as well as dynamic stabilization techniques for soft contact lenses that attempt to control on-eye lens rotation to minimize lens movement.
Other alternatives like the SynergEyes (SynergEyes, Inc.) lens line, which offers a stable rigid central surface for wavefront correction and a large overall lens diameter to reduce on-eye movement, seems like another excellent alternative to deliver consistent wavefront corrections with contact lenses.
Studies show an improvement in photopic high-contrast vision and photopic low-contrast vision when custom aberration control is used in normal eyes, and keratoconic patients show significant improvements in visual performance using three-dimensional aberration-customized soft lenses provided they maintain on-eye centration.
The vast majority of contact lenses in today's market are mass-produced, molded soft or silicone hydrogel lenses. To achieve aberration control on these lenses, manufacturers must make approximations for the general population's higher-order distortions. Moreover, spherical lens designs can correct only rotationally symmetrical aberrations such as spherical aberration. Subsequently, some patients will experience improvement in their vision while others may actually suffer a detriment.
For aberration-controlled optics to be successful with contact lenses, the fit must achieve consistent geometric centration over the line of sight. Decentration of the contact lens will result in additional unusual aberrations. In addition, some aberrations such as coma and trefoil require rotationally stable contact lenses, while other aberrations such as spherical aberration are symmetrical and can be corrected by non-rotationally stable lens designs. Each aberration-controlled lens, however, would have to be individually manufactured for the patient's specific LOAs and HOAs, resulting, by today's standards, in a more expensive lens. Regardless, the quest for superior vision will drive patients toward more individualized lens corrections and continue to grow this marketplace. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #156.
Contact Lens Spectrum, Issue: November 2008