MUSCULOSKELETAL PAIN
Effectiveness of Interferential CurrentTherapy in the
Management of Musculoskeletal Pain: A Systematic
Review and Meta-Analysis
+Author Affiliations
- Address all correspondence to Mr Fuentes at: jorgef@ualberta.ca.
Abstract
Background Interferential current (IFC) is a common electrotherapeutic modality used to treat pain. Although IFC is widely used, the available information regarding its clinical efficacy is debatable.
Purpose The aim of this systematic review and meta-analysis was to analyze the available information regarding the efficacy of IFC in the management of musculoskeletal pain.
Data Sources Randomized controlled trials were obtained through a computerized search of bibliographic databases (ie, CINAHL, Cochrane Library, EMBASE, MEDLINE, PEDro, Scopus, and Web of Science) from 1950 to February 8, 2010.
Data Extraction Two independent reviewers screened the abstracts found in the databases. Methodological quality was assessed using a compilation of items included in different scales related to rehabilitation research. The mean difference, with 95% confidence interval, was used to quantify the pooled effect. A chi-square test for heterogeneity was performed.
Data Synthesis A total of 2,235 articles were found. Twenty studies fulfilled the inclusion criteria. Seven articles assessed the use of IFC on joint pain; 9 articles evaluated the use of IFC on muscle pain; 3 articles evaluated its use on soft tissue shoulder pain; and 1 article examined its use on postoperative pain. Three of the 20 studies were considered to be of high methodological quality, 14 studies were considered to be of moderate methodological quality, and 3 studies were considered to be of poor methodological quality. Fourteen studies were included in the meta-analysis.
Conclusion Interferential current as a supplement to another intervention seems to be more effective for reducing pain than a control treatment at discharge and more effective than a placebo treatment at the 3-month follow-up. However, it is unknown whether the analgesic effect of IFC is superior to that of the concomitant interventions. Interferential current alone was not significantly better than placebo or other therapy at discharge or follow-up. Results must be considered with caution due to the low number of studies that used IFC alone. In addition, the heterogeneity across studies and methodological limitations prevent conclusive statements regarding analgesic efficacy.
Successful management of musculoskeletal pain is a major challenge in clinical practice. One of the electrotherapeutic techniques used for managing musculoskeletal pain is interferential current therapy (IFC). The results of questionnaire surveys in England,1 Canada,2 and Australia3,4 have shown that IFC is widely used by diverse clinicians throughout the world.
Interferential current therapy is the application of alternating medium-frequency current (4,000 Hz) amplitude modulated at low frequency (0–250 Hz).5–7 A claimed advantage of IFC over low-frequency currents is its capacity to diminish the impedance offered by the skin.6 Another advantage speculated for IFC is its ability to generate an amplitude-modulated frequency (AMF) parameter, which is a low-frequency current generated deep within the treatment area.6,8–10 Several theoretical physiological mechanisms such as the “gate control” theory,11increased circulation, descending pain suppression, block of nerve conduction, and placebo have been proposed in the literature to support the analgesic effects of IFC.5,8,12
Despite IFC's widespread use, information about it is limited. A review of the literature reveals incomplete and controversial documentation regarding the scientific support of IFC in the management of musculoskeletal pain. For example, a systematic review about the use of electrotherapy for neck disorders13 excluded the analysis of IFC. Moreover, much of the IFC information is not written in English,10,14–22 and most articles appear to be based on case reports,23–25 clinical studies not including a randomization process,26,27 letters to the editor,28,29clinical notes,30 experimental settings,31–37 descriptive studies,8,12,38,39 or experience in the field40,41 instead of methodologically qualified studies.
Thus, the objective of this systematic review and meta-analysis was to determine the analgesic effectiveness of IFC compared with control, placebo, or other treatment modalities for decreasing pain in patients with painful musculoskeletal conditions.
Method
Search Strategy
Relevant studies of IFC in musculoskeletal pain management from 1950 to February 8, 2010, were obtained through an extensive computerized search of the following bibliographic databases: MEDLINE (1950 through week 4 of 2010), EMBASE (1988 through week 5 of 2010), CINAHL (1970 through February 8, 2010), Scopus (1970 through February 8, 2010), Cochrane Library (1991 through the first quarter of 2010), ISI Web of Science (1970 through February 8, 2010), and PEDro (Physiotherapy Evidence Database) (1970 through February 8, 2010). The key words “interferential,” “interferential therapy,” “interferential current,” “musculoskeletal pain,” “electrotherapy,” “electroanalgesia,” “muscle pain,” “low back pain,” “shoulder pain,” “hip pain,” “knee pain,” “neck pain,” “osteoarthritis pain,” and “joint pain” were used in the search, including combinations of these words. For details regarding the search terms and combinations, see eAppendix 1. The literature search procedure was complemented by manually searching the bibliographies of the identified articles for key authors and journals.
Study Selection and Inclusion/Exclusion Criteria
Studies that met the following criteria were considered for inclusion: (1) randomized controlled trials (RCTs) from journal publications in the English language (because the clinical application of IFC often is based on its coadjutant effect, studies in which IFC was used as a cointervention also were included); (2) studies of male and female humans between 18 and 80 years of age; (3) studies of subjects clinically diagnosed with a painful musculoskeletal condition, such as muscle (eg, low back pain, neck pain), soft tissue (eg, tendinosis/tendinitis), or joint (eg, osteoarthritis) disorders; (4) regarding the type of interventions, all randomized comparisons of isolated or coadjutant IFC applications versus placebo, control, another physical therapy intervention, or another type of intervention; and (5) studies in which the outcome of interest was pain, as measured by the use of a visual analog scale (VAS) or numeric pain rating scale (NRS). Exclusion criteria for this study were: (1) studies based on animal data, (2) studies published in languages other than English, and (3) studies including subjects who were healthy in experimental settings.
Data Extraction and Quality Assessment
Two independent reviewers screened the abstracts of the publications found in the databases. The reviewers analyzed all articles initially selected by the abstract or title for the inclusion and exclusion criteria. Each criterion was graded on a yes/no basis. In case of discrepancies between reviewers regarding whether a particular article met a criterion, the ratings were compared and the criterion forms were discussed until a consensus was reached.
A critical appraisal was conducted to determine the methodological quality of the final selected studies. We used 7 scales (ie, Delphi List, PEDro, Maastricht, Maastricht-Amsterdam List, Bizzini, van Tulder, and Jadad) commonly used in thephysical therapy field to evaluate the methodological quality of the included studies, compiled in a set of 39 items.42 These items were grouped into 5 categories: patient selection, blinding, intervention, outcomes, and statistics. Based on a recent systematic review,42 no one scale effectively determines the overall methodological quality of individual studies. For this reason, we used all of them in a compiled fashion.
The articles were evaluated on the basis of only the information available in the articles using the critical appraisal sheet (eAppendix 2). For each item listed on the critical appraisal sheet, a score of 1 was given when the item was included in the article, and a score of 0 was given when the item was not included or the information provided by the authors was not sufficient to make a clear statement. In cases where the study did not consider a particular item, the item was marked as not applicable on the critical appraisal sheet. The scoring for each study was calculated by dividing the number of items included by the number of applicable items. Finally, each study was graded as having low, moderate, or high methodological quality based on how many items from the critical appraisal were met. The cutoff was determined as follows: 0–0.40=low methodological quality, 0.41–0.70=moderate methodological quality, and 0.71–1.00=high methodological quality. This criterion was determined a priori to the quality assessment. Similar criteria for cutoffs have been used in correlational studies to determine reference values for quality of association or agreement.43,44
The critical appraisal was independently completed by the 2 reviewers, and the results were compared. At this stage, the intraclass correlation coefficient (ICC) was calculated using SPSS version 17 software* in order to determine the agreement between the reviewers for article grading. Any discrepancies were settled through discussion.
Data Synthesis and Analysis
Studies investigating similar outcomes and interventions and those providing clear quantitative data were grouped, evaluated for heterogeneity, and pooled, if possible. When combining outcome data was not possible, narrative, descriptive, and qualitative summaries were completed. In the present study, a meta-analysis was performed to quantify the pooled effect of IFC alone or as an adjunct treatment on pain intensity when compared with placebo, control group, or comparison intervention. Because the pooled effect was based on the results of the VAS or NRS, the mean difference was used to quantify the pooled effect. RevMan 5.0 software† was used to summarize the effects (ie, pooled mean differences) and construct the forest plots for all comparisons. For this analysis, the 95% confidence interval (CI) was used. A chi-square test for heterogeneity was performed (P<.10).45 In the presence of clinical heterogeneity in the study population or intervention, the DerSimonian and Laird random-effects model of pooling was used based on the assumption of the presence of interstudy variability to provide a more conservative estimate of the true effect.45,46 If there was relative homogeneity, a fixed-effects model was used to pool data.45
Results
A total of 2,235 articles were found in the database search. Of these, 154 were selected as potential studies of interest based on abstract review (Fig. 1). After fullarticle review, only 20 studies were deemed to fulfill the initial selection criteria.47–66 The kappa agreement between the reviewers in selecting articles after applying the inclusion and exclusion criteria was perfect at κ=1.0.
Figure 1.
Study screening process. IFC=interferential current therapy.
Seventy-seven studies were rejected after applying the inclusion and exclusion criteria. The primary reasons for exclusion from the study were: (1) the use of subjects who were healthy in an experimental setting31–37,67–82; (2) descriptive studies in the form of case reports, dissertations, or clinical notes,8,12,23–25,30,38–41,69,83–96; (3) studies not published in the English language10,14–22; (4) the absence of pain outcomes97–105; (5) randomized trial not used26,27,106–108; (6) use of a current other than IFC109,110; (7) use of animal data111; and (8) unavailability of the full text of the article.112–114 At theend of the critical appraisal stage, there was an agreement of κ=.83 between the 2 raters. This ICC value is considered as “excellent” agreement according to the approach described by McDowell.115
Characteristics of the Studies
All 20 studies reviewed in detail were RCTs that examined the pain-reducing effectiveness of IFC. These studies analyzed the effects of IFC for several diagnoses considered to be either acute or chronic painful conditions. Only 6 articles (30%)48,54,56,57,61,63 examined the clinical analgesic effectiveness of IFC as a single therapeutic modality. The rest of the articles included the application of IFC as a cointervention along with other therapeutic alternatives such as exercise,47,49,53,58–60,62,64–66 shortwave diathermy,51,59 hot packs,55,60 ice,58myofascial release,55 neuromuscular electrical stimulation,52 infrared radiation,51and ultrasound.50,60,62 Details of the studies' characteristics are shown in Table 1.
Table 1.
Characteristics of the Studiesa
Methodological Quality of the Studies
The results of the critical appraisal for the selected studies are presented in Table 2. Three of the 20 studies were considered to be of high methodological quality, 14 studies were considered to be of moderate quality, and 3 studies were considered to be of poor quality. Even though the quality of most of the studies was rated as acceptable (17 studies were rated as being of moderate or high quality), there are some points regarding quality that need to be highlighted. Study flaws regarding patient selection were mainly related to description and appropriateness of the randomization procedure and concealment of allocation, with only 9 and 5 of the studies meeting these criteria, respectively. Items related to blinding were not achieved by the majority of the studies. Only 3 of the studies used a double-blinded design.
Table 2.
Methodological Quality of the Studiesa
Testing subjects' adherence to intervention or having adequate adherence was another issue that was not accomplished by many studies (only 8 and 6 studies, respectively). Furthermore, adverse effects were reported in only 3 of the studies, and none of the studies provided details of the follow-up period.
Despite the fact that the adequate handling of dropouts is considered an important method used to prevent bias in data analysis, only 11 of the analyzed studies included information regarding the rate of withdrawals/dropouts. The outcome measures were not described well in terms of validity, reliability, or responsiveness.
Regarding statistical issues, it was uncertain whether sample size was adequate in 15 of the studies. Intention-to-treat analysis was used only in 11 of the studies. Finally, it also was unclear whether extraneous factors such as equipment calibration or medications during the study could affect the treatment responsiveness for IFC. For example, only 2 studies (10%) reported that the IFC equipment was calibrated during the study procedure.
IFC and Type of Pain Management
The effect of IFC has been studied predominantly in patients with chronic painful conditions (16 of 20 trials examined). These conditions included knee osteoarthritis,47,49,51,52,54,59 chronic low back pain,48,63,–,65 shoulder soft tissue pain,53,60,62 fibromyalgia,50 chronic jaw pain,61 and myofascial syndrome pain.55In contrast, the analysis of IFC in acute pain included just 4 articles, 3 of them related to acute low back pain and 1 to postoperative knee pain.
Meta-analysis Results
Fourteen studies were included in the meta-analysis (Fig. 1),47,49–56,60,61,63–66with an overall sample size of 1,114 patients. Six studies were excluded for the following reasons: information regarding data variability (ie, mean and standard deviation) was not present,58,59 the unit of variability included was different than the standard deviation (ie, interquartile range, median),57,62 the comparison included in the trial was not relevant for the study's purpose,48 and the interventions included in the trial were too heterogeneous51 (ie, IFC, infrared radiation, shortwave diathermy, and 2 drugs [sodium hyaluronate and hylan G-F 20]).
The 14 selected studies were chosen because they provided complete information on the outcomes evaluated and homogeneity regarding outcome measures. Of these studies, 4 studies54,56,61,63 addressed the analgesic effect of IFC alone and 10 studies47,49,50,52,53,55,60,64–66 evaluated the effect of IFC applied as adjunct in a multimodal treatment protocol. In addition, of these 14 studies, 3 studies53,54,66 compared the effectiveness of IFC with a control group, 6 studies47,50,54,61,64,65 investigated IFC against placebo, and 7 studies49,52,53,55,56,60,63 compared IFC with another intervention such as manualtherapy or exercise.
Comparison 1: IFC Alone Versus Placebo Group on Pain Intensity at Discharge
Two studies54,61 were included in this comparison. One study54 measured outcomes at discharge after 4 weeks of therapy, and the other study61 measured outcomes after 1 week of therapy. One trial54 studied the effect of IFC on knee osteoarthritis, and the other trial61 studied the effect of IFC on temporomandibular joint pain. One study54 was rated of moderate methodological quality, and the other study61 was rated of poor quality.61 In this comparison, both studies had opposite results regarding the effectiveness of IFC when compared with a placebo group (Fig. 2). The pooled mean difference (MD) obtained for this analysis was 1.17 (95% CI=1.70–4.05). These results indicate that IFC alone was not significantly better than placebo at discharge.
Figure 2.
Forest plot of comparison: interferential current therapy (IFC) alone versus placebo treatment on pain intensity at 1 week and 4 weeks (data presented as change scores). IV=inverse variance, 95% CI=95% confidence interval.
Comparison 2: IFC Alone Versus Comparison Group on Pain Intensity at Discharge
Two studies56,63 were included in this comparison. One study63 measured outcomes at discharge after 2 to 3 weeks of treatment, and the other study56measured outcomes after 8 weeks. One trial56 studied the effect of IFC on acute low back pain, and the other trial63 studied the effect of IFC on chronic low back pain. Both studies were of moderate methodological quality. In this comparison, both studies agreed that IFC was not significantly better than manual therapy or traction and massage (Fig. 3). The pooled MD obtained for this analysis was −0.16 (95% CI=−0.62, 0.31). These results indicate that IFC alone was not significantly better than any of the comparisons at discharge from therapy.
Figure 3.
Forest plot of comparison: interferential current therapy (IFC) alone versus comparison treatment on pain intensity at 3 weeks and 8 weeks (data presented as change scores). IV=inverse variance, 95% CI=95% confidence interval.
Comparison 3: IFC as a Supplement to Another Treatment Versus Control Group on Pain Intensity at Discharge
Three studies53,54,66 were included in this comparison. Two studies53,54 used a 4-week discharge period, and one study66 used a one-day discharge period. One trial54 studied the effect of IFC on knee osteoarthritis, another trial53 studied the effect of IFC on frozen shoulder, and the third trial66 studied the effect of IFC on acute low back pain. Two studies included in this comparison were of moderate methodological quality,53,54 and one study was considered to be of high quality.66In this comparison, the 3 studies tended to significantly favor IFC applied as a cointervention when compared with the control group (Fig. 4). The pooled MD obtained for this analysis was 2.45 (95% CI=1.69, 3.22). Thus, IFC applied as a cointervention was more than 2 points better, as measured with the VAS, in reducing pain intensity when compared with a control group in these conditions.
Figure 4.
Forest plot of comparison: interferential current therapy (IFC) as a supplemental treatment versus control treatment on pain intensity at 1 day and 4 weeks (data presented as change scores). IV=inverse variance, 95% CI=95% confidence interval.
Comparison 4: IFC as a Supplement to Another Treatment Versus Placebo on Pain Intensity at Discharge
Five studies47,50,54,64,65 were included in this comparison. Different times of discharge were used in the studies, ranging from 2 weeks64,65 to 4 weeks.47,50,54Mean difference to pool the data was used. In addition, 95% CI and the random-effects model were chosen. In this comparison, 3 studies47,50,54 of moderate quality tended to significantly favor IFC as a cointervention when compared with placebo. One study64 of moderate methodological quality tended to significantly favor the placebo group. One study of moderate quality did not favor either IFC as a cointervention or placebo (Fig. 5, upper part).65 The pooled MD obtained for this analysis was 1.60 (95% CI=−0.13, 3.34). This finding indicates that although IFC as a cointervention was statistically significantly better than a placebo at decreasing pain intensity at discharge in conditions such as osteoarthritis, chronic low back pain, and fibromyalgia, IFC tended to reduce pain in these conditions when compared with a placebo condition. In addition, the heterogeneity among studies was I2=96%, which is considered substantial according to Cochrane group guidelines.45 Therefore, these results should be interpreted with caution.
Figure 5.
Forest plot of comparison: interferential current therapy (IFC) as a supplemental treatment versus placebo treatment on pain intensity at 1-week, 2-week, 4-week, and 3-month follow-ups (data presented as change scores). IV=inverse variance, 95% CI=95% confidence interval.
In this comparison, 2 studies64,65 provided follow-up data (3 months). Thus, an analysis at the 3-month follow-up was performed (Fig. 5, lower part). The pooled MD obtained for this analysis was 1.85 (95% CI=1.47, 2.23). The 2 studies significantly favored IFC when compared with the placebo. This finding indicates that IFC as a cointervention was better than a placebo at decreasing pain intensity at the 3-month follow-up.
Comparison 5: IFC as a Supplement to Another Treatment Versus Comparison on Pain Intensity at Discharge
Five studies49,52,53,55,60 were included in this comparison (Fig. 6). Different times of discharge were used, ranging from 1 day55 to 4 weeks49,53,60 to 2 months.52Two studies49,52 evaluated the effectiveness of IFC as a cointervention for knee osteoarthritis, 2 studies53,60 evaluated the effectiveness of IFC as a cointervention for shoulder pain, and 1 study55 evaluated the effectiveness of IFC as a cointervention for myofascial pain.
Figure 6.
Forest plot of comparison: interferential current therapy (IFC) as a supplemental treatment versus comparison treatment on pain intensity at 1 day, 2 weeks, 4 weeks, and 2 months (data presented as change scores). IV=inverse variance, 95% CI=95% confidence interval. B1=hot pack + active range of motion.
One study55 compared IFC plus hot packs, active range of motion, and myofascial release with 5 different treatment modalities; thus, different analyses were run in order to determine the effect of IFC as a cointervention when compared with all of these modalities (sensitivity analysis). We used the MD to pool the data. In addition, 95% CI and the random-effects model were chosen.
In this comparison, no clear trend favoring either IFC as a cointervention or the comparison treatments was observed for any of the analyses performed (Fig. 6). The pooled MD obtained for the various analyses was 0.55 (95% CI=−0.33, 1.44). The mean difference indicated that IFC as a cointervention was no better than other conventional interventions such as exercise, transcutaneous electrical nerve stimulation, or ultrasound plus hot packs at decreasing pain intensity at discharge.
Discussion
Analysis of the Analgesic Effect of IFC Alone
The results of this meta-analysis indicate that IFC applied alone as an intervention for musculoskeletal pain is not significantly better than placebo or comparisontherapy (ie, manual therapy, traction, massage) at discharge from physicaltherapy treatment. However, few included studies (27%) examined the clinical analgesic effectiveness of IFC as a single therapeutic modality, and most did not focus on a specific musculoskeletal disorder. We also observed differences in length of treatment (ie, 1, 2, 3, and 8 weeks) and type of pain (ie, acute or chronic), indicating no consensus on optimal treatment parameters, which potentially contributed to the nonsignificance of the results.
Analysis of the Analgesic Effect of IFC as Part of a Multimodal Protocol (Cointerventions)
An important factor in this meta-analysis was the inclusion and analysis of studies including the application of IFC as a cointervention in a multimodal treatment protocol. This decision was clinically sound because IFC is used mainly as an adjunct treatment. The results of this study indicate that IFC as a cointervention is significantly better than control and placebo for reducing chronic musculoskeletal pain at discharge and at 3 months posttreatment, respectively. The pooled effect for IFC as a cointervention versus control was 2.45 on the VAS (95% CI=1.69, 3.22). According to some authors, this change is considered a clinically meaningful effect for acute painful conditions.116–119 However, in chronic pain, a more stringent criterion seems to operate because a relative pain reduction of 50% or at least 3 cm on a VAS has been recommended for detecting a clinically successful pain reduction.120,121
In addition, when IFC as a cointervention was compared with placebo at discharge, there was no statistically significant difference between the groups. At 3-month follow-up, IFC as a cointervention obtained a better effect on the VAS, although less pronounced than when compared with a control group (pooled effect=1.85, 95% CI=1.47, 2.23). Thus, it seems that although IFC applied as a cointervention may have a modest analgesic effect, the magnitude of the effect is not large enough to be considered clinically relevant when compared with placebo or comparison interventions.
Because this is the first meta-analysis looking at the analgesic effect of IFC, direct comparisons cannot be made. In a previous study, Johnson and Martinson122concluded that transcutaneous electrical nerve stimulation, used mainly as an isolated intervention, provided significant pain relief when compared with a placebo intervention in a variety of chronic musculoskeletal conditions. Although methodological differences are present between both meta-analyses, some similarities such as the final sample sizes included, the focus on chronic musculoskeletal conditions, and clinical heterogeneity make the comparison between these 2 meta-analyses worth considering.
Some factors regarding IFC treatment may have accounted for the modest effect size observed. For example, although the stimulation of small-diameter fibers has been demonstrated to produce a more positive effect for chronic pain when compared with the stimulation of large-diameter fibers (Aβ),54 the included studies, regardless of the type of pain, used stimulation parameters that were related mainly to the stimulation of Aβ fibers and the pain gate mechanism.11,47–50,52,53,56–58,61,62 Although the stimulation of large-diameter fibers is acknowledged to produce a fast onset of analgesia, an important shortcoming is its brief analgesic effect.123–125 Thus, it is plausible that in chronic pain, which was the dominant condition in this review, the effectiveness of IFC under these stimulation parameters may have been attenuated, resulting in a small effect in reported pain reduction. Further research is needed to evaluate the effect of noxious stimulation (eg, small-diameter fibers) on IFC effectiveness, especially in chronic pain.
Additionally, IFC has not been applied using a consistent treatment protocol. For example, similar AMF settings (≥80 Hz) were considered for treating either acute56,57 or chronic47,50,53,55,64,65 conditions. Moreover, under the same condition (eg, osteoarthritis), the authors inconsistently applied fixed AMF frequencies (ie, 80 Hz)49 or sweep AMF frequencies (ie, 1–150 Hz, 30–60 Hz, 0–100 Hz).52,54,59 Although experimental evidence has challenged the role of AMF as the main analgesic component of IFC,36,37,85,126 inconsistency in the use of this parameter in clinical settings warrants consideration. Based on the current evidence, recommendations for optimal dosage when using IFC are not clear. It seems, however, that clinical evidence supports the fact that AMF should not be the most important parameter for clinical decision making. This fact has been corroborated by recent experimental evidence as well.80 Instead, the use of a sensory level of intensity appears to be a consistent factor for the majority of the studies. Although some variations in the number of treatments and the treatment time exist, it seems that 10 to 20 minutes of application for 2 to 4 weeks with a total of 12 sessions is the most common treatment protocol for IFC.47–51,53,54,59,60,62,64,65
In this systematic review, 16 out of 20 studies evaluated the role of IFC in chronic rather than acute pain. Based on this fact, it seems that IFC has been applied more often in the management of chronic painful conditions. Interestingly, and apparently in contrast to current clinical practice in which IFC is used mostly for short-term pain relief, this meta-analysis provided information regarding potential positive long-term benefits from IFC.64,65
Adverse Effects
An important safety feature when applying electrotherapy modalities is the report of adverse effects. Although IFC is considered a safe modality, its application has been associated with local adverse effects such as blisters, burns, bruising, and swelling.127,128 Interestingly, only 3 studies52,56,60 included reports of adverse effects as a result of IFC treatment. Two studies56,60 reported no complications, and one study52 reported the presence of muscle soreness in one subject. Reporting adverse effects must be mandatory, not only for the safety of patients, but also for the professional integrity of therapists.
Methodological Elements Affecting Observed Effect
Even though the quality of the trials appraised generally was moderate, there are some methodological biases common to these studies that could have had an impact on the results. Selection bias could have existed, as only 9 trials reported appropriate randomization and only 5 trials reported concealment of allocation. Another potentially important bias was the lack of blinding, especially of the patients (9 studies) and assessors (11 studies). The outcome measure for this meta-analysis was pain, which is a subjective outcome and dependent on the subject's report. Trials without appropriate randomization, concealment of allocation, and blinding tend to report an inaccurate treatment effect compared with trials that include these features.129–131
Other potential biases that could have affected the observed effects were the lack of an appropriate sample size (only 5 of the trials reported adequate sample size) and the inappropriate handling of withdrawals and dropouts (only 11 trials used intention-to-treat analysis). Reporting clinical significance of results has become a relevant issue to demonstrate the effectiveness of an intervention. Clinical significance provides the clinician with adequate information regarding the clinical impact of an intervention because it can identify when a meaningful change is produced.132 Despite this message, the report of clinically meaningful changes in the present study was largely neglected, with only 3 studies including this component.56,57,62
The present study used a compilation of items from all of the scales used in the studies in the physical therapy literature. Although some of the scales used inphysical therapy (ie, PEDro, Jadad) have been validated in some way, our recent analysis of health scales used to evaluate methodological quality determined that none of these scales are adequate for that use alone.42 Therefore, it was decided that all of these scales would be used to assess methodological quality, and we used a compilation of items to provide a comprehensive and sensitive evaluation of the quality of individual trials. However, further research investigating methodological predictors for determining trial quality in physical therapy is needed.
Summary of Evidence
As an isolated treatment, IFC was not significantly better than placebo or other interventions. Conversely, when included in a multimodal treatment plan, IFC displayed a pain-relieving effect (VAS reduction of over 2 points) compared with a control condition.
Strengths
This meta-analysis is the first systematic investigation regarding the pain-reducing effectiveness of IFC on musculoskeletal pain. A comprehensive search was made of all the published research in this area over a wide range of years (1950–2010). In addition, authors were contacted in an attempt to have complete information about the selected studies. The 20 RCT articles included in this review covered a broad spectrum of acute and chronic musculoskeletal conditions. Interferential current therapy was analyzed as isolated intervention, as well as part of a multimodal treatment plan. In addition, the study provided multiple analyses, including the comparison between IFC and placebo, the comparison between IFC and control, and IFC contrasted to different types of interventions.
Limitations
Outcome level.
A main limitation of this meta-analysis is the presence of clinical heterogeneity in the study population in most of the comparisons, casting some doubt on the validity of our results.
Study and review level.
A potential limitation is the omission of non–English-language publications; however, English is considered the primary scientific language. It also has been reported that language-restricted meta-analyses only minimally overestimate treatment effects (∼2% on average) compared with language-inclusive meta-analyses.114 Therefore, language-restricted meta-analyses do not appear to lead to biased estimates of intervention effectiveness.133,134 Applicability of results about the isolated effect of IFC on musculoskeletal pain also is limited, as only 4 studies addressed this issue. Another important limitation is that this study included only pain as an outcome measure. It would be important to know whether outcomes such as disability or function could have been modified by the application of IFC.
Conclusions
Implications for Practice
Interferential current therapy included in a multimodal treatment plan seems to produce a pain-relieving effect in acute and chronic musculoskeletal painful conditions compared with no treatment or placebo. Interferential current therapycombined with other interventions was shown to be more effective than placebo application at the 3-month follow-up in subjects with chronic low back pain. However, it is evident that under this scenario, the unique effect of IFC is confounded by the impact of other therapeutic interventions. Moreover, it is still unknown whether the analgesic effect of IFC is superior to that of these concomitant interventions.
When IFC is applied alone, its effect does not differ from placebo or other interventions (ie, manual therapy, traction, or massage). However, the small number of trials evaluating the isolated effect of IFC, heterogeneity across studies, and methodological limitations identified in these studies prevent conclusive statements regarding its analgesic efficacy.
Implications for Research
Because only 4 studies that evaluated the isolated effect of IFC were identified, and these studies had mixed results, further research examining this issue is needed, ideally in homogeneous clinical samples. Further research also is needed to study the effect of IFC on acute painful conditions. Also of interest would be the study of the effect of IFC in chronic conditions using a theoretical framework for the selection of parameters associated with suprasegmental analgesic mechanisms (ie, noxious stimulus) instead of sensory stimulation.
The Bottom Line
What do we already know about this topic?
Despite the widespread use of interferential current (IFC), information about its clinical effectiveness is limited and controversial. The pain-reducing effect of IFC, when applied alone or as part of a multimodal treatment plan to treat musculoskeletal pain, has not been determined.
What new information does this study offer?
The application of IFC as part of a multimodal treatment plan appears to produce a modest pain-relieving effect in a broad spectrum of acute and chronic musculoskeletal conditions when compared with no treatment or placebo. In addition, the potential long-term effects of IFC versus placebo observed at 3-month follow-up are of interest.
Interferential current alone was not significantly better than placebo and other interventions (ie, manual therapy, traction, or massage). However, heterogeneity across the included studies, along with methodological limitations identified in these studies, prevents conclusive statements regarding the analgesic efficacy of IFC.
If you're a patient, what might these findings mean for you?
If you are seeking pain treatment, IFC could be potentially effective in reducing musculoskeletal pain; however, its application should be included as part of a multimodal treatment plan.
Footnotes
- Mr Fuentes, Dr Armijo Olivo, and Dr Gross provided concept/idea/research design and writing. Mr Fuentes and Dr Armijo Olivo provided data collection and analysis. Mr Fuentes provided project management. Dr Magee and Dr Gross provided consultation (including review of manuscript before submission).
- This study was supported by the Alberta Provincial CIHR Training Program in Bone and Joint Health, an Izaak Walton Killam scholarship to Dr Armijo Olivo from the University of Alberta, the Provost Doctoral Entrance Award to Mr Fuentes from the University of Alberta, the Canadian Institutes of Health Research, the government of Chile (MECESUP Program), the University Catholic of Maule, Chile, and the Physiotherapy Foundation of Canada through an Ann Collins Whitmore Memorial Award to Dr Armijo Olivo.
- ↵* SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.
- ↵† Copenhagen, Denmark: The Nordic Cochrane Centre, The Cochrane Collaboration, 2008.
- Received October 13, 2009.
- Accepted May 14, 2010.
- © 2010 American Physical Therapy Association
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