Dear editor,

We would like to give a response to the letter of Dr. Anshuman Sinha concerning our article "23 Gauge versus 20 Gauge System for Pars Plana Vitrectomy: A Prospective Randomized Clinical Trial".

For analysis of postoperative pain we used a simplified verbal rating scale (ranging from 0, meaning no pain, to 3, meaning severe pain). The standardized verbal rating scale reaches from 0 (no pain) to 10 ("the worst pain possible"), which we think is too extensive for our patients and purpose. In literature verbal rating scales are described to be a valid, reliable and appropriate tool for determination of postoperative pain in clinical practice [1,2] Furthermore we believe that the assessment of postoperative pain is not likely to be biased if the assessor is not masked, because the perception of pain is a completely subjective parameter.

Regarding the sample size evaluation: The effect size was assumed to be 75% of the standard deviation and the power to be 80%. These assumptions were extrapolated from the observations of our previous study [3]. Unfortunately, this information had been dropped from a previous version of the manuscript for brevity.

It is well known that studies with single surgeons have better results than with multiple surgeons. On the other hand, it is recommended to choose a realistic situation to achieve more realistic results. Our surgeons were randomized and there was a general surgical protocol to be followed. This also is the reason why we used two different vitrectomy machines. Both of them work in a comparable way and the settings were preset and the same in both machines. We agree, that the lens status has an influence on the amount of vitreous removed and that an intraocular tamponade influences postoperative eye pressure. However, both factors were equally distributed in both surgical groups and therefore have no influence on the results. Concerning the way of suturing the 20 gauge wounds, this was done the same way in all patients using vicryl 7.0 sutures as described in the methods section.

A summary of nominal categorical data would have to be done by contingency tables and neither by mean nor by median and interquartile range. Since the data mentioned are not of nominal but ordinal nature, we assume that the author of this letter refers to ordinal data. We have to note that the summary presented in table 2 was done for the area under the curve (AUC) for the conjunctival injection and postoperative pain measured at days 0, 1, 2, 3, one week, one month and three months after surgery as indicated in figures 1 and 2. Although theses measurements are, strictly speaking, still of ordinal nature they have a large range of values (see table 2) and hence can be treated as metric variables. Moreover, it is common practice to summarize AUCs by mean and standard deviation. We also note that in a previous version of the paper we have given in addition to the mean and standard deviation also the median, first and third quartile. Unfortunately, these numbers as well as the information in the text that the AUCs are summarized have been omitted in the edited manuscript.

We furthermore agree that each complication represents a serious situation. However, from the data of our study we can not conclude that a clear indication for higher risk is given using 23 gauge vitrectomy. This question remains open and a larger prospective randomized study is needed to get a clear result of the safety of 23 gauge surgery. Since the data do not indicate a difference in the development of serious complications between the two treatment groups we made the cited careful assertion "seems to be a safe system".

Regarding the Consolidated Standards for Reporting Trials (CONSORT) statement [4], we certainly know and appreciate this publication and followed these recommendations. Moreover, our article represents the only prospective randomized study concerning this topic. We do appreciate your considerations, but we strongly believe that we have performed a well-designed study. Still, there are always things that can be improved.

References

1.Williamson A, Hoggart B. Pain: a review of three commonly used pain rating scales. J Clin Nurs. 2005 Aug;14(7):798-804.

2.Randall C. Cork, Ihab Isaac, Ahmad Elsharydah, Sarosh Saleemi, Frank Zavisca and Lori Alexander: A comparison of the verbal rating scale and the visual analog scale for pain assessment. The Internet Journal of Anesthesiology. 2004: Volume 8, Number1.

3.Kellner L, Wimpissinger B, Stolba U, Brannath W, Binder S. 25-gauge vs 20-gauge system for pars plana vitrectomy: a prospective randomised clinical trial. Br J Ophthalmol. 2007 Jul;91(7):945-8.

4.Moher D, Schulz KF, Altman DG. The CONSORT statement: revised recommendations for improving the quality of reports of parallel-group randomised trials. Lancet 2001;357:1191-4.