Methods of Quantifying Posterior Capsule Opacification (2003)

Reply: It is correct that our photographic setup is based on Dr. Spalton's system in London. We cited this accordingly in our paper. We use the same image-acquisition technique with minor modifications. However, our image-analysis software (AQUA) was developed in cooperation with the Technical University of Graz, Austria, and is not based on the POCO system. To test the reproducibility of the different analysis methods, we chose to duplicate 10 of the images in the image set. For obvious reasons, neither the examiners performing the subjective scoring and EPCO analysis nor the examiners performing the POCO analysis in London were informed of the purpose of these duplicated images. The results of the study were presented at the meetings of the American Society of Cataract and Refractive Surgery and the European Society of Cataract and Refractive Surgeons in 2000, and we sent Dr. Spalton a printout of the poster presented at those meetings. That the POCO system measures the area and not the severity of PCO and that it is semiautomated were not well known to many (or at least to us). In the publication that Dr. Spalton cites, which describes the POCO analysis algorithm,1 neither the title nor the abstract mentions that area is assessed but instead uses terminology such as “objective assessment of the amount of PCO.” The paper also does not mention that the POCO system is semiautomated (as stated in the letter) but instead calls the method “objective,” even though matching the segmented to the raw images by an operator cannot be objective by definition. We found it informative to show a scatterplot comparing our subjective PCO grading and the POCO results. From that graph, it becomes clear that the POCO system assesses something different than PCO intensity, showing an early saturation of values in mild to moderate cases of PCO intensity. This also explains why some of the PCO differences found in former studies by the London group between hydrophobic acrylic and silicone intraocular lenses (IOLs) (unfortunately, also differing in edge design—“apples and pears”) were much larger than the difference in visual function or the neodymium:YAG capsulotomy rate.2 We did not draw a plot comparing AQUA and POCO. We printed the correlation coefficient (AQUA versus POCO) in the correlation table to show all correlation values. The AQUA measurements are not limited to the central 4.0 mm zone of the posterior capsule. It is possible to evaluate the entire capsulorhexis area (or any other area of interest). However, for this study, we cut the central 4.0 mm area out of the test-set images to ensure that exactly the same area would be assessed by all quantification methods. These masked images were sent to London, and therefore the POCO software evaluated the identical central 4.0 mm area of each case. We agree that the AQUA software only works well with high-quality images and that the score can be influenced by various artifacts. However, we were able to get images of sufficient quality from all our study patients, documented by the good correlation of subjective and AQUA scores. We believe that patients with severe corneal or tear-film problems should generally not be included in studies of PCO. If the data (in this case the image) collected are of poor quality, the outcome will be questionable. Concerning multicenter studies, with images taken with different photographic setups, probably all objective systems would have to be adapted accordingly beforehand. Possibly the POCO system, with the operator-dependent matching of segmented and raw images, may be more robust to differences in image type—at the cost of not being objective. Dr. Spalton correctly points out that a large area of mild PCO could have the same AQUA score as a small area of moderate PCO. However, if one is interested in predicting visual function, one would only assess the area of the posterior capsule that corresponds to the pupil area under those lighting conditions and is, therefore, relevant to visual function. If one is more interested in the morphological changes with an IOL over time, one would choose to analyze the entire posterior capsule area not obscured by the capsulorhexis rim. We do not see a methodological problem with this approach. Contrarily, when assessing only the PCO area, as does the POCO system, the correlation with visual function would be rather poor. This is illustrated in a recent paper by Dr. Spalton's group,3 in which the area of PCO 2 years after surgery was the same as that found 6 months after surgery with silicone and hydrophilic IOLs. From the photographs in this paper (Figures 4 and 6), as well as data from numerous longitudinal studies of PCO and the general clinical experience, PCO, if present, usually increases from 6 months to 2 years postoperatively. This is obviously not detected by assessing the PCO area. We acknowledge the pioneer work by Dr. Spalton and his coauthors in the field of PCO assessment. He was one of the first to introduce randomized clinical trials with long-term follow-up to PCO assessment. However, we believe that Dr. Spalton's method of PCO assessment has brought some confusion into the literature since PCO area does not mean PCO intensity. The intensity of PCO is what is relevant to the patient and what all clinicians are confronted with in daily practice. We believe that our paper and these letters have helped to clarify this important point. We are aware that the current AQUA version is not the ultimate solution for PCO analysis, but it allows an objective evaluation of PCO intensity in clinical trials and the score correlates well with the subjective evaluation of PCO intensity. We believe that further, parallel development of different analysis systems and their discussion will lead to a broadly accessible assessment system of PCO intensity in the future. Oliver Findl MD