Comparison of the effects of transcutaneous electrical nerve stimulation and microcurrent electrical neuromuscular stimulation after total knee arthroplasty

Abstract

[Purpose] This study aimed to compare the effects of transcutaneous electrical nerve stimulation and microcurrent electrical neuromuscular stimulation on pain relief and knee function following total knee arthroplasty. [Participants and Methods] This was a prospective, single-center, three-group parallel study. Thirty-five patients scheduled for total knee arthroplasty were divided into transcutaneous electrical nerve stimulation, microcurrent electrical neuromuscular stimulation, and control groups. Interventions began on postoperative day 3 and continued for two weeks. Pain intensity during walking, maximum walking speed, timed up-and-go test, and isometric knee extension strength were assessed preoperatively, and two and four weeks postoperatively. [Results] Two weeks postoperatively, pain during walking was lower in the transcutaneous electrical nerve stimulation group than in other groups, and maximum walking speed was significantly faster in the transcutaneous electrical nerve stimulation group than in the microcurrent electrical neuromuscular stimulation group but not in the control group. Timed up-and-go and isometric knee extension strength improvements were observed gradually but were not significantly different between the groups. [Conclusion] Transcutaneous electrical nerve stimulation effectively relieved pain and improved walking speed early after total knee arthroplasty, whereas microcurrent electrical neuromuscular stimulation did not show significant benefits compared to the control.

INTRODUCTION

Transcutaneous electrical nerve stimulation (TENS) is an electrical stimulation therapy used after total knee arthroplasty (TKA)^1,2,3). During TENS, two or more electrodes are applied to the skin, and electrical stimulation is performed using a special device. TENS is thought to affect the peripheral, spinal, and supraspinal structures for pain relief¹⁾. A systematic review and meta-analysis reported that TENS could reduce pain and decrease the doses of morphine and opioids after TKA^{2, 3)}. Although most studies on pain relief after TKA have focused on TENS, microcurrent electrical neuromuscular stimulation (MENS), another electrical stimulation method, has also been studied⁴⁾. MENS is an electrical stimulation therapy that mimics the electric current that is generated during the repair process after tissue damage and has been used in the treatment of bone and skin⁵⁾. A previous study showed that MENS decreased pain and the dose of tramadol required after TKA⁶⁾.

TENS and MENS have been reported to have analgesic effects after TKA, despite their differing treatment mechanisms. One study compared the two methods in patients with temporomandibular disorders⁷⁾. However, no study comparing the two methods in cases involving surgical invasiveness, such as TKA, has been conducted. Therefore, we aimed to investigate and compare the analgesic effects of TENS and MENS in patients after TKA. We hypothesized that 1) both TENS and MENS would show analgesic effects, but 2) MENS would show greater improvement in knee function (i.e., muscle strength and walking ability) than TENS because of its effect on soft tissue healing.

PARTICIPANTS AND METHODS

This was a prospective, single-center, three-group, parallel study. We enrolled 35 patients with end-stage knee osteoarthritis scheduled for primary TKA (mean age: 70.9 ± 8.1 years, weight: 60.9 ± 9.3 kg, height: 151.4 ± 7.0 cm). The exclusion criteria were as follows: 1) end-stage circulatory disease, 2) cardiac or respiratory disease to the extent that postoperative rehabilitation would be compromised, 3) history of central nervous system disease, and 4) history of cognitive dysfunction. This study was approved by the Ethics Committee of Hirosaki Memorial Hospital (approval number 28-1) before initiation. Written informed consent was obtained from all the participants.

Cruciate-retaining (CR)-type TKAs were performed using the Triathlon Knee CR System (Stryker Japan K.K., Tokyo, Japan). Two senior surgeons (YW and TS) performed all the procedures. The measured resection technique was used in this study. The medial parapatellar approach was used for the knee. Finally, the components were fixed using a cementless technique. All participants received conventional physical therapy on postoperative day one. On postoperative day three, knee range of motion, quadriceps strength, and gait exercises were performed. Stair climbing exercises were initiated on postoperative day eight. All participants were hospitalized for 4 weeks after TKA.

Participants who met the eligibility criteria were assigned to the 1) TENS group, 2) MENS group, or 3) control group after TKA. We used the envelope method of randomization; cost and time constraints precluded the use of a computerized randomization system. Participants were blinded to the allocation results. The intervention in each group began on postoperative day three. TENS and MENS were performed using a portable electrical stimulation device (Ito ESPURGE; Ito Co., Ltd., Kawaguchi City, Saitama, Japan). The TENS parameters were as follows: symmetrical biphasic square wave with a pulse duration of 200 µs, frequency of 100 Hz, and time of 60 min. The TENS stimulus intensity was set above the sensory threshold and below the motor threshold to account for the aggravation of discomfort caused by postoperative swelling. The MENS parameters were as follows: symmetrical biphasic square wave with a pulse duration of 250 µs, frequency of 0.2 Hz, and time of 60 min. The intensity of the MENS stimulus was set at 200 µA. Two self-adhesive electrodes (PALS L, 50 mm × 90 mm; Axelgaard, Fallbrook, CA, USA) were used for all groups. The electrodes were placed on the skin just above the vastus medialis and medial tibial tuberosity at the L3 and L4 dermatomes. We considered that the osteotomy area in TKA primarily corresponded to the sclerotome at the L3 and L4 levels. We positioned the electrodes considering the soft tissue invasion associated with the medial parapatellar approach. In the control group, the electrodes were only applied for 60 min and did not receive electrical stimulation. All interventions were conducted face-to-face in the hospital, five times a week for two weeks, by T.M., a physical therapist with 5 years of clinical experience.

Pain intensity during walking (w-pain), maximum walking speed (MWS), timed up-and-go test (TUG), isometric knee extension strength (IKES), and pain catastrophizing score (PCS) were evaluated. W-pain was assessed using a visual analog scale (VAS). The VAS is a rating method that expresses pain intensity by drawing a line on a 100-mm straight line with “no pain” at the left end and “worst pain imaginable” at the right end. The farther to the right of the line (i.e., the higher the VAS score), the greater the pain reported while walking. The MWS was defined as the time required to walk a 10-m flat, straight path at maximum speed. The TUG test began with participants seated in chairs. When the evaluator said “go”, they stood up, and the timing was started with a stopwatch. Participants then walked 3 meters, turned around obstacles, returned to their chairs at a comfortable gait speed, and sat down. The timing was stopped as soon as they were seated again. The MWS and TUG were measured twice, and the participants used a cane as needed. The analysis was carried out in a short time. Finally, IKES was measured with a handheld dynamometer (microFET2; Hoggan Scientific, Salt Lake City, UT, USA) using the H-fixation method, which has been reported to have good inter-rater reliability⁸⁾. Participants were seated in a wheelchair with their knees flexed at 90°. The evaluator fixed the handheld dynamometer at the level of the medial and lateral malleoli on each participant’s lower legs. All IKES measurements were performed by the same physiotherapist, and the participants were instructed to demonstrate maximum knee extension muscle strength for 5 s. Measurements were performed twice after each training session. The larger measurement was multiplied by the lower leg length (from the lateral knee joint space to the lateral malleolus) and divided by the body weight for analysis. The PCS is a 13-item, self-administered questionnaire that assesses pain catastrophizing. Dave et al.⁹⁾ reported that the preoperative PCS is a good predictor of pain after TKA. We evaluated w-pain, MWS, TUG, and IKES for all participants preoperatively, 2 weeks postoperatively (i.e., after completion of all interventions), and 4 weeks postoperatively. The PCS was evaluated only before TKA.

For the w-pain, MWS, TUG, and IKES, statistical analyses were performed using a mixed-effects model for repeated measures, with split-plot analysis of variance using a random intercept model. The three groups were compared using preoperative PCS as a covariate. If the interaction was significant, comparisons between groups at each time point were performed using the Tukey–Kramer method. The significance threshold was set at p=0.05. All statistical analyses were performed using R 4.3.1 (R Foundation for Statistical Computing, Vienna, Austria).

RESULTS

Of the 35 patients, 13 were assigned to the TENS group, 11 to the MENS group, and 11 to the control group. The preoperative patient characteristics of each group are shown in Table 1. Some data were missing due to ward closures caused by influenza virus infections. Missing data were considered completely missing at random. The data were registered in figshare (DOI: https://doi.org/10.6084/m9.figshare.26315014.v1). The results of each outcome measure are presented in Table 2. Only w-pain and MWS showed a significant interaction and main effect on the repeated measures (p<0.01). There were significant differences between groups: w-pain was significantly lower in the TENS group than in the other groups at 2 weeks after surgery (p<0.05). MWS was faster in the TENS group than in the MENS group at 2 weeks after surgery (p<0.05). For the TUG and IKES, only the main effect was significant for repeated measures. Details of the effect sizes (Cohen’s d) in the analysis of variance and group comparisons are provided in Supplementary Tables 1 and 2.

Table 1. Characteristics of the participants of each group.

Group	Number (Men)	Age (years)	Height (cm)	Weight (kg)
TENS	13 (0)	66.7 ± 10.1	151.7 ± 13.4	61.4 ± 10.2
MENS	11 (2)	71.5 ± 7.2	150.1 ± 8.4	58.7 ± 6.7
Control	11 (4)	75.1 ± 2.9	152.4 ± 7.5	62.5 ± 10.7

TENS: transcutaneous electrical nerve stimulation; MENS: microcurrent electrical neuromuscular stimulation.

Table 2. The results of all data and 3-group comparisons.

	Group	Pre-operation	2 weeks after TKA	4 weeks after TKA
w-pain	TENS	76.2 ± 18.1	21.5 ± 13.4	11.5 ± 10.6
(mm)	MENS	62.1 ± 22.0	41.1 ± 16.6*	26.6 ± 13.2
	Control	66.9 ± 26.8	43.6 ± 26.6*	13.0 ± 11.4
MWS	TENS	8.78 ± 3.08	9.85 ± 2.92	7.98 ± 2.18
(s)	MENS	8.09 ± 1.62	13.4 ± 5.55*	9.13 ± 2.97
	Control	9.70 ± 3.35	12.5 ± 4.08	9.46 ± 3.21
TUG	TENS	10.2 ± 3.82	11.4 ± 3.37	9.00 ± 2.44
(s)	MENS	8.87 ± 1.54	13.2 ± 4.15	9.48 ± 2.65
	Control	11.8 ± 3.85	14.0 ± 4.46	11.1 ± 3.91
IKES	TENS	0.896 ± 0.282	0.604 ± 0.211	0.739 ± 0.211
(Nm/kg)	MENS	0.882 ± 0.305	0.483 ± 0.128	0.712 ± 0.183
	Control	0.995 ± 0.416	0.547 ± 0.183	0.800 ± 0.168
PCS	TENS	30.8 ± 9.90
	MENS	27.0 ± 9.63
	Control	27.0 ± 16.7

*: indicated p<0.05 comparison with the TENS group.

w-pain: pain intensity during walking; MWS: maximum walking speed; TUG: timed up-and-go test; IKES: isometric knee extension strength; PCS: pain catastrophizing score; TENS: transcutaneous electrical nerve stimulation; MENS: microcurrent electrical neuromuscular stimulation; TKA: total knee arthroplasty.

DISCUSSION

In this study, we investigated the effects of TENS and MENS after TKA and found that TENS could relieve pain and improve walking speed during the early period following TKA. This indicates that TENS acted on the analgesic system to control pain in the early postoperative period after TKA. In a previous study by Rakel et al.¹⁰⁾, TENS was continued until 6 weeks after TKA, and an analgesic effect was observed. Based on the results of our study, given that analgesic effects were observed for up to 2 weeks after TKA, we assumed that prolonged TENS would be effective. This reduction in w-pain could explain why the TENS group had a faster MWS at 2 weeks postoperatively. Early initiation of walking after TKA has been reported to shorten hospital stay, improve motor function, and decrease the incidence of complications¹¹⁾, suggesting that TENS may promote early initiation of walking after TKA. In contrast, TUG and IKES only showed changes over time. The TUG test consists of standing and sitting activities in addition to walking. A correlation between sit-to-stand performance, TUG test results, and knee extensor strength has been previously reported¹²⁾. We believe that the lack of improvement in IKES in the TENS and MENS groups influenced the TUG results. Thus, although we expected improvement of IKES in the MENS group compared to the other groups as the muscle tissue healed, this was not the case in our study. Most studies on MENS in muscles have focused on delayed onset muscle soreness^{5, 13, 14)}, and there are no reports on muscle function after surgical procedures such as TKA. Based on the present results, caution should be exercised when using MENS in muscles that have undergone surgical invasion.

This study has several limitations. First, regarding the intervention of MENS, although the intervention time was standardized to 1 h for all groups for blinding purposes, a review stated a treatment time for delayed onset muscle soreness (DOMS) of up to 3 h¹⁵⁾. A longer adaptation time may have been necessary because the muscle tissue damage associated with TKA is considered more severe than that associated with DOMS. Also, in the present study, 10 interventions were carried out over the first 2 weeks post-surgery. In another study demonstrating analgesic effects, 10 interventions were conducted over 10 days; thus, the intervention frequency in our study was lower. Further, the electrode position was the same in all groups to ensure blinding; it was positioned considering soft tissue invasion associated with TKA. However, in the previous study⁶⁾, the electrodes were applied across the surgical wound at the patella level. This difference may have affected treatment effectiveness. Second, due to the small sample size, there was no significant difference in MWS at 2 weeks postoperatively between the TENS and Control groups (Supplementary Table 2, p=0.114). However, the effect size of Cohen’s d was not small (0.79). Due to the influence of infection, a sufficient sample size could not be ensured; this should be improved in future studies. Furthermore, we evaluated only pain intensity and physical function in this study; patient-reported outcome measures related to activities of daily living and quality of life, such as the Knee Injury and Osteoarthritis Outcome Score^{16, 17)}, were not evaluated and should thus be included in future studies.

In conclusion, this study examined the effects of TENS and MENS after TKA and found that the TENS group had better results in terms of pain intensity while walking and maximum walking speed at two weeks postoperatively compared with the MENS group. In contrast, there was no significant difference between MENS group and the control group. These results suggest that TENS is more effective for pain control in the early period after TKA than MENS.

Conflict of interest

None.