Pressure-induced muscle pain and tissue biomechanics: A computational and experimental study

Corrugator Muscle Activity Associated with Pressure Pain in Adults with Neck/Shoulder Pain

Background and Objectives: No studies have reported corrugator muscle activity associated with pain in people with pain. This study aimed to develop an objective pain assessment method using corrugator muscle activity with pressure pain stimulation to the skeletal muscle. Methods: Participants were 20 adults (a mean ± SD age of 22.0 ± 3.1 years) with chronic neck/shoulder pain. Surface electromyography (sEMG) of corrugator muscle activity at rest (baseline) and without and with pressure pain stimulation applied to the most painful tender point in the shoulder was recorded. Participants evaluated the intensity of the neck/shoulder pain and the sensory and affective components of pain with pressure stimulation using a visual analogue scale (VAS). The percentages of integrated sEMG (% corrugator activity) without and with pressure pain stimulation to the baseline integrated sEMG were compared, and the relationships between the % corrugator activity and the sensory and affective components of pain VAS scores were evaluated. Results: Without pressure stimulation, an increase in corrugator muscle activity due to chronic neck/shoulder pain was not observed. The % corrugator activity with pressure pain stimulation was significantly higher than that without stimulation (p < 0.01). A significant positive correlation between corrugator muscle activity and the affective components of pain VAS scores with pressure stimulation was found (ρ = 0.465, p = 0.039) and a tendency of positive correlation was found for the sensory component of pain VAS scores (ρ = 0.423, p = 0.063). Conclusions: The increase in corrugator muscle activity with pressure pain stimulation to the tender point in adults with chronic neck/shoulder pain was observed, although increased corrugator muscle activity resulting from the chronic neck/shoulder pain was not. These findings suggest that corrugator muscle activity with pressure pain stimulation can be a useful objective indication for tender point sensitivity assessment in the skeletal muscle with pain.

Inter-individual variability in mechanical pain sensation in patients with cervicogenic headache: an explorative study

Currently, evidence for effective physiotherapy interventions in patients with cervicogenic headache (CeH) is inconsistent. Although inter-individual variability in pain response is predictive for successful physiotherapy interventions, it was never explored in patients with CeH. Therefore the objective of the current study was to explore inter-individual variability in mechanical pain sensation, and its association with biopsychosocial-lifestyle (BPSL) characteristics in patients with CeH. A cross-sectional explorative analysis of inter-individual variability in mechanical pain sensation in 18 participants with CeH (29-51 years) was conducted. Inter-individual variability in mechanical pain sensation (standard deviations (SDs), F-statistics, Measurement System Analysis) was deducted from bilateral pressure pain thresholds of the suboccipitals, erector spine, tibialis anterior. BPSL-characteristics depression, anxiety, stress (Depression Anxiety Stress Scale-21), quality of life (Headache Impact Test-6), sleep-quality (Pittsburgh Sleep Quality Index), and sedentary time (hours/week) were questioned. Inter-individual variability in mechanical pain sensation explained 69.2% (suboccipital left), 86.8% (suboccipital right), 94.6% (erector spine left), 93.2% (erector spine right), 91.7% (tibialis anterior left), and 82% (tibialis anterior right) of the total variability in patients with CeH. The significant p-values and large F-statistic values indicate inter-individual differences in SDs. Significant associations between (1) lower quality of life and lower SDs of the suboccipital left PPT (p .005), and (2) longer sedentary time and higher SDs of the suboccipital left PPT (p .001) were observed. Results from our explorative study could suggest inter-individual variability in mechanical pain sensation at the left suboccipitals which associates with quality of life and sedentary time. These novel findings should be considered when phenotyping patients and 'individually' match interventions.

Pressure pain threshold map of thoracolumbar paraspinal muscles after lengthening contractions in young male asymptomatic volunteers

This study aimed to characterise topographic distribution of pressure pain thresholds (PPTs) of thoracolumbar paraspinal muscles and its change after lengthening contractions (LCs) of the back muscles. Using young male asymptomatic participants in Experiment 1, we systematically examined the distribution of PPTs bilaterally in the range of Th1–L5 at measurement points 2 and 4 cm from the midline. PPTs were found to be higher in the lumbar segments of the paraspinal muscles than in the thoracic segments, and in muscles closer to the vertebrae (2 vs. 4 cm from the midline). The PPTs did not differ between the left and right sides in each segment. In Experiment 2, LC was applied by asking a part of participants recruited in Experiment 1 to fall their trunk from a starting position (parallel to the floor) to 40° flexed position, and then made it back as quickly as possible to the starting position. This cycle was repeated until participants could not keep contractions (30 times/set, 25.4 ± 10.6 sets). PPTs of the LC group decreased prominently in the lower thoracic and lumbar segments, and the decrease was more evident 24 h after LC compared to that 48 h after. In contrast, PPTs in the control group without LC remained unchanged. These results provided broad topographic images of PPTs in the thoracolumbar paraspinal muscles of young male participants with and without LC, and the obtained PPT maps could be a useful guide for better treatment of exercise-induced myofascial pain in the lower back.

Acute effects of dermal suctioning on back pain in racehorses: a pilot study

Back pain is a common clinical condition that leads to poor performance in racehorses. Therefore, horse owners would benefit from a suitable and effective treatment that results in the early recovery of their horses. Dermal suctioning significantly improves chronic lower back pain in humans. Thus, if a similar effect were to be found in racehorses, it could become a new treatment for back pain in horses. In this study, we examined the acute effects of dermal suctioning on back pain in racehorses. Twelve Thoroughbred racehorses with back pain underwent 10 min of dermal suctioning in the thoracolumbar region. The pain score, mechanical nociceptive threshold (MNT), heart rate variability (HRV), and plasma cortisol concentrations were measured. Results showed that pain scores were significantly improved immediately after dermal suctioning (P=0.028), while MNT, HRV, and plasma cortisol concentrations did not show significant changes (P>0.05). These results indicate that dermal suctioning immediately relieves pain but has a limited effect on the other three parameters.

A Statistical Model to Determine Biomechanical Limits for Physically Safe Interactions With Collaborative Robots

Collaborative robots (cobots) provide a wide range of opportunities to improve the ergonomics and efficiency of manual work stations. ISO/TS 15066 defines power and force limiting (PFL) as one of four safeguarding modes for these robots. PFL specifies biomechanical limits for hazardous impacts and pinching contacts that a cobot must not exceed to protect humans from serious injuries. Most of the limits in ISO/TS 15066 are preliminary, since they are based on unverified data from a literature survey. This article presents a human-subject study that provides new and experimentally verified limits for biomechanically safe interactions between humans and cobots. The new limits are specifically tailored to impact and pinching transferred through blunt and semi-sharp surfaces as they can occur in the event of human error or technical failures. Altogether 112 subjects participated in the study and were subjected to tests with emulated impact and pinching loads at 28 different body locations. During the experiments, the contact force was gradually increased until the load evoked a slightly painful feeling on the subject’s body location under test. The results confirm that the pain thresholds of males and females are different in specific body regions. Therefore, when defining biomechanical limits, the gender difference must be taken into account. A regression model was utilized to incorporate the gender effect as a covariate into a conventional statistical distribution model that can be used to calculate individual limits, precisely fitted to a specific percentile of a mixed group of male and female workers which interacting with cobots.

Taking the strain: An examination of upper back musculoskeletal tissue sensitivity in relation to upper back pain and breast size. A cross-sectional study

BACKGROUND

The physiological basis for upper back pain experienced by women with large breasts is unclear but could relate to sensitivity of musculoskeletal tissues strained from the postural adaptations to large breasts. The aim of this cross-sectional study was to examine if upper back pain and breast size were associated with greater localised sensitivity of upper back musculoskeletal tissues.

METHODS

119 healthy postmenopausal women (mean age 61 years) had their upper back pain (numerical rating scale), breast size (breast size score), and upper back tissue sensitivity (pressure pain thresholds (digital algometry, kPa)) assessed. The pressure pain thresholds of six skeletal sites (T2, T4, T6, T8, T10 and T12) and six muscular sites (pectoralis major, levator scapulae, sternocleidomastoid, and upper, middle, and lower trapezius muscles) were examined. Linear mixed models with random subject effects were used to evaluate differences in sensitivity at each anatomical site between participants grouped by upper back pain (nil-mild, moderate-severe) and breast size (small, large).

FINDINGS

For most sites, the differences in sensitivity between upper back pain groups were highly significant (P < 0.002) with significantly lower pressure pain thresholds (Mean difference (MD): 74.6 to 151.1 kPa) recorded for participants with moderate-severe upper back pain. There were no differences in sensitivity between breast size groups.

INTERPRETATION

Increased upper back musculoskeletal sensitivity is related to perceived upper back pain but not to breast size. It remains unclear if and how structural or mechanical factors related to breast size contribute to upper back pain in women with large breasts.

Effect of different postures of the scapular girdle and arm on the pressure pain threshold in the infraspinatus muscle

BACKGROUND

Mechanosensitivity changes and trigger points in the infraspinatus muscle are associated with several painful conditions of the upper limb. The aim of this study was to assess the effect of different postures of the upper quadrant on the pressure pain threshold (PPT) of the infraspinatus muscle.

METHODS

This was an observational, cross-sectional study. Fifty-four subjects with and without shoulder pain (Asymptomatic subjects = 27, mean age 26.9 ± 4.92 years, BMI 23.73 ± 3.87), (symptomatic subjects = 27, mean age 27.6 ± 3.68 years, BMI 24.35 ± 3.86) were evaluated with a pressure algometer on the infraspinatus muscle belly, in four different positions of the upper quadrant: rest position (P1), passive scapular retraction position (P2), cervical contralateral inclination position (P3), and suprascapular nerve provocation position (P4). The assessed side was randomly chosen and all measurements were taken in sitting position.

RESULTS

No differences were observed between groups. The within-group analysis showed differences for both factors: "Positions" (F = 69.91; p = 0.001) and the interaction "Positions^Group" (F = 3.36; p = 0.02). The pairwise post-hoc analysis showed differences for the retracted position (P2) compared to others P1 (p = 0.001), P3 (p = 0.001), and P4 (p = 0.001), with higher PPT results achieved on the retracted position. Differences between P4 vs. P1 (p = 0.03) were also observed, with higher values for P4.

CONCLUSION

Placing the scapular girdle in a passive scapular retraction position significantly reduces the pressure sensitivity at the infraspinatus muscle. Physiotherapists can take into account these results when assessing and treating patients with upper quadrant pain syndromes.

Fracture behaviour of human skin in deep needle insertion can be captured using validated cohesive zone finite-element method

Medical needles have shown an appreciable contribution to the development of novel medical devices and surgical technologies. A better understanding of needle-skin interactions can advance the design of medical needles, modern surgical robots, and haptic devices. This study employed finite element (FE) modelling to explore the effect of different mechanical and geometrical parameters on the needle's force-displacement relationship, the required force for the skin puncture, and generated mechanical stress around the cutting zone. To this end, we established a cohesive FE model, and identified its parameters by a three-stage parameter identification algorithm to closely replicate the experimental data of needle insertion into the human skin available in the literature. We showed that a bilinear cohesive model with initial stiffness of 5000 MPa/mm, failure traction of 2 MPa, and separation length of 1.6 mm can lead to a model that can closely replicate experimental results. The FE results indicated that while the coefficient of friction between the needle and skin substantially changes the needle reaction force, the insertion velocity does not have a noticeable effect on the reaction force. Regarding the geometrical parameters, needle cutting angle is the prominent factor in terms of stress fields generated in the skin tissue. However, the needle diameter is more influential on the needle reaction force. We also presented an energy study on the frictional dissipation, damage dissipation, and strain energy throughout the insertion process.

Exploring multidimensional characteristics in cervicogenic headache: Relations between pain processing, lifestyle, and psychosocial factors

OBJECTIVE

Although multidimensional interventions including physiotherapy, psychology, and education are generally recommended in managing headache, and to prevent chronification, such approach is lacking in cervicogenic headache (CeH). Therefore, exploring CeH within a biopsychosocial framework is deemed an essential first step.

METHODS

Non-randomized cross-sectional design to compare pain processing, lifestyle, and psychosocial characteristics between 18 participants with CeH (CeH group) (40.2 ± 10.9 years) and 18 matched controls (control group) (39.2 ± 13.1 years). Pain processing characteristics included degree of central sensitization (Central Sensitization Inventory), and (extra)-cephalic pressure pain thresholds (kPa/cm²/s). Lifestyle characteristics included sleep quality (Pittsburgh Sleep Quality Index), physical activity, screen time, and sedentary time (hours a week). Psychosocial characteristics included degree of depression, anxiety and stress (Depression Anxiety Stress Scale-21), and quality of life (Headache Impact Test-6).

RESULTS

Pain processing characteristics: More (p = .04) participants in the CeH group showed higher degrees of central sensitization compared to the control group. Lower (p < .05) (extra)-cephalic pressure pain thresholds were revealed in the CeH group compared to the control group for each muscle. Lifestyle and psychosocial characteristics: Compared to the control group, sleep quality and headache-related quality of life were worse (p < .0001) in the CeH group. Severe to extreme stress was experienced by more participants in the CeH group (p = .02). Further, significant relations between pain processing and (1) lifestyle characteristics and (2) psychosocial characteristics were seen in the CeH group.

CONCLUSION

Exploring multidimensional characteristics in CeH exposed relations between pain processing, lifestyle, and psychosocial characteristics. These novel findings fill a gap in the current scientific literature, and highlight the need for outcome research targeting lifestyle and psychosocial factors.

Spinal postural variability relates to biopsychosocial variables in patients with cervicogenic headache

Patients with cervicogenic headache (CeH) showed lower spinal postural variability (SPV). In a next step, the complex character of such SPV needs to be analysed. Therefore, variables influencing SPV need to be explored. A non-randomized repeated-measure design was applied to analyse relations between biopsychosocial variables and SPV within a CeH-group (n = 18), 29–51 years, and matched control-group (n = 18), 26–52 years. Spinal postural variability, expressed by standard deviations, was deducted from 3D-Vicon motion analysis of habitual spinal postures (degrees). Interactions between SPV and pain processing, lifestyle, psychosocial characteristics were analysed. Pain processing characteristics included symptoms of central sensitization (Central Sensitization Inventory), (extra)-cephalic pressure pain thresholds (kPa/cm²/s). Lifestyle characteristics included sleep quality (Pittsburgh Sleep Quality Index), physical activity, screen-time, sedentary-time (hours a week), position (cm) and inclination (degrees) of the laptop (= desk-setup). Psychosocial characteristics included degree of depression, anxiety and stress (Depression Anxiety Stress Scale-21), impact of headache on quality of life (Headache Impact Test-6). Spinal postural variability related significantly to intrinsic (stress, anxiety, extra-cephalic pressure pain thresholds, sleep-duration) and extrinsic (desk-setup, screen-time) variables in the CeH-group. In the control-group, SPV related significantly to extra-cephalic pressure pain thresholds. Spinal postural variability related to diverse variables in the CeH-group compared to the control-group. More research is needed into a possible causal relationship and its clinical implication.

Controlled manual loading of body tissues: towards the next generation of pressure algometer

Assessing the responses of body tissue subjected to mechanical load is a fundamental component of the clinical examination, psychophysical assessments and bioengineering research. The forces applied during such assessments are usually generated manually, via the hands of the tester, and aimed at discreet tissue sites. It is therefore desirable to objectively quantify and optimise the control of manually applied force. However, current laboratory-grade manual devices and commercial software packages, in particular pressure algometer systems, are generally inflexible and expensive. This paper introduces and discusses several principles that should be implemented as design goals within a flexible, generic software application, given currently available force measurement hardware. We also discuss pitfalls that clinicians and researchers might face when using current pressure algometer systems and provide examples of these. Finally, we present our implementation of a pressure algometer system that achieves these goals in an efficient and affordable way for researchers and clinicians. As part of this effort, we will be sharing our configurable software application via a software repository.

A comparison of pain responses, hemodynamic reactivity and fibre type composition between Bergström and microbiopsy skeletal muscle biopsies

This study tested the hypotheses that 1) skeletal muscle biopsies performed with the Bergström needle evoke larger perceptions of pain and greater hemodynamic reactivity compared to biopsies performed with the microbiopsy needle, and 2) both needles yield samples with similar fibre type compositions when samples are collected at similar skeletal muscle depths. Fourteen healthy (age: 21.6 ± 3.2 years; VO₂peak: 41.5 ± 5.8 mL/kg/min) males (n = 7) and females (n = 7) provided two resting skeletal muscle biopsies, one with each needle type, following a randomized crossover design. Participants completed the short-form McGill Pain Questionnaire and the Brief Pain Inventory before, during, and after the skeletal muscle biopsies. Hemodynamic reactivity was assessed by measuring heart rate (HR) and mean arterial pressure (MAP) at rest and during the biopsy procedures. Immunofluorescence analysis was used to assess fibre type composition in vastus lateralis samples. Compared to the microbiopsy needle, the Bergström needle elicited a larger perception of pain but similar hemodynamic reactivity during the biopsy. Both needles yielded skeletal muscle samples with similar fibre type composition and resulted in similar perceptions of pain and pain-related interference during the post-biopsy recovery period. Collectively, these findings suggest that studies should consider using the microbiopsy needle rather than the Bergström needle unless large amounts of muscle tissue or certain muscle fibre lengths are required. However, future work should determine whether our findings are generalizable to biopsies performed with different procedures and/or types of Bergström/microbiopsy needles.

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The first characterization of responses to two muscle biopsy techniques.

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Compared to the Bergström, the microbiopsy needle evokes smaller pain responses.

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Both needles resulted in similar hemodynamic reactivity and fibre type composition.

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If analysis permits, future work should consider using the microbiopsy needle.

Touch and Pain Sensations in Diadynamic Current (DD) and Transcutaneous Electrical Nerve Stimulation (TENS): A Randomized Study

The study investigated touch and pain sensations and the correlation between them in diadynamic current (DD) and transcutaneous electrical nerve stimulation (TENS), electrotherapies commonly applied in musculoskeletal disorders and occupational rehabilitation medicine. Forty healthy subjects were treated with either DD (n=20) or TENS (n=20). Each treatment consisted of three sessions with one-week interval. Touch sensation was determined with the JVP Domes esthesiometer, pain sensation with pressure pain threshold (PPT), and pressure pain tolerance threshold (PPTO) by an algometer. During each session the measurements were performed before the application of the procedure (T0), immediately after it (T1), and 30 minutes after the end of the procedure (T2). Both DD and TENS increased touch sensation (p<0.01) and did not significantly alter PPT and PPTO (p>0.05). No statistically significant differences in short-term effects, i.e., 3 weeks of the trial, were noted between DD and TENS in their influence on touch and pain sensations (p>0.05). There was a high significant correlation between touch and pain sensations in DD (r=0.86). TENS and DD caused similar analgesic effects. DD, which is shorter in the duration of the treatment, may comprise a realistic alternative to TENS in clinical practice of pain management.

The impact of anxiety on chronic musculoskeletal pain and the role of astrocyte activation

Supplemental Digital Content is Available in the Text.

Anxiety predicts onset of knee pain and drives greater osteoarthritis pain in humans. Our validated preclinical model identifies supraspinal astrocytosis as a potential mechanism.

Anxiety and depression are associated with increased pain responses in chronic pain states. The extent to which anxiety drives chronic pain, or vice versa, remains an important question that has implications for analgesic treatment strategies. Here, the effect of existing anxiety on future osteoarthritis (OA) pain was investigated, and potential mechanisms were studied in an animal model. Pressure pain detection thresholds, anxiety, and depression were assessed in people with (n = 130) or without (n = 100) painful knee OA. Separately, knee pain and anxiety scores were also measured twice over 12 months in 4730 individuals recruited from the general population. A preclinical investigation of a model of OA pain in normo-anxiety Sprague-Dawley (SD) and high-anxiety Wistar Kyoto (WKY) rats assessed underlying neurobiological mechanisms. Higher anxiety, independently from depression, was associated with significantly lower pressure pain detection thresholds at sites local to (P < 0.01) and distant from (P < 0.05) the painful knee in patients with OA. Separately, high anxiety scores predicted increased risk of knee pain onset in 3274 originally pain-free people over the 1-year period (odds ratio = 1.71; 95% confidence interval = 1.25-2.34, P < 0.00083). Similarly, WKY rats developed significantly lower ipsilateral and contralateral hind paw withdrawal thresholds in the monosodium iodoacetate model of OA pain, compared with SD rats (P = 0.0005). Linear regressions revealed that baseline anxiety-like behaviour was predictive of lowered paw withdrawal thresholds in WKY rats, mirroring the human data. This augmented pain phenotype was significantly associated with increased glial fibrillary acidic protein immunofluorescence in pain-associated brain regions, identifying supraspinal astrocyte activation as a significant mechanism underlying anxiety-augmented pain behaviour.

Preclinical Signs of a Temporomandibular Disorder in Female Patients With Episodic Cervicogenic Headache Versus Asymptomatic Controls: A Cross-Sectional Study

BACKGROUND

The International Classification of Headache Disorders, 3rd Edition, accepted "headache attributed to temporomandibular disorders" as a valid headache. The neurophysiologic interplay between cervical structures and the temporomandibular joint, however, might also suggest that a temporomandibular disorder could develop in patients with cervicogenic headache.

OBJECTIVE

To compare the presence of preclinical temporomandibular signs between female patients with episodic cervicogenic headache and a control group.

DESIGN

Case-controlled cross-sectional design.

SETTING

Institutional setting: Hasselt University.

PARTICIPANTS

Twenty-two women (mean age ± SD: 20.7 ± 2.5 years) with episodic cervicogenic headache, without temporomandibular signs and 22 matched (gender, age, level of education, occupation) asymptomatic controls (21 ± 2.3 years).

METHODS

(Un)assisted temporomandibular range of motion, pressure pain thresholds, painful palpations were examined and the level of perceived stress was measured.

MAIN OUTCOME MEASUREMENTS

Temporomandibular range of motion (mm), pressure pain thresholds (kPa/cm²), painful palpations (yes/no), and level of perceived stress (Perceived Stress Scale).

RESULTS

Maximal mouth-opening was significantly smaller in the headache group (P <.05; effect size [ES] -0.45). Palpation of the masseter resulted in significantly more positive pain responses in the headache group at the left (P = .009; ES ∞) and right (P = .002;ES 17.5) origin, left (P = .004; ES 14.54) and right (P = .03; ES 5.71) body and left (P < .001; ES 12) insertion. Significantly lower pressure pain thresholds on the left and right anterior (P = .03; ES -0.33 resp. P = .02; ES -0.35), central (P = .003; ES 1.02 resp. P = .02; ES 0.79) and right posterior (P = .03; ES 0.62) temporalis and right tibialis anterior (P = .03; ES -0.33) were measured in the headache group. The level of perceived stress was significantly higher (P = .02) in the headache group.

CONCLUSIONS

Patients with episodic cervicogenic headache present with signs of a preclinical temporomandibular disorder and sensitization. The smaller range of motion, lower pressure pain thresholds, and higher levels of stress accentuate the multidimensionality of the problem.

LEVEL OF EVIDENCE

IV.

Cuff Pressure Pain Detection Is Associated with Both Sex and Physical Activity Level in Nonathletic Healthy Subjects

Purpose

The aim of this study was to evaluate pressure pain sensitivity on leg and arm in 98 healthy persons (50 women) using cuff algometry. Furthermore, associations with sex and physical activity level were investigated.

Method

Normal physical activity level was defined as Godin Leisure-Time Exercise Questionnaire (GLTEQ) score ≤ 45 and high activity level as GLTEQ > 45. A pneumatic double-chamber cuff was placed around the arm or leg where a single chamber was inflated. The cuff inflation rate (1 kPa/s) was constant, and pain intensity was registered continuously on a 10 cm electronic visual analogue scale (VAS). The pain detection threshold (PDT) was defined as when the pressure was perceived as painful, and pain tolerance (PTT) was when the subject terminated the cuff inflation. For PTT, the corresponding VAS score was recorded (VAS-PTT). The protocol was repeated with two chambers inflated.

Result

Only single cuff results are given. For women compared with men, the PDT was lower when assessed in the arm ( P = 0.002), PTTs were lower in the arm and leg ( P < 0.001), and the VAS-PTT was higher in the arm and leg ( P < 0.033). Highly active participants compared with less active had higher PDT ( P = 0.027) in the leg. Women showed facilitated spatial summation ( P < 0.014) in the arm and leg and a steeper VAS slope (i.e., the slope of the VAS pressure curve between PDT and PPT) in the arm and leg ( P < 0.003).

Conclusion

This study indicates that reduced pressure pain sensitivity is associated both with male sex and physical activity level.

PK/PD modeling of flunixin meglumine in a kaolin-induced inflammation model in piglets

Flunixin is marketed in several countries for analgesia in adult swine but little is known about its efficacy in piglets. Thirty-two piglets (6-8 days old) were randomized to receive placebo saline (n = 11, group CONTROL) or flunixin meglumine intravenously at 2.2 (n = 11, group MEDIUM) or 4.4 (n = 10, group HIGH) mg/kg, 10 hr after subcutaneous injection of kaolin in the left metacarpal area. A hand-held algometer was used to determine each piglet's mechanical nociceptive threshold (MNT) from both front feet up to 50 hr after treatment (cut-off value of 24.5 newton). Serial venous blood samples were obtained to quantify flunixin in plasma using LC-MS/MS. A PKPD model describing the effect of flunixin on the mechanical nociceptive threshold was obtained based on an inhibitory indirect response model. A two-compartmental PK model was used. A significant effect of flunixin was observed for both doses compared to control group, with 4.4 mg/kg showing the most relevant (6-10 newton) and long-lasting effect (34 hr). The median IC50 was 6.78 and 2.63 mg/ml in groups MEDIUM and HIGH, respectively. The ED50 in this model was 6.6 mg/kg. Flunixin exhibited marked antinociceptive effect on kaolin-induced inflammatory hyperalgesia in piglets.

Tissue mechanics during temporal summation of sequentially cuff pressure-induced pain in healthy volunteers and patients with painful osteoarthritis

BACKGROUND

The phenomenon of temporal summation to repetitive pressure pain stimuli is an important central neural mechanism for pain intensity encoding. This study evaluated the time-dependent behaviour of mechanical characteristics of soft tissue during repeated cuff stimulation used for eliciting temporal summation of cuff pressure-evoked pain. Such information of tissue mechanics is important for the interpretation of the pain response evoked during sequential stimulations.

METHODS

Temporal summation was assessed in 16 subjects separated into two groups (healthy controls and severe knee osteoarthritis patients) using a visual analogue scale during 10 repetitive painful cuff stimuli (1-s duration, 1-s break) of the lower leg. The geometry of the lower leg was constructed based on magnetic resonance image (MRI) data. The loading boundary condition of the finite element model was defined according to the parabolic pattern of the interface pressure around the limb and the time-dependent profile of the cuff pressure during repetitive stimuli.

RESULTS

The pain intensity significantly increased with an increasing number of stimuli (p < 0.001), and facilitated temporal summation of pain was observed in patients compared with healthy controls (p < 0.001). The maximal deep tissue stress and strain during stimuli 1-4 varied 43% and 9%, respectively. No variation was observed for stimuli 5-10.

CONCLUSIONS

The study concludes that the temporal summation of pain response during sequential cuff pressure is not explicable by a specific time-dependent behaviour of stress and strain in the activated deep tissue and hence not due to changes in tissue biomechanics.

SIGNIFICANCE

The temporal summation of pain during sequential cuff stimulation is inexplicable by the time-dependent response of mechanical stress and strain in soft tissue.

A combination of experimental and finite element analyses of needle-tissue interaction to compute the stresses and deformations during injection at different angles

One of the main clinical applications of the needles is its practical usage in the femoral vein catheterization. Annually more than two million peoples in the United States are exposed to femoral vein catheterization. How to use the input needles into the femoral vein has a key role in the sense of pain in post-injection and possible injuries, such as tissue damage and bleeding. It has been shown that there might be a correlation between the stresses and deformations due to femoral injection to the tissue and the sense of pain and, consequently, injuries caused by needles. In this study, the stresses and deformations induced by the needle to the femoral tissue were experimentally and numerically investigated in response to an input needle at four different angles, i.e., 30°, 45°, 60°, and 90°, via finite element method. In addition, a set of experimental injections at different angles were carried out to compare the numerical results with that of the experimental ones, namely pain score. The results revealed that by increasing the angle of injection up to 60°, the strain at the interaction site of the needle-tissue is increased accordingly while a significant falling is observed at the angle of 90°. In contrast, the stress due to injection was decreased at the region of needle-tissue interaction with showing the lowest one at the angle of 90°. Experimental results were also well confirmed the numerical observations since the lowest pain score was seen at the angle of 90°. The results suggest that the most effective angle of injection would be 90° due to a lower amount of stresses and deformations compared to the other angles of injection. These findings may have implications not only for understating the stresses and deformations induced during injection around the needle-tissue interaction, but also to give an outlook to the doctors to implement the most suitable angle of injection in order to reduce the pain as well as post injury of the patients.

Interaction between ultraviolet B-induced cutaneous hyperalgesia and nerve growth factor-induced muscle hyperalgesia

BACKGROUNDS AND OBJECTIVES

Clinical observations indicate that cutaneous hyperalgesia may arise from pain located in deep structures. The objective of this study was to investigate whether combined sensitization of deep and superficial somatic tissues facilitates skin hyperalgesia.

METHODS

The interaction between muscle and cutaneous hyperalgesia was investigated in 16 healthy volunteers. Skin sensitization was induced unilaterally on the same randomly selected part of the body by ultraviolet B (UVB) irradiation above the upper trapezius and low back muscles. The next day, muscle hyperalgesia was induced bilaterally in low back muscles by injections of nerve growth factor (NGF). Thus, 1 day after irradiation there was skin sensitization, whereas after 2 days both skin and muscle sensitizations were present. Cutaneous blood flow, pin-prick thresholds, pressure pain thresholds (PPTs), temporal summation to repetitive painful pressure stimulation, and stimulus-response functions of graded pressure stimulations and pain intensity were assessed within the irradiated skin area and in the surrounding area before and 1, 2 and 3 days after irradiation.

RESULTS

Comparing baseline with 1 day after irradiation, UVB and UVB+NGF locations demonstrated: (1) Increased superficial blood flow inside the irradiated area (p < 0.01); (2) Reduced pin-prick (p < 0.01) and PPTs (p < 0.05) within the irradiated area and in the surrounding area; (3) Left-shifted pressure stimulus-response function within the irradiated area (p < 0.01); (4) Facilitated temporal summation inside the irradiated area (p < 0.01).

CONCLUSIONS

Using skin and deep tissue pain sensitization models simultaneously, no significant synergistic effects were found within the 3-day investigation suggesting little integration between the two phenomena in this period.

Single-point but not tonic cuff pressure pain sensitivity is associated with level of physical fitness--a study of non-athletic healthy subjects

Exercise is often used for pain rehabilitation but the link between physical activity level and pain sensitivity is still not fully understood. Pressure pain sensitivity to cuff algometry and conditioned pain modulation (CPM) were evaluated in highly active men (n=22), normally active men (n=26), highly active women (n=27) and normally active women (n=23) based on the Godin Leisure-Time Exercise Questionnaire. Cuff pressure pain sensitivity was assessed at the arm and lower leg. The subjects scored the pain intensity on an electronic Visual Analogue Scale (VAS) during ten minutes with 25 kPa constant cuff pressure and two minutes with zero pressure. The maximal VAS score and area under the VAS-curve were extracted. Pressure pain thresholds (PPT) were recorded by manual pressure algometry on the ipsilateral tibialis anterior muscle before, during and after the tonic arm stimulation. Tonic cuff stimulation of the arm and leg resulted in higher VAS peak scores in women compared with men (p<0.04). In all groups the PPTs were reduced during and after the cuff stimulation compared with baseline (p=0.001). PPT were higher in men compared with women (p=0.03) and higher in highly physical active compared with normal active (p=0.048). Besides the well-known gender difference in pressure pain sensitivity this study demonstrates that a high physical fitness degree in non-athletic subjects is associated with increased pressure pain thresholds but does not affect cuff pressure pain sensitivity in healthy people.

An MRI-based leg model used to simulate biomechanical phenomena during cuff algometry: a finite element study

Cuff pressure stimulation is applicable for assessing deep-tissue pain sensitivity by exciting a variety of deep-tissue nociceptors. In this study, the relative transfer of biomechanical stresses and strains from the cuff via the skin to the muscle and the somatic tissue layers around bones were investigated. Cuff pressure was applied on the lower leg at three different stimulation intensities (mild pressure to pain). Three-dimensional finite element models including bones and three different layers of deep tissues were developed based on magnetic resonance images (MRI). The skin indentation maps at mild pressure, pain threshold, and intense painful stimulations were extracted from MRI and applied to the model. The mean stress under the cuff position around tibia was 4.6, 4.9 and around fibula 14.8, 16.4 times greater than mean stress of muscle surface in the same section at pain threshold and intense painful stimulations, respectively. At the same stimulation intensities, the mean strains around tibia were 36.4, 42.3 % and around fibula 32.9, 35.0 %, respectively, of mean strain on the muscle surface. Assuming strain as the ideal stimulus for nociceptors the results suggest that cuff algometry is less capable to challenge the nociceptors of tissues around bones as compared to more superficially located muscles.

Dynamic mechanical assessment of muscle hyperalgesia in humans: the dynamic algometer

BACKGROUND

Musculoskeletal pain is often associated with a nonhomogeneous distribution of mechanical hyperalgesia. Consequently, new methods able to detect this distribution are needed.

OBJECTIVE

To develop and test a new method for assessing muscle hyperalgesia with high temporal and spatial resolution that provides complementary information compared with information obtained by traditional static pressure algometry.

METHODS

The dynamic pressure algometer was tested bilaterally on the tibialis anterior muscle in 15 healthy subjects and compared with static pressure algometry. The device consisted of a wheel that was rolled over the muscle tissue with a fixed velocity and different predefined forces. The pain threshold force was determined and pain intensity to a fixed-force stimulation was continuously rated on a visual analogue scale while the wheel was rolling over the muscle. The pressure pain sensitivity was evaluated before, during, and after muscle pain and hyperalgesia induced unilaterally by either injection of hypertonic saline (0.5 mL, 6%) into the tibialis anterior or eccentric exercise evoking delayed-onset muscle soreness (DOMS).

RESULTS

The intraclass correlation coefficient was >0.88 for the dynamic thresholds; thus, the method was reliable. Compared with baseline, both techniques detected hyperalgesia at the saline injection site and during DOMS (P<0.05). The dynamic algometer also detected the widespread, patchy distribution of sensitive loci during DOMS, which was difficult to evaluate using static pressure algometry.

DISCUSSION AND CONCLUSION

The present study showed that dynamic pressure algometry is a reliable tool for evaluating muscle hyperalgesia (threshold and pain rating) with high temporal and spatial resolution. It can be applied as a simple clinical bed-side test and as a quantitative tool in pharmacological profiling studies.

Hyperalgesia and allodynia to superficial and deep-tissue mechanical stimulation within and outside of the UVB irradiated area in human skin

Background and aims The ultraviolet-B (UVB) inflammatory model is a well-established model of inflammatory pain. This study investigated whether UVB-induced cutaneous inflammation would enhance pain responses from the underlying deep somatic areas. Methods Skin inflammation was induced, in 24 healthy volunteers, by UVB irradiation (three times of the individual minimal erythema UVB dose) in square-shaped areas on the forearm and lower back. Assessments of cutaneous blood flow, pin-prick thresholds, pressure pain thresholds and tolerance, stimulus-response functions relating graded pressure stimulations and pain intensity (visual analogue scale, VAS) were performed within and outside the irradiated area. Results Twenty-four hours after UVB irradiation, a significant increase in superficial blood flow in the irradiated skin area was demonstrated compared with baseline (P < 0.01) indicating that inflammation was induced. Compared with baseline, UVB irradiation significantly reduced the pin-prick thresholds, pressure pain thresholds and tolerance within and outside of the irradiated area (P < 0.05). The stimulus-response function was left-shifted compared with baseline both within and outside the irradiated area (P < 0.05) with a more pronounced left-shift within the irradiated area (P < 0.01). Application of topical anaesthesia 24h after irradiation in 5 subjects, both within and outside the irradiated area, could only increase the pin-prick thresholds outside the irradiated area. Conclusion The UVB irradiation of the skin not only provokes cutaneous primary and secondary hyper-algesia but also causes hyperalgesia to blunt pressure stimulations 24h after the UVB exposure. Implications The presented UVB model can be used as a translational model from animals into healthy subjects. This model can potentially be used to screen drug candidates with anti-inflammatory properties in early stages of drug development.

Vibration and rotation during biaxial pressure algometry is related with decreased and increased pain sensations

OBJECTIVE

During palpation, the pressure intensity and direction include minor deviations suggesting that standardized variations of the pressure intensity during pressure algometry may optimize the stimulus efficacy. This study examined the perceived pain outcome and reliability of a biaxial (bidirectional) algometer exerting rotational and vibratory stimulation on top of the basic pressure.

METHODS

In 24 healthy subjects, pressure pain thresholds (PPTs) were recorded with a linear pressure gradient (30 kPa/s) applied by a 1-cm(2) probe bilaterally on the tibialis anterior muscle via biaxial and handheld algometers. During constant pressure stimulation (5 seconds, 75%, 100%, 125% PPT), rotational (45°, 90°, and 180°), linear vibrational (15, 25, and 50 Hz), and radial vibrational stimulations (5, 15, and 25 Hz) were applied randomly via regular and fanning rounded probes (1 cm(2)). Subjects rated perceived pain on a 10-cm visual analogue scale on two occasions separated by 1 week period.

RESULTS

Repeated measures analysis of variance revealed enhanced effect of rotation angle (P < 0.001), probe (P < 0.001), and radial vibration frequency (P < 0.02), and suppressing effect of axial vibration frequency (P < 0.03) on pain perception, relative to basic pressure alone. PPT reliability of biaxial and handheld algometers showed averaged intraclass correlation coefficient of 0.94 and 0.945, and coefficient of variations of 15.4 and 13.5%, respectively.

CONCLUSIONS

PPT assessment and multidirectional stimulations can be exerted reliably via biaxial algometer. Linear vibrational stimulation effect on pressure pain perception verified the inhibitory interaction between simultaneous pressure stimulation of low-threshold mechanoreceptors and nociceptors, while radial vibration and rotational stimulation showed facilitatory effects.

Bone hyperalgesia after mechanical impact stimulation: a human experimental pain model

Hyperalgesia in different musculoskeletal structures including bones is a major clinical problem. An experimental bone hyperalgesia model was developed in the present study. Hyperalgesia was induced by three different weights impacted on the shinbone in 16 healthy male and female subjects. The mechanical impact pain threshold (IPT) was measured as the height from which three weights (165, 330, and 660 g) should be dropped to elicit pain at the shinbone. Temporal summation of pain to repeated impact stimuli was assessed. All these stimuli caused bone hyperalgesia. The pressure pain threshold (PPT) was assessed by a computerized pressure algometer using two different probes (1.0 and 0.5 cm(2)). All parameters were recorded before (0), 24, 72, and 96 h after the initial stimulations. The IPTs were lowest 24 h after hyperalgesia induction for all three weights and the effect lasted up to 72 h (p < 0.05). The PPT obtained with the 1.0 cm(2) probe was significantly lower than the PPT obtained with the 0.5 cm(2) probe, regardless of the time. Females developed more pronounced hyperalgesia reflected in reduced IPTs and PPTs (p < 0.05). Temporal summation was significantly (p < 0.05) facilitated after induction of hyperalgesia with the strongest facilitation in males. The developed bone pain and hyperalgesia model may provide the basis for studying this fundamental mechanism of bone-related hyperalgesia and be used for profiling compounds developed for this target.

An improvement of mechanical pain sensitivity measurement method: the smaller sized probes may detect heterogeneous sensory threshold in healthy male subjects

OBJECTIVE

On the basis of our experience in the application of the mechanical algometer and a number of pilot experiments, we speculated that 0.1- and 0.01-cm(2) probes might improve the measurement of mechanical pain sensitivity relative to the conventional 1-cm(2) probe. Here, we examined the accuracy, feasibility, and applicability of these probes in detecting the mechanical pain sensitivity.

DESIGN

Mechanical pain threshold and tolerance tests were performed on subjects using the three probes of 1, 0.1, and 0.01 cm(2) in random order. We compared the application of these probes.

SETTING

The study was set at the Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China.

SUBJECTS

Fifty healthy male Han Chinese subjects were recruited.

OUTCOME MEASURES

We compared the qualities of stimulus-evoked pain, test stability, the measuring time, the subjects' acceptance level of the procedure, the validity of pain measurement, and the arduousness of the task for the investigator among the three different size probes.

RESULTS

Compared with the conventional 1-cm(2) probe, the 0.01- and 0.1-cm(2) probes resulted in the subjects responding to stimulus-evoked pain more quickly, accurately, and consistently, and also made the measurement more comfortable for investigators. Up to 80% of the subjects reported the pain quality as a pricking sensation when the 0.01-cm(2) probe was used.

CONCLUSION

The use of the 0.1-cm(2) probe might be more suitable as an optimized method for the detection of pressure pain sensitivity in clinical studies. In addition, the 0.01-cm(2) probe could potentially serve as an alternative to the weighted needle pinprick, providing continuous quantizing detection for pricking pain sensitivity.

Factors associated with the pressure pain threshold in healthy Chinese men

OBJECTIVE

The goal of present study was to examine the influence of demographic characteristics, lifestyle, and metabolic factors on pressure-induced pain threshold.

DESIGN AND SETTING

The study population comprised 2,517 healthy men at second-phase recruitment of a population-based cohort in China. The pressure pain threshold (PPT) at two locations, triceps and inguinal lines, was obtained using a digital pressure algometer. Education and occupation, as well as lifestyle factors, were self-reported in a face-to-face interview. Blood lipid and fasting glucose were measured as a routine healthy check-up program.

RESULTS

A lower PPT was found among men with younger age, higher level of education, and non-manual occupation. In addition, men with central obesity and moderate leisure time physical exercise were more sensitive to muscle mechanical stimuli. PPT was significantly correlated with high-density lipoprotein cholesterol (β for triceps = 0.064, and β for inguinal lines = 0.052) in a multivariate linear regression model, after controlling for multiple variables. Elevated PPT was also found among subjects with hyperglycemia (both P for triceps and inguinal lines <0.001) and excess drinking (P for triceps = 0.005).

CONCLUSIONS

It is important for physicians and researchers to consider these variables when evaluating pain sensitivity in clinic and in research. The underling mechanisms between these factors and pressure pain perception are worthy of further exploration.

Development and validation of a pressure-type automated quantitative sensory testing system for point-of-care pain assessment

Quantitative sensory testing (QST) can provide useful information about the underlying mechanisms involved in chronic pain. However, currently available devices typically employed suffer from operator-dependent effects, or are too cumbersome for routine clinical care. This paper presents the design and initial validation of a novel automated pressure-pain type QST platform, termed the multi-modal automated sensory testing (MAST) system. The MAST configuration presented consists of wireless, hand-held thumbnail pressure stimulators (with circular 10 mm² rubber tips) and graphical touch screen interface devices to manage the QST process and obtain patient feedback. Validation testing of the custom-designed force sensor showed a 1 % error for low forces increasing to 2 % error for larger loads up to 100 N (full-scale). Validation of the controller using three ramp rates (64, 248, and 496 kPa/s) and six pressures (32, 62, 124, 273, 620, and 1116 kPa) showed an overall mean error of 1.7 % for applied stimuli. Clinical evaluation revealed decreased pressure pain thresholds in chronic pain patients (98.07 ± SE 16.34 kPa) compared to pain free, healthy control subjects (259.88 ± SE 33.54 kPa, p = 0.001). The MAST system is portable and produces accurate, repeatable stimulation profiles indicating potential for point-of-care applications.

Safety system for moving coil pressure algometer

The threat of safety failure during use of potent actuators is a known problem. The use of such actuators in the field of pressure algometry requires adaptation of safety measures since stimulation is applied to human beings. This design provides an additional safety level required in the field of computer-controlled pressure algometry but in principle its usage is not restricted just to this area. The fuse consists of four parts (inner cylinder, outer cylinder lid, outer cylinder guide, and the gauge screw) which are simple and cheap to manufacture, easy to reassemble once the fuse has been triggered, and gaugeable with commercially available tools. The prototype showed acceptable levels of performance given the intended usage of the stimulation setup, namely increasing and repeated musculoskeletal stimulation. Repeatable range of holding force has been attained for the particular application against a rubber mat surface mimicking musculoskeletal tissue (96% for forces F < 20 kg, and 30% for forces 25 kg < F ≤ 35 kg).

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

NGF-evoked sensitization of muscle fascia nociceptors in humans

Nerve growth factor (NGF) induces local hyperalgesia for a few days after intramuscular injection, but longer-lasting muscle pain upon systemic administration. As the muscle fascia is densely innervated by free nerve endings, we hypothesized a lasting sensitization of fascia nociceptors by NGF. We administered 1 μg NGF (dissolved in 100 μL saline) ultrasound-guided to the fascia of the Musculus erector spinae muscle at the lumbar level of 14 male volunteers and assessed hypersensitivity after 6 hours, and 1, 3, 7, 14, and 21 days. Pain upon mechanical stimuli (constant pressure and dynamic impact), upon exercise and electrically induced M. erector spinae contraction, and upon injection of 100 μL phosphate buffer pH4 (at day 7 and 14 only) to the fascia of both NGF- and saline-treated muscles, was investigated. Injections into the muscle fascia did not cause acute pain. Local heat pain thresholds were unchanged following NGF and saline (control) administration. NGF evoked a lasting (days 1-7) and significant reduction of pressure pain, pressure thresholds, exercise-evoked muscle pain, and hyperalgesia to impact stimuli (12 m/s). Pain upon injected protons was significantly elevated (P<0.04) for 2 weeks. NGF induced a sensitization of the muscle fascia to mechanical and chemical stimuli lasting for up to 2 weeks. As nociceptors in the fascia appear to be particularly prone to sensitization, they may contribute to acute or chronic muscle pain.

Moving coil pressure algometer produces consistent force gradient and repeated stimulation

Computer-controlled pressure stimulation (algometry) offers seemingly good reliability when it comes to pain assessment methods. It is therefore important to ensure through methodological quantification that moving coil pressure algometer (MCPA) exhibits accurate, fast, and precise tissue stimulation techniques. This study 1) demonstrates that MCPA satisfies force gradient capabilities of a conventional computer-controlled algometry, and 2) reports on effectiveness of the MCPA to produce sustained, fast, and repeated stimulation of a known pulse duration (600 ms) and force magnitude (10 kg). Solicited force gradients of 500, 1000, and 1800 g/s showed high correlation values (R(2) > 0.99) for both rubber mat and direct probe-to-sensor contact cases. Through fast switching between different modes of operation of the actuator, force overshoot was reduced from as much as 300 to 20% for the same force magnitude, at the expense of a slight delay in repeated pulse delivery scheme.