Extracorporeal Shock Wave Versus Interferential Current in Tennis Elbow

Tennis Elbow (TE) encompasses pain and tenderness due
to overuse or injury of the extensor wrist tendons...


Introduction
Tennis Elbow (TE) encompasses pain and tenderness due to overuse or injury of the extensor wrist tendons [1]. It is a degenerative rather than inflammatory tendinopathy causing persistent elbow pain [2]. Moreover, TE usually affects 4-7 per 1000 individuals [3]. A typical episode lasts 6-24 months and recurrence is common. They also reported that conditions that are not resolved within six months of the onset, may need surgical interventions.
Modalities include but are not limited to acupuncture, splinting, therapeutic exercises, shock wave therapy, laser, ultrasound, transcutaneous electrical nerve stimulation, manipulation, and manual therapy [4,5]. Unfortunately, there is no consensus on the optimum intervention strategy for TE. As such, research for new effective interventions for TE is warranted. Extracorporeal Shock Wave Therapy (ESWT) is used to treat many musculoskeletal disorders such as plantar fasciitis, calcific tendinitis of the shoulder, epicondylitis, patellar tendinopathy, Achilles's tendinopathy, nonunion and delayed union of long bone fracture [6]. ESWT may impede progression of OA, improve function, and depress chondro protective effects in animals [7,8]. A few researchers elucidated that ESWT reduced pain and improved knee functions in patients with knee OA [9,10]. Therefore, ESWT has been suggested effective for TE. Interferential Current (IFC) has been used for treating musculoskeletal disorders for its analgesic, anti-inflammatory, sympatholytic, local vasodilating, and muscle stimulating effects and muscular pain [12]. However, there has been little research comparing between ESWT and IFC in patients with knee OA. Also, little is known about the efficacy of these two therapies in treating TE. The purpose of this study was to determine the effectiveness of ESWT compared with IFC for patients with TE.

Design
This study was a randomized controlled trial with subjects randomly assigned to one of three intervention groups: The therapist who did the testing and data analyses was not aware of group allocation. However, the treating therapist was aware of group allocation.

Participants
Eighty-eight participants with TE who visited the outpatient setting for a minimum of 3 months pain duration participated in the study. All participants signed informed consent forms detailing procedure and risks involved with participating in this research.
Inclusion criteria included subjects with lateral elbow and forearm pain that lasted for more than six weeks. Subjects had a medical diagnosis of TE by a physician. Subjects experienced less pain during resistance of supination with the elbow in 90° of flexion rather than in full extension. Patients also had to complain of increased tenderness upon extending the wrist against resistance with the forearm in supination. The exclusion criteria for the study were bilateral TE, systemic metabolic diseases, elbow surgery, medial epicondylopathy, radial nerve entrapment, carpal tunnel syndrome, chronic inflammatory and neoplastic disease; other criteria included: radial tunnel syndrome, cervical radiculopathy, thoracic outlet syndrome, dysfunction in the shoulder, and treatment with corticosteroid or local anesthetic injection in the previous three months. Also, subjects with any contraindication for the physical therapy modalities were excluded. Participants were asked to refrain from receiving other forms of physical therapy and analgesics during the study.

Measurements
A 10 -cm Visual Analog Scale (VAS) was used to measure pain intensity during rest, activity of the painful elbow, and nighttime. It has an excellent test-retest reliability of 0.95 [13]. The disabilities of the arm, shoulder and hand (DASH) self-report questionnaire was also used to measure disability. It is a valid and reliable 30 items questionnaire where 1 point was given for performance of the specific activity without difficulty and 5 points for disability [14]. The total score ranged between 30 (best) and 150 (worst).
Handheld dynamometer (Jamar, J.A. Preston Co., MI) was also used as a valid and reliable tool to measure hand grip strength [15]. The assessor used the mean of three measurements. The Shock Master was used to produce shock waves. It is a low to medium-energy range. Evotron RFL0300 (Swiss Tech Medical AG, Switzerland) was used to deliver shock waves. It has a depth of penetration between 0 to 30 mm. Pulse rates were 60, 120, 180, 240 impulses per minute.

Interventions
Patients signed an informed consent before data collection. The to exercise within their pain tolerance. Stretching exercises were done by asking each subject to maintain the elbow in full extension, forearm in full pronation, and the wrist in flexion and ulnar deviation within pain tolerance for 45 seconds. The therapist asked participants to record frequency and repetition of each exercise to measure adherence to exercises. The therapist asked each subject to avoid activities that may irritate the elbow such as shaking hands, grasping, or lifting. The therapist advised the subjects not to take pain medications one week before the visits. The therapist also instructed the subjects to do the home exercises on the days they do not come to the clinic.

Data Analysis
All statistical tests were performed using SPSS for windows Version 20 (Chicago, IL, USA. Mann-Whitney U test was used to compare between both groups. Kruskal-Wallis test was also used to analyze differences among the groups.

Results
Eighty-eight participants eligible for inclusion visited the clinic within the study period. Seventy-two participants completed the study and were randomly assigned to one of the three groups:   (Table 1). There were statistically significant differences for the values of pain (during rest, activity, nighttime), disability and grip strength on the epicondyle among all groups at four weeks ( Table   2). Pain intensity, disability and grip strength improved in all groups at the end of the intervention. Patients in Group 1 had lower pain during rest, activity, and nighttime, disability and higher grip strength when compared with Group 2 or Group 3 subjects.  ESWT uses electrical energy to direct a series of magnetic pulses through injured tissues whereby each magnetic pulse induces a tiny electrical signal that stimulates cellular repair [16]. ESWT is effective for healing soft-tissue wounds; suppressing inflammatory responses at the cell membrane level to reduce pain and increase mobility [16]. The mechanism of this method is not yet completely understood. ESWT may alleviate pain by over stimulating the axons (gate-control theory) and increasing the pain threshold [17]. Also, it may induce the release of endorphins [18]. Besides, it may reduce substance P in the target tissue and the dorsal root ganglia cells and selective destruction of unmyelinated nerve fibers within the focal zone of ESWT [19].
IFC is commonly used to reduce discomfort, muscle incitement, increases blood flow and decreases of edema [20]. Various authors reported that IFC are successful in the reduction of pain in various musculoskeletal issues [21][22][23]. Its produces low-recurrence (0-250 Hz) give impact inside the tissue through the obstruction of 2 higher freq. (4000 Hz) and acts fundamentally on the sensitive (nerve) tissues with the most grounded impacts prone to be those which are an immediate consequence of such stimulation [20]. IFC has been shown to increase production of endogenous opioids, at a spinal level to hinder the reduce pain transmission [22][23][24]. In this study, patients in group 2 (IFC and conservative program) showed improvement in all parameters. There is widespread use of electrical stimulation to reduce pain. However, there are a few research for using IFC. IFC is based on the creation of a low frequency current effect in deep tissues by two medium-frequency alternative currents at different frequencies. Since the skin resistance decreases with the increasing current frequency, IFC may exert its effect on deeper tissue planes without patient discomfort [25].
IFC allows an increased dosage applied in a greater depth because of the body tissue's better tolerance of medium-frequency currents. IFC could stimulate local nerve cells that can have an analgesic effect due to blocking the transmission of the pain signals or by stimulating the release of endorphins [26]. IFC has been used for patients with low back pain and OA. However,

Fuentes et al. conducted a meta-analysis and found that IFC is not
better than placebo or any other modality when presented as an isolated treatment in musculoskeletal pain [27]. They proposed that therapists may use it as part of a multi-modal treatment plan. Therefore, the author used exercises along with the IFC for participants with TE. Our results were supported by Zhou and associates who found that ESWT reduced pain and neuralgia in diabetic rats [28]. Our results also agreed with those of Yan et al.
who conducted a meta-analysis comparing between ESWT and ultrasound in TE [29]. Yan et al repoted that the ESWT had significant difference in decrease of pain values measured by visual analogue scale at 1 month, 3 months, and 6 months follow-ups, and raised grip strength compared with the group that received ultrasound [29]. The findings of this trial also agreed with those of Mohamed in Osgood Schlatter disease of the knee at one month and 3 months follow up [44]. The author results also agree with those of Al Sawalha who proposed that IFT improves outcomes of patients with Tennis elbow [45]. The results also were supported by Albornoz et al. who found significant between-group differences were found for IFC on pain perception by VAS ( p = 0.032) and disability level by ODQ ( p = 0.002) in chronic low back. All subjects received up to 10 treatment sessions of 25 minutes over a two-week period and completed the intervention and follow-up [46].
This study has some limitations. Placebo effects were not evaluated since no control groups were included in this research.
The sample size was relatively small to detect differences between groups. Further, the follow-up period was short to demonstrate whether the improvements in each group are sustained in longterm. The author did not evaluate parameters such as intensity and treatment intervals affecting the treatment effects. Also, the daily living activities of the patients could not be completely controlled. Last but not least, objective laboratory tests of lactic acid, prostaglandin E, substance P and akinin were not taken. Based on the results of this randomized, clinical trial, ESWT may be more effective than IFC in reducing pain and disability and improving function in patients with TE.