Morphological and immunohistochemical examination of nerves in normal and injured collateral ligaments of rat, rabbit, and human knee joints

Central Sensitization and Neuropathic Pain Cumulatively Affect Patients Reporting Inferior Outcomes Following Total Knee Arthroplasty

UPDATE

This article was updated on November 17, 2023, because of previous errors, which were discovered after the preliminary version of the article was posted online. On page 102, the text that had read "In a post hoc analysis of the preoperative results, Group 1 showed significantly inferior WOMAC pain, function, and total scores compared with Group 4 (p < 0.05 for all). Groups 2 and 3 showed worse preoperative WOMAC pain, function, and total subscores compared with Group 4 (p < 0.05 for all). These results remained the same at 2 years after surgery." now reads "In a post hoc analysis of the preoperative results, Groups 1, 2, and 3 showed significantly inferior WOMAC pain, function, and total scores compared with Group 4 (p < 0.05 for all). At 2 years postoperatively, Group 1 showed inferior WOMAC pain, function, and total scores compared with the other groups (p < 0.05 for all). Also, Groups 2 and 3 had worse WOMAC pain, function and total scores compared with Group 4 (p < 0.05 for all)." Also, on page 106, the title of Table IV, which had previously read "Inter-Group Comparison of Preoperative Scores (Post Hoc Analysis)" now reads "Inter-Group Comparison of Postoperative Scores (Post Hoc Analysis)."

Autonomic Nervous System Dysregulation and Osteoarthritis Pain: Mechanisms, Measurement, and Future Outlook

PURPOSE OF REVIEW

The autonomic nervous system is an important regulator of stress responses and exhibits functional changes in chronic pain states. This review discusses potential overlap among autonomic dysregulation, osteoarthritis (OA) progression, and chronic pain. From this foundation, we then discuss preclinical to clinical research opportunities to close gaps in our knowledge of autonomic dysregulation and OA. Finally, we consider the potential to generate new therapies for OA pain via modulation of the autonomic nervous system.

RECENT FINDINGS

Recent reviews provide a framework for the autonomic nervous system in OA progression; however, research is still limited on the topic. In other chronic pain states, functional overlaps between the central autonomic network and pain processing centers in the brain suggest relationships between concomitant dysregulation of the two systems. Non-pharmacological therapeutics, such as vagus nerve stimulation, mindfulness-based meditation, and exercise, have shown promise in alleviating painful symptoms of joint diseases, and these interventions may be partially mediated through the autonomic nervous system. The autonomic nervous system appears to be dysregulated in OA progression, and further research on rebalancing autonomic function may lead to novel therapeutic strategies for treating OA pain.

Real-world evidence of mesenchymal stem cell therapy in knee osteoarthritis: a large prospective two-year case series

Objective: To evaluate the long-term safety and efficacy of adipose-derived mesenchymal stem cell (ADMSC) therapy in the treatment of knee osteoarthritis (OA). Methods: 329 participants with knee OA underwent intra-articular ADMSC therapy. Participants were followed up for 24 months and were separated based on radiological OA grade. Results: Treatment was well tolerated with no related serious adverse events. All participant groups reported clinically and statistically significant pain improvement. Clinical outcome was not influenced by patients' age or BMI. Conclusion: ADMSC therapy is an effective, safe and long-lasting treatment option for knee OA with the potential to delay total joint replacement. In addition to the observed clinical benefits, ADMSC therapy promises to reduce the global economic burden of OA. Trial registration number: ACTRN12617000638336.

Joint Damage and Neuropathic Pain in Rats Treated With Lysophosphatidic Acid

Joint pain is a complex phenomenon that involves multiple endogenous mediators and pathophysiological events. In addition to nociceptive and inflammatory pain, some patients report neuropathic-like pain symptoms. Examination of arthritic joints from humans and preclinical animal models have revealed axonal damage which is likely the source of the neuropathic pain. The mediators responsible for joint peripheral neuropathy are obscure, but lysophosphatidic acid (LPA) has emerged as a leading candidate target. In the present study, male and female Wistar rats received an intra-articular injection of LPA into the right knee and allowed to recover for 28 days. Joint pain was measured by von Frey hair algesiometry, while joint pathology was determined by scoring of histological sections. Both male and female rats showed comparable degenerative changes to the LPA-treated knee including chondrocyte death, focal bone erosion, and synovitis. Mechanical withdrawal thresholds decreased by 20-30% indicative of secondary allodynia in the affected limb; however, there was no significant difference in pain sensitivity between the sexes. Treatment of LPA animals with the neuropathic pain drug amitriptyline reduced joint pain for over 2 hours with no sex differences being observed. In summary, intra-articular injection of LPA causes joint degeneration and neuropathic pain thereby mimicking some of the characteristics of neuropathic osteoarthritis.

Methylene blue prevents osteoarthritis progression and relieves pain in rats via upregulation of Nrf2/PRDX1

Oxidative stress-related cartilage degeneration, synovitis, and joint pain play vital roles in the progress of osteoarthritis (OA). Anti-oxidative stress agents not only prevent structural damage progression but also relieve OA-related pain. In this study, we investigated the therapeutic effect of methylene blue (MB), a classical and important anti-oxidant with strong neural affinity. Experimental OA was established in rats by radial transection of medial collateral ligament and medial meniscus (MCLT + MMT) of the right knee joint. The OA rats received intra-articular injection of MB (1 mg/kg) every week starting one week after surgery. We showed that MB administration exerted significant cartilage protection, synovitis inhibition as well as pain relief in OA rats. In human chondrocytes and fibroblast-like synoviocytes, MB significantly attenuated tert-butyl hydroperoxide (TBHP)-induced inflammatory response and oxidative stress. We demonstrated that these effects of MB resulted from dual targets of important antioxidant enzymes, Nrf2 and PRDX1, which also mutually reinforcing and participated in an interaction. Furthermore, we found that calcitonin gene-related peptide (CGRP), a neural inflammatory mediator, was accumulated around the vessel in synovium and subchondral bone in OA rats and in TBHP-treated primary cortical neurons; MB administration significantly inhibited CGRP expression through upregulation of Nrf2 and PRDX1. Taken together, these results suggest that MB ameliorates oxidative stress via Nrf2/PRDX1 regulation to prevent progression and relieve pain of OA.

Prevalence of Persistent Pain of the Neuropathic Subtype after Total Hip or Knee Arthroplasty

Purpose: This study aimed to (1) estimate the point prevalence of persistent postoperative pain (PPP) identified using the Self-Administered Leeds Assessment of Neuropathic Symptoms and Signs (S-LANSS) after unilateral primary total hip arthroplasty (THA) and total knee arthroplasty (TKA) using data from a registry of total joint arthroplasty (TJA) patients in Ontario, (2) estimate the effect of PPP on function, (3) estimate the prevalence of neuropathic pain (NP) features among patients with persistent pain, (4) determine participant characteristics in order to estimate the potential predictors of NP classification among individuals with persistent pain after TJA, (5) estimate the extent to which the estimates of prevalence depended on the measure used (i.e., S-LANSS vs. NP sub-scale of the Short-Form McGill Pain Questionnaire 2 [NP-SF-MPQ-2]), and (6) determine the difference in characteristics between those with and without NP. Method: This was a prospective follow-up study of a historical cohort of individuals who had undergone primary unilateral THA or TKA. Persistent pain was operationally defined as pain rated as 3 or more (out of 5) on the Oxford Pain Questionnaire 6 months or 1 year after THA or TKA. Participants with persistent pain completed the S-LANSS and the NP-SF-MPQ-2. Results: A total of 1,143 participants were identified as having had a TJA, 148 (13%) of whom had PPP. A total of 67 recipients completed the S-LANSS and the NP-SF-MPQ-2. Of these, an NP subtype was identified among 19 (28%; those with an S-LANSS score ≥ 12) to 29 (43%; those with an NP-SF-MPQ-2 score ≥ 0.91). Individuals with persistent pain of the NP subtype after TJA reported severe pain intensity and higher disability levels 1.5–3.5 years after surgery compared with those without persistent pain. Conclusions: A significant proportion of patients have persistent pain post-unilateral THA or TKA.

Peripheral pain mechanisms in osteoarthritis

There is a well-established historical observation that structural joint damage by plain X-ray correlates poorly with symptomatic disease in osteoarthritis (OA). This is often attributed to the inability to visualise soft-tissue pathology within the joint and the recognition of heterogeneous patient factors that drive central pain sensitisation. A major issue is the relative paucity of mechanistic studies in which molecular pathogenesis of pain is interrogated in relation to tissue pathology. Nonetheless, in recent years, three broad approaches have been deployed to attempt to address this: correlative clinical studies of peripheral and central pain outcomes using magnetic resonance imaging, where soft-tissue processes can be visualised; molecular studies on tissue from patients with OA; and careful molecular interrogation of preclinical models of OA across the disease time course. Studies have taken advantage of established clinical molecular targets such as nerve growth factor. Not only is the regulation of nerve growth factor within the joint being used to explore the relationship between tissue pathology and the origins of pain in OA, but it also provides a core model on which other molecules present within the joint can modulate the pain response. In this narrative review, how molecular and pathological tissue change relates to joint pain in OA will be discussed. Finally, a model for how tissue damage may lead to pain over the disease course will be proposed.

Basic Mechanisms of Pain in Osteoarthritis: Experimental Observations and New Perspectives

The specific changes in the peripheral neuronal pathways underlying joint pain in osteoarthritis are the focus of this review. The plasticity of the nociceptive system in osteoarthritis and how this involves changes in the structural, physiologic, and genetic properties of neurons in pain pathways are discussed. The role of the neurotrophin, nerve growth factor, in these pathogenic processes is discussed. Finally, how neuronal pathways are modified by interaction with the degenerating joint tissues they innervate and with the innate immune system is considered. These extensive cellular interactions provide a substrate for identification of targets for osteoarthritis pain.

Cannabinoid control of neurogenic inflammation

A significant number of cannabinoids are known to have analgesic and anti-inflammatory properties in various diseases. Due to their presynaptic/terminal location, cannabinoid receptors can inhibit synaptic transmission and have the potential to regulate neurogenic inflammation. Neurogenic inflammation occurs when a noxious signal is detected in the periphery initiating an antidromic axon reflex in the same sensory neurone leading to depolarization of the afferent terminal. Neuropeptides are subsequently released and contribute to vasodilation, plasma extravasation and modulation of immune cells. Endocannabinoids, synthetic cannabinoids and phytocannabinoids can reduce neuroinflammation by inhibiting afferent firing and inflammatory neuropeptide release. Thus, in addition to a direct effect on vascular smooth muscle and inflammatory cells, cannabinoids can reduce inflammation by silencing small diameter neurones. This review examines the neuropharmacological processes involved in regulating antidromic depolarization of afferent nerve terminals by cannabinoids and the control of neurogenic inflammation in different diseases.

Cruciate ligament healing and injury prevention in the age of regenerative medicine and technostress: homeostasis revisited

PURPOSE

This clinical concepts paper discusses the essential elements of cruciate ligament recuperation, micro-trauma repair, and remodeling.

METHODS

Cruciate ligament mechanobiology and tissue heterogeneity, anatomy and vascularity, and synovial membrane and fluid functions are discussed in relationship to deficiency-induced inflammatory responses, nervous and immune system function, recuperation, repair and remodeling, and modern threats to homeostasis.

RESULTS

Cruciate ligament surgical procedures do not appreciate the vital linked functions of the central, peripheral, and autonomic nervous systems and immune system function on knee ligament injury recuperation, micro-trauma repair, and remodeling. Enhanced knowledge of these systems could provide innovative ways to decrease primary non-contact knee injury rates and improve outcomes following reconstruction or primary repair.

CONCLUSIONS

Restoration of knee joint homeostasis is essential to cruciate ligament recuperation, micro-trauma repair, and remodeling. The nervous and immune systems are intricately involved in this process. Varying combinations of high-intensity training, under-recovery, technostress, and environmental pollutants (including noise) regularly expose many athletically active individuals to factors that abrogate the environment needed for cruciate ligament recuperation, micro-trauma repair, and remodeling. Current sports training practice, lifestyle psychobehaviors, and environmental factors combine to increase both primary non-contact knee injury risk and the nervous and immune system dysregulation that lead to poor sleep, increased anxiety, and poorly regulated hormone and cytokine levels. These factors may create a worst-case scenario leading to poor ligament recuperation, micro-trauma repair, and remodeling. Early recognition and modification of these factors may decrease knee ligament injury rates and improve cruciate ligament repair or reconstruction outcomes.

LEVEL OF EVIDENCE

V.

Enhanced T-type calcium channel 3.2 activity in sensory neurons contributes to neuropathic-like pain of monosodium iodoacetate-induced knee osteoarthritis

The monosodium iodoacetate knee osteoarthritis model has been widely used for the evaluation of osteoarthritis pain, but the pathogenesis of associated chronic pain is not fully understood. The T-type calcium channel 3.2 (CaV3.2) is abundantly expressed in the primary sensory neurons, in which it regulates neuronal excitability at both the somata and peripheral terminals and facilitates spontaneous neurotransmitter release at the spinal terminals. In this study, we investigated the involvement of primary sensory neuron-CaV3.2 activation in monosodium iodoacetate osteoarthritis pain. Knee joint osteoarthritis pain was induced by intra-articular injection of monosodium iodoacetate (2 mg) in rats, and sensory behavior was evaluated for 35 days. At that time, knee joint structural histology, primary sensory neuron injury, and inflammatory gliosis in lumbar dorsal root ganglia, and spinal dorsal horn were examined. Primary sensory neuron-T-type calcium channel current by patch-clamp recording and CaV3.2 expression by immunohistochemistry and immunoblots were determined. In a subset of animals, pain relief by CaV3.2 inhibition after delivery of CaV3.2 inhibitor TTA-P2 into sciatic nerve was investigated. Knee injection of monosodium iodoacetate resulted in osteoarthritis histopathology, weight-bearing asymmetry, sensory hypersensitivity of the ipsilateral hindpaw, and inflammatory gliosis in the ipsilateral dorsal root ganglia, sciatic nerve, and spinal dorsal horn. Neuronal injury marker ATF-3 was extensively upregulated in primary sensory neurons, suggesting that neuronal damage was beyond merely knee-innervating primary sensory neurons. T-type current in dissociated primary sensory neurons from lumbar dorsal root ganglia of monosodium iodoacetate rats was significantly increased, and CaV3.2 protein levels in the dorsal root ganglia and spinal dorsal horn ipsilateral to monosodium iodoacetate by immunoblots were significantly increased, compared to controls. Perineural application of TTA-P2 into the ipsilateral sciatic nerve alleviated mechanical hypersensitivity and weight-bearing asymmetry in monosodium iodoacetate osteoarthritis rats. Overall, our findings demonstrate an elevated CaV3.2 expression and enhanced function of primary sensory neuron-T channels in the monosodium iodoacetate osteoarthritis pain. Further study is needed to delineate the importance of dysfunctional primary sensory neuron-CaV3.2 in osteoarthritis pain.

The nociceptive innervation of the normal and osteoarthritic mouse knee

OBJECTIVES

To document the nociceptive innervation of the normal and osteoarthritic murine knee.

METHODS

Knees were collected from naïve male C57BL/6 Na_V1.8-tdTomato reporter mice aged 10, 26, and 52 weeks (n = 5/group). Destabilization of the medial meniscus (DMM) or sham surgeries (n = 5/group) were performed in the right knee of 10-week old male Na_V1.8-tdTomato mice, and knees were harvested 16 weeks later. Twenty 20-μm frozen sections from a 400-μm mid-joint region were collected for confocal microscopy. Integrated density of the tdTomato signal was calculated using Image J by two independent observers blinded to the groups. Consecutive sections were stained with hematoxylin & eosin. C57BL/6-Pirt-GCaMP3 mice (n = 5/group) and protein gene product 9.5 (PGP9.5) immunostaining of C57BL/6 wild type (WT) mice (n = 5/group) were used to confirm innervation patterns.

RESULTS

In naive 10-week old mice, nociceptive innervation was most dense in bone marrow cavities, lateral synovium and at the insertions of the cruciate ligaments. By age 26 weeks, unoperated knees showed a marked decline in nociceptors in the lateral synovium and cruciate ligament insertions. No further decline was observed by age 1 year. Sixteen weeks after DMM, the medial compartment of OA knees exhibited striking changes in Na_V1.8+ innervation, including increased innervation of the medial synovium and meniscus, and nociceptors in subchondral bone channels. All results were confirmed through quantification, also in Pirt-GCaMP3 and PGP9.5-immunostained WT mice.

CONCLUSIONS

Nociceptive innervation of the mouse knee markedly declines by age 26 weeks, before onset of spontaneous OA. Late-stage surgically induced OA is associated with striking plasticity of joint afferents in the medial compartment of the knee.

Lateral epicondylalgia: A primary nervous system disorder

Lateral epicondylalgia (LE) is the most common chronic painful condition affecting the elbow in the general population. Although major advances have been accomplished in recent years in the understanding of LE, the underlying physiopathology is still a reason for debate. Differences in clinical presentation and evolution of the symptoms among patients, suggest the need for revisiting the current knowledge about subjacent mechanisms that attempt to explain pain and functional loss. Previous models have suggested that the condition is mainly a degenerative tendinopathy, associated with changes in pain pathways and the motor system. The hypothesis of this work is that LE is the clinical manifestation of a primary nervous system disorder, characterized by an abnormal increase in neuronal activity and a subsequent loss of homeostasis, which secondarily affects the musculoskeletal tissues of the elbow-forearm-hand complex. A new model for LE is presented, supported by an in-deep analysis of basic sciences, epidemiological and clinical studies.

Epiligament Tissue of the Medial Collateral Ligament in Rat Knee Joint: Ultrastructural Study

Introduction Recent studies stressed the importance of the epiligament in ligament nutrition and healing. While ligaments of the knee joint have been the subject of extensive research, the epiligament of the medial collateral ligament has received only limited attention. The aim of our study was to present the ultrastructural morphological features of the epiligament of the medial collateral ligament in a rat knee joint. Materials and methods For the present study, we used eight eight-month-old male Wistar rats. A transmission electron microscopic study of the epiligament was conducted according to standard protocol. Results In the epiligament, we described the presence of fibroblasts with the typical features of protein-synthesizing cells, as well as fibrocytes and adipocytes. We noted an abundance of blood vessels and nerve elements. Collagen fibers were organized in multidirectional bundles. Conclusions Our findings confirm that the cells and structures of the epiligament play an important role in the nutrition and healing of the medial collateral ligament.

Synovial nerve fiber density decreases with naturally-occurring osteoarthritis in horses

OBJECTIVE

To measure the nerve fiber density in synovial membranes from healthy and OA equine joints and to investigate the relationship between synovial innervation and OA severity, synovial vascularity and synovitis.

DESIGN

Twenty-five equine metacarpophalangeal joints were collected post-mortem. The joints were dissected and the macroscopic lesions of the articular cartilage were scored. Synovial membrane specimens (n = 50) were harvested, fixed, sectioned and scored histologically. Immunohistochemical staining and immunofluorescence with S-100 protein, that identifies nerve fibers, and ⍺-actin, that stains vascular smooth muscle, were also performed on site-matched specimens and the relationships between these tissues was interrogated.

RESULTS

The nerve fiber density was higher in the superficial layer (≤200 μm) of the synovium when compared to the deeper layer in control equine joints (mean difference (95% C.I.): 0.054% (0.018%, 0.11%)). In osteoarthritic joints, synovial innervation decreased in the superficial layer with increasing macroscopic OA score (β (SEM), 95% C.I.: -0.0061 (0.00021), -0.0011, -0.00017). The blood vessel density was also higher in the superficial layer of the synovium compared to the deep layer in the control (mean difference (95% C.I.): 1.1% (0.36%, 2.3%)) and OA (mean difference (95% C.I.): 0.60% (0.22%, 1.2%)) equine joints. Moreover, considering all synovial specimens, higher nerve fiber density in the deep layer positively correlated with blood vessel density (β (SEM), 95% C.I.: 0.11 (0.036), 0.035, 0.18).

CONCLUSION

The reduction in nerve fiber density with advanced cartilage degeneration suggests that peripheral neuropathy is associated with equine OA. Whether this link is associated with neuropathic pain, requires further investigation.

Peripheral Mechanisms Contributing to Osteoarthritis Pain

PURPOSE OF REVIEW

Osteoarthritis (OA) is the most common form of arthritis and a major source of pain and disability worldwide. OA-associated pain is usually refractory to classically used analgesics, and disease-modifying therapies are still lacking. Therefore, a better understanding of mechanisms and mediators contributing to the generation and maintenance of OA pain is critical for the development of efficient and safe pain-relieving therapies.

RECENT FINDINGS

Both peripheral and central mechanisms contribute to OA pain. Clinical evidence suggests that a strong peripheral nociceptive drive from the affected joint maintains pain and central sensitization associated with OA. Mediators present in the OA joint, including nerve growth factor, chemokines, cytokines, and inflammatory cells can contribute to sensitization. Furthermore, structural alterations in joint innervation and nerve damage occur in the course of OA. Several interrelated pathological processes, including joint damage, structural reorganization of joint afferents, low-grade inflammation, neuroplasticity, and nerve damage all contribute to the pain observed in OA. It can be anticipated that elucidating exactly how these mechanisms are operational in the course of progressive OA may lead to the identification of novel targets for intervention.

Development and Persistence of Suspected Neuropathic Pain After Total Knee Arthroplasty in Individuals With Osteoarthritis

BACKGROUND

Despite the effectiveness of total knee arthroplasty (TKA) for osteoarthritis (OA), up to 20% will report knee pain 1 year after surgery. One possible reason is the development of neuropathic pain before or after TKA.

OBJECTIVE

To longitudinally describe suspected neuropathic pain in patients pre- and post-TKA and to explore relations between pre-TKA suspected neuropathic pain and post-TKA outcomes.

DESIGN

Prospective observational study.

SETTING

Participants were recruited from orthopedic surgery clinics prior to inpatient elective primary TKA.

PARTICIPANTS

Convenience sample of 135 patients were assessed for eligibility; 99 were enrolled and 74 completed the 6-month follow-up.

METHODS

Participants completed the Self-Leeds Assessment of Neuropathic Symptoms and Signs (S-LANSS) and outcome measures at baseline (pre-TKA) and 1 and 6 months post-TKA by postal survey. Demographic variables included age, gender, and comorbidities. Descriptive statistics were calculated for the presence of suspected neuropathic pain at each assessment and course of outcomes for various suspected neuropathic pain trajectories. Further, t-tests were used to compare outcomes between those with and without suspected neuropathic pain at each assessment. Multiple linear regressions assessed the relationship between baseline suspected neuropathic pain and 6-month outcomes.

MAIN OUTCOME MEASUREMENTS

Intermittent and Constant Osteoarthritis Pain (ICOAP), Pain Catastrophizing Scale (PCS), and the Patient Health Questionnaire (PHQ-9) for depression.

RESULTS

Suspected neuropathic pain was present in 35.5% of pre-TKA patients, 39.0% at 1 month, and 23.6% at 6 months post-TKA. Those with suspected neuropathic pain had higher scores for ICOAP total pain (P = .05), pain catastrophizing (P < .01), and depression (P < .01) at each assessment. After adjusting for potential confounding, pre-TKA suspected neuropathic pain did not predict ICOAP total pain or PHQ-9 depression scores at 6 months.

CONCLUSIONS

Although 14% of individuals with knee OA had suspected neuropathic pain that persisted 6 months post-TKA and those with suspected neuropathic pain had higher levels of pain, catastrophizing, and depression, the clinical identification of neuropathic pain remains enigmatic. Preoperative suspected neuropathic pain, as measured by S-LANSS, may have limited prognostic value for post-TKA outcomes.

LEVEL OF EVIDENCE

II.

Pain quality descriptors in community-dwelling older adults with nonmalignant pain

This study aimed to characterize the prevalence of various pain qualities in older adults with chronic nonmalignant pain and determine the association of pain quality to other pain characteristics namely: severity, interference, distribution, and pain-associated conditions. In the population-based MOBILIZE Boston Study, 560 participants aged ≥70 years reported chronic pain in the baseline assessment, which included a home interview and clinic exam. Pain quality was assessed using a modified version of the McGill Pain Questionnaire (MPQ) consisting of 20 descriptors from which 3 categories were derived: cognitive/affective, sensory, and neuropathic. Presence of ≥2 pain-associated conditions was significantly associated with 18 of the 20 pain quality descriptors. Sensory descriptors were endorsed by nearly all older adults with chronic pain (93%), followed by cognitive/affective (83.4%) and neuropathic descriptors (68.6%). Neuropathic descriptors were associated with the greatest number of pain-associated conditions including osteoarthritis of the hand and knee. More than half of participants (59%) endorsed descriptors in all 3 categories and had more severe pain and interference, and multisite or widespread pain than those endorsing 1 or 2 categories. Strong associations were observed between pain quality and measures of pain severity, interference, and distribution (P < 0.0001). Findings from this study indicate that older adults have multiple pain-associated conditions that likely reflect multiple physiological mechanisms for pain. Linking pain qualities with other associated pain characteristics serve to develop a multidimensional approach to geriatric pain assessment. Future research is needed to investigate the physiological mechanisms responsible for the variability in pain qualities endorsed by older adults.

Lysophosphatidic acid provides a missing link between osteoarthritis and joint neuropathic pain

OBJECTIVE

Emerging evidence suggests that osteoarthritis (OA) has a neuropathic component; however, the identity of the molecules responsible for this peripheral neuropathy is unknown. The aim of this study was to determine the contribution of the bioactive lipid lysophosphatidic acid (LPA) to joint neuropathy and pain.

DESIGN

Male Lewis rats received an intra-articular injection of 50 μg of LPA into the knee and allowed to recover for up to 21 days. Saphenous nerve myelination was assessed by g-ratio calculation from electron micrographs and afferent nerve damage visualised by activation transcription factor-3 (ATF-3) expression. Nerve conduction velocity was measured electrophysiologically and joint pain was determined by hindlimb incapacitance. The effect of the LPA antagonist Ki-16425 was also evaluated. Experiments were repeated in the sodium monoiodoacetate (MIA) model of OA.

RESULTS

LPA caused joint nerve demyelination which resulted in a drop in nerve conduction velocity. Sensory neurones were ATF-3 positive and animals exhibited joint pain and knee joint damage. MIA-treated rats also showed signs of demyelination and joint neuropathy with concomitant pain. Nerve damage and pain could be ameliorated by Ki-16425 pre-treatment.

CONCLUSION

Intra-articular injection of LPA caused knee joint neuropathy, joint damage and pain. Pharmacological blockade of LPA receptors inhibited joint nerve damage and hindlimb incapacitance. Thus, LPA is a candidate molecule for the development of OA nerve damage and the origin of joint neuropathic pain.

Early blockade of joint inflammation with a fatty acid amide hydrolase inhibitor decreases end-stage osteoarthritis pain and peripheral neuropathy in mice

Background

The endocannabinoid system has been shown to reduce inflammatory flares and pain in rodent models of arthritis. A limitation of endocannabinoids is that they are rapidly denatured by hydrolysing enzymes such as fatty acid amide hydrolase (FAAH) which renders them physiologically inert. Osteoarthritis (OA) is primarily a degenerative joint disease; however, it can incorporate mild inflammation and peripheral neuropathy. The aim of this study was to determine whether early blockade of FAAH bioactivity could reduce OA-associated inflammation and joint neuropathy. The ability of this treatment to prevent end-stage OA pain development was also tested.

Methods

Physiological saline or sodium monoiodoacetate (MIA; 0.3 mg) was injected into the right knee of male C57Bl/6 mice (20–42 g) and joint inflammation (oedema, blood flow and leukocyte trafficking) was measured over 14 days. Joint inflammation was also measured in a separate cohort of animals treated on day 1 with either saline or the FAAH inhibitor URB597 (0.03–0.3 mg/kg topical onto the knee joint). In other experiments, von Frey hair tactile sensitivity was determined on days 1 and 14 in MIA-injected mice treated prophylactically with URB597 (0.3 mg/kg s.c. over the knee joint on days 0–3). Saphenous nerve myelination was also assessed in these animals on day 14 by G-ratio analysis.

Results

Intra-articular injection of MIA caused an increase in joint oedema (P < 0.0001), blood flow (P < 0.05), leukocyte rolling (P < 0.05) and adherence (P < 0.001) on day 1 after treatment which subsequently resolved over later time points. This acute inflammatory response was ameliorated by local URB597 treatment. Prophylactic local administration of URB597 prevented MIA-induced saphenous nerve demyelination, and chronic joint pain was also attenuated.

Conclusions

These data indicate that local inhibition of FAAH in MIA-injected knees can reduce acute inflammatory changes associated with the model. Prophylactic treatment of OA mice with the endocannabinoid hydrolysis inhibitor URB597 was also shown to be neuroprotective and prevented the development of joint pain at later time points.

Nociceptive neuronal Fc-gamma receptor I is involved in IgG immune complex induced pain in the rat

Antigen-specific immune diseases such as rheumatoid arthritis are often accompanied by pain and hyperalgesia. Our previous studies have demonstrated that Fc-gamma-receptor type I (FcγRI) is expressed in a subpopulation of rat dorsal root ganglion (DRG) neurons and can be directly activated by IgG immune complex (IgG-IC). In this study we investigated whether neuronal FcγRI contributes to antigen-specific pain in the naïve and rheumatoid arthritis model rats. In vitro calcium imaging and whole-cell patch clamp recordings in dissociated DRG neurons revealed that only the small-, but not medium- or large-sized DRG neurons responded to IgG-IC. Accordingly, in vivo electrophysiological recordings showed that intradermal injection of IgG-IC into the peripheral receptive field could sensitize only the C- (but not A-) type sensory neurons and evoke action potential discharges. Pain-related behavioral tests showed that intradermal injection of IgG-IC dose-dependently produced mechanical and thermal hyperalgesia in the hindpaw of rats. These behavioral effects could be alleviated by localized administration of non-specific IgG or an FcγRI antibody, but not by mast cell stabilizer or histamine antagonist. In a rat model of antigen-induced arthritis (AIA) produced by methylated bovine serum albumin, FcγRI were found upregulated exclusively in the small-sized DRG neurons. In vitro calcium imaging revealed that significantly more small-sized DRG neurons responded to IgG-IC in the AIA rats, although there was no significant difference between the AIA and control rats in the magnitude of calcium changes in the DRG neurons. Moreover, in vivo electrophysiological recordings showed that C-nociceptive neurons in the AIA rats exhibited a greater incidence of action potential discharges and stronger responses to mechanical stimuli after IgG-IC was injected to the receptive fields. These results suggest that FcγRI expressed in the peripheral nociceptors might be directly activated by IgG-IC and contribute to antigen-specific pain in pathological conditions.

Spatiotemporal gait compensations following medial collateral ligament and medial meniscus injury in the rat: correlating gait patterns to joint damage

Introduction

After transection of the medial collateral ligament and medial meniscus (MCLT + MMT) in the rat, focal cartilage lesions develop over 4–6 weeks; however, sham surgery (MCLT alone) does not result in cartilage damage over a similar period. Thus, comparison of MCLT + MMT with the MCLT sham group offers an opportunity to investigate behavioral modifications related to focal cartilage and meniscus damage in the rat.

Methods

MCLT or MCLT + MMT surgery was performed in the right knees of male Lewis rats, with spatiotemporal gait patterns and hind limb sensitivity assessed at 1, 2, 4, and 6 weeks postsurgery (n = 8 rats per group per time point, n = 64 total). After the animals were euthanized, Histology was performed to assess joint damage.

Results

MCLT + MMT animals had unilateral gait compensations at early time points, but by week 6 bilateral gait compensations had developed in both the MCLT sham and MCLT + MMT groups. Conversely, heightened tactile sensitivity was detected in both MCLT sham and MCLT + MMT animals at week 1, but only the MCLT + MMT animals maintained heightened sensitivity to week 6. Cartilage lesions were found in the MCLT + MMT group but not in the MCLT sham group. Correlations could be identified between joint damage and gait changes in MCLT + MMT animals; however, the same gait changes were found with MCLT sham animals despite a lack of joint damage.

Conclusions

Combined, our data highlight a common conundrum in osteoarthritis (OA) research: Some behavioral changes correlate to cartilage damage in the OA group, but the same changes can be identified in non-OA controls. Of the behavioral changes detected, allodynia was maintained in MCLT + MMT animals but not in the MCLT sham group. However, the correlation between cartilage damage and hind limb sensitivity is relatively weak (R = −0.4498), and the range of sensitivity measures overlaps between groups. The factors driving gait abnormalities in MCLT and MCLT + MMT animals also remain uncertain. The gait modifications are similar between groups and do not appear until weeks after surgery, despite cartilage damage being focused in the MCLT + MMT group. Combined, our data highlight the need to evaluate the links between noncartilage changes and behavioral changes following joint injury in the rat.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0791-2) contains supplementary material, which is available to authorized users.

Mechanisms and Mediators That Drive Arthritis Pain

There are over 100 different types of arthritis and each can differ greatly in their aetiology and pathophysiology; however, one characteristic that is common to all arthritic conditions is joint pain. Musculoskeletal pain is the leading cause of disability in the world, and the number one reason arthritis patients visit their primary care physician. Despite the prevalence and burden of arthritis pain, current analgesics lack sufficient efficacy and are plagued by multiple adverse side effects. In this review, we outline the current landscape of research concerning joint pain, drawing from both preclinical and clinical studies. Specifically, this review is a discussion of the different neurophysiological processes that occur during joint disease and how inflammatory and neuropathic aspects contribute to the development of arthritis pain.

Involvement of the endocannabinoid system in osteoarthritis pain

Osteoarthritis is a degenerative joint disease associated with articular cartilage degradation. The major clinical outcome of osteoarthritis is a complex pain state that includes both nociceptive and neuropathic mechanisms. Currently, the therapeutic approaches for osteoarthritis are limited as no drugs are available to control the disease progression and the analgesic treatment has restricted efficacy. Increasing evidence from preclinical studies supports the interest of the endocannabinoid system as an emerging therapeutic target for osteoarthritis pain. Indeed, pharmacological studies have shown the anti-nociceptive effects of cannabinoids in different rodent models of osteoarthritis, and compelling evidence suggests an active participation of the endocannabinoid system in the pathophysiology of this disease. The ubiquitous distribution of cannabinoid receptors, together with the physiological role of the endocannabinoid system in the regulation of pain, inflammation and even joint function further support the therapeutic interest of cannabinoids for osteoarthritis. However, limited clinical evidence has been provided to support this therapeutic use of cannabinoids, despite the promising preclinical data. This review summarizes the promising results that have been recently obtained in support of the therapeutic value of cannabinoids for osteoarthritis management.

Systematic Review of Biological Modulation of Healing in Anterior Cruciate Ligament Reconstruction

Background:

Whether biological modulation is effective to promote healing in anterior cruciate ligament (ACL) reconstruction remains unclear.

Purpose:

To perform a systematic review of both clinical and experimental evidence of preclinical animal studies on biological modulation to promote healing in ACL reconstruction.

Study Design:

Systematic review; Level of evidence, 2.

Methods:

A systematic search was performed using the PubMed, Ovid, and Scopus search engines. Inclusion criteria were clinical and animal studies involving subjects with ACL injury with the use of biological modulation to promote healing outcomes. Methodological quality of clinical studies was evaluated using the Critical Appraisal Skill Programme (CASP) appraisal tool, and animal studies were evaluated by a scoring system based on a published checklist of good animal studies.

Results:

Ten clinical studies and 50 animal studies were included. Twenty-five included studies were regarded as good quality, with a methodological score ≥5. These studies suggested that transforming growth factor–beta (TGF-β), mesenchymal stem cells, osteogenic factors, and modalities that reduce local inflammation may be beneficial to promote graft healing in ACL reconstruction.

Conclusion:

This systematic review suggests that biological modulation is able to promote healing on top of surgical treatment for ACL injuries. This treatment strategy chiefly works through promotion of healing at the tunnel-graft interface, but the integrity of the intra-articular midsubstance of the graft would be another target for biological modulation.

Towards a mechanism-based approach to pain management in osteoarthritis

Pain is the defining symptom of osteoarthritis (OA), yet available treatment options, of which NSAIDs are the most common, provide inadequate pain relief and are associated with serious health risks when used long term. Chronic pain pathways are subject to complex levels of control and modulation, both in the periphery and in the central nervous system. Ongoing clinical and basic research is uncovering how these pathways operate in OA. Indeed, clinical investigation into the types of pain associated with progressive OA, the presence of central sensitization, the correlation with structural changes in the joint, and the efficacy of novel analgesics affords new insights into the pathophysiology of OA pain. Moreover, studies in disease-specific animal models enable the unravelling of the cellular and molecular pathways involved. We expect that increased understanding of the mechanisms by which chronic OA-associated pain is generated and maintained will offer opportunities for targeting and improving the safety of analgesia. In addition, using clinical and genetic approaches, it might become possible to identify subsets of patients with pain of different pathophysiology, thus enabling a tailored approach to pain management.

A commentary on modelling osteoarthritis pain in small animals

OBJECTIVE

To describe the currently used animal models for the study of osteoarthritis (OA) pain, with an emphasis on small animals (predominantly mice and rats).

OUTLINE

Narrative review summarizing the opportunities and limitations of the most commonly used small animal models for the study of pain and pain pathways associated with OA, and discussing currently used methods for pain assessment. Involvement of neural degeneration in OA is briefly discussed. A list of considerations when studying pain-related behaviours and pathways in animal models of OA is proposed.

CONCLUSIONS

Animal models offer great potential to unravel the complex pathophysiology of OA pain, its molecular and temporal regulation. They constitute a critical pathway for developing and testing disease-specific symptom-modifying therapeutic interventions. However, a number of issues remain to be resolved in order to standardize pre-clinical OA pain research and to optimize translation to clinical trials and patient therapies.

Neurophysiology of arthritis pain

Arthritis pain is a complex phenomenon involving intricate neurophysiological processing at all levels of the pain pathway. The treatment options available to alleviate joint pain are fairly limited and most arthritis patients report only modest pain relief with current treatments. A better understanding of the neural mechanisms responsible for musculoskeletal pain and the identification of new targets will help in the development of future pharmacological therapies. This article reviews some of the latest research into factors which contribute to joint pain and covers areas such as cannabinoids, proteinase activated receptors, sodium channels, cytokines and transient receptor potential channels. The emerging hypothesis that osteoarthritis may have a neuropathic component is also discussed.

Neuropathic pain symptoms in a community knee OA cohort

OBJECTIVE

A neuropathic pain (NP) questionnaire may facilitate the identification of a neuropathic component to osteoarthritis (OA) pain. An existing questionnaire, the painDETECT, was modified for use in knee OA and administered to measure the prevalence and correlates of NP symptoms among adults with this condition.

METHOD

Sensibility of the modified painDETECT (mPD-Q) was assessed in 20 OA subjects followed by mail administration in an established knee OA cohort. NP symptoms were defined using a previously established, painDETECT cut-point. Correlates of NP symptoms, including OA severity (Western Ontario and McMaster Universities Osteoarthritis Index, Von Korff Chronic Pain Grade pain subscale score), psychological factors (Centre for Epidemiological Studies Depression Scale, Pain Catastrophizing Scale), and concomitant medical conditions, were evaluated using logistic regression. Construct validity of the mPD-Q was evaluated through co-administration with another NP questionnaire (S-LANSS).

RESULTS

The mPD-Q had face and content validity. Of 259 eligible cohort members, 171 (66%) completed the questionnaire; 28% had NP symptoms on the mPD-Q (19% among those without neurological conditions). Independent correlates of NP symptoms were: pain intensity (adjusted odds ratio [OR]=2.1 per 10 unit increase, P<0.0001), the presence of referred back/hip pain (adjusted OR=2.9, P=0.024), number of painful joints (OR=1.2, P=0.20) and one or more self-reported neurological condition (OR=3.0, P=0.026).

CONCLUSIONS

Among older adults with chronic symptomatic knee OA, over one-quarter had NP symptoms localized to their knees using the mPD-Q. The mPD-Q may facilitate the identification of a neuropathic component to pain in adults with knee OA who may benefit from further evaluation and/or treatment for NP.

Electroacupuncture reduces the evoked responses of the spinal dorsal horn neurons in ankle-sprained rats

Acupuncture is shown to be effective in producing analgesia in ankle sprain pain in humans and animals. To examine the underlying mechanisms of the acupuncture-induced analgesia, the effects of electroacupuncture (EA) on weight-bearing forces (WBR) of the affected foot and dorsal horn neuron activities were examined in a rat model of ankle sprain. Ankle sprain was induced manually by overextending ligaments of the left ankle in the rat. Dorsal horn neuron responses to ankle movements or compression were recorded from the lumbar spinal cord using an in vivo extracellular single unit recording setup 1 day after ankle sprain. EA was applied to the SI-6 acupoint on the right forelimb (contralateral to the sprained ankle) by trains of electrical pulses (10 Hz, 1-ms pulse width, 2-mA intensity) for 30 min. After EA, WBR of the sprained foot significantly recovered and dorsal horn neuron activities were significantly suppressed in ankle-sprained rats. However, EA produced no effect in normal rats. The inhibitory effect of EA on hyperactivities of dorsal horn neurons of ankle-sprained rats was blocked by the α-adrenoceptor antagonist phentolamine (5 mg/kg ip) but not by the opioid receptor antagonist naltrexone (10 mg/kg ip). These data suggest that EA-induced analgesia in ankle sprain pain is mediated mainly by suppressing dorsal horn neuron activities through α-adrenergic descending inhibitory systems at the spinal level.

Neural stimulation does not mediate attenuated vascular response in ACL-deficient knees: potential role of local inflammatory mediators

Chronic inflammation associated with osteoarthritis (OA) alters normal responses and modifies the functionality of the articular vasculature. Altered responsiveness of the vasculature may be due to excessive neural activity associated with chronic pain and inflammation, or from the production of inflammatory mediators which induce vasodilation. Using laser speckle perfusion imaging (LSPI), blood flow to the medial collateral ligament (MCL) of adult rabbits was measured in denervated ACL transected knees (n = 6) and compared to unoperated control (n = 6) and 6-week anterial cruciate ligament (ACL)-transected knees (n = 6). Phenylephrine and neuropeptide Y were applied to the MCL vasculature in topical boluses of 100 microL (dose range 10(-14) to 10(-8) mol and 10(-14) to 10(-9) mol, respectively). Denervation diminished vasoconstrictive responsiveness to phenylephrine compared to both control and ACL-transected knees. Denervation minimally enhanced vascular responses to neuropeptide Y (NPY) compared to ACL deficiency alone, which nevertheless remained significantly diminished from control responses. To evaluate the potential role of inflammatory dilators in the diminished contractile responses, phenylephrine was coadministered with histamine, substance P, and prostaglandin E(2). High-dose histamine, and low-dose substance P and PGE(2) were able to inhibit contractile responses in the MCL of control knees. Excessive neural input does not mediate diminished vasoconstrictive responses in the ACL transected knee; inflammatory mediators may play a role in the deficient vascular responsiveness of the ACL transected knee.

Insights from imaging on the epidemiology and pathophysiology of osteoarthritis

This article highlights recent studies, particularly those with an emphasis on MR imaging, that are providing unique insights into the relation between structures identified on imaging and symptoms and disease genesis. It is becoming increasingly apparent that the subchondral bone, periosteum, periarticular ligaments, periarticular muscle spasm, synovium, and joint capsule are all richly innervated and are the likely source of nociception in osteoarthritis. It is also apparent that local tissue alterations in the bone and meniscus and alignment of the lower extremity are important in terms of disease genesis. This article represents the literature in that much of the focus and understanding is knee centric with less focus on the hip and hand.

The symptoms of osteoarthritis and the genesis of pain

This article delineates the characteristic symptoms and signs associated with OA and how they can be used to make the clinical diagnosis. The predominant symptom in most patients is pain. The remainder of the article focuses on what we know causes pain in OA and contributes to its severity. Much has been learned over recent years; however, for the budding researcher much of this puzzle remains unexplored or inadequately understood.

The symptoms of osteoarthritis and the genesis of pain

Symptomatic osteoarthritis (OA) causes substantial physical and psychosocial disability. This article delineates the characteristic symptoms and signs associated with OA and how they can be used to make the clinical diagnosis. The predominant symptom in most patients is pain. The remainder of the article focuses on what is known about the causes of pain in OA and factors that contribute to its severity. Much has been learned during recent years, but much of this puzzle remains unexplored or inadequately understood.

In vivo effects of CB2 receptor-selective cannabinoids on the vasculature of normal and arthritic rat knee joints

BACKGROUND AND PURPOSE

Cannabinoids (CBs) are known to be vasoactive and to regulate tissue inflammation. The present study examined the in vivo vasomotor effects of the CB2 receptor agonists JWH015 and JWH133 in rat knee joints. The effect of acute and chronic joint inflammation on CB2 receptor-mediated responses was also tested.

EXPERIMENTAL APPROACH

Blood flow was assessed in rat knee joints by laser Doppler imaging both before and following topical administration of CB2 receptor agonists. Vasoactivity was measured in normal, acute kaolin/carrageenan inflamed and Freund's complete adjuvant chronically inflamed knees.

KEY RESULTS

In normal animals, JWH015 and JWH133 caused a concentration-dependent increase in synovial blood flow which in the case of JWH133 was blocked by the selective CB2 receptor antagonist AM630 as well as the transient receptor potential vanilloid-1 (TRPV1) antagonist SB366791. The vasodilator effect of JWH133 was significantly attenuated in both acute and chronically inflamed knees. Given alone, AM630 had no effect on joint blood flow.

CONCLUSION AND IMPLICATIONS

In normal joints, the cannabinomimetic JWH133 causes hyperaemia via a CB2 and TRPV1 receptor mechanism. During acute and chronic inflammation, however, this vasodilatatory response is significantly attenuated.

Neuropeptides regulate expression of matrix molecule, growth factor and inflammatory mediator mRNA in explants of normal and healing medial collateral ligament

Denervation degrades normal ligament properties and impairs ligament healing. This suggests that secreted neuromediators, such as neuropeptides, could be modulating cell metabolism in ligament and scar tissue. To test this hypothesis we investigated the effect of exogenous substance P (SP), neuropeptide Y (NPY) or calcitonin gene-related peptide (CGRP) on the mRNA levels for proteins associated with inflammation, angiogenesis, and matrix production in tissue-cultured specimens of normal and injured medial collateral ligament. SP and NPY induced increased mRNA levels for several inflammatory mediators in the 2-week post-injury specimens. All three neuropeptides induced decreases in mRNA levels for healing-associated growth factors and matrix molecules, including basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF) and collagen types I and III. The results indicate that neuropeptides strongly influence the metabolic activity of cells in healing ligament, particularly at early time points after injury.

Neurovascular invasion at the osteochondral junction and in osteophytes in osteoarthritis

BACKGROUND

Normal adult articular cartilage is thought to be avascular and aneural.

OBJECTIVE

To describe neurovascular structures at the osteochondral junction and in osteophytes in tibiofemoral osteoarthritis (OA) displaying a range of severity of cartilage changes.

METHODS

Articular surfaces were obtained from 40 patients at total knee joint replacement surgery for tibiofemoral OA (TKR) and seven patients post mortem (PM). Antibodies directed against CD34 (vascular endothelium), protein gene product 9.5 (pan-neuronal marker), substance P and calcitonin gene-related peptide (sensory nerves) and C-flanking peptide of neuropeptide Y (sympathetic nerves) were used to localise blood vessels and nerves by immunohistochemistry. Severity of OA cartilage changes was graded histologically.

RESULTS

TKR and PM samples displayed a range of OA cartilage changes including tidemark breaching by vascular channels. Sympathetic and sensory nerves were both present within vascular channels in the articular cartilage, in both mild and severe OA. Perivascular and free nerve fibres, and nerve trunks were observed within the subchondral bone marrow and within the marrow cavities of osteophytes. Sensory and sympathetic nerves displayed similar distributions in each region studied.

CONCLUSION

Vascularisation and the associated innervation of articular cartilage may contribute to tibiofemoral pain in OA across a wide range of structural disease severity.

Arthritis and pain. Neurogenic origin of joint pain

Arthritis pain affects millions of people worldwide yet we still have only a limited understanding of what makes our joints ache. This review examines the sensory innervation of diarthroidal joints and discusses the neurophysiological processes that lead to the generation of painful sensation. During inflammation, joint nerves become sensitized to mechanical stimuli through the actions of neuropeptides, eicosanoids, proteinase-activated receptors and ion channel ligands. The contribution of immunocytes to arthritis pain is also reviewed. Finally, the existence of an endogenous analgesic system in joints is considered and the reasons for its inability to control pain are postulated.

Physical activity modulates nerve plasticity and stimulates repair after Achilles tendon rupture

In a rat model of tendon rupture using semiquantitative methodology, healing was assessed according to the diameter of newly organized collagen and the occurrence of the sensory neuropeptides (SP, CGRP) in relation to different levels of physical activity. Normally, innervation of the Achilles tendon is confined to the paratenon. After rupture new nerve fibers grow into the tendon proper, but disappear after healing. In a first experiment to establish peak tissue and nerve regeneration after rupture, tendon tissues from freely moving rats were collected consecutively over 16 weeks. A peak increase in organized collagen and nerve ingrowth was observed between week 2 to 4 post rupture. Therefore, in a second experiment week 4 was chosen to assess the effect of physical activity on tendon healing in three groups of rats, that is, wheel running, plaster treated, and freely moving (controls). In the wheel-running group, the diameter of newly organized collagen was 94% ( p = 0.001) greater than that in the plaster-treated group and 48% ( p = 0.02) greater than that in the controls. Inversely, the neuronal occurrence of CGRP in the tendon proper was 57% ( p = 0.02) lower in the wheel-running group than that in the plaster-treated group and 53% ( p = 0.02) lower than that in the controls, suggesting an earlier neuronal in-growth and disappearance in the more active group. Physical activity speeds up tendon healing, which may prove to be linked to accelerated neuronal plasticity.

The Pathomechanics of Plantar Fasciitis

Plantar fasciitis is a musculoskeletal disorder primarily affecting the fascial enthesis. Although poorly understood, the development of plantar fasciitis is thought to have a mechanical origin. In particular, pes planus foot types and lower-limb biomechanics that result in a lowered medial longitudinal arch are thought to create excessive tensile strain within the fascia, producing microscopic tears and chronic inflammation. However, contrary to clinical doctrine, histological evidence does not support this concept, with inflammation rarely observed in chronic plantar fasciitis. Similarly, scientific support for the role of arch mechanics in the development of plantar fasciitis is equivocal, despite an abundance of anecdotal evidence indicating a causal link between arch function and heel pain. This may, in part, reflect the difficulty in measuring arch mechanics in vivo. However, it may also indicate that tensile failure is not a predominant feature in the pathomechanics of plantar fasciitis. Alternative mechanisms including 'stress-shielding', vascular and metabolic disturbances, the formation of free radicals, hyperthermia and genetic factors have also been linked to degenerative change in connective tissues. Further research is needed to ascertain the importance of such factors in the development of plantar fasciitis.

Neurokinin 1-receptors and sensory neuropeptides in tendon insertions at the medial and lateral epicondyles of the humerus. Studies on tennis elbow and medial epicondylalgia

There is no information on the sensory innervation at the flexor muscle origin at the medial epicondyle of the humerus and it is not known if substance P receptors (Neurokinin 1-receptors, NK1-R) are present in tendon insertions in general. In the present investigation, we have studied the muscle origin in patients suffering from medial epicondylalgia and tennis elbow. Immunohistochemistry and antibodies to substance P (SP) and CGRP as well as the general nerve marker PGP 9.5 were used. Specific immunoreactions were observed in nerve bundles and as free nerve fibers. The immunoreactive structures were partly seen in association with some of the blood vessels. The observations constitute a morphological correlate for the occurrence of nerve mediated effects in this region. By using immunohistochemistry and antibodies to NK1-R, the distribution of this receptor was studied at the insertion of the proximal tendon of the extensor carpi radialis brevis muscle at the lateral epicondyle. Specific immunoreactions were seen as varicose fibers occurring as single fibers or grouped into bundles, indicating that SP has effects in the nerves in this region. The results give further evidence for a possible neurogenic involvement in the pathophysiology of tennis elbow and in medial epicondylalgia.

Denervation impairs healing of the rabbit medial collateral ligament

Little is known about the contribution of innervation to ligament healing after traumatic disruption, although there is good evidence of an important role for the peripheral nervous system in the healing of fractures and skin injuries. Tissues such as ligament, with a low resting blood supply, are dependent on substantial increases in blood flow and vascular volume during the initial stages of repair. We hypothesized that this initial healing response would be strongly promoted by neurogenic inflammation. Since the saphenous nerve (a major sensory branch of the femoral nerve) supplies the medial half of the knee joint, we elected to use femoral nerve transection as a model to determine the role of sensory and autonomic innervation in the initial outcome of repair of the injured medial collateral ligament. Twelve adult, female NZW rabbits underwent right medial collateral ligament transection. Of these, six rabbits underwent right femoral nerve transection to disrupt the somatic sensory and autonomic nerve supply to the knee joint and six were kept neurologically intact (controls). At six weeks post-injury, the animals were assessed by laser Doppler perfusion imaging (LDI) to determine the local blood flow, at both the injury site and at the uninjured contralateral ligament. The animals were then killed, the knee joints were removed and the biomechanical characteristics of the healing bone-median collateral ligament (MCL)-bone complexes assessed. In a separate cohort of 16 rabbits, vascular volumes of the injured ligaments were measured by infusion of a carmine red/gelatin solution. At six weeks post-injury, in vivo measurement of perfusion with LDI revealed that normally innervated ligaments had an almost three-fold higher average blood flow. Carmine red/gelatin infusion revealed a 50% higher density of blood vessels as compared to denervated ligaments. The force required for ultimate failure was found to be 50% higher in normally innnervated MCL's as compared to denervated MCL's: 153.14 +/- 20.71 N versus 101.29 +/- 17.88 N (p < 0.05). Static creep was increased by 66% in denervated MCL's: 2.83 +/- 0.45% versus 1.70 +/- 0.12% (p < 0.05). Total creep was increased by 45% in denervated MCL's: 5.29 +/- 0.62% compared to 3.64 +/- 0.31% in innervated MCL's (p < 0.05). We conclude that intact innervation makes a critical contribution to the early healing responses of the MCL of adult rabbits.