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Comment on ‘Neuromuscular electrical stimulation in the treatment of knee osteoarthritis: a systematic review and meta-analysis’

We read with great interest the recent article by Giggins et al.1 entitled ‘Neuromuscular electrical stimulation in the treatment of knee osteoarthritis: a systematic review and meta-analysis’. We appreciate their effort, but feel some worthwhile issues need to be explored. First, the authors acknowledged that the current review lacked of consistency among studies in terms of neuromuscular electrical stimulation (NMES) groups because there were four studies combined NMES with other therapeutic interventions.2–5 It also should be noted that four included studies did not have a placebo or blank control when excluding the same interventions between the NMES group and control group.5–8 Among them, three studies used exercise as the control group.5,6,8 Even more remarkable was that transcutaneous electrical nerve stimulation (TENS) had been used as a control intervention in another study.7 Of course, the differences of the control group between studies may have an influence on the accuracy of the overall results with overestimation or underestimation. Second, a study that used functional electrical stimulation (FES) as the experimental group was included.9 However, FES usually refers to the process of pairing the electrical stimulation simultaneously or intermittently with a functional task as initially described by Moe and Post.10 Moreover, according to a recently published review entitled ‘Physical therapy interventions for knee pain secondary to osteoarthritis’, electrical stimulation was divided into five categories including electrical muscle stimulation, FES, high-voltage pulsed current, NMES and TENS.11 Therefore, inclusion of FES without explanation may confuse the reader. Third, nine randomized controlled trials and one controlled clinical trial were included in this current review. The authors reported a level D of evidence, along with the inconsistency among the trials. Our question is why the authors decided to include this only non-randomized controlled trial,12 which downgraded the evidence level of the whole meta-analysis as a consequence. Finally, publication bias has a tendency, on average, to produce results that appear significant, because negative or near neutral results are hard to be published. In addition, sensitivity analysis is the study of how the uncertainty in the output of a mathematical model or system (numerical or otherwise) can be apportioned to different sources of uncertainty in its inputs. However, in this study the risk of publication bias was not reported, as well as the sensitivity analysis was not conducted. According to chapter four of the Cochrane Handbook for Systematic Reviews of Interventions 5.1.0, assessing publication bias and conducting sensitivity analysis are both recommended.13 We respect the contribution of the authors, but wish to draw attention to these concerns.