A Cochrane review [Abstract] 1 included 18 small studies with a total of 813 subjects. Eleven studies used transcutaneous electrical nerve stimulation (TENS), 4 interferential current stimulation, 1 both TENS and interferential current stimulation, and 2 pulsed electrostimulation. The largest trial had 71 subjects. The analysis suggested an overall large SMD of -0.86 (95% CI -1.23 to -0.49), which corresponds to a difference in pain scores of 2.1 cm on a 10 cm VAS between electrostimulation and control. Four studies showed unrealistically large SMDs of twice to three times the magnitude of what would be expected for total joint replacement. The funnel plot was highly asymmetric. The authors discuss as follows: If reporting is inadequate, as was the case in this systematic review, then the standard error as a proxy for study size may be a more precise measure of trial quality than formal assessments of methodological quality. When modelling effects expected in trials as large as the largest trial included in our systematic review, we found effects on pain near null -0.07 (95% CI -0.46 to 0.32), which were clearly smaller than the pooled SMD actually found for pain in the meta-analysis. This corresponds to a difference in pain scores between electrostimulation and control of 0.2 cm on a 10 cm visual analogue scale.
There was little evidence that SMDs differed on the type of electrostimulation (P = 0.94). The relative risk of being withdrawn or dropping out due to adverse events was 0.97 (95% CI 0.2 to 6.0).
The analysis of function suggested a small SMD of -0.34 (95% CI -0.54 to -0.14), which corresponds to a difference in function scores of 0.7 units on a standardised WOMAC disability scale ranging from 0 to 10, favouring electrostimulation. The funnel plot did not appear asymmetrical.
Comment: The quality of evidence is downgraded by potential reporting bias, by study quality (inadequate or unclear allocation concealment and inadequate intention-to-treat adherence), by inconsistency (heterogeneity in interventions and outcomes and variability in results across studies), and by imprecise results (limited study size for each comparison).
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