Wavefront testing

I.        List the indications

A.     To perform wavefront guided surgery

B.     To evaluate postoperative quality of vision symptoms

C.    To evaluate procedure outcome by comparing postoperative to preoperative tests

II.      List the alternatives to this procedure

A.     There is no alternative procedure which can be used to quantitate all of the optical aberrations of the eye. Corneal topography can be used to generate the wavefront pattern of the anterior cornea surface.

B.     Retinoscopy can be used to qualitatively assess ocular aberrations, such as scissoring in keratoconus.

III.    Describe the instrumentation and technique

A.     Wavefront techniques. All wavefront devices evaluate aberrations for the entire optical system of the eye (i.e. anterior and posterior corneal surface as well as the crystalline lens). As such, they cannot distinguish those aberrations that originate in the cornea from those in the crystalline lens.

1.      Hartmann Shack. Most common system in use today; using a lenslet array to analyze the wavefront emanating from the eye

2.      Ray tracing

3.      Scanning skiascopy

4.      Tscherning

B.     Wavefront measurement

1.      Measurements are made over the pupillary area

2.      Adequate fixation, centration, and focus are required

3.      Highly aberrated eyes, such as severe keratoconus, may not be able to be imaged

4.      The quality of the measurement can be assessed by (Hartman Shack):

a.      The reproducibility of the sphere and cylinder terms on repeated testing

b.      Sharpness of the lenslet array

c.      Lenslet dropout (inadequate tear film or optical opacity)

d.      Cross-over of lenslets (highly aberrated eyes)

5.      The wavefront is usually reconstructed using Zernike polynomials. These polynomials are then used to describe the wavefront by its components

6.      Aberrations are divided into lower and higher order

a.      Lower order are sphere (defocus) and cylinder

b.      Higher order encompass all other ocular aberrations

7.      The display of the wavefront:

a.      Typically made with a color coded map, with elevation above and below a perfect “flat” wavefront. A map is usually displayed for all aberrations and a second map of higher aberrations.

b.      Can also be displayed as a point spread function. This is the calculated appearance of a point source of light.

C.    Higher order aberrations

1.      The most common in a normal population are:

a.      Coma (a third order term), so named for the “comet” like pattern produced by a point source of light

b.      Spherical aberration (a fourth order term)

D.    Higher order aberrations are generally increased after conventional laser in situ keratomileusis. In particular, spherical aberration can be increased when treating myopia. The amount of increase is related to the level of treated myopia.

E.     Significant amounts of higher order aberrations can be responsible for visual disturbances, such as glare and halos.

IV.   Describe the considerations in interpretation of this diagnostic procedure

A.     A manufacturer’s wavefront unit is mated to its laser for custom procedure

B.     The tear film greatly influences the quality of the wavefront

C.    Accommodation can be a significant factor and needs to be controlled during the measurement

D.    Optical opacities, such as cataracts, make the measurement unreliable

E.     The wavefront can only be calculated over the entrance pupil size

F.     A larger pupil size generally increases higher order aberrations

Additional Resources

1. AAO, Basic and Clinical Science Course. Section 14: Refractive Surgery, 2004-2005.