Centration axis in refractive surgery

The human eye is an asymmetric optical system and the real cornea is not a rotationally symmetrical volume.Each optical element in the eye has its own optical and neural axes.Defining the optimum center for laser ablation is difficult with many available approaches.We explain the various centration approaches(based on these reference axes)in refractive surgery and review their clinical outcomes.The line-of-sight(LOS)(the line joining the entrance pupil center with the fixation point)is often the recommended reference axis for representing wavefront aberrations of the whole eye(derived from the definition of chief ray in geometrical optics);however pupil centration can be unstable and change with the pupil size.The corneal vertex(CV)represents a stable preferable morphologic reference which is the best approximate for alignment to the visual axis.However,the corneal light reflex can be considered as non-constant,but dependent on the direction of gaze of the eye with respect to the light source.A compromise between the pupil and CV centered ablations is seen in the form of an asymmetric offset where the manifest refraction is referenced to the CV while the higher order aberrations are referenced to the pupil center.There is a need for a flexible choice of centration in excimer laser systems to design customized and non-customized treatments optimally.

The art of nomograms

Background:To retrospectively analyse strategies for adjusting refractive surgery plans with reference to the preoperative manifest refraction.Methods:We constructed seven nomograms based on the refractive outcomes(sphere,cylinder,axis[SCA])of 150 consecutive eyes treated with laser in situ keratomileusis for myopic astigmatism.We limited the initial data to the SCA of the manifest refraction.All nomograms were based on the strategy:if for x diopters(D)of attempted metric,y D is achieved;we can reverse this sentence and state for achieving y D of change in the metric,x D will be planned.The effects of the use of plus or minus astigmatism notation,spherical equivalent,sphere,principal meridians notation,cardinal and oblique astigmatism,and astigmatic axis were incorporated.Results:All nomograms detected subtle differences in the spherical component(p<0.0001).Nomograms 5 and 7(using power vectors)and 6(considering axis shifts)detected significant astigmatic differences(nomogram 5,p<0.001;nomogram 6,p<0.05;nomogram 7,p<0.005 for cardinal astigmatism,p=0.1 for oblique astigmatism).We observed mild clinically relevant differences(~0.5 D)in sphere or astigmatism among the nomograms;differences of~0.25 D in the proposals for sphere or cylinder were not uncommon.All nomograms suggested minor improvements versus actual observed outcomes,with no clinically relevant differences among them.Conclusions:All nomograms anticipated minor improvements versus actual observed outcomes without clinically relevant differences among them.The minimal uncertainties in determining the manifest refraction(~0.6 D)are the major limitation to improving the accuracy of refractive surgery nomograms.

Corneal functional optical zone under monocular and binocular assessment

Background:In this retrospective randomized case series,we compared bilateral symmetry between OD and OS eyes,intercorneal differences and Functional Optical Zone(FOZ)of the corneal aberrations.Methods:Sixty-seven normal subjects(with no ocular pathology)who never had any ocular surgery were bilaterally evaluated at Augenzentrum Recklinghausen(Germany).In all cases,standard examinations and corneal wavefront topography(OPTIKON Scout)were performed.The OD/OS bilateral symmetry was evaluated for corneal wavefront aberrations,and FOZ-values were evaluated from the Root-Mean-Square(RMS)of High-Order Wavefront-Aberration(HOWAb).Moreover,correlations of FOZ,spherical equivalent(SE),astigmatism power,and cardinal and oblique astigmatism for binocular vs.monocular,and binocular vs.intercorneal differences were analyzed.Results:Mean FOZ was 6.56±1.13 mm monocularly,6.97±1.34 mm binocularly,and 7.64±1.30 mm intercorneal difference,with all strongly positively correlated,showing that the diameter of glare-free vision is larger in binocular than monocular conditions.Mean SE was 0.78±1.30 D,and the mean astigmatism power(magnitude)was 0.46±0.52 D binocularly.The corresponding monocular values for these metrics were 0.78±1.30 D and 0.53±0.53 D respectively.SE,astigmatism magnitude,cardinal astigmatism component,and FOZ showed a strong correlation and even symmetry;and oblique astigmatism component showed odd symmetry indicating Enantiomorphism between the left and right eye.Conclusions:These results confirm OD-vs.-OS bilateral symmetry(which influences binocular summation)of HOWAb,FOZ,defocus,astigmatism power,and cardinal and oblique astigmatism.Binocular Functional Optical Zone calculated from corneal wavefront aberrations can be used to optimize refractive surgery design.