Article Date: 12/1/2002

Wavefront Technology
Wavefront Analysis: The Next Frontier in Refractive Care
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 keratectomy.

Using wavefront to diagnose

The following is a list of some of the actual and potential diagnostic refractive applications of wavefront technology:

Therapeutic applications

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.

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.

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.

ANATOMICAL  REFRACTIVE TECHNOLOGICAL
+ tear dysfunction + mild defocus + conventional LASIK (2x to 11x increases)
+ large pupils + lens changes (>40) + decentered ablation (> coma & trefoil)
+ corneal biomechanics + accommodation + 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
- youth + oblate corneal shape (> sph. aberrations) - femtosecond lasers
- normal tear film  - hyperopes (prolate) - surface ablation (PRK)
- small pupils (<4 mm) - no cylinder - "staged" procedures
- minimal lenticular astig. - pre-procedure lens analysis (e.g. PreVue) - smoothest ablation
- higher density of cones in the fovea - prolate corneal shape - custom ablation
  - lensectomy - 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.

STRENGTHS WEAKNESSES
Accurate to 0.005D Retinal/neural limitations
Maximizes retinal resolving potential LASIK flap irregularities
Totally objective refractive assessment Lens/corneal opacities
Fast readings (as fast as two seconds per eye) Variable accommodation
Measures higher order aberrations to 6th order Tear film aberrations
Identifies smallest incremental cylinders 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

Dr. Catania 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