Functional properties of single motor units in the human medial pterygoid muscle: Thresholds.

BACKGROUND: Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements. OBJECTIVES: The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal-directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle. METHODS: Intra-muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening-closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid-incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal-Wallis tests; significance accepted at P < .05. RESULTS: A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n = 62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n = 208); there were no significant associations for the protrusive (n = 131) and opening-closing (n = 58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties. CONCLUSIONS: The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening-closing jaw movements.

Clinical and laboratory surface finishing procedures for zirconia on opposing human enamel wear: A laboratory study.

AIM: To investigate the effect of laboratory and clinical finishing procedures for zirconia on antagonistic enamel wear. MATERIALS AND METHODS: Forty-eight yttria-tetragonal partially stabilised zirconia (Y-TZP) specimens were prepared and divided into four groups according to their surface preparation: laboratory polished (LP); laboratory polished and glazed (G); clinically adjusted (CA); and clinically adjusted and repolished (CAR). Enamel opposing enamel was used as a control. Pre-testing surface roughness for each group was determined using contact profilometry. Two-body wear resistance tests were conducted using a masticatory simulator. Enamel specimens were subjected to 120,000 cycles in distilled water (frequency 1.6 Hz, loading force of 49 N). Volumetric and vertical enamel losses were measured by superimposition of pre- and post-testing images using a three-dimensional laser scanner and software analysis. Scanning electron microscopy was used for qualitative surface analysis of pre- and post-testing zirconia and enamel surfaces. One-way ANOVA and multiple comparisons with Bonferroni corrections were used for statistical analysis at a significance level of α=0.05. RESULTS: There was no statistical difference in volumetric and vertical enamel loss between CAR, G and LP. CAR produced statistically significantly less volumetric enamel loss compared with CA and control, and statistically significantly less vertical enamel loss compared with CA. Volumetric and vertical enamel loss were highly correlated in all groups. CONCLUSIONS: Enamel wear by clinically ground zirconia is comparable to that of opposing enamel surfaces and greater than clinically repolished zirconia. Repolishing of zirconia restorations following clinical adjustment with diamond burs is effective in reducing antagonistic enamel wear.

Changes in human primary motor cortex activity during acute cutaneous and muscle orofacial pain.

AIMS: To use functional magnetic resonance imaging (fMRI) to determine whether orofacial cutaneous or muscle pain is associated with changes in primary motor cortex (M1) activity that outlast the duration of perceived pain, and whether these M1 changes are different during cutaneous pain compared with muscle pain. METHODS: fMRI was used in healthy subjects experiencing orofacial muscle (n = 17) or cutaneous (n = 15) pain induced by bolus injections of hypertonic saline (4.5%) into the belly of the masseter muscle (0.5 ml) or subcutaneously (0.2 ml) into the overlying skin, respectively. To determine the effects of the injection volume, isotonic saline (n = 4) was injected into the masseter muscle. RESULTS: Similar pain scores were observed following subcutaneous (mean [± SEM]; 4.73 ± 0.51) or intramuscular injections (4.35 ± 0.56). Orofacial muscle but not cutaneous pain was associated with a transient increase in signal intensity in the contralateral M1. Cutaneous and muscle orofacial pains were associated with similar signal intensity decreases within the contralateral M1 that continued to decrease for the entire scanning period. Isotonic saline did not evoke pain or changes in M1 signal intensity. CONCLUSION: The transient contralateral M1 signal intensity increase during orofacial muscle pain may underlie escape-like motor patterns. However, once the initial threat has subsided, longer-term reductions in M1 activity and/or excitability may occur to aid in minimizing movement of the affected part, an effect consistent with the general proposals of the Pain Adaptation Model.

Differential activation of the human trigeminal nuclear complex by noxious and non-noxious orofacial stimulation.

There is good evidence from animal studies for segregation in the processing of non-nociceptive and nociceptive information within the trigeminal brainstem sensory nuclear complex. However, it remains unknown whether a similar segregation occurs in humans, and a recent tract tracing study suggests that this segregation may not exist. We used functional magnetic resonance imaging (fMRI) to define and compare activity patterns of the trigeminal brainstem nuclear complex during non-noxious and noxious cutaneous and non-noxious and noxious muscle orofacial stimulation in humans. We found that during cutaneous pain, signal intensity increased within the entire rostrocaudal extent of the spinal trigeminal nucleus (SpV), encompassing the ipsilateral oralis (SpVo), interpolaris (SpVi) and caudalis (SpVc) subdivisions. In contrast, muscle pain did not activate SpVi, but instead activated a discrete region of the ipsilateral SpVo and SpVc. Further, muscle noxious stimulation activated a region of the ipsilateral lateral pons in the region of the trigeminal principal sensory nucleus (Vp). Innocuous orofacial stimulation (lip brushing) also evoked a significant increase in signal intensity in the ipsilateral Vp; however, non-noxious muscle stimulation showed no increase in signal in this area. The data reveal that orofacial cutaneous and muscle nociceptive information and innocuous cutaneous stimulation are differentially represented within the trigeminal nuclear complex. It is well established that cutaneous and muscle noxious stimuli evoke different perceptual, behavioural and cardiovascular changes. We speculate that the differential activation evoked by cutaneous and muscle noxious stimuli within the trigeminal sensory complex may contribute to the neural basis for these differences.

Experimental jaw-muscle pain has a differential effect on different jaw movement tasks.

AIMS: To determine the effects of experimental jaw-muscle pain on jaw movements. METHODS: Mandibular mid-incisor point was tracked in 22 asymptomatic subjects during standardized (at 2.2 mm/s) protrusion, contralateral excursion, and open jaw movements, as well as free, right-sided chewing and chewing standardized for timing (900 ms/cycle). Tonic infusion of 4.5% hypertonic saline into the right masseter muscle maintained pain intensity between 30 and 60 mm on a 100-mm visual analog scale. Subjects performed tasks in 3 sessions on the same experimental day: control condition (baseline trials), test condition 1 (during hypertonic or 0.9% isotonic saline infusion), and test condition 2 (during isotonic or hypertonic saline infusion). RESULTS: In comparison with control, there were no significant effects of hypertonic saline infusion on amplitude or velocity for protrusion or contralateral jaw movements or on velocity for jaw opening. Jaw-opening amplitude was significantly smaller in comparison with control during hypertonic, but not isotonic, saline infusion. During free but not standardized chewing, subjects chewed faster and exhibited larger amplitude gapes during hypertonic and isotonic infusion in comparison with control. Therefore, it was unlikely that pain had an effect on the kinematic parameters of jaw movement during free chewing. Qualitatively, individual subject data revealed considerable variability in the effects of hypertonic saline on movement parameters, which suggests that the effect of pain on jaw movement may not be uniform between individuals. CONCLUSIONS: The data indicate that the effect of pain on jaw movement may vary with the task performed.

Pain intensity, illness duration, and protein catabolism in temporomandibular disorder patients with chronic muscle pain.

AIMS: To investigate whether the duration of chronic pain in temporomandibular disorder (TMD) patients is associated with a net depletion of amino acids, and a distinct process from pain intensity. METHODS: Twenty-nine patients defined by the research diagnostic criteria/TMD as having Type 1a muscle pain (TMD1A group), and 34 age- and sex-matched control subjects, were assessed for variation in urinary organic and amino acid excretion by gas chromatography-mass spectrometry. RESULTS: The TMD1A patients' mean pain intensity, assessed on a visual analog scale (VAS), was 5.4 (95% confidence limits: 4.5 to 6.3), TMD1A illness duration was 5.0 +/- 1.2 (SD) years, number of body areas with pain/subject was 6.3 +/- 2.4 (range 0 to 10), and symptom prevalence from the Symptom Check List-90-Revised (SCL-90-R) was 25.5 +/- 11.3 symptoms/subject, which was higher than the controls (5.2 +/- 5.0 symptoms/subject, P < .001). TMD1A patient illness duration was positively correlated with symptom prevalence and body pain distribution, and all were independent of pain intensity. The TMD1A patients had: (1) and increased tyrosine:leucine ratio; and (2) reduced leucine concentrations (both P < .001), which suggests deregulated catabolism. Pain intensity was associated with: (1) changes in the multivariate urinary metabolite excretion patterns (P < .001); (2) reduced leucine concentrations (P < .001); and (3) increases in total urinary metabolites (P < .04), and in 2 unidentified molecules, UM28 (P < .001) and CFSUM1 (P < .002). TMD1A illness duration was associated with lower (1) urinary metabolite concentrations and (2) succinic acid and combined glutamine + glutamic acid levels, suggesting a progressive depletion of metabolite reserves. CONCLUSION: In TMD1A patients, total amino acid excretion was positively correlated with pain intensity and negatively correlated with illness duration, which indicated that illness duration was associated with a different set of metabolic anomalies compared with those identified for pain intensity.

Coagulase-negative staphylococcal membrane-damaging toxins, pain intensity, and metabolic changes in temporomandibular disorder patients with chronic muscle pain.

AIMS: To investigate the association between toxin-producing staphylococci, symptom expression, and changes in urinary excretion of metabolites in temporomandibular disorder (TMD) patients and age- and sex-matched control subjects. METHODS: Twenty-nine patients defined by the research diagnostic criteria/TMD as having Type 1a muscle pain (TMD1A), and 34 age- and sex-matched control subjects were assessed for the carriage of staphylococcal species, staphylococcal toxin production, expression of symptoms, and changes in urinary excretion of amino and organic acids. RESULTS: TMD1A patients had an increased incidence of carriage of toxin-producing coagulase-negative staphylococcus (MDT-CoNS, P < .004), which produced increased levels of delta-like membrane-damaging toxins. The TMD1A patients also had a reduction in the incidence of carriage of Staphylococcus aureus (P < .02). Increased incidence of MDT-CoNS was positively associated with increased pain intensity as assessed by a visual analog scale (P < .001). Odds ratio analysis revealed a 9.2-fold increase in MDT-CoNS recovery from the nose of TMD1A patients compared with the control subjects (odds ratio = 9.2, > 95% confidence limits: 2.3 to 37.5, P < .001). Increases in the carriage incidence of MDT-CoNS were also associated with increases in the urinary tyrosine:leucine ratio (P < .004), which represents a change in the balance of proteolysis and protein synthesis. The toxin production by these CoNS species was also associated with an increased urinary excretion of glutamic acid (P < .03). CONCLUSION: These data suggest that an increased colonization of MDT-CoNS on skin and mucosal membranes was associated with changed proteolysis, increased pain intensity, and an increase in excitatory amino acids consistent with events associated with the development of chronic orofacial muscle pain in TMD patients.