Optimization of biometry for intraocular lens implantation using the Zeiss IOLMaster

Abstract

ABSTRACT.Purpose: To compare the accuracy of biometry using conventional A-scan ultrasonography and partial coherence interferometry, and to improve the accuracy of biometry by sequential audit of postoperative refractive error. Methods: The study was performed in three phases. In phase 1, 20 consecutive patients undergoing routine phacoemulsification underwent biometry using both A-scan ultrasonography and the Zeiss IOLMaster (ZIOLM). A single experienced optometrist refracted all patients 2 weeks after surgery. The errors between expected and achieved refraction were calculated and compared between the two methods. In phases 2 and 3, a further 22 and 20 patients, respectively, were recruited and only the ZIOLM was used for biometry. The manufacturer's suggested A-constant was refined and the error between expected and achieved refraction was calculated. Results: In phase 1, the median unexpected error for the ZIOLM was +0.63 (interquartile range +0.368 to +1.015) and for A-scan biometry was -0.24 (interquartile range -1.335 to +0.802). In phase 1 65% of patients' postoperative refractions were found to be within 1.0 D of emmetropia using the ZIOLM (55% using A-scan biometry). Refinements to the A-constant improved this to 95% by phase 3. Conclusion: An error was identified in IOL power estimation with the ZIOLM, when using the manufacturer's recommended A-constant (recommended and previously optimized ultrasound A-constant 118.0; ZIOLM optimized A-constant 118.6). Serial modifications to the A-constant were successful in reducing the unexpected error to well within the tolerance limits of published international standards.

Intraocular lens formula constant optimization and partial coherence interferometry biometry: Refractive outcomes in 8108 eyes after cataract surgery

“…15 When PCI biometry became commercially available, there were expectations that this technology would allow improved precision in refractive outcomes compared with US techniques. 16 This appears to be true, 8,10,16,17 but only if optimized IOL constants that eliminate systematic differences in AL measurement between PCI and applanation US are used. Indeed, using the manufacturer's IOL constants with PCI biometry can deliver worse refractive outcomes than using manufacturer's IOL constants with US biometry.…”

“…Indeed, using the manufacturer's IOL constants with PCI biometry can deliver worse refractive outcomes than using manufacturer's IOL constants with US biometry. 10 To our knowledge, no published study is adequately powered to assess differences in refractive outcomes between PCI and immersion US. 18 Although IOL manufacturers do not publish details of the data from which their IOL constants are derived, in the past they were probably from contact US AL measurements.…”

“…Similar results have been reported by other studies when using optimized IOL constants. 10,11,14 The PCI technology used in the IOLMaster to measure AL is more precise (resolution 10 mm) than immersion US (10 MHz US affords 100 mm resolution) or contact US. The IOLMaster device was calibrated against a high-precision 40 MHz immersion US system with a conversion algorithm to transform PCI measurements to values "almost identical" to those obtained using the immersion US, even though the PCI biometer measures to the retinal pigment epithelium rather than the internal limiting membrane of the retina, as is the case with US measurements.…”

“…Despite studies of the refractive outcomes following cataract surgery, there are few publications assessing the benefits of optimization of IOL constants compared with using the manufacturer's value. 10,11 Surgeons optimize their IOL constant using data from a sample of operated eyes. The IOLMaster manual recommends the use of 200 eyes with deviations in postoperative refractions within 2 standard deviations (SD) of the mean A ; however, although there is no published evidence on the minimum number of eyes required for IOL constant optimization.…”

Distribution and visual impact of postoperative refractive error after cataract surgery in rural China

“…[8][9][10] New technologies using partial coherence interferometry to measure axial length may provide more accurate prediction of postoperative refraction. 6,11,12 Risk factors for deviation from desired emmetropia include older age, 2 clear corneal (as opposed to limbal) incisions, 2 and use of anterior chamber IOLs. 13 Some studies in the developing world 14,15 report acceptable accuracy in the prediction of postoperative refraction, even using average-power IOLs, 14,15 with 45% to 54% of patients having a postoperative refraction within G1.00 D. 14 However, postoperative refractive error has been implicated as one of the most significant risk factors for poor visual and functional outcomes after cataract surgery in many reports from rural Asia, including those from Bangladesh, 16 Pakistan, 17 China, 18 and India.…”