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Immediate Effects of Spinal Manipulative Therapy on Regional Antinociceptive Effects in Myofascial Tissues in Healthy Young Adults
ORIGINAL ARTICLES IMMEDIATE EFFECTS OF SPINAL MANIPULATIVE THERAPY REGIONAL ANTINOCICEPTIVE EFFECTS IN MYOFASCIAL TISSUES IN HEALTHY YOUNG ADULTS
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John Z. Srbely, DC PhD, a Howard Vernon, DC, PhD, b David Lee, DC, c and Miranda Polgar, BSc d
ABSTRACT Objective: The purpose of this study was to investigate if spinal manipulative therapy (SMT) can evoke immediate regional antinociceptive effects in myofascial tissues by increasing pressure pain thresholds (PPTs) over myofascial trigger points in healthy young adults. Methods: A total of 36 participants (19 men, 17 women; age, 28.0 [5.3] years; body mass index, 26.5 [5.7] kg/m 2) with clinically identifiable myofascial trigger points in the infraspinatus and gluteus medius muscles were recruited from the University of Guelph, Ontario, Canada. Participants were randomly allocated to 2 groups. Participants in the test group received chiropractic SMT targeted to the C5-C6 spinal segment. Participants in the control group received sham SMT. The PPT was recorded from the right infraspinatus and gluteus medius muscles at baseline (preintervention) and 1, 5, 10, and 15 minutes postintervention. Results: Three participants were disqualified, resulting in a total of 33 participants analyzed. Significant increases in the PPT (decreased pain sensitivity) were observed in the test infraspinatus group when compared with test gluteus medius, control infraspinatus, and control gluteus medius groups (P b .05). No significant differences in PPT were observed at any time point when comparing test gluteus medius, control infraspinatus, and control gluteus medius groups (P N .05). Conclusions: This study showed that SMT evokes short-term regional increases in PPT within myofascial tissues in healthy young adults. (J Manipulative Physiol Ther 2013;36:333-341) Key Indexing Terms: Manipulation; Myofascial Pain Syndrome; Myofascial Trigger Point; Pain Threshold; Chiropractic
hronic myofascial pain syndrome (MPS) is one of the most commonly encountered pain conditions and represents a significant burden to our health care system. Population-based studies report that chronic regional pain, such as MPS, impacts up to 25% of the population at any given time. 1 The prevalence of MPS
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a Assistant Professor, Department of Human Health and Nutritional Science, University of Guelph. b Professor, Canadian Memorial Chiropractic College. c Clinical Professor, Canadian Memorial Chiropractic College. d Student, Department of Human Health and Nutritional Science, University of Guelph. Submit requests for reprints to: John Z. Srbely, DC, PhD, Assistant Professor, Department of Human Health and Nutritional Science, University of Guelph, 50 Stone Rd E, Guelph, Ontario, Canada N1G2W1 (e-mail: [email protected]). Paper submitted May 5, 2012; in revised form January 15, 2013; accepted January 15, 2013. 0161-4754/$36.00 Copyright © 2013 by National University of Health Sciences. http://dx.doi.org/10.1016/j.jmpt.2013.01.005
among the middle-aged population (30-60 years) is reported at 37% and 65% in men and women, respectively, 2 and increases to 85% in the elderly (N 65 years). 3 Given that the elderly population is expected to double by the year 2040, 4 chronic MPS is poised to become one of the most significant health challenges in the future. Consequently, advancement of conservative and cost-effective therapeutic approaches is imperative to easing its impending burden on our health delivery system. The application of spinal manipulative therapy (SMT) is a cost-effective and widely recognized manual intervention used by a variety of health care professionals in the management of musculoskeletal pain. 5,6 A growing body of empirical evidence supports the use of SMT for the treatment of a broad scope of musculoskeletal disorders, 7,8 citing short-term antinociceptive (pain-relieving) effects 9 and restoration of normal joint mechanics. 10,11 Although the practice of manipulative therapy dates back to 2700 BCE, 12 its underlying physiological mechanisms of action are still poorly understood. One of the 333
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Fig 1. Flow diagram of participant recruitment, testing, and analysis. unresolved questions is whether SMT is predominantly governed by regional (segmental, heterosegmental) or general (nonsegmental) physiological mechanisms, or a complex interaction of both. The limited data addressing this question are equivocal, supporting either one or both of these mechanisms. 13,14 One objective of SMT research is to explore the neurophysiologic mechanisms and potential therapeutic applications of SMT in the treatment and management of MPS. Although the pathophysiology of MPS is still unclear, research suggests that myofascial trigger points (MTPs) play an integral role in the pathophysiology and clinical manifestation of MPS. 15,16 It is currently unknown if the antinociceptive effects of SMT in myofascial tissues are manifest predominantly via regional or general mechanisms, or a combination of both. A study is needed to specifically investigate the hypothesis that SMT evokes robust antinociceptive effects in MTPs preferentially located within neurosegmentally linked myofascial tissues. To our knowledge, no previous study has explicitly examined the direct impact of regional antinociceptive mecha-
nisms post-SMT in myofascial tissues using a singlesession, validated, sham-controlled design. Therefore, the purpose of this study was to investigate if SMT can evoke immediate regional antinociceptive effects in myofascial tissues by increasing pressure pain thresholds (PPT) over MTPs in healthy young adults.
METHODS This single-session, single-blinded, randomized, controlled intervention study was approved by the University of Guelph Ethics Board and was conducted in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans. All participants provided written informed consent before participating, and no one withdrew from the study. Figure 1 summarizes the stages of patient recruitment, testing, and analysis. A power analysis using previously published data 17 (which demonstrated an effect size of 1.14) determined that a sample size of 18 participants per
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Table 1. Demographic profile of study participants Test Control P value
Height (m)
Weight (kg)
Age (y)
Body mass index (kg/m2)
Male (%)
1.64 (0.09) 1.67 (0.05) .250
73.96 (15.33) 70.31 (11.64) .450
28.83 (7.04) 27.1 (2.52) .361
27.68 (6.72) 25.29 (4.30) .236
61.1 44.4
Data presented as mean (standard deviation).
group (test, control) is necessary to provide 90% power to detect the same effect size at an α of .05. A total of 44 prospective participants were randomly recruited between May 2010 and January 2011 from the University of Guelph (Ontario, Canada) and were assessed for eligibility. The primary aim of this study was to investigate the basic neurophysiological mechanisms of SMT, with a specific focus on evaluating the regional antinociceptive effects of SMT in myofascial tissues. Previous research has demonstrated that age significantly impacts pain pressure threshold (PPT) values 18; accordingly, we chose to study these effects in a young population. Each prospective participant was assessed by completing a confidential health questionnaire and undergoing a brief physical assessment by one of the clinician investigators. The primary inclusion criterion was the presence of a clinically identifiable MTP locus (active or latent) within the right infraspinatus and right gluteus medius muscles. The primary diagnostic criterion used to clinically identify the trigger point locus was a palpable hyperirritable nodule nested within a taut band of skeletal muscle; sustained ischemic pressure over the trigger point locus elicited a dull achy regional pain or discomfort. 19 Exclusion criteria were applied to identify conditions that could affect normal somatosensory processing in the cervical spine. These include serious medical conditions (cancer), central or peripheral neurologic conditions, recent acute injury to the spine, and/or medication use. A total of 36 participants (Table 1) qualified for the study and were randomly allocated to test or control groups by a research assistant drawing concealed labeled (test, control) slips of paper from a bin. The same research assistant was also responsible for charting all PPT values during the study. Two chiropractic clinicians, each with more than 17 years of clinical experience with SMT in the field of rehabilitation medicine, participated in this study. All testing was conducted at an urban outpatient rehabilitation clinic setting. The first clinician (“treating” clinician) was responsible for performing the history and physical assessment on all prospective participants and administering all SMT interventions. The second clinician (“assessing” clinician) was responsible for clinically identifying the trigger points within the infraspinatus and gluteus medius muscles as well as measuring all PPT values. The assessing clinician was blinded to participant group, whereas the treating clinician and research assistant were not.
The primary outcome measure of this study was the PPT value recorded from trigger points within the infraspinatus and gluteus medius muscles. To test our hypothesis, we used a trigger point within the infraspinatus muscle, chosen because of its common innervation to the manipulated segment (C5-C6). We also used a trigger point within the gluteus medius muscle (L4-S1) to act as a regional control point, given its segmental distance from the manipulated segment (C5-C6). Previous research using a similar design was used to demonstrate segmental antinociceptive effects postneedling 17 and postultrasonography. 20 Pain pressure threshold measures were quantified using a Chatillon DFE Series Force Gauge (AMETEK TCI, Largo, FL) with a gauge tip contact area of 285 mm 2 (19 × 15 mm). For this study, we defined the PPT as the magnitude of applied force (Newtons) over the trigger point locus (infraspinatus, gluteus medius) necessary to elicit the onset of a deep dull achy local discomfort and/or referred pain. 21 To quantify the pressure sensitivity of the trigger point, an increasing force was applied over the identified trigger point with the force gauge at an approximate rate of 5 N/s 22 until the participant verbally indicated the onset of the local and/or referred deep, dull, or achy sensation. All PPT readings were performed by an investigator with previous experience in the technique and application of pressure algometry. The instantaneous reading on the force gauge was recorded as the raw PPT value. The average of 2 PPT readings 20 was used as the raw PPT value at each time interval (0, 1, 5, 10, and 15 minutes postintervention). Participants were asked to lie prone while trigger points were clinically identified in both the right infraspinatus and gluteus medius muscles by the assessing clinician. The 2 trigger point loci were marked on the skin with a nontoxic marker to allow for easy identification throughout the study. All PPT readings were recorded from the right side. Before actual data recording, participants were first trained to reliably identify the PPT using trigger points using the homologous contralateral muscle. This was achieved when 2 consecutive PPT readings fell within a 2 N range. Baseline (preintervention) PPT measures were then recorded from the infraspinatus and gluteus medius muscles by the assessing clinician. The assessing clinician then left the examination room while the treating clinician entered to perform the intervention. Participants were instructed to lie supine during the manipulation procedure with the head resting on a mechanical “drop headpiece,” which is designed to increase
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Table 2. Raw PPT values recorded at each muscle over time Time (min) 0 Infraspinatus test Infraspinatus control Gluteus medius test Gluteus medius control
28.7 (9.6) 28.8 (4.8) 52.4 (13.5) 40.7 (10.3)
1
5
10
15
34.4 (9.6) 30.7 (7.5) 52.0 (15.0) 42.2 (11.5)
37.5 (11.9) 28.7 (6.0) 51.4 (19.0) 41.5 (12.6)
37.9 (14.4) 28.9 (6.3) 51.5 (18.5) 42.4 (10.8)
34.3 (11.5) 28.6 (7.0) 51.9 (16.0) 42.6 (12.3)
Raw PPT values (in newtons) were recorded at each muscle (infraspinatus, gluteus medius) in test vs controls at 0 (preintervention), 1, 5, 10, and 15 minutes postintervention. Spinal manipulation was applied to the C5-C6 spinal segment in test participants, whereas controls received a sham placebo manipulation. The PPT values are reported as average (standard deviation).
the acceleration of the manual thrust. Participants assigned to the “manipulation” group received a bilateral rotary manipulation targeting the C5-C6 segment. This procedure initially involves preloading the target tissues by inducing slight flexion, lateral flexion, and rotation (b 40°) of the cervical spine. A high-velocity, low-amplitude rotary thrust (b 10°) was then manually applied through the C5-C6 spinal segment with the contact hand. 23 The drop headpiece mechanism was engaged during this procedure and created a noise during the spinal thrust. The head/neck complex was then returned to the neutral position. Although a small number of studies have failed to validate the audible cavitation as a marker for manipulation success, 24 we adopted the criterion that cavitation did confirm successful administration of the procedure. 25 Control participants received a validated manual sham manipulation procedure. 26 This maneuver involves identical preloading of the cervical tissues as in the real manipulation protocol. In the sham procedure, however, the participant's head is supported by the clinician's forearm that rests directly on the headpiece. The quick release and rapid acceleration of the headpiece during the sham maneuver evoke the sensation of a real manipulation in participants. During this procedure, the clinician thrusts downward into the headpiece with the supporting arm to produce the sensation of a rapid manual thrust to the neck; however, no real thrust is made by the contact hand, and no intersegmental manipulation is achieved at any cervical segment. Vernon et al 26 have reported that a modified version of this procedure is perceived by at least 50% of patients with neck pain as a “real manipulation.” Furthermore, these participants report no clinically important or statistically significant change in pain or tenderness following this control procedure. 27 The treating clinician left the room while the assessing clinician returned to record PPT values from both the infraspinatus and gluteus medius trigger points at 1, 5, 10, and 15 minutes postintervention. The participant remained prone and resting on the treatment table throughout this time. Differences in group demographic characteristics (age, height, weight, body mass index) were assessed with t tests. Raw PPT measures recorded at time intervals 1, 5, 10, and 15 minutes were first normalized to baseline (time 0) values
Table 3. Significant interactions of normalized PPT scores Interaction
P value
Time Intervention Muscle Muscle × intervention Time × muscle Muscle × time × intervention
.040 b.001 b.001 b.001 .030 .010
to account for regional differences in PPT values due to differences in tissue thickness and/or receptor density. 28 We analyzed for PPT differences by performing a 2-way analysis of variance using PPT as the dependent variable and time, intervention, and muscle location as the independent variables. Significance was then followed up with post hoc individual comparisons at each time interval (Tukey test). In addition, to specifically compare regional antinociceptive effects between intervention groups, we calculated the PPT difference (PPTdiff) between infraspinatus and gluteus medius trigger points at each time interval within each participant. We then performed a 2-way analysis of variance using PPTdiff as the dependent variable and time and intervention as the independent variables. Significance was followed up at each time interval with post hoc individual comparisons (Tukey test). Statistical analyses were performed with SPSS Statistical Software (Version 18.0; SPSS Inc, Chicago, IL). Test for normality was performed using the Anderson-Darling test. Level of significance was set at .05.
RESULTS No significant differences in demographic characteristics between groups were observed (Table 1). A total of 36 participants were tested. Technical issues with the force gauge resulted in the exclusion of 1 test participant; poor reliability in gluteus medius trigger point identification during the PPT recording protocol resulted in the exclusion of a further 2 controls. As a result, a total of 33 participants (17 manipulation test, 16 sham controls) were analyzed. Average raw PPT scores are listed as a function of time and group in Table 2. Significant effects of time (P = .04),
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Fig 2. Normalized PPT vs time. Raw PPT values (N) were recorded from a trigger point in the infraspinatus and gluteus medius muscles at 0 (baseline preintervention), 1, 5, 10, and 15 minutes postintervention and normalized to baseline values. Test participants received spinal manipulation to the C5-C6 spinal segment, whereas controls received a sham placebo intervention. Significance denoted by asterisk. GM, gluteus medius; IS, infraspinatus; PPT, pain pressure threshold. Table 4. Significant interactions of PPTdiff scores Time Intervention Time × intervention
.020 b.001 .002
including 1 (P = .01), 5 (P b .001), 10 (P b .001), and 15 (P = .001) minutes (Fig 3).
DISCUSSION intervention (P b .001), and muscle (P b .001) as well as muscle × intervention (P b .001), time × muscle (P = .03), and muscle × time × intervention (P = .01) interactions were observed with normalized PPT scores (Table 3). Post hoc comparisons demonstrated significantly increased PPT scores at the test infraspinatus trigger point site vs all other groups (test gluteus medius, control infraspinatus, control gluteus medius) (P b .001) at all time intervals beyond baseline. No significant differences in PPT scores were observed at any time interval when comparing test gluteus medius, control infraspinatus, and control gluteus medius groups (P N .05). Figure 2 illustrates the normalized PPT values as a function of group and time. Significant differences between test and control conditions were also observed with the PPTdiff outcome. Analysis of variance revealed significant effects of time (P = .02) and intervention (P b .001) as well as time × intervention interaction (P = .002) (Table 4). Post hoc comparisons demonstrated significant increases in PPTdiff in the test group vs controls at all time intervals beyond baseline
Our data suggest that SMT evokes statistically significant short-term increases in PPT in segmentally related myofascial tissues in young adults. Decreased pressure sensitivity (increased PPT score) was observed at all time intervals beyond baseline within neurologically linked infraspinatus muscle after real, but not sham, manipulation; no difference in pressure sensitivity from baseline was observed at either muscle in controls or from the gluteus medius muscle in test participants. Furthermore, no difference in pressure sensitivity (PPTdiff) was observed at the gluteus medius muscle between test and control conditions, suggesting that SMT did not evoke significant generalized antinociceptive responses. The peak antinociceptive effect was measured as a 36% decrease in pressure sensitivity from baseline values and was recorded at 5 minutes postSMT. Significant antinociceptive effects in the test group were maintained throughout the 15-minute record period, whereas no significant antinociceptive effects were observed at any time point in controls. The findings of this preliminary single-session, manual sham-controlled study illustrate the robust regional mechanisms of SMT in
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Fig 3. Comparison of intraparticipant difference between PPT scores at infraspinatus and gluteus medius trigger points. Raw PPT values (N) were recorded from a trigger point in the infraspinatus and gluteus medius muscles at 0 (baseline preintervention), 1, 5, 10, and 15 minutes postintervention and normalized to baseline values. The intraparticipant difference in PPT (PPTdiff) between the infraspinatus and gluteus medius muscles was calculated and compared at each time point. Test participants received spinal manipulation to the C5-C6 spinal segment, whereas controls received a sham placebo intervention. Significance denoted by asterisk. PPT, pain pressure threshold. myofascial tissues and may offer a basis for further research into potential applications of SMT in the conservative treatment and management of MPS. Historically, SMT has been regarded and used as a technique to primarily restore joint mechanics. However, studies investigating changes in joint mobility post-SMT have challenged this paradigm. 29 Recent research suggests that the primary physiological effect of SMT may be neurophysiological; yet, the mechanisms are still poorly understood. In particular, it is still unclear whether the predominant mechanism of action of SMT in myofascial tissues is regional (segmental, heterosegmental), general (nonsegmental, supraspinal), or a combination of both. This is an important, yet unresolved, question that forms a foundational principle in understanding potential therapeutic applications for SMT in the management of MPS. Our data support the existing literature showing that SMT elicits regional inhibitory mechanisms in myofascial tissues. Previous work has demonstrated that areas of secondary hyperalgesia surrounding painful lumbar spinal segments reduce in size to a greater extent after manipulation when compared with other interventions. 30 In addition, Terrett and Vernon 31 report significant increases (2.4fold) in electrical cutaneous pain tolerance post-SMT in
skin regions immediately surrounding painful spinal segments, whereas 2 other studies demonstrate reduced PPT in MTP in tissues surrounding painful spinal lesions by up to 45%, when compared with other interventions. 14,32 Furthermore, regional decreases in thermal pain sensitivity, 33 increased electromyographic amplitude/fatigue resistance, 34 and bilateral increase in PPT in the wrist extensors of patients with lateral epicondylitis 35 have all been reported; however, none of these studies addressed the relative contributions of segmental vs nonsegmental mechanisms to their findings. Furthermore, there have been a considerable number of single-session studies investigating the effect of SMT; however, none of these have used a validated manual sham procedure. 36 Vernon and Humphreys 37 also note that “there is moderate-to-high quality evidence that immediate clinically important improvements (in pain scores) are obtained from a single session of (cervical) spinal manipulation.” However, most of these studies are so poorly controlled that the differential effect of the index therapy was subject to the confounding effects of unblinding of controls. Despite the strong regional antinociceptive responses observed in our study, existing research suggests that SMT can potentially evoke both regional and supraspinal antinociceptive effects. 13,14 Studies report increases of up to 25%
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in PPTs at remote anatomical sites post-SMT. 9 In addition, SMT applied to the upper cervical spine segments (atlantooccipital joint C0-C1) has been shown to reduce the PPT over the sphenoid bone in women with mechanical neck pain. 38 To distinguish between the relative impact of regional (segmental, heterosegmental) and supraspinal mechanisms, we compared the PPTdif) between the infraspinatus and gluteus medius muscles over time under test vs control conditions. Descending pain modulation is not somatotopically organized, but a generalized response mediated by serotonergic fibers within the dorsolateral funiculus. 39 The dorsolateral funiculus descends from the nucleus raphe magnus in the midbrain and terminates at all levels of the spinal cord. 40,41 Numerous studies have shown that descending inhibitory mechanisms can block afferent pain transmission and nociresponsive mechanisms at all levels of the spinal cord and trigeminal pathway. 42,43 Based on this rationale, we assumed that SMT-evoked supraspinal effects would impact pain sensitivity at both trigger point sites comparably; therefore, observed differences in the PPT profile between the infraspinatus and gluteus medius trigger points would be attributed predominantly to direct spinal segmental mechanisms acting at the infraspinatus trigger point site. A study investigating regional thermal pain sensitivity and using a similar design/rationale has been previously published. 33 Our study contributes to the existing body of literature, as it is the first single-session, validated manual sham-controlled study directly investigating the impact of segmental mechanisms of SMT on pain sensitivity in MTPs. Although one other study similarly compares PPT changes in segmentally related tissues using a controlled (not sham-controlled) design, 32 it did not distinguish between the relative contributions from segmental vs central mechanisms. We quantified the PPT using pressure algometry. Pressure algometry is a simple and reliable technique of quantifying trigger point sensitivity. 44 Previous research has shown a high inter- and intraexaminer reliability 45 in the PPT measure as well as a strong correlation to pain perception. 46 The PPT measure is consistent in normal individuals, 47 and measured changes in the PPT reflect changes in pain processing that could reflect changes in segmental and/or nonsegmental mechanisms. To compare segmental vs nonsegmental mechanisms, our study design used a regional control point within the gluteus medius muscle. This site was chosen because of its segmental distance from the manipulated C5C6 segment, precluding it from direct segmental and/or heterosegmental effects. Based on this rationale, we assumed that the measureable differences in the PPT response between the 2 trigger point sites would be predominantly attributable to direct local spinal segmental and/or heterosegmental mechanisms acting on the infraspinatus trigger point. Further research along this line is needed to enhance our understanding of these mechanisms and how they may contribute to the conservative management of MPS.
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Limitations and Future Studies There are several limitations to consider when interpreting the results of this study. The primary limitation is the potential for participant group bias. Although we used a validated sham SMT procedure, 26 the reliability of the sham could potentially be affected by previous experience with SMT. We did not exclude participants with previous SMT experience, and participants taking part in this study may have been able to identify their assigned intervention. The potential impact of such bias on our outcome, however, is significantly mitigated by our study design. Participants were informed before the study that the aim of this study was to investigate the effect of a spinal manipulative intervention on muscle pain; they were not aware of the specific manual technique(s) used, nor were they preconditioned to expect either outcome (regional, general). Furthermore, our specific outcome measure was the difference in PPT (PPT diff) scores between the infraspinatus and gluteus medius trigger points at each time interval. The interpretation of our results relies on the assumption that the effects of participant bias would manifest as a generalized response and, therefore, be comparable at both trigger point sites. Consequently, participant bias would not have a meaningful impact on the primary outcome of this study. Age and sex are important considerations in the interpretation of pain research. Given that age has a significant impact on PPT, 18 we chose to investigate these mechanisms in a young population; accordingly, the findings of this study cannot be extrapolated to clinical populations. Moreover, previous research investigating the effects of sex on pain is equivocal. Most studies in this field do not report a sex bias, 49 and those that do demonstrate small and inconsistent differences. 50 Previous research suggests that the specificity of SMT can vary between practitioners, potentially impacting additional spinal segment(s) above and below the target segment. 48 Although, the treating clinician in our study was experienced in delivering the intervention, manipulation of additional segment(s) above and/or below the target C5-C6 segment would not significantly impact our specific objective of investigating regional effect(s) post-SMT in myofascial tissues given that regional antinociceptive effects evoked via heterosegmental effects are governed by similar neurophysiologic mechanisms. In addition, we only studied the short-term effects (15 minutes) of SMT. Although we observed significant regional antinociceptive effects throughout the 15-minute record period, these findings cannot be extrapolated long term. A final limitation is the possibility for modulating trigger point sensitivity via repetitive pressure testing; however, previous research has shown that repeated pressure algometry over a period of 1 hour does not significantly impact the PPT. 22 Cost-effective manual therapies, such as SMT, are widely used in the management of a broad spectrum of musculoskeletal conditions, although their mechanisms and
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clinical applications are still poorly understood. An important outstanding question remains whether the physiologic mechanism governing SMT is predominantly regional (segmental, heterosegmental), generalized, or a combination of both. These data provide insight into the physiological effects of SMT and support the need for further research into potential clinical applications of SMT in the conservative treatment and management of MPS. Further research is needed in clinical populations to better understand the clinical relevance and application of these findings.
CONCLUSION This study demonstrated that SMT evoked robust shortterm segmental antinociceptive effects in myofascial tissues in a young population. The findings of this study contribute toward a better understanding of the basic physiological mechanisms of SMT and provide a foundation for further research investigating the potential therapeutic applications of SMT in the treatment and management of MPS.
Practical Applications • This study found that SMT applied to the C5C6 segments resulted in significant increases in the PPT (ie, decreased pain sensitivity) of MTPs located within segmentally related myofascial tissues (infraspinatus muscle). • Significant antinociceptive effects were not observed at trigger points located within nonsegmentally related myofascial tissues (gluteus medius). • These findings suggest that SMT evokes robust regional antinociceptive effects in myofascial tissues. • This study provides important foundational data to support further research into the potential benefit and role of SMT in the management of chronic widespread pain syndromes (myofascial pain, fibromyalgia).
FUNDING SOURCES AND POTENTIAL CONFLICTS OF INTEREST No funding sources or conflicts of interest were reported for this study.
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increases in trigger point sensitivity in humans. J Pain 2010; 11:636-43. Chesterton LS, Barlas P, Foster NE, Baxter GD, Wright CC. Gender differences in pressure pain threshold in healthy humans. Pain 2003;101:259-66. Triano JJ, Schultz AB. Motions of the head and thorax during neck manipulations. J Manipulative Physiol Ther 1994;17: 573-83. Sillevis R, Cleland J. Immediate effects of the audible pop from a thoracic spine thrust manipulation on the autonomic nervous system and pain: a secondary analysis of a randomized clinical trial. J Manipulative Physiol Ther 2011; 34:37-45. Herzog W, Zhang YT, Conway PJ, Kawchuk GN. Cavitation sounds during spinal manipulative treatments. J Manipulative Physiol Ther 1993;16:523-6. Vernon H, MacAdam K, Marshall V, Pion M, Sadowska M. Validation of a sham manipulative procedure for the cervical spine for use in clinical trials. J Manipulative Physiol Ther 2005;28:662-6. Vernon HT, Triano JJ, Ross JK, Tran SK, Soave DM, Dinulos MD. Validation of a novel sham cervical manipulation procedure. Spine J 2012;12:1021-8. Kanitakis J. Anatomy, histology and immunohistochemistry of normal human skin. Eur J Dermatol 2002;12:390-9. Nilson N, Christejsen HW, Hartvigsen DC. Lasting changes in passive range of motion after spinal manipulation: a randomised, blind, controlled trial. J Manipulative Physiol Ther 1996;19:165-8. Glover JR, Morris JG, Khosla T. Back pain: a randomized clinical trial of rotational manipulation of the trunk. Br J Ind Med 1974;31:59-64. Terrett AC, Vernon H. Manipulation and pain tolerance. A controlled study of the effect of spinal manipulation on paraspinal cutaneous pain tolerance levels. Am J Phys Med 1984;63:217-25. Ruiz-Saez M, Fernandez-de-Las-Penas C, Blanco CR, Martinez-Segura R, Garcia-Leon R. Changes in pressure pain sensitivity in latent myofascial trigger points in the upper trapezius muscle after a cervical spine manipulation in pain-free subjects. J Manipulative Physiol Ther 2007;30: 578-83. George SZ, Bishop MD, Bialosky JE, Zeppieri G, Robinson ME. Immediate effects of spinal manipulation on thermal pain sensitivity: an experimental study. BMC Musculoskelet Disord 2006;7:68. de CV, Burquerque-Sendin F, Berzin F, Stefanelli VC, de Souza DP, Fernandez-de-Las-Penas C. Immediate effects on electromyographic activity and pressure pain thresholds after a cervical manipulation in mechanical neck pain: a randomized controlled trial. J Manipulative Physiol Ther 2011;34: 211-20.
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35. Fernandez-Carnero J, Fernandez-de-Las-Penas C, Cleland JA. Immediate hypoalgesic and motor effects after a single cervical spine manipulation in subjects with lateral epicondylalgia. J Manipulative Physiol Ther 2008;31:675-81. 36. Vernon H, Puhl A, Reinhart C. Systematic review of clinical trials of cervical manipulation: control group procedures and pain outcomes. Chiropr Man Therap 2011;19:3. 37. Vernon H, Humphreys BK. Chronic mechanical neck pain in adults treated by manual therapy: a systematic review of change scores in randomized controlled trials of a single session. J Man Manipulative Ther 2008;16:E42-52. 38. Mansilla-Ferragut P, Fernandez-de-Las PC, BurquerqueSendin F, Cleland JA, Bosca-Gandia JJ. Immediate effects of atlanto-occipital joint manipulation on active mouth opening and pressure pain sensitivity in women with mechanical neck pain. J Manipulative Physiol Ther 2009; 32:101-6. 39. Porreca F, Ossipov MH, Gebhart GF. Chronic pain and medullary descending facilitation. Trends Neurosci 2002;25: 319-25. 40. Basbaum AI, Fields HL. Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry. Annu Rev Neurosci 1984;7:309-38. 41. Vanderah TW, Suenaga NM, Ossipov MH, Malan TP, Lai J, Porreca F. Tonic descending facilitation from the rostral ventromedial medulla mediates opioid-induced abnormal pain and antinociceptive tolerance. J Neurosci 2001;21:279-86. 42. Le Bars D. The whole body receptive field of dorsal horn multireceptive neurones. Brain Res Brain Res Rev 2002;40: 29-44. 43. Morgan MM, Heinricher MM, Fields HL. Inhibition and facilitation of different nocifensor reflexes by spatially remote noxious stimuli. J Neurophysiol 1994;72:1152-60. 44. Park G, Kim CW, Park SB, Kim MJ, Jang SH. Reliability and usefulness of the pressure pain threshold measurement in patients with myofascial pain. Ann Rehabil Med 2011;35: 412-7. 45. Fischer AA. Pressure algometry over normal muscles. Standard values, validity and reproducibility of pressure threshold. Pain 1987;30:115-26. 46. Fischer AA. Reliability of the pressure algometer as a measure of myofascial trigger point sensitivity. Pain 1987;28:411-4. 47. O'Driscoll S, Jayson M. The clinical significance of the pain threshold measurement. Rheumatol Rehabil 1982;21:31-5. 48. Ross JK, Bereznick DE, McGill SM. Determining cavitation location during lumbar and thoracic spinal manipulation: is spinal manipulation accurate and specific? Spine (Phila Pa 1976) 2004;29:1452-7. 49. Fillingim R, Maixner W. Gender diferences in the response to noxious stimuli. Pain Forum 1995;4:209-21. 50. Berkley K. Sex differences in pain. Behav Brain Sci 1997;20: 371-80 [discussion 435–13].
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John Z. Srbely, DC PhD, a Howard Vernon, DC, PhD, b David Lee, DC, c and Miranda Polgar, BSc d
ABSTRACT Objective: The purpose of this study was to investigate if spinal manipulative therapy (SMT) can evoke immediate regional antinociceptive effects in myofascial tissues by increasing pressure pain thresholds (PPTs) over myofascial trigger points in healthy young adults. Methods: A total of 36 participants (19 men, 17 women; age, 28.0 [5.3] years; body mass index, 26.5 [5.7] kg/m 2) with clinically identifiable myofascial trigger points in the infraspinatus and gluteus medius muscles were recruited from the University of Guelph, Ontario, Canada. Participants were randomly allocated to 2 groups. Participants in the test group received chiropractic SMT targeted to the C5-C6 spinal segment. Participants in the control group received sham SMT. The PPT was recorded from the right infraspinatus and gluteus medius muscles at baseline (preintervention) and 1, 5, 10, and 15 minutes postintervention. Results: Three participants were disqualified, resulting in a total of 33 participants analyzed. Significant increases in the PPT (decreased pain sensitivity) were observed in the test infraspinatus group when compared with test gluteus medius, control infraspinatus, and control gluteus medius groups (P b .05). No significant differences in PPT were observed at any time point when comparing test gluteus medius, control infraspinatus, and control gluteus medius groups (P N .05). Conclusions: This study showed that SMT evokes short-term regional increases in PPT within myofascial tissues in healthy young adults. (J Manipulative Physiol Ther 2013;36:333-341) Key Indexing Terms: Manipulation; Myofascial Pain Syndrome; Myofascial Trigger Point; Pain Threshold; Chiropractic
hronic myofascial pain syndrome (MPS) is one of the most commonly encountered pain conditions and represents a significant burden to our health care system. Population-based studies report that chronic regional pain, such as MPS, impacts up to 25% of the population at any given time. 1 The prevalence of MPS
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a Assistant Professor, Department of Human Health and Nutritional Science, University of Guelph. b Professor, Canadian Memorial Chiropractic College. c Clinical Professor, Canadian Memorial Chiropractic College. d Student, Department of Human Health and Nutritional Science, University of Guelph. Submit requests for reprints to: John Z. Srbely, DC, PhD, Assistant Professor, Department of Human Health and Nutritional Science, University of Guelph, 50 Stone Rd E, Guelph, Ontario, Canada N1G2W1 (e-mail: [email protected]). Paper submitted May 5, 2012; in revised form January 15, 2013; accepted January 15, 2013. 0161-4754/$36.00 Copyright © 2013 by National University of Health Sciences. http://dx.doi.org/10.1016/j.jmpt.2013.01.005
among the middle-aged population (30-60 years) is reported at 37% and 65% in men and women, respectively, 2 and increases to 85% in the elderly (N 65 years). 3 Given that the elderly population is expected to double by the year 2040, 4 chronic MPS is poised to become one of the most significant health challenges in the future. Consequently, advancement of conservative and cost-effective therapeutic approaches is imperative to easing its impending burden on our health delivery system. The application of spinal manipulative therapy (SMT) is a cost-effective and widely recognized manual intervention used by a variety of health care professionals in the management of musculoskeletal pain. 5,6 A growing body of empirical evidence supports the use of SMT for the treatment of a broad scope of musculoskeletal disorders, 7,8 citing short-term antinociceptive (pain-relieving) effects 9 and restoration of normal joint mechanics. 10,11 Although the practice of manipulative therapy dates back to 2700 BCE, 12 its underlying physiological mechanisms of action are still poorly understood. One of the 333
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Fig 1. Flow diagram of participant recruitment, testing, and analysis. unresolved questions is whether SMT is predominantly governed by regional (segmental, heterosegmental) or general (nonsegmental) physiological mechanisms, or a complex interaction of both. The limited data addressing this question are equivocal, supporting either one or both of these mechanisms. 13,14 One objective of SMT research is to explore the neurophysiologic mechanisms and potential therapeutic applications of SMT in the treatment and management of MPS. Although the pathophysiology of MPS is still unclear, research suggests that myofascial trigger points (MTPs) play an integral role in the pathophysiology and clinical manifestation of MPS. 15,16 It is currently unknown if the antinociceptive effects of SMT in myofascial tissues are manifest predominantly via regional or general mechanisms, or a combination of both. A study is needed to specifically investigate the hypothesis that SMT evokes robust antinociceptive effects in MTPs preferentially located within neurosegmentally linked myofascial tissues. To our knowledge, no previous study has explicitly examined the direct impact of regional antinociceptive mecha-
nisms post-SMT in myofascial tissues using a singlesession, validated, sham-controlled design. Therefore, the purpose of this study was to investigate if SMT can evoke immediate regional antinociceptive effects in myofascial tissues by increasing pressure pain thresholds (PPT) over MTPs in healthy young adults.
METHODS This single-session, single-blinded, randomized, controlled intervention study was approved by the University of Guelph Ethics Board and was conducted in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans. All participants provided written informed consent before participating, and no one withdrew from the study. Figure 1 summarizes the stages of patient recruitment, testing, and analysis. A power analysis using previously published data 17 (which demonstrated an effect size of 1.14) determined that a sample size of 18 participants per
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Table 1. Demographic profile of study participants Test Control P value
Height (m)
Weight (kg)
Age (y)
Body mass index (kg/m2)
Male (%)
1.64 (0.09) 1.67 (0.05) .250
73.96 (15.33) 70.31 (11.64) .450
28.83 (7.04) 27.1 (2.52) .361
27.68 (6.72) 25.29 (4.30) .236
61.1 44.4
Data presented as mean (standard deviation).
group (test, control) is necessary to provide 90% power to detect the same effect size at an α of .05. A total of 44 prospective participants were randomly recruited between May 2010 and January 2011 from the University of Guelph (Ontario, Canada) and were assessed for eligibility. The primary aim of this study was to investigate the basic neurophysiological mechanisms of SMT, with a specific focus on evaluating the regional antinociceptive effects of SMT in myofascial tissues. Previous research has demonstrated that age significantly impacts pain pressure threshold (PPT) values 18; accordingly, we chose to study these effects in a young population. Each prospective participant was assessed by completing a confidential health questionnaire and undergoing a brief physical assessment by one of the clinician investigators. The primary inclusion criterion was the presence of a clinically identifiable MTP locus (active or latent) within the right infraspinatus and right gluteus medius muscles. The primary diagnostic criterion used to clinically identify the trigger point locus was a palpable hyperirritable nodule nested within a taut band of skeletal muscle; sustained ischemic pressure over the trigger point locus elicited a dull achy regional pain or discomfort. 19 Exclusion criteria were applied to identify conditions that could affect normal somatosensory processing in the cervical spine. These include serious medical conditions (cancer), central or peripheral neurologic conditions, recent acute injury to the spine, and/or medication use. A total of 36 participants (Table 1) qualified for the study and were randomly allocated to test or control groups by a research assistant drawing concealed labeled (test, control) slips of paper from a bin. The same research assistant was also responsible for charting all PPT values during the study. Two chiropractic clinicians, each with more than 17 years of clinical experience with SMT in the field of rehabilitation medicine, participated in this study. All testing was conducted at an urban outpatient rehabilitation clinic setting. The first clinician (“treating” clinician) was responsible for performing the history and physical assessment on all prospective participants and administering all SMT interventions. The second clinician (“assessing” clinician) was responsible for clinically identifying the trigger points within the infraspinatus and gluteus medius muscles as well as measuring all PPT values. The assessing clinician was blinded to participant group, whereas the treating clinician and research assistant were not.
The primary outcome measure of this study was the PPT value recorded from trigger points within the infraspinatus and gluteus medius muscles. To test our hypothesis, we used a trigger point within the infraspinatus muscle, chosen because of its common innervation to the manipulated segment (C5-C6). We also used a trigger point within the gluteus medius muscle (L4-S1) to act as a regional control point, given its segmental distance from the manipulated segment (C5-C6). Previous research using a similar design was used to demonstrate segmental antinociceptive effects postneedling 17 and postultrasonography. 20 Pain pressure threshold measures were quantified using a Chatillon DFE Series Force Gauge (AMETEK TCI, Largo, FL) with a gauge tip contact area of 285 mm 2 (19 × 15 mm). For this study, we defined the PPT as the magnitude of applied force (Newtons) over the trigger point locus (infraspinatus, gluteus medius) necessary to elicit the onset of a deep dull achy local discomfort and/or referred pain. 21 To quantify the pressure sensitivity of the trigger point, an increasing force was applied over the identified trigger point with the force gauge at an approximate rate of 5 N/s 22 until the participant verbally indicated the onset of the local and/or referred deep, dull, or achy sensation. All PPT readings were performed by an investigator with previous experience in the technique and application of pressure algometry. The instantaneous reading on the force gauge was recorded as the raw PPT value. The average of 2 PPT readings 20 was used as the raw PPT value at each time interval (0, 1, 5, 10, and 15 minutes postintervention). Participants were asked to lie prone while trigger points were clinically identified in both the right infraspinatus and gluteus medius muscles by the assessing clinician. The 2 trigger point loci were marked on the skin with a nontoxic marker to allow for easy identification throughout the study. All PPT readings were recorded from the right side. Before actual data recording, participants were first trained to reliably identify the PPT using trigger points using the homologous contralateral muscle. This was achieved when 2 consecutive PPT readings fell within a 2 N range. Baseline (preintervention) PPT measures were then recorded from the infraspinatus and gluteus medius muscles by the assessing clinician. The assessing clinician then left the examination room while the treating clinician entered to perform the intervention. Participants were instructed to lie supine during the manipulation procedure with the head resting on a mechanical “drop headpiece,” which is designed to increase
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Table 2. Raw PPT values recorded at each muscle over time Time (min) 0 Infraspinatus test Infraspinatus control Gluteus medius test Gluteus medius control
28.7 (9.6) 28.8 (4.8) 52.4 (13.5) 40.7 (10.3)
1
5
10
15
34.4 (9.6) 30.7 (7.5) 52.0 (15.0) 42.2 (11.5)
37.5 (11.9) 28.7 (6.0) 51.4 (19.0) 41.5 (12.6)
37.9 (14.4) 28.9 (6.3) 51.5 (18.5) 42.4 (10.8)
34.3 (11.5) 28.6 (7.0) 51.9 (16.0) 42.6 (12.3)
Raw PPT values (in newtons) were recorded at each muscle (infraspinatus, gluteus medius) in test vs controls at 0 (preintervention), 1, 5, 10, and 15 minutes postintervention. Spinal manipulation was applied to the C5-C6 spinal segment in test participants, whereas controls received a sham placebo manipulation. The PPT values are reported as average (standard deviation).
the acceleration of the manual thrust. Participants assigned to the “manipulation” group received a bilateral rotary manipulation targeting the C5-C6 segment. This procedure initially involves preloading the target tissues by inducing slight flexion, lateral flexion, and rotation (b 40°) of the cervical spine. A high-velocity, low-amplitude rotary thrust (b 10°) was then manually applied through the C5-C6 spinal segment with the contact hand. 23 The drop headpiece mechanism was engaged during this procedure and created a noise during the spinal thrust. The head/neck complex was then returned to the neutral position. Although a small number of studies have failed to validate the audible cavitation as a marker for manipulation success, 24 we adopted the criterion that cavitation did confirm successful administration of the procedure. 25 Control participants received a validated manual sham manipulation procedure. 26 This maneuver involves identical preloading of the cervical tissues as in the real manipulation protocol. In the sham procedure, however, the participant's head is supported by the clinician's forearm that rests directly on the headpiece. The quick release and rapid acceleration of the headpiece during the sham maneuver evoke the sensation of a real manipulation in participants. During this procedure, the clinician thrusts downward into the headpiece with the supporting arm to produce the sensation of a rapid manual thrust to the neck; however, no real thrust is made by the contact hand, and no intersegmental manipulation is achieved at any cervical segment. Vernon et al 26 have reported that a modified version of this procedure is perceived by at least 50% of patients with neck pain as a “real manipulation.” Furthermore, these participants report no clinically important or statistically significant change in pain or tenderness following this control procedure. 27 The treating clinician left the room while the assessing clinician returned to record PPT values from both the infraspinatus and gluteus medius trigger points at 1, 5, 10, and 15 minutes postintervention. The participant remained prone and resting on the treatment table throughout this time. Differences in group demographic characteristics (age, height, weight, body mass index) were assessed with t tests. Raw PPT measures recorded at time intervals 1, 5, 10, and 15 minutes were first normalized to baseline (time 0) values
Table 3. Significant interactions of normalized PPT scores Interaction
P value
Time Intervention Muscle Muscle × intervention Time × muscle Muscle × time × intervention
.040 b.001 b.001 b.001 .030 .010
to account for regional differences in PPT values due to differences in tissue thickness and/or receptor density. 28 We analyzed for PPT differences by performing a 2-way analysis of variance using PPT as the dependent variable and time, intervention, and muscle location as the independent variables. Significance was then followed up with post hoc individual comparisons at each time interval (Tukey test). In addition, to specifically compare regional antinociceptive effects between intervention groups, we calculated the PPT difference (PPTdiff) between infraspinatus and gluteus medius trigger points at each time interval within each participant. We then performed a 2-way analysis of variance using PPTdiff as the dependent variable and time and intervention as the independent variables. Significance was followed up at each time interval with post hoc individual comparisons (Tukey test). Statistical analyses were performed with SPSS Statistical Software (Version 18.0; SPSS Inc, Chicago, IL). Test for normality was performed using the Anderson-Darling test. Level of significance was set at .05.
RESULTS No significant differences in demographic characteristics between groups were observed (Table 1). A total of 36 participants were tested. Technical issues with the force gauge resulted in the exclusion of 1 test participant; poor reliability in gluteus medius trigger point identification during the PPT recording protocol resulted in the exclusion of a further 2 controls. As a result, a total of 33 participants (17 manipulation test, 16 sham controls) were analyzed. Average raw PPT scores are listed as a function of time and group in Table 2. Significant effects of time (P = .04),
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Fig 2. Normalized PPT vs time. Raw PPT values (N) were recorded from a trigger point in the infraspinatus and gluteus medius muscles at 0 (baseline preintervention), 1, 5, 10, and 15 minutes postintervention and normalized to baseline values. Test participants received spinal manipulation to the C5-C6 spinal segment, whereas controls received a sham placebo intervention. Significance denoted by asterisk. GM, gluteus medius; IS, infraspinatus; PPT, pain pressure threshold. Table 4. Significant interactions of PPTdiff scores Time Intervention Time × intervention
.020 b.001 .002
including 1 (P = .01), 5 (P b .001), 10 (P b .001), and 15 (P = .001) minutes (Fig 3).
DISCUSSION intervention (P b .001), and muscle (P b .001) as well as muscle × intervention (P b .001), time × muscle (P = .03), and muscle × time × intervention (P = .01) interactions were observed with normalized PPT scores (Table 3). Post hoc comparisons demonstrated significantly increased PPT scores at the test infraspinatus trigger point site vs all other groups (test gluteus medius, control infraspinatus, control gluteus medius) (P b .001) at all time intervals beyond baseline. No significant differences in PPT scores were observed at any time interval when comparing test gluteus medius, control infraspinatus, and control gluteus medius groups (P N .05). Figure 2 illustrates the normalized PPT values as a function of group and time. Significant differences between test and control conditions were also observed with the PPTdiff outcome. Analysis of variance revealed significant effects of time (P = .02) and intervention (P b .001) as well as time × intervention interaction (P = .002) (Table 4). Post hoc comparisons demonstrated significant increases in PPTdiff in the test group vs controls at all time intervals beyond baseline
Our data suggest that SMT evokes statistically significant short-term increases in PPT in segmentally related myofascial tissues in young adults. Decreased pressure sensitivity (increased PPT score) was observed at all time intervals beyond baseline within neurologically linked infraspinatus muscle after real, but not sham, manipulation; no difference in pressure sensitivity from baseline was observed at either muscle in controls or from the gluteus medius muscle in test participants. Furthermore, no difference in pressure sensitivity (PPTdiff) was observed at the gluteus medius muscle between test and control conditions, suggesting that SMT did not evoke significant generalized antinociceptive responses. The peak antinociceptive effect was measured as a 36% decrease in pressure sensitivity from baseline values and was recorded at 5 minutes postSMT. Significant antinociceptive effects in the test group were maintained throughout the 15-minute record period, whereas no significant antinociceptive effects were observed at any time point in controls. The findings of this preliminary single-session, manual sham-controlled study illustrate the robust regional mechanisms of SMT in
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Fig 3. Comparison of intraparticipant difference between PPT scores at infraspinatus and gluteus medius trigger points. Raw PPT values (N) were recorded from a trigger point in the infraspinatus and gluteus medius muscles at 0 (baseline preintervention), 1, 5, 10, and 15 minutes postintervention and normalized to baseline values. The intraparticipant difference in PPT (PPTdiff) between the infraspinatus and gluteus medius muscles was calculated and compared at each time point. Test participants received spinal manipulation to the C5-C6 spinal segment, whereas controls received a sham placebo intervention. Significance denoted by asterisk. PPT, pain pressure threshold. myofascial tissues and may offer a basis for further research into potential applications of SMT in the conservative treatment and management of MPS. Historically, SMT has been regarded and used as a technique to primarily restore joint mechanics. However, studies investigating changes in joint mobility post-SMT have challenged this paradigm. 29 Recent research suggests that the primary physiological effect of SMT may be neurophysiological; yet, the mechanisms are still poorly understood. In particular, it is still unclear whether the predominant mechanism of action of SMT in myofascial tissues is regional (segmental, heterosegmental), general (nonsegmental, supraspinal), or a combination of both. This is an important, yet unresolved, question that forms a foundational principle in understanding potential therapeutic applications for SMT in the management of MPS. Our data support the existing literature showing that SMT elicits regional inhibitory mechanisms in myofascial tissues. Previous work has demonstrated that areas of secondary hyperalgesia surrounding painful lumbar spinal segments reduce in size to a greater extent after manipulation when compared with other interventions. 30 In addition, Terrett and Vernon 31 report significant increases (2.4fold) in electrical cutaneous pain tolerance post-SMT in
skin regions immediately surrounding painful spinal segments, whereas 2 other studies demonstrate reduced PPT in MTP in tissues surrounding painful spinal lesions by up to 45%, when compared with other interventions. 14,32 Furthermore, regional decreases in thermal pain sensitivity, 33 increased electromyographic amplitude/fatigue resistance, 34 and bilateral increase in PPT in the wrist extensors of patients with lateral epicondylitis 35 have all been reported; however, none of these studies addressed the relative contributions of segmental vs nonsegmental mechanisms to their findings. Furthermore, there have been a considerable number of single-session studies investigating the effect of SMT; however, none of these have used a validated manual sham procedure. 36 Vernon and Humphreys 37 also note that “there is moderate-to-high quality evidence that immediate clinically important improvements (in pain scores) are obtained from a single session of (cervical) spinal manipulation.” However, most of these studies are so poorly controlled that the differential effect of the index therapy was subject to the confounding effects of unblinding of controls. Despite the strong regional antinociceptive responses observed in our study, existing research suggests that SMT can potentially evoke both regional and supraspinal antinociceptive effects. 13,14 Studies report increases of up to 25%
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in PPTs at remote anatomical sites post-SMT. 9 In addition, SMT applied to the upper cervical spine segments (atlantooccipital joint C0-C1) has been shown to reduce the PPT over the sphenoid bone in women with mechanical neck pain. 38 To distinguish between the relative impact of regional (segmental, heterosegmental) and supraspinal mechanisms, we compared the PPTdif) between the infraspinatus and gluteus medius muscles over time under test vs control conditions. Descending pain modulation is not somatotopically organized, but a generalized response mediated by serotonergic fibers within the dorsolateral funiculus. 39 The dorsolateral funiculus descends from the nucleus raphe magnus in the midbrain and terminates at all levels of the spinal cord. 40,41 Numerous studies have shown that descending inhibitory mechanisms can block afferent pain transmission and nociresponsive mechanisms at all levels of the spinal cord and trigeminal pathway. 42,43 Based on this rationale, we assumed that SMT-evoked supraspinal effects would impact pain sensitivity at both trigger point sites comparably; therefore, observed differences in the PPT profile between the infraspinatus and gluteus medius trigger points would be attributed predominantly to direct spinal segmental mechanisms acting at the infraspinatus trigger point site. A study investigating regional thermal pain sensitivity and using a similar design/rationale has been previously published. 33 Our study contributes to the existing body of literature, as it is the first single-session, validated manual sham-controlled study directly investigating the impact of segmental mechanisms of SMT on pain sensitivity in MTPs. Although one other study similarly compares PPT changes in segmentally related tissues using a controlled (not sham-controlled) design, 32 it did not distinguish between the relative contributions from segmental vs central mechanisms. We quantified the PPT using pressure algometry. Pressure algometry is a simple and reliable technique of quantifying trigger point sensitivity. 44 Previous research has shown a high inter- and intraexaminer reliability 45 in the PPT measure as well as a strong correlation to pain perception. 46 The PPT measure is consistent in normal individuals, 47 and measured changes in the PPT reflect changes in pain processing that could reflect changes in segmental and/or nonsegmental mechanisms. To compare segmental vs nonsegmental mechanisms, our study design used a regional control point within the gluteus medius muscle. This site was chosen because of its segmental distance from the manipulated C5C6 segment, precluding it from direct segmental and/or heterosegmental effects. Based on this rationale, we assumed that the measureable differences in the PPT response between the 2 trigger point sites would be predominantly attributable to direct local spinal segmental and/or heterosegmental mechanisms acting on the infraspinatus trigger point. Further research along this line is needed to enhance our understanding of these mechanisms and how they may contribute to the conservative management of MPS.
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Limitations and Future Studies There are several limitations to consider when interpreting the results of this study. The primary limitation is the potential for participant group bias. Although we used a validated sham SMT procedure, 26 the reliability of the sham could potentially be affected by previous experience with SMT. We did not exclude participants with previous SMT experience, and participants taking part in this study may have been able to identify their assigned intervention. The potential impact of such bias on our outcome, however, is significantly mitigated by our study design. Participants were informed before the study that the aim of this study was to investigate the effect of a spinal manipulative intervention on muscle pain; they were not aware of the specific manual technique(s) used, nor were they preconditioned to expect either outcome (regional, general). Furthermore, our specific outcome measure was the difference in PPT (PPT diff) scores between the infraspinatus and gluteus medius trigger points at each time interval. The interpretation of our results relies on the assumption that the effects of participant bias would manifest as a generalized response and, therefore, be comparable at both trigger point sites. Consequently, participant bias would not have a meaningful impact on the primary outcome of this study. Age and sex are important considerations in the interpretation of pain research. Given that age has a significant impact on PPT, 18 we chose to investigate these mechanisms in a young population; accordingly, the findings of this study cannot be extrapolated to clinical populations. Moreover, previous research investigating the effects of sex on pain is equivocal. Most studies in this field do not report a sex bias, 49 and those that do demonstrate small and inconsistent differences. 50 Previous research suggests that the specificity of SMT can vary between practitioners, potentially impacting additional spinal segment(s) above and below the target segment. 48 Although, the treating clinician in our study was experienced in delivering the intervention, manipulation of additional segment(s) above and/or below the target C5-C6 segment would not significantly impact our specific objective of investigating regional effect(s) post-SMT in myofascial tissues given that regional antinociceptive effects evoked via heterosegmental effects are governed by similar neurophysiologic mechanisms. In addition, we only studied the short-term effects (15 minutes) of SMT. Although we observed significant regional antinociceptive effects throughout the 15-minute record period, these findings cannot be extrapolated long term. A final limitation is the possibility for modulating trigger point sensitivity via repetitive pressure testing; however, previous research has shown that repeated pressure algometry over a period of 1 hour does not significantly impact the PPT. 22 Cost-effective manual therapies, such as SMT, are widely used in the management of a broad spectrum of musculoskeletal conditions, although their mechanisms and
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clinical applications are still poorly understood. An important outstanding question remains whether the physiologic mechanism governing SMT is predominantly regional (segmental, heterosegmental), generalized, or a combination of both. These data provide insight into the physiological effects of SMT and support the need for further research into potential clinical applications of SMT in the conservative treatment and management of MPS. Further research is needed in clinical populations to better understand the clinical relevance and application of these findings.
CONCLUSION This study demonstrated that SMT evoked robust shortterm segmental antinociceptive effects in myofascial tissues in a young population. The findings of this study contribute toward a better understanding of the basic physiological mechanisms of SMT and provide a foundation for further research investigating the potential therapeutic applications of SMT in the treatment and management of MPS.
Practical Applications • This study found that SMT applied to the C5C6 segments resulted in significant increases in the PPT (ie, decreased pain sensitivity) of MTPs located within segmentally related myofascial tissues (infraspinatus muscle). • Significant antinociceptive effects were not observed at trigger points located within nonsegmentally related myofascial tissues (gluteus medius). • These findings suggest that SMT evokes robust regional antinociceptive effects in myofascial tissues. • This study provides important foundational data to support further research into the potential benefit and role of SMT in the management of chronic widespread pain syndromes (myofascial pain, fibromyalgia).
FUNDING SOURCES AND POTENTIAL CONFLICTS OF INTEREST No funding sources or conflicts of interest were reported for this study.
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