The effect of continuous ultrasound on chronic non-specific low back pain: a single blind placebo-controlled randomized trial

Low back pain (LBP) is a major cause of morbidity in high, middle and low-income countries and affects 80–85% of people over their lifetime[1]. LBP is a common health and socioeconomic problem in Iran. In a cross-sectional study in one of the largest car-manufacturing companies in Iran, the 1-year prevalence of self reported LBP was 21% (20% for males and 27% for females). The prevalence rate of absence due to LBP was 5% per annum[2]. As part of a World Health Organization study, LBP was detected in 15.4% of the population under survey in Tehran (urban area)[3] and in 23.4% of the population in rural areas in Iran[4].

Specific back pain occurs in approximately 2% of all patients with back complaints[5]. For the majority of patients with LBP a specific diagnosis cannot be defined on the basis of anatomical or physiological abnormalities. Non-specific LBP (NSLBP) is assumed to be inflammatory or mechanical in nature[6]. Chronic NSLBP refers to an episode of activity-limiting LBP (with no pain referred into either lower limb) that lasts for 3 months or more[7].

Non-pharmacological methods including a variety of physical agents are the cornerstone of the management of chronic LBP. Therapeutic ultrasound (US) is among the commonly used physical modalities for treating soft tissue injuries[8]. There is a dearth of evidence for the clinical use of therapeutic US in patients with LBP[9].

Therapeutic US is delivered in two modes: 1) Continuous mode in which the delivery of US is non-stop throughout the treatment period; 2) Pulsed mode in which the delivery of US is intermittently interrupted[10].

Therapeutic effects of US are classified as thermal and non-thermal. Ultrasonic energy causes soft tissue molecules to vibrate from exposure to the acoustic wave. This increased molecular motion generates frictional heat and consequently increases tissue temperature. This increased temperature, named thermal effects, is thought to cause changes in nerve conduction velocity, increase in enzymatic activity, changes in contractile activity of skeletal muscles, increase in collagen tissue extensibility, increase in local blood flow, increase in pain threshold, and reducing muscle spasm[11].

Acoustic waves cause normally present minute gas pockets in the tissue to develop into microscopic bubbles or cavities. With therapeutic US, stable acoustic cavitation results, whereby the microbubbles pulsate without imploding. This pulsation leads to microstreaming of fluid around the pulsating bubbles. When occurring around cells, this process, referred to as non-thermal effects, is reported to alter cell membrane activity, vascular wall permeability, and facilitate soft tissue healing[12]. Traditionally, continuous US is used for its thermal effects. Pulsing the US is thought to minimize its thermal effects[10]. In fact, it is not possible to truly isolate the thermal and non-thermal effects as both effects occur with US application[13].

Studies on the efficacy of continuous US in chronic LBP are lacking[8] and there is little evidence of its effectiveness in physiotherapy practice[14, 15]. However, lack of evidence is not evidence of lack of effect. Therefore, the main objective of the current study was to compare the effect of continuous US to placebo US combined with exercise therapy on the primary outcomes, functional status and pain of a group of patients with NSCLBP, as well as on the secondary outcomes, endurance of paravertebral and hip muscles, and lumbar range of motion.

In everyday clinical practice the application of US is often combined with other physiotherapeutical interventions, usually with exercise therapy[41]. The aim of this study was to investigate whether continuous US can add to the effects of exercise therapy in patients suffering from NSCLBP compared to placebo US.

The results showed that both FRI and VAS have improved after 10 sessions of treatment and over time after 1 month in both groups. FRI improvement was significantly greater in the group receiving continuous US. This finding is consistent with Ansari et al.[20] who demonstrated a better functional outcome in a continuous US group in comparison with a placebo US group. In their study patients did not receive any treatment in addition to continuous and placebo US. Other randomized trials in which the effect of US is directly compared with placebo US in NSCLBP are lacking. US is usually studied in comparison with other modalities[42, 43] or is presented in a package of physiotherapy[44] and is also investigated in other subgroups of patients with LBP other than non-specific LBP, such as lumbar disk herniation[45–47].

Durmus et al.[42] in comparing 3 groups of NSCLBP patients who received US + exercise, Electrical Stimulation (ES) + exercise and exercise only, showed significantly greater improvement in pain and function of the ES and US groups in comparison with the control group. The study found no difference in function between groups receiving either ES, or US but the US group had significantly better scores regarding pain improvement.

Mohseni et al.[43] compared manipulation and exercise treatment with US and exercise treatment in a randomized clinical trial. One hundred and twenty patients with chronic LBP were given a program of exercises. In addition, one group received spinal manipulation therapy and the other group received therapeutic US. Pain intensity, functional disability, lumbar movements measured by Modified Modified Schober Test and muscle endurance were measured shortly before treatment, at the end of the treatment program and 6 months after randomization using surface electromyography. Although improvements were recorded in both groups, patients receiving manipulation/exercise showed a greater improvement compared with those receiving US/exercise at both the end of the treatment period and at 6-month follow-up. The authors did not report on the details of the exercise program, and US delivery was inconsistent (continuous 1MHz, 1.5-2.5W/cm² for 5 to 10 minutes, average 6 sessions, one or two times a week) which could both be possible sources of difference with our study.

Since the current study lacks a third group with no US, it is impossible to explore the effects of exercise and US separately except in parts where the continuous US group has shown significant differences in comparison to the placebo group. As both groups in our study improved significantly regarding pain, we can conclude that the treatment common to both groups (exercise and mechanical application of US head) have attributed to the outcome. There is strong evidence that exercise is an effective treatment in chronic low-back pain[48]. Exercise programs for CLBP may be designed to reverse deconditioning or the fear of movement associated with pain. Such exercises typically include aerobic exercises like walking as well as strengthening and stretching regimens[27]. The specific exercises administered to patients in this study may have been of benefit in improving pain. Since many items of the FRI questionnaire are indirectly related to the pain experienced by patients during that specific task, the decreased pain achieved with treatment could have caused the patients in both groups to perform better during those tasks as well.

However, the individual role of the placebo effects of US in the placebo group as well as the individual effect of mechanical movement of US head and exercising in both groups cannot be specified although each one may have played a part in the outcome. A placebo effect of US can be the result of moving the applicator head thus benefitting from the effects of massaging[20, 25]. Continuous movement of the applicator may increase the temperature of the area under treatment and may stimulate the skin receptors causing the pain gate control mechanism to become active[20]. It has been shown that moving the applicator of US on the affected area can change the level of serum cortisol, which in turn can affect inflammation and swelling[25]. Patients in both groups could have benefitted from the Placebo effects of the treatment[49].

A significant difference in the improvement of FRI scores in favor of the continuous US group can be related to the thermal and mechanical effects of continuous US.

Morrisette et al.[50] showed that continuous 1 MHz US given at either 1.5 W/cm² or 2.0 W/cm² intensity has the capability of heating lumbar periarticular tissue while the intervening muscle may heat as well. Morrisette stated that the temperature elevation was at a level thought to be sufficient to produce the theoretical therapeutic effects proposed with an elevation in temperature.

Regarding secondary outcome measures, although lumbar flexion and extension ROM increased in both groups after treatment, the increase did not reach statistical significance within groups. Nevertheless, the amount of improvement in ROM was significantly greater in the continuous US group. Durmus et al.[42] reported significant improvement in Modified Schober scores in the group receiving US + exercise. However, this improvement was not significantly different from the two other treatment groups receiving ES + exercise and exercise only. In the study carried out by Mohseni et al.[43], lumbar flexion and extension ROM as measured by MMST (Modified Modified Schober Test) improved significantly in US + exercise group but this improvement was significantly lower in comparison with the manipulation + exercise group. Though none of the studies above, had reported the exact exercises prescribed, their difference with our study can be possibly explained by the differences in exercise type and intensity and patient population as well as the difference in the dosage of US.

Clinical assessment of movement impairment in low back pain is predominantly done by measuring changes in lumbar ROM in order to investigate patient’s response to treatment[51]. The reduction in pain alongside stretching and strengthening exercises prescribed could have contributed to the increase of ROM in both groups. The significant additional increase of ROM in the continuous US group may be due to the thermal and mechanical effects of continuous US. It has been shown that temporary increases in range of movement can be produced by US treatment[52]. There is considerable evidence that the extensibility of collagen based tissues will change with ultrasound thermal applications as long as sufficient temperature change is achieved[53]. Since the therapeutic window for stretching following US application is limited to some 3 minutes immediately after treatment[54], our participants that performed exercises after the treatment sessions, at home, barely could have benefited from such thermal effect. Given that patients suffering from chronic low back pain usually have spasm[7], using continuous US could have been effective in decreasing spasm[10] and consequently resulting in greater ROM increase in comparison with placebo US.

Considering surface EMG parameters, no significant effect of Time or Group was found on median frequency slope of all measured muscles.

The assessment of fatigue based on SEMG techniques during a fatiguing contraction can be demonstrated by a trend of the power spectrum to lower frequencies usually measured by the decrease in median frequency. It has been proposed that better endurance would exhibit a less precipitous decay rate of the median frequency[55], though conflicting opinions exist[56]. It has been indicated that trunk muscle endurance can be increased by using specific exercises[57].

Sung[58] investigated changes in multifidi muscle endurance and functional status after a 4-week supervised spinal stabilization exercise program in 16 patients presenting with chronic low back dysfunction (LBD). Results showed that Oswestry scores improved significantly from pre to post treatment. Significant pre- to post treatment increase in multifidi muscle fatigue for men coupled with a nonsignificant improvement in multifidi muscle endurance for women was also seen. Sung[58] concluded that a 4-week spinal stabilization exercise program significantly improved functional status in patients presenting with LBD but the program was insufficient to effect muscle fatigue. In another study, Mohseni et al.[43] did not find any significant change in median frequency slope or endurance time in the group of patients with low back pain who received continuous US plus exercise for an average of 6 sessions. We also witnessed a nonsignificant change in MF slope of measured paravertebral muscles, which may imply that the usefulness and sensitivity of this parameter was limited in our study.

Regarding endurance time, the group receiving continuous US showed a significantly greater increase than the placebo group. Traditionally, endurance is thought of as the time for sustaining a nonstationary activity, which ceases with fatigue[59]. One of the main reasons for muscle fatigue is the accumulation of metabolite wastes in the region and the inability of the system to provide adequate blood circulation to supply oxygen to the tissue and deplete it from wastes[60]. Additionally, ischemia due to inflammation and spasm is a common finding in chronic low back pain[7, 28, 61]. It is possible that continuous US has improved low back muscle fatigue by increasing blood circulation in the region and helping improve blood supply[17, 23, 61] which in turn have caused more sufficient and longer muscle contraction during the test.

This single blind, placebo -controlled, randomized clinical trial showed that adding 1 MHz, 1.5 W/cm² US to a semi-supervised regimen of exercise had significantly beneficial effects on function, lumbar flexion and extension ROM, and endurance time in patients with NSCLBP.

Further studies including a third group of no US are needed to explore the differential effects of each intervention on patients with NSCLBP. In addition, it would be helpful to measure other surface electromyography parameters other than median frequency slope, such as mean frequency, initial median frequency and normalized median frequency slope to explore the possible effects of the method used in this study on these parameters.

Studies, in which the methodological shortcomings of this study and similar studies are addressed, are needed to verify a dose response relation in patients with chronic low back pain.