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LETTERS

A Problem of Power

Mahabir, Raman C. M.D., M.Sc.

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Plastic and Reconstructive Surgery: January 2009 - Volume 123 - Issue 1 - p 428
doi: 10.1097/PRS.0b013e31819055c9
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Sir:

I read with interest the article recently published in Plastic and Reconstructive Surgery about pain control with a continuous local anesthetic infusion.1 The authors' goal was to examine the effectiveness of the pain pump after cosmetic breast augmentation. There are two concerns with this article that I feel need to be addressed.

First, the authors conducted a randomized controlled trial without performing a sample size calculation. The literature has been weighed down by underpowered studies and this is a classic example of a type II (β) error. The authors noted this limitation but did not discuss it in detail. Taking the data from the article (mean and SD for the saline group of 5.4 and 2.8, respectively, and a correlation of 0.08), using an α of 0.05 and a power of 90 percent, the sample size would only have been adequate to detect a difference of 55 percent (2.95). To detect a clinically relevant 20 percent difference using an α of 0.05, a power of 90 percent, and the numbers from the article again, the authors would have needed 128 patients. When a negative result is misinterpreted as showing no effect (type II error), the authors consume valuable resources, discourage further research, and inhibit advances in clinical treatment.2 For a review of the concepts of calculation of sample size and power in clinical research,3 as well as the role of randomized controlled trials in plastic surgery, the reader is directed to the most recent Clinics in Plastic Surgery.4

The second issue is in the methodology. The authors used the patients as their own controls, with saline in one side and bupivacaine in the other breast. In my experience,5,6 women are not able to discriminate between sides with regard to amount of pain. In the study by Kazmier et al., 30 percent of the patients reported more pain on the side with the local anesthetic, lending support to the observation in my previous articles.5,6 Placing the same solution in both sides increases the sample size, but it provided more reproducible data at my center.

Raman C. Mahabir, M.D., M.Sc.

Division of Plastic Surgery

Scott and White and Texas A&M

2401 South 31st Street

Temple, Texas 76508

rmahabir@swmail.sw.org

REFERENCES

1. Kazmier FR, Henry SL, Christiansen D, Puckett CL. A prospective, randomized, double-blind, controlled trial of continuous local anesthetic infusion in cosmetic breast augmentation. Plast Reconstr Surg. 2008;121:711–715.
2. Chung KC, Kallianen LK, Spilson SV, et al. The prevalence of negative studies with inadequate statistical power: An analysis of the plastic surgery literature. Plast Reconstr Surg. 2002;109:1–6.
3. Sheps S. Sample size and power. J Invest Surg. 1993;6:469–475.
4. Thoma A. Evidence-based plastic surgery: Design, measurement and evaluation. Clin Plast Surg. 2008;35(theme issue):189–316.
5. Mahabir RC, Peterson BD, Williamson JS, Valnicek SM, Williamson DG, East WE. Locally administered ketorolac and bupivacaine for control of postoperative pain in breast augmentation patients. Plast Reconstr Surg. 2004;114:1910–1916.
6. Mahabir RC, Peterson BD, Williamson J, et al. Locally administered ketorolac and bupivacaine for control of postoperative pain in breast augmentation patients. Part II. Ten-day follow-up. Plast Reconstr Surg. 2008;121:638–643.

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