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. Author manuscript; available in PMC: 2026 Jun 12.
Published in final edited form as: Man Ther. 2014 Dec 22;20(4):540–546. doi: 10.1016/j.math.2014.12.003

Immediate changes in pressure pain sensitivity after thoracic spinal manipulative therapy in patients with subacromial impingement syndrome: A randomized controlled study

Joseph R Kardouni a,*, Scott W Shaffer b, Peter E Pidcoe c, Sheryl D Finucane c, Seth A Cheatham d, Lori A Michener e
PMCID: PMC13257939  NIHMSID: NIHMS2178587  PMID: 25595413

Abstract

Background:

Thoracic SMT can improve symptoms in patients with subacromial impingement syndrome. However, at this time the mechanisms of SMT are not well established. It is possible that changes in pain sensitivity may occur following SMT.

Objectives:

To assess the immediate pain response in patients with shoulder pain following thoracic spinal manipulative therapy (SMT) using pressure pain threshold (PPT), and to assess the relationship of change in pain sensitivity to patient-rated outcomes of pain and function following treatment.

Design:

Randomized Controlled Study.

Methods:

Subjects with unilateral subacromial impingement syndrome (n = 45) were randomly assigned to receive treatment with thoracic SMT or sham thoracic SMT. PPT was measured at the painful shoulder (deltoid) and unaffected regions (contralateral deltoid and bilateral lower trapezius areas) immediately pre- and post-treatment. Patient-rated outcomes were pain (numeric pain rating scale – NPRS), function (Pennsylvania Shoulder Score – Penn), and global rating of change (GROC).

Results:

There were no significant differences between groups in pre-to post-treatment changes in PPT (p ≥ 0.583) nor were there significant changes in PPT within either group (p ≥ 0.372) following treatment. NPRS, Penn and GROC improved across both groups (p < 0.001), but there were no differences between the groups (p ≥ 0.574).

Conclusion:

There were no differences in pressure pain sensitivity between participants receiving thoracic SMT versus sham thoracic SMT. Both groups had improved patient-rated pain and function within 24–48 h of treatment, but there was no difference in outcomes between the groups.

Keywords: Subacromial impingement, Thoracic manipulation, Pressure pain threshold

1. Introduction

Shoulder pain is one of the most common musculoskeletal pain complaints in general medical practice, with a prevalence ranging from 16 to 48% (Pope et al., 1997; Broadhurst et al., 2006). Treatment of shoulder pain with manual therapy techniques that include thoracic spinal manipulative therapy (SMT) is reported to produce positive clinical outcomes (Winters et al., 1997; Bang and Deyle, 2000; Bergman et al., 2004; Boyles et al., 2009; Strunce et al., 2009; Mintken et al., 2010a). Although clinical efficacy is reported with thoracic SMT for the treatment of shoulder pain, the mechanisms underlying the clinical improvements have not been well established. Recent studies have found improvements in patient-rated pain and function after a single treatment of thoracic SMT in patients with subacromial impingement syndrome (SIS), but did not find mechanical changes in thoracic spine or shoulder mobility (Muth et al., 2012; Haik et al., 2014).

Pain relief after thoracic SMT may be due to neurophysiologic changes in pain sensitivity at the peripheral and/or central nervous system (Bialosky et al., 2009). Decreased sensitivity to pressure pain (increase in PPT) has been reported after SMT in patients with musculoskeletal pain (Vernon et al., 1990; Fernandez-Carnero et al., 2008; Mansilla-Ferragut et al., 2009; de Camargo et al., 2011; Martinez-Segura et al., 2012). To date, no studies have characterized the neurophysiologic effects of pain sensitivity after thoracic SMT in patients with SIS.

The primary purpose of this study was to assess the effects of thoracic SMT on central and peripheral pain sensitivity measured with PPT in patients with SIS. The secondary purpose of this study was to examine the relationship between change in the pain sensitivity following thoracic SMT and patient-rated outcomes of pain and function. It is hypothesized that patients receiving thoracic SMT compared to sham thoracic SMT will show: 1) increased PPT (decreased sensitivity to pressure pain) at the affected shoulder, indicating a decreased peripheral and/or central sensitivity to pain, 2) increased PPT at regions away from the affected shoulder (unaffected shoulder and over the lower trapezius muscle bilaterally) indicating decreased central sensitivity to pain, and 3) decreased pressure pain sensitivity will be related to improved patient-rated pain and function.

2. Methods

2.1. Participants

Participants (n = 48) with SIS were recruited from local physical and occupational therapy offices, physicians’ clinics, as well as by advertisement at a university gym during the period from November 2012 to April 2013. This study took place in a research laboratory in the Physical Therapy Department at Virginia Commonwealth University, and the study protocol was approved by the university’s Institutional Review Board. Inclusion criteria for patients with SIS were: 1) pain for ≥6 weeks, 2) typical daily shoulder pain ≥2/10 on an 11-point numeric pain rating scale (NPRS), and 3) 18–60 years of age. Subjects with shoulder pain also had to have 3 of the following 5 clinical signs of SIS: 1) positive Hawkin’s Test, 2) positive Neer Test, 3) pain during active elevation >60 in the scapular or sagittal plane, 4) positive Jobe/Empty Can test for pain or weakness, 5) pain or weakness with resisted shoulder external rotation with the arm at the side (Michener et al., 2009). Subjects were excluded from this study if they had 1) a history of shoulder, cervical spine, or thoracic spine surgery, 2) a primary complaint of neck or thoracic pain, 3) signs of central nervous system involvement, 4) signs of cervical nerve root involvement, 5) contraindications to manipulative therapy such as osteoporosis, metastatic disease, or systemic arthritis, 6) adhesive capsulitis, 7) instability of the shoulder, or 8) shoulder or arm pain with cervical rotation to the ipsilateral side, axial compression, or Spurling’s Test.

2.2. Procedures

All participants were provided verbal and written explanation of study procedures and signed an informed consent approved by the Institutional Review Board of the university. The participants completed an intake questionnaire (health screening questions, demographic information, and symptom history), a Fear Avoidance Beliefs Questionnaire (FABQ) (Mintken et al., 2010b) a baseline numeric pain rating scale (NPRS) and a baseline Pennsylvania Shoulder Score (Penn) (Leggin et al., 2006).

The NPRS consisted of an 11-point scale ranging from 0 (“no pain at all”) to 10 (“pain as bad as it can be”). The NPRS has shown to be reliable and responsive, with a minimal detectable change (MDC) of 2.5 points and a minimal clinically important difference (MCID) of 1.1 points in patients with shoulder pain (Mintken et al., 2009). The baseline NPRS asked patients to “Please rate your shoulder pain at the present time.” The NPRS following treatment read: “Now that you have had the manual therapy treatment to your thoracic spine, please rate your shoulder pain.” The Penn is a patient-rated shoulder function/disability questionnaire that has been found to be reliable and responsive (Leggin et al., 2006), where scores range from 0 to 100 (100 = no pain or functional loss). The MDC for the Penn is 12.1, and the=MCID is 11.4 points (Leggin et al., 2006). Global rating of change (GROC) was assessed following treatment. The GROC is a 15-point scale ranging from −7 (a great deal worse), through 0 (no change), to +7 (a great deal better) and was given at the 24–48 h follow-up to assess change in quality of life following treatment. GROC with an absolute value of 1–3 represent a small change, while change of 4–5 represents moderate change, and change of 6–7 represents large change (Jaeschke et al., 1989).

Upon initiation of testing, baseline PPT measurements were taken at the bilateral deltoid and lower trapezius muscles by an investigator that would remain blinded to treatment group assignments (non-treating investigator). Participants were then randomly assigned to receive thoracic SMT or sham thoracic SMT treatment. Both the thoracic SMT and sham thoracic SMT treatments were administered by a licensed physical therapist with 11 years of orthopedic physical therapy experience (treating investigator). Immediately following the treatment, PPT measures and the NPRS were administered again by the non-treating investigator. At 24–48 h after treatment, participants completed another NPRS and Penn, as well as the GROC. Fig. 1 depicts the experimental procedures.

Fig. 1.

Fig. 1.

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Flow diagram for experimental procedures. Abbreviations: PPT = Pressure Pain Threshold, NPRS = numeric pain rating scale, Penn = Penn Shoulder Scale, GROC = global rating of change.

2.3. Randomization and blinding

A randomization list for treatment group assignments of the participants was computer generated with random blocking using nQuery Advisor software (Statistical Solutions, Saugus, MA). Treatment assignments were placed into sequentially numbered privacy envelopes to conceal treatment group allocation. Participants were blinded to treatment assignment, and were told prior to the start of testing they could receive an active or a placebo treatment. In an effort to help maintain blinding and prevent participants from knowing that they were receiving the active or placebo treatment, both treatment groups were assigned names representative of active treatments. Participants randomized to the thoracic SMT group were told that they were receiving “spinal manipulative therapy” while those randomized to the sham thoracic SMT group were told they would receive a “therapist-assisted range of motion” treatment. An investigator blinded to treatment took all PPT measurements. This non-treating investigator was not in the room during treatment, and participants were asked to not discuss their treatment with the non-treating investigator. Blinding was assessed after treatment by asking the patient if they believed they received an active or placebo treatment.

2.4. Thoracic manipulation and sham thoracic manipulation

The SMT interventions were applied to the lower thoracic spine, mid thoracic spine, and cervicothoracic junction (Fig. 2). Each technique was applied 2 times at each of the 3 regions, for a total of 6 thoracic SMT or sham thoracic SMT maneuvers. During administration of the thoracic SMT, a high velocity, low-amplitude thrust was applied at the end of available spinal motion after the patient exhaled. For the mid and lower thoracic SMT, the participants were prone, and the thrust was directed in the posterior to anterior direction. For the cervicothoracic junction SMT, participants were seated, and the thrust was an axial (cephalad) distraction. The sham thoracic SMT was performed with identical body positioning of both the subject and therapist. During the sham thoracic SMT, the therapist maintained manual contact through the range of motion during exhalation, but no manipulative thrust was delivered. The sham thoracic SMT was previously reported as a believable active treatment (Michener et al., 2014).

Fig. 2.

Fig. 2.

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A: Prone mid and lower thoracic manipulation techniques. The therapist asked the participant to inhale fully and exhale completely. As the participant exhaled, the therapist applied posterior to anterior pressure to take out soft tissue slack. Upon complete exhalation, the therapist applied a high-velocity, low-amplitude manipulative thrust. B: Seated cervicothoracic manipulation technique. The therapist laced their arms through the participant’s arms and clasped their hands near the region of C7–T1. The therapist applied one side of the chest to the participants upper thoracic region as a fulcrum. The therapist took the participant into thoracic extension during exhalation, and applied a high-velocity, low-amplitude thrust in the cephalad direction.

2.5. Pressure pain threshold measurements

PPT was measured using a mechanical pressure algometer (Wagner Instruments, Greenwich, CT) with a flat rubber covered 1 cm2 round force gage (Fig. 3). PPT was measured bilaterally (affected and unaffected sides) over the middle deltoid and the lower trapezius muscle belly (between the spine and scapula at a spinal level between T5 and T7). The locations for PPT testing were chosen to represent the affected shoulder (middle deltoid), unaffected shoulder, and a region away from the affected region, near the application of treatment (lower trapezius). PPT measures at the affected shoulder were used to examine changes in peripheral and/or central pain sensitivity, while PPT at the unaffected shoulder and over the bilateral lower trapezius muscle were used to define changes in central pain sensitivity. The participant was instructed to say “pain” when the pressure applied through the algometer changed from a sensation of pressure to that of pain. At this point the pressure reading was recorded from the algometer. The order of PPT measurements at each location was randomized, and each site was tested 3 times. There was approximately 1 min between repetitions at each site. The average of the final 2 readings at each location was used for data analysis. Test/re-test reliability and error values were calculated on 10 healthy volunteers. The intraclass correlation coefficient (ICC(3,1)) values were excellent, at 0.98 and 0.93 for the deltoid and the lower trapezius, respectively. The standard error of the measure (SEM) was 0.24 kg/cm2 at the deltoid and 0.50 kg/cm2 at the lower trapezius, while the MDC was 0.34 kg/cm2 at the deltoid and 0.71 kg/cm2 at the lower trapezius.

Fig. 3.

Fig. 3.

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Pressure pain threshold measurements at the deltoid (A) and the lower trapezius (B) muscles.

2.6. Sample size

Sample size was calculated based on the primary aim, with 80% power and a significance level of α = 0.05. Preliminary data from 6 individuals with SIS showed a pre-to post-treatment effect size of 0.83 for PPT. Therefore, a sample size of 24 subjects per group for a total of n = 48 was required for the study.

2.7. Data analysis

All statistical analyses were performed using JMP Pro 10.0.0 software (SAS Institute, Cary, NC) with level of significance set at α = 0.05. PPT results were compared using a 2 × 2 mixed model ANCOVA, with gender used as a covariate, as the magnitude of PPT has been shown to vary by gender (Chesterton et al., 2003; Binderup et al., 2010; Coronado et al., 2011; Martinez-Segura et al., 2012). NPRS results were compared using a 2 × 3 mixed-model ANOVA using the factors of treatment group (thoracic SMT and sham thoracic SMT) and time (pre-treatment, post-treatment, and 24–48 h follow-up). Penn scores were compared using a 2 × 2 mixed-model ANOVA using the factors of treatment group and time (pre-treatment and 24–48 h follow-up). Distribution of GROC scores was assessed for normality as visualized on histogram and normal quantile plots, and a t-test was used to compare GROC scores between the groups at the 24–48 h follow-up. Correlations were calculated between the pre-to post-treatment change in PPT and patient-rated outcome variables. A Bonferroni corrected α = 0.013 (0.05/4) was used for correlations, as the relationship between PPT and each outcome variable was assessed at the 4 PPT test locations. A two-sample test of proportions was used to compare the proportion of participants in each group who believed they received an active or placebo treatment.

3. Results

Forty-eight (n = 48) individuals with SIS were randomly assigned to receive thoracic SMT (n = 24) or sham thoracic SMT (n = 24). Three participants were excluded from the final analysis (all in the sham thoracic SMT group) because it was discovered after testing that they had pain in both shoulders, leaving n = 45 for final analysis (Table 1). PPT measurements for both groups are shown in Table 2, and patient-rated outcomes are shown in Table 3. The mixed-model ANCOVA (Table 4) showed no differences in PPT between the treatment groups and no differences in PPT in the participants from pre-treatment to post-treatment (p ≥ 0.337). There were no differences between the two groups in pre-treatment to post-treatments changes (Group × Time interactions) in patient-rated outcomes of pain (NPRS) or function (Penn), with p-values ≥0.278. There was a main effect for Time for the NPRS and the Penn (p < 0.001), indicating that scores improved in both groups from pre-to post-treatment (Table 5). NPRS decreased across the groups 1.1, 95% CI [0.6, 1.6] points from pre-treatment to post-treatment measures and 1.5 [95% CI = 0.9, 2.0] points from pre-treatment to the 24–48 h follow-up. Penn scores improved across the groups 10.1, 95% CI [7.3, 12.9] points from pre-treatment to 24–48 h follow-up. A t-test revealed no statistically significant difference in the GROC (Fig. 4) between the two treatment groups, (t(43) 1.2, p = 0.235). The two-sample test for proportions was not significant (p = 0.312), indicating no difference between the groups for participants who felt they received an active form of treatment; 76% of participants in the thoracic SMT group and 62% of patients within the sham thoracic SMT group felt they received an active form of treatment.

Table 1.

Participant characteristics are provided as mean ± standard deviation for the variables of age (years), symptom duration (months), BMI (kg/m2), Fear Avoidance Beliefs Questionnaire (FABQ) work scale (scored 0–24), and FABQ physical activity scale (scored 0–42).

Participant characteristics
SMT (n = 24) Sham SMT (n = 21) p-value
Age (years) 31.1 ±12.3 31.2 ±12.1 p = 0.97
 Range 18–59 18–56
Symptom duration (months) 40.2 ±65.9 41.2 ±56.5 p = 0.96
 Acute/subacute (0–12 weeks) 3 (13%) 2 (10%)
 Chronic (>12 weeks) 21 (87%) 19 (90%)
Female, n (%) 14 (58%) 9 (43%) p = 0.38
BMI (kg/m2) 25.7 ±5.8 27.1 ±5.9 p = 0.44
 Range 18.2–39.6 19.6–40.5
FABQ work 4.5 ±5.2 8.0 ±10.4 p = 0.14
FABQ physical activity 14.5 ±4.3 15.2 ±5.5 p = 0.64
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Table 2.

Pre-treatment (Pre), post-treatment (Post), and change in pressure pain threshold (Change) at the deltoid and lower trapezius (Lower Trap) in the thoracic SMT and sham SMT treatment groups. Values are in kg/cm2 ± standard deviation.

Pressure Pain threshold
Thoracic SMT Sham SMT
Location Pre Post Change Pre Post Change
Deltoid
 Affected 3.68 ±1.25 3.72 ±1.54 0.04 ±0.61 3.71 ±1.79 3.65 ±1.47 −0.05 ±0.76
 Unaffected 3.57 ±1.21 3.73 ±1.59 0.16 ±0.79 3.79 ±1.79 3.82 ±1.66 0.04 ±0.62
Lower Trap
 Affected 4.19 ±1.30 4.32 ±1.32 0.13 ±0.83 4.58 ±2.28 4.59 ±1.96 0.00 ±0.69
 Unaffected 4.15 ±1.46 4.21 ±1.36 0.05 ±0.60 4.72 ±2.32 4.69 ±2.16 −0.03 ±0.68
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Table 3.

Patient-rated outcomes on the numeric pain rating scale (NPRS) and Penn Shoulder Score (Penn) are shown at pre-treatment (Pre), post-treatment (Post), and 24–48 h follow-up (24–48 h) for each of the treatment groups. The global rating of change (GROC) at the 24–28 h follow-up is also listed. The scores are reported ± standard deviation. Score range is indicated next to each outcome instrument.

Patient-rated outcomes
Pre Post 24–48 h
NPRS (0–10)
 Thoracic SMT 3.5 ±1.4 2.6 ±1.8 2.4 ±1.6
 Sham SMT 4.0 ±1.4 2.5 ±2.1 2.0 ±1.5
Penn (0–100)
 Thoracic SMT 71.4 ±11.2 80.6 ±11.1
 Sham SMT 72.0 ±12.1 83.0 ±9.8
GROC (−7 to 7)
 Thoracic SMT 1.3 ±2.0
 Sham SMT 2.0 ±2.2
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Table 4.

Mixed-model ANCOVA results for pressure pain threshold (PPT) are reported in this table according to the region and the side tested (affected or unaffected). Deltoid = Middle Deltoid. LT = Lower Trapezius. Group = treatment group. Time = pre-to post-treatment.

Factor df Pressure Pain thresholds
Deltoid - affected Deltoid - unaffected LT - affected LT - unaffected
F Ratio p-value F Ratio p-value F Ratio p-value F Ratio p-value
Group 1,42 0.3 0.579 0.0 0.886 0.2 0.678 0.5 0.503
Time 1,43 0.0 0.940 0.8 0.372 0.3 0.569 0.0 0.890
Group × Time 1,43 0.2 0.655 0.3 0.583 0.3 0.580 0.2 0.671
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Table 5.

Mixed-model ANOVA results for patient-rated outcomes of pain (NPRS) and function (Penn). Group = treatment group. Time = pre-treatment, post-treatment, and 24–48 h follow-up for NPRS. Time = pre-treatment to 24–48 h follow-up for Penn.

Factor df F Ratio p-value
Numeric pain rating scale (NPRS)
Group 1,43 0.0 0.984
Time 2,86 15.8 <0.001*
Group*Time 2,86 1.3 0.278
Penn shoulder score (Penn)
Group 1,43 0.2 0.627
Time 1,43 53.5 <0.001*
Group*Time 1,43 0.4 0.518
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Fig. 4.

Fig. 4.

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Frequency counts for Global Rating of Change (GROC) responses 24–48 h following treatment with either thoracic spinal manipulative therapy (SMT) or sham SMT.

Correlations between the pre-to post-treatment change in PPT and patient-rated outcome variables (NPRS, Penn, GROC) were calculated within the entire study sample and in the thoracic SMT group alone. Relationships were assessed between the change in PPT at each location and baseline NPRS and Penn scores, as well as between the change in PPT and the change in NPRS and Penn. The only significant correlation was for the thoracic SMT group, with a moderate correlation (0.52, p = 0.009) between change in PPT at the unaffected lower trapezius and baseline Penn. There were no other significant relationships (within the 20 comparisons) between change in PPT and the patient-rated outcomes of baseline pain and Penn, pre-to post-treatment change in pain and Penn, or GROC for the entire study sample or the SMT group.

4. Discussion

Thoracic SMT did not have any greater effect than the sham thoracic SMT on measures of pain sensitivity or patient-rated outcomes. While there was no pre-to post-treatment change in pain sensitivity, patient-rated outcomes improved across both groups. When we examined the relationship between mechanism (PPT) and outcome in the thoracic SMT group, a higher level of function (higher Penn score) at baseline was related to increased pain threshold at the unaffected lower trapezius following treatment. This was the only significant correlation of patient-rated outcomes with PPT mechanistic measures, limiting the implications of this finding.

Changes in patient-rated outcomes indicated small, but potentially meaningful, improvements. Changes across both groups met or exceeded the MCID for the NPRS of 1.1 points (1.1-point improvement immediately following treatment and 1.5-point improvement at 24–48 h follow-up) (Mintken et al., 2009). There were 8 (33.3%) participants in the SMT group and 10 (47.6%) in the sham thoracic SMT group that had a reduction in pain greater than the MCID. The mean improvement of 10.0 points in Penn score falls slightly short of the MCID of 11.4 (Leggin et al., 2006). There were 6 (25%) participants in the SMT group and 9 (42.9%) participants in the sham thoracic SMT group with a change in Penn score greater than the MCID. The fact that both groups were similar in perception that they received an active form of treatment indicates they were adequately blinded and knowledge of treatment likely did not have an effect on outcomes.

Since both the sham thoracic SMT and the thoracic SMT groups in this study showed improvement in patient-rated outcomes, the mechanisms of improvement from manual therapy to the thoracic spine may be independent of the use of a manipulative thrust. The mechanism of SMT could be related to factors of manual contact, positioning of the subject and moving them through the range of motion, interaction with a healthcare provider, or placebo effects (Bialosky et al., 2008; Benz and Flynn, 2013; Bishop et al., 2013). Since only immediate effects were assessed in this study, it is also possible that greater benefits and mechanistic changes with SMT would be seen with multiple treatments or over a greater time period following treatment. Prior clinical trials using thoracic SMT and/or mobilization as part of a manual therapy regimen in the treatment of SIS have reported positive effects compared to exercise interventions (Winters et al., 1997; Bang and Deyle, 2000) or combinations of exercise, oral medications, and/or physical modalities (Bergman et al., 2004). These studies used multiple treatments with manual therapy over weeks, rather than the single treatment performed in our study.

Single-arm clinical trials examining short-term effects of thoracic SMT as a stand-alone treatment for shoulder pain have reported positive effects in patient-rated outcomes, but it is difficult to draw cause and effect conclusions from these studies due to the lack of a comparator treatment or control group (Boyles et al., 2009; Strunce et al., 2009; Mintken et al., 2010a; Muth et al., 2012). Our study incorporated the use of a sham SMT comparator, and as we have reported, clinical improvements were seen across both treatment groups. The improvements we found in patient-rated pain are similar in magnitude to the results of Boyles et al. (1.1–1.2 point improvement of pain elicited during impingement signs), and Muth et al. (1.1–2.8 point improvements with arm elevation and impingement signs), but our results did not reach the magnitude of the pain reduction reported by Strunce et al. (31.9 mm improvement on visual analog scale) (Boyles et al., 2009; Strunce et al., 2009; Muth et al., 2012). The improvements in Penn score in our study were similar to the 7.6 points reported by Muth et al. (Muth et al., 2012). Although the Penn score results were significant, both studies failed to reach the threshold for clinically meaningful difference in Penn (Leggin et al., 2006). Interestingly, Muth et al. reported that 80% of participants had clinically meaningful reductions in pain, but only 33% had clinically meaningful improvements in Penn in their study. In our study, 40% of participants had meaningful reductions in pain and 33% had meaningful improvements in Penn scores.

Mean GROC across both groups in this study (1.6) was similar to that reported by Boyles et al. (1.4), indicating a small improvement (Jaeschke et al., 1989; Boyles et al., 2009). However, GROC in this study was smaller than the moderate improvement (4.2) reported by Strunce et al. (Strunce et al., 2009). Our study and that of Boyles et al. used a standardized treatment approach, whereas Strunce et al. used a pragmatic approach in applying thoracic SMT based on joint restrictions noted during examination. It is possible that the use of a pragmatic approach could improve outcomes, especially if the therapist engaged the patient with the treatment rationale and leveraged expectations prior to the application of particular techniques (Benz and Flynn, 2013).

4.1. Limitations

Pressure is a nonspecific stimulus that triggers mechanoreceptors and nociceptors in the skin and underlying tissues (Staahl and Drewes, 2004). It also does not allow for the noxious stimulus to be delivered rapidly and briefly, as it is applied gradually until the pain threshold is reached. Other pain eliciting modalities such as electrical, thermal, or chemical stimulation may produce different results when assessing the effects of SMT by providing variable stimuli to nociceptors and respective pathways and induce pain by means other than mechanoreception (e.g. chemical or thermal induced) (Staahl and Drewes, 2004). Our study included only a single session (dose) of thoracic SMT. Characterizing mechanisms and outcomes following multiple doses would more closely mirror clinical practice and may yield different results. We also did not assess subjects enrolling into this study for peripheral or central sensitization to pain, and prior research (Coronado et al., 2014) suggests that there may be a heterogeneous mix of patients with shoulder pain exhibiting peripheral and/or central pain sensitization.

Although our initial power analysis indicated that 48 participants were required for this study, we examined 45. Our pre-to post-treatment measurements within each group and the difference in pre-to post-treatment measurements between the groups were much less than the measurement error of 0.34 kg/cm2 for the deltoid and 0.71 kg/cm2 for the lower trapezius. Therefore, the addition of 3 more subjects to the sham thoracic SMT group likely would not have led to a change in our overall results.

4.2. Future studies

Since pain is an effect from peripheral nociceptive stimulation (from multiple potential stimuli) and processing mechanisms at multiple central nervous system structures, future studies should consider using alternate experimental pain modalities or multiple experimental pain modalities to characterize pain sensitivity following treatment. It would also be of benefit to measure mechanistic pain responses over longer duration treatment studies.

5. Conclusion

Thoracic SMT did not alter sensitivity to pressure pain at the affected shoulder or remote regions compared to sham thoracic SMT. There was also no difference in patient-rated outcomes between the groups. Interestingly, both groups had improved patient-rated outcomes following treatment. Clinically, thoracic SMT leads to improvements in pain and function in patients with SIS, but since effects were similar to sham thoracic SMT they may be related to factors such as manual contact, positioning of the subject and moving them through spinal range of motion, interaction with a healthcare provider, or placebo effect, rather than the manipulative thrust.

Acknowledgements

This research was funded through Clinical and Translational Science Award No. UL1TR000058 from the National Center for Advancing Translational Sciences and the AD Williams’ Fund of the Virginia Commonwealth University. The authors certify that they have no affiliations with or financial involvement in any organization or entity with a direct financial interest in this study.

Footnotes

Disclaimer

The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or reflecting the views of the National Institutes of Health, the Department of the Army or the Department of Defense.

Any citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations.

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