Refractive error introduced in the application of a refractive surgical treatment and in the subsequent response of the ocular tissue Charles Campbell

 Refractive error introduced during surgical treatment  Refractive error introduced following treatment

Refractive error introduced during surgical treatment  Misplacement of the treatment  Rotation error  Centration error  Change in ablation rate

Refractive error due to rotation error Consider the familiar case of cylinder axis error – dA, for corrective cylinder power C (cross cylinder power = C/2) Blurring effect of the error is expressed by

Refractive error due to rotation error Blurring effect of the error is the product of the magnitude of the correction times the rotation error times its vector angle space value The aberration induced is of the same type as the correction itself

Refractive error due to rotation error Blurring effect dE of a rotation error any refractive term – expressed as a Zernike polynomial – is in similar fashion the product of the magnitude of the term times the rotation error dA times the value of it angle vector space m

Comments on rotation error  If the magnitude of the correction is small large rotational errors have little effect in degrading vision  For aberrations expressed in terms of Zernike coefficients, terms with larger meridional indices, m, are affected more by rotational error  In most cases common astigmatism has by far the largest magnitude and so it is usually the only aberration really affected by rotation error

An example of the size of the effect Let dA be 5 degrees so that the value that is multiplied times the correction value is If 4 D of astigmatism are to be corrected, induced dioptric error is then

Cause and prevention of rotation error  Rotational misalignment can occur in the placement of the head beneath the laser system  The eye often rotates from its normal – head vertical – orientation when the head becomes horizontal during surgery (cyclo-rotation)  Iris registration has proved to be an effective way optically sensing the rotational orientation of the eye during surgery and providing a correction to the treatment routine

Refractive error due to centration error Consider the familiar case of decentration of lens with sphere power S decentered by a distance dx Prentice’s rule gives the magnitude of aberration introduced – prism Δ in this case

Similarity between rotation and decentration errors Similar to the case of rotation error the magnitude of the aberration is the product of the correction times the displacement error The same type of rule also applied to misplacement of higher order corrections

Difference between rotation and centration errors For decentration the full corrective effect is still applied. Whereas for rotation the desired refraction effect is changed. For decentration errors the induced errors are always of a lower order than the correction. Whereas for rotation the induced error is of the same order as the correction.

Decentration effects on Zernike terms Defocus  prism Coma  defocus  astigmatism and prism Trefoil  astigmatism and prism Spherical aberration  coma defocus astigmatism prism

A more important effect of treatment misplacement  The effect of the transition zone of the ablation when it enters the pupil of the eye Ablation centered on the mesopic pupil Ablation decentered on the mesopic pupil Crescent shaped astigmatic lenslet Ablation zone Optical zone Pupil edge Optical zone

Cause and effect of a transition zone lenslet Caused by high radial curvature in transition zone Creates a secondary, displaced image The power of lenslet is proportional to the change in corneal curvature in the optical zone, i.e. to the magnitude of correction Visibility of the secondary image is proportional to ratio of the area of the lenslet to the rest of the pupil area and the contrast of object and background

Minimizing decentration  Plan the treatment to have the optical zone centered on the mesopic pupil center  It not advisable to center the ablation on the corneal apex because the corneal apex is usually displaced from the mesopic pupil center  Use iris registration or centration on the pupil center as viewed through the operation microscope  The pupil restricts during treatment which usually decenters it from the mesopic pupil. So centering on the treatment pupil will usually cause some ablation decentration. But this effect is very small in general ~ 1/16 mm to 1/8 mm

Change in ablation rate during treatment  Tissue dehydration or an excessive fluid film on the stromal tissue will cause the ablation rate to change  Dehydration – causing extra ablation – is most likely due to air flow over the treatment area during the procedure

Refractive error introduced following treatment  Flap effects for LASIK cases  Aberrations induced by the corneal epithelial cell layer

Considerations on the LASIK flap  The flap acts like a zero power soft contact lens that is fit tight (myopic correction) or loose (hyperopic correction)  The typical flap is about 125 microns thick of which 50 is the epithelial cell layer and 75 microns stromal lamella and Bowman’s membrane  Hydrogel lenses are typically about 125 microns in the center and composed of homogeneous material

Considerations on the LASIK flap  Myopic flap after treatment but before pressing against the stromal tissue

An example  A myopic correction of 6 D with a 6 mm optical zone  Change in flap arc length ~ 1%  Change in flap area ~ 1%  Changes are too small to affect outcome

Effect of flap thickness  Typical ablation surfaces exhibit low spatial frequencies  This type of surface shape ‘prints through’ the flap well  Flaps indeed act as zero power lenses - if they are created with even thickness in the first place

Aberrations induced by the corneal epithelial cell layer Spherical aberration is found to be induced following laser refractive treatments

Corneal epithelial anatomy  Cross section showing epithelial cells at different stages of differentiation and the junctions that hold them together

Squamous cell layer as an elastic membrane or mesh The red hexagons represent the cell plasma membrane. The black spots are the tight junctions. The green hexagons are the elastic actin fibrils attached to the tight junction proteins within the cell.

Evidence for the presence of an elastic membrane at the corneal epithelium surface If an incision is made in the corneal epithelium, the wound gapes

Effect of the transition zone in combination with an outer elastic membrane The transition zone introduces an area of higher mean curvature with respect to neighboring areas which difference increases with increased refractive correction An elastic membrane under tension will attempt to reform its shape to equalize mean curvature throughout As the epithelium replenishes itself the cells will move so as to minimize mean curvature difference

Effect of the transition zone in combination with an outer elastic membrane Stroma Epithelium Optical zone Transition zone Epithelial remodeling Stroma Epithelium Optical zone Transition zone

Simulation versus experimental findings

What can be done to decrease this effect? Move the transition zone outward Estimate the effect and include an opposite aberration into the treatment

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