Association between elasticity of tissue and pain pressure threshold in the tender points present in subjects with fibromyalgia: a cross-sectional study

Fibromyalgia (FM) is a multicomponent illness and despite its worldwide prevalence, a complete understanding of its aetiology and pathogenesis remains unclear. The goal of the study is to analyze the level of association between elastic properties of tissue measured by strain elastography (SEL) and pain pressure threshold (PPT) in the characteristic painful points described in patients suffering from FM. This was a cross-sectional, observational study. A sample comprised of 42 subjects with FM was recruited from a private care centre. The occiput, low cervical, trapezius, supraspinatus, paraspinous, lateral pectoral, second rib, lateral epicondyle, medial epicondyle, gluteus, greater trochanter, knee, and anterior tibial PPTs were bilaterally assessed using a standard pressure algometer and elastic properties of tissue were evaluated by SEL. Linear regression analysis showed significant associations between SEL and dominant trapezius PPT (β = 0.487, 95% CI [0.045, 0.930], p = 0.032) after adjustments for the age, body mass index, and menopause status (higher SEL and higher pain sensitivity). No significant associations between SEL and the other PPTs variables were found in women diagnosed with FM. The PPT of the dominant trapezius is associated with SEL measurements in subjects suffering from FM. More studies are required to fully explain the underlying mechanisms.

Association between elasticity of tissue and pain pressure threshold in the tender points present in subjects with fibromyalgia: a cross-sectional study Santiago Navarro-Ledesma 1 , María Aguilar-García 1,2 , Ana González-Muñoz 1,3,4 , Antonio Casas-Barragán 5,6* & Rosa María Tapia-Haro 5,6 Fibromyalgia (FM) is a multicomponent illness and despite its worldwide prevalence, a complete understanding of its aetiology and pathogenesis remains unclear.The goal of the study is to analyze the level of association between elastic properties of tissue measured by strain elastography (SEL) and pain pressure threshold (PPT) in the characteristic painful points described in patients suffering from FM.This was a cross-sectional, observational study.A sample comprised of 42 subjects with FM was recruited from a private care centre.The occiput, low cervical, trapezius, supraspinatus, paraspinous, lateral pectoral, second rib, lateral epicondyle, medial epicondyle, gluteus, greater trochanter, knee, and anterior tibial PPTs were bilaterally assessed using a standard pressure algometer and elastic properties of tissue were evaluated by SEL.Linear regression analysis showed significant associations between SEL and dominant trapezius PPT (β = 0.487, 95% CI [0.045, 0.930], p = 0.032) after adjustments for the age, body mass index, and menopause status (higher SEL and higher pain sensitivity).No significant associations between SEL and the other PPTs variables were found in women diagnosed with FM.The PPT of the dominant trapezius is associated with SEL measurements in subjects suffering from FM.More studies are required to fully explain the underlying mechanisms.Exclusion criteria (i) Presenting any inflammatory, (ii) orthopedic, (iii) or neurological disease which may affect cognitive, balance, hearing, and vision impairment in terms of the ability to answer questions.

Recruitment procedures
A physiotherapist collaborating in the project was in contact with the participants at the time of recruitment and carrying out the treatment, as well as providing them the information about the study and eligibility criteria.
Those who fit in the study and remained interested in the participation signed a consent form.

Outcome measures
Pain pressure threshold (PPT) PPT were bilaterally assessed over the 12 initial TPs considered by the ACR for FM diagnosis 6 , together with the anterior tibial, other characteristic painful point presented in central sensitization conditions 25 , with a standard pressure algometer of 1 cm2 (FPK 20; Wagner Instruments, Greenwich, CT, USA).The TPs assessed were: (i) at the suboccipital muscle insertions, (ii) at the anterior aspects of the intertransverse spaces at low cervical C5-C7, (iii) at the midpoint of the trapezius (upper border), (iv) at the supraspinatus origins, above the scapula spine near the medial border, (v) paraspinous, at the level of the mid-scapula, 3 cm lateral to the midline, (vi) at the second costochondral junctions (second rib), (vii) at the level of the fourth rib at the anterior axillary line (lateral pectoral), (viii) 2 cm distal to the epicondyles, (ix) medial epicondyle, (x) at the distal dorsal third of the forearm, (xi) at the upper outer quadrants of buttocks in the anterior fold of gluteal muscle, (xii) greater trochanter just posterior to the trochanteric prominence and (xiii) at the medial knee fat pad, proximal, to the joint line.We have followed the measurement protocol previously described in literature 26 .We positioned the algometer perpendicular to the TP, to assess PPT, the pain threshold was set as the minimum pressure that caused pain.Positive evidence of a TP was considered to exist when participants reported "pain" at or below a pressure of 4 kg.The mean of three measurements at each TP was used for the analysis.The total count of positive TPs was recorded for each participant 6,26,27 .Pressure algometry is considered a valid and reliable method for the evaluation of pain sensitivity 28 .

SEL measurements
The Logiq S7 using a 15 MHz linear probe (GE Healthcare, Milwaukee, WI) was used to carry out all the measurements by an expert physiotherapist with eleven years of experience in ultrasound imaging.All participants were positioned in the same orientation used for the MTrPs) identification protocol 29 .All images were obtained with the transducer placed longitudinally with the muscle fibers and positioning the center of the probe over the TP of interest and the control point locations, following recommendations from reliability studies 30 .Then, the tissue was compressed, a software incorporated quality control evaluated the recommended compression size, being approximately 2-5 mm.The exact raw strain value (0-6; with 0 being softest and 6 being the hardest tissue) was calculated using A 5 mm circular region of the area of interest was used to calculate the exact raw strain value, as indicated by the manufacturer's instructions and following previous studies 26,27 .In order to minimize intra-observer variation, a mean of the three measured areas at each was calculated.Following manufacture's recommendations, only sequences with the highest image quality were used (Fig. 1).

Sample size
Sample size was determined using G * Power 3.1.9.7 software (Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany).Based on results of PPT through sonoelastograhpy in patients with nonspecific lumbopelvic pain of a previous cross-sectional study 31 , it is necessary to include a minimum of 18 subjects with FM to obtain a power of 80% and an alpha level (α) of 0.05.The sample size was increased to 48 participants after taking into account an expected loss rate of approximately 62.5%.The final sample of the study was 42 women with FM.

Statistical analysis
The statistical analysis was conducted by SPSS Statistics Version© 24 software for Windows (IBM Corporation, Armonk, NY, USA).The data for continuous and categorical variables were expressed as mean ± standard deviation (SD) and frequency (%), respectively.The normality and distribution of the variables was performed using the Kolmogorov-Smirnov test (α-value = 0.05).To test homoscedasticity and independence of the variables, we also performed the Levene test (α-value = 0.05) and Durbin-Watson test.We used a One Sample T-test with 95% confidence interval (95% CI; α-value = 0.05) to test the demographic and clinical data.Linear regression analysis was performed to check the associations among SEL and PPTs in FM women after adjustment for age, menopause status, body mass index (BMI).The results of the linear regression analysis were shown as estimate (β) with 95% CI and p-value.We fixed the level of statistical significance at p < 0.05.

Ethics approval and consent to participate
This study has received ethical approval by the Ethics Committee of Human Research of the University of Granada, Spain (approval number: 1044/CEIH/2020).All participants signed a consent form to participate.

Sociodemographic characteristics
Forty-two women with FM (mean age, 52.83 ± 8.04) were recruited and met the inclusion and exclusion criteria established in this study.The sociodemographic data of women with FM are summarised in Table 1.Finally, a flow diagram of study was included (Fig. 2).

Associations between strain elastography and pain pressure thresholds in women diagnosed with FM
Table 2 illustrates the associations among SEL and PPT variables in women with FM.Linear analysis regression only revealed that SEL was significantly associated with the dominant trapezius PPT (β = 0.487, 95% CI [0.045, 0.930], p = 0.032) after adjustment by age, menopause status, and BMI.There were no significant correlations between SEL and the other TPs in the women with FM.

Discussion
The aim of this study was to analyse the degree of association between the elasticity of the tissues and the level of the PPT in the characteristic TPs established for patients with FM.We hypothesized that SEL would show an increased stiffness measurement in TPs and this would be positively related to the level of the PPT.Our results showed a significant regressions association between SEL measurements and dominant trapezius PPT in patients with FM, but there were no significant regressions between SEL and the rest of the PPTs.Central sensitization is proposed as a primary key factor in the development of FM 7 .Given that the elastic properties of soft tissue are known to be influenced or possibly determined by the activity of the sympathetic nervous system, it is plausible that any dysfunction in the sympathetic nervous system could result in alterations in the tensegrity of the tissue.This, in turn, may significantly impact an individual's perception of pain 10,11 .It is our hypothesis that if a correlation can be established between PPT and SEL in individuals with FM, it opens up the possibility of using SEL and PPT in the assessment of subjects with FM, and this theory would warrant further investigation.The fact that significant results have been found in the dominant trapezius may indicate that the presented theory only occurs in that point from the 13 TP assessed.Therefore, presenting a painful and stiff dominant trapezius may www.nature.com/scientificreports/be a common characteristic and sign in those with FM, which could serve as a part of the explanatory assessment in such condition.However, this is a first research approach, and more studies are needed to corroborate our findings.The use of SEL as a non-invasive technique to measure the mechanical properties of tissue has been increasing in recent years 13,32,33 .Previous studies have described using SEL in different organs, such as liver fibrosis, breast, thyroid, prostate, kidney and lymph node or low back pain 13,34,35 .In this regard, SEL has been used to analyse the effectiveness of different interventions such as radiofrequency 27,36 ;radial extracorporeal shock wave therapy 37 , manual therapy 38 and conservative physical therapy 39,40 in diverse pathologies.Additionally, there is a previous study in which SEL was used to evaluate the effects of photobiomodulation on tissues in patients with FM 27 .SEL has also been used to perform a differential diagnosis between patients with psoriasis that suffers chronic widespread pain and patients with FM [41][42][43] .
To the best of our knowledge, this is the first study assessing the tissue elasticity of the TPs in female patients with FM and the relationship of these TPs to the SEL; therefore, a comparison with other studies is difficult.Muro-Culebras and Cuesta-Vargas evaluated and compared the stiffness of TPs in women with FM and in healthy subjects using sono-myography and sono-myoelastography.They observed hypoechoic areas in the ultrasound images of the upper trapezius in both groups but sono-myoelastographydid not reveal greater stiffness in these areas compared with the rest of the muscle 16 , which is in line with our results.Also, in this regard, a recent study by Karayol and Sibel analysed the level of association between the objectivity of visual analog scale (VAS) values and elasticity values of rhomboid major muscles using Shear Wave Elastography (SWE) in Table 1.Summary of sociodemographic data and pain pressure threshold of the women with Fibromyalgia.Data are expressed as mean ± SD for quantitative variables and as frequency (%) for qualitative variables.SD standard deviation, CI confidence interval, BMI body mass index, D dominant, ND no dominant.www.nature.com/scientificreports/FM and healthy subjects, determining that there was no significant association between VAS scores and SWE values 44 .In line with our results, which show a correlation between the PPT in the trapezius muscle and SEL, the evidence suggests a higher prevalence of MTrPs in the upper quarter muscles 29 , with the trapezius being the most affected muscle in MTrPS 40 .
Recent studies have described that SEL can provide an objective and reproducible measure of the changes in elasticity in MTrPs, showing that stiff nodules vibrate with less amplitude than healthy tissue 39,40,45 .In this context, similarities between TPs and MTsPs have been described, since both are related to pathologies that occurs with non-inflammatory myalgia; TPs in FM and MTrPs in MPS 16 , and are the result of the interaction between the autonomic nervous system and anon-specific response of the central nervous system 46 .In this line, the study by Ge et al. in 2010 support the similarities between TPs and MTrPs, since they found that TPs sites coincide with MTrPs in people with FM 47 .
Since TPs can be associated with a variety of diseases such as infections, inflammation, or lesions and MTrPs are common in patients with a variety of conditions ranging from cancer, neck and shoulder disorders to spinal and musculoskeletal disorders 14,17 , it would be very interesting to carry out future studies that compare TPs between patients with FM and healthy subjects or that analyse and compare the characteristics of TPs/MTrPs in other pathologies.
Tissue quality is understood based on the elastic properties of connective tissue, which means proper loading capacity as well as aging.The muscle elasticity measurement indicates the stiffness of the tissue assessed.The interpretation of a "good" muscle SEL quality assessment would be one showing soft elastic characteristics rather than hard elastic characteristics.This is because of vascularity mainly and sympathetic tone.However, SEL measurements are highly evaluator-dependent and results should be interpreted with caution.
The present study shows strengths that should be highlighted.Firstly, this is the first study that analyses the relationship between the elasticity of tissue and all the PPTs in patients with FM.Secondly, all SEL measurements were carried out by an expert who has more than 10 years of experience in ultrasound and SEL.Finally, this study opens new possibilities of research in relation with soft tissue elasticity and the perception of pain.

Study limitations
The present study also has some limitations that should be recognized.Sample size is small, with FM participants only, and the design of the study is observational and cross-sectional, which means our results should be interpreted with caution.Besides, SEL measurements, although widely accepted and used, are highly dependent on www.nature.com/scientificreports/ the operator, which must be taken into account when interpreting our results 48,49 .Future studies should utilise a larger sample size and include a control group in order to corroborate our findings and extrapolate the data.Furthermore, future studies comparing the results of SEL and PPT in TPs from patients with FM and a healthy population, as well as changes in SEL of the PPTs after a treatment program, are needed.

Conclusions
In the light of the results, there was an association between SEL measurements and dominant trapezius PPT in subjects with FM, but there was no significant association between SEL and the rest of the PPT points.Further studies to corroborate our findings and to fully explain the underlying mechanism are needed.

Figure 1 .
Figure 1.Quantitative strain elastography (SEL) assessment in the upper trapezius muscle in a woman with Fibromyalgia.Different circles represent assessments of different regions of interest (ROI) from superficial to deep tissues.
Aged between 34 and 64 years; ii) FM diagnosis following the ACR classification 2 .

Table 2 .
Correlations between strain elastography (SEL) and pressure pain thresholds (PPTs) in women with Fibromyalgia.Beta (β) represents the regression coefficient.Adjusted for age, menopause status and body mass index.SEL strain elastography, SD standard deviation, CI confidence interval, D dominant, ND no dominant.*Significance level: p < 0.05.