[Pitfalls of IOL power prediction after photorefractive keratectomy for high myopia -- case report, practical recommendations and literature review]

Calculating Intraocular Lens Geometry by Real Ray Tracing

PURPOSE

An implementation of real ray tracing based on Snell's law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs).

METHODS

The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae.

RESULTS

Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11 +/- 0.43 diopters (D), cylinder -0.18 +/- 0.52 D, and axis 5.13 degrees +/- 30.19 degrees. Pearson's correlation coefficient was sphere r = 0.92, P < .01; cylinder r = 0.79, P < .01; and axis r = 0.91, P < .01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae.

CONCLUSIONS

It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history.

Intraocular lens power calculation using ray tracing following excimer laser surgery

PURPOSE

To evaluate intraocular lens (IOL) power calculation using ray tracing in patients presenting with cataract after excimer laser surgery.

METHODS

Ten eyes of seven consecutive patients who presented for cataract surgery following excimer laser treatment without any pre-refractive biometry data were enrolled in this prospective clinical study. Preoperatively, IOL power calculation was performed using a ray tracing software called OKULIX. Keratometry data (C-Scan) were imported and axial length (IOLMaster) was entered manually. Accuracy of IOL power calculation was investigated by subtracting attempted and achieved spherical equivalent.

RESULTS

Mean spherical equivalent was -3.51+/-2.77 D (range -10.38 to -0.5 D) preoperatively and -1.01+/-1.08 D (range -2.5 to +0.75 D) postoperatively. Mean error was 0.31+/-0.84 D, mean absolute error was 0.74+/-0.46 D, and IOL calculation errors ranged from -1.39 to +1.47 D. A total of 40% of eyes were within +/-0.5 D, 70% within +/-1.0 D, and 100% within +/-1.5 D. Three eyes with corneal radii over 10 mm showed calculation errors exceeding +/-1.0 D. Mean best-corrected visual acuity increased from 20/60 to 20/30 postoperatively.

CONCLUSIONS

IOL power calculation after excimer laser surgery can be difficult, especially when pre-refractive keratometry values are not available. In these cases, ray tracing combined with corneal topography measurements provides reliable and satisfactory postoperative results. However, it is advisable to be careful when calculating IOL power for eyes with corneal radii exceeding 10 mm because of slightly higher prediction errors.

A New Formula for Intraocular Lens Power Calculation After Refractive Corneal Surgery

PURPOSE

When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed.

METHODS

A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial keratotomy [n = 5]) and compared our results to other formulas.

RESULTS

Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosa's method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D.

CONCLUSIONS

In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery

PURPOSE

To test the reliability of a corneal radius correcting factor (R factor) in calculating intraocular lens (IOL) power in eyes that developed cataract after refractive surgery and compare it with the clinical history (CHM) and double-K (DKM) methods.

SETTING

Department of Ophthalmology, Second University of Naples, Naples, Italy.

METHODS

Nineteen eyes from the literature that underwent cataract extraction and IOL implantation after refractive surgery were used to compare actual postoperative and expected refractive errors utilizing the R factor, CHM, and DKM. Intraocular lens powers were calculated with 3 formulas: SRK/T, Hoffer Q and Holladay 1. The differences were evaluated with the Wilcoxon test and Spearman correlation.

RESULTS

With the R factor SRK/T and Holladay 1 formulas gave the best results; 16 (84.2%) and 17 (89.5%) eyes were within +/-2 diopters (D) of emmetropia. With CHM, the best results were obtained using the SRK/T and Holladay 1 formulas; with both formulas 12 (63.2%) eyes were within +/-2 D of emmetropia. With DKM, the best results were obtained using SRK/T and Holladay 1 formulas; with both formulas 10 eyes (52.63%) were in the range of +/-2 D from emmetropia.

CONCLUSIONS

The R factor can be used with the SRK/T or Holladay 1 formula because this method seems comparable or superior to DKM and CHM.

Intraocular lens calculations after refractive surgery

PURPOSE

To evaluate the effect of refractive surgery on intraocular lens (IOL) power calculation, compare methods of IOL power calculation after refractive surgery, evaluate the effect of pre-refractive surgery refractive error on IOL deviation, review the literature on determining IOL power after refractive surgery, and introduce a formula for IOL calculation for use after refractive surgery for myopia.

SETTING

Laser & Corneal Surgery Associates and Center for Ocular Tear Film Disorders, New York, New York, USA.

METHODS

This retrospective noncomparative case series comprised 21 patients who had uneventful cataract extraction and IOL implantation after previous uneventful myopic refractive surgery. Six methods of IOL calculation were used: clinical history (IOL(HisK)), clinical history at the spectacle plane (IOL(HisKs)), vertex (IOL(vertex)), back-calculated (IOL(BC)), calculation based on average keratometry (IOL(avgK)), and calculation based on flattest keratometry (IOL(flatK)). Each method result was compared to an "exact" IOL (IOL(exact)) that would have resulted in emmetropia and then compared to the pre-refractive surgery manifest refraction using linear regression. The paired t test was used to determine statistical significance.

RESULTS

The IOL(HisKs) was the most accurate method for IOL calculations, with a mean deviation from emmetropia of -0.56 diopter +/-1.59 (D), followed by the IOL(BC) (+1.06 +/- 1.51 D), IOL(vertex) (+1.51 +/- 1.95 D), IOL(flatK) (-1.72 +/- 2.19 D), IOL(HisK) (-1.76 +/- 1.76 D), and IOL(avgK) (-2.32 +/- 2.36 D). There was no statistical difference between IOL(HisKs) and IOL(exact) in myopic eyes. The power of IOL(flatK) would be inaccurate by -(0.47x+0.85), where x is the pre-refractive surgery myopic SE (SEQ(m)). Thus, without adjusting IOL(flatK), most patients would be left hyperopic. However, when IOL(flatK) is adjusted with this formula, it would not be statistically different from IOL(exact).

CONCLUSIONS

For IOL power selection in previously myopic patients, a predictive formula to calculate IOL power based only on the pre-refractive surgery SEQ(m) and current flattest keratometry readings was not statistically different from IOL(exact). The IOL(HisKs), which was also not statistically different from IOL(exact), requires pre-refractive surgery keratometry readings that are often not available to the cataract surgeon.

Accuracy of intraoperative retinoscopy in corneal power and axial length estimation using a high plus soft contact lens

AIM

To assess the accuracy of intraoperative retinoscopy with high plus soft contact lens (CL) in estimating corneal power and the axial length compared with standard keratometry and biometry measurement.

METHODS

Intraoperative retinoscopy was performed in 30 eyes prior to the implantation of an intraocular lens (IOL). A +10 D disposable soft contact lens was applied on the cornea to minimize retinoscopic error. Corneal power was derived from refraction using biometric axial length while the axial length was derived using keratometric measurement. Refraction derived corneal powers and axial lengths from 26 eyes within +/-1.00 D of the target postoperative refraction were compared with preoperative standard keratometry and biometry measurements.

RESULTS

In the 26 eyes, the mean difference between the corneal powers derived from refraction and keratometry was 0.35 D (S.D.=1.678). The 95% limits of agreement were -3.006 to +3.706. The mean difference between the axial lengths derived from refraction and biometry was 0.15 mm (S.D.=0.721). The 95% limits of agreement were -1.292 to 1.542.

CONCLUSION

Intraoperative retinoscopy with a high plus soft contact lens after phacoemulsification is useful but not accurate in estimating corneal power or axial length of the eye. It should be used cautiously in IOL power calculation as a substitute for standard keratometry or biometry machines when either of these is not available or in error.

Difficult lens power calculations

PURPOSE OF REVIEW

Although cataract extraction seems to be feasible without major technical obstacles, the surgical technique has changed completely, and patients are no longer satisfied with good spectacle-corrected vision but anticipate complete visual rehabilitation after cataract surgery, without correction. To fulfill this desire, toric or accommodative intraocular lenses are of increasing popularity, and the intraocular lens power calculation after keratorefractive surgery has been improved.

RECENT FINDINGS

In this review article, we provide an overview of different mathematical strategies of calculating the intraocular lens power with standard formulas and with new algorithms, such as paraxial or numeric ray-tracing. These enhanced techniques may improve the validity of lens power calculation due to reduction of the prediction error, especially in cases with high or excessive corneal astigmatism and after refractive laser surgery. Furthermore, a new calculation scheme for the determination of bitoric eikonic intraocular lenses allows a distortion-free imaging in astigmatic eyes. The biometric determinants for the different formulas and calculation schemes are discussed in detail.

SUMMARY

In difficult cases, standard calculation schemes are overemployed and new mathematical algorithms are necessary to adequately address these problems. Ray-tracing algorithms and other complex mathematical computation schemes are of increasing interest and will more and more replace conventional calculation formulas for determination of intraocular lens power.