Accuracy comparison of tomography devices for ray tracing-based intraocular lens calculation

PURPOSE

To evaluate the interchangeability of different tomography devices used for ray tracing-based intraocular lens (IOL) calculation.

SETTING

Eye clinic, Castrop-Rauxel, Germany.

DESIGN

Retrospective analysis.

METHOD

Measurements from 3 Placido-Scheimpflug devices and 3 optical coherence tomography (OCT) devices were compared in 83 and 161 other eyes after cataract surgery, respectively. 2-dimensional matrices of anterior local corneal curvature and local corneal thickness are transferred to the ray-tracing software OKULIX. Calculations are performed with the same IOL in the same position of an eye with the same axial length. Differences in spherical equivalent (SE), astigmatism, and spherical aberration are evaluated. Furthermore, the influence of the size of the matrices (optical zone) on the accuracy is quantified.

RESULTS

For the Placido-Scheimpflug devices, the deviations from the average of three measurements taken for each eye in SE (mean ± SD) were 0.17 ± 0.24 diopters (D), -0.26 ± 0.29 D, and 0.08 ± 0.39 D ( P < .001, analysis of variance [ANOVA]), for the centroids of the astigmatic differences 0.04 D/173 degrees, 0.14 D/93 degrees, and 0.10 D/7 degrees, and for the median of the absolute values of the vector differences 0.31 D, 0.33 D, and 0.29 D. For OCT devices, the corresponding results were 0.01 ± 0.21 D, -0.03 ± 0.21 D, and 0.02 ± 0.20 D ( P = .005, ANOVA); 0.18 D/120 degrees, 0.07 D/70 degrees, and 0.22 D/4 degrees; and 0.26 D, 0.30 D, and 0.33 D. The accuracy of the calculated spherical aberrations allows for an individual selection of the best fitting IOL model in most cases.

CONCLUSIONS

The differences are small enough to make the devices interchangeable regarding astigmatism and spherical aberration. Although there are significant differences in SE between Scheimpflug and OCT devices, the differences between OCT devices are also small enough to make them interchangeable, but the differences between Placido-Scheimpflug devices are too large to make these devices interchangeable.

IOL Power Calculations and Cataract Surgery in Eyes with Previous Small Incision Lenticule Extraction

Small incision lenticule extraction (SMILE), with over 5 million procedures globally performed, will challenge ophthalmologists in the foreseeable future with accurate intraocular lens power calculations in an ageing population. After more than one decade since the introduction of SMILE, only one case report of cataract surgery with IOL implantation after SMILE is present in the peer-reviewed literature. Hence, the scope of the present multicenter study was to compare the IOL power calculation accuracy in post-SMILE eyes between ray tracing and a range of empirically optimized formulae available in the ASCRS post-keratorefractive surgery IOL power online calculator. In our study of 11 post-SMILE eyes undergoing cataract surgery, ray tracing showed the smallest mean absolute error (0.40 D) and yielded the largest percentage of eyes within ±0.50/±1.00 D (82/91%). The next best conventional formula was the Potvin–Hill formula with a mean absolute error of 0.66 D and an ±0.50/±1.00 D accuracy of 45 and 73%, respectively. Analyzing this first cohort of post-SMILE eyes undergoing cataract surgery and IOL implantation, ray tracing showed superior predictability in IOL power calculation over empirically optimized IOL power calculation formulae that were originally intended for use after Excimer-based keratorefractive procedures.

Ray tracing biometry in post radial keratotomy eye

OBJECTIVE

To report a case of post radial keratotomy (RK) cataract in a 55-year-old lady wherein biometry was done by ray-tracing method incorporated in scheimpflug topographer (Sirius + Scheimpflug Analyzer, CSO, Italy).

METHOD

In our case, we performed intraocular lens (IOL) power calculation using a recent concept of ray tracing with scheimpflug topographer and compared with traditional methods available at American Society of Cataract and Refractive Surgery(ASCRS) website (www.ascrs.org) for eyes with prior RK. Phacoemulsification was performed and a monofocal + 24.5D IOL implanted in the capsular bag.

RESULT

Manifest refraction at six weeks postoperative period was + 1.0DS/-2.0DC × 75° with spherical equivalence of 0. On comparison of all the methods used to calculate IOL power, the absolute errors of ray tracing and Barrett true K were found to be the least, 0.14 and 0.18 respectively.

CONCLUSION

Ray tracing biometry with scheimpflug topographer seems to provide accurate IOL power in post RK eyes.

Predictability of intraocular lens power calculation after small-incision lenticule extraction for myopia

PURPOSE

To evaluate and compare the predictability of intraocular lens (IOL) power calculation after small-incision lenticule extraction (SMILE) for myopia and myopic astigmatism.

SETTING

Department of Ophthalmology, Philipps University of Marburg, Marburg, Germany.

DESIGN

Retrospective comparative case series.

METHODS

Preoperative evaluation included optical biometry using IOLMaster 500 and corneal tomography using Pentacam HR. The corneal tomography measurements were repeated at 3 months postoperatively. The change of spherical equivalent due to SMILE was calculated by the manifest refraction at corneal plane (SMILE-Dif). A theoretical model, involving the virtual implantation of the same IOL before and after SMILE, was used, and the IOL power calculations were performed using ray tracing (OKULIX, version 9.06) and third- (Hoffer Q, Holladay 1, and SRK/T) and fourth-generation (Haigis-L and Haigis) formulas. The difference between the IOL-induced refractive error at corneal plane before and after SMILE (IOL-Dif) was compared with SMILE-Dif. The prediction error (PE) was calculated as the difference between SMILE-Dif-IOL-Dif.

RESULTS

The study included 204 eyes that underwent SMILE. The PE with ray tracing was -0.06 ± 0.40 diopter (D); Haigis-L, -0.39 ± 0.62 D; Haigis, 0.70 ± 0.48 D; Hoffer Q, 0.84 ± 0.47 D; Holladay 1, 1.21 ± 0.51 D; and SRK/T, 1.46 ± 0.54 D. The PE with ray tracing was significantly smaller compared with that of all formulas (P ≤ .001). The PE variance with ray tracing was σ2 = 0.159, being significantly more homogenous compared with that of all formulas (P ≤ .011, F ≥ 6.549). Ray tracing resulted in an absolute PE of 0.5 D or lesser in 81.9% of the cases, followed by Haigis-L (53.4%), Haigis (35.3%), Hoffer Q (25.5%), Holladay 1 (6.4%), and SRK/T (2.9%) formulas.

CONCLUSIONS

Ray tracing was the most accurate approach for IOL power calculation after myopic SMILE.

Project hyperopic power prediction: accuracy of 13 different concepts for intraocular lens calculation in short eyes

PURPOSE

To evaluate the accuracy of intraocular lens (IOL) power calculation in a patient cohort with short axial eye length to assess the performance of IOL power calculation schemes in strong hyperopes.

METHODOLOGY

The study was a single centre, single surgeon retrospective consecutive case series at the Augen- und Laserklinik, Castrop-Rauxel, Germany. Inclusion of patients after uneventful cataract surgery implanting either spherical (SA60AT) or aspheric (ZCB00) IOLs. Inclusion criteria were axial eye length <21.5 mm and/or emmetropising IOL power >28.5 D. Lens constants were optimised on a separate patient cohort considering the full bandwidth of axial eye length. Data of one single eye per patient were randomly included. The outcome measures were: mean absolute prediction error (MAE), median absolute prediction error, mean prediction error with SD and median prediction error and the percentage of eyes with an MAE within 0.25 D, 0.5 D, 0.75 D and 1.0 D.

RESULTS

A total of 150 eyes from 150 patients were assessed. Okulix, PEARL-DGS, Kane and Castrop provided a statistically significantly smaller MAE compared with the Hoffer Q and SRK/T formulae.

CONCLUSION

In our patient cohort with short axial eye length, the use of PEARL-DGS, Okulix, Kane or Castrop formulae showed the lowest MAE. The Castrop formula has not been published before, but will be disclosed with a ready-to-use Excel sheet as an addendum to this paper.

Intraocular Lens Power Calculation after Small Incision Lenticule Extraction

With more than 1.5 million Small Incision Lenticule Extraction (SMILE) procedures having already been performed worldwide in an ageing population, intraocular lens (IOL) power calculation in post-SMILE eyes will inevitably become a common challenge for ophthalmologists. Since no refractive outcomes of cataract surgery following SMILE have been published, there is a lack of empirical data for optimizing IOL power calculation. Using the ray tracing as the standard of reference - a purely physical method that obviates the need for any empirical optimization - we analyzed the agreement of various IOL power calculation formulas derived from the American Society of Cataract and Refractive Surgeons (ASCRS) post-keratorefractive surgery online calculator. In our study of 88 post-SMILE eyes, the Masket formula showed the smallest mean prediction error [-0.36 ± 0.32 diopters (D)] and median absolute error (0.33D) and yielded the largest percentage of eyes within ±0.50D (70%) in reference to ray tracing. Non-inferior refractive prediction errors and ±0.50D accuracies were achieved by the Barrett True K, Barrett True K No History and the Potvin-Hill formula. Use of these formulas in conjunction with ray tracing is recommended until sufficient data for empirical optimization of IOL power calculation after SMILE is available.

Suture Removal After Trabeculectomy With Fornix-based Conjunctival Flap Leads to Faster Visual Recovery but Not Reduced Astigmatism

PRéCIS:: The closing limbal suture after trabeculectomy with a fornix-based conjunctival flap plays no critical role in the development of corneal astigmatism and intraocular pressure (IOP). A standard removal is not recommended.

PURPOSE

To investigate the effect of removal of the conjunctival suture after trabeculectomy with fornix-based conjunctival flap on corneal astigmatism, visual acuity, and IOP.

METHODS

Eighty-seven cases of trabeculectomy with mitomycin C with a fornix-based conjunctival flap performed in the eyes of 82 patients (5 patients underwent bilateral trabeculectomy) were enrolled in a prospective randomized study. All surgeries were conducted by the same surgeon (J.W.) in the Ophthalmology Department of the University Medical Center of Mainz, Germany. All eyes received a corneal-conjunctival, continuous, mattress, interlocked suture for closing the conjunctiva at the limbus. After randomization, in 46 cases the suture was removed 6 weeks postoperatively; in 41 patients, the suture was left in place. All patients were examined preoperatively, and at 6 weeks, 3 months, 6 months, and 12 months after surgery. Astigmatism was measured using objective refraction and corneal topography, IOP and visual acuity were also assessed. Results were compared using a Wilcoxon test or Mann-Whitney U test for single time-points.

RESULTS

During follow-up, no significant differences between the 2 study groups regarding refractive or topographic values were found. Patients in the suture removal group had a significantly higher visual acuity than controls at 3 months, 6 months, and 1 year after surgery. IOP was similar in both groups throughout the study.

CONCLUSIONS

Removal of the conjunctival suture in trabeculectomy with a fornix-based conjunctival flap leads to a faster rehabilitation of visual acuity but does not significantly affect corneal astigmatism or IOP.

Accuracy of minus power intraocular lens calculation using OKULIX ray tracing software

PURPOSE

The purpose of this retrospective study was to assess the accuracy of minus power intraocular lens calculation using partial coherence interferometry and OKULIX ray tracing software.

METHODS

We included 25 consecutive, myopic eyes with axial length ≥ 30 mm (25 patients, 13 males and 12 females, and 57.6 ± 10.3 years old), which underwent phacoemulsification and implantation of a minus power intraocular lens in the capsular bag. Axial length measurement and corneal topography were performed using the OA-1000 optical biometer and Topographic Modeling System TMS-5, respectively. The IOL power was calculated using SRK/T formula and OKULIX ray tracing software. The implanted IOL power was chosen based on OKULIX ray tracing software calculation aiming for - 2 diopters (D) of myopia.

RESULTS

SRK/T calculated IOL power (- 6.3 ± 2.8 D) showed statistically significant difference compared to OKULIX calculated IOL power (- 4.7 ± 2.6 D), r_s 0.994 p < 0.001. The expected refraction with implanted IOL was - 1.7 ± 0.9 D based on OKULIX ray tracing software calculation. A statistically significant difference was reported between implanted IOL and OKULIX calculated IOL power (2.7 ± 1.4 D), r_s 0.981 p < 0.001. A statistically significant difference was reported between the expected refraction with implanted IOL and the achieved spherical refraction at 1 month postoperatively (1.4 ± 0.7 D), r_s 0.77 p < 0.001. The achieved spherical refraction at 1 month postoperatively was 0.2 ± 0.2 D.

CONCLUSIONS

Although OKULIX ray tracing software yielded more accurate minus power intraocular lens calculation in extreme myopia, compared to SRK/T formula, yet it still shows tendency toward hyperopia.

Full 3-D OCT-based pseudophakic custom computer eye model

We compared measured wave aberrations in pseudophakic eyes implanted with aspheric intraocular lenses (IOLs) with simulated aberrations from numerical ray tracing on customized computer eye models, built using quantitative 3-D OCT-based patient-specific ocular geometry. Experimental and simulated aberrations show high correlation (R = 0.93; p<0.0001) and similarity (RMS for high order aberrations discrepancies within 23.58%). This study shows that full OCT-based pseudophakic custom computer eye models allow understanding the relative contribution of optical geometrical and surgically-related factors to image quality, and are an excellent tool for characterizing and improving cataract surgery.

C constant: new concept for ray tracing-assisted intraocular lens power calculation

PURPOSE

To evaluate the accuracy of the C constant for ray tracing-assisted intraocular lens (IOL) power calculation.

DESIGN

Case series.

SETTING

Public university hospital and private clinic.

METHODS

Preoperatively, all intraocular distances were measured using laser biometry. Various IOL designs were studied; powers ranged from -5.0 diopters (D) to +38.0 D. The IOL power calculation was performed with the Olsen formula using the C constant and compared with the Haigis, Hoffer Q, Holladay 1, and the SRK/T formulas on optimized datasets. Outcome measures were the error of the prediction, expressed as the arithmetic error, and the absolute error between the observed refraction and the predicted refraction.

RESULTS

Two thousand forty-three cases from the 2 centers were studied. No significant differences were found between the Haigis, Hoffer Q, Holladay 1, and SRK/T formulas with the exception of the SRK/T formula, which performed better than the other thin-lens formulas in eyes with an axial length (AL) greater than 27.0 mm (P<.01). Compared with the SRK/T formula, the Olsen formula showed an improvement of 15% and 14% in the mean absolute error and a 39% and 85% reduction in the number of large errors (>1.0 D) for the 2 series, respectively (P<.0001). Contrary to the Olsen formula, all thin-lens formulas showed a significant bias in terms of the AL, keratometry reading, and anterior segment length (P<.0001).

CONCLUSION

The C constant is a promising concept for ray tracing-assisted IOL power calculation.

Intraocular lens power calculation following laser refractive surgery

Refractive outcomes following cataract surgery in patients that have previously undergone laser refractive surgery have traditionally been underwhelming. This is related to several key issues including the preoperative assessment (keratometry) and intraocular lens power calculations. Peer-reviewed literature is overwhelmed by the influx of methodology to manipulate the corneal or intraocular lens (IOL) powers following refractive surgery. This would suggest that the optimal derivative formula has yet been introduced. This review discusses the problems facing surgeons approaching IOL calculations in these post-refractive laser patients, the existing formulae and programs to address these concerns. Prior published outcomes will be reviewed.

Prediction of residual astigmatism after cataract surgery using swept source fourier domain optical coherence tomography

PURPOSE

To compare corneal measurements obtained by a swept source fourier domain OCT (CASIA SS-1000), an autokeratometer (Haag-Streit Lenstar), a hybrid topographer (Tomey TMS-5), a Placido topographer (Tomey TMS-5 in Placido mode) and a Scheimpflug tomographer (Oculus Pentacam) to manifest subjective refraction.

METHODS

One hundred and four pseudophacic patients with non-toric IOLs were measured at least 6 months after surgery. Corneal astigmatism as measured on the anterior corneal surface as well as total corneal astigmatism including posterior surface data was compared to manifest refractive cylinder (cross-cylinder strategy) by computing difference vectors and correlation analysis of power vectors.

RESULTS

The OCT (0.43 ± 0.25 D) and the hybrid topographer (0.44 ± 0.25 D) yielded the smallest difference vector to subjective cylinder and by far the lowest percentage of outliers >0.75 D (≈10%). The rotating Scheimpflug camera showed the largest (0.70 ± 0.41 D) difference vector. The best predictive precision (0.37 ± 0.22) could be achieved by vector averaging Lenstar keratometry and OCT.

CONCLUSIONS

Autokeratometry yielded the least measuring noise but OCT as well as hybrid topography had better predictive precision due to posterior curvature data. Scheimpflug tomography suffered from high measuring noise. Combination of keratometry and OCT data yielded the best precision for planning of toric IOL implantation. To get a reliable target cylinder for TIOL calculation, accuracy of the measuring device is crucial. Keratometry and Placido topography lack the information of the posterior corneal curvature while Scheimpflug devices suffer from higher measuring noise. In this paper, a combination of ssOCT with autokeratometry yielded the best predictive quality.

A ray tracing approach to calculate toric intraocular lenses

PURPOSE

To quantify the precision of astigmatic correction in routine cataract surgery with toric intraocular lenses (IOLs) and to evaluate the predictability of keratometric and anterior/posterior topographic measurement for the improvement of the overall accuracy.

METHODS

Seventy-eight eyes of 56 patients were implanted with toric IOLs. Data acquired by the Lenstar optical biometer (Haag-Streit, Bern, Switzerland) and TMS5 topography (Tomey, Nagoya, Japan) were processed with the ray tracing software Okulix (Tedics, Dortmund, Germany) to predict the residual refraction. Four different inputs were examined: keratometry only, anterior topography, anterior and posterior topography/ tomography, and combination of keratometry only and anterior and posterior topography/tomography. Four weeks postoperatively, the spherical prediction error and the cylindrical prediction error (difference vector between predicted and achieved cylindrical refraction) were determined.

RESULTS

Mean absolute error of spherical prediction error was 0.27 diopter (D). Cylindrical prediction errors were 0.57 D (keratometry only), 0.56 D (anterior topography), 0.56 D (anterior and posterior topography/ tomography), and 0.50 D (combination of keratometry only and anterior and posterior topography/tomography). Differences between intraocular lens groups were statistically significant (Friedman test, P < .05).

CONCLUSION

The combination of keratometry and anterior and posterior topography/tomography of anterior and posterior surface yielded the best results for toric IOL power calculations.