Operator-induced errors in Hartmann-Shack wavefront sensing: Model eye study

Influence of perfluorohexyloctane (Evotears®) on higher order aberrations

PURPOSE

To prospectively assess the effect of regular application of perfluorohexyloctane (F6H8; Evotears®) on the tear film lipid layer, higher order aberrations (HOA) and the repeatability of measurements in healthy eyes.

METHODS

This prospective clinical study included 104 eyes treated with F6H8 four times daily for four weeks (group A) and 101 eyes that served as controls (group B). Measurements were performed with the WASCA aberrometer (Carl Zeiss Meditec GmbH, Jena, Germany). Main outcome measurement in addition to subjective refraction were the root mean square values of HOA measured before and after the intervention.

RESULTS

Regular use of F6H8 over a period of four weeks significantly increases HOA in healthy eyes (p < 0.05). In addition, the repeatability of measurement increases after the application of F6H8.

CONCLUSION

F6H8 may be a suitable treatment option to improve the accuracy of refractive assessment, although it increases HOA. Further studies are needed to confirm the effect on HOA and the repeatability of measurement.

A pilot study on the differences in wavefront aberrations between two ethnic groups of young generally myopic subjects

A comparative population-based cross-sectional study design was used to examine the prevalence of wavefront patterns in two different ethnic groups, and the relationship of these patterns with ocular biometrics and gender. The Shin-Nippon SRW5000 open field autorefractor, the Wavefront Analysis Supported Customized Ablation (WASCA) wavefront analyser and the IOLMaster were used to determine wavefront aberrations, mean spherical equivalent (SE) refractive error and axial length (AL). Seventy-four eyes from 74 young healthy subjects (44 British Asians, 30 Caucasians; 36 men, 38 women; mean age 22.51 +/- 3.89 years) with mean SE averaging -1.90 +/- 2.76 D (range -10.88 to +2.19 D) were examined. Relationships between ethnicity, gender, AL and SE, against the wavefront high-order root mean square, and aberration components up to the fifth order, were assessed by using multiple regression and correlation analysis. AL on its own accounted for 4.7% of the variance in trefoil component Z(-3)(3) (F(1,72) = 4.602; p = 0.035), 13.7% of coma component Z(1)(3) (F(1,72) = 12.536; p = 0.001), 6.1% of trefoil component Z(3)(3) (F(1,72) = 5.705; p = 0.020) and 9.8% of coefficient Z(-2)(4) (F(1,72) = 8.908; p = 0.004). A significant model emerged (F(2,71) = 6.164; p = 0.003) for ethnicity and axial length, accounting for 12.4% of variance in primary spherical aberration with ethnicity accounting for 8.4% of that variance. For Caucasian subjects, a significant correlation was found between axial length and (Pearson's correlation coefficient -0.500; p = 0.005) and Z(0)(4) (Pearson's correlation coefficient -0.423; p = 0.020). For British Asian subjects, AL was only correlated with coefficient Z(-2)(4) (Pearson's correlation coefficient -0.358; p = 0.017). Ethnicity is a factor to be considered in the variability of wavefront aberration, particularly spherical aberration. Relationship between AL and wavefront aberrations seems to vary between ethnicities. If higher order aberrations play a role in the emmetropization process, this may be different for different populations.

Can uncompensated higher order aberration profile, or aberropia be responsible for subnormal best corrected vision and pseudo-amblyopia

BACKGROUND

Higher order aberrations (HOA) are components of wavefront distortion which cannot be corrected by conventional spectacle correction. Normally they constitute up to 15% of wavefront distortion in the ocular media. The HOA interact in a nonlinear method and tend to compensate each other in most conditions.

HYPOTHESES

We hypothesize that abnormally increased and uncompensated higher order aberration profile in certain cases may limit the amount of best corrected visual acuity to subnormal levels and produce a pseudo-ambylopia like picture. We term this entity as aberropia.

EVALUATION OF THE HYPOTHESIS

Higher order aberrations (HOA) have been proven to have supporting role in visual function in association with lower order aberrations. Normally HOA tend to compensate for each other and have an effect much less than what would be expected if they would have acted separately. This unique state of compensation may be lost in certain cases, for example, in patients who have undergone laser refractive surgery without consideration for HOA correction or in partial correction of a pathological eye like severe keratoconus. Occasionally investigators have noticed that patients with high induced aberrations after refractive surgery cannot achieve spectacle correction to get a 20/20 vision. Conversely, there is evidence for gain of vision after wavefront guided refractive surgery and phakic intraocular lens implantations in some cases which cannot be explained only by retinal image change.

CHALLENGES TO CONCLUSIVELY PROVE THE HYPOTHESES

With the current level of advancement in refractive surgery, we can correct lower order aberration to a near zero level. However it is not as easy to do the same with higher order aberrations because of variability in assessment, evolving laser beam profiles and factors associated with the surgery itself and wound healing. All the four can alter higher order aberrations.

IMPLICATIONS OF THE HYPOTHESES

Understanding of uncompensated higher order profiles leading to loss of BCVA, or aberropia, would be a paradigm shift in understanding of the role of higher order aberrations on visual function. With improvement in diagnostic and treatment methods, these patients with a subnormal vision, may be treated to achieve their maximum visual potential.

Apodized diffractive versus refractive multifocal intraocular lenses: optical and visual evaluation

PURPOSE

To evaluate the optical and visual performance after implantation of refractive or apodized diffractive multifocal intraocular lenses (IOLs).

SETTING

Military Health Service Institute, Warsaw, Poland.

METHODS

Uncorrected distance visual acuity, best distance-corrected visual acuity, best distance-corrected near visual acuity, distance contrast sensitivity under photopic conditions (CSV-1000), residual refractive error, and wavefront aberrations (LADARWave Hartmann-Shack wavefront analyzer) were measured in 23 patients who had bilateral implantation of the AcrySof ReSTOR SN60D3 IOL and 23 patients who had bilateral implantation of the ReZoom IOL....

RESULTS

At the 6-month postoperative visit, the mean photopic uncorrected distance acuity was 0.03+/-0.05 (SD) in the ReSTOR group and 0.02+/-0.06 logMAR in the ReZoom group (both approximately 20/20) (P= .569). In all patients, the mean photopic best distance-corrected acuity was 0.00 logMAR (approximately 20/20) and the mean photopic best distance-corrected near acuity at 35 cm was 0.10 logMAR. The photopic contrast sensitivity was within the standard normal range in both IOL groups. The difference in photopic contrast sensitivity between groups was statistically significant (P<.001). Higher-order aberrations, in particular coma and spherical aberrations, were significantly higher in the ReZoom group (all P<.001).

CONCLUSIONS

AcrySof ReSTOR SN60D3 and ReZoom IOLs provided good visual performance at distance and near under photopic conditions. Optical quality measures were significantly worse in patients with ReZoom IOLs.

Wavefront sensing using speckles with fringe compensation

Wavefront sensing with numerical phase-error correction system is carried out using a random phase plate and phase retrieval using multiple intensity measurements of axially-displaced speckle patterns and the wave propagation equation. Various wavefronts with smooth curvatures incident on the developed phase plate (DPP) are examined: planar, spherical, cylindrical, and a wavefront passing through the side of a bare optical fiber. Spurious fringe pattern in the wavefront reconstructions due to a small tilt (Delta theta=0.212 degrees) in the plane illumination wave is detected and numerically corrected for. Fringe pattern of the illumination wave obtained for the setup without the phase object being investigated is used as reference fringe pattern. Fringe compensation yields wavefronts with the correct shape and numerical value based on the specifications of the setup. The numerical phase-error correction system described in this study can be extended to other types of phase errors such as those due to aberrations if optical elements are present in the setup or due to perturbations in the environment.

Accuracy of cornea and lens biometry using anterior segment optical coherence tomography

We assess the accuracy of the Visante anterior segment optical coherence tomographer (AS-OCT) and present improved formulas for measurement of surface curvature and axial separation. Measurements are made in physical model eyes. Accuracy is compared for measurements of corneal thickness (d(1)) and anterior chamber depth (d(2)) using-built-in AS-OCT software versus the improved scheme. The improved scheme enables measurements of lens thickness (d(3)) and surface curvature, in the form of conic sections specified by vertex radii and conic constants. These parameters are converted to surface coordinates for error analysis. The built-in AS-OCT software typically overestimates [mean+/-standard deviation(SD)]d(1) by +62+/-4 mum and d(2) by + 4 +/- 88 microm. The improved scheme reduces d(1) (-0.4 +/- 4 microm) and d(2) (0 +/- 49 microm) errors while also reducing d(3) errors from +218 +/- 90 (uncorrected) to +14 +/- 123 microm (corrected). Surface x coordinate errors gradually increase toward the periphery. Considering the central 6-mm zone of each surface, the x coordinate errors for anterior and posterior corneal surfaces reached +3 +/- 10 and 0 +/- 23 microm, respectively, with the improved scheme. Those of the anterior and posterior lens surfaces reached +2 +/- 22 and +11 +/- 71 microm, respectively. Our improved scheme reduced AS-OCT errors and could, therefore, enhance pre- and postoperative assessments of keratorefractive or cataract surgery, including measurement of accommodating intraocular lenses.