Background
Patellofemoral pain syndrome, commonly known as anterior knee pain, is characterised by short- or long-term pain in the front part of the knee or behind the kneecap. Muscle stimulation has been proposed as a treatment for this condition. This involves the use of a device that produces a muscle contraction by placing electrodes on the skin of the leg. Muscle stimulation is often used together with exercises and other treatments but can also be used on its own.
Results of the search
We searched the medical literature up to May 2017 and found eight studies reporting results for a total of 345 participants who had anterior knee pain for at least one month, and sometimes over several years. Most of the participants were female. The average age of participants in the studies ranged from 25 to 43 years. All the studies were small and had flaws that meant they were at risk of bias. There was very little evidence on longer-term outcome. The results of one study that reported on immediate pain after a single 15-minute session of muscle stimulation are not reported here as these are of questionable clinical relevance.
Key results
Each of the seven remaining studies tested one of three comparisons.
Four studies compared a multiple-session muscle stimulation programme combined with exercise over several weeks with exercise on its own for the same period. All participants in two studies had adhesive tape applied across their knee cap, with ice also being applied in one study. We found very low-quality evidence that muscle stimulation with exercise may slightly reduce knee pain at the end of a treatment period of between 3 and 12 weeks better than exercise alone. However, very low-quality evidence did not show an effect on knee function. None of the studies reported on harms such as muscle fatigue and discomfort. There was very little useful information on longer-term effects.
One study compared muscle stimulation lasting four hours each day for four weeks with exercise. Very low-quality evidence showed no important difference between the two groups in knee function at the end of the four-week treatment. Of note is that the duration of muscle stimulation is much longer than used nowadays.
Two studies compared different types of muscle stimulation. Very low-quality evidence showed no important differences at the end of the six-week treatment programme between the different types of muscle stimulation.
Quality of the evidence
The overall quality of the evidence for all reported outcomes was very low. This means that we are very uncertain about the findings of these studies.
Conclusions
We found insufficient evidence to inform on the role of neuromuscular electrical stimulation for treating people with anterior knee pain. Further research is needed.
This review found insufficient and inconclusive evidence from randomised controlled trials to inform on the role of NMES for treating people with PFP in current clinical practice. The very low-quality evidence available means that we are uncertain whether or not a multiple-session programme of NMES combined with exercise over several weeks versus exercise alone results in clinically important differences in knee pain and function at the end of the treatment period or at one year. There were no data on adverse effects such as muscle fatigue and discomfort. High-quality randomised clinical trials are needed to inform on the use of NMES for people with PFP. However, professional and stakeholder consensus is required on prioritisation of the research questions for interventions for treating people with PFP, including on the NMES treatment protocol for trials testing NMES.
Patellofemoral pain syndrome, now generally referred to as patellofemoral pain (PFP), is one of the most common orthopaedic disorders, characterised by pain in the anterior or retropatellar knee region. Neuromuscular electrical stimulation (NMES) has been proposed generally as a complementary treatment, associated with other interventions such as exercise, or as a single treatment to increase muscle force, reduce knee pain, and improve function.
To assess the effects (benefits and harms) of neuromuscular electrical stimulation for people with patellofemoral pain.
We searched the Cochrane Bone, Joint and Muscle Trauma Group Specialised Register, the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, PEDro, CINAHL, SPORTDiscus, AMED, LILACS, trial registers, conference abstracts, and reference lists. We carried out the search in May 2017.
We included randomised controlled clinical trials that evaluated the use of NMES for people with PFP.
Two review authors independently performed the process of study selection, data extraction, and 'Risk of bias' assessment in duplicate. The primary outcomes were knee pain, knee function, and adverse events. The timing of outcome measurements was up to three months (short term), three to 12 months (medium term), and 12 months and above from trial entry (long term). We calculated risk ratios for dichotomous data and mean differences or standardised mean differences for continuous data. Where appropriate, we pooled data using the fixed-effect model.
We included eight randomised clinical trials, reporting results for 345 participants with PFP. The mean ages of trial populations ranged from 25 to 43 years, and the majority (53% to 100%) of participants were female. There was a wide duration of symptoms, with the minimum duration of symptoms for trial inclusion ranging from one to six months. In addition to the study inclusion criteria, studies varied widely in the characteristics of the NMES and its application, and associated co-interventions. We assessed all trials as at high risk of bias in at least one domain, particularly blinding and incomplete outcome data. The results of a laboratory-based trial reporting knee pain immediately after a single 15-minute session of NMES are not reported here as these are of questionable clinical relevance. The seven remaining trials provided evidence for three comparisons. We assessed the overall quality of the evidence, using GRADE, for all primary outcomes for all comparisons as very low, thus we are very unsure of the findings.
Four studies compared NMES plus exercise versus exercise alone. Patellar taping was applied as well as exercise to all participants of one study, and patellar taping and ice were also applied in another study. Each trial tested a different multiple-session NMES programme. Pooled data from three studies (118 participants) provided very low-quality evidence that NMES is associated with reduced pain at the end of treatment (ranging from 3 to 12 weeks): mean difference -1.63, 95% confidence interval (CI) -2.23 to -1.02; visual analogue scale (VAS) 0 to 10; higher scores = worse pain. However, this result may not be clinically relevant since the minimal clinically important difference for VAS during activities (1.5 to 2.0, out of 10 points) lies within the 95% CI. We found very low-quality evidence from pooled data from two trials of little effect of NMES on knee function, as measured by two knee function rating systems. We found inconclusive and very low-quality evidence from one trial (29 participants) of little effect of NMES on pain and function at one-year follow-up. None of the four trials reported on adverse effects of treatment.
One study (94 participants) compared NMES, applied four hours per day on a daily basis for four weeks, with two types of exercises (isometric and isokinetic). The study did not report on knee pain or adverse events. The study provided very low-quality evidence of no important difference between the two groups in knee function at the end of the four-week treatment. Of note is the potentially onerous NMES schedule in this study, which does not correspond to that typically used in clinical practice.
Two studies compared different types of NMES. Simultaneously delivered high-low frequencies NMES was compared with sequentially delivered high-low frequencies NMES in one trial (14 participants) and with fixed frequency NMES in the second trial (64 participants). The studies provided very low-quality evidence of no important differences at the end of the six-week treatment programme between the simultaneous frequencies NMES and the two other NMES programmes in overall knee pain, knee function, or in quadriceps fatigue (an adverse event).