Abstract
This mixed-method blinded research study used high-frequency diagnostic medical sonography (DMS) to document myofascial trigger points (MTrPs) associated with ankle/foot pain. A total of 17 symptomatic and 8 asymptomatic participants provided 500 MTrP areas for palpation. Forty-nine of these MTrP areas (including 16 tender points, 15 palpable MTrPs, and 18 palpated and imaged MTrPs) were compared with the patient’s ankle/foot pain, collected with the SF-12 Health Survey, Victorian Institute of Sports Assessment–Achilles questionnaire, and a visual analog scale. Qualitative analyses of the participants’ histories were used to understand the context of the data collected. The mean area of the 18 imaged MTrPs was 0.09 cm2, and they appeared inhomogeneous and hypoechoic. Those with right-sided MTrPs were statistically significant for those with reported pain. Participants with left-sided MTrPs did not demonstrate a significant correlation to reported pain. The results demonstrated a promising possible diagnostic approach using sonography to evaluate MTrPs in the ankle/foot for documenting myofascial pain. In addition, elastography and transducer frequencies >12 MHz are proposed as a DMS technique to increase overall diagnostic yield.
Musculoskeletal pain is a frequently reported health condition, and in 2004, the cost of treatment and lost wages associated with these ailments was estimated to be $849 billion (8% of the US gross domestic product). 1 Musculoskeletal pain can lead to a loss of physical function and decline in mental health, which can also adversely affect gainful employment. 2 According to a large cross-sectional survey of the general population, musculoskeletal pain has a prevalence of 20%. 3 Suggesting myofascial trigger points (MTrPs) as a major source of musculoskeletal pain is not as controversial an idea as when it was originally proposed. 4 Physicians, by in large, have become accepting of the concept of and interested in myofascial pain, even though the full pathophysiology of MTrPs remains contested. It is now widely recognized by the therapeutic massage community, with a specific certification for myofascial trigger point therapists. In fact, many estimate that chronic musculoskeletal pain affects about 23 million people or about 10% of the US population. 5 Yet, according to experts, physicians fail to consider MTrPs when investigating pain. 6 If no abnormality is found on diagnostic imaging, such as radiographs or magnetic resonance imaging (MRI), the pain being reported is often looked upon as being indeterminate. While being reassured nothing serious has been found with traditional diagnostic imaging, a patient most likely continues to experience pain. Moreover, should a patient pursue a diagnosis, there is always the risk of misdiagnosing the pain as being of a psychogenic origin, without the activation of myofascial trigger points even being considered. 6
MTrPs are considered hyperirritable spots within taut bands of skeletal muscle fibers, often producing a palpable nodule. Biopsies of MTrPs have shown contraction knots and giant round muscle fibers. 7 Yet currently in clinical practice, physical examination is the sole way to make a definite diagnosis of an MTrP, which is most often defined as palpation of a tender point in a taut band. 4 Complicating matters, experts have understandably refined their criteria in attempts to improve diagnostic sensitivity. For example, the local twitch response, which has shown to be the least reliable of the original criteria, is not considered a diagnostic essential for a diagnosis, and referred pain is now considered nonspecific. 4 A 2007 literature review of 93 published articles found the diagnostic criteria were unreliable and inconsistent for an MTrP diagnosis. 3 In addition, to palpate an MTrP, a high level of skill is required, 7 and research has revealed poor interrater reliability based on the current diagnostic criteria. 8 It is for reasons such as these that MTrPs are excluded from the current physical examination. It is obvious a more reliable and objective diagnostic test is needed to help physicians identify MTrPs.
Until recently, a limited amount of research has been published with regard to evaluating MTrPs with medical imaging. Over 15 years ago, the first study to evaluate MTrPs with diagnostic medical sonography (DMS) by Lewis and Tehan 9 found no correlation between clinically identified active MTrPs and DMS findings. Of 11 subjects with MTrPs of the upper trapezius, only one hyperechoic area (5 mm in diameter) was discussed, and it related to the superficial fascia of the superficial erector spinae (longissimus/iliocosalis). That pilot study did not mention how many MTrPs were observed in total.
The most recent studies have used elastography in conjunction with two-dimensional (2D) DMS. Studies published by Sikdar et al10,11 evaluated the upper limb (upper trapezius) and described MTrPs identified with 2-D DMS as inhomogeneous hypoechoic areas. With these criteria, the nodule visualized more than 15 years earlier in Lewis and Tehan’s study 9 was likely not a trigger point, since the nodule was seen as hyperechoic. It is unclear from the Sikdar et al studies, however, how identifiable the MTrPs are without the use of elastography, which continues to evolve with its clinical application.
Most of the imaging research literature deals with MTrPs in the upper extremity or back. While it is most common in the upper extremity, little has been done to investigate the lower extremity through diagnostic imaging. Therefore, the objective of this current study was to uncover DMS’s potential for imaging ankle/foot MTrPs. To provide a balanced approach to the evaluation of MTrPs, a qualitative research paradigm was also used to capture and analyze compatible data. A complete understanding of pain is not limited to just physical findings; therefore, added dimensions of the patient’s complaint should be explored. It is important to underscore that physical findings may account for only a portion of musculoskeletal pain.
Methods
Prior to initiation, the study was approved by the local institutional review board. The study recruited 25 patients via a flyer on a large college campus. All recruited patients gave appropriate informed consent and received a $25 grocery gift card for participation. A single sonographer with 5 years of clinical experience evaluated every participant by means of a bilateral sonographic investigation. The sonographer did all of the image analysis and was blinded to the participants’ status of being symptomatic or asymptomatic until the analysis was completed. A licensed massage therapist and educator was consulted for MTrP palpation methods and study feedback; once trained, this same sonographer did trigger point palpation following each sonographic examination. The sonographer has over 12 years of traditional martial arts experience, which provided an intimate understanding of MTrP palpation. The study accepted patients 18 to 65 of age, with the exclusion criteria of diabetes, past ankle surgery, or current pregnancy to control for confounding factors. The study was completed with 17 symptomatic and 8 asymptomatic patients.
Survey Instrumentation
All participants were first asked to complete a questionnaire consisting of four written sections. The first section consisted of the widely used SF-12 Health Survey (modified) to access their overall health, both mentally and physically. 12 The second section was the Victorian Institute of Sports Assessment–Achilles questionnaire (VISA-A), which is a valid and reliable tool for measuring Achilles tendinopathy. The scores are summated for a total of 100, where a score of 100 equates to an asymptomatic person. While the VISA-A is an index of severity of Achilles tendinopathy, other conditions that influence the lower limb function, such as an ankle sprain, will reduce a person’s VISA-A score. 13 The third section included a visual analog scale (VAS) for participants to rate their ankle/foot pain for each limb on a scale of 0 to 10 (Figure 1). 14 The fourth section included demographic questions, along with occupation and sports activity inquiries. In addition to the written data collected, another researcher not blinded to the participant’s symptomology obtained a thorough history about the person’s ankle/foot pain, inclusive of current and past injuries.
The visual analog scale (VAS) of pain.
Scanning Protocol
After written and oral data were collected, a scanning protocol based on the American Institute of Ultrasound in Medicine (AIUM) protocol for ankle/foot DMS was used to evaluate the bilateral ankle/foot structures. 15 A 5- to 12-MHz linear probe was used with a handheld GE Logic i (GE Healthcare Ultrasound, Milwaukee, WI) portable sonography machine; this equipment receives bimonthly quality control testing with both a 2D and Doppler flow phantom. Each evaluation was done using musculoskeletal setting defaults with spatial compounding (Cross X Beam) technology and harmonics, upshifted to the highest frequency. A standoff pad was used for the DMS portion but was not used for evaluating MTrPs, due to better visualization of the muscle at various depths.
A bilateral manual assessment of 10 MTrP areas was performed. The assessment complied with the latest criteria, relevant to a blinded study, for the identification of MTrPs (including both a palpable nodule, within a taut band of muscle and pain upon palpation) in conjunction with well-known lower extremity MTrP locations (Figure 2). 4 It should be noted the delineation between active and latent MTrPs could not be explicitly determined, as the main difference between these is the presence of persistent pain to which the sonographer was blinded and could not ascertain. Ten areas were diagnostically evaluated: the quadratus plantae, soleus I, tibialis anterior and posterior, extensor digitorum longus and hallicus, and the peroneus longus, brevis, and tertius I and II.
Selected myofascial trigger points (MTrPs) of the ankle/foot used in the study. Five of the 10 MTrPs palpated in the study are marked with an “X,” including the tibialis anterior MTrP, extensor digitorum longus MTrP, extensor hallicus longus MTrP, tibialis posterior MTrP, and the soleus I MTrP. (Note: The referred pain locations are not depicted, only the trigger points.)
Palpation Technique
Both flat and pincer palpation techniques were used. The selected 10 MTrP areas correlated with the physical structures evaluated with sonography. A four-tier scoring system (Table 1) was devised to assess MTrPs and patient pain for the purposes of assigning numerical value to imaging findings. A score of 0 was used for no pain or palpation on examination. A score of 1 was given for when the patient reported pain upon palpation, but no taut nodule was palpated by the examiner. It was determined a score should be given, although no MTrP was identified, as a way to account for the patient’s pain and not simply dismiss it. No score was given when a possible MTrP was felt by the examiner and was not reported as pain by the participant. In this case, a latent or inactive MTrP may have been identified but does not relate to the dependent variable of the participant’s self-reported pain. A score of 2 was used when both the participant reported pain and the examiner felt an MTrP. Finally, a score of 3 was given when the criteria for a score of 2 were met and a sonographic nodule could be identified in two planes, both longitudinal and orthogonal to the muscle fiber. An area tracing was obtained from the longitudinal plane (Figure 3).
Trigger Point Scoring System.
Longitudinal two-dimensional gray-scale sonographic image of a visualized myofascial trigger point, showing the tracing used to determine the cross-sectional area.
Statistical Analysis
Linear mixed models were chosen to analyze the relationship between the dependent variable of the pain scores (SF-12, VISA-A, or VAS) and the independent variable, MTrP status (palpated, DMS, or both). The subject was included as a random effect in the linear mixed model to account for correlation between responses (i.e., right limb and left limb). Comparisons of trigger status for each limb were tested as contrasts within the mixed model. Diagnostic measures such as residual plots were proposed to assess model assumptions. In addition, linear regression was used to model the relationship between SF-12 scores (physical or mental) and combined MTrp total. Model assumptions were assessed by the plotting of residuals against predicted values. P values <.05 were chosen a priori and considered statistically significant. All analyses were conducted using SAS version 9.2 (SAS Institute, Cary, NC).
Results
Descriptive information on the 17 symptomatic and 8 asymptomatic patients detailed varying ages and sexes. The participant pool included a total of 11 men and 14 women, aged 18 to 63 years, with a mean age of 33.6 years and median age of 28 years. There were a total of 14 painful self-reported right limbs and 9 painful left limbs, with six individuals reporting bilateral pain.
Quantitative Analysis
Based on the 500 total MTrP areas palpated on both symptomatic and asymptomatic participants, the majority of the MTrP areas (441, 88.2%) had a score of 0 according to the above devised MTrP scoring system. There was no MTrP area in which the examiner felt a nodule and no pain was noted. A total of 49 MTrP locations met one of the criteria for the devised scoring system; 16 were scored as 1, 15 were scored as 2, and 18 met the strict criteria to be scored as 3. Based on current MTrP diagnostic criteria and meeting the criteria for a score of 2 or 3, 18 of 33 (55%) were identified with 2D DMS, fitting with a score of 3. The area of the visualized MTrPs ranged from 0.05 to 0.21 cm2, with an average of 0.09 cm2. The MTrPs appeared as inhomogeneous hypoechoic areas by 2D gray-scale DMS (Figures 3–5).
Longitudinal two-dimensional gray-scale sonographic image of the peroneus tertius II myofascial trigger point showing its inhomogeneous, hypoechoic character.
Transverse two-dimensional gray-scale sonographic image of the peroneus tertius II myofascial trigger point showing its inhomogeneous, hypoechoic character.
Limb Statistical Analysis
Statistical analysis was conducted to evaluate trigger status and reported pain by limb, as taken from the participant history. The right-side trigger status for each scoring algorithm, which included total MTrPs (score 1, 2, or 3), palpable MTrPs (score of 2 or 3), and DMS MTrPs (score of 3 only), was statistically significant for those with a history of limb pain, while the all left-side trigger statuses were not significant (Fisher exact test, P = 1.00 for each status on the left side) compared with the participant’s reported history of ankle/foot pain (Tables 2–4). For example, reviewing the palpable MTrPs, 10 of 14 (71%) of those who reported a history of pain in their right limb also had positive triggers. This is in comparison to only 2 of 11 (18%) of those who reported having no history of pain with positive MTrPs. The left limb, however, did not demonstrate a significant difference, and palpable MTrPs were 33% for pain and 31% for no pain in terms of positive assessment. It should be noted there were more painful right limbs (total of 14) than left (total of 9).
Right Limb History of Pain and Right Trigger (Any).
Fisher exact test: P = .02.
Right Limb History of Pain and Right Trigger (Palpable).
Fisher exact test: P = .02.
Right Limb History of Pain and Right Trigger (Ultrasound).
Fisher exact test: P = .04.
The participants’ palpable and DMS MTrP statuses relative to the VISA-A questionnaire results were analyzed. For the right limb, a total of 13 negative palpable trigger points yielded a mean (SD) VISA-A score of 85.4 (14.20), in comparison to a total of 12 positive palpable MTrPs with a mean (SD) score of 65.8 (15.70). The left limb had a total of 17 negative palpable MTrPs with a mean (SD) VISA-A score of 82.00 (13.73), compared with a total of 8 positive MTrPs with a mean (SD) score of 73.00 (20.86) on the VISA-A. The DMS MTrPs yielded similar results with participants with right negative DMS MTrPs (n = 17), with a mean (SD) of 82.00 (17.33) on the VISA-A, versus those with positive DMS MTrPs (n = 8), with a mean (SD) VISA-A score of 63.25 (10.69). For those with negative left limb DMS MTrPs (n = 20), the mean (SD) VISA-A score was 81.85 (13.13), compared with those with positive DMS MTrPs (n = 5), with a mean (SD) score of 68.20 (24.90).
The palpated trigger overall, not specific to limb, does show there is a statistically significant difference in VISA-A for positive and negative triggers (P < .02). There is not a statistically significant interaction between trigger and limb (P < .64), meaning the effect of the trigger status does not depend on the limb. The mean adjusted difference in VISA-A between negative and positive palpable triggers is 10.2311 (95% confidence interval [CI], 2.02–18.45). To avoid skewing the distribution of the data, the adjusted difference accounts for the fact that the VISA-A scores associated with each trigger status come from a different limb. The raw score difference for the overall VISA-A scores between the negative and positive palpable MTrPs is approximately 15 (83.47 – 68.70 = 14.77), indicating the negative trigger status had a higher mean VISA-A score than did the positive trigger status.
Similarly, while there is not a statistically significant interaction between DMS MTrPs and limb (P < .98), the DMS trigger overall, while not statistically significant (since the 95% CI contains zero), does show a difference in VISA-A for positive and negative triggers (P < .07), with the trend showing negative DMS triggers having higher mean VISA-A scores than DMS positive triggers, similar to the findings for palpable triggers.
The palpable and DMS MTrPs were also analyzed in comparison to the VAS. The VAS rates pain on a scale from 0 to 10, with 0 meaning the person experiences no pain at all and 10 representing the highest level of pain. On the right limb, those with negative palpable MTrPs (n = 13) had a mean (SD) VAS score of 1.38 (2.84), compared with those with positive palpable MTrPs (n = 12) who had a mean (SD) VAS score of 4.08 (2.50). Participants with negative left limb palpable MTrPs (n = 17) had a mean (SD) VAS score of 1.71 (2.76), while the participants with positive left limb MTrPs (n = 8) had a mean (SD) VAS score of 0.88 (1.46). For those with negative right limb DMS MTrPs (n = 17), the mean (SD) VAS score was 1.82 (2.90), compared with participants with positive DMS MTrPs (n = 8) who had a mean (SD) VAS score of 4.50 (2.33). Finally, those with negative DMS MTrPs on the left limb (n = 20) had a mean (SD) VAS score of 1.65 (2.64), compared with those with a positive DMS MTrPs (n = 5) who had a mean (SD) score of 0.60 (0.89).
There was a statistically significant interaction between palpable MTrP and limb (P < .03), as well as DMS MTrP and limb (P < .02), which indicates that the trigger status does depend on the limb. For the left limb, the mean adjusted difference between negative and positive palpable triggers was 0.2 (95% CI, –1.7 to 2.2; P = .8111). No statistical significance was reported for DMS MTrPs on the left limb as well. For the right limb, the mean adjusted difference between negative and positive palpable triggers was −2.6 (95% CI, −4.5 to −0.78), and for DMS MTrPs, the mean adjusted difference was −2.5 (95% CI, −4.5 to −0.51). This corresponds to the VAS being significantly lower for negative triggers versus positive palpable and DMS MTrPs in the right limb.
Additional Statistical Analysis
The overall SF-12 mental health and physical health scores were above average for these participants in general, and no differences were seen between the symptomatic and asymptomatic groups. The national mean, based on a scale of 0 (poorest health) to 100 (highest health), is 50.0. The mean (SD) of the SF-12 physical component score of all participants was 50.42 (7.08), with a median of 52.30 (Figure 6). The mean (SD) SF-12 mental component score was 56.34 (5.04), with a median of 57.89 (Figure 7).
Graph of the SF-12 physical component score versus combined myofascial trigger point (MTrP) total showing the negative correlation (as the combined MTrP increases, the SF-12 physical score trends to decrease).
Graph of the SF-12 mental component score versus combined myofascial trigger point (MTrP) total showing the positive correlation (as the combined MTrP increases, the SF-12 Mental Health score trends to increase).
There was evidence (P < .02) of a significant linear relationship between the combined MTrp total and SF-12 physical score (Figure 6). The negative linear relationship shows that as overall MTrP increases, the mean SF-12 physical score decreases. When the MTrP total increases by one unit, the mean SF-12 physical score decreases between 0.15 and 1.4 points (95% CI). In this model, a relatively weak correlation was found, with only 22% (r2 = .22 by linear regression) of the variation in the physical score being explained by the combined MTrP total. However, while the correlation is not strong for predictive purposes, this relationship does account for some of the variation and may be helpful in a clinical setting. There is also evidence (P < .02) of a significant positive linear relationship between combined MTrP total and SF-12 mental health score; when the MTrP total increases by one unit, the mean SF-12 mental health score increases between 0.12 and 1.00 points (95% CI). Again, while a relatively weak correlation is shown and thus would not be used for predictive purposes, it does clinically indicate that 23% (r2 = .23) of the variation in the mental health score can be explained by the regression line using the combined MTrP total. To summarize, as the total trigger score increases, so does the mental health score, but the physical score decreases.
Qualitative Analysis
For the eight asymptomatic participants, only four (4/49 total) painful MTrP areas were palpated in three people. Participant 6, who had a score of 1, reported pain on the right tibialis anterior MTrP. This same participant had a previous hairline fracture of her right tibia near the ankle. Participant 9 scored a 2 bilaterally, with palpation of both extensor digitorum (anterior) MTrPs without visualization with DMS. On the history intake form, it was recorded that the participant had pain bilaterally six months ago from bad shoes while walking on pavement but was not experiencing pain currently since wearing new shoes. Finally, participant 18 had an MTrP that was identified with DMS and a score of 3 on the left tibialis posterior. While this person did not have any pain, a scar near the tibia midway down the lower leg was noted by the sonographer. Patient history did not reveal how this scar was obtained, and the person stated no prior surgery or trauma to either ankle or foot. DMS revealed highly abnormal left bone erosion at the peroneus longus insertion.
Of the remaining five true asymptomatic participants, participants 12, 13, 23, and 24 reported no previous injuries, pain, or surgery of either foot. Participant 19 reported no trauma or surgeries, although a chronic tendinosis was reported about a year ago on the right until undergoing massage and physical therapy, which resolved the pain.
A total of five limbs were given an MTrP score of 2 or 3 in an asymptomatic limb (5/17 or 29%); three were given a score of 2 or 3 on the opposite limb in the same area. One MTrP did not match any other side, and another positive MTrP score of 3 (participant 5) of the asymptomatic right limb had a positive MTrP on the right and left in the same tibialis anterior MTrP area on both limbs.
Of the total 17 symptomatic participants with self-reported ankle/foot pain, 14 had a score of 1, 2, or 3 (14/17 or 82%) with palpation of the MTrP areas. In nine of the participants and 10 limbs, a score of 3 correlated with pain in the same limb (10/17 or 59%). In six symptomatic limbs (6/17 or 35%), a score of 2 was given. Only one had a score of 1 in which a nodule could not be located manually. This participant (16) reported painful palpation of the right tertius MTrPs (Figures 4 and 5) (lateral) and complained of twisting his lateral right ankle five years ago. DMS demonstrated a bony abnormality of the right tibiofibular ligament, which sits just superior to this MTrP.
There were three symptomatic participants without any reported pain on MTrP palpation. The first was participant 4, a 27-year-old who was physically active and had point tenderness of the left metatarsal and complained mostly of bilateral toe pain. Some pain relief was attributed to purchasing new shoes. Participant 11 had extensive abnormalities shown by DMS of the left lateral ankle. This participant had rolled the left ankle in the distant past and had resprained this same ankle only 12 days prior to examination. Finally, the third participant without active MTrPs (participant 14) was a football player with bilateral pain and swelling. The participant reported a history of a left sprained ankle about 10 years ago and a stress fracture in the right foot over 4 years ago. DMS revealed right and left bony abnormalities with venous clusters on the left, as well as right edema on the anterior sweep.
Of the remaining 13 participant MTrPs with active score of 2 or 3 palpated (13/17 or 76%), there were 17 total symptomatic limbs. Combined, a 2 or 3 MTrP score was given correctly with the same limb 76% of the time (13/17). Of the remaining four who were symptomatic and had no MTrP score, one was a 24-year-old dancer (participant 7); this patient’s history documented left knee surgery in 2010 for a torn meniscus and a gait change causing pain in the left great toe. DMS showed a decrease in echogenicity of the left extensor hallux. Another participant had a negative MTrP on the right symptomatic lateral limb but had a positive MTrP on the left lateral limb. This 24-year-old participant, who had been very active prior to having an injury, had sharp pain on the right lateral ankle six years ago and has complained of right lateral ankle instability since; however, DMS demonstrated subluxation of his left lateral peroneal tendons. DMS also showed increased vascularity and decreased echogenicity of the right talofibular tendon. The participant reported no pain on the left. Furthermore, this participant had a negative radiograph; an MRI shortly after the injury, which showed fluid buildup and a possible chip on the right lateral side; and no lasting relief with cortisone shots, physical therapy, or shock therapy. Third, a 38-year-old active participant (participant 22) with bilateral pain had chronic pain on the right for three to four years and had just sprained his ankle on the left two weeks prior to scanning. An MTrP score of 3 was detected on the right and none on the left. DMS revealed abnormalities on both sides, right greater than left. The last of these had a missed MTrP on the bottom of his foot and is discussed below.
The quadratus plantae MTrP on the bottom of the foot could be visualized with DMS in four of five (participants 15, 17, 20, 21, 22) participants with pain in this area. The one participant (participant 17) in whom no MTrP within the quadratus plantae could be palpated had reported “subtle heel pain.” This same older participant gave a description of having general foot pain in both feet as well. By DMS, there was extensive swelling and venous channels noted bilaterally, as well as bony abnormalities noted in both feet. In the remaining four patients with heel pain, one person had bilateral pain, and DMS detected five MTrPs. All of these complaints of heel pain were chronic, and three had undergone previous treatments, such as cortisone injections, massage, and physical therapy.
Discussion
Enabled by a qualitative portion of the methodology, this blinded study is the first of its kind to compare trigger point palpation in the lower extremity, in conjunction with DMS, with self-reported patient ankle/foot pain. While only 55% of the MTrPs palpated could be confirmed with DMS, this is a vast improvement from Lewis and Tehan’s study in 1999, 9 which used 2D DMS and failed to identify a single MTrP. Perhaps even more significant, the present study used portable sonography equipment, whereas the Lewis and Tehan study used a large, now outdated ATL UltraMark9 machine (ATL/Philips Ultrasound, Bothel, WA). This increase in visualized MTrPs is most likely attributed to the advances of sonography imaging technology and increased transducer frequencies, which greatly increases spatial resolution. The older study used a transducer frequency of only 5 MHz, whereas the current study used a broad bandwidth 12-MHz transducer. Similar to the studies by Sikdar et al10,11 and by Thomas and Shankar, 8 the current study found that MTrPs evaluated with 2D DMS appeared as inhomogeneous elliptical hypoechoic structures.
While this study demonstrates 2D DMS has improved in its detection of MTrPs, additional imaging choices can increase visualization and documentation. The portable ultrasound machine used in this study is limited compared with the most cutting-edge DMS available on the market, although it does represent current technology in clinical use with handheld equipment. While 17- to 18-MHz transducers have become increasingly available, there are now newly Food and Drug Administration (FDA)–approved portable ultrasound machines with transducer frequencies that shift up to 20 MHz. It is highly likely these newer higher frequencies would be more sensitive at detecting MTrPs with 2D DMS. In addition, the use of color and spectral Doppler can evaluate blood flow in and around MTrPs. While available, these technologies were not used in the screening protocol because of time constraints. The evaluation of the blood flow with Doppler, however, in several studies has yielded differing results.
While 2D DMS has greatly improved, many other DMS advances were unavailable for this study. Sikdar et al10,11 and Thomas and Shankar 8 have demonstrated that elastography can evaluate tissue density and allow for better visualization of MTrPs in general. The study by Sikdar et al 11 in 2009 study detected MTrPs at all 33 sites in the upper trapezius in nine participants. While stating they were able to identify all MTrPs with both 2D and three-dimensional (3D) DMS in B-mode, it is unclear how much elastography helped in identifying the focal areas. From the images, elastography surely may have helped visualize the nodule, as it can delineate tissue density much better than traditional 2D echoes. The investigators did mention that quantification of the density of MTrPs was not possible with elastography, although it increased visualization. 11 Clinically, elastography is in its infancy, often limited by its lack of reproducibility because of the use of freehand pressure and operator-dependent variations. 16 A recent publication by Klauser et al 17 advocates the use of freehand elastography for the evaluation of the Achilles tendon. Future studies would likely benefit from using acoustic radiation force impulse (ARFI) elastography imaging, which can actually quantify tissue stiffness by sending a vertical pulse signal into the tissue and record density as reflected by shear wave velocity (m/s), unlike classic elastography, which can only record the received horizontal reflected signal, expressed in Pascal corresponding to a color code. By quantifying the actual stiffness of these MTrPs, it may help in determining more about their pathophysiology. Recent FDA approval of ARFI will likely propel this technology, and its clinical utility will increase. Already, large ultrasound manufacturers are equipping their machines with this new technology. 18
Another limitation of this study was the inability to distinguish latent from active MTrPs since the researcher was blinded to the participant’s symptoms. It is important to discuss the two types of MTrPs. Active MTrPs (A-MTrPs) are described as always causing a persistent pain response, usually in a referred pain pattern, whereas latent (L-MTrPs) ones do not produce this same constant pain. However, L-MTrPs, when detected and pressed, may produce a similar focal tenderness. Often, latent MTrPs are associated with older age, stiffness, and restricted motion. It is important to note that healthy muscle tissue does not contain active or latent MTrPs, and both may cause dysfunction. 4 Clearly, a thorough patient history and evaluation of pain symptoms is critical to differentiate active from latent MTrPs, and this would be an advantage for clinicians.
Sikdar et al 11 looked at both active and latent MTrPs and found no significant difference in their size, with a mean (SD) size of 0.16 (0.11) cm2 for all MTrPs, and this was relatively close to the size of the MTrPs found in this study, in which the mean size was 0.09 cm2, with a range of 0.05 to 0.21 cm2. Furthermore, they often found MTrPs to exist with multiple nodules in close proximity. Differences in the muscles being examined may account for the smaller sizes and the fact that most of the MTrPs in this study were found alone.
A disadvantage of this study was that there was only one researcher to do the evaluation and analysis. However, the detection of MTrPs in the field has poor interrater reliability, and therefore a screening scenario was chosen and may be a strength given the results. Another limitation of this study may also be the experience of the researcher at detecting MTrPs in the ankle/foot, as the researcher had much more experience in detecting MTrPs in the upper extremity. However, a massage therapist with a specific focus on MTrPs was consulted and found both active and latent MTrPs in the same locations on an outside participant as the researcher.
When evaluating the analysis by limb, most statistical significance was reported or driven (in the case when the two were combined together) by the right limb. The participant’s reported history of ankle/foot pain may be due to more painful right limbs (total of 14) than left (total of 9). Although a matched-controlled study was originally planned for this study, it was not possible, and certainly future research should employ a match-controlled study for this reason. Also, the examination was done in conjunction with standard DMS positioning with the patient on the examiner’s right side. It is hypothetically possible that MTrP palpation was limited on the left due to this reason, and future studies with DMS may evaluate MTrPs on both sides prior to scanning. Finally, since the sample size and power analysis is limited in this pilot study, more participants would be needed to see if this same right versus left lower limb discrepancy persists.
Based on these preliminary data, it may be worthwhile to have patients fill out the VISA-A form prior to evaluating for MTrPs. The VAS may not be as sensitive for MTrPs, however. In this study, the VAS data are a bit skewed, as seen by the standard deviations being larger than the means reported. Therefore, as a sensitivity analysis, this was looked at using a square root transformation on VAS to help improve the normality, but it did not greatly help. The linear mixed model for the VAS square root transformation also had similar conclusions (significant interaction as well as the same trends for right and left limbs).
The SF-12 physical score may be helpful in evaluating patients with suspected lower limb MTrPs. While the negative linear relationship shows that about 22% of the variation in the physical score can be explained by the combined MTrP total, this low correlation is not necessarily a poor result. Since it is not used for predictive purposes, there are still clinically meaningful relationships. The positive linear relationship observed between the combined MTrp total and SF-12 mental score is counterintuitive, since a higher score on the SF-12 equates to better mental health. In this limited population, participants reported robust mental health regardless of a higher combined MTrP score. Again, the limitation of not being able to ascertain potential differences between A-MTrPs and L-MTrPs may have been influential.
Reviewing the individual participants and their MTrP status according to the descriptive results section, it is highly recommended in the future to evaluate a cohort of patients and their lower limb MTrP status longitudinally with DMS. This is based on the four asymptomatic participants with positive MTrPs who all had prior injuries related to the MTrPs palpated versus the four of five asymptomatic participants with no prior pain who had no palpable MtrPs. It would be interesting to see what separates those who go on to have presumably latent MTrPs versus those who have presumed active MTrPs postinjury to evaluate which demarcates injury type, length, and so forth.
Evaluation of the musculoskeletal structures by DMS may aid in better understanding the poorly understood pathophysiology of MTrPs in general. While the focus of this study was on MTrPs, other trigger points may exist in the skin, fascia, or tendon. Musculoskeletal DMS offers the added benefit of evaluating these trigger points as well.
Since several asymptomatic limbs were given an MTrP score opposite of a painful limb or injury in the same area, more thought should be given to development of MTrPs in opposing limbs since antagonist muscles may play a role. Finally, future similar studies using 2D DMS to detect MTrPs of the lower extremity may yield more results by evaluating those participants with heel pain, as the quadratus plantae MTrP was readily visualized in four of five such participants.
Conclusions
This mixed-method approach has led the authors to believe a complete assessment is necessary that incorporates both quantitative and qualitative factors that may influence ankle/foot pain. Given that the medical community struggles with the clinical options and efficacy of diagnosing and treating MTrPs, this study offers an integrated approach to investigating ankle/foot pain and its etiology. While producing some statistical significance within a limited cohort of a convenience sample, this study has also provided many unexplained results, demonstrating clear pitfalls with current MTrP imaging with DMS, and prompts future endeavors to evaluate MTrPs to be carried out in a more dedicated manner incorporating elastography. Certainly, by using advances in DMS, more objective MTrP criteria may be developed, and this would allow their identification and classification to be more reliable, sensitive, and specific. The potential advantages of having a low-cost, reliable way to visualize trigger points are many, including their treatment and evaluation over time. Qualitative analyses also must be brought to the forefront in addition to adhering to quantitative results with regard to ankle/foot pain. An integrative approach should guide not only research in this area but also the clinical evaluation of patients experiencing this kind of pain.
Footnotes
Declaration of Conflicting Interests
The authors declared no financial conflicts of interest with respect to the research, authorship, and/or publication.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The primary author received funding for research through The Ohio State University’s School of Health and Rehabilitation Sciences’ M. Rosita Schiller Fund, as well as working in a CDC/NIOSH-funded laboratory.