According to the World Health Organization, chronic pain is a worldwide public health problem  . It is defined as “pain lasting longer than normal tissue healing time”, generally taken to be 12 weeks  . In looking at the prevalence of chronic low back pain in US citizens over a period of 14 years, there was a significant increase of about 6.3% with no changes in symptom severity or general health  . Chronic pain impairs quality of life, work performance and increases healthcare costs  . The socio-economic burden of low back pain has been evaluated by several studies    . Approximately 80% of low back pain cases are non-specific, meaning that no definite anatomical structure can be associated with neither the medical history nor the clinical examination  . Despite existing recommendations for avoiding chronicity of low back pain  , non-specific chronic low back pain (NSCLBP) continues to be increasing in prevalence in both developed and developing countries  . Several countries provide corresponding treatment and behaviour guidelines  highlighting the benefits of physical activity  . Strong evidence exists to encourage chronic low back pain patients to follow an active lifestyle and to assume self-responsibility for their health  . Despite conflicting evidence     , thermal therapy is applied in practice and by patients themselves as a self-management strategy  . Thermal therapy comprises the implementation of any superficial heat or warmth application to the skin via conduction or convection  . Thermal therapy is believed to influence pain  , to increase superficial and deep muscle tissue temperature and blood flow  affecting muscle nerve conduction velocity  . Electrophysical agents generate heat within the tissue by means of energy conversion (e.g. ultrasound, diathermy)  . Allen (2009) stated that “physical agents may serve as useful adjunctive modalities of pain relief or to enhance the effectiveness of other elements in therapy geared toward resolution of movement impairments and restoration of physical function”  . Despite these findings, the European guidelines for the management of NSCLBP (2006) do not consider thermal therapy to be more effective than placebo or other treatments neither to relieve pain, nor to improve functional outcome parameters  .
Thus, the authors of this review wanted to update the evidence for thermal therapy applications in patients with NSCLBP with respect to pain reduction and improvement of functional outcome parameters and global health and to rate the methodological quality of the included studies.
2.1. Research Question
The research question was defined by the PICOS-model  in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement  : Population: patients diagnosed with NSCLBP (>3 months symtomatic) aged 18 years or older; Intervention: thermal therapy applications except balneo/spa therapy; Comparator: any kind of treatment except thermal therapy; Outcomes: pain ratings, physical function, global health; Study design: RCT, CCT, CT.
2.2. Literature Search Strategies and Data Sources
An electronic systematic search, according to the guidelines of the PRISMA statement  , was conducted between May 2016 and February 2018 on the MEDLINE (PubMed), PEDro (Physiotherapy Evidence Database), CENTRAL (Cochrane Central Register of Controlled Trials), and CINHAL (Cumulative Index to Nursing and Allied Health Literature) databases. The additional filters “randomized controlled trials, clinical trials, controlled clinical trials, English language, full text” were applied in PubMed, the filter “clinical trials” in PEDro, “trials” in CENTRAL and “full text, academic journal” in CINHAL. Grey literature was searched on the websites of Google Scholar, Aspetar Sports Medicine Journal and by screening the Book of Abstracts of the European College of Sports Science (2014-2017). The function “similar articles” on PubMed and screening the reference lists were applied to attain additional literature. MeSH-proven (Medical Subject Headings) keywords were applied where possible (eAddenda Appendix I). The keywords which best fitted the research question among others were: “low back pain”, “heat application”, “hot packs”, “thermal therapy”, “warmth application”. The keywords representing the thermal therapy application were always combined with “AND” and “low back pain” (Table 1). The a priori set inclusion criteria were: 1) RCT, CCT, CT, 2) English full-text availability, 3) participants of 18 years or older diagnosed with NSCLBP, 4) control intervention of any treatment including placebo/sham without using thermal therapy, 5) outcome parameters comprising pain ratings and/or physical function and/or global health. The rationale for inclusion of these outcome variables was comparability. After screening and processing all the articles found (n = 164), a total of n = 9 articles was included in the final data analysis. Figure 1 shows the flow-chart of the selection process.
2.3. Data Extraction and Quality Assessment
The methodological quality of the studies was assessed using the 11-item PEDro scale   and the Cochrane Risk of Bias Tool (ROB)  . Two researchers (SW, RS) rated independently from each other the n = 9 studies. In case of disagreement or doubt consensus was reached by a third investigator (EH). For trials which PEDro scores were originally listed on the PEDro website
Table 1. Summary of keywords and its combinations of each database.
(https://www.pedro.org.au/)  , the presented PEDro score was used. The data extraction was performed in the same manner. Data on study design, participants, intervention method, assessment and each outcome parameter were split into corresponding columns on spreadsheets to extract relevant data. The accepted level of significance was set at alpha < 0.05. The between-group difference results were extracted following the completion of the whole treatment in order to observe the overall effect of the specified thermal therapy versus the control intervention.
3.1. Total PEDro Score and Risk of Bias Analysis
The total PEDro scores (eAddenda Appendix II) of the included studies were in line with the Risk of Bias results. Figure 2 depicts the Risk of Bias results of each included study and Figure 3 demonstrates the overview of all included studies for this analysis. A low risk of reporting bias was observed in 100% of the analysed studies. A high risk of bias with > 50% contributed to performance and detection bias (Personnel and Outcome assessor) as well as attrition bias. A low risk of bias with > 75% was attained for selection and other bias.
Figure 1. Flow-chart describing the selection process.
3.2. Study Characteristics
All included studies exclusively applied electrophysical agents as intervention treatment; n = 6 studies used ultrasound treatment  -  , n = 2 studies applied short-wave diathermy   and n = 1 study microwave diathermy  (see Figure 4 for detailed overview on the results). All studies, except Licciardone et al. (2013)  (USA), were performed in Eastern countries (n = 3 Iran    , n = 3 Turkey    , n = 2 Bangladesh   . The gender
Figure 2. Risk of bias graph for each included study.
Figure 3. Risk of bias summary for all included studies.
Figure 4. Detailed overview on the results of the included studies. Legend. BDI = back depression index, BMI(#) = body mass index(self-calculated weighted means), CG = control group, DO = drop outs, (H)EP = (home) exercise program, ERA = effective radiating area, FRI = functional rating index, FU = follow-up, IG = intervention group, Meth. = methodological, MP = manipulation, mod. = modality, MWD = microwave diathermy, 6 MWT = 6-min walk test, N = number of participants, n/a = not applicable, ODQ = Oswestry Disability questionnaire, p = placebo, PDI = pain disability index, RMDQ = Roland-Morris disability questionnaire, ROM = range of motion, s = sham, SF-36 = medical outcomes study short form-36 health survey, SI = sacro-iliac, SWD = short-wave diathermy, US = ultrasound, VAS = visual analog scale, §= significant at alpha < 0.05, *= within-group results over time.
distribution was not mentioned by n = 3 studies    , n = 2 studies were conducted with female participants only   , and n = 4 studies included male and female participants     . Age, BMI (except one)  and symptomatic back pain time before enrolment could be extracted from all studies. Treatment sessions ranged from 2 to 18 sessions over 4 to 8 weeks. Four of the included studies did a follow-up measurement, out of them n = 3 one month after the end of treatment    and n = 1 after six months post treatment  .
The studies comprised a wide range of assessment tools to rate the outcome parameters: pain was assessed by the visual analog scale (VAS)       , Lattinnen’s test score   and tenderness score  . Functional parameters were assessed by trunk mobility (Range of Motion (ROM)    , (modified) lumbar Schober test    , fingertip to floor distance    , trunk muscle strength  , endurance    , activation   , 6-min Walk Test (6MWT))   , and questionnaires (Functional Rating Index (FRI)   , Pain Disability Index (PDI)    , Oswestry Disability Questionnaire (ODQ)     , Back Depression Index (BDI)  , Medical Outcomes Study Short Form-36 Health survey general health scale (SF-36 GH)   , Roland-Morris Disability Questionnaire (RMDQ))  .
3.3. Treatment Effects on Outcome Parameters
Out of n = 6 studies on ultrasound treatment  -  a significant within and between-group pain reduction was reported by n = 2 studies, compared to exercise training alone (p = 0.012)  or to manipulation treatment (p = 0.001)  . Trunk mobility was mentioned in n = 6 studies       of which all reported flexion mobility with n = 1 study including lateral flexion  and n = 3 studies evaluating extension mobility    . Three studies reported significant within-group ROM improvements in flexion    for both the intervention and the control group. None of the n = 6 studies showed significant between-group results. Out of the n = 3 studies on extension ROM, n = 2 showed significant within-group results   , none of them found significant between-group results in favour of ultrasound treatment compared to placebo ultrasound treatment  or manipulation treatment  . Right lateral flexion ROM increased significantly within and between the placebo and ultrasound group (p = 0.04, p = 0.032, respectively), whereas left lateral flexion ROM significantly improved within-group (p = 0.04)  . 6MWT results, evaluated by n = 1 study, were significant for within and between-groups (p = 0.001, p = 0.024, respectively)  . Questionnaire outcomes were used in n = 6 studies  -  . The FRI score was significant within the placebo and treatment groups, no data on between-group results were mentioned  . The time (p < 0.001) and group (p = 0.004) main effects for the ultrasound treatment compared to the placebo intervention were significant in Ebadi et al. (2012), with a non-significant group*time interaction (p = 0.31)  . The PDI, ODQ and BDI showed significant within-group results (p = 0.001) for the combined ultrasound and exercise treatment group and exercise group alone and non-significant between group results   . Mohseni-Bandpei et al. (2006) reported significant within-group results for the ultrasound group and manipulative control group and significant between-group results (p = 0.001) in favour of manipulation treatment  . The SF-36 subscale items showed significant within-group differences for all items (p = 0.001)   with significant between-group results for “physical function” (p = 0.009)  , “social function” (p = 0.016)  and “physical function” (p = 0.021)  , “pain” (p = 0.046)  and “energy” (p = 0.009)  compared to exercise treatment alone. Licciardone et al. (2013) mentioned non-significantly different total SF-36 scores (p = 0.53) and RMDQ outcomes (p = 0.76) between ultrasound treatment and placebo intervention (eAddenda Appendix III)   .
Three studies focused on diathermy treatment, n = 2 studies on short-wave diathermy   and n = 1 on microwave diathermy  . For short-wave diathermy treatment, significant results in pain reduction were mentioned compared to the placebo group (p = 0.0)   . Shakoor et al. (2008) reported significant within-group results at all time intervals for both the intervention and the placebo group  . Non-significant between-group results for microwave diathermy combined with exercise versus exercise alone (p = 0.496) were mentioned  . Trunk mobility in flexion showed no significant difference between the microwave diathermy and exercise group compared to the exercise group alone (p > 0.05)  , as well as for 6MWT performance and all reported questionnaire outcomes (p > 0.05)  .
The aim of this systematic review was to assess the evidence for the use of thermal therapy in NSCLBP patients on pain and physical outcome parameters and to evaluate the methodological quality of the corresponding studies.
4.1. Evidence for Thermal Therapy
Despite the search strategy no studies on heat wraps, hot or mud packs fulfilled the inclusion criteria. To evaluate solely the thermal effect of local interventions, unbiased by water pressure or whole-body thermal applications and its physiological reactions, balneo/spa therapy was deliberately omitted as a keyword. However, several clinical trials showed positive short and long-term effects for balneo therapy in CLBP patients on pain, quality of life and physical function   .
The findings of this review, for a fraction of electrophysical agents only (continuous ultrasound, short-wave diathermy and microwave diathermy), was that all observed treatment interventions except microwave diathermy  significantly improved pain symptoms, with only two studies documenting the intake of oral pain killers   . The ineffectiveness of microwave diathermy on pain relief is supported by a study on non-specific chronic neck pain patients  . The significant improvement in pain between the groups was underpinned by the SF-36 subscale items physical function, pain and energy  . The SF-36 subscale items “physical function, social function, energy” may be positively affected by thermal therapy applied by means of electrophysical agents, of which ultrasound treatment may be superior to diathermy application. Surprisingly, mental health and general health were unaffected by pain reduction  . However, control group results showed significant pain reduction over time, limiting the positive effect of any electrophysical treatment for NSCLBP on pain. Supporting this finding, Koldas et al. (2008) reported significant improvements in global health and disability questionnaire results rather than pain reduction for physical therapy intervention group (hot pack, ultrasound and TENS) compared to home exercise alone  .
The included studies demonstrated that trunk mobility in the frontal and sagittal planes was indeed reduced in NSCBLP patients but improved independently of treatment, supporting the guidelines’ recommendations to remain physically active, rather than applying thermal therapy   . Additionally, NSCLBP patients attained walking distances comparable to healthy subjects  , assuming only a marginal negative effect of NSCLBP on walking distance performance and questioning the efficacy of the 6MWT distance as sensitive outcome parameter for NSCLBP. Earlier literature has reported that all forms of therapy have shown a positive effect on depression and global health  . This was supported by the questionnaire score results of this review. Moreover, Kurklinsky et al. (2016) challenged pain reduction in chronic pain patients proposing aiming at functional and quality of life improvements instead  .
All ultrasound studies applied continuous mode to ensure a thermal effect. Unfortunately, there was a wide heterogeneity in the reported ultrasound settings, although existing literature on ultrasound modalities and corresponding heat rates in specific depths for muscle and connective tissue exist     . The participants’ characteristic between the studies was heterogenic in terms of being symptomatic with NSCLBP for a period ranging from 3 months to 12 years. To presume that the duration of symptoms may negatively affect treatment outcomes cannot be supported since even the control groups showed improvement in the assessed outcome parameters.
4.2. Methodological Quality
The mean methodological quality of the included studies did not attain the total PEDro scores proposed by Moseley et al. (2011)  . Higher total PEDro scores would have been achievable through thorough reporting of statistical procedures and resolute blinding of personnel and/or outcome assessors   . However, the methodological quality did not affect the studies’ outcomes.
4.3. Limitations and Future Research
Figure 2 and Figure 3 depict that the included studies had a high risk of bias for blinding personnel and outcome assessors and an unclear risk of bias for random sequence generation, allocation concealment and other bias. The risk of a selection bias could be reduced by consistent reporting of random sequence generation and allocation concealment. Depending on the chosen thermal therapy treatment blinding of participants may be impossible due to the warming effect, nevertheless, > 50% low risk of bias was achieved. Surprisingly, as accessible, is the high risk of performance, detection bias and attrition bias, since hardly any follow-up measurements were conducted. Future studies are recommended to specify their study protocols and procedures to further reduce the risk of selection, reporting and other bias. Blinding of outcome assessors should become a matter of course, blinding of participants and personnel highly advised wherever possible to ensure a low risk of performance and detection bias. Further, there is a lack of studies using hot packs or heat wraps as applications method, although commonly utilized by patients at home.
In line with previous findings   , this review included recently published studies reaffirmed the contradictory evidence for thermal therapy on NSCLBP. Only electrophysical treatment methods fulfilled the inclusion criteria. Little evidence for direct post treatment pain reduction exists for continuous ultrasound treatment of 10 minutes’ duration, three days per week for 6 weeks, and short-wave diathermy treatment of 15 minutes’ duration, three days per week for 2 to 3 weeks. This short-term pain reduction achieved may be utilized as preparation for physical activity and exercising   . Remaining physically active and attending exercising are the main statements of international guidelines for chronic low back pain. Nevertheless, with respect to the complexity of neurophysiological mechanisms behind chronic pain syndromes, individual therapy and advice should be sought  . Although the effect of thermal therapy has been described in literature  , as applied by means of electrophysical agents (continuous ultrasound, short-wave and microwave diathermy), the authors conclude that there is only marginal evidence to support its application in patients with NSCLBP.
The authors thank Mrs. Lindsay Melcher-Kunz for providing language help and proof reading of the manuscript, and the “Thim van der Laan foundation” for the financial support.
Conflict of Interest
The authors declare no conflict of interest.
Conceived and designed the experiments: SW, RS, RC. Data extraction and quality assessment: SW, RS, EH. Risk of bias assessment: SW, RS, EH. Analysis of the data: SW, RS. Wrote the paper: SW, RS. Read and approved final version of manuscript: SW, RS, EH, RC.
Table A1. Overview on the MeSH-Term proven keywords.
Table A2. PEDro-score of the included studies.
Legend. + = criteria fulfilled, − = criteria not fulfilled.
Table A3. Detailed overview on the SF-36 questionnaire results.
Legend. M0, M1, M2, M3 = measurement time points, see Figure 4 for clarification, IG = intervention group, CG = control group, US = ultrasound, EP = exercise program, sUS = sham ultrasound, MWD = microwave diathermy. §= significant at α < 0.05.