Significant errors were still encountered because of assumptions necessary about axial length (AL) and depth of the anterior chamber (AC). Based on the experiences of many, pirical formulae were developed, but it was not until 1980 that improvents were made in the calculation with the release of the SRK (Saunders, Retz, Kraff) formula. It used lens constants, keratometry (K), and AL. Later still, SRK II adjusted for long and short eyes.{{image4-a:r-w:300}}The later release of ocular biometers that measured the key ocular parameters gave the formulae creators the information lacking previously. The two mainstays of that instrument class are the various incarnations of the Haag-Streit Lenstar, which use low-coherence reflectometry, and the Zeiss IOL Master. The latter uses partial coherence interferometry, of which the latest incarnation [700] is in effect a swept-source OCT device.Not surprisingly, the data provided relate to corneal thickness, AC depth, crystalline lens thickness, and AL (measured into the foveal pit). Confidence in the outputs is enhanced with fixation checking, multi-zone measurents, and the overlaying of instrument reference marks over on-screen images of the eye, indicating HVID, pupil sizes and locations.The asymmetric and complex shape of the cornea is another challenge that renders simple keratometry inadequate for modern IOL calculations. Wavefront analysis, ray tracing, reflectance topography, and ocular biometry are superior approaches to the challenge of estimating the corneas contribution to total ocular power. Painter showed a complex bar chart indicating accuracy ranges for the most common formulae plotted against AL.The recent formulae compared included: Olsen, Barrett, Hill-RBF, T2, and Super Formula. Using a table taken from Cooke and Cooke (2016) that donstrated several of the recent and some older formulae used with the Lenstar and the IOL Master, Painter showed that the Olsen Stand Alone (0.361/0.446) and the Barrett Universal II (0.365/0.387) performed best overall. The combination of the Olsen and Lenstar proved to be the most accurate in their test, but many combinations of instrument and formula performed credibly.In the case of a larger AL, calculations tend to underestimate AL resulting in a hyperopic final Rx. In those cases that have undergone previous refractive surgery, a growing sub-set of patients, good results have been reported using the ASCRS Post Keratorefractive Surgery Calculator.When an IOL has to be implanted in the ciliary sulcus rather than the more desirable in-the-bag location, an adjustment of up to –1.50 D to the back vertex power (BVP) of the IOL calculated for a capsular bag location has to be made for the higher-powered IOLs (+9.50 to +30 D).When an unwelcome refractive surprise occurs, the options are: spectacles, explantation and IOL exchange (resisted if at all possible), a Sulcoflex supplental lens implantation, or refractive surgery. If the second eye is to be operated on, lessons learned from the first’s refractive surprise should be applied.When planning an IOL implantation, the three calculators referenced were the online Alcon Toric Calculator, Dr Noel Alpins’ ASSORT Toric Calculator, and the commercial Holladay IOL Consultant.A confounding factor is posterior corneal astigmatism, which has been estimated to reduce total corneal astigmatism by 13.4% but which still has no valid way of being measured reliably. Painter also mentioned the ORA device, now owned by Alcon, which facilitated intra-operative measurents and suggestions.He also commented on some differences between the various ftosecond laser cataract surgery platforms, noting that the LenSx syst’s negative-pressure coupling to the eye prevented movent of the eye relative to the device, whereas such relative movent was possible with some other systs. However, he did acknowledge that LenSx can have difficulties with high astigmatism.
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Associate Professor GEOFF PAINTER is co-founder of Gordon Eye Surgery, a multi-surgeon, comprehensive ophthalmic practice where he specialises in cataract surgery and glaucoma. He is also Head of Section at Dalcross Adventist Hospital in Killara and is a VMO at Royal North Shore Hospital. |
