Use of Low-Frequency Electrical Stimulation for the Treatment of Plantar Fasciitis
To the Editor:
It was with great interest that I read the article by Michael Stratton, PT, DSc, ATC, Thomas G. McPoil, PT, PhD, ATC, Mark W. Cornwall, PT, PhD, CPed, and Kyle Patrick, DO, published in the November/December 2009 issue of JAPMA, entitled “Use of Low-Frequency Electrical Stimulation for the Treatment of Plantar Fasciitis.”
However, I am concerned that the authors may have made a type II error, as I was unable to find a power calculation for a clinically important treatment effect. The results of the outcomes were not expressed as a point estimate with 95% confidence intervals. I wonder if a clinically important effect was missed because the study was underpowered. It was unclear to me if the authors addressed this issue in the Discussion section of the article.
UAuthor’s Response
To the Editor:
We would like to thank Dr. Turlik for his comments regarding our recent article, “Use of Low-Frequency Electrical Stimulation for the Treatment of Plantar Fasciitis” that appeared in the November/December 2009 issue of JAPMA.
Dr. Turlik’s observation is correct; we did not have sufficient power to detect a significant difference or a possible interaction between the two groups (control and electrical stimulation) and the treatment time. The authors, however, believe that even if sufficient statistical power had been present, the question of clinical significance remains. For example, in order to demonstrate the mean difference between the two experimental groups, approximately 1,900 participants would have been needed (950 in each group) to show a statistical difference of 1.2 mm on the visual analog score (VAS). The authors do not believe that such a difference, regardless of its statistical significance, is clinically relevant, nor is it practical to conduct a study with that many participants simply to show such a small change. On the other hand, the number of participants needed to show a statistical interaction between the two groups and treatment time would be approximately 100 (50 in each group). This is certainly more reasonable compared to 1,900, but again, the detected change is relatively small. Repeating our study with 100 participants might have shown a statistical interaction but only at the 4-week time period and not after 3 months. As such, low-frequency electrical stimulation would result in a 9.4% improvement on the patient’s VAS in the morning after 4 weeks, but not at 3 months. The authors would concede that some clinicians might feel that a 9.4% change on a patient’s VAS is clinically significant. When the cost of the treatment is taken into account and the fact that there is not a difference between the two groups after 3 months, we did not believe that the use of additional participants would be worthwhile.
MICHAEL STRATTON, PT, DSC, ATC
Continuum Wellness Clinic
3941 E. Baseline Rd
Ste 101
Gilbert, AZ 85234
