Purpose To identify and quantify sources of error in the refractive outcome of cataract surgery. Setting AMO Groningen BV, Groningen, The Netherlands. Methods Means and standard deviations (SDs) of parameters that influence refractive outcomes were taken or derived from the published literature to the extent available. To evaluate their influence on refraction, thick-lens ray tracing that allowed for asphericity was used. The numerical partial derivative of each parameter with respect to spectacle refraction was calculated. The product of the partial derivative and the SD for a parameter equates to its SD, expressed as spectacle diopters, which squared is the variance. The error contribution of a parameter is its variance relative to the sum of the variances of all parameters. Results Preoperative estimation of postoperative intraocular lens (IOL) position, postoperative refraction determination, and preoperative axial length (AL) measurement were the largest contributors of error (35%, 27%, and 17%, respectively), with a mean absolute error (MAE) of 0.6 diopter (D) for an eye of average dimensions. Pupil size variation in the population accounted for 8% of the error, and variability in IOL power, 1%. Conclusions Improvement in refractive outcome requires better methods for predicting the postoperative IOL position. Measuring AL by partial coherence interferometry may be of benefit. Autorefraction increases precision in outcome measurement. Reducing these 3 major error sources with means available today reduces the MAE to 0.4 D. Using IOLs that compensate for the spherical aberration of the cornea would eliminate the influence of pupil size. Further improvement would require measuring the asphericity of the anterior surface and radius of the posterior surface of the cornea.

Sources of error in intraocular lens power calculation.

This review describes the principles and practices involved in the calculation of intraocular lens (IOL) power The theories behind formulas for calculating IOL power are described, using regression and optical methods employing 'thin lens' and 'thick lens' models, as well as exact ray-tracing methods Numerical examples are included to illustrate the points made The paper emphasizes the importance of establishing an accurate estimation of corneal power as well as an accurate technique for the measurement of axial length and accurate methods of predicting postoperative anterior chamber depth (ACD) It is concluded that current improvements in diagnostic and surgical technology, combined with the latest generation IOL power formulas, make the calculation and selection of appropriate IOL power among the most effective tools in refractive surgery today

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-0420.2007.00879.x

Calculation of intraocular lens power: a review.

Purpose To present and analyze biometry data sets and prevalence data for corneal astigmatism in a large population. Setting High-volume eye surgery center, Castrop-Rauxel, Germany. Methods Axial length (AL), corneal radii, anterior chamber depth (ACD), and horizontal corneal diameter (white-to-white [WTW] distance) were optically measured by partial coherence interferometry (IOLMaster). Patient data sets acquired between 2000 and 2006 were reviewed and analyzed. Results The study evaluated 23 239 data sets of 15 448 patients with a median age of 74 years. The mean values were as follows: AL, 23.43 mm ± 1.51 (SD); corneal radius, 7.69 ± 0.28 mm; WTW distance, 11.82 ± 0.40 mm; and ACD, 3.11 ± 0.43 mm. The ACD and axis of astigmatism were correlated with age. The AL, corneal radius, ACD, and WTW were correlated with one other. Eight percent of eyes had corneal astigmatism greater than 2.00 diopters (D), and 2.6% had more than 3.00 D. Astigmatism was with the rule (WTW) in 46.8% of eyes, against the rule in 34.4%, and oblique in 18.9%. High astigmatism was predominantly WTW. Conclusions The results in this analysis might provide normative data for cataract patients and a useful reference for multiple purposes. The correlation of AL with corneal radius, ACD, and corneal diameter in normal eyes was not present in eyes with extreme myopia or hyperopia. Financial Disclosure Neither author has a financial or proprietary interest in any material or method mentioned.

Analysis of biometry and prevalence data for corneal astigmatism in 23,239 eyes.

Accuracy of Intraocular Lens Calculation Formulas

• PURPOSE: To compare biometry performed by an enhanced version of dual beam partial coherence interferometry and applanation ultrasound in a prospective study of 85 cataract eyes to improve refractive outcome of cataract surgery due to a more accurate calculation of intraocular lens power. • METHODS: The SRK II formula using ultrasound biometry data was employed. Three months after surgery, partial coherence interferometry biometry was repeated and refractive outcome was determined. Preoperative partial coherence interferometry biometry data were used to determine the refractive power of the intraocular lenses retrospectively and to calculate the possible refractive outcome. • RESULT: Precision of partial coherence interferometry biometry was more than 10 times better than that of ultrasound. Therefore, the possible mean absolute error for postoperative refraction achieved with partial coherence interferometry biometry was 0.49 diopters (compared with 0.67 diopters with ultrasound biometry), resulting in an improvement of 27%. Axial eye length measured with the two techniques differed by a mean of 460 μm. The difference in lens thickness measured with partial coherence interferometry and ultra-sound significantly correlated with cataract grade. A mean shortening of 120 μm of axial eye length following cataract surgery was also detected by partial coherence interferometry. • CONCLUSIONS: The enhanced version of partial coherence interferometry offers biometry with un, precedented precision (<10 μm) and resolution (~12 μm), therefore improving the refractive outcome in cataract surgery. This noninvasive technique provides a high degree of comfort for the patient, with no need for local anesthesia or pupil dilation and minimized risk of corneal infection.

Partial Coherence Interferometry: A Novel Approach to Biometry in Cataract Surgery

Background: The precision of intraocular lens (IOL) calculation is essentially determined by the accuracy of the measurement of axial length. In addition to classical ultrasound biometry, partial coherence interferometry serves as a new optical method for axial length determination. A functional prototype from Carl Zeiss Jena implementing this principle was compared with immersion ultrasound biometry in our laboratory. Patients and methods: In 108 patients attending the biometry laboratory for planning of cataract surgery, axial lengths were additionally measured optically. Whereas surgical decisions were based on ultrasound data, we used postoperative refraction measurements to calculate retrospectively what results would have been obtained if optical axial length data had been used for IOL calculation. For the translation of optical to geometrical lengths, five different conversion formulas were used, among them the relation which is built into the Zeiss IOLMaster. IOL calculation was carried out according to Haigis with and without optimization of constants. Results: On the basis of ultrasound immersion data from our Grieshaber Biometric System (GBS), postoperative refraction after implantation of a Rayner IOL type 755U was predicted correctly within ±1 D in 85.7% and within ±2 D in 99% of all cases. An analogous result was achieved with optical axial length data after suitable transformation of optical path lengths into geometrical distances. Conclusions: Partial coherence interferometry is a non- contact, user- and patient-friendly method for axial length determination and IOL planning with an accuracy comparable to that of high-precision immersion ultrasound.

Nicole Miller

Papers

Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis