Ultrasound therapy: Dose-dependent effects in LBP treatment

1. Introduction

Lumbar pain syndrome, or low back pain (LBP), affects most people at least once in their lives [1, 2, 3, 4]. Despite the high prevalence of LBP, more questions than answers remain regarding the causes of its pathogenesis, risk factors and treatment. The literature indicates that LBP incidence is between 15–30% and 45–50% annually [4, 5, 6].

Many patients who experience acute back pain develop chronic pain [4]. The high incidence and prevalence of LBP implies the need for various treatment options; however, no unified standard currently exists for treating LBP [7, 8]. Some options for treating LBP include pharmacotherapy, physical therapy (including ultrasound therapy), surgery and acupuncture.

Ultrasound therapy (UD) is one method of treating back pain, which is most often used in conjunction with other physical modalities or exercises.

Ultrasound is an oscillating sound pressure wave with a frequency above 20 kHz. Therapeutic UD frequencies range from 0.5 to 5 MHz [9, 10]. Ultrasound penetration is determined by the half-value depth, which represents the tissue depth at which half the surface energy is available. UD penetration differs for each tissue as well as with UD frequencies. The half-value depth when using 1 MHz UD for muscle is 12 cm, while for 3 MHz, it is only 4 cm. UD waves penetrate deeper into fat tissue, while bone tissue absorbs energy at the superficial layer [9, 10].

Wave energy depends on the frequency (higher frequencies yield higher energy) and the amplitude (higher amplitudes yield higher energy). For example, ultrasound waves of high intensities from 5,000 to 15,000 W/cm 2 are used to treat tumours causing necrosis, while those used in physical therapy are less intense at 0.5 W/cm 2 to 3 W/cm 2 [11].

The UD application’s effect depends on the application method. Applying UD in pulse mode will generate a smaller thermal effect than will a continuous wave [12]. Continuous UD results in a local temperature increase, thus increasing the local blood flow to reduce pain, relax the muscles and improve cellular metabolism [9, 12]. When the pulse UD method is used, nonthermal effects dominate, i.e., unstable cavitation, accelerated protein degradation, stimulated fibroblast proliferation, increased collagen production, and macrophage stimulation [9, 10, 12].

In our study, the impacts of ultrasound therapy and different UD intensities on patients’ clinical conditions were analysed.

2. Methods

The study design was a randomized prospective trial that evaluated the efficacy of two LBP treatments. Sixty-two patients, 48 women and 14 men, aged 30 to 65 years, with chronic lumbar pain lasting more than 8 weeks and with signs of osteoarthritis on imaging studies, were examined. Exclusion criteria were radicular pain; a nonmechanical cause of pain (cancer, inflammation); other LBP therapy conducted during the study; and contraindications for UD such as cancer, pregnancy, electronic implants, acute infections, or areas subjected to radiation therapy in previous months.

We assessed the presence of intervertebral space narrowing, sclerotization, cysts in the subchondral bone layer, and the presence of osteophytes. If there was a narrowing of the synovial space suggestive of discopathy but without symptoms of nerve root compression, the patient was qualified for therapy.

Patients were randomly divided into two groups by drawing numbered cards. The groups did not significantly differ in age. The number of men and women was similar in both groups.

Both groups were subjected to ultrasound therapy for two weeks. The treatment took place at the Department of Rehabilitation at the Antoni Jurasz University Hospital in Bydgoszcz, Poland.

All persons participating in the study were asked to refrain from other physical therapy or manual therapy for low back pain, as well as to stop analgesic use during the study period.

Work on this research was conducted from 2009 to 2011. All participants were patients of Antoni Jurasz University Hospital in Bydgoszcz, Poland. The study was approved by the local ethics committee, and all patients provided written informed consent.

The two patient groups were as follows. The first group (A; n = 30) underwent ten ultrasound treatment sessions at an intensity of 1 W/cm 2 , lasting 6 minutes each, with a continuous stream to the lumbar paraspinal muscles. The second group (B; n = 32 subjects) underwent ten ultrasound treatment sessions at an intensity of 0.5 W/cm 2 , lasting 6 minutes each, with a continuous stream to the lumbar paraspinal muscles.

In both groups, treatment was performed using a 5-cm 2 head, emitting ultrasound at a frequency of 1 MHz, which has a half-depth penetration in the muscle of approximately 1.2 cm [10]. Both groups received a continuous wave, and the treatments were performed using a dynamic technique.

The above parameters were used based on the principle that to treat chronic pathologies, higher UD doses are needed. The continuous stream was used in both study groups to achieve a thermal effect resulting in decreased pain [9, 10, 12].

All stages of the subjects’ examination (i.e., the clinical examination and form completion) were performed twice: once at the beginning of treatment and again after the last procedure.

Persons involved in this project were medically examined to determine their clinical symptoms, duration, and previous treatment methods. A thorough interview was conducted to determine whether ultrasound therapy was contraindicated. Patients were checked for the presence of Lasegue’s sign, deep reflexes, spinal mobility, paraspinal muscle tension, and tenderness upon palpation in the lumbar region. Test images provided by the patients were evaluated to exclude serious pathologies.

The Oswestry Disability Index (ODI) and the Roland-Morris Disability Questionnaire (RM) were used to evaluate subjects’ disability due to LBP. The ODI questionnaire was selected as a measure because it evaluates the use of painkillers, independence, capability of lifting objects, walking, sitting, standing, sleeping, social life, sexual activity, and travel. Each question has 6 answers; answers indicating that patients had no restrictions performing that activity were scored as 0 points, while answers indicating that the activity was impossible to perform were scored as 5 points. Respondents could choose only one answer for each question. The number of points was totalled, divided by the maximum number of points possible, and the result was multiplied by 100 to obtain a percentage score [13]. For example, if the respondent answered only nine questions, the denominator was 45 instead of 50. High scores indicated high degrees of disability. This questionnaire has been used for multiple clinical outcomes of LBP.

The second scale, which was used to assess the subjects’ degree of disability, was the Polish version of the Roland-Morris Disability Questionnaire, MAPI 2004. The RM form contains 24 statements relating to restrictions on the subject’s functioning due to LBP. The interviewed subject selected all answers related to their situation, and the result was obtained by summing all selected answers. As in the ODI questionnaire, higher scores indicated greater reductions in the subjects’ functioning.

The Visual Analog Scale (VAS) of 0–10 points was used as a self-report measure of pain at each assessment point.

Two researchers conducted the procedures. One conducted the examination and administered the questionnaires and Visual Analogue Scale, while the second applied the ultrasound therapy.

A chi-square test and Student’s t-test were used to analyse the data. P-values ⩽ 0.05 were considered statistically significant.

3. Results

Sixty-two subjects provided informed consent to participate in the study. The shortest duration of LBP reported by participants was approximately 20 weeks, and the longest was several years (the patient could not specify the exact number). Both groups were comparable in age and gender. Thirty persons received UD treatment at an intensity of 1 W/cm 2 (group A), and 32 subjects received UD at an intensity of 0.5 W/cm 2 (group B). The remaining UD parameters were equal for both groups. No major adverse events were reported for any participants during treatment. All patients tolerated the procedures well, with no complications.

The ODI form was completed by subjects on the first (pODI) and last days of the study (after ten days, kODI).

Group A had a larger reduction in ODI scores (13.16%). A smaller, but also significant change in disability level occurred in group B (8.84%) ( 0.0001) (Table 1).

The chi-square test demonstrated that subjects achieved significant improvements in the following: pain relief, self-reliance, lifting objects, walking, sleeping, social life, sexual activity and travel ( p ⩽ 0.05). Symptom reduction in the standing position was near significance ( p = 0.0566). The smallest improvement was in the sitting position ( p = 0.3618). In eight of ten categories, group A showed more improvement than group B (Table 2), but the difference was not significant.

In addition to the ODI form, subjects completed the RM form on the first day (pRM) and the last day of the study (after ten days, kRM).

The final scores showed that the RM score decreased significantly in both study groups (3.37 points in group A vs 3.59 points in group B). The RM score decreased slightly more in group B than in group A, but this difference was not statistically significant (Table 3).

The chi-square test was used to analyse specific answers on the RM questionnaire, which showed statistically significant improvements in the following responses:

3.

I walk more slowly than usual because of my back.

Pain was assessed using the VAS scale. After the therapeutic ultrasound sessions, at the end of the study, the average VAS pain score had decreased in both groups. Pain remission was greater in group A (approximately 20.28 mm) than in group B (approximately 16.31 mm). Student’s t-test analysis showed significant reductions in pain intensity in both groups (Table 4).

We found no studies comparing UD parameters in the literature. Most studies on the effectiveness of UD compared ultrasound treatments to a placebo or other physiotherapeutic interventions [14]. Therefore, we will refer to studies conducted using one dose of UD and discuss their clinical results.

In standard therapy, ultrasound dosages differ from 0.05 (weak) to 1.2 W/cm 2 (strong) [9], but the literature also includes studies conducted using dosages of 1.5 W/cm 2 to 2.5 W/cm 2 (Mohseni-Bandpei) [15] or 1.5 W/cm 2 (Unlu) [16]. Durmus used 1 MHz at 1 W/cm 2 for 10 minutes [17], and Fiore used 2 W/cm 2 for 10 minutes [18].

Our study demonstrated a statistically significant decrease in disability as measured by the ODI score in both groups. Results differed by 8% to 10% in prior studies that found minimally important clinical improvements via the ODI [19, 20]. Therefore, the ultrasound dosage of 1 W/cm 2 can be confirmed as effective because the decline in this group was 13.16%, while it was 8.84% in the group who received 0.5 W/cm 2 ; thus, these results were clinically significant.

Statistical analysis showed that after receiving therapy, participants obtained significant improvements in pain intensity, self-reliance, lifting objects, walking, sleeping, social life, sexual activity and travel. The standing position achieved near significant improvement, while sitting scores showed the least improvement. Importantly, in eight of ten questions on the Oswestry Disability Questionnaire, improvement was greater in the group who received the 1 W/cm 2 ultrasound treatments. At the beginning of the study, the patient population in both groups was moderately disabled due to LBP. After the therapy, group A reached a minimum level of disability.

Ultrasound treatment significantly decreased the disability scores measured by the Roland-Morris Scale. The average RM score decreased by 3.37 points in the 1 W/cm 2 group and by 3.59 points in the 0.5 W/cm 2 group. The minimally important change that was previously established in the literature is 3.5 points [19, 20]. The group treated with UD at 0.5 W/cm 2 achieved the minimal clinically important change (MCIC), while the group treated with 1 W/cm 2 showed a strong tendency to achieve clinical significance.

Our findings showed that patients subjected to ultrasound treatment, irrespective of the applied intensity, showed significant improvement in most RM features. The 1 W/cm 2 group achieved greater improvement in thirteen symptoms, while the other study group achieved better improvement in eight of the symptoms. Significance was achieved on six questions.

Similar results were obtained for the VAS scores. Our study found that both UD dosages (0.5 W/cm 2 and 1 W/cm 2 ) achieved pain relief, although receiving the higher intensity resulted in greater average improvements. We also referenced the MCIC, for which the literature revealed values ranging from 15 mm to 20 mm [19, 21]. These values, along with the clinical significance of the VAS score, confirmed improvement in group A. In the group treated with the lower ultrasound intensity, the improvement’s clinical significance is debatable.

Most previously conducted research on ultrasound efficiency in LBP used the ODI, RM and VAS scale questionnaires to assess the outcomes. Despite many conducted trials, no consensus has been reached regarding UD’s effectiveness and proper dosages. We believe that future studies assessing the effectiveness of specific UD parameters will answer the question of whether UD improves patients’ conditions, and, if so, these results will help to adjust UD therapy in clinical practice.

5. Conclusions

Many issues related to LBP, including its diagnosis and treatment, remain to be solved. Ultrasound therapy yields promising results in decreasing the disability level and pain intensity, but the effective treatment parameters must be determined because effectiveness appears to depend on the dosage. Our research showed that the patients who received the higher UD intensity achieved better results, but because of the small number of participants in this study, further research is needed.