Thomas Olsen

Sources of error in intraocular lens power calculation.

ABSTRACT The hypothesis that the minimum error in predicted refraction after implantation of an intraocular lens (IOL) of calculated power is the sum of the random error in (1) the measurement of the axial length, (2) the measurement of the corneal power, and (3) the estimation of the psuedophakic anterior chamber depth (ACD) is proposed. Based on preoperative and postoperative biometry of 584 IOL implantations, 54% of the error was attributed to axial length errors, 8% to corneal power errors, and 38% to errors in the estimation of the postoperative ACD, when a fired ACD was used in the IOL calculations. However, if the ACD was predicted according to a previously described regression method, the contribution of error from the ACD source was reduced to 22%, thereby reducing the total refractive prediction error from ±1.03 diopters (D) (±SD) to ±0.92 D (±SD). These predictions accord with clinical results.

Intraocular lens power calculation with an improved anterior chamber depth prediction algorithm

Abstract The accuracy of intraocular lens (IOL) power calculation was evaluated in a multicenter study of 822 IOL implantations using the Binkhorst II, Sanders/ Retzlaff/Kraff (SRK I™, SRK II™, SRK/T™), Holladay, and Olsen formulas. All but the first of these were optimized in retrospect with calculation of the SRK A‐constant, the Holladay surgeon factor, and the Olsen pseudophakic anterior chamber depth (ACD) for each lens style. The ACD prediction of the Olsen formula was based on a previously described regression formula incorporating preoperative ACD, corneal height, axial length, and lens thickness. Among the optical IOL power calculation formulas, the highest IOL power prediction error was found with Binkhorst’s and the lowest with Olsen’s, which was more accurate than the SRK/T and the Holladay formulas (P < .05). The SRK/T formula was significantly more accurate than the original SRK regression formulas (P < .001). When analyzed for axial length dependence, all formulas showed the least error in the normal range. Error of the Olsen formula was lower than that of the others in the axial length interval 20 mm to 26 mm. No differences in accuracy were found between the optical IOL calculation formulas in eyes with an axial length above 26 mm (P < .05). The accuracy of IOL power calculation can be improved with optical formulas using newer‐generation ACD‐prediction algorithms.

Prediction of the effective postoperative (intraocular lens) anterior chamber depth

PURPOSE: To investigate methods to predict the effective postoperative anterior chamber depth (ACD) based on a large patient sample. SETTING: University Eye Clinic, Aarhus Kommunehospital, Aarhus, Denmark. METHODS: Based on 6698 consecutive cataract operations with recorded postoperative refractive results, the postoperative effective ACD was calculated in each case and studied by multiple linear regression for covariance with a number of preoperatively defined variables including the axial length by ultrasonography, preoperative ACD, lens thickness, corneal radius by keratometry, subjective refraction, patient age, and corneal white‐to‐white diameter, the latter of which was available in a subgroup of 900 cases. RESULTS: The postoperative effective ACD was significantly correlated with 6 preoperative variables (in decreasing order): axial length, preoperative ACD, keratometry reading, lens thickness, refraction, and patient age (R = 0.49, P<.000001). Age showed the weakest correlation (P = .02) and could be omitted with no significant decrease in the total correlation coefficient. Using the 5 most significant variables, the ACD could be predicted according to a regression formula with an accuracy of 82.1% of the predictions within 0.5 mm. When this ACD algorithm was used in retrospect in the intraocular lens (IOL) power calculation, the refractive prediction error decreased by 10% from the error associated with a previously published 4‐variable algorithm and decreased 28% from the error using no individual ACD method other than the average ACD (P<.00001). CONCLUSIONS: The postoperative ACD was significantly correlated with and hence predictable by a 5‐variable regression method incorporating the preoperative axial length, ACD, keratometry reading, lens thickness, and refraction as the most significant variables. The statistical relationship can be used to create a new ACD prediction algorithm to incorporate in a modern “thick lens” IOL power calculation formula with significant improvement in the accuracy of the refractive predictions as a result.

Theoretical approach to intraocular lens calculation using Gaussian optics

ABSTRACT A computer‐assisted method of calculating intraocular lens (IOL) power using formulas according to Gaussian optics is described. The method is more exact than current theoretical formulas, especially in calculating corneal power and in dealing with principal planes, and tends to give somewhat higher values for IOL power. One advantage of the theoretical approach is that the eye can be analyzed optically in a conventional manner, not only for the IOL power of emmetropia but also for the power of the biological lens, the total refractive power of the eye, and the magnification of the entire system. The relation to other theoretical formulas is discussed.

Accuracy of the newer generation intraocular lens power calculation formulas in long and short eyes.

ABSTRACT The accuracy of two newer generation theoretical intraocular lens (IOL) power calculation formulas and of the empirical SRK I and II formulas was evaluated in a series of 500 IOL implantations including a series of unusually long and short eyes. The prediction error of the theoretical formulas was found to be largely unaffected by the variation in axial length and corneal power, while the prediction of the SRK I formula was less accurate in the short and long eyes. The prediction of the SRK II formula was more accurate than the SRK I in that no systematic of Tset error with axial length could be demonstrated. However, because of a relatively larger scatter in the long eyes and a significant bias with the corneal power, the absolute error of the SRK II formula was higher than that of the theoretical formulas in the long eyes. The higher accuracy of the newer generation theoretical formulas was attributed to their improved prediction of the pseudophakic anterior chamber depth.

Contrast sensitivity as a function of focus in patients with the diffractive multifocal intraocular lens

ABSTRACT Contrast sensitivity as a function of focus and visual acuity as a function of contrast were investigated in 19 patients with a diffractive multifocal intraocular lens and compared with 19 control patients with a conventional monofocal implant. The contrast sensitivity of the multifocal patients followed a bimodal curve with a maximum sensitivity at the far focus and a second peak at the near focus, corresponding to about +3 diopters in the spectacle plane. The maximum sensitivity of the multifocal group was 0.14 log units lower than the control group (PÅ.05). In the near region, the contrast sensitivity of the multifocal patients exceeded that of the control group from + 2 diopters and inward. No difference in distance visual acuity was found with high contrast letters. With intermediate contrast letters, the visual acuity of the multifocal patients was lower than that of the control group (PÅ.05).

Corneal versus scleral tunnel incision in cataract surgery: A randomized study

Purpose: To compare the induced regular and irregular astigmatism after scleral and corneal tunnel incision. Setting: University hospital outpatient cataract clinic. Methods: One hundred phacoemulsification patients with less than 1.0 diopter (D) of preoperative astigmatism were randomly assigned to have a clear corneal incision (50 patients) or a scleral tunnel incision (50 patients). All incisions were 3.5 to 4.0 mm wide and were made in the steepest axis of the corneal astigmatism. The surgically induced astigmatism was analyzed by vector analysis from keratometric data, as well as by Fourier harmonic series analysis of the topographic data. Results: One day after surgery, the surgically induced astigmatism (vector analysis, keratometry) was 1.41 D ± 0.66 (SD) and 0.55 ± 0.31 D in the corneal incision group and the scleral incision group, respectively (P < .01). Six months after surgery, the induced astigmatism was 0.72 ± 0.35 D and 0.36 ± 0.21 D in the two groups, respectively (P < .01). The corneal topography data confirmed the regular astigmatism changes found by conventional keratometry. However, in addition, Fourier harmonic series analysis of the topography data showed significantly more irregular induced astigmatism with the corneal approach than with the scleral approach. Conclusion: The clear corneal incision induces significantly more regular as well as irregular astigmatism than the scleral tunnel incision.

Calibration of axial length measurements with the Zeiss IOLMaster.

Purpose: To study the conditions for consistent axial length measurements with partial coherence interferometry (PCI) performed with the Zeiss IOLMaster. Setting: University Eye Clinic, Aarhus, Denmark. Methods: A consecutive, unselected series of 1289 cataractous eyes were measured with the optical technique of PCI according to the IOLMaster as well as with conventional (contact) A‐scan ultrasound (US) for the measurement of axial length. For each PCI reading, the signal‐to‐noise ratio (SNR) was recorded and used for comparison with the US measurement. All patients had routine phacoemulsification with implantation of an intraocular lens (IOL). In 284 cases, the patients were reexamined 2 to 3 months after surgery and the axial length was again measured using PCI. The readings of the IOLMaster, which had been calibrated against immersion US from the manufacturer, were recalculated to represent the true optical length and used in the analysis of the consistency of the measurements. Results: Not all readings obtainable with the IOLMaster were of good quality, and large differences with conventional US were found. The error between US and PCI decreased significantly with increasing SNR, showing a minimum error at an SNR value above 2.1. The SNR correlated significantly with the visual acuity with considerable scatter, however. Excluding readings with a poor quality (SNR <2.1), the postoperative PCI measurements showed a high correlation with the preoperative measurement (r = 0.99), showing a mean difference of 0.08 mm ± 0.12 (SD). The difference was highly significantly different than zero (P<.001) and may be explained by a higher refractive index of the biological lens than assumed in the original calibration of the IOLMaster. Conclusions: The quality of the axial length readings of the IOLMaster was influenced by the SNR value. However, with proper SNR evaluation and recalibration of the PCI measurements, it is possible to achieve consistent PCI readings with little variation between preoperative and postoperative measurements. These results are promising for a higher accuracy of the IOL power calculation.

Intraocular lens calculation accuracy limits in normal eyes

PURPOSE: To quantify the current accuracy limits, analyze the residual errors, and propose the next steps for prediction accuracy improvements. SETTING: Eye hospitals in Germany, Denmark, and Austria. METHOD: Numerical ray tracing using manufacturers intraocular lens (IOL) data (vertex radii, central thickness, refractive index) was used for all calculations. Postoperative lens position was predicted by a simple scaling model based on measurements in 1 patient collective. The model was compared with 2 other approaches in 2 patient collectives at 2 hospitals (1121 eyes with 13 IOL models; 936 eyes with 2 models). Axial lengths were measured optically (IOLMaster, Zeiss). No parameter adjustments or individualization of IOL types or of surgeons/localizations were done. The prediction errors and measures of systematic bias for short or long eyes were used to quantify the outcome. RESULTS: The mean prediction errors in the 2 collectives were +0.13 diopter (D) and −0.13 D and the mean absolute errors were 0.44 D and 0.50 D without bias for long or short eyes, but depending on the IOL position model approach. The differences in the mean prediction errors for the IOL types were below the allowed manufacturing tolerances and below human recognition thresholds. CONCLUSIONS: The need to individualize and fudge parameters decreases with better physical models of the pseudophakic eye. Further improvements are possible by individual topography to extract corneal asphericity and measured pupil size to calculate the best focus, by improved position predictions based on individual measurements of the crystalline lens and by smaller tolerances for IOL manufacturing.

On the optical measurement of corneal thickness. II. The measuring conditions and sources of error.

The optical measurement of corneal thickness based on oblique viewing of the optical section of the cornea is complicated by the finite width of the incident slit beam. In this report the theoretical and practical aspects of the effect of the slit width on the thickness reading are analysed. In practice, it was not possible to make slit‐width independent thickness readings which were reproducible from one observer to another. In addition, the observed slit‐width error was found to vary from one patient to another. The lack of a reproducible estimate of the corneal thickness is attributed to difficulties associated with an exact definition of the edges of the visible bands of the optical section, which are determined by biological properties of the cornea as well as perceptive properties of the observer.