Wavefront Analysis: The Next Frontier in Refractive
Learn about some existing and upcoming applications for wavefront technology.
BY LOUIS J. CATANIA, O.D., F.A.A.O.
Among the dynamic advances in vision care over the past 25 years, the most dramatic is wavefront analysis, which also has the broadest diagnosis and treatment applications. Spawned from the refractive surgery revolution of the past eight to 10 years and based on the principles of adaptive optics used in astrophysics since the 1970s, wavefront analysis is rapidly changing the very fundamentals of refractive care for the human eye.
The progress of the excimer laser and the laser-assisted in situ keratomileusis
(LASIK) procedure has introduced eyecare practitioners to the quantitative and qualitative effects of higher-order aberrations such as spherical aberrations, coma, trefoil,
quadrefoil, etc. on vision.
The exquisite accuracy of the excimer laser has made it apparent that the weakest link in refractive surgery is indeed the refraction, or at least the standard refraction being provided by contemporary eye care practitioners
(O.D.s and M.D.s).
It has also become apparent that photoablation and the LASIK procedure, particularly the keratectomy or cutting of a corneal lamellar flap, provide adequate correction for lower-order aberrations (basic sphere and cylinder) but in fact aggravate and induce higher-order aberrations in the eye.
Understanding the science
The standard refraction that eyecare clinicians have performed for more than 40 years has consisted of subjective and objective measurements of lower-order aberrations (sphere and cylinder), generally in 0.25-D steps. However, the total aberrations of the eye produced by the cornea, aqueous, lens, vitreous and the numerous changes in indices of refraction of light rays, include up to 20% higher-order aberrations. These aberrations represent refractive abnormalities well below 0.25D (or 3 µm) and thus, higher-order aberrations require measuring systems and instrumentation well beyond contemporary, standard refractive technologies.
Adaptive optics provides such measuring capabilities through its use of root mean square deviations, or RMS units, which measure light deviations down to 0.01 µm, the equivalent to about 0.005D. Through specialized, measuring sensors (deformable
"lenslet" systems) and computer analysis, RMS values are converted into 3-D mathematical models or maps called Zernicke polynomials. These values and maps can provide analysis of virtually 100% of the aberrations of an eye.
The evolving ability to accurately measure and map the full complement of visual aberrations, particularly the higher-order aberrations of the human eye is, and will continue to enhance a fuller understanding of human vision and its most effective correction. The continuing evolution of refractive surgery has also helped provide a better understanding of those elements that can increase and decrease higher-order aberrations in a patient.
Efforts to avoid or reduce elements that increase higher-order aberrations while clinically exploiting those elements that decrease higher-order aberrations should help improve refractive and vision sciences in the future. "Elements That Affect Higher-Order Aberrations" (pg. 50) lists some of the major anatomical, refractive and technological elements that increase and decrease higher-order aberrations.
Using wavefront clinically
Numerous sensing and measuring methods are available for wavefront analysis (also referred to as
aberrometry) including Shack-Hartmann (e.g., Alcon's LadarWare, Bausch & Lomb's
Zyoptix, VISX's WavePrint), Tscherning (e.g., Allegretto Wavelight), ray tracing (e.g., Tracey
VFA) and spatial skiametry (e.g., Nidek's OPD- Scan). You should familiarize yourself with all of these commercially available systems and keep in mind their strengths and weaknesses.
Just as each commercial wavefront analyzer has its own strengths and weaknesses, so too does the general science of wavefront technology (see "Weighing the Pros and Cons" on pg. 52). A review of these considerations leads to an awareness of the potentials and limitations of wavefront analysis in diagnosis and treatment of visual conditions.
These characteristics will have a profound impact on the scope and precision of diagnostic refractive care, on the various current and evolving forms of refractive treatments attempting to correct both lower- and higher-order aberrations and on the future practice of optometry and ophthalmology.
The application of wavefront technologies in human vision care evolved from the refractive surgery revolution over the past 10 years. However, it's now becoming apparent that in fact, its place in refractive care may be as valuable (or more valuable) in refractive diagnosis as in refractive treatment applications such as custom corneal ablation.
The original goals for custom ablation (i.e., the correction of higher-order aberrations) have now been restated to more realistically describe their attempt to reduce the induced higher-order aberrations associated with the corneal biomechanical insult of laser photoablation and the LASIK
Using wavefront to diagnose
The following is a list of some of the actual and potential diagnostic refractive applications of wavefront technology:
- With wavefront analysis, we can expand the accuracy of routine refraction from 80% to capture 100% of the eye's total aberrations. Thus, when a patient asks, "Can you get me better than 20/20?" you could answer definitively by maximizing his best corrected visual acuity according to his retinal/neural limitations. Can the patient's cortex handle that full potential? Careful consideration and studies into the perceptual aspects of higher-order corrections will be an interesting and evolving science of the future.
- You could analyze eyes not correctable to 20/20 so accurately as to segregate out each of the visual system's components (eyeball aberrations, retinal/neural function and cortical perception) and efficiently and precisely diagnose normal potentials from the physiological and/or pathological factors.
- Differentiating corneal from crystalline lens aberrations will become an important part of determining what type of refractive procedure, if any, is best for a patient. This is achievable through simple regression analysis of the wavefront readings and corneal topography maps.
While topography can help interpret corneal aberrations, its subtraction from the total wavefront aberrations will essentially quantify the residual aberrations from the lens. This will allow precise assessment of aging effects in the lens well before cataract formation.
- Dynamic accommodation changes the profile of higher-order aberrations in the eye. Assessing and comparing
tions under static and dynamic visual conditions will allow us to better determine, diagnose and prescribe near corrections.
- Pre-testing refractive surgery candidates to determine their precise lower- and higher-order aberration profile will allow eyecare practitioners to identify at-risk patients for certain
dures that may not adequately correct or that may even aggravate their principal visual
problem(s). Corneal refractive procedures for patients who have significant or advancing lens aberrations may not be appropriate when you consider immediate or long-term results.
- Post refractive procedural or corrective device complaints of a visual nature will be able to be diagnosed precisely and comparatively to pre-procedure or device correction. This will enable the surgeon and the optometrist to precisely assess what variables may have been introduced from the target correction and/or from the virgin eye (e.g., induced aberrations in corneal refractive surgery).
- Retreatment considerations and decisions in any form of surgical or non-surgical correction will be made through wavefront analysis with the greatest accuracy and assessment of potential outcomes for such post procedure or device interventions.
Wavefront technology will also provide an array of therapeutic applications in vision care that will improve the accuracy and predictability of both surgical and non-surgical refractive alternatives. The following list includes some, but not all of these current and potential treatment applications.
- Custom corneal photoablation uses a narrow beam excimer laser precisely guided by wavefront computer software programmed from the lower and higher-order aberrations of the patient's eye. For two reasons, this procedure will soon become standard for all corneal photoablation procedures. First, outcome data shows that custom corneal ablation minimizes the induction of higher-order aberrations associated with laser photoablation and the LASIK
Second, photoablation of prescription powers that are difficult or impossible to treat with conventional procedures are achievable with custom corneal ablation.
- Fixing previous surgical "disasters" appears to be an effective use of custom corneal ablation. Decentered ablations, irregular astigmatism, even induced higher-order aberrations, are within the realm of repair with the precision of
wavefront-guided custom ablation.
- Wavefront-guided custom ablations have been effective in treating certain corneal abnormalities such as keratoconus and other topographic and corneal surface abnormalities. Results may not often be optimal, but alone or with other modalities
(bioptics), such treatments have shown better outcomes than previous alternative approaches.
- Wavefront technologies can even construct non-surgical refractive treatment options using companion optical materials and devices such as synthetic optics, intraocular lenses
(IOLs), contact lenses and even spectacles lenses.
Examples include corneal inlays and onlays; light-adjustable IOLs capable of corrections down to 1 µm accuracy; replaceable contact lenses,
wavefront-constructed and bioadhesively applied to the corneal surface; and "intelligent spectacles" that have central or even the entire distance to near channel programmed with wavefront corrected optics. These materials and designs will become more refined and commercially available through sophisticated technologies such as
nanoconstruction, which will build them (and anything!) from the molecular level up.
- Genetic engineering methods of manipulating corneal cells, tissues and proteins (e.g., glycosaminoglycan or ground substance) are being developed to provide controlled healing and adjustments of corneal curvatures and surface characteristics to produce wavefront accuracy (less than 1 µm) for refractive correction.
Banking on a sure thing
Wavefront technology represents a major clinical advance in the science of vision care for us as well as our patients. Its immediate and long-term applications will significantly change the way we diagnose and treat refractive errors in the future.
That's why it's imperative that we're fully knowledgeable and directly involved in the current and ongoing development and implementation of this important science in clinical care. Your patients, your practice and the optometric profession need it and will benefit significantly from it. Just wait and see.
Elements that Affect Higher-Order Aberrations
The following factors either increase (+) or decrease (-) higher order aberrations.
|+ tear dysfunction
||+ mild defocus
||+ conventional LASIK (2x to 11x increases)
|+ large pupils
||+ lens changes (>40)
||+ decentered ablation (> coma & trefoil)
|+ corneal biomechanics
||+ conventional PRK (about 3x increases)
|+ corneal hydration
||+ higher cylinders
||+ flap hinge in LASIK (> sph aberrations)
|+ corneal remodeling
||+ irregular astigmatism
||+ flap complications
|+ corneal anisetropy
||+ mesopic conditions
||- spectacles and contact lenses
||+ oblate corneal shape
(> sph. aberrations)
||- femtosecond lasers
|- normal tear film
|| - hyperopes
||- surface ablation
|- small pupils (<4 mm)
||- no cylinder
||- "staged" procedures
|- minimal lenticular
||- pre-procedure lens
analysis (e.g. PreVue)
||- smoothest ablation
|- higher density of cones
in the fovea
||- prolate corneal shape
||- custom ablation
||- centered ablation
||- significant defocus
||- small ablation optical zone
||- refining nomograms
||- Intacs (intracorneal rings)
||- aspheric IOLs
Weighing the Pros and Cons
Consider both sides of wavefront technology
before using it on a patient.
|Accurate to 0.005D
||LASIK flap irregularities
|Fast readings (as fast
as two seconds per eye)
|Measures higher order
aberrations to 6th order
||Tear film aberrations
||Designing spectacles or
contact lenses to wavefront accuracy
||Technology too accurate for
accurate corneal remodel ing and cortical "perception"
- e.g., corneal healing @5 µm
- e.g., LGB and cortical limits
is an internationally acclaimed clinical educator, author and recognized expert in anterior segment care, refractive surgery and new eyecare technologies.
Optometric Management, Issue: December 2002