Whiplash-like facet joint loading initiates glutamatergic responses in the DRG and spinal cord associated with behavioral hypersensitivity

Facet joint disorders: from diagnosis to treatment

One of the most common sources of spinal pain syndromes is the facet joints. Cervical, thoracic, and lumbar facet joint pain syndromes comprise 55%, 42%, and 31% of chronic spinal pain syndromes, respectively. Common facet joint disorders are degenerative disorders, such as osteoarthritis, hypertrophied superior articular process, and facet joint cysts; septic arthritis; systemic and metabolic disorders, such as ankylosing spondylitis or gout; and traumatic dislocations. The facet pain syndrome from osteoarthritis is suspected from a patient's history (referred pain pattern) and physical examination (tenderness). Other facet joint disorders may cause radicular pain if mass effect from a facet joint cyst, hypertrophied superior articular process, or tumors compress the dorsal root ganglion. However, a high degree of morphological change does not always provoke pain. The superiority of innervating nerve block or direct joint injection for diagnosis and treatment is still a controversy. Treatment includes facet joint injection in facet joint osteoarthritis or whiplash injury provoking referred pain or decompression in mass effect in cases of hypertrophied superior articular process or facet joint cyst eliciting radicular pain. In addition, septic arthritis is treated using a proper antibiotic, based on infected tissue or blood culture. This review describes the diagnosis and treatment of common facet joint disorders.

Characterization of the L4/L5 rat facet capsular ligament macromechanical and microstructural responses to tensile failure loading

Low back pain is a prevalent condition that affects the global population. The lumbar facet capsular ligament is a source of pain since the collagenous tissue of the ligament is innervated with sensory neurons that deform with the capsule's stretch. Regional differences in the microstructural and macrostructural anatomy of the spinal facets affect its capsule's mechanical behavior. Although there are many studies of the cervical facet in human and rodent models, the lumbar capsular ligament's multiscale behavior is less well-defined. This study characterizes the macroscale and fiber-scale changes of the rat lumbar facet capsule during tensile failure loading. An integrated polarized light imaging setup captured local fiber alignment during 0.08 mm/s distraction of 7 lumbar facets. Force, displacement, strain, and circular variance were measured at several points along the failure curve: the first instance when the local collagen fiber network realigns differentially (anomalous realignment), yield, the first peak in force corresponding to the capsule's first failure, and peak force, defined as ultimate rupture. Those outcomes were compared across events. While each of force, displacement, and average maximum principal strain increased with applied tension, so did the circular variance of the collagen, suggesting that the fibers were becoming more disorganized. From the fiber alignment maps collected at each mechanical event, the number of anomalous realignment events were counted and found to increase dramatically with loading. The increased collagen disorganization and increasing regions of such disorganization in the facet capsule during loading can provide insights about how loading to the ligament afferent nerves may be activated and thereby produce pain.

Local tissue heterogeneity may modulate neuronal responses via altered axon strain fields: insights about innervated joint capsules from a computational model

In innervated collagenous tissues, tissue scale loading may contribute to joint pain by transmitting force through collagen fibers to the embedded mechanosensitive axons. However, the highly heterogeneous collagen structures of native tissues make understanding this relationship challenging. Recently, collagen gels with embedded axons were stretched and the resulting axon signals were measured, but these experiments were unable to measure the local axon strain fields. Computational discrete fiber network models can directly determine axon strain fields due to tissue scale loading. Therefore, this study used a discrete fiber network model to identify how heterogeneous collagen networks (networks with multiple collagen fiber densities) change axon strain due to tissue scale loading. In this model, a composite cylinder (axon) was embedded in a Delaunay network (collagen). Homogeneous networks with a single collagen volume fraction and two types of heterogeneous networks with either a sparse center or dense center were created. Measurements of fiber forces show higher magnitude forces in sparse regions of heterogeneous networks and uniform force distributions in homogeneous networks. The average axon strain in the sparse center networks decreases when compared to homogeneous networks with similar collagen volume fractions. In dense center networks, the average axon strain increases compared to homogeneous networks. The top 1% of axon strains are unaffected by network heterogeneity. Based on these results, the interaction of tissue scale loading, collagen network heterogeneity, and axon strains in native musculoskeletal tissues should be considered when investigating the source of joint pain.

Macromolecular changes in spinal cord white matter characterize whiplash outcome at 1-year post motor vehicle collision

Each year, whiplash injuries from motor vehicle collisions (MVC) affect millions worldwide, with no strong evidence of pathology. While the majority recover soon after the injury, the same is not true for roughly 20% reporting higher levels of pain and distress, without diagnostic options. This study used magnetization transfer (MT) imaging to quantify white matter integrity in 78 subjects with varying levels of pain, 1 year after MVC. MT images of the cervical spinal cord were collected parallel to the intervertebral disks. MT ratios (MTR) were calculated in select white matter tracts along with MTR homogeneity (MTRh) at each level. Significant differences were observed between clinical outcome groups in the left and right spinothalamic tracts (p = 0.003 and 0.020) and MTRh (p = 0.009). MTRh was elevated in females with poor recovery versus females reporting recovery (p < 0.001) or milder symptoms (p < 0.001), and in males reporting recovery (p = 0.007) or no recovery (p < 0.001). There was a significant interaction between recovery status and sex (p = 0.015). MT imaging identified tract specific and regional changes in white matter integrity suggesting potential insults to the cord. Additionally, significant MTRh differences between sexes were observed, characterizing the heterogeneity of whiplash recovery and worse outcomes in females.

The bivalent ligand, MMG22, reduces neuropathic pain after nerve injury without the side effects of traditional opioids

ABSTRACT

Functional interactions between the mu opioid receptor (MOR) and the metabotropic glutamate receptor 5 (mGluR5) in pain and analgesia have been well established. MMG22 is a bivalent ligand containing MOR agonist (oxymorphamine) and mGluR5 antagonist (MPEP) pharmacophores tethered by a 22-atom linker. MMG22 has been shown to produce potent analgesia in several models of chronic inflammatory and neuropathic pain (NP). This study assessed the efficacy of systemic administration of MMG22 at reducing pain behavior in the spared nerve injury (SNI) model of NP in mice, as well as its side-effect profile and abuse potential. MMG22 reduced mechanical hyperalgesia and spontaneous ongoing pain after SNI, with greater potency early (10 days) as compared to late (30 days) after injury. Systemic administration of MMG22 did not induce place preference in naive animals, suggesting absence of abuse liability when compared to traditional opioids. MMG22 also lacked the central locomotor, respiratory, and anxiolytic side effects of its monomeric pharmacophores. Evaluation of mRNA expression showed the transcripts for both receptors were colocalized in cells in the dorsal horn of the lumbar spinal cord and dorsal root ganglia. Thus, MMG22 reduces hyperalgesia after injury in the SNI model of NP without the typical centrally mediated side effects associated with traditional opioids.

Electroacupuncture Regulates Pain Transition by Inhibiting the mGluR5-PKCε Signaling Pathway in the Dorsal Root Ganglia

Background

Acute pain can transition to chronic pain, presenting a major clinical challenge. Electroacupuncture (EA) can partly prevent the transition from acute to chronic pain. However, little is known about the mechanisms underlying the effect of EA. This study investigated the effect of EA on pain transition and the activation of metabotropic glutamate receptor 5 (mGluR5)–protein kinase C epsilon (PKCε) signaling pathway in the dorsal root ganglia (DRG).

Methods

The hyperalgesic priming model was established by the sequential intraplantar injection of carrageenan (1%, 100 μL) and prostaglandin E2 (PGE2) into the left hind paw of rats. EA treatment (2/100 Hz, 30 min, once/day) was applied at bilateral Zusanli (ST36) and Kunlun (BL60) acupoints in rats. Von Frey filaments were used to investigate the mechanical withdrawal threshold (MWT) at different time points. The protein expression levels of mGluR5 and PKCε in the ipsilateral L4-L6 DRGs of rats were detected by Western blot. Some pharmacological experiments were performed to evaluate the relationship between mGluR5, PKCε and the MWT. It was also used to test the effects of EA on the expression levels of mGluR5 and PKCε and changes in the MWT.

Results

Sequential injection of carrageenan and PGE2 significantly decreased the MWT of rats and up-regulated the expression level of mGluR5 and PKCε in the ipsilateral L4-L6 DRGs. EA can reverse the hyperalgesic priming induced by sequential injection of carrageenan/PGE and down-regulate the protein expression of mGluR5 and PKCε. Glutamate injection instead of PGE2 can mimic the hyperalgesic priming model. Pharmacological blocking of mGluR5 with specific antagonist MTEP can prevent the hyperalgesic priming and inhibit the activation of PKCε in DRGs. Furthermore, EA also produced analgesic effect on the hyperalgesic priming rats induced by carrageenan/mGluR5 injection and inhibited the high expression of PKCε. Sham EA produced none analgesic and regulatory effect.

Conclusion

EA can regulate pain transition and it may relate with its inhibitory effect on the activation of mGluR5-PKCε signaling pathway in the DRGs.

Dietary polyphenols as a safe and novel intervention for modulating pain associated with intervertebral disc degeneration in an in-vivo rat model

Developing effective therapies for back pain associated with intervertebral disc (IVD) degeneration is a research priority since it is a major socioeconomic burden and current conservative and surgical treatments have limited success. Polyphenols are naturally occurring compounds in plant-derived foods and beverages, and evidence suggests dietary supplementation with select polyphenol preparations can modulate diverse neurological and painful disorders. This study tested whether supplementation with a select standardized Bioactive-Dietary-Polyphenol-Preparation (BDPP) may alleviate pain symptoms associated with IVD degeneration. Painful IVD degeneration was surgically induced in skeletally-mature rats by intradiscal saline injection into three consecutive lumbar IVDs. Injured rats were given normal or BDPP-supplemented drinking water. In-vivo hindpaw mechanical allodynia and IVD height were assessed weekly for 6 weeks following injury. Spinal column, dorsal-root-ganglion (DRG) and serum were collected at 1 and 6 weeks post-operative (post-op) for analyses of IVD-related mechanical and biological pathogenic processes. Dietary BDPP significantly alleviated the typical behavioral sensitivity associated with surgical procedures and IVD degeneration, but did not modulate IVD degeneration nor changes of pro-inflammatory cytokine levels in IVD. Gene expression analyses suggested BDPP might have an immunomodulatory effect in attenuating the expression of pro-inflammatory cytokines in DRGs. This study supports the idea that dietary supplementation with BDPP has potential to alleviate IVD degeneration-related pain, and further investigations are warranted to identify the mechanisms of action of dietary BDPP.

Physiologic facet capsule stretch can induce pain & upregulate matrix metalloproteinase-3 in the dorsal root ganglia when preceded by a physiological mechanical or nonpainful chemical exposure

BACKGROUND

Neck pain from cervical facet loading is common and induces inflammation and upregulation of nerve growth factor (NGF) that can sensitize the joint afferents. Yet, the mechanisms by which these occur and whether afferents can be pre-conditioned by certain nonpainful stimuli are unknown. This study tested the hypothesis that a nonpainful mechanical or chemical insult predisposes a facet joint to generate pain after a later exposure to typically nonpainful distraction.

METHODS

Rats were exposed to either a nonpainful distraction or an intra-articular subthreshold dose of NGF followed by a nonpainful distraction two days later. Mechanical hyperalgesia was measured daily and C6 dorsal root ganglia (DRG) tissue was assayed for NGF and matrix metalloproteinase-3 (MMP-3) expression on day 7.

FINDINGS

The second distraction increased joint displacement and strains compared to its first application (p = 0.0011). None of the initial exposures altered behavioral sensitivity in either of the groups being pre-conditioned or in controls; but, sensitivity was established in both groups receiving a second distraction within one day that lasted until day 7 (p < 0.024). NGF expression in the DRG was increased in both groups undergoing a pre-conditioning exposure (p < 0.0232). Similar findings were observed for MMP-3 expression, with a pre-conditioning exposure increasing levels after an otherwise nonpainful facet distraction.

INTERPRETATION

These findings suggest that nonpainful insults to the facet joint, when combined, can generate painful outcomes, possibly mediated by upregulation of MMP-3 and mature NGF.

Image-based multi-scale mechanical analysis of strain amplification in neurons embedded in collagen gel

A general multi-scale strategy is presented for modeling the mechanical environment of a group of neurons that were embedded within a collagenous matrix. The results of the multi-scale simulation are used to estimate the local strains that arise in neurons when the extracellular matrix is deformed. The distribution of local strains was found to depend strongly on the configuration of the embedded neurons relative to the loading direction, reflecting the anisotropic mechanical behavior of the neurons. More importantly, the applied strain on the surrounding extracellular matrix is amplified in the neurons for all loading configurations that are considered. In the most severe case, the applied strain is amplified by at least a factor of 2 in 10% of the neurons' volume. The approach presented in this paper provides an extension to the capability of past methods by enabling the realistic representation of complex cell geometry into a multi-scale framework. The simulation results for the embedded neurons provide local strain information that is not accessible by current experimental techniques.

Pre-treatment with Meloxicam Prevents the Spinal Inflammation and Oxidative Stress in DRG Neurons that Accompany Painful Cervical Radiculopathy

Painful neuropathic injuries are accompanied by robust inflammatory and oxidative stress responses that contribute to the development and maintenance of pain. After neural trauma the inflammatory enzyme cyclooxygenase-2 (COX-2) increases concurrent with pain onset. Although pre-treatment with the COX-2 inhibitor, meloxicam, before a painful nerve root compression prevents the development of pain, the pathophysiological mechanisms are unknown. This study evaluated if pre-treatment with meloxicam prior to painful root injury prevents pain by reducing spinal inflammation and peripheral oxidative stress. Glial activation and expression of the inflammatory mediator secreted phospholipase A2 (sPLA2) in the spinal cord were assessed at day 7 using immunohistochemistry. The extent of oxidative damage was measured using the oxidative stress marker, 8-hydroxyguanosine (8-OHG) and localization of 8-OHG with neurons, microglia and astrocytes in the spinal cord and peripherally in the dorsal root ganglion (DRG) at day 7. In addition to reducing pain, meloxicam reduced both spinal microglial and astrocytic activation at day 7 after nerve root compression. Spinal sPLA2 was also reduced with meloxicam treatment, with decreased production in neurons, microglia and astrocytes. Oxidative damage following nerve root compression was found predominantly in neurons rather than glial cells. The expression of 8-OHG in DRG neurons at day 7 was reduced with meloxicam. These findings suggest that meloxicam may prevent the onset of pain following nerve root compression by suppressing inflammation and oxidative stress both centrally in the spinal cord and peripherally in the DRG.

Repeated High Rate Facet Capsular Stretch at Strains That are Below the Pain Threshold Induces Pain and Spinal Inflammation With Decreased Ligament Strength in the Rat

Repeated loading of ligamentous tissues during repetitive occupational and physical tasks even within physiological ranges of motion has been implicated in the development of pain and joint instability. The pathophysiological mechanisms of pain after repetitive joint loading are not understood. Within the cervical spine, excessive stretch of the facet joint and its capsular ligament has been implicated in the development of pain. Although a single facet joint distraction (FJD) at magnitudes simulating physiologic strains is insufficient to induce pain, it is unknown whether repeated stretching of the facet joint and ligament may produce pain. This study evaluated if repeated loading of the facet at physiologic nonpainful strains alters the capsular ligament's mechanical response and induces pain. Male rats underwent either two subthreshold facet joint distractions (STFJDs) or sham surgeries each separated by 2 days. Pain was measured before the procedure and for 7 days; capsular mechanics were measured during each distraction and under tension at tissue failure. Spinal glial activation was also assessed to probe potential pathophysiologic mechanisms responsible for pain. Capsular displacement significantly increased (p = 0.019) and capsular stiffness decreased (p = 0.008) during the second distraction compared to the first. Pain was also induced after the second distraction and was sustained at day 7 (p < 0.048). Repeated loading weakened the capsular ligament with lower vertebral displacement (p = 0.041) and peak force (p = 0.014) at tissue rupture. Spinal glial activation was also induced after repeated loading. Together, these mechanical, physiological, and neurological findings demonstrate that repeated loading of the facet joint even within physiologic ranges of motion can be sufficient to induce pain, spinal inflammation, and alter capsular mechanics similar to a more injurious loading exposure.

Multiscale mechanics of the cervical facet capsular ligament, with particular emphasis on anomalous fiber realignment prior to tissue failure

The facet capsular ligaments encapsulate the bilateral spinal facet joints and are common sources of painful injury due to afferent innervation. These ligaments exhibit architectural complexity, which is suspected to contribute to the experimentally observed lack of co-localization between macroscopic strain and microstructural tissue damage. The heterogeneous and multiscale nature of this ligament, combined with challenges in experimentally measuring its microscale mechanics, hinders the ability to understand sensory mechanisms under normal or injurious loading. Therefore, image-based, subject-specific, multiscale finite-element models were constructed to predict the mechanical responses of the human cervical facet capsular ligament under uniaxial tensile stretch. The models precisely simulated the force-displacement responses for all samples ([Formula: see text]) and showed promise in predicting the magnitude and location of peak regional strains at two different displacements. Yet, there was a loss of agreement between the model and experiment in terms of fiber organization at large tissue stretch, possibly due to a lack of accounting for tissue failure. The mean fiber stretch ratio predicted by the models was found to be significantly higher in regions that exhibited anomalous fiber realignment experimentally than in regions with normal realignment ([Formula: see text]). The development of microstructural abnormalities was associated with the predicted fiber-level stretch ([Formula: see text]), but not with the elemental maximum principal stress or maximum principal strain by logistic regression. The multiscale models elucidate a potential mechanical basis for predicting injury-prone tissue domains and for defining the relationships between macroscopic ligament stretch and microscale pathophysiology in the subfailure regime.

Collagen organization regulates stretch-initiated pain-related neuronal signals in vitro: Implications for structure-function relationships in innervated ligaments

Injury to the spinal facet capsule, an innervated ligament with heterogeneous collagen organization, produces pain. Although mechanical facet joint trauma activates embedded afferents, it is unclear if, and how, the varied extracellular microstructure of its ligament affects sensory transduction for pain from mechanical inputs. To investigate the effects of macroscopic deformations on afferents in collagen matrices with different organizations, an in vitro neuron-collagen construct (NCC) model was used. NCCs with either randomly organized or parallel aligned collagen fibers were used to mimic the varied microstructure in the facet capsular ligament. Embryonic rat dorsal root ganglia (DRG) were encapsulated in the NCCs; axonal outgrowth was uniform and in all directions in random NCCs, but parallel in aligned NCCs. NCCs underwent uniaxial stretch (0.25 ± 0.06 strain) corresponding to sub-failure facet capsule strains that induce pain. Macroscopic NCC mechanics were measured and axonal expression of phosphorylated extracellular signal-regulated kinase (pERK) and the neurotransmitter substance P (SP) was assayed at 1 day to assess neuronal activation and nociception. Stretch significantly upregulated pERK expression in both random and aligned gels (p < 0.001), with the increase in pERK being significantly higher (p = 0.013) in aligned than in random NCCs. That increase likely relates to the higher peak force (p = 0.025) and stronger axon alignment (p < 0.001) with stretch direction in the aligned NCCs. In contrast, SP expression was greater in stretched NCCs (p < 0.001) regardless of collagen organization. These findings suggest that collagen organization differentially modulates pain-related neuronal signaling and support structural heterogeneity of ligament tissue as mediating sensory function. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:770-777, 2018.

Collagen Organization in Facet Capsular Ligaments Varies With Spinal Region and With Ligament Deformation

The spinal facet capsular ligament (FCL) is primarily comprised of heterogeneous arrangements of collagen fibers. This complex fibrous structure and its evolution under loading play a critical role in determining the mechanical behavior of the FCL. A lack of analytical tools to characterize the spatial anisotropy and heterogeneity of the FCL's microstructure has limited the current understanding of its structure-function relationships. Here, the collagen organization was characterized using spatial correlation analysis of the FCL's optically obtained fiber orientation field. FCLs from the cervical and lumbar spinal regions were characterized in terms of their structure, as was the reorganization of collagen in stretched cervical FCLs. Higher degrees of intra- and intersample heterogeneity were found in cervical FCLs than in lumbar specimens. In the cervical FCLs, heterogeneity was manifested in the form of curvy patterns formed by collections of collagen fibers or fiber bundles. Tensile stretch, a common injury mechanism for the cervical FCL, significantly increased the spatial correlation length in the stretch direction, indicating an elongation of the observed structural features. Finally, an affine estimation for the change of correlation length under loading was performed which gave predictions very similar to the actual values. These findings provide structural insights for multiscale mechanical analyses of the FCLs from various spinal regions and also suggest methods for quantitative characterization of complex tissue patterns.

A Nociceptive Role for Integrin Signaling in Pain After Mechanical Injury to the Spinal Facet Capsular Ligament

Integrins modulate chemically-induced nociception in a variety of inflammatory and neuropathic pain models. Yet, the role of integrins in mechanically-induced pain remains undefined, despite its well-known involvement in cell adhesion and mechanotransduction. Excessive spinal facet capsular ligament stretch is a common injury that induces morphological and functional changes in its innervating afferent neurons and can lead to pain. However, the local mechanisms underlying the translation from tissue deformation to pain signaling are unclear, impeding effective treatment. Therefore, the involvement of the integrin subunit β1 in pain signaling from facet injury was investigated in complementary in vivo and in vitro studies. An anatomical study in the rat identified expression of the integrin subunit β1 in dorsal root ganglion (DRG) neurons innervating the facet, with greater expression in peptidergic than non-peptidergic DRG neurons. Painful facet capsule stretch in the rat upregulated the integrin subunit β1 in small- and medium-diameter DRG neurons at day 7. Inhibiting the α2β1 integrin in a DRG-collagen culture prior to its stretch injury prevented strain-induced increases in axonal substance P (SP) in a dose-dependent manner. Together, these findings suggest that integrin subunit β1-dependent pathways may contribute to SP-mediated pain from mechanical injury of the facet capsular ligament.

The Interface of Mechanics and Nociception in Joint Pathophysiology: Insights From the Facet and Temporomandibular Joints

Chronic joint pain is a widespread problem that frequently occurs with aging and trauma. Pain occurs most often in synovial joints, the body's load bearing joints. The mechanical and molecular mechanisms contributing to synovial joint pain are reviewed using two examples, the cervical spinal facet joints and the temporomandibular joint (TMJ). Although much work has focused on the macroscale mechanics of joints in health and disease, the combined influence of tissue mechanics, molecular processes, and nociception in joint pain has only recently become a focus. Trauma and repeated loading can induce structural and biochemical changes in joints, altering their microenvironment and modifying the biomechanics of their constitutive tissues, which themselves are innervated. Peripheral pain sensors can become activated in response to changes in the joint microenvironment and relay pain signals to the spinal cord and brain where pain is processed and perceived. In some cases, pain circuitry is permanently changed, which may be a potential mechanism for sustained joint pain. However, it is most likely that alterations in both the joint microenvironment and the central nervous system (CNS) contribute to chronic pain. As such, the challenge of treating joint pain and degeneration is temporally and spatially complicated. This review summarizes anatomy, physiology, and pathophysiology of these joints and the sensory pain relays. Pain pathways are postulated to be sensitized by many factors, including degeneration and biochemical priming, with effects on thresholds for mechanical injury and/or dysfunction. Initiators of joint pain are discussed in the context of clinical challenges including the diagnosis and treatment of pain.

Ablation of IB4 non-peptidergic afferents in the rat facet joint prevents injury-induced pain and thalamic hyperexcitability via supraspinal glutamate transporters

The facet joint is a common source of neck pain, particularly after excessive stretch of its capsular ligament. Peptidergic afferents have been shown to have an important role in the development and maintenance of mechanical hyperalgesia, dysregulated nociceptive signaling, and spinal hyperexcitability that develop after mechanical injury to the facet joint. However, the role of non-peptidergic isolectin-B4 (IB4) cells in mediating joint pain is unknown. Isolectin-B4 saporin (IB4-SAP) was injected into the facet joint to ablate non-peptidergic cells, and the facet joint later underwent a ligament stretch known to induce pain. Behavioral sensitivity, thalamic glutamate transporter expression, and thalamic hyperexcitability were evaluated up to and at day 7. Administering IB4-SAP prior to a painful injury prevented the development of mechanical hyperalgesia that is typically present. Intra-articular IB4-SAP also prevented the upregulation of the glutamate transporters GLT-1 and EAAC1 in the ventral posterolateral nucleus of the thalamus and reduced thalamic neuronal hyperexcitability at day 7. These findings suggest that a painful facet injury induces changes extending to supraspinal structures and that IB4-positive afferents in the facet joint may be critical for the development and maintenance of sensitization in the thalamus after a painful facet joint injury.

How Do We Meet the Challenge of Whiplash?

A previous special issue of JOSPT (October 2016) discussed whiplash in terms of the clinical problems and current research surrounding prevention, biomechanics of injury, emergent care, imaging advancements, recovery pathways and prognosis, pathogenesis of posttrauma pain, acute and chronic management, and new predictive clinical tools. While great strides have been made in the field of whiplash and are continuing in earnest, a key group of clinicians and academics have recognized that inconsistent outcomes in published literature hamper our ability to meaningfully synthesize research findings, leading to results of systematic reviews that provide very few concrete clinical recommendations. We are optimistic that improved outcomes for people with whiplash-associated disorder (WAD) are attainable in the near future, as interdisciplinary research efforts continue to align internationally, new mechanisms are identified and explored, and advanced statistical techniques allow complex questions to be answered in clinically meaningful ways. J Orthop Sports Phys Ther 2017;47(7):444-446. doi:10.2519/jospt.2017.0106.

The Physiological Basis of Cervical Facet-Mediated Persistent Pain: Basic Science and Clinical Challenges

Synopsis Chronic neck pain is a common condition and a primary clinical symptom of whiplash and other spinal injuries. Loading-induced neck injuries produce abnormal kinematics between the vertebrae, with the potential to injure facet joints and the afferent fibers that innervate the specific joint tissues, including the capsular ligament. Mechanoreceptive and nociceptive afferents that innervate the facet have their peripheral terminals in the capsule, cell bodies in the dorsal root ganglia, and terminal processes in the spinal cord. As such, biomechanical loading of these afferents can initiate nociceptive signaling in the peripheral and central nervous systems. Their activation depends on the local mechanical environment of the joint and encodes the neural processes that initiate pain and lead to its persistence. This commentary reviews the complex anatomical, biomechanical, and physiological consequences of facet-mediated whiplash injury and pain. The clinical presentation of facet-mediated pain is complex in its sensory and emotional components. Yet, human studies are limited in their ability to elucidate the physiological mechanisms by which abnormal facet loading leads to pain. Over the past decade, however, in vivo models of cervical facet injury that reproduce clinical pain symptoms have been developed and used to define the complicated and multifaceted electrophysiological, inflammatory, and nociceptive signaling cascades that are involved in the pathophysiology of whiplash facet pain. Integrating the whiplash-like mechanics in vivo and in vitro allows transmission of pathophysiological mechanisms across scales, with the hope of informing clinical management. Yet, despite these advances, many challenges remain. This commentary further describes and highlights such challenges. J Orthop Sports Phys Ther 2017;47(7):450-461. Epub 16 Jun 2017. doi:10.2519/jospt.2017.7255.

Painful Cervical Facet Joint Injury Is Accompanied by Changes in the Number of Excitatory and Inhibitory Synapses in the Superficial Dorsal Horn That Differentially Relate to Local Tissue Injury Severity

STUDY DESIGN

Immunohistochemistry labeled pre- and postsynaptic structural markers to quantify excitatory and inhibitory synapses in the spinal superficial dorsal horn at 14 days after painful facet joint injury in the rat.

OBJECTIVE

The objective of this study was to investigate the relationship between pain and synapse density in the spinal cord after facet injury.

SUMMARY OF BACKGROUND DATA

Neck pain is a major contributor to disability and often becomes chronic. The cervical facet joints are susceptible to loading-induced painful injury, initiating spinal central sensitization responses. Although excitatory synapse plasticity has been reported in the superficial dorsal horn early after painful facet injury, whether excitatory and/or inhibitory synapse density is altered at a time when pain is maintained is unknown.

METHODS

Rats underwent either a painful C6/C7 facet joint distraction or sham surgery. Mechanical hyperalgesia was measured and immunohistochemistry techniques for synapse quantification were used to quantify excitatory and inhibitory synapse densities in the superficial dorsal horn at day 14. Logarithmic correlation analyses evaluated whether the severity of facet injury correlated with either behavioral or synaptic outcomes.

RESULTS

Facet joint injury induces pain that is sustained until day 14 (P <0.001) and both significantly greater excitatory synapse density (P = 0.042) and lower inhibitory synapse density (P = 0.0029) in the superficial dorsal horn at day 14. Injury severity is significantly correlated with pain at days 1 (P = 0.0011) and 14 (P = 0.0002), but only with inhibitory, not excitatory, synapse density (P = 0.0025) at day 14.

CONCLUSION

This study demonstrates a role for structural plasticity in both excitatory and inhibitory synapses in the maintenance of facet-mediated joint pain, and that altered inhibitory, but not excitatory, synapse density correlates to the severity of painful joint injury. Understanding the functional consequences of this spinal structural plasticity is critical to elucidate mechanisms of chronic joint pain.

LEVEL OF EVIDENCE

N /A.

Upper Cervical Spine Loading Simulating a Dynamic Low-Speed Collision Significantly Increases the Risk of Pain Compared to Quasi-Static Loading With Equivalent Neck Kinematics

Dynamic cervical spine loading can produce facet capsule injury. Despite a large proportion of neck pain being attributable to the C2/C3 facet capsule, potential mechanisms are not understood. This study replicated low-speed frontal and rear-end traffic collisions in occiput-C3 human cadaveric cervical spine specimens and used kinematic and full-field strain analyses to assess injury. Specimens were loaded quasi-statically in flexion and extension before and after dynamic rotation of C3 at 100 deg/s. Global kinematics in the sagittal plane were tracked at 1 kHz, and C2/C3 facet capsule full-field strains were measured. Dynamic loading did not alter the kinematics from those during quasi-static (QS) loading, but maximum principal strain (MPS) and shear strain (SS) were significantly higher (p = 0.028) in dynamic flexion than for the same quasi-static conditions. The full-field strain analysis demonstrated that capsule strain was inhomogeneous, and that the peak MPS generally occurred in the anterior aspect and along the line of the C2/C3 facet joint. The strain magnitude in dynamic flexion continued to rise after the rotation of C3 had stopped, with a peak MPS of 12.52 ± 4.59% and a maximum SS of 5.34 ± 1.60%. The peak MPS in loading representative of rear-end collisions approached magnitudes previously shown to induce pain in vivo, whereas strain analysis using linear approaches across the facet joint was lower and may underestimate injury risk compared to full-field analysis. The time at which peak MPS occurred suggests that the deceleration following a collision is critical in relation to the production of injurious strains within the facet capsule.

Tissue Strain Reorganizes Collagen With a Switchlike Response That Regulates Neuronal Extracellular Signal-Regulated Kinase Phosphorylation In Vitro: Implications for Ligamentous Injury and Mechanotransduction

Excessive loading of ligaments can activate the neural afferents that innervate the collagenous tissue, leading to a host of pathologies including pain. An integrated experimental and modeling approach was used to define the responses of neurons and the surrounding collagen fibers to the ligamentous matrix loading and to begin to understand how macroscopic deformation is translated to neuronal loading and signaling. A neuron-collagen construct (NCC) developed to mimic innervation of collagenous tissue underwent tension to strains simulating nonpainful (8%) or painful ligament loading (16%). Both neuronal phosphorylation of extracellular signal-regulated kinase (ERK), which is related to neuroplasticity (R2 ≥ 0.041; p ≤ 0.0171) and neuronal aspect ratio (AR) (R2 ≥ 0.250; p < 0.0001), were significantly correlated with tissue-level strains. As NCC strains increased during a slowly applied loading (1%/s), a "switchlike" fiber realignment response was detected with collagen reorganization occurring only above a transition point of 11.3% strain. A finite-element based discrete fiber network (DFN) model predicted that at bulk strains above the transition point, heterogeneous fiber strains were both tensile and compressive and increased, with strains in some fibers along the loading direction exceeding the applied bulk strain. The transition point identified for changes in collagen fiber realignment was consistent with the measured strain threshold (11.7% with a 95% confidence interval of 10.2-13.4%) for elevating ERK phosphorylation after loading. As with collagen fiber realignment, the greatest degree of neuronal reorientation toward the loading direction was observed at the NCC distraction corresponding to painful loading. Because activation of neuronal ERK occurred only at strains that produced evident collagen fiber realignment, findings suggest that tissue strain-induced changes in the micromechanical environment, especially altered local collagen fiber kinematics, may be associated with mechanotransduction signaling in neurons.

Stretch-induced network reconfiguration of collagen fibres in the human facet capsular ligament

Biomaterials can display complex spatial patterns of cellular responses to external forces. Revealing and predicting the role of these patterns in material failure require an understanding of the statistical dependencies between spatially distributed changes in a cell's local biomechanical environment, including altered collagen fibre kinematics in the extracellular matrix. Here, we develop and apply a novel extension of network science methods to investigate how excessive tensile stretch of the human cervical facet capsular ligament (FCL), a common source of chronic neck pain, affects the local reorganization of collagen fibres. We define collagen alignment networks based on similarity in fibre alignment angles measured by quantitative polarized light imaging. We quantify the reorganization of these networks following macroscopic loading by describing the dynamic reconfiguration of network communities, regions of the material that display similar fibre alignment angles. Alterations in community structure occur smoothly over time, indicating coordinated adaptation of fibres to loading. Moreover, flexibility, a measure of network reconfiguration, tracks the loss of FCL's mechanical integrity at the onset of anomalous realignment (AR) and regions of AR display altered community structure. These findings use novel network-based techniques to explain abnormal collagen fibre reorganization, a dynamic and coordinated multivariate process underlying tissue failure.

Whole-body Vibration at Thoracic Resonance Induces Sustained Pain and Widespread Cervical Neuroinflammation in the Rat

BACKGROUND

Whole-body vibration (WBV) is associated with back and neck pain in military personnel and civilians. However, the role of vibration frequency and the physiological mechanisms involved in pain symptoms are unknown.

QUESTIONS/PURPOSES

This study asked the following questions: (1) What is the resonance frequency of the rat spine for WBV along the spinal axis, and how does frequency of WBV alter the extent of spinal compression/extension? (2) Does a single WBV exposure at resonance induce pain that is sustained? (3) Does WBV at resonance alter the protein kinase C epsilon (PKCε) response in the dorsal root ganglia (DRG)? (4) Does WBV at resonance alter expression of calcitonin gene-related peptide (CGRP) in the spinal dorsal horn? (5) Does WBV at resonance alter the spinal neuroimmune responses that regulate pain?

METHODS

Resonance of the rat (410 ± 34 g, n = 9) was measured by imposing WBV at frequencies from 3 to 15 Hz. Separate groups (317 ± 20 g, n = 10/treatment) underwent WBV at resonance (8 Hz) or at a nonresonant frequency (15 Hz). Behavioral sensitivity was assessed throughout to measure pain, and PKCε in the DRG was quantified as well as spinal CGRP, glial activation, and cytokine levels at Day 14.

RESULTS

Accelerometer-based thoracic transmissibility peaks at 8 Hz (1.86 ± 0.19) and 9 Hz (1.95 ± 0.19, mean difference [MD] 0.290 ± 0.266, p < 0.03), whereas the video-based thoracic transmissibility peaks at 8 Hz (1.90 ± 0.27), 9 Hz (2.07 ± 0.20), and 10 Hz (1.80 ± 0.25, MD 0.359 ± 0.284, p < 0.01). WBV at 8 Hz produces more cervical extension (0.745 ± 0.582 mm, MD 0.242 ± 0.214, p < 0.03) and compression (0.870 ± 0.676 mm, MD 0.326 ± 0.261, p < 0.02) than 15 Hz (extension, 0.503 ± 0.279 mm; compression, 0.544 ± 0.400 mm). Pain is longer lasting (through Day 14) and more robust (p < 0.01) after WBV at the resonant frequency (8 Hz) compared with 15 Hz WBV. PKCε in the nociceptors of the DRG increases according to the severity of WBV with greatest increases after 8 Hz WBV (p < 0.03). However, spinal CGRP, cytokines, and glial activation are only evident after painful WBV at resonance.

CONCLUSIONS

WBV at resonance produces long-lasting pain and widespread activation of a host of nociceptive and neuroimmune responses as compared with WBV at a nonresonance condition. Based on this work, future investigations into the temporal and regional neuroimmune response to resonant WBV in both genders would be useful.

CLINICAL RELEVANCE

Although WBV is a major issue affecting the military population, there is little insight about its mechanisms of injury and pain. The neuroimmune responses produced by WBV are similar to other pain states, suggesting that pain from WBV may be mediated by similar mechanisms as other neuropathic pain conditions. This mechanistic insight suggests WBV-induced injury and pain may be tempered by antiinflammatory intervention.

Thrombospondin-4 and excitatory synaptogenesis promote spinal sensitization after painful mechanical joint injury

Facet joint injury induces persistent pain that may be maintained by structural plasticity in the spinal cord. Astrocyte-derived thrombospondins, especially thrombospondin-4 (TSP4), have been implicated in synaptogenesis and spinal sensitization in neuropathic pain, but the TSP4 response and its relationship to synaptic changes in the spinal cord have not been investigated for painful joint injury. This study investigates the role of TSP4 in the development and maintenance of persistent pain following injurious facet joint distraction in rats and tests the hypothesis that excitatory synaptogenesis contributes to such pain. Painful facet joint loading induces dorsal horn excitatory synaptogenesis along with decreased TSP4 in the DRG and increased astrocytic release of TSP4 in the spinal cord, all of which parallel the time course of sustained tactile allodynia. Blocking injury-induced spinal TSP4 expression with antisense oligonucleotides or reducing TSP4 activity at its neuronal receptor in the spinal cord with gabapentin treatment both attenuate the allodynia and dorsal horn synaptogenesis that develop after painful facet joint loading. Increased spinal TSP4 also facilitates the development of allodynia and spinal hyperexcitability, even after non-painful physiological loading of the facet joint. These results suggest that spinal TSP4 plays an important role in the development and maintenance of persistent joint-mediated pain by inducing excitatory synaptogenesis and facilitating the transduction of mechanical loading of the facet joint that leads to spinal hyperexcitability.

Early afferent activity from the facet joint after painful trauma to its capsule potentiates neuronal excitability and glutamate signaling in the spinal cord

Cervical facet joint injury induces persistent pain and central sensitization. Preventing the peripheral neuronal signals that initiate sensitization attenuates neuropathic pain. Yet, there is no clear relationship among facet joint afferent activity, development of central sensitization, and pain, which may be hindering effective treatments for this pain syndrome. This study investigates how afferent activity from the injured cervical facet joint affects induction of behavioral sensitivity and central sensitization. Intra-articular bupivacaine was administered to transiently suppress afferent activity immediately or 4 days after facet injury. Mechanical hyperalgesia was monitored after injury, and spinal neuronal hyperexcitability and spinal expression of proteins that promote neuronal excitability were measured on day 7. Facet injury with saline vehicle treatment induced significant mechanical hyperalgesia (P<.027), dorsal horn neuronal hyperexcitability (P<.026), upregulation of pERK1/2, pNR1, mGluR5, GLAST, and GFAP, and downregulation of GLT1 (P<.032). However, intra-articular bupivacaine immediately after injury significantly attenuated hyperalgesia (P<.0001), neuronal hyperexcitability (P<.004), and dysregulation of excitatory signaling proteins (P<.049). In contrast, intra-articular bupivacaine at day 4 had no effect on these outcomes. Silencing afferent activity during the development of neuronal hyperexcitability (4 hours, 8 hours, 1 day) attenuated hyperalgesia and neuronal hyperexcitability (P<.045) only for the treatment given 4 hours after injury. This study suggests that early afferent activity from the injured facet induces development of spinal sensitization via spinal excitatory glutamatergic signaling. Peripheral intervention blocking afferent activity is effective only over a short period of time early after injury and before spinal modifications develop, and is independent of modulating spinal glial activation.

Ablating spinal NK1-bearing neurons eliminates the development of pain and reduces spinal neuronal hyperexcitability and inflammation from mechanical joint injury in the rat

The facet joint is a common source of pain, especially from mechanical injury. Although chronic pain is associated with altered spinal glial and neuronal responses, the contribution of specific spinal cells to joint pain is not understood. This study used the neurotoxin [Sar(9),Met(O2)(11)]-substance P-saporin (SSP-SAP) to selectively eliminate spinal cells expressing neurokinin-1 receptor (NK1R) in a rat model of painful facet joint injury to determine the role of those spinal neurons in pain from facet injury. Following spinal administration of SSP-SAP or its control (blank-SAP), a cervical facet injury was imposed and behavioral sensitivity was assessed. Spinal extracellular recordings were made on day 7 to classify neurons and quantify evoked firing. Spinal glial activation and interleukin 1αα (IL1α) expression also were evaluated. SSP-SAP prevented the development of mechanical hyperalgesia that is induced by joint injury and reduced NK1R expression and mechanically evoked neuronal firing in the dorsal horn. SSP-SAP also prevented a shift toward wide dynamic range neurons that is seen after injury. Spinal astrocytic activation and interleukin 1α (IL1α) expression were reduced to sham levels with SSP-SAP treatment. These results suggest that spinal NK1R-bearing cells are critical in initiating spinal nociception and inflammation associated with a painful mechanical joint injury.

PERSPECTIVE

Results demonstrate that cells expressing NK1R in the spinal cord are critical for the development of joint pain, spinal neuroplasticity, and inflammation after trauma to the joint. These findings have utility for understanding mechanisms of joint pain and developing potential targets to treat pain.

The roles of mechanical compression and chemical irritation in regulating spinal neuronal signaling in painful cervical nerve root injury

Both traumatic and slow-onset disc herniation can directly compress and/or chemically irritate cervical nerve roots, and both types of root injury elicit pain in animal models of radiculopathy. This study investigated the relative contributions of mechanical compression and chemical irritation of the nerve root to spinal regulation of neuronal activity using several outcomes. Modifications of two proteins known to regulate neurotransmission in the spinal cord, the neuropeptide calcitonin gene-related peptide (CGRP) and glutamate transporter 1 (GLT-1), were assessed in a rat model after painful cervical nerve root injuries using a mechanical compression, chemical irritation or their combination of injury. Only injuries with compression induced sustained behavioral hypersensitivity (p≤0.05) for two weeks and significant decreases (p<0.037) in CGRP and GLT-1 immunoreactivity to nearly half that of sham levels in the superficial dorsal horn. Because modification of spinal CGRP and GLT-1 is associated with enhanced excitatory signaling in the spinal cord, a second study evaluated the electrophysiological properties of neurons in the superficial and deeper dorsal horn at day 7 after a painful root compression. The evoked firing rate was significantly increased (p=0.045) after compression and only in the deeper lamina. The painful compression also induced a significant (p=0.002) shift in the percentage of neurons in the superficial lamina classified as low- threshold mechanoreceptive (sham 38%; compression 10%) to those classified as wide dynamic range neurons (sham 43%; compression 74%). Together, these studies highlight mechanical compression as a key modulator of spinal neuronal signaling in the context of radicular injury and pain.

Spinal neuronal plasticity is evident within 1 day after a painful cervical facet joint injury

Excessive stretch of the cervical facet capsular ligament induces persistent pain and spinal plasticity at later time points. Yet, it is not known when such spinal modifications are initiated following this painful injury. This study investigates the development of hyperalgesia and neuronal hyperexcitability in the spinal cord after a facet joint injury. Behavioral sensitivity was measured in a model of painful C6/C7 facet joint injury in the rat, and neuronal hyperexcitability in the spinal cord was evaluated at 6h and 1 day after injury or a sham procedure, in separate groups. Extracellular recordings of C6/C7 dorsal horn neuronal activity (229 neurons) were used to quantify spontaneous and evoked firing. Rats exhibited no change in sensitivity to mechanical stimulation of the forepaw at 6h, but did exhibit increased sensitivity at 1 day after injury (p=0.012). At 6h, both spontaneous neuronal activity and firing evoked by light brushing, pinch, and von Frey filaments (1.4-26g) applied at the forepaw were not different between sham and injury. At 1 day, spontaneous firing was noted in a greater number of neurons after injury than sham (p<0.04). Evoked firing was also increased 1 day after injury compared to normal and sham (p<0.03). Dorsal horn hyperexcitability and increased spontaneous firing developed between 6 and 24h after painful facet injury, suggesting that the development of hyperalgesia parallels dorsal horn hyperexcitability following mechanical facet joint injury, and these spinal mechanisms are initiated as early as 1 day after injury.

Muscle pathologies after cervical spine distortion-like exposure--a porcine model

OBJECTIVE

Histological evaluation of porcine posterior cervical muscles after a forceful translational and extensional head retraction simulating high-speed rear end impact.

METHODS

Four anesthetized pigs were exposed to a cervical spine distortion (CSD)-like motion in a lying position. After 2 different survival times of 4 and 6 h (posttrauma), the pigs were euthanized and tissue sampling of posterior cervical muscles was performed. A standard histological staining method involving paraffin-embedded sections was used to analyze the muscles, focusing on injury signs like hemorrhage and inflammatory cell reaction. A pig that was not subjected to impact was used as a control pig and was subjected to the same procedure to exclude any potential artifacts from the autopsy.

RESULTS

The differentiation of 8 different posterior neck muscles in the dissection process was successful in more than 50 percent for each muscle of interest. Staining and valid analysis was possible from all extracted samples. Muscle injuries to the deepest posterior neck muscles could be found, especially in the musculus obliquus samples, which showed laminar bleedings in 4 out of 4 samples. In addition, in 4 out of 4 samples we were able to see increased cellular reactions. The splenius muscle also showed bleeding in all 4 samples. All animals showed muscle injury signs in more than three quarters of analyzed neck muscles. Differences between survival times of 4 and 6 h in terms of muscular injury were not of primary interest and could not be found.

CONCLUSIONS

By simulating a CSD-like motion we were able to confirm injuries in the posterior cervical muscles under severe loading conditions. Further studies need to be conducted to determine whether these muscle injuries also occur under lower exposure forces.