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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

Objective

To determine the additional effects of transcutaneous electrical nerve stimulation (TENS) for knee osteoarthritis (OA) when combined with a group education and exercise program (knee group).

Methods

The study was a randomized, sham-controlled clinical trial. Patients referred for physiotherapy with suspected knee OA (confirmed using the American College of Rheumatology clinical criteria) were invited. Exclusion criteria included comorbidities preventing exercise, previous TENS experience, and TENS contraindications. Prospective sample size calculations required 67 participants in each trial arm. A total of 224 participants (mean age 61 years, 37% men) were randomized to 3 arms: TENS and knee group (n = 73), sham TENS and knee group (n = 74), and knee group (n = 77). All patients entered an evidence-based 6-week group education and exercise program (knee group). Active TENS produced a “strong but comfortable” paraesthesia within the painful area and was used as much as needed during the 6-week period. Sham TENS used dummy devices with no electrical output. Blinded assessment took place at baseline and 3, 6, 12, and 24 weeks. The primary outcome was the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function subscale at 6 weeks. Secondary outcomes included WOMAC pain, stiffness, and total scores; extensor muscle torque; global assessment of change; exercise adherence; and exercise self-efficacy. Data analysis was by intent to treat.

Results

All outcomes improved over time (P < 0.05), but there were no differences between trial arms (P > 0.05). All improvements were maintained at 24-week followup.

Conclusion

There were no additional benefits of TENS, failing to support its use as a treatment adjunct within this context.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

Knee osteoarthritis (OA) is associated with pain, decreased range of movement, and muscle weakness, which over time may lead to function loss ([1]) and substantial economic burden ([2]). Of those ages >55 years, 16.3% of men and 29.1% of women exhibit radiographic evidence of knee OA, with age, pain, and stiffness contributing to locomotor disability ([3]). Knee OA therefore presents a significant source of morbidity. Cochrane reviews of the evidence for knee OA interventions have indicated that exercise is effective for both pain and function ([4, 5]). The latest Cochrane review exploring transcutaneous electrical nerve stimulation (TENS), however, was unable to support its effectiveness for pain relief ([6]). The latter review highlighted poor methodologic quality and inadequate statistical power of existing trials, supporting the need for more robust randomized controlled trials (RCTs).

Therapists often employ complex intervention packages, but research into the effectiveness of combining treatments has been neglected ([7]). TENS specifically has been advocated as an adjunct to other treatments ([8-10]), and those who use TENS often do so in conjunction with exercise ([7]). Clinical guidelines also recommend exercise and patient education as core treatments, with TENS as an adjunct ([9]). Elucidation of the effects of TENS when combined with exercise and education is therefore an important clinical research question. Such effects have yet to be clearly established, with a limited number of previous studies in knee OA.

One study found that TENS in combination with exercise was no better for OA knee pain intensity than either in isolation ([11]). Another trial that met its prospectively calculated sample size found that neither TENS nor interferential current provided additional benefits (on pain and function) over exercise alone ([12]). A third trial reported a trend toward TENS and exercise producing better improvements in a range of physical outcome measures (isometric peak torque, gait parameters, and knee range of movement) when compared with either intervention in isolation or sham (no current) stimulation ([13]), although the results failed to reach statistical significance. Common features of these studies are low participant numbers (total n = 46–62, n = 15 or 16 per trial arm) and the intermittent and brief (20–60 minutes) application of TENS where effectiveness may be maximized when used for extended periods throughout the day ([10]). Each study applied TENS for only 4 weeks, which is shorter than the minimum 6 weeks suggested by an earlier Cochrane review ([14]). Further, TENS was administered by therapists in the clinic when TENS is designed for self-administration at home ([6]). Suboptimal TENS dosing and inappropriate outcome assessment have been identified as particularly prevalent weaknesses indicating low fidelity in RCTs, and this contributes to negative findings ([15]). A high-quality trial was therefore required, with close attention to issues of statistical power, TENS application, and outcomes assessment.

This study is a randomized, sham-controlled clinical trial designed to determine the additional effects of TENS in knee OA when combined with a 6-week group education and exercise regimen (knee group).

Box 1. Significance & Innovations

  • People with knee osteoarthritis improved over time with a group education and exercise intervention.
  • Improvements included pain, stiffness, function, strength, exercise adherence, exercise self-efficacy, and global assessment of change.
  • Transcutaneous electrical nerve stimulation failed to provide any additional clinical benefit and cannot be recommended as a treatment adjunct in this context.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

Study design and population

The study was conducted in accordance with the Declaration of Helsinki and received a favorable opinion from the Cambridgeshire 2 Research Ethics Committee (07/H0308/209). This was a randomized, sham-controlled trial with 3 parallel arms: 1) TENS and knee group, 2) sham TENS and knee group, and 3) knee group.

Patients referred to physiotherapy at University Hospitals Bristol with confirmed or suspected knee OA were eligible. Referrals were screened by a research associate (MD). Telephone screening was then conducted by the research associate (MD) or 1 of 2 other therapists trained in the recruitment procedure. The telephone interview offered referral into the knee group (routine care for knee OA patients at University Hospitals Bristol) and gave initial verbal information about the research and an opportunity to ask questions. Patients who confirmed that they were interested were sent a patient information sheet and an appointment for a full standardized physiotherapy assessment with the research associate (MD).

The physiotherapy assessment aimed to ensure the knee group was clinically appropriate, to screen for study inclusion and exclusion criteria, and to provide patients with an opportunity to ask further questions. Those agreeing to enter the trial provided formal written consent, completed the baseline outcome measures (see below), and were assigned a consecutive study number in the order in which they attended for assessment. They were given a date to commence the knee group, and the participant's study number and contact details were forwarded to the TENS instructors. The inclusion criteria comprised being age ≥18 years with knee OA confirmed by the American College of Rheumatology (ACR) clinical criteria ([16]). The ACR criteria are knee pain accompanied by at least 3 of 6 signs and symptoms (age >50 years, stiffness <30 minutes, crepitus, bony tenderness, bony enlargement, and no palpable warmth). Exclusion criteria were comorbidities preventing participation in the knee group ([17]), contraindications to TENS ([18]), and previous TENS experience. Those not fulfilling the eligibility criteria or declining entry to the trial were referred either for individual physiotherapy treatment or to the knee group as normal (but not as a study participant), according to which was considered most clinically appropriate.

Randomization

Participants were randomly assigned to 1 of the 3 trial arms. An independent monitor prepared sealed opaque envelopes containing treatment allocations. A total of 261 envelopes were prepared, with 87 allocations to each of the 3 treatment arms. Envelopes were mixed and then mixed again before being numbered consecutively. Once prepared, the trained TENS instructors were the only individuals to have access to the envelopes and were responsible for assigning all participants by opening the appropriately numbered envelope corresponding to their study number. All other members of the research team and clinical staff were blinded to allocation throughout. Before participants commenced the knee group, the TENS instructors contacted those allocated to active or sham TENS to arrange TENS training. Participants allocated to receive only the knee group intervention simply attended the knee group as originally scheduled.

TENS interventions

TouchTENS devices (model XL-Y1, TensCare) were used throughout. Sham devices were deactivated by the manufacturer. All devices were checked by the Medical Equipment Management Organisation at University Hospitals Bristol and were randomly assigned a device number before being employed in the trial. Only the TENS instructors and one member of the research team (FC) had access to the identity (active or sham) of individual devices. This research team member (FC) provided all training for the TENS instructors according to standardized operating procedures. This team member (FC) also audited the processes associated with randomization and treatment allocation, including periodic observation of TENS instruction sessions.

TENS instructors were all physiotherapy technical instructors working with patients with musculoskeletal disorders. Four TENS instructors were employed on the project to reflect the pragmatic nature of the trial. TENS instruction comprised a 30-minute appointment during which patients were also assessed for competency to self-administer. Patients received a TENS device for their personal use from the date of TENS instruction and throughout the 6-week duration of the knee group. Written and verbal information detailed the existence of different TENS devices, that some do not produce perceptible sensations, and that participants may receive an active or inactive device (in line with the principles of fully informed consent). Patients were taught to position 4 electrodes around the knee joint: 2 on the medial and 2 on the lateral aspect on either side of the joint line (such that each of the 2 electrical circuits diagonally crossed the knee). For the purposes of instruction, the devices were set to a continuous mode (program A: 110 Hz, 50 μs). All electrical pulses were asymmetric and biphasic. Dummy devices (the displays were active but there was no current output) were used to administer sham TENS. All written and verbal instructions were standardized as far as possible, although patients were allowed to ask questions and received further instruction as required to ensure adequate understanding. Due to a lack of evidence for the specific effects of many TENS parameters ([19-22]), patients were instructed to use the device as much as needed ([23, 24]) and encouraged to try different TENS programs. TENS could be used before, during, or after exercise or physical function. Stimulus intensity is one parameter related to treatment efficacy ([19, 22]); therefore, those receiving active TENS were instructed to select a stimulation intensity that generated a “strong but comfortable” tingling sensation within or close to the site of pain. Those receiving sham TENS were instructed to select an intensity of 7 or 8 (within the middle range of the 15 settings available). TENS devices were returned to the TENS instructor at the end of the 6-week intervention. Participants were requested not to discuss their TENS treatment with the research staff, clinical staff, or fellow patients during the trial.

Exercise intervention

Full details of the evidence-based education and exercise program previously have been reported by Domaille et al ([17]), and it has been shown to be as effective as similar programs. To reflect the pragmatic nature of the trial, 7 therapists were trained to facilitate the knee groups. To maximize consistency, the same therapist aimed to lead all 6 sessions of each group. The education component was supported by a standardized presentation and protocol. All participants took part in the 6-week knee group ([17]). This involved a group of up to 12 patients attending for 1 hour (30 minutes of education and 30 minutes of group exercise) on 6 consecutive weeks. The education program aimed to enhance patients' abilities to self-manage their condition. It included information on setting personal objectives, pacing, managing flares, diet, medical management of OA, local community exercise opportunities, and long-term exercise adherence. The exercise component included a 5-minute warm-up, followed by a circuit of exercises aimed at improving lower extremity strength, proprioception, and function. Each exercise had specific ideas for progression that patients advanced as able over the 6-week program. During the first session, each exercise was performed for 1 minute, with 1 minute between exercises to move to the next station. On the subsequent 5 sessions, each exercise was performed for 2 minutes, which included the time to move to the next station. All patients were taught home exercises during the second session and advised to perform them daily. These included step-ups, sit to stand, balancing on one leg, and heel to toe walking. Increase in general physical function was a key aim of the intervention and was supported by individual action plans. The knee group was supported by a booklet containing written advice on the topics covered in the education session, details of the home exercises, and tools to aid goal setting.

Outcomes

Baseline outcome measures were taken by a research associate (MD) at the end of the initial physiotherapy assessment. Two blinded assessors (MD and another senior physiotherapist specializing in rheumatology) took all other outcome measures at weeks 3, 6, 12, and 24. Assessments took place in the same treatment cubicle to standardize the environment. Participants were asked to complete all paper-based outcome measures prior to attending. Outcomes were chosen to meet the recommendations of the Outcome Measures in Rheumatology core set ([25]).

The primary outcome measure was the function subscale of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) ([26]) at 6 weeks. Although TENS is primarily used for pain relief, function was chosen to reflect the overall aim of the combined TENS, education, and exercise package to enhance physical function. The total WOMAC score and pain and stiffness subscales were included as secondary outcome measures. The WOMAC was administered at all outcome points.

Maximum knee extensor torque (as a measure of quadriceps strength) was measured in upright sitting (knees and hips at 90° flexion) at all outcome points using a digital myometer (MIE Medical Research). One knee was tested, chosen by the patient as being the most painful. Participants undertook one practice followed by one assessment measure (to avoid aggravating patients' pain). The assessor and participant were blinded to the myometer reading by a flap that was later uncovered, recorded (in N), and divided by the moment arm length to give a torque value (in Nm). Extensor torque was the only outcome measure that was not patient reported; therefore, an interrater reliability study was conducted using the first 12 participants entering the trial. Reliability was found to be excellent (intraclass correlation coefficient 0.95) and supported the use of 2 blinded assessors.

A patient global assessment of change scale ([27]) was used at weeks 3, 6, 12, and 24. Patients were asked to state whether they were the same, better, or worse compared to when they first attended the knee group. If patients chose better or worse, they were asked to indicate on a 7-point Likert scale by how much (ranging from a tiny bit, almost the same to a very great deal).

Self-efficacy for exercise ([28]) was assessed at baseline and at week 24. Patients were asked to respond to 4 statements about their ability and confidence to exercise using a 5-point Likert scale (ranging from strongly disagree to strongly agree). Self-reported exercise adherence was assessed by asking patients to indicate how often they undertook the daily exercises using a 5-point Likert scale (ranging from never to always).

Unknown to patients, the TENS devices logged the cumulative duration of use, which was retrieved on return of the device. A brief questionnaire about the settings used was also completed by all TENS users. To assess the effectiveness of blinding, assessors and exercise therapists were asked to guess whether or not each participant had received a TENS machine.

Sample size

Sample size calculations were based on the WOMAC function subscale data reported by Tubach and colleagues ([29]), with data converted to scores out of 68 (the WOMAC function subscale total) rather than 100. This gave a mean ± SD of 29.1 ± 10.9, with the minimum clinically significant improvement being 6.2 of 68. At an alpha level of 0.05 and with 90% power, a minimum sample size of 67 in each trial arm was calculated. Allowing for 30% attrition, a target recruitment of 261 participants was set (87 in each trial arm).

Statistical analysis

Primary data analysis was by intent to treat using IBM SPSS Statistics, version 19. Missing values were imputed using multiple imputation techniques. With the exception of age and the WOMAC function subscale and WOMAC total score, data were not normally distributed. Log, square root, and reciprocal transformation techniques were applied, with little improvement in data distributions. Although deviation from normality does not preclude parametric analysis, a more conservative approach was taken and it was decided to use nonparametric analyses on all data that deviated from a normal distribution. Changes over time and between trial arms were investigated using a general linear model repeated-measures test or nonparametric Kruskal-Wallis and Friedman's tests, as appropriate. All data analysis was performed blind to trial arm allocations.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

Recruitment and retention

Figure 1 shows the recruitment and retention of study participants. Telephone screening effectively identified potential participants who were ineligible, no longer needed treatment, were unwilling, or had difficulty attending. Table 1 shows that randomization resulted in largely comparable trial arms (on the basis of demographic information and baseline outcome measures). The only potential exception was extensor torque, with the participants who received active TENS displaying slightly lower baseline values. Total loss to followup was 13% at 6 weeks and 22% at 24 weeks. Loss to followup was slightly higher in the knee group when compared to either of the TENS groups. The reasons for withdrawal failed to identify why, although more people in this trial arm failed to give a reason and some may have been disappointed not to have received TENS. Five patients were not issued with TENS devices: 1 (active TENS) due to scheduling problems and 4 (2 active and 2 sham TENS) who withdrew after baseline assessment. All were still included as part of an intent-to-treat analysis.

image

Figure 1. Flow chart of recruitment and retention of study participants. Data analysis was by intent to treat using multiple imputation; therefore, 224 data sets were available for analysis. TENS = transcutaneous electrical nerve stimulation.

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Table 1. Participant characteristics and baseline values for outcome measures*
 Active TENS and knee group (n = 73)Sham TENS and knee group (n = 74)Knee group (n = 77)Total (n = 224)
  1. Values are the median (interquartile range) unless indicated otherwise. TENS = transcutaneous electrical nerve stimulation; WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index.

Sex, women:men47:2649:2545:32141:83
Study knee, left:right37:3636:3838:39111:113
Age, mean ± SD years61.2 ± 11.460.9 ± 10.862.0 ± 9.461.4 ± 10.5
Body mass index, kg/m229.7 (11.1)29.1 (9.0)29.8 (7.4)29.6 (8.4)
Pain duration, years4.4 (8.7)4.0 (8.7)3.8 (9.8)4.0 (8.7)
WOMAC total (max 96), mean ± SD41.7 ± 19.241.0 ± 17.639.0 ± 18.140.5 ± 18.2
WOMAC function (max 68), mean ± SD29.3 ± 14.028.8 ± 13.027.5 ± 13.528.5 ± 13.5
WOMAC pain9.0 (6.0)9.0 (5.0)8.0 (5.8)9.0 (6.0)
WOMAC stiffness4.0 (2.0)4.0 (2.0)4.0 (2.0)4.0 (2.0)
Extensor torque, Nm42.0 (40.2)47.8 (34.9)45.1 (54.6)45.0 (45.1)
Exercise self-efficacy (max 20)14.6 (4.0)15.0 (3.9)14.6 (3.5)15.0 (4.0)
Exercise beliefs62.2 (8.1)63.0 (10.2)62.0 (10.8)63.0 (9.6)

Primary outcome

Results for the primary outcome measure of WOMAC function, along with the total WOMAC score, are shown in Table 2. There was a clear trend of improvement over time in both outcomes, but little obvious difference between the groups. Improvements over time for each score were statistically significant (both P < 0.001), but differences between trial arms or time × trial arm interaction effects were not (all P > 0.05). Table 3 shows the proportion of patients in each trial arm who met the clinically significant threshold of improvement of >6.2 points on the WOMAC function subscale at the primary outcome point of 6 weeks. A slightly greater proportion of those who received exercise in isolation achieved this magnitude of improvement. The pooled standardized effect size from baseline (using data for all 224 participants) was 0.30 at both week 6 and week 24.

Table 2. WOMAC function (primary outcome measure) and WOMAC total scores over time for each trial arm*
Outcome/trial armBaselineWeek 3Week 6Week 12Week 24Effects, P
TimeTrial armTime × trial arm
  1. Values are the mean ± SD. A reduction in scores indicates an improvement in the condition. Effects of time, trial arm, and time × trial arm interaction were tested using a general linear model repeated-measures test. Post hoc analyses of changes over time did not provide any novel findings and have therefore been excluded in the interests of brevity and because this was not the primary focus of the investigation. Since the overall effect of the trial arm was nonsignificant, between-group comparisons at each time point have not been explored. WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index; TENS = transcutaneous electrical nerve stimulation.

  2. a

    Statistically significant.

WOMAC function (max 68)     < 0.001a0.4130.528
Active TENS and knee group (n = 73)29.3 ± 14.026.2 ± 13.826.4 ± 15.025.3 ± 14.125.8 ± 13.8   
Sham TENS and knee group (n = 74)28.8 ± 13.026.9 ± 14.025.1 ± 13.925.7 ± 14.125.3 ± 15.0   
Knee group (n = 77)27.5 ± 13.524.4 ± 11.622.2 ± 12.124.3 ± 11.922.6 ± 13.4   
WOMAC total (max 96)     < 0.001a0.3630.541
Active TENS and knee group (n = 73)41.7 ± 19.237.4 ± 18.837.3 ± 20.436.2 ± 19.436.7 ± 19.5   
Sham TENS and knee group (n = 74)41.0 ± 17.638.7 ± 18.435.7 ± 18.936.4 ± 19.535.7 ± 20.6   
Knee group (n = 77)39.0 ± 18.134.9 ± 15.931.7 ± 16.734.4 ± 16.631.8 ± 18.4   
Table 3. Proportion of patients in each trial arm who met the clinically significant difference of 6.2 of 68 points on the Western Ontario and McMaster Universities Osteoarthritis Index function subscale at 6 weeks*
 Active TENS and knee group (n = 73)Sham TENS and knee group (n = 74)Knee group (n = 77)
  1. TENS = transcutaneous electrical nerve stimulation.

Improving by >6.2 points, %33.235.941.6

Secondary outcomes

Table 4 shows clear improvements over time in the secondary outcomes of WOMAC pain, WOMAC stiffness, extensor torque, self-reported global change, and exercise self-efficacy, but no obvious differences between the groups. Reported exercise adherence did not seem to change over time. Statistical analysis confirmed that there were no statistically significant differences between trial arms (all P > 0.05 at each time point), although each of the secondary outcomes improved over time (all P < 0.05).

Table 4. Secondary outcome scores over time for each trial arm*
Outcome/trial armBaselineWeek 3Week 6Week 12Week 24Effect of time, P
  1. Values are the median (interquartile range) unless indicated otherwise. A reduction in Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain and stiffness scores indicates an improvement in the condition. An increase in all other scores indicates improvement. Effects of trial arm were tested using Kruskal-Wallis tests and time effects were tested using Friedman's tests. Since there were no trial arm effects for any outcome measure, the effects of time were calculated using all participants (n = 224). Post hoc analyses of changes over time did not provide any novel findings and have therefore been excluded in the interests of brevity and because this was not the primary focus of the investigation. TENS = transcutaneous electrical nerve stimulation.

  2. a

    Statistically significant.

WOMAC pain     < 0.001a
Active TENS and knee group (n = 73)9.0 (6.0)7.5 (7.0)6.0 (5.0)7.0 (7.8)7.0 (8.0) 
Sham TENS and knee group (n = 74)9.0 (5.0)8.0 (6.0)8.0 (7.0)7.0 (7.0)6.0 (8.0)
Knee group (n = 77)8.0 (5.8)7.0 (5.8)6.0 (5.0)6.0 (6.3)6.0 (7.0)
Effect of trial arm, P0.5900.5370.4650.3860.226 
WOMAC stiffness     < 0.001a
Active TENS and knee group (n = 73)4.0 (2.0)4.0 (2.0)3.0 (3.0)4.0 (3.0)3.5 (3.0) 
Sham TENS and knee group (n = 74)4.0 (2.0)4.0 (3.0)4.0 (2.0)4.0 (3.0)3.0 (3.0)
Knee group (n = 77)4.0 (2.0)3.0 (2.0)3.0 (2.5)3.0 (3.0)3.0 (2.5)
Effect of trial arm, P0.5030.5380.2010.5270.510 
Extensor torque, Nm     < 0.001a
Active TENS and knee group (n = 73)42.0 (40.2)47.6 (56.0)49.2 (51.5)51.5 (45.2)53.9 (44.6) 
Sham TENS and knee group (n = 74)47.8 (34.9)49.4 (41.9)56.6 (35.8)53.9 (44.4)58.9 (39.7)
Knee group (n = 77)45.1 (54.6)55.6 (52.6)62.0 (60.5)58.7 (53.0)62.2 (62.2)
Effect of trial arm, P0.4400.7480.7880.6600.419 
Global assessment of change (range −7 to +7)     < 0.001a
Active TENS and knee group (n = 73)2.0 (3.1)3.0 (3.5)2.0 (5.6)2.8 (5.8) 
Sham TENS and knee group (n = 74)2.0 (3.7)2.9 (4.4)2.7 (5.4)2.9 (6.4)
Knee group (n = 77)2.0 (3.0)3.0 (4.8)2.0 (5.0)3.2 (5.3)
Effect of trial arm, P0.2120.5900.7850.595 
Exercise self-efficacy (range 5–20)     0.031a
Active TENS and knee group (n = 73)14.6 (4.0)15.8 (4.5) 
Sham TENS and knee group (n = 74)15.0 (3.9)16.0 (4.1)
Knee group (n = 77)14.6 (3.5)16.0 (5.5)
Effect of trial arm, P0.6930.259 
Exercise adherence (range 0–4)     < 0.001a
Active TENS and knee group (n = 73)3.0 (1.1)3.0 (1.0)3.0 (1.0)3.0 (1.0) 
Sham TENS and knee group (n = 74)3.0 (1.0)3.0 (0.3)3.0 (1.0)3.0 (1.0)
Knee group (n = 77)3.0 (1.6)3.0 (1.9)3.0 (1.0)3.0 (1.2)
Effect of trial arm, P0.5550.3910.2920.159 

Blinding

Questionnaires about how patients used TENS were returned by 74% of participants (n = 54 of 73) in the active TENS arm and 81% (n = 60 of 74) in the sham TENS arm. Of those who predicted whether they had received an active or inactive device (participants had to choose one or the other), 92% of active TENS users (n = 46 of 50) guessed correctly. Correct identification in the sham TENS arm was only slightly better than chance at 60% (n = 36 of 60). Data on blinding of the assessor and exercise therapists were available for 51% (n = 114 of 224) and 35% (n = 78 of 224), respectively. They were asked whether or not they thought individual participants had a TENS machine (regardless of whether it was active or sham). The proportion of correct responses was worse than chance, at 43% for the assessor (n = 49 of 114) and 30% for the exercise therapists (n = 23 of 78).

TENS usage

TENS usage (as logged by the devices) was available for 88% of active TENS users (n = 64 of 73) and 93% of sham TENS users (n = 69 of 74). Median logged usage was 46.5 and 39.0 hours for active and sham TENS, respectively (Mann-Whitney U test P = 0.861). The self-reported settings “normally used” used by participants are shown in Table 5. Data for this question were available for 67% (n = 49 of 73) and 72% (n = 53 of 74) of active and sham TENS users, respectively. The results show a large variation in settings used by participants in each trial arm.

Table 5. Settings reported by TENS users as those that they “normally used”*
TENS programActive TENS, %Sham TENS, %
  1. Note that patients were asked to choose the transcutaneous electrical nerve stimulation (TENS) program letter identifier, since this was all that was visible to participants on the device display. Additional TENS parameter details are included here for clarity.

  2. a

    Setting A was used for initial instruction, but participants were encouraged to try other settings throughout the 6-week TENS intervention.

A. Continuous: 110 Hz (50 μs)a2217
B. Continuous: 4 Hz (200 μs)88
C. Burst: frequency within burst 100 Hz (200 μs); repetition frequency of bursts 2 Hz208
D. Continuous: 10 Hz (200 μs)1211
E. Continuous: 110 Hz (200 μs)26
F. Intensity modulation: 110 Hz (200 μs)29
G. Frequency modulation: 2 Hz, 10 Hz, 50 Hz, 80 Hz, 90 Hz, 100 Hz, and 110 Hz (200 μs)421
Combination2921

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

This investigation found that using TENS as an adjunct to a 6-week group education and exercise program failed to elicit additional clinical benefits. Thirty-three percent of active TENS users achieved the prespecified clinically significant improvement in function, slightly lower than the sham TENS group (36%) or the knee group in isolation (42%).

These results support previous but inconclusive findings of a lack of an additional effect of TENS when combined with exercise for pain intensity ([11]) and for pain and function ([12]). Another study observed a non–statistically significant trend toward TENS and exercise improving a range of physical outcome measures ([13]). The present investigation failed to substantiate those trends. The present findings concur with those of the most recent Cochrane review on the effectiveness of TENS for OA knee pain ([6]), although it should be noted that our study focused on identifying adjunctive rather than isolated benefits of TENS.

Alternative explanations for the findings could be that the trial was not sensitive enough to detect any additional benefit of TENS from that achieved by the knee group, or that the knee group reduced symptoms to a level that would not benefit from TENS. However, although the variability of baseline WOMAC function scores was slightly greater than the values used for the prospective sample size calculation (reducing the statistical power to 80%), this was considered acceptable and there was certainly no trend suggestive of TENS benefits.

The knee group intervention seems effective, with improvements in all outcome measures over time, although it should be noted that this investigation was not specifically designed to assess the efficacy of the knee group. Nonetheless, the standardized effect size of 0.30 for the primary function outcome at both week 6 and week 24 suggests moderate treatment effects similar in magnitude to nonsteroidal antiinflammatory drugs ([30]). It is slightly less than the effect size of 0.37 at 6 weeks observed by Domaille and colleagues ([17]) using the same knee group intervention, although the present cohort was slightly less disabled and arguably had less scope for improvement. The effect size was comparable to that observed by Hurley and colleagues (0.28 at 6 months) ([31]), whose intervention was adapted ([17]) and employed in the present investigation.

The study effectively blinded the outcome assessor and exercise therapists to treatment allocation. Blinding patients to the sham intervention was relatively successful, with 40% believing that it was active (compared to 50% expected by chance). This was, however, much less than the 92% of active TENS users who correctly identified that their device was active and the 84% of sham TENS users who thought they had an active device achieved by Deyo and colleagues ([32]). Because of the lack of additional effect of either active or sham TENS, patient blinding issues are not thought to have affected the trial outcome. Interestingly, the logged usage of sham and active TENS devices was comparable and a wide range of programs were used (Table 5), suggesting a high level of interaction with, and use of, sham TENS devices.

The lack of standardization of TENS parameters may be seen by some as a limitation, but was designed to encourage patients to engage with the devices. It is known that individual patients experiment with TENS devices to find settings that suit them ([19]), and this was evidenced by the wide range of settings used in the present study (Table 5). There is currently a lack of evidence for the effects of many specific parameters ([19-22]), and therefore prescribed use may discourage engagement with TENS devices. More formal verification of device parameters than was conducted in this study would be required to identify their importance.

It would be interesting in the future to more fully explore the interaction between TENS use and exercise adherence. Anecdotal evidence suggested some practical issues with TENS use, such as leads pulling and pads peeling off. It is also conceivable that TENS use might engender a slightly more passive approach to self-management. Both issues have the potential to reduce engagement with exercise. Although the limited data suggested no difference in exercise adherence between the groups, a much more nuanced approach to assessing exercise adherence might help to determine if and how TENS affects engagement with exercise. Identification of predictors of TENS outcome could be a further important focus for future research ([33, 34]).

In conclusion, the findings fail to support the use of TENS as an adjunct to a group education and exercise intervention.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Palmer had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Palmer, Domaille, Cramp, Walsh, Kirwan, Johnson.

Acquisition of data. Domaille, Pollock.

Analysis and interpretation of data. Palmer, Domaille, Cramp, Kirwan, Johnson.

Acknowledgments

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES

The authors would like to thank the patient research partners who contributed to the design and conduct of this trial. We would also like to acknowledge all of the patients who took part and the clinical staff who contributed to the trial conduct.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. Acknowledgments
  9. REFERENCES
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