Effectiveness of Acupuncture in the Treatment of Temporomandibular Disorders of Muscular Origin: A Systematic Review of the Last Decade

Abstract

Objective:

The purpose of this review is to evaluate the effectiveness of using acupuncture treatment for temporomandibular disorders (TMD) of muscular origin according to research published in the last decade.

Methods:

The information was gathered using the MEDLINE, EMBASE, CINAHL, and CISCOM databases. The inclusion criteria for selecting the studies were the following: (1) only randomized controlled trials (RCTs) were selected; (2) studies had to be carried out on patients with TMD of muscular origin; (3) studies had to use acupuncture treatment; and (4) studies had to be published in scientific journals between 1997 and 2008. Two (2) independent reviewers analyzed the methodological quality of the studies using the Delphi list. A total of four RCTs were chosen once the methodological quality was judged as being acceptable. All of the studies included in the review compared the acupuncture treatment with a placebo treatment. All of them described results that were statistically significant in relation to short-term improvement of TMD signs and symptoms of a muscular origin, except one of the analyzed studies that found no significant difference between acupuncture and sham acupuncture.

Conclusions:

In the authors' opinion, research into the long-term effects of acupuncture in the treatment of TMD is needed. We also recommend larger samples sizes for future studies, so the results will be more reliable.

Introduction

T he term temporomandibular disorders (TMD) is a general one that encompasses a group of clinical conditions that affect the temporomandibular joint, the masticatory musculature, and the associated structures.¹ Muscular conditions are the main cause of TMD.² One of muscular conditions that affect the craniomandibular system is myofascial pain (MP), which is the most frequent type of TMD.³

MP is described as a musculoskeletal condition presenting a series of signs and symptoms caused by myofascial trigger points.^4,5

There are numerous studies that analyzed MP treatment using botulinum toxin injections. However, the systematic reviews of randomized controlled trials (RCTs) conclude that there is currently not enough evidence to support the use of this method for MP treatment.^6,7 Physiotherapy and manual therapy have proven to be somewhat effective in treating MP,^8,9 but more clinical trials are needed to confirm the effectiveness of these interventions.

Occlusal splint therapy in MP related to the craniomandibular system has been shown to be effective when compared with a control group not receiving treatment. However, these results must be interpreted with great care, as the research design of some of the studies was not completely reliable.^10,11 Besides, these results are based on short-term results, but not on long-term relief of painful symptoms.

Acupuncture is frequently used for treating TMD, and studies using this type of therapy have been systematically reviewed previously.^12,13 Although positive results were found in those reviews, the authors concluded that the research design of the trials was seriously inconsistent, and that the samples of the study, in some cases, were not very representative. Moreover, the comparisons were carried out with control groups in which no placebo was used.^12,13 It is also important to note that most of the studies analyzed in this review took place before 1994 and a systematic review related to the effectiveness of acupuncture in the treatment of TMD of muscular origin is lacking.

For this reason, the objective of this review is to evaluate the effectiveness of acupuncture treatment in TMD of muscular origin, according to RCT results published in the last decade.

Materials and Methods

Inclusion criteria

The inclusion criteria used to select studies for this review were the following: (1) only RCT were selected; (2) studies had to be carried out with TMD patients, especially those with orofacial pain of muscular origin; (3) studies had to include acupuncture treatment; and (4) studies had to be published in scientific journals between 1997 and 2007. No restrictions were used in terms of the language of publication.

We excluded all the studies that presented multiple interventions or were not RCT. Figure 1 ^14

–19 shows how the methodology was used and which studies were excluded.

FIG. 1.

Representation of the systematic review phases. RCT, randomized controlled trial.

Search strategy

The following databases were used to search for articles: MEDLINE,^® EMBASE, CISCOM, and CINAHL. The terms used for the search derived from a combination of the following words: acupuncture, temporomandibular joint dysfunction, temporomandibular disorders, orofacial pain, myofascial pain, and randomized controlled trial. Ten (10) potential studies were found. The studies were first analyzed based on the information provided in the summary, the title, and the key words. The articles selected were analyzed in great depth using the complete text in the evaluation phase.

Methodologic quality and data extraction

The evaluation of the methodological quality of these studies was carried out using the Delphi list.²⁰ This list is a generic criteria list developed by international consensus and consists of (1) randomization, (2) adequate allocation concealment, (3) groups similar at baseline, (4) specification of eligibility criteria, (5), blinding of outcome assessor, (6) blinding of care provider, (7) blinding of patient, (8) presentation of point estimates and measures of variability, and (9) intention-to-treat analysis. The additional item concerned (10) withdrawal/dropout rate (>20% or selective dropout) unlikely to cause bias, which was found relevant for these studies. The methodological criteria were scored as yes (1), no (0), or don't know (0). A quality score for individual Delphi items, resulting in possible score of 0–10, high quality is defined as six or more criteria fulfilled.²¹

Two (2) independent reviewers analyzed the quality of all of the articles selected using the same methodology throughout the analysis. Disagreements between reviewers were resolved by including the criteria of a third reviewer so that a consensus could be reached.

The intra-assessor reliability of the overall quality assessment and clinical relevance assessment was derived by κ coefficient statistics (>0.7 means high level of agreement between assessor; between 0.5 and 0.7 a moderate level of agreement, and <0.5 a poor level of agreement).

The characteristics of the treatment applied, as well as the results and conclusions presented in the studies analyzed, are fully described in the Results section.

Results

In the first phase of the analysis, 4 RCTs were selected^22

–25 where acupuncture was applied to treat TMD of muscular origin. Table 1 represents the general characteristics of the studies in a descriptive way.

Table 1.

General Characteristics of the Studies

Authors	Design	Subjects used for study	Treatment	Outcome measures	Results
Shen and Goddard²²	RCT Double-blinded	N = 15 Age: average 43.1 ± 13.6 14 women, 1 man	Study group (SG) = Acupuncture Control group (CG) = Sham acupuncture	Numeric scale Visual analogue scale (VAS) Pressure algometer	Decrease in facial pain (p = 0.003), neck pain (p = 0.011) and headache (p = 0.0159) Increasing the pain tolerance (p = 0.027)
Smith et al.²³	RCT Double-blinded	N = 27 Age: average 40.5 ± 13.63 24 women, 3 men	SG = Acupuncture CG = Sham acupuncture	VAS Incisor opening and lateral movement measurement/measured in millimeters using Vernier style bite TMJ sounds/stereo-stethoscope Pain distribution	Improvement in functional impairment (p = 0.003) Improvement in pain intensity (p = 0.001) Improvement in pain distribution, left (p = 0.0003), right (p = 0.005) Improvement in maximum interincisor opening (p = 0.029) Improved maximum pain-free opening (p = 0.029) Extent of lateral movement, left (p = 0.06), right (p = 0.008) Improvement in tenderness of masticatory muscles (p = <0.05) No improvement in joint sounds (p = 0.317)
Schmid-Schwap et al.²⁴	RCT Single-blinded	N = 23 Age: average 35 ± 14 23 women	SG = Acupuncture CG = Sham laser	VAS Manual palpation Electronic axiography	No mouth opening improvement (p = 0.114) No improvement in retrusion and protrusion movements (p = 0.084) Improvement in pain (p = 0.033) Improvement in muscular pressure-point tolerance (p = <0.05)
Goddard et al.²⁵	RCT Double-blinded	N = 18 Age: EG: 35.4 ± 10.63 CG: 34.53 ± 6.78 15 women, 3 men	SG = Acupuncture CG = Sham-acupuncture	VAS Pressure algometer	Both groups showed a statistically significant reduction in VAS pain scores (p = 0.001) There was no significant difference between the 2 groups (p = 0.255)

RCT, randomized controlled trial; VAS, visual analogue scale; TMJ, temporomandibular joint.

Methodological quality

When evaluating the quality design of the studies, the results showed that the methodology of all those included trials was acceptable.^22

–25 Table 2 shows the methodological quality of the studies. The two reviewers had a discrepancy in the evaluation of three studies.^22,24,25 The discrepancy for those three studies concerned their scores for items 2, 6, and 10 on the Delphi list. A consensus was reached after the third reviewer intervened. The intra-assessor reliability of the methodological quality assessment was high (κ = 0.85).

Table 2.

Representation of Evaluation Scores on the Methodological Quality Applied to the Delphi List in the Four Studies

Study	Year	1	2	3	4	5	7	8	9	10	Total
Shen and Goddard²²	2007	1	0	1	1	1	1	1	1	0	7
Smith et al.²³	2007	1	0	1	1	1	1	1	1	1	8
Schmid-Schwap et al.²⁴	2006	1	0	1	1	0	1	1	1	1	7
Goddard et al.²⁵	2002	1	1	1	1	1	1	1	1	1	9

Participants

According to the inclusion criteria of each of the studies, we interpreted that all the subjects had a TMD of muscular origin and, more specifically, had orofacial pain of myofascial origin.

There were no dropout subjects in three of the studies^22,24,25 during the whole research process, and all of their results were analyzed. The other study²³ only had 1 subject drop out during its course.

Interventions

The results of the studies analyzed showed positive effects of acupuncture in improving the signs and symptoms of TMD of muscular origin (Table 1). In all of the studies, the only treatment used in the study group was acupuncture, and in the control group, all of the studies used a placebo. In three of the studies, sham acupuncture^22,23,25 was used, and in the other, sham laser.²⁴ The sham needle acupuncture interventions included minimal penetration (2–4 mm) of points 1 cm distal to the verum acupoints bilaterally in one study,²⁵ while two studies^22,23 had the needle touch the skin at or near the verum acupoint without piercing the skin. The fourth study²⁴ used sham laser (inactivated) applied randomly at several acupoints.

The verum acupuncture points chosen for treatment of TMD pain varied widely between the four studies. One study examined only one local acupoints, Stomach 7 (ST-7).²³ Two (2) studies examined only distal acupoints. Both examined Large Intestine 4 (LI-4),^24,25 but one study²⁴ also included Small Intestine 2 (SI-2) and Small Intestine 3 (SI-3) in its acupuncture protocol. The fourth study²⁵ examined both local (ST-6) and distal (LI-4) acupoints. Three (3) studies described depth of needle insertion,^22,23,25 which varied from 6 mm to 30 mm. Duration of acupuncture treatment varied from 15 minutes to 30 minutes.^22,23,24,25

Discussion

The four studies analyzed consistently showed that acupuncture is effective in the treatment of TMD. All studies showed TMD pain reduction for both verum and sham acupuncture interventions, and three of the four studies^22
–24 demonstrated that verum acupuncture produced statistically significant improvements in pain compared to sham acupuncture.

The study designs of the four RCT were appropriate, and their methodological quality was acceptable. Some of the highlights of these studies were that both the randomized method and the blinding technique were described in detail. Most significantly, all studies used a placebo treatment in their control groups, which is essential to try to confirm that the verum acupuncture analgesic effects are due to the acupuncture intervention rather than due to placebo effects, as other studies have suggested.^26,27 A recent review has demonstrated that the placebo effect in analgesia is a real phenomenon measurable with brain-imaging analyses, and that every pain treatment has a placebo component that can be very strong.²⁸ The study of Goddard et al.²⁵ reviewed herein suggest this, as both placebo (sham) and verum acupuncture treatment groups had improvements of their TMD pain that were not statistically significantly different.

Three (3) of the four studies included in this review found that verum acupuncture produced statistically significant visual analogue scale (VAS) pain reduction compared to sham acupuncture, yet only two studies also demonstrated clinically significant reductions in VAS pain scores, with one study showing a VAS improvement of 33 mm²³ (baseline VAS = 62 mm) and the other an improvement of 19.1 mm²⁴ (baseline VAS: 44 mm). Bird and Dickson²⁹ have postulated that clinically significant changes in pain are not uniform along the VAS scale, and recommend defining a clinically significant change as a decrease of 17 mm when the baseline VAS score is between 34 and 66 mm, and a decrease of 28 mm when the baseline VAS is ≥67 mm. Todd et al.,³⁰ however, have suggested that a 13-mm alteration in the VAS score is the minimum change that should be considered clinically significant.

Three (3) studies reviewed herein examined the effect on TMD pain of distal acupoints, most notably LI-4.^22,24,25 Hui et al.³¹ observed that the stimulation of LI-4 modulates specific areas of the subcortical gray matter and limbic system involved in pain perception. Hsieh et al.³² demonstrated that the stimulation of LI-4 activates different areas of the hypothalamus on single photon emission computed tomography scanning, and suggested that this could be an important mechanism to explain the acupuncture's analgesic efficacy. The present systematic review provides limited evidence that stimulation of LI-4 alone reduces TMD pain, though the improvement is of uncertain clinical significance. More clinical studies using distal points alone (e.g., LI-4) are needed to confirm these findings and determine whether these pain improvements are large enough to be clinically important.

Two (2) studies reviewed herein used specific local acupoints located in the craniomandibular region (ST-6²⁵ and ST-7²³) in their verum acupuncture for TMD pain, and the study by Smith et al.,²³ which treated only ST-7, had the most statistically (and clinically) significant reductions of TMD pain. These points may be coincident with masseter muscle trigger points.³³ Further research is needed to determine whether addition of a distal acupuncture point enhances the efficacy of pain reduction produced by stimulation of a local craniomandibular acupoint in TMD pain conditions.

Shortcomings of the studies^22

–25 analyzed in this review include their very small sample sizes, though their methodological quality were higher than studies carried out in the previous decade.^12,13 These studies also only examined the short-term effects of acupuncture for TMD pain, and future research should include analysis of the long-term effect of acupuncture on TMD pain of muscular origin, including the duration of analgesia produced by acupuncture.

Conclusions

Based on this review, it would seem the evidence is (1) limited in amount, (2) shows short-term benefit for acupuncture for TMD pain of muscular origin; the study also showed that (3) local acupuncture had greatest pain reduction, (4) distal points had efficacy, and (5) a lot more research is needed as to which points and/or combination of points to use and duration of efficacy of acupuncture. The authors of the present review suggest the use of ST-6, ST-7, and LI-4 acupoints for the treatment of TMD pain of muscular origin.

Footnotes

Disclosure Statement

No competing financial interests exist.

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