What can human experimental pain models teach us about clinical TMD?

Local anaesthesia decreases nerve growth factor induced masseter hyperalgesia

The aim of this investigation was to evaluate the effects of local anaesthesia on nerve growth factor (NGF) induced masseter hyperalgesia. Healthy participants randomly received an injection into the right masseter muscle of either isotonic saline (IS) given as a single injection (n = 15) or an injection of NGF (n = 30) followed by a second injection of lidocaine (NGF + lidocaine; n = 15) or IS (NGF + IS; n = 15) in the same muscle 48 h later. Mechanical sensitivity scores of the right and left masseter, referred sensations and jaw pain intensity and jaw function were assessed at baseline, 48 h after the first injection, 5 min after the second injection and 72 h after the first injection. NGF caused significant jaw pain evoked by chewing at 48 and 72 h after the first injection when compared to the IS group, but without significant differences between the NGF + lidocaine and NGF + IS groups. However, the mechanical sensitivity of the right masseter 5 min after the second injection in the NGF + lidocaine group was significantly lower than the second injection in the NGF + IS and was similar to the IS group. There were no significant differences for the referred sensations. Local anaesthetics may provide relevant information regarding the contribution of peripheral mechanisms in the maintenance of persistent musculoskeletal pain.

Modulation of experimental facial pain via somatosensory stimuli targeting sensations of different valence

BACKGROUND

Knowledge of pain modulation from oro-facial somatosensory stimuli with different valence (pleasant-unpleasant) is limited.

OBJECTIVES

To investigate (a) the modulatory effects of painful, pleasant and unpleasant somatosensory stimuli on two models of experimental facial pain, (b) whether modulation could be changed by blocking peripheral nerves via application of a local anaesthetic, EMLA, or blocking endogenous opioid receptors via naltrexone and (c) whether pain ratings were significantly correlated with participant psychological profiles.

METHODS

Thirty-eight healthy women received experimental facial skin burning pain or jaw myalgia for four randomised sessions on different days. The painful region was stimulated with mechanical or thermal painful, pleasant, unpleasant and control stimuli, with ratings recorded before and during stimulation. Sessions differed in pre-treatment: EMLA/naltrexone/placebo tablet/cream.

RESULTS

Significant effects of thermal or mechanical stimuli (P < .017), but not session (P > .102), were found on pain ratings for both models. In myalgia, painful cold resulted in a greater reduction in pain ratings than unpleasant cold, pleasant cold, control and pleasant warmth (P < .004). Decreases in pain ratings from painful, unpleasant and pleasant mechanical stimuli were greater than control (P < .002). In burning pain, painful cold resulted in a greater reduction in pain ratings than all but one of the other thermal stimuli (P < .033). The pleasant mechanical stimulus reduced pain ratings more than all other mechanical stimuli (P ≤ .003). There were no significant correlations between pain and psychometrics.

CONCLUSION

Valence-targeted thermal and mechanical stimuli modulated experimental myalgia and skin burning pain (P < .017). Partially blocking peripheral afferents or opioid receptors did not affect modulation.

Use of orthopedic manual physical therapy to manage chronic orofacial pain and tension-type headache in an adolescent

Temporomandibular dysfunction (TMD) is frequently associated with multiple headache types. While the efficacy of orthopedic manual physical therapy (OMPT) intervention for TMD with/without headache symptoms has been investigated, it has received less attention than other musculoskeletal conditions. This case describes the OMPT treatment and outcome of a 14 year-old female with a 2-year history of facial pain surrounding the right temporomandibular joint (TMJ), and bilateral occipital and temporal tension-type headaches (TTHs). Intervention included manual mobilization of the TMJ and surrounding cervical/facial soft tissue structures, postural re-education, and patient/family education on prevention, self-treatment, and postural adaptations. Outcomes included pain free and maximal mouth opening (MMO), pressure pain threshold (PPT) over the masseter muscle belly, fear avoidance beliefs questionnaire-physical activity subscale (FABQ-PA), jaw pain and function questionnaire (JPFQ), visual analog pain scale (VAS) for facial pain, and headache frequency, duration, and intensity on a VAS. Results included increased pain free and MMO by 22 mm, improved PPT bilaterally by 403.05 kPa on the affected, and 360.88 kPa on the unaffected side, and decreased reported headache frequency, duration, and intensity. Score decreases of 5 points on VAS with MMO, 8 points on FABQ-PA, and 5 points on JPFQ were also noted. Further research is warranted to investigate the effects of OMPT on pain processing and functional outcomes in patients with TMD and headache.

Neurobiology of estrogen status in deep craniofacial pain

Pain in the temporomandibular joint (TMJ) region often occurs with no overt signs of injury or inflammation. Although the etiology of TMJ-related pain may involve multiple factors, one likely risk factor is female gender or estrogen status. Evidence is reviewed from human and animal studies, supporting the proposition that estrogen status acts peripherally or centrally to influence TMJ nociceptive processing. A new model termed the "TMJ pain matrix" is proposed as critical for the initial integration of TMJ-related sensory signals in the lower brainstem that is both modified by estrogen status, and closely linked to endogenous pain and autonomic control pathways.

Surface electromyography in the assessment of jaw elevator muscles

Surface electromyography (EMG) allows the non-invasive investigation of the bioelectrical phenomena of muscular contraction. The clinical application of surface EMG recordings has been long debated. This paper reviews the main limitations and the current applications of the surface EMG in the investigation of jaw elevator muscles. Methodological factors associated with the recording of the surface EMG may reduce the reliability and sensitivity of this technique and may have been the cause of controversial results reported in different studies. Despite these problems, several clinical applications of surface EMG in jaw muscles are promising. Moreover, technological advances in signal detection and processing have improved the quality of the information extracted from the surface EMG and furthered our understanding of the anatomy and physiology of the stomatognathic apparatus.

Glutamate-evoked jaw muscle pain as a model of persistent myofascial TMD pain?

OBJECTIVE

Compare pain-related measures and psychosocial variables between glutamate-evoked jaw muscle pain in healthy subjects (HS) and patients with persistent myofascial temporomandibular disorder (TMD) pain.

DESIGN

Forty-seven female HS and 10 female patients with persistent myofascial TMD pain participated. The HS received an injection of glutamate into the masseter muscle to model persistent myofascial TMD pain. Participants filled out a coping strategies questionnaire (CSQ), the symptom checklist 90 (SCL-90) and McGill pain questionnaire (MPQ). Pain intensity was assessed on an electronic visual analogue scale (VAS). Pain-drawing areas, numerical rating scale (NRS) scores of unpleasantness, pressure pain thresholds (PPTs) and pressure pain tolerance (PPTOL) were measured. Unpaired t-tests and correlation tests were used for analyses.

RESULTS

The groups were significantly different when comparing the CSQ scores of control, decrease, diverting attention, increase of behavioural activities and somatization. The peak VAS pain, NRS of unpleasantness and MPQ scores were not significantly different between groups, but PPT and PPTOL were significantly lower in the TMD patients. Significant positive correlations were found in the TMD patients between peak VAS pain and CSQ catastrophizing score and SCL-90 somatization. The scores of PPTs and PPTOLs, in patients showed positive correlations with CSQ reinterpreting pain sensations scores and PPTs correlated with CSQ praying/hoping scores.

CONCLUSIONS

Glutamate-evoked pain responses in HS and persistent myofascial TMD pain have similar sensory-discriminative and affective-unpleasantness components but differ in psychosocial features. This study suggests that experimental designs based on glutamate injection into muscle can provide an appropriate model for elucidating persistent myofascial pain conditions.

Review of clinical EMG studies related to muscle and occlusal factors in healthy and TMD subjects

Several electronic instruments have been developed as adjuncts to objectively record the dysfunctional features of temporomandibular disorders and to study the effectiveness of various treatment interventions. The aim of this review was to assess the value and contribution of clinical electromyographic research in the understanding of asymptomatic and dysfunctional muscle function and the therapeutic effects of interocclusal appliances. For this purpose MedLine and PubMed searches were conducted with the following main keywords alone and in various combinations: electromyography, muscles of mastication, masseter, temporalis, temporomandibular, TMD, utility, validity, repeatability, rest, postural, vertical dimension, occlusal, splint, treatment. The review includes critical evaluation, discussion and conclusions regarding electromyographic studies in asymptomatic and dysfunctional muscles, rest position, occlusal parameters and interocclusal appliances, as well as a critical summary and proposals for further research. Much of earlier critique of many electromyographic studies still applies regarding comparative sample selections, research designs, analyses and conclusions. The areas not well-understood include normal biological variation, capacity for adaptation, fluctuations regarding the clinical course and multidimensional features of temporomandibular disorders and long-term follow-up data, especially in studies that evaluate the effectiveness of therapeutic measures. Considering the required improvements in technical and research designs features and critical appraisal electromyographic research could have value as an adjunct research tool to study features of craniofacial muscle-related dysfunction. Until electromyographic measures are correlated with other multidimensional, especially subjective and pain-related methods, the clinical use of this method for diagnostic purposes of temporomandibular disorders remains in doubt, and is not at present recommended.