Back-calculation to model strategies for pretreatment adjustment of the ablation sphere in myopic wavefront laser in situ keratomileusis

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.

Multiple regression analysis in nomogram development for myopic wavefront laser in situ keratomileusis: Improving astigmatic outcomes

PURPOSE

To describe and evaluate a new multiple regression-derived nomogram for myopic wavefront laser in situ keratomileusis (LASIK).

SETTING

Moorfields Eye Hospital, London, United Kingdom.

DESIGN

Prospective comparative case series.

METHODS

Multiple regression modeling was used to derive a simplified formula for adjusting attempted spherical correction in myopic LASIK. An adaptation of Thibos' power vector method was then applied to derive adjustments to attempted cylindrical correction in eyes with 1.0 diopter (D) or more of preoperative cylinder. These elements were combined in a new nomogram (nomogram II).

RESULTS

The 3-month refractive results for myopic wavefront LASIK (spherical equivalent ≤11.0 D; cylinder ≤4.5 D) were compared between 299 consecutive eyes treated using the earlier nomogram (nomogram I) in 2009 and 2010 and 414 eyes treated using nomogram II in 2011 and 2012. There was no significant difference in treatment accuracy (variance in the postoperative manifest refraction spherical equivalent error) between nomogram I and nomogram II (P = .73, Bartlett test). Fewer patients treated with nomogram II had more than 0.5 D of residual postoperative astigmatism (P = .0001, Fisher exact test). There was no significant coupling between adjustments to the attempted cylinder and the achieved sphere (P = .18, t test).

CONCLUSIONS

Discarding marginal influences from a multiple regression-derived nomogram for myopic wavefront LASIK had no clinically significant effect on treatment accuracy. Thibos' power vector method can be used to guide adjustments to the treatment cylinder alongside nomograms designed to optimize postoperative spherical equivalent results in myopic LASIK. mentioned.

[LASIK nomogram modifications for the treatment of myopic astigmatism]

PURPOSE

The predictability of laser-assisted in situ keratomileusis (LASIK) can be improved by adjustments to treatment nomograms. The aim of this research was to clinically evaluate two different methods of nomogram optimization, to compare them with a reference and to analyze the effectiveness of individual cylinder modifications on the distribution of the postoperative spherical equivalent.

METHODS

Patients with myopic astigmatism formed three patient collectives and were treated consecutively with LASIK. In total 146 Femto-LASIK procedures were performed with the VISX S 4 excimer laser and the Intralase FS 60 Femto laser. Patients in the first collective (group A) received identical spherical nomogram adjustments, independent of the preoperative refraction. Individual adjustments of the treatment sphere and treatment cylinder in relation to the preoperative manifest refraction, were used for the treatment of a second patient collective (group B). Patients in a third collective were treated with the internal standard nomograms of the excimer laser without further modifications (reference group). The 3 months results were used for the outcome analysis.

RESULTS

Both methods of nomogram adjustment significantly improved the predictability of the postoperative spherical equivalent. Of the attempted spherical adjustment 82% was achieved by group A and 70% by group B. The postoperative astigmatism and spherical equivalent did not show a significant improvement due to the individual adjustments of the treatment cylinder.

CONCLUSION

A spherical modification of the treatment sphere was effective in the optimization of refractive outcomes for myopic astigmatic patients undergoing Femto-LASIK. The more demanding individual treatment adjustments with cylinder optimizations were safe but not capable of further improvement of the postoperative spherical equivalent distribution.