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Resolving nebulous data
Improvements in techniques, instrumentation, and intraocular lens design appear to have been associated with declining rates of posterior capsule opacification following cataract surgery.1 While morbidity associated with posterior capsule opacification and its treatment, Nd:YAG laser capsulotomy, is limited, progressive opacification affects quality of vision and visual function, while capsulotomy requires additional patient visits, consumes additional surgical resources, and introduces the potential risks of cystoid macular oedema and retinal detachment. Therefore, a better understanding of the mechanisms of posterior capsule opacification and its retardation would have obvious immediate benefits. Moreover, control of capsular optical and biomechanical characteristics following cataract extraction is essential if dynamic accommodative intraocular lenses, which change in optical conjugation power in response to ciliary body motion translated via the zonules and capsular bag, are to provide consistent and sustained performance.
In addition to providing further evidence suggesting an increased risk for posterior capsule opacity associated with specific features of intraocular lens design independent of material, the study presented by Mian and co-authors in this issue of the BJO (p 1453) supports the observation that we as yet do not have a complete understanding of all of the features that govern the behaviour of the capsular bag following cataract surgery; in this case, inability to exercise control over this process resulted in laser capsulotomy in up to 7.5% of cases at 24 months. Moreover, the comprehensive review provided by the authors draws attention to inconsistency in the literature with regard to its assessment of risk factors and strategies for the development of posterior capsule opacification after cataract surgery. This, in turn, obfuscates research directions for strategies to control the process of capsule opacification.
Well informed and sceptical readers will point out, as the authors acknowledge, that some previous studies, which have specifically addressed the issue of the relative risk for posterior capsule opacification associated with one piece acrylic compared with three piece acrylic intraocular lenses, have failed to demonstrate a significant difference in risk for posterior capsule opacity.2–4 As a previous study by Wallin and colleagues5 supports the authors’ observations of an increased risk associated with the one piece design, one must consider explanations for these discrepancies, in so far as they may represent inconsistencies in study design or reporting.
One must first ask if a negative study that fails to identify a difference in outcomes is based upon the evaluation of relatively insensitive or unreliable parameters. This is a particular problem in the evaluation of posterior capsule opacification. Laser capsulotomy represents but a proxy for the process of opacification; it might be argued that it is a perfectly reasonable measure to study if our goal is to reduce the additional burden that the procedure places on patients and the medical system but, in the event that opacification which fails to meet criteria for treatment induces some compromise of visual function, there remains benefit to the reduction of these sub-interventional threshold levels of capsule opacity. Moreover, the goal of capsule control for reliable long term accommodative intraocular lens function requires an understanding of the mechanisms of posterior capsule fibrosis at an earlier stage than that associated with the need for laser intervention.
The problem of posterior capsular opacification has not yet been conquered
Aslam and colleagues6 have presented, in the BJO, a systematic analysis of strategies to evaluate posterior capsule opacity and have pointed out that at present there is no entirely satisfactory method of comprehensively quantifying capsule opacity in vivo. In addition to regional quantitation of the optical density of capsule opacity, typically scored subjectively or by analysis of digitised photographs, capsule opacity has fibrotic and lens epithelial proliferative components that require three dimensional measurement. In order to assess the extent of opacification, researchers such as Sacu and colleagues have resorted to multiple modes of evaluation including a subjective score that ranks subtypes of fibrotic opacity, automated analysis of digitised photographs, and need for Nd:YAG laser capsulotomy.4 Hayashi and Hayashi7 have argued that multiple methods are indeed required to adequately describe these various features, and have suggested that retroillumination photography is helpful in describing the area of involvement, but Scheimpflug photography can further provide a measure of density. Aslam and colleagues have contended, however, that Scheimpflug imaging is hampered by its restriction to the examination of slit images and its analysis of back scattered rather than forward scattered light. Alternatively, this group has suggested an analysis system that attempts to analyse the three dimensional quality of capsular opacity (“texture”) through quantitation of measures of entropy in the intensity histogram of digitised retroilluminated photographic images.8 While these provide more direct measures than capsulotomy rates, the validity of their description of the complex patterns and varying degrees of capsule opacity have yet to be broadly confirmed.
Another explanation for inconsistent findings in studies of risks for capsule opacity would be that characteristics of the populations under study might vary significantly. For example, along with material properties and lens design, there is evidence that surgical technique, including factors such as the degree of capsulorhexis cover over the optic or cortical clean up might influence the course and pattern of subsequent capsule opacity.9 While Mian et al did not provide details of their management of the relation between the capsulorhexis edge and optic, or of cortical clean up, Smith and colleagues,10 in a study of the relation between capsule and lens edge overlap found that incomplete anterior capsule overlap, was associated with a significant increase in capsule opacity, and that this effect was indeed moderated by lens design. In order to compare studies and refine our protocols, these factors must be controlled as best we can.
Significant advances have been made in the design of intraocular lenses for implantation after cataract surgery, and a wide range of options are now available for patients and surgeons. However, the problem of posterior capsular opacification has not yet been conquered. It is to be hoped that more sophisticated methods of assessing and comparing the development of capsule opacity will help to guide research to this end.
Note in Proof
Resolving nebulous data