Enhanced Pain Sensitivity Among Individuals With Symptomatic Knee Osteoarthritis: Potential Sex Differences in Central Sensitization

Diminished vibration perception and greater pressure pain sensitivity are associated with worse knee osteoarthritis outcomes across sex and race

OBJECTIVE

To examine associations of vibration sensitivity and pressure pain sensitivity with knee osteoarthritis (OA) outcomes across sex and race, which may relate to known sex and race disparities in clinical outcomes.

DESIGN

Data were from the 2013-2015 visit of the Johnston County Osteoarthritis Project. Exposures were vibration perception threshold (VPT) measured at the bilateral medial femoral condyle (MFC) and first metatarsophalangeal joint (MTP), and pressure pain threshold (PPT) measured at the bilateral upper trapezius. Outcomes were knee pain severity and presence of knee symptoms, radiographic knee OA, and symptomatic knee OA in each knee. Cross-sectional associations of the exposures with the outcomes were examined using logistic regression models, overall and separately by sex and race.

RESULTS

In the VPT and PPT analyses, 851 and 862 participants (mean age 71 years, 68% female, 33% Black, body mass index 31 kg/m2) and 1585 and 1660 knees were included, respectively. Higher VPT (lower vibration sensitivity) at the MFC and first MTP joint was associated with all outcomes. Lower PPT (greater pressure pain sensitivity) was associated with greater knee pain severity. Associations of VPT and PPT with all outcomes were similar among females and males and Black and White individuals.

CONCLUSIONS

Diminished vibration perception and greater pressure pain sensitivity were cross-sectionally associated with worse knee OA outcomes. Despite differences in VPT and PPT among females and males and Black and White adults, associations with knee OA outcomes did not differ by sex or race, suggesting neurophysiological differences do not relate to established disparities.

Examining Reactivity and Recovery Patterns of Pain-Evoked Cortisol and Alpha-Amylase Trajectories: Relations Between Psychological Markers of Risk and Resilience

Chronic low back pain (cLBP) is one of the leading causes of disability globally and represents an enormous burden to aging adults. While numerous factors contribute to cLBP, dysregulation in the hypothalamic-pituitary-adrenal axis and autonomic nervous system functioning have been implicated in its pathogenesis. It is well documented that negative psychological states can modulate biological stress responsivity in chronic pain; however, little is known regarding the influence of positive psychological factors in this relationship. The aim of this study was to examine the association between psychological risk and resilience factors with patterns of physiological stress reactivity and recovery in 60 older adults with cLBP. Participants completed measures of hope, optimism, pain catastrophizing, and perceived stress, and underwent psychophysical pain testing assessing responses to painful pressure, heat, and cold stimuli. Salivary samples were obtained prior to pain induction and at 7 time points spanning 90 minutes after pain testing terminated. To examine reactivity and recovery profiles in hypothalamic-pituitary-adrenal axis and autonomic nervous system function, samples were assayed for cortisol and alpha-amylase, respectively. Results revealed higher levels of hope and optimism were associated with increased cortisol reactivity (p's < .003) and more rapid recovery (p's = .001). Further, pain catastrophizing and perceived stress were associated with cortisol reactivity, with lower levels of these factors predicting larger increases in cortisol from baseline to peak levels (p's < .04). No significant differences in reactivity or recovery patterns emerged for alpha-amylase. Overall, findings highlight the role of psychological risk and resilience factors in modulating physiological stress reactivity. PERSPECTIVE: This article investigated whether psychosocial risk and resilience factors were associated with stress reactivity and recovery in response to laboratory-based pain testing in older adults with chronic low back pain. Results indicate that high resilience factors may be protective by modulating adrenocortical reactivity and recovery profiles.

Healthy women show more experimentally induced central sensitization compared with men

ABSTRACT

Women more often experience chronic pain conditions than men. Central sensitization (CS) is one key mechanism in chronic pain that can differ between the sexes. It is unknown whether CS processes are already more pronounced in healthy women than in men. In 66 subjects (33 women), a thermal CS induction protocol was applied to the dorsum of one foot and a sham protocol to the other. Spatial extent [cm 2 ] of secondary mechanical hyperalgesia (SMH) and dynamic mechanical allodynia were assessed as subjective CS proxy measures, relying on verbal feedback. Changes in nociceptive withdrawal reflex magnitude (NWR-M) and response rate (NWR-RR) recorded through surface electromyography at the biceps and rectus femoris muscles were used as objective CS proxies. The effect of the CS induction protocol on SMH was higher in women than in men (effect size 2.11 vs 1.68). Nociceptive withdrawal reflex magnitude results were statistically meaningful for women (effect size 0.31-0.36) but not for men (effect size 0.12-0.29). Differences between men and women were not meaningful. Nociceptive withdrawal reflex response rate at the rectus femoris increased in women after CS induction and was statistically different from NWR-RR in men (median differences of 13.7 and 8.4% for 120 and 140% reflex threshold current). The objective CS proxy differences indicate that dorsal horn CS processes are more pronounced in healthy women. The even larger sex differences in subjective CS proxies potentially reflect greater supraspinal influence in women. This study shows that sex differences are present in experimentally induced CS in healthy subjects, which might contribute to women's vulnerability for chronic pain.

Clinical Assessment of Osteoarthritis Pain: Contemporary Scenario, Challenges, and Future Perspectives

The multifaceted nature of osteoarthritis (OA) pain presents a challenge in understanding and managing the condition. The diverse pain experiences, progression rates, individual responses to treatments, and complex disease mechanisms contribute to heterogeneity in the clinical studies outcomes. The lack of a standardized methodology for assessing and classifying OA pain challenges healthcare practitioners. This complicates the establishment of universally applicable protocols or standardized guidelines for treatment. This article explores the heterogeneity observed in clinical studies evaluating OA pain treatments, highlighting the necessity for refined methodologies, personalized patient categorization, and consistent outcome measures. It discusses the role of the multidimensional nature of OA pain, underlying pain mechanisms, and other contributing factors to the heterogeneity in outcome measures. Addressing these variations is crucial to establishing a more consistent framework for evidence-based treatments and advancing care of the patient with OA pain.

Unraveling sex-specific risks of knee osteoarthritis before menopause: Do sex differences start early in life?

OBJECTIVE

Sufficient evidence within the past two decades have shown that osteoarthritis (OA) has a sex-specific component. However, efforts to reveal the biological causes of this disparity have emerged more gradually. In this narrative review, we discuss anatomical differences within the knee, incidence of injuries in youth sports, and metabolic factors that present early in life (childhood and early adulthood) that can contribute to a higher risk of OA in females.

DESIGN

We compiled clinical data from multiple tissues within the knee joint-since OA is a whole joint disorder-aiming to reveal relevant factors behind the sex differences from different perspectives.

RESULTS

The data gathered in this review indicate that sex differences in articular cartilage, meniscus, and anterior cruciate ligament are detected as early as childhood and are not only explained by sex hormones. Aiming to unveil the biological causes of the uneven sex-specific risks for knee OA, we review the current knowledge of sex differences mostly in young, but also including old populations, from the perspective of (i) human anatomy in both healthy and pathological conditions, (ii) physical activity and response to injury, and (iii) metabolic signatures.

CONCLUSIONS

We propose that to close the gap in health disparities, and specifically regarding OA, we should address sex-specific anatomic, biologic, and metabolic factors at early stages in life, as a way to prevent the higher severity and incidence of OA in women later in life.

EEG-Based Cortical Alterations in Individuals With Chronic Knee Pain Secondary to Osteoarthritis: A Cross-sectional Investigation

Chronic painful knee osteoarthritis (OA) is a disabling physical health condition. Alterations in brain responses to arthritic changes in the knee may explain persistent pain. This study investigated source localized, resting-state electroencephalography activity and functional connectivity in people with knee OA, compared to healthy controls. Adults aged 44 to 85 years with knee OA (n = 37) and healthy control (n = 39) were recruited. Resting-state electroencephalography was collected for 10 minutes and decomposed into infraslow frequency (ISF) to gamma frequency bands. Standard low-resolution electromagnetic brain tomography statistical nonparametric maps were conducted, current densities of regions of interest were compared between groups and correlation analyses were performed between electroencephalography (EEG) measures and clinical pain and functional outcomes in the knee OA group. Standard low-resolution electromagnetic brain tomography nonparametric maps revealed higher (P = .006) gamma band activity over the right insula (RIns) in the knee OA group. A significant (P < .0001) reduction in ISF band activity at the pregenual anterior cingulate cortex, whereas higher theta, alpha, beta, and gamma band activity at the dorsal anterior cingulate cortex, pregenual anterior cingulate cortex, the somatosensory cortex, and RIns in the knee OA group were identified. ISF activity of the dorsal anterior cingulate cortex was positively correlated with pain measures and psychological distress scores. Theta and alpha activity of RIns were negatively correlated with pain interference. In conclusion, aberrations in infraslow and faster frequency EEG oscillations at sensory discriminative, motivational-affective, and descending inhibitory cortical regions were demonstrated in people with chronic painful knee OA. Moreover, EEG oscillations were correlated with pain and functional outcome measures. PERSPECTIVE: This study confirms alterations in the rsEEG oscillations and its relationship with pain experience in people with knee OA. The study provides potential cortical targets and the EEG frequency bands for neuromodulatory interventions for managing chronic pain experience in knee OA.

Sex differences in osteoarthritis prevalence, pain perception, physical function and therapeutics

OBJECTIVE

Women have a higher prevalence of osteoarthritis (OA) and worse clinical courses than men. However, the underlying factors and therapeutic outcomes of these sex-specific differences are incompletely researched. This review examines the current state of knowledge regarding sex differences in OA prevalence, risk factors, pain severity, functional outcomes, and use and response to therapeutics.

METHODS

PubMed database was used with the title keyword combinations "{gender OR sex} AND osteoarthritis" plus additional manual search of the included papers for pertinent references, yielding 212 references. Additional references were added and 343 that were reviewed for appropriateness.

RESULTS

Globally, women account for 60% of people with osteoarthritis with a greater difference after age 40. The higher risk for women may be due to differences in joint anatomy, alignment, muscle strength, hormonal influences, obesity, and/or genetics. At the same radiographic severity, women have greater pain severity than men, which may be explained by biologically distinct pain pathways, differential activation of central pain pathways, differences in pain sensitivity, perception, reporting, and coping strategies. Women have greater limitations of physical function and performance than men independent of BMI, OA severity, injury history, amount of weekly exercise. Women also have greater use of analgesic medications than men but less use of arthroplasty and poorer prognosis after surgical interventions.

CONCLUSIONS

The recognition of sex differences in OA manifestations and management could guide tailoring of sex-specific treatment protocols, and analysis of sex as a biological variable in future research would enhance development of precision medicine.

Exploring the association of gender role expectations of pain and measures of pain sensitization in people with knee osteoarthritis: A cross-sectional study

OBJECTIVES

First, we explored the association between Gender Role Expectations of Pain (GREP), and psychophysical measures of sensitization in people with knee osteoarthritis (OA). Second, we explored whether the association differed by level of GREP items (high vs low scores).

DESIGN

We conducted secondary analyses of a cohort study. Those who were (i) age of ≥40, English or French speaking, ii) diagnosed with knee OA using American College of Rheumatology criteria and iii) consulting with an orthopedic surgeon were included. GREP items pertaining to pain sensitivity and pain endurance of the typical man or woman were rated by males and females respectively. Psychophysical tests consisted of pressure pain thresholds (PPTs), Temporal Summation (TS), and Conditioned Pain Modulation (CPM). Multiple linear regression models for males and females were run with GREP scores (independent variables) and psychophysical tests (dependent variables). Next models stratified on the median split of GREP scores were run. Models were adjusted for age, BMI, pain catastrophizing, anxio-depressive symptoms, and radiographic severity.

RESULTS

280 participants (57% females; age (SD): 63.9 (9.6) and BMI (SD): 31.3 (8.40)) were included. GREP pain sensitivity scores in males were associated with CPM values (β: 95% CI: 0.09 (0.01 to 0.17)). Males with low GREP pain sensitivity or pain endurance had very small to small positive associations with PPT and CPM values.

CONCLUSION

This first exploration of gendered pain sensitivity and pain endurance by males and females has small and clinically unimportant associations with measures of pain sensitization requiring further validation.

Gender Differences in Pain Threshold, Unpleasantness, and Descending Pain Modulatory Activation Across the Adult Life Span: A Cross Sectional Study

The neurobiological underpinnings of gender differences in pain perception, and how these differences may be modified by age, are incompletely understood, placing patients at risk of suboptimal pain management. Using functional magnetic resonance imaging, we examined brain responses in the descending pain modulatory system (DPMS, specifically, dorsolateral prefrontal cortex, anterior cingulate cortex, insula, hypothalamus, amygdala, and periaqueductal gray, during an evoked pain task. We investigated the interaction of age and gender in our sample of healthy adults (27 females, 32 males, 30-86 years) on DPMS response. In a perceptually matched thermal pain paradigm, we investigated pain unpleasantness and neural responses for 3 heat pain percepts: just noticeable pain, weak pain, and moderate pain (MP). Females reported just noticeable pain at a lower temperature, but reported less unpleasantness at weak pain and MP percepts, compared to males. There was a significant age-by-gender interaction during moderate pain in the right anterior cingulate cortex and bilateral insula, such that, males had a stronger positive relationship between DPMS response and age compared to females in these regions. Our results indicate that differences in DPMS responses may explain some gender differences in pain perception and that this effect may change across the adult lifespan. PERSPECTIVE: Gender differences in pain have been well-documented but the brain mechanisms for these differences are still unclear. This article describes potential differences in brain functioning during different levels of pain that could explain differences in pain responses between men and women across the adult lifespan.

Exploring different models of pain phenotypes and their association with pain worsening in people with early knee osteoarthritis: The MOST cohort study

OBJECTIVE

To determine i) pain phenotypes (PP) in people with early-stage knee osteoarthritis (EKOA); ii) the longitudinal association between the phenotypes and pain worsening at two years.

DESIGN

We studied participants with EKOA from the Multicenter Osteoarthritis Study defined as pain intensity ≤3/10, Kellgren and Lawrence grade ≤2, intermittent pain none to sometimes, and no constant pain. Two models of PP were explored. Model A included pressure pain thresholds, temporal summation, conditioned pain modulation, pain catastrophizing, sleep quality, depression, and widespread pain (WSP). In Model B, gait characteristics, quadriceps strength, comorbidities, and magnetic resonance imaging features were added to Model A. Latent Class Analysis was used to create phenotypes, and logistic regression was used to determine their association with pain worsening.

RESULTS

750 individuals (60% females), mean age [standard deviation (SD)]: 60.3 (9.4) were included in Model A and 333 individuals (60% females), mean age (SD): 59.4 (8.1) in Model B. 3-class and 4-class solutions were chosen for Model A and Model B. In Model A, the most "severe" phenotype was dominated by psychosocial factors, WSP, and measures of nervous system sensitization. Similarly in Model B, the Model A phenotype plus gait variables, quadriceps strength, and comorbidities were dominant. Surprisingly, none of the phenotypes in either model had a significant relationship with pain worsening.

CONCLUSION

Phenotypes based upon various factors thought to be important for the pain experience were identified in those with EKOA but were not significantly related to pain worsening. These phenotypes require validation with clinically relevant endpoints.

Factors associated with pain pressure threshold in both local and remote sites in knee osteoarthritis

BACKGROUND

Knee osteoarthritis (KOA) is a prevalent condition, and its most frequent symptom is pain that often leads to disability. Pain sensitization is a core feature of KOA, and it can be measured through quantitative sensory testing protocols such as pain pressure threshold (PPT). However, there is a lack of understanding about the factors that may influence changes in PPTs in the KOA population.

OBJECTIVE

To explore the clinical and functional factors associated with PPTs in a sample of people with chronic KOA pain and to compare models of local (knees) and remote (thenar regions) sites.

DESIGN

Cross-sectional analysis of a prospective cohort.

SETTING

Primary care in public institution.

PARTICIPANTS

113 adults with KOA.

INTERVENTION

N/A.

MAIN OUTCOME MEASURES

Multivariable regression analyses evaluating demographic, clinical, and functional variables that could be associated with local and remote PPTs (main outcomes) were performed.

RESULTS

Both thenar region (adjusted-R2 : 0.29) and knee (adjusted-R2 : 0.45) models had the same significant negative association with being a female, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain levels (thenar: β: -0.15, p = .002; knee: β: -0.2, p < .001), and the 10-Meter Walking Test (thenar: β: -0.05, p = .038; knee: β: -0.08, p = .004). A small significant positive association with depressive symptoms was identified in both models, which acted as a confounder for WOMAC pain and was likely affected by unmeasured confounders.

CONCLUSIONS

PPTs in KOA pain are associated with functional outcomes such as the 10-Meter Walking Test and activity-related pain intensity; thus more disability is associated with smaller pain thresholds. Similarity between models may suggest central sensitization.

Self-regulation training for people with knee osteoarthritis: a protocol for a feasibility randomised control trial (MiNT trial)

Introduction

Knee osteoarthritis (OA) is a chronic secondary musculoskeletal pain condition resulting in disability, reduced quality of life, and high societal costs. Pain associated with knee OA is linked to increased sensitivity in sensory, cognitive, and emotional areas of the brain. Self-regulation training targeting brain functioning related to pain experience could reduce pain and its associated disability. Self-regulatory treatments such as mindfulness meditation (MM) and electroencephalography neurofeedback (EEG-NF) training improve clinical outcomes in people with knee OA. A feasibility clinical trial can address factors that could inform the design of the full trial investigating the effectiveness of self-regulation training programmes in people with knee OA. This clinical trial will evaluate the feasibility, safety, acceptability, experience and perceptions of the self-regulatory training programmes.

Methods

The proposed feasibility trial is based on a double-blind (outcome assessor and investigators), three-arm (MM usual care, EEG-NF + usual care and usual care control group) randomised controlled parallel clinical trial. Participants with knee OA will be recruited from the community and healthcare practices. A research assistant (RA) will administer both interventions (20-min sessions, four sessions each week, and 12 sessions over three successive weeks). Feasibility measures (participant recruitment rate, adherence to interventions, retention rate), safety, and acceptability of interventions will be recorded. An RA blinded to the group allocation will record secondary outcomes at baseline, immediately post-intervention (4th week), and 3 months post-intervention. The quantitative outcome measures will be descriptively summarised. The qualitative interviews will evaluate the participants' experiences and perceptions regarding various aspects of the trial, which includes identifying the barriers and facilitators in participating in the trial, evaluating their opinions on the research procedures, such as their preferences for the study site, and determining the level of acceptability of the interventions as potential clinical treatments for managing knee OA. Māori participant perceptions of how assessment and training practices could be acceptable to a Māori worldview will be explored. The interviews will be audio-recorded and analysed thematically.

Discussion

This trial will provide evidence on the feasibility, safety, and acceptability of the MM and EEG-NF training in people with knee OA, thus informing the design of a full randomised clinical control trial.

Association of mechanical temporal summation of pain with muscle co-contraction during walking in people with knee osteoarthritis

BACKGROUND

People with knee osteoarthritis walk with excessive muscle co-contraction that can accelerate disease progression. Central pain sensitization is common in people with knee osteoarthritis and may be related to walking patterns. The objective of this study was to examine the relation of central pain sensitization with muscle co-contraction during walking in people with knee osteoarthritis.

METHODS

This study reports secondary analysis from baseline data of two clinical trials (n = 90 participants with knee osteoarthritis). The presence of central pain sensitization was measured by mechanical temporal summation at the patella and the wrist. Quadriceps and hamstrings activation was assessed using surface electromyography during walking at self-selected and fast paces. Muscle co-contraction indices for vastus medialis-medial hamstrings and vastus lateralis-lateral hamstrings muscle pairs were calculated during stance phases. Co-contraction outcomes were compared between people with and without mechanical temporal summation at each site, adjusting for age, sex, and body mass index.

FINDINGS

People with mechanical temporal summation at the knee had greater vastus lateralis-lateral hamstrings co-contraction while walking at a fast pace (P = 0.04). None of the other differences was statistically significant, but the overall trends and effect sizes indicated greater co-contraction in people with temporal summation at the knee irrespective of gait phase, walking speed, or muscle pairs.

INTERPRETATION

Central pain sensitization, assessed as mechanical temporal summation at the knee, is related to greater knee muscle co-contraction during fast walking in people with knee osteoarthritis. Thus, mitigating central sensitization may be an interventional target to reduce muscle co-contraction for people with knee osteoarthritis.

Sex Differences in Pain and Quantitative Sensory Testing in Patients With Rheumatoid Arthritis

OBJECTIVE

Women with rheumatoid arthritis (RA) have higher pain and worse functional outcomes compared to men, even when treated with similar medications. The objective of this study was to identify sex differences in pain intensity, pain interference, and quantitative sensory tests (QST), which are independent of inflammation, in patients with RA.

METHODS

This study is a post hoc analysis of participants in the Central Pain in Rheumatoid Arthritis cohort. Pain intensity was assessed using a 0-10 numeric rating scale. Pain interference was measured using a Patient-Reported Outcomes Measurement Information System computerized adaptive test. QST included pressure pain detection thresholds, temporal summation, and conditioned pain modulation. Women and men were compared using multiple linear regression, adjusted for age, education, race, research site, depression, obesity, RA disease duration, swollen joint count, and C-reactive protein.

RESULTS

Mean ± SD pain intensity was 5.32 ± 2.29 among women with RA, compared to 4.60 ± 2.23 among men with RA (adjusted difference 0.83 [95% confidence interval (95% CI) 0.14, 1.53]). Women with RA had lower pressure pain detection thresholds at the trapezius (adjusted difference -1.22 [95% CI -1.73, -0.72]), wrist (adjusted difference -0.57 [95% CI -1.07, -0.06]), and knee (adjusted difference -1.10 [95% CI -2.00, -0.21]). No statistically significant differences in pain interference, temporal summation, and conditioned pain modulation were observed.

CONCLUSION

Women reported higher pain intensity and lower pressure pain detection thresholds (higher pain sensitivity) than men. However, pain interference, temporal summation, and conditioned pain modulation did not differ between men and women.

Innate Immunity and Sex: Distinct Inflammatory Profiles Associated with Murine Pain in Acute Synovitis

Joint pain severity in arthritic diseases differs between sexes and is often more pronounced in women. This disparity is thought to stem from biological mechanisms, particularly innate immunity, yet the understanding of sex-specific differences in arthritic pain remains incomplete. This study aims to investigate these disparities using an innate immunity-driven inflammation model induced by intra-articular injections of Streptococcus Cell Wall fragments to mimic both acute and pre-sensitized joint conditions. Nociceptive behavior was evaluated via gait analysis and static weight-bearing, and inflammation was evaluated via joint histology and the synovial gene expression involved in immune response. Although acute inflammation and pain severity were comparable between sexes, distinct associations between synovial inflammatory gene expression and static nociceptive behavior emerged. These associations delineated sex-specific relationships with pain, highlighting differential gene interactions (Il6 versus Cybb on day 1 and Cyba/Gas6 versus Nos2 on day 8) between sexes. In conclusion, our study found that, despite similar pain severity between sexes, the association of inflammatory synovial genes revealed sex-specific differences in the molecular inflammatory mechanisms underlying pain. These findings suggest a path towards more personalized treatment strategies for pain management in arthritis and other inflammatory joint diseases.

Relation of pain sensitization to self-reported and performance-based measures of physical functioning: the Multicenter Osteoarthritis (MOST) study

OBJECTIVE

It is unclear if alterations in nociceptive signaling contribute to poor physical functioning in persons with knee osteoarthritis (OA). We aimed to characterize the relation of pain sensitization to physical functioning in persons with or at risk for knee OA, and determine if knee pain severity mediates these relationships.

MATERIALS AND METHODS

We used cross-sectional data from the Multicenter Osteoarthritis Study, a cohort study of persons with or at risk for knee OA. Pressure pain thresholds (PPTs) and temporal summation (TS) were assessed with quantitative sensory testing. Self-reported function was quantified with the Western Ontario and McMaster Universities Arthritis Index function subscale (WOMAC-F). Walking speed was determined during a 20-m walk. Knee extension strength was assessed with dynamometry. Relations of PPTs and TS to functional outcomes were examined with linear regression. The mediating role of knee pain severity was assessed with mediation analyses.

RESULTS

Among 1560 participants (60.5% female, mean age (SD) 67 (8), body mass index (BMI) 30.2 (5.5) kg/m2), lower PPTs and the presence of TS were associated with worse WOMAC-F scores, slower walking speeds, and weaker knee extension. The extent of mediation by knee pain severity was mixed, with the greatest mediation observed for self-report function and only minimally for performance-based function.

CONCLUSIONS

Heightened pain sensitivity appears to be meaningfully associated with weaker knee extension in individuals with or at risk for knee OA. Relations to self-reported physical function and walking speed do not seem clinically meaningful. Knee pain severity differentially mediated these relationships.

Early pain in females is linked to late pathological features in murine experimental osteoarthritis

Background

Osteoarthritis (OA) is a progressive joint disease and a major cause of chronic pain in adults. The prevalence of OA is higher in female patients, who tend to have worse OA outcomes, partially due to pain. The association between joint pain and OA pathology is often inconclusive. Preclinical research studies have largely overlooked sex as a potential determinant in joint pain during OA. This study aimed to investigate the role of sex in joint pain in the collagenase-induced OA (CiOA) model and its link with joint pathology.

Methods

Multiple aspects of pain were evaluated during identically executed experiments of CiOA in male and female C57BL/6J mice. Cartilage damage, osteophyte formation, synovial thickness, and cellularity were assessed by histology on day 56. The association between pain and pathology was investigated, disaggregated by sex.

Results

Differences in pain behavior between sexes were found in the majority of the evaluated pain methods. Females displayed lower weight bearing ability in the affected leg compared to males during the early phase of the disease, however, the pathology at the end stage was comparable between sexes. In the second cohort, males displayed increased mechanical sensitivity in the affected joint compared to females but also showed more cartilage damage at the end stage of the model. Within this cohort, gait analysis showed varied results. Males used the affected paw less often and displayed dynamic weight-bearing compensation in the early phase of the model. These differences were not observed in females. Other evaluated parameters displayed comparable gait behavior between males and females. A detailed analysis of individual mice revealed that seven out of 10 pain measurements highly correlated with OA histopathology in females (Pearson r range: 0.642-0.934), whereas in males this measurement was only two (Pearson r range: 0.645-0.748).

Conclusion

Our data show that sex is a determinant in the link between pain-related behavior with OA features. Therefore, to accurately interpret pain data it is crucial to segregate data analysis by sex to draw the correct mechanistic conclusion.

Pain in osteoarthritis from a symptom to a disease

Pain is the most frequent symptom of osteoarthritis (OA), occurring much more commonly than stiffness or disability. Classically, OA-related pain has been considered to be a nociceptive pain condition and an alarm signal correlated to the intensity of joint degradation. However, OA-related pain is a specific disease, with a complex pathophysiology, including neuropathic peripheral and central abnormalities, together with local inflammation involving all joint structures. Clinical findings emphasize that it is not a stable and linear condition, that pain experience is poorly correlated to structural modifications, and that the quality of pain in OA is important to consider, aside from its intensity. OA-related pain is modulated by many factors, including the individual patient's psychological and genetic factors, as well as the theoretical role of meteorological influences. Recent findings have improved our knowledge about the central mechanisms of OA pain, especially in persistent cases. A specific questionnaire on OA pain is currently being developed to assess more precisely the patient's experience and target specific pain mechanisms. In conclusion, OA-related pain should be analyzed specifically aside from OA, taking into account the complexity of OA pain as a disease, distinguishing different OA pain phenotypes, to guide more precisely analgesic treatment and OA global management.

Anserine bursa palpation tenderness is a risk factor for knee osteoarthritis progression and arthroplasty: data from the Osteoarthritis Initiative

OBJECTIVE

Anserine bursa pain (ABP) is defined as the presence of palpation tenderness medially below the joint line, which is 2 cm from the tibial tuberosity. This study aimed to determine a link between ABP and three knee outcomes: frequent pain, joint space narrowing (JSN) progression, and total knee arthroplasty (TKA).

METHODS

Participants from the Osteoarthritis Initiative cohort were included in this study. Frequent ABP was defined as presenting thrice at four-time points. The Chi-square test and binary logistic regression analyses examined the associations between ABP and the three knee outcomes. Furthermore, Cox Proportional Hazards Model explored the association between ABP and TKA.

RESULTS

Baseline ABP was linked to a higher risk of frequent pain (odds ratio (OR): 2.28, 95% confidence interval (CI): 1.76-2.97, P < 0 .001) and TKA (OR: 1.54, 95% CI 1.01-2.36, P = 0 .044) after adjusting for gender, baseline age, body mass index (BMI), and Kellgren-Lawrence (KL) grade. In the frequent ABP group from baseline to the 4-year follow-up (≥ 3 of four-time points), frequent pain (OR: 3.14, 95% CI: 2.34-4.22, P < 0 .001) and TKA (OR: 1.79, 95% CI: 1.11-2.90, P = 0 .017) had a high association with ABP after adjusting for gender, baseline age, BMI, and KL grade.

CONCLUSION

This study highlights the association between ABP and knee outcomes; therefore, clinicians should pay closer attention during the physical examination, especially in middle-aged and older female patients. Moreover, understanding ABP cause aids in better diagnosis and treatment. Key Points • This is the first study to identify an association between anserine bursa palpation tenderness and symptomatic knee osteoarthritis. • As opposed to most studies, which focus on intra-articular symptoms and signs, this study focused on extra-articular symptoms and signs. • Clinically, anserine bursa palpation tenderness can be utilized to determine patients at risk for the progression of knee osteoarthritis, thereby aiding in providing early therapeutic intervention.

Prevalence of and factors associated with unmanageable pain levels in people with knee or hip osteoarthritis: a cross-sectional population-based study

BACKGROUND

Pain due to knee and / or hip osteoarthritis (HKOA) is the most common symptom for seeking healthcare. Pain interferes on daily activities, social and occupational participation in people with HKOA. The goal of this study is to estimate the prevalence of unmanageable pain levels (UPL) among people with HKOA), characterize this population and identify factors associated with UPL, and compare therapeutic strategies used by people with UPL versus manageable pain levels (MPL).

METHODS

We analysed data from the EpiReumaPt study (n = 10,661), that included a representative sample of the Portuguese population. Among these, 1081 participants had a validated diagnosis of HKOA by a rheumatologist.. Sociodemographic, lifestyle and health-related data were collected in a structured interview. Pain intensity (NPRS) data were collected in a medical appointment. Painmedication (last month), physiotherapy and surgery were considered as therapies for pain management. UPL was defined as a mean pain intensity in the previous week of ≥5 points on 11-point numeric pain rating scale. The factors associated with UPL were analyzed with logistic regression (p < 0.05, 95%CI). The effect of unmanageable pain levels was assessed by the HOOS/KOOS activities of daily living and quality of life subscales. Symptoms of anxiety and depression were assessed by the Hospital Anxiety and Depression Scale (HADS). Analysis was completed with linear and logistic regression. All analysis were weighted.

RESULTS

The estimated prevalence of UPL among people with HKOA was 68.8%. UPL was associated with being female (odds ratio (OR) = 2.36, p < 0.001), being overweight (OR = 1.84, p = 0.035) or obese (OR = 2.26, p = 0.006), and having multimorbidity (OR = 2.08, p = 0.002). People with UPL reported worse performance in activities of daily living and lower quality of life (β = - 21.28, p < 0.001 and β = - 21.19, p < 0.001, respectively) than people with MPL. People with UPL consumed more NSAIDs (22.0%, p = 0.003), opioids (4.8%, p = 0.008), paracetamol (2.7%, p = 0.033), and overall analgesics (7.3%, p = 0.013) than people with MPL. A higher proportion of people with UPL underwent physiotherapy (17.5%, p = 0.002) than people with MPL.

CONCLUSION

Two-thirds of people with HKOA in Portugal have poor management of their pain levels. Clinical and lifestyle factors, that are highly presented in individuals with HKOA, are associated with unmanageable pain. Our results highlighting the need for further research and implementation of effective interventions to improve pain, function and quality of life in people with HKOA.

Development of a Clinical Prediction Rule for Treatment Success with Transcranial Direct Current Stimulation for Knee Osteoarthritis Pain: A Secondary Analysis of a Double-Blind Randomized Controlled Trial

The study’s objective was to develop a clinical prediction rule that predicts a clinically significant analgesic effect on chronic knee osteoarthritis pain after transcranial direct current stimulation treatment. This is a secondary analysis from a double-blind randomized controlled trial. Data from 51 individuals with chronic knee osteoarthritis pain and an impaired descending pain inhibitory system were used. The intervention comprised a 15-session protocol of anodal primary motor cortex transcranial direct current stimulation. Treatment success was defined by the Western Ontario and McMaster Universities’ Osteoarthritis Index pain subscale. Accuracy statistics were calculated for each potential predictor and for the final model. The final logistic regression model was statistically significant (p < 0.01) and comprised five physical and psychosocial predictor variables that together yielded a positive likelihood ratio of 14.40 (95% CI: 3.66−56.69) and an 85% (95%CI: 60−96%) post-test probability of success. This is the first clinical prediction rule proposed for transcranial direct current stimulation in patients with chronic pain. The model underscores the importance of both physical and psychosocial factors as predictors of the analgesic response to transcranial direct current stimulation treatment. Validation of the proposed clinical prediction rule should be performed in other datasets.

Lean Mass is Associated with, but Does Not Mediate Sex Differences in Pressure Pain Sensitivity in Healthy Adults

Background

Sex differences exist in pain sensitivity, however, the underlying mechanism(s) that explain these differences are not fully understood. Pain sensitivity has been shown to be influenced by body mass index, but limited data exist on the role of body composition on pain sensitivity. The purpose was to examine the influence of body composition on pain sensitivity in males and females.

Methods

This cross-sectional study design used pressure pain thresholds (PPT) of 87 participants (45 female) who were assessed in the vastus lateralis (leg PPT) and brachioradialis (arm PPT) using a pressure algometer. Fat and lean tissue were assessed via dual-energy X-ray absorptiometry (DXA). A two group by two limb, repeated measured ANOVA was used to assess differences between limbs and sex. Spearman correlations and hierarchical regression analyses were employed to determine the association between body composition and PPT.

Results

Males had higher PPTs then females (P<0.05) and had higher DXA assessed lean and lower levels fat mass (P<0.05). Total body and limb specific lean mass was associated with PPTs (r≥0.34; P<0.05). Hierarchical regression analysis revealed lean mass was a significant predictor of 8% of the variance in arm PPT (P<0.006) and 18% of the variance in leg PPT (P<0.001). However, lean mass was not found to statistically mediate the observed sex differences in PPT.

Conclusion

This finding suggests lean mass may play a previously unknown role in sex differences in pressure pain sensitivity. Future studies are needed to confirm this finding and a larger sample size is likely required to have sufficient power to perform the mediation analysis.

Liposomal Bupivacaine Versus Continuous Nerve Block: Liposomal Bupivacaine May be Non-inferior and More Cost Effective

BACKGROUND

Regional anesthesia with adductor canal block has become the standard of care for pain management after total knee arthroplasty (TKA). We hypothesized that liposomal bupivacaine (LB) may be noninferior to continuous nerve block with a pain pump in terms of average pain scores, 30- and 90-day readmissions, and emergency department (ED) visits while reducing cost.

METHODS

A retrospective chart review was performed on primary TKA patients from 2015 to 2020 by 23 orthopaedic surgeons at a single institution. The inclusion criteria was treatment with LB or a pain pump, and exclusion criteria was receipt of both anesthetics and revision surgery. A total of 2,378 patients met the inclusion criteria with 1,640 patients treated with LB and 738 treated with the pain pump. Demographic differences were not statistically significant. Primary outcomes were average pain scores, 30- and 90-day readmissions, and ED visits. Secondary outcomes were average milligram morphine equivalents per admission, hospital lengths of stay , and costs.

RESULTS

There was no significant difference in pain scores on postoperative days 0, 1, 2, or 3 (P = .77, .86, 0.08, and 0.40, respectively), 30- or 90- day readmissions (P = .527 and P = .374), ED visits (P = .129 and P = .108), milligram morphine equivalents utilizations (P = .194), or average hospital LOS (P = .348). We estimated a potential cost savings of $95 per patient and $155,800 over the course of the study.

CONCLUSION

LB was found to be noninferior to a pain pump, and the transition to this medication was associated with cost savings.

Efficacy of Home-Based Transcranial Direct Current Stimulation on Experimental Pain Sensitivity in Older Adults with Knee Osteoarthritis: A Randomized, Sham-Controlled Clinical Trial

Although transcranial direct current stimulation (tDCS) is encouraging regarding clinical pain intensity for individuals with knee osteoarthritis, very few studies have explored its impact on experimental pain sensitivity, which may hinder our understanding of underlying therapeutic mechanisms. The purpose of this study was to assess the efficacy of 15 home-based tDCS sessions on experimental pain sensitivity and explore its relationships with clinical pain intensity. We randomly assigned 120 participants to active tDCS (n = 60) and sham tDCS (n = 60). Quantitative sensory testing (QST) was used, including heat pain threshold and tolerance, pressure pain threshold, and conditioned pain modulation. Patients in the active tDCS group exhibited reduced experimental pain sensitivity as reflected by all QST measures at the end of treatment. Furthermore, correlations were observed between changes in clinical pain intensity and experimental pain sensitivity. These findings warrant further studies on tDCS and experimental pain sensitivity in patients with knee osteoarthritis and exploring the magnitude and sustainability of effects on a longer term.

Dietary Inflammatory Index (DII) is Associated with Movement-Evoked Pain Severity in Adults with Chronic Low Back Pain: Sociodemographic Differences

Chronic low back pain (CLBP) is one of the leading causes of pain and disability in adults in the United States and disproportionately burdens non-Hispanic Black (NHB) individuals and females. Approximately 90% of CLBP cases are of unknown cause, and it is imperative that potential causes be explored. It has been reported that diet quality can influence pain state via diet-induced inflammation. The present study assessed the relationship between Dietary Inflammatory Index (DII) and movement evoked-pain severity in people with CLBP and investigated whether race/sex moderated the relationship between DII and movement-evoked pain. Results revealed no significant differences in DII scores between males and females, or between NHB and non-Hispanic White (NHW) participants. Participant sex significantly modified the relationship between DII and movement-evoked pain severity (P = .0155), such that movement-evoked pain severity was significantly impacted by DII scores in females, but not males. Participant race did not significantly moderate the DII - movement-evoked pain severity relationship. These results suggest that diet-induced inflammation may impact the CLBP experiences of females to a greater degree than males. Further research is needed to determine whether dietary interventions that reduce inflammation improve CLBP outcomes and whether these interventions may be differentially-beneficial based on sex. PERSPECTIVE: This article highlights the impact of diet-induced inflammation in a community-based sample as a whole, as well as stratified in various sociodemographic groups. This work expands our understanding of the influence of diet on pain experience and suggests that modifications to diet may be efficacious treatments for reducing chronic pain.

Source localized infraslow neurofeedback training in people with chronic painful knee osteoarthritis: A randomized, double-blind, sham-controlled feasibility clinical trial

Persistent pain is a key symptom in people living with knee osteoarthritis (KOA). Infra-slow Neurofeedback (ISF-NF) training is a recent development focusing on modulating cortical slow-wave activity to improve pain outcomes. A parallel, two-armed double-blinded, randomized sham-controlled, feasibility clinical trial aimed to determine the feasibility and safety of a novel electroencephalography-based infraslow fluctuation neurofeedback (EEG ISF-NF) training in people with KOA and determine the variability of clinical outcomes and EEG changes following NF training. Eligible participants attended nine 30-min ISF-NF training sessions involving three cortical regions linked to pain. Feasibility measures were monitored during the trial period. Pain and functional outcomes were measured at baseline, post-intervention, and follow-up after 2 weeks. Resting-state EEG was recorded at baseline and immediate post-intervention. Participants were middle-aged (61.7 ± 7.6 years), New Zealand European (90.5%), and mostly females (62%) with an average knee pain duration of 4 ± 3.4 years. The study achieved a retention rate of 91%, with 20/22 participants completing all the sessions. Participants rated high levels of acceptance and “moderate to high levels of perceived effectiveness of the training.” No serious adverse events were reported during the trial. Mean difference (95% CI) for clinical pain and function measures are as follows for pain severity [active: 0.89 ± 1.7 (−0.27 to 2.0); sham: 0.98 ± 1.1 (0.22–1.7)], pain interference [active: 0.75 ± 2.3 (−0.82 to 2.3); Sham: 0.89 ± 2.1 (−0.60 to 2.4)], pain unpleasantness [active: 2.6 ± 3.7 (0.17–5.1); sham: 2.8 ± 3 (0.62–5.0)] and physical function [active: 6.2 ± 13 (−2.6 to 15); sham: 1.6 ± 12 (−6.8 to 10)]. EEG sources demonstrated frequency-specific neuronal activity, functional connectivity, and ISF ratio changes following NF training. The findings of the study indicated that the ISF-NF training is a feasible, safe, and acceptable intervention for pain management in people with KOA, with high levels of perceived effectiveness. The study also reports the variability in clinical, brain activity, and connectivity changes following training.

Body Composition does not influence Conditioned Pain Modulation and Exercise Induced Hyperalgesia in Healthy Males and Females

BACKGROUND

Obese individuals report a higher susceptibility to chronic pain. Females are more likely to have chronic pain and excess adipose tissue. Chronic pain is associated with dysfunctional pain modulatory mechanisms. Body composition differences may be associated with pain modulation differences in males and females. The purpose of this study was to investigate body composition (lean vs fat mass) differences and pain modulatory functioning in healthy males and females.

METHODS

Pressure pain thresholds (PPT) of 96 participants (47M; 49F) were assessed in both arms and legs before and after a double footed ice bath (2°C) for 1min and an isometric knee extension, time to failure task. The difference between post and pre measures was defined conditioned pain modulatory (CPM) response (ice bath) and exercise induced hypoalgesia (EIH) response. Whole body and site-specific fat and lean tissue were assessed via DXA scan.

RESULTS

Sex differences were found in whole body lean mass (61.5±6.7kg vs 41.2±5.4kg; P<0.001) but not fat mass amount (17.2±10.5kg vs 21.0±9.7kg; P=0.068). No effect of sex was found between limb CPM (P=0.237) and limb EIH (P=0.512). When controlling for lean mass there was no significant effect of sex on CPM (P=0.732) or EIH (P=0.474) response. Similar findings were found for fat mass.

CONCLUSION

The lack of difference suggests that males and females have similar modulatory functioning. It appears that in healthy adults free from chronic pain, neither fat mass nor lean mass has an influence on endogenous pain modulatory function.

Associations between joint pathologies and central sensitization in persons with hand osteoarthritis: results from the Nor-Hand study

OBJECTIVE

Pain sensitization is associated with pain severity in persons with hand OA. What contributes to pain sensitization is unclear. This study explores whether hand OA pathologies and symptom duration are related to central sensitization.

METHOD

Participants with hand OA in the Nor-Hand study underwent bilateral hand radiography and US examination. Central sensitization was assessed with pressure pain thresholds (PPT) at remote sites (wrist, trapezius and tibialis anterior muscles) and temporal summation. We examined whether hand OA pathologies, independent of each other, including structural severity (Kellgren-Lawrence sum score, presence of erosive hand OA), inflammatory severity (greyscale synovitis and power Doppler activity sum scores) and symptom duration, were related to central sensitization, adjusting for age, sex, BMI, comorbidities and OA-severity of knee/hip.

RESULTS

In 291 participants (88% women, median age 61 years, interquartile range 57-66 years) Kellgren-Lawrence, greyscale synovitis and power Doppler activity sum scores were not associated with lower PPTs at remote sites. Persons with erosive hand OA had lower PPTs at the wrist (adjusted beta -0.75, 95% CI -1.32, -0.19) and tibialis anterior (adjusted beta -0.82, 95% CI -1.54, -0.09) and had greater temporal summation (adjusted beta 0.56, 95% CI 0.12, 1.01) compared with persons with non-erosive disease. No associations were found for symptom duration.

CONCLUSIONS

A person's overall amount of structural or inflammatory hand OA pathologies was not associated with central sensitization. Although persons with erosive hand OA showed greater signs of central sensitization, the small differences suggest that central sensitization is mainly explained by factors other than joint pathologies.

High Prevalence of Pain Sensitization in Knee Osteoarthritis: A Meta-Analysis with Meta-Regression

OBJECTIVE

The aim of this meta-analysis was to study the evidence on pain sensitization in knee osteoarthritis (OA), providing a quantitative synthesis of its prevalence and impact. Factors associated with pain sensitization were also investigated.

METHODS

Meta-analysis; PubMed (MEDLINE), Cochrane Central Register (CENTRAL), and Web of Science were searched on February 2021. Level I to level IV studies evaluating the presence of pain sensitization in patients with symptomatic knee OA, documented through a validated method (questionnaires or quantitative sensory testing), were included. The primary outcome was the prevalence of pain sensitization. Factors influencing the prevalence were also evaluated, as well as differences in terms of pain thresholds between knee OA patients and healthy controls.

RESULTS

Fifty-three articles including 7,117 patients were included. The meta-analysis of proportion documented a prevalence of pain sensitization of 20% (95% confidence interval [CI] = 16%-26%) with a significant heterogeneity of results (I² = 89%, P < 0.001). The diagnostic tool used was the main factor influencing the documented prevalence of pain sensitization (P = 0.01). Knee OA patients presented higher pain sensitivity compared with healthy controls, both in terms of local pressure pain threshold (standardized mean difference [SMD] = -1.00, 95% CI = -1.67 to -0.32, P = 0.007) and distant pressure pain threshold (SMD = -0.54, 95% CI = -0.76 to -0.31, P < 0.001).

CONCLUSIONS

Knee OA pain presents features that are consistent with a significant degree of pain sensitization. There is a high heterogeneity in the reported results, mainly based on the diagnostic tool used. The identification of the best methods to detect pain sensitization is warranted to correctly evaluate and manage symptoms of patients affected by knee OA.

REGISTRATION

PROSPERO CRD42019123347.

Pain descriptors and determinants of pain sensitivity in knee osteoarthritis: a community-based cross-sectional study

Objectives

To explore pain characteristics in individuals with knee osteoarthritis (KOA), to compare pain sensitivity across individuals with KOA, chronic back pain (CBP) and pain-free individuals (NP), and to examine the relationship between clinical and pain characteristics with pain sensitivity in KOA.

Methods

Cross-sectional, community-based online survey. Two datasets were combined comprising Dutch individuals of ≥ 40 years of age, experiencing chronic knee pain (KOA, N = 445), chronic back pain (CBP, N = 504), or no pain (NP, N = 256). Demographic and clinical characteristics, global health, physical activity/exercise, and pain characteristics including intensity, spreading, duration, quality (SF-MPQ), and sensitivity (PSQ) were assessed. Differences between (sub)groups were examined using analyses of variance or Chi-square tests. Regression analyses were performed to examine determinants of pain sensitivity in the KOA group.

Results

Quality of pain was most commonly described as aching, tender, and tiring-exhausting. Overall, the KOA group had higher levels of pain sensitivity compared to NP group, but lower levels than the CBP group. Univariately, pain intensity, its variability and spreading, global health, exercise, and having comorbidities were weakly related to pain sensitivity (standardized betas: 0.12-0.27). Symptom duration was not related to pain sensitivity. Older age, higher levels of continuous pain, lower levels of global health, and exercise uniquely contributed, albeit modest, to pain sensitivity (P&lt;0.05).

Conclusion

Continuous pain such as aching and tenderness in combination with decreased physical activity may be indicative for a subgroup of individuals at risk for pain sensitivity and, ultimately, poor treatment outcomes.

Patient Characteristics Independently Associated With Knee Osteoarthritis Symptom Severity at Initial Orthopedic Consultation

BACKGROUND/OBJECTIVE

The objective of this study was to identify patient and disease characteristics associated with the symptomatic severity of knee osteoarthritis (OA) at the time of initial knee OA diagnosis by an orthopedist.

METHODS

This medical records review included patients initially diagnosed with knee OA during 2016 to 2017 by a single orthopedic surgeon in a university-based tertiary care setting. All variables were assessed at first OA diagnosis. Main outcomes were subscales of the Knee Injury and Osteoarthritis Outcome Score-Pain, other Symptoms, knee-related quality of life, and function in daily living. Multivariable regression analyses examined the following predictors of main outcomes: sex, race, age, insurance type, body mass index, Charlson comorbidity index, and radiographic OA severity (Kellgren-Lawrence grade).

RESULTS

Of the 559 patients included in the study, most were African American (52.1%), female (71.7%), and had severe radiographic OA (Kellgren-Lawrence grade, 4; 68.7%). Female sex, African American racial/ethnic group, Medicaid insurance, younger age, and severe radiographic OA were independently statistically significantly associated with worse symptoms, pain, and function (p < 0.05 for all). Body mass index and Charlson comorbidity index were not statistically significant predictors of any outcome.

CONCLUSIONS

This study identified disparities in the perception of knee OA problems at initial orthopedist diagnosis based on sex, age, race, insurance, and radiographic OA severity. Because most of these variables are also associated with more rapid progression of OA, identifying their biopsychosocial underpinnings may help determine which interventions are most likely to redress these disparities and delay progression to end-stage knee OA.

Stress-induced analgesia: an evaluation of effects on temporal summation of pain and the role of endogenous opioid mechanisms

Acute stress reduced the initial pain rating in a temporal summation protocol via nonopioid mechanisms but did not affect temporal summation slope, an indicator of central sensitization.

Introduction:

Objectives:

Methods:

Results:

Conclusions:

Effects of neuroplasticity in people with knee osteoarthritis: A systematic review of the literature

BACKGROUND

Knee osteoarthritis (OA) is associated with chronic inflammation in somatic structures, which alters sensory afferents and leads to plastic changes in the nervous system.

METHODS

A systematic literature review was carried out, without language restrictions, period, or status of publication. The database used were Medline, EMBASE, Cochrane Library and clinicaltrials.gov. Extra bibliographic references were extracted through the discussion with specialists, and through scientific researches in conference papers.

RESULTS

The electronic search found 938 articles. When excluding duplicates and applying the inclusion/exclusion criteria, 5 studies were considered: 2 using EEG and 3 using TMS. Significant reduction of EEG activity in the cingulate medium cortex, reduction of conditioned pain modulation (CPM) in studies with EEG, as well as the occurrence of an association between pain and motor response threshold/intracortical pain facilitation in studies with TMS were observed.

CONCLUSIONS

The study contributes to a better understanding of the neurophysiological changes seen in the cingulate medium cortex, decrease in CPM and motor response threshold/intracortical pain facilitation. Advances in neuroplasticity studies may aid in the screening for early diagnosis of knee OA in the future. However, more studies are necessary.

Quantitative sensory testing: identifying pain characteristics in patients with osteoarthritis

OBJECTIVE

This review outlines the most commonly used quantitative sensory tests to identify pain sensitization. We examine cross-sectional associations between quantitative sensory testing (QST) measures and OA symptoms and severity, along with longitudinal associations between QST findings and response to surgical and non-surgical treatments for OA.

DESIGN

We conducted a search in PubMed for English language papers including 'osteoarthritis' and 'quantitative sensory testing' as search terms. Papers that did not pertain specifically to OA or QST were excluded.

RESULTS

Pressure Pain Threshold (PPT), Conditioned Pain Modulation (CPM), and Temporal Summation (TS) are the QST measures used most frequently to identify pain sensitization. Findings indicate that persons with knee OA often exhibit lower PPT thresholds, inefficient CPM, and facilitated TS as compared with controls who do not have OA, supporting the discriminant validity of QST. Pre-treatment QST has shown some success in identifying persons who experience less pain relief from surgical and non-surgical treatments for knee OA. Post-treatment QST has shown that sometimes PPT and CPM can normalize (PPT thresholds increase, and CPM becomes efficient) in patients for whom joint replacement is successful. Recent studies indicate that QST measures are more closely associated with pain severity than OA radiographic severity, suggesting that sensitization may be a trait rather than a state.

CONCLUSIONS

QST may have a role in identifying persons who are susceptible to chronic pain and may offer an opportunity for personalized, more effective treatment of OA.

Association of Weight Change and Physical Activity with Knee Pain and Health-Related Quality of Life in East Asian Women Aged 50 Years and Older with Knee Osteoarthritis: A Population-Based Study

bjectives: This study aimed to investigate the association of self-reported weight change and physical activity with the level of knee pain and health-related quality of life in East Asian women with knee osteoarthritis using population-based data.Methods: A total of 564 women (mean age, 68.2 years, standard deviation, 8.9 years) aged 50 years or older with knee osteoarthritis (Kellgren-Lawrence (K-L) grade ≥ 2) were included in the data analyses from the fifth Korea National Health and Nutrition Examination Survey. Data regarding the radiographic grade, self-reported weight change during the past year, physical activity, level of knee pain, and health-related quality of life (EuroQOL five- dimension (EQ-5D) index) were collected. Multiple regression analysis was performed to identify factors significantly associated with the level of knee pain and health-related quality of life in subgroups according to the body mass index (BMI) range (≤ 22.5, between 22.6 and 27.5, and > 27.5 kg/m ² ).Results: In the whole group, the level of knee pain was significantly associated with K-L grade (P < 0.001), and EQ-5D was negatively associated with age (P < 0.001), the level of knee pain (P < 0.001), and weekly hours of vigorous-intensity activity (P = 0.026). In the subgroup analysis, weight gain showed significant association with the level of knee pain only in women with 22.5 kg/m² < BMI ≤ 27.5 kg/m² (P = 0.006). Weight gain showed significant association with EQ-5D in women with BMI ≤ 22.5 kg/m²(P = 0.047) whereas weekly hours of moderate-intensity activity was negatively associated with EQ-5D in women with BMI > 27.5 kg/m² .Conclusions: The association of weight change and physical activity with knee pain and health-related quality of life might be different according to BMI ranges. Well-designed interventions to improve both knee pain and health-related quality of life need to be investigated in future studies that would strictly control physical activity, diet, and weight changes.

Psychological profiles in adults with knee OA-related pain: a replication study

Introduction

Psychological factors have been associated with knee osteoarthritis pain severity and treatment outcomes, yet their combined contribution to phenotypic heterogeneity is poorly understood. In particular, empirically derived psychological profiles must be replicated before they can be targeted or considered for treatment studies. The objectives of this study were to (1) confirm previously identified psychological profiles using unsupervised clustering methods in persons with knee osteoarthritis pain, (2) determine the replicability of profiles using supervised machine learning in a different sample, and (3) examine associations with clinical pain, brain structure, and experimental pain.

Methods

Participants included two cohorts of individuals with knee osteoarthritis pain recruited as part of the multisite UPLOAD1 (n = 270, mean age = 56.8 ± 7.6, male = 37%) and UPLOAD2 (n = 164, mean age = 57.73 ± 7.8, male = 36%) studies. Similar psychological constructs (e.g. optimism, coping, somatization, affect, depression, and anxiety), sociodemographic and clinical characteristics, and somatosensory function were assessed across samples. UPLOAD2 participants also completed brain magnetic resonance imaging. Unsupervised hierarchical clustering analysis was first conducted in UPLOAD1 data to derive clusters, followed by supervised linear discriminative analysis to predict group membership in UPLOAD2 data. Associations among cluster membership and clinical variables were assessed, controlling for age, sex, education, ethnicity/race, study site, and number of pain sites.

Results

Four distinct profiles emerged in UPLOAD1 and were replicated in UPLOAD2. Identified psychological profiles were associated with psychological variables (ps < 0.001), and clinical outcomes (ps = 0.001-0.03), indicating good internal and external validation of the cluster solution. Significant associations between psychological profiles and somatosensory function and brain structure were also found.

Conclusions

This study highlights the importance of considering the biopsychosocial model in knee osteoarthritis pain assessment and treatment.

Quantitative Sensory Testing Protocols to Evaluate Central and Peripheral Sensitization in Knee OA: A Scoping Review

OBJECTIVE

This scoping review analyzed various quantitative sensory testing methodologies used in the assessment of sensitization and how sensitization is defined in people with knee osteoarthritis.

DESIGN

A scoping review.

SETTING

All clinical and research settings.

SUBJECTS

Non-surgical adults with knee osteoarthritis.

METHODS

This scoping review was guided by existing scoping review methodologies. Relevant studies were extracted from the following electronic databases: Medical Literature Analysis and Retrieval System Online, Excerpta Medica Database, Allied and Complementary Medicine Database and the Cumulative Index to Nursing Allied Health Literature. Abstract and full article screening and data extraction were performed in pairs. Information on quantitative sensory testing techniques and parameters was extracted and summarized in tables. General and technique specific definitions of sensitization were extracted from included texts.

RESULTS

Our search yielded 4199 articles, of which 50 were included in our review. The most common quantitative sensory test was pressure pain threshold. In total 28 unique testing sites were found speaking to the high degree of variability between studies. Sensitization was poorly defined with only 8 studies fully operationalizing it, 22 partially, and the remainder did not provide sufficient information to meet our criteria.

CONCLUSION

This scoping review has provided an overview of the most common methods of quantitative sensory testing being implemented in the assessment of nervous system sensitization to nociceptive signaling in people with knee osteoarthritis. This study provides a foundation for future development of quantitative sensory testing methodology for research and clinical practice in the osteoarthritis population.

Scoping Review of Pain and Patient Characteristics and Physical Function Associated with Intermittent and Constant Pain in People with Knee Osteoarthritis

Purpose: We identified the pain and patient characteristics and physical function associated with intermittent and constant pain in people with knee osteoarthritis (OA). Method: The search strategy was conducted in MEDLINE, CINAHL, Embase, SPORTDiscus, Cochrane, and the Physiotherapy Evidence Database for articles published up to October 2018. We included observational studies, randomized controlled trials, reviews, qualitative studies, and case reports in English and French of adults with knee OA. Articles were screened by two independent reviewers, and disagreements were resolved by a third researcher. Results: Our search yielded 1,232 articles, and 4 articles were included in the review. Constant pain was related to limitations in daily activities, a higher Kellgren and Lawrence grade, a higher prevalence of comorbidities, and symptomatic knee OA and depressive symptoms. Intermittent pain was associated with limitations in high-impact activities, but greater time spent in moderate activities was reported. The evidence is contradictory as to whether intermittent or constant pain has a greater average severity. Conclusions: There was limited evidence available to determine the association of pain and patient characteristics and physical function with constant and intermittent pain in people with knee OA. Future research will help improve our understanding of these two pain patterns by examining their associations across a variety of pain and functional outcomes in people with knee OA.

The Central Aspects of Pain in the Knee (CAP-Knee) questionnaire; a mixed-methods study of a self-report instrument for assessing central mechanisms in people with knee pain

OBJECTIVES

Pain is the prevailing symptom of knee osteoarthritis. Central sensitisation creates discordance between pain and joint pathology. We previously reported a Central Pain Mechanisms trait derived from eight discrete characteristics: Neuropathic-like pain, Fatigue, Cognitive-impact, Catastrophising, Anxiety, Sleep disturbance, Depression, and Pain distribution. We here validate and show that an 8-item questionnaire, Central Aspects of Pain in the Knee (CAP-Knee) is associated both with sensory- and affective- components of knee pain severity.

METHODS

Participants with knee pain were recruited from the Investigating Musculoskeletal Health and Wellbeing study in the East Midlands, UK. CAP-Knee items were refined following cognitive interviews. Psychometric properties were assessed in 250 participants using Rasch-, and factor-analysis, and Cronbach's alpha. Intra-class correlation coefficients tested repeatability. Associations between CAP-Knee and McGill Pain questionnaire pain severity scores were assessed using linear regression.

RESULTS

CAP-Knee targeted the knee pain sample well. Cognitive interviews indicated that participants interpreted CAP-Knee items in diverse ways, which aligned to their intended meanings. Fit to the Rasch model was optimised by rescoring each item, producing a summated score from 0 to 16. Internal consistency was acceptable (Cronbach's alpha = 0.74) and test-retest reliability was excellent (ICC2,1 = 0.91). Each CAP-Knee item contributed uniquely to one discrete 'Central Mechanisms trait' factor. High CAP-Knee scores associated with worse overall knee pain intensity, and with each of sensory- and affective- McGill Pain Questionnaire scores.

CONCLUSION

CAP-Knee is a simple and valid self-report questionnaire, which measures a single 'Central Mechanisms' trait, and may help identify and target centrally-acting treatments aiming to reduce the burden of knee pain.

Sex Differences, Sleep Disturbance and Risk of Persistent Pain Associated With Groin Hernia Surgery: A Nationwide Register-Based Cohort Study

Persistent pain after groin hernia repair is a major health problem. Sleep disturbance is associated with heightened pain sensitivity. The main objective of this study was to examine the role of sleep disturbance in the development and long-term maintenance of chronic postherniorrhaphy inguinal pain (CPIP), with exploration of sex differences. From 2012 to 2017, a national cohort of patients with prior groin hernia repair (n = 2084;45.8% females) were assessed for the development of CPIP 12 months after surgery. Patients then underwent long-term (median 5.0 years) follow-up to evaluate the contribution of sex and sleep disturbance on the maintenance of CPIP. Associations between pre- and postoperative sleep problems (assessed at long-term follow-up) and CPIP were tested using logistic regression. Females had higher rates of CPIP with negative impact on daily activities 12 months after surgery as compared to males (14.6 vs 9.2%, P < .0005), and were more likely to have moderate-severe CPIP in the long-term (3.1 vs 1.2%, P = .003). Preoperative sleep problems predicted development of CPIP 12 months after surgery (adjusted odds ratio [aOR] 1.76 [95%CI 1.26-2.46], P = .001) and CPIP in the long-term (aOR 2.20 [1.61-3.00] , P < .0001). CPIP was associated with insomnia and depression. Sleep disturbance may increase the risk for CPIP, and contribute to maintenance of postsurgical pain. PERSPECTIVE: Females are at heightened risk for CPIP as compared to males. Increased severity of pain symptoms are linked to poorer sleep and psychiatric morbidity. Given the robust associations between sleep disturbance and CPIP, interventions which consolidate and promote sleep, especially in females, may improve long-term pain control.

Sex-dependent Cav2.3 channel contribution to the secondary hyperalgesia in a mice model of central sensitization

Central sensitization (CS) is characteristic of difficult to treat painful conditions, such as fibromyalgia and neuropathies and have sexual dimorphism involved. The calcium influx in nociceptive neurons is a key trigger for CS and the role of Cav2.1 and Cav2.2 voltage gated calcium channels (VGCC) in this role were evidenced with the use of ω-agatoxin IVA and ω-agatoxin MVIIA blockers, respectively. However, the participation of the α1 subunit of the voltage-gated channel Cav2.3, which conducts R-type currents, in CS is unknown. Furthermore, the role of sexual differences in painful conditions is still poorly understood. Thus, we investigated the role of Cav2.3 in capsaicin-induced secondary hyperalgesia in mice, which serve as a CS model predictive of the efficacy of novel analgesic drugs. Capsaicin injection in C57BL/6 mice caused secondary hyperalgesia from one to five hours after injection, and the effects were similar in male and female mice. In female but not male mice, intrathecal treatment with the Cav2.3 inhibitor SNX-482 partially and briefly reversed secondary hyperalgesia at a dose (300 pmol/site) that did not cause adverse effects. Moreover, Cav2.3 expression in the dorsal root ganglia (DRG) and spinal cord was reduced by intrathecal treatment with an antisense oligonucleotide (ASO) targeting Cav2.3 in female and male mice. However, ASO treatment was able to provide a robust and durable prevention of secondary hyperalgesia caused by capsaicin in female mice, but not in male mice. Thus, our results demonstrate that Cav2.3 inhibition, especially in female mice, has a relevant impact on a model of CS. Our results provide a proof of concept for Cav2.3 as a molecular target. In addition, the result associated to the role of differences in painful conditions linked to sex opens a range of possibilities to be explored and needs more attention. Thus, the relevance of testing Cav2.3 inhibition or knockdown in clinically relevant pain models is needed.

The physiopathology of osteoarthritis: Paleopathological implications of non-articular lesions from a modern surgical sample

OBJECTIVES

This research focused on osteoarthritis (OA) lesions on modern patients to 1) identify consistently observed lesions not included within current paleopathological measures of OA, 2) assess the correspondence of bone and cartilage lesions with clinical OA diagnostic criteria, and 3) discuss the correspondence of bone lesions with sources of pain reported in clinical literature.

MATERIALS

Tibial plateaus from 62 patients undergoing total knee replacement surgery due to OA were examined.

METHODS

Plateaus were scored for several non-standard OA criteria, including non-articular and X-ray visible lesions and pre-maceration cartilage lesions, as well as articular surface criteria standard in paleopathology.

RESULTS

Proliferative bone in the intercondylar region was present in 95 % of specimens, while areas of dense trabecular bone and lytic defects, both on the inferior side of the plateaus, were present in 98 % and 83 %, respectively.

CONCLUSIONS

The inferior lytic defects may be physical evidence of bone marrow lesions (BML), a clinical OA indicator visible via MRI. Previous research has linked BML to pain, inflammation, and ligament pathology. The latter conditions have also been associated with intercondylar enthesophytes and third intercondylar tubercle of Parsons (TITP), both of which were observed in the intercondylar regions.

SIGNIFICANCE

Several non-articular lesions not currently included in paleopathological measures of OA were consistently observed.

SUGGESTIONS FOR FUTURE RESEARCH

A similar analysis of a control sample of non-OA tibial plateaus would better contextualize these results.

LIMITATIONS

The sample's high average age (65.8 years) and severe OA stage may hamper generalizability to archaeological collections.

Poor diet quality is associated with self-reported knee pain in community-dwelling women aged 50 years and older

In the current study, we aimed to examine the association between knee pain and diet quality in women aged ≥ 50 years using data from the Korea National Health and Nutrition Examination Survey. This was a population-based, cross-sectional study. Diet quality was assessed using the Diet Quality Index-International (DQI-I), and knee pain and osteoarthritis were self-reported. A multivariate logistic regression model was used to adjust for age, body mass index, household income, marital status, education, occupation, smoking status, hazardous alcohol use, regular physical activity, menopause, and chronic diseases, including hypertension, diabetes, dyslipidemia, osteoarthritis, and depression. A total of 3,881 women were included in this study, and the prevalence of knee pain was 25.4%. The intakes of total energy, protein, and fat were lower in women with knee pain than in those without (all P < 0.01), while the carbohydrate intake was higher (P = 0.01). No significant differences were noted in the scores for variety, overall balance, and moderation components, except for the item of total fat intake, between the DQI-I scores for women with and without knee pain, after adjusting for age. Women without knee pain showed higher scores in several items of the adequacy component (P < 0.05) than did women with knee pain. The total DQI-I scores were lower in women with knee pain than in women without knee pain, after adjusting for covariates, including osteoarthritis (OR = 0.985, 95% CI = 0.973–0.997, P = 0.01). Knee pain independent of osteoarthritis was associated with poor diet quality in community-dwelling women aged ≥ 50 years.

Understanding the Impact of Sex and Gender in Osteoarthritis: Assessing Research Gaps and Unmet Needs

Osteoarthritis (OA) affects more than 300 million individuals globally, with higher prevalence in women than in men. In addition, OA affects women and men differently, with women demonstrating both increased disease severity and disability. The Society for Women's Health Research (SWHR) convened an interdisciplinary group of expert researchers and clinicians for a roundtable meeting to review the current state of the science on OA and to identify knowledge gaps in the scientific literature, especially as they relate to the topics of sex and gender. The current review summarizes discussions from the roundtable and prioritizes areas of need that warrant further attention in OA research, diagnosis, care, and education. Improvements in basic and clinical research, clinical practice, patient education, and policy are needed to allow for better understanding as to the pathogenesis of sex- and gender-related disparities in OA.

Absence of pain in subjects with advanced radiographic knee osteoarthritis

Background

To investigate the frequency of pain among subjects with advanced radiographic knee osteoarthritis (OA) defined as Kellgren–Lawrence (KL) grade 4 and clinical features associated with pain.

Methods

Subjects from the Hallym Aging Study (HAS), the Korean National Health and Nutrition Examination Survey (KNHANES), and the Osteoarthritis Initiative (OAI) were included. Participants were asked knee-specific questions regarding the presence of knee pain. Clinical characteristics associated with the presence of pain were evaluated with multivariable logistic regression analysis.

Results

The study population consisted of 504, 10,152 and 4796 subjects from HAS, KNHANES, and OAI, respectively. KL grade 4 OA was identified in 9.3, 7.6, and 11.5% of subjects, while pain was absent in 23.5, 31.2, and 5.9% of subjects in KL grade 4 knee OA, respectively. After multivariable analysis, female gender showed a significant association with pain in the KNHANES group, while in the OAI group, younger age did. Advanced knee OA patients without pain did not differ from non-OA subjects in most items of SF-12 in both Korean and OAI subjects. Total WOMAC score was not significantly different between non-OA and advanced knee OA subjects without pain in the OAI.

Conclusions

Our study showed that a considerable number of subjects with KL grade 4 OA did not report pain. In patients whose pain arises from causes other than structural damage of the joint, therapeutic decision based on knee X-ray would lead to suboptimal result. In addition, treatment options focusing solely on cartilage engineering, should be viewed with caution.

Sleep and pain management: a review

There is a complex interplay between sleep disturbance and patients in pain. There is an increasing appreciation of the direct effects of analgesic drugs and sleep quality. This review provides an overview of the effects of different analgesic drugs and their effects on phases of sleep. The effects of different pain conditions and their direct effects on sleep physiology are also discussed. A structured search of the scientific literature using MEDLINE and PubMed databases. Original human and animal studies were included. A multi-search term strategy was employed. An appreciation of the physiological effects of these drugs will allow a more considered prescription of them to better manage sleep disturbance.

Sex and Gender are Not the Same: Why Identity Is Important for People Living with HIV and Chronic Pain

Background

Sex differences in pain sensitivity have been well documented, such that women often report greater sensitivity than men. However, clinical reports highlighting sex differences often equate gender and sex. This is a particularly critical oversight for those whose gender identity is different than their genetic sex.

Methods

This preliminary study sets to analyze differences in pain responses between cisgender and transgender individuals living with HIV and chronic pain. A total of 51 African-American participants (24 cisgender men, 20 cisgender women, 7 transgender women) with similar socioeconomic status were recruited. Genetic sex, gender identity, depression and anxiety, pain severity, pain interference and pain-related stigma were recorded. Participants also completed a quantitative sensory testing battery to assess pain in response to noxious heat and mechanical stimuli.

Results

Transgender women and cisgender women demonstrated a greater magnitude of temporal summation for heat pain stimuli or mechanical stimuli compared to cisgender men. Specifically, transgender women reported greater mechanical summation than either cisgender women or cisgender men. Transgender women and cisgender women similarly reported greater chronic pain severity compared to cisgender men.

Conclusion

These data support the notion that gender identity may play a more significant role in pain sensation than genetic sex. These results further maintain that not only gender identity and genetic sex are distinct variables but that treatment should be based on identity as opposed to genetic sex.

Pain and knee damage in male and female mice in the medial meniscal transection-induced osteoarthritis

OBJECTIVE

To investigate sex effects on pain-related behaviors in the medial meniscal transection (MMT) knee osteoarthritis (OA) model.

METHODS

Experiments were performed in male and female C57BL/6J mice (12/group/sex). MMT was induced by transection of the medial collateral ligament and the medial meniscus. Sham-operated and naïve mice served as controls. Mechanical and heat sensitivity in hind paws, hind limb use, and locomotor activity were measured for 3 months. Knee histology was performed on week 12.

RESULTS

In males, MMT triggered a bi-phasic mechanical hypersensitivity and decreased load on OA limb, with an acute post-operative (1-5 days) and chronic (3-12 weeks) OA phases separated by a remission in the intermediate phase (1-2 weeks). Females showed a less pronounced bi-phasic pattern, with a greater mechanical hypersensitivity, but not poorer limb use, than males in the intermediate phase (maximal difference: 1.1 g, 95% confidence intervals (CI) [0.7, 1.5]). There were no major sex differences in the chronic phase. MMT did not induce heat hypersensitivity or change in locomotor activity in the chronic phase in both sexes. MMT caused more severe cartilage damage in males than in females (maximal difference: 1.1 score points, 95% CI [1.9, 0.3]), and a comparable between sexes osteophyte formation. The knee damage did not correlate with pain.

CONCLUSIONS

MMT modelled human knee OA well, capturing cartilage destruction and osteophyte formation, mechanical pain, and poorer limb use in both sexes. Sex differences in pain were modality- and time-dependent, reflecting complex sex-related features of human OA.

Pain thresholds and intensities of CRPS type I and neuropathic pain in respect to sex

BACKGROUND AND AIMS

Healthy women have generally been found to have increased experimental pain perception and chronic pain has a higher prevalence in female as compared to male patients. However, no study has investigated whether pain intensity and pain perception thresholds are distinct or similar between sexes within various chronic pain entities. We investigated whether average pain intensities and pain thresholds assessed using quantitative sensory testing (QST) differed between women and men suffering from three distinct chronic pain conditions: Complex Regional Pain Syndrome (CRPS type I), peripheral nerve injury (PNI) or polyneuropathy (PNP), as compared to paired healthy volunteers.

METHODS

QST data of 1,252 patients (669 female, 583 male) with PNI (n = 342), PNP (n = 571) or CRPS (n = 339), and average pain intensity reports from previously published studies were included. Absolute and z-values (adjusted for age and body region) of cold, heat, pressure (PPT) and pinprick pain thresholds were compared in generalized linear models with aetiology, duration of underlying pain disease and average pain intensity as fixed effects.

RESULTS

Average pain intensity during the past four weeks did not differ between women and men, in both mean and range. In women absolute pain thresholds for cold, heat and pinprick were lower than in males across all diagnoses (p < .05). However, after z-transformation these differences disappeared except for PPT in CRPS (p = .001).

DISCUSSION

Pain thresholds in patients show only minor sex differences. However, these differences mimic those observed in healthy subjects and do not seem to be linked to specific pathophysiological processes.

SIGNIFICANCE

Female healthy participants and female patients with neuropathic pain conditions or CRPS I report lower pain thresholds compared to males, but pain intensity is similar and there is no sex difference in the extent to which the thresholds are altered in neuropathic pain or CRPS. Thus, the sex differences observed in various chronic pain conditions mimic those obtained in healthy participants, indicating that these differences are not linked to specific pathophysiological processes and are of minor clinical relevance.