Phosphorus magnetic resonance spectroscopy of human masseter muscle

Activation of rat masticatory muscle afferent fibres by acidic pH

Previous research findings have suggested an important role for acid sensing ion channels (ASICs) in muscle pain mechanisms. This study was conducted to determine if masticatory muscle afferent fibres express ASICs, if there are sex differences in this expression, and to compare the effects of low pH and hypertonic saline on afferent fibres that innervate the masticatory muscle in vivo. Immunohistochemistry methods were applied to examine the expression of ASICs in trigeminal ganglion neurons, while in vivo electrophysiology techniques were employed to examine changes in masticatory muscle afferent fibre excitability. Both ASIC₁ and ASIC₃ were expressed by predominantly larger masticatory muscle ganglion neurons, but the frequency of ASIC₃ expression (56%) was significantly greater than ASIC₁ (35%). No sex-related differences in expression were identified. Injection of pH 5.8, but not pH 6.8, phosphate buffered saline evoked afferent discharges that were significantly greater than those evoked by pH 7.4 buffer (control). Since ASIC₃ channels are not activated until the pH is around 6, these results indicate that activation of both channels contributes to excitation of masticatory muscle afferent fibres. The results further show that many masticatory muscle afferent fibres, which respond to low pH, are low threshold mechanoreceptors. These findings may explain why injection of low pH solutions into the masticatory muscles of healthy humans is not associated with significant muscle pain.

The usefulness of diagnostic imaging for the assessment of pain symptoms in temporomandibular disorders

The causes of pain symptoms in the temporomandibular joint (TMJ) and masticatory muscle (MM) regions may not be determined by clinical examination alone. In this review, we document that pain symptoms of the TMJ and MM regions in patients with temporomandibular disorders (TMDs) are associated with computed tomography and magnetic resonance (MR) findings of internal derangement, joint effusion, osteoarthritis, and bone marrow edema. However, it is emphasized that these imaging findings must not be regarded as the unique and dominant factors in defining TMJ pain. High signal intensity and prominent enhancement of the posterior disk attachment on fat saturation T2-weighted imaging and dynamic MR imaging with contrast material are closely correlated with the severity of TMJ pain. Magnetic transfer contrast, MR spectroscopy, diffusion tensor imaging, and ultrasonography findings have helped identify intramuscular edema and contracture as one of the causes of MM pain and fatigue. Recently, changes in brain as detected by functional MR neuroimaging have been associated with changes in the TMJ and MM regions. The thalamus, the primary somatosensory cortex, the insula, and the anterior and mid-cinglate cortices are most frequently associated with TMD pain.

Acidic saline-induced pain as a model for experimental masseter myalgia in healthy subjects

BACKGROUND

Repeated injection of acidic saline into skeletal muscles of the leg in rodents induces a prolonged bilateral mechanical hyperalgesia that persists for up to 30 days and may be useful to model widespread muscle pain conditions. In this study, repeated injection of acidic (pH 3.3) saline solution into the masseter muscle of healthy human subjects was undertaken to determine if these injections are painful and whether they would induce a prolonged period of muscle sensitization to artificial and/or natural mechanical stimulation of the masseter and temporalis muscles.

METHODS

Eighteen subjects (10 male, 8 female) participated in the study. Subjects received two injections of 0.5 mL acidic or regular isotonic saline 2 days apart, in a randomized, double blind, crossover design.

RESULTS

There was no significant difference in pain intensity ratings when acidic saline injections were compared with regular saline injections. Pain area drawings were, however, significantly larger in response to the first injection of acidic saline than to the second injection of acidic saline or to either the first or second injection of regular saline. Repeated injection of acidic saline did not significantly alter pressure pain thresholds from the masseter or temporalis muscles on either the injected side or the opposite side over the 10-day post injection monitoring period. There was also no effect of injections on chewing.

CONCLUSION

These findings indicate that, unlike in some rodent models, repeated injection of low pH solutions into jaw muscles of humans fails to induce a period of prolonged muscle hyperalgesia.

A ³¹P-magnet resonance spectroscopy study on the metabolism of human masseter in individuals with different vertical facial pattern

OBJECTIVE

The objective of this study was to investigate differences in masseter metabolism by (31)P-Chemical Shift Imaging (CSI) in adult individuals with different vertical facial patterns. The clinical study should be supported by functional findings at the mRNA level after orthognathic surgery.

STUDY DESIGN

Twenty-two male volunteers (mean age 24.6) were divided into a deep-bite (NL/ML 11.8 ± 2.3°) and open-bite group (NL/ML 34.1 ± 2.6°). Vertical jaw relationship, gonial angle, and masseter volume were defined and compared with the phosphate values obtained from the (31)P spectra. Student t test and regression analysis were used.

RESULTS

Phosphocreatine related strongly to muscle volume (P < .001), gonial angle (P < .001), and ML/NL angle (P < .01). Pi was found to be related to gonial angle (P < .05). Muscle volume was found to be inversely related to ML/NL (P < .01) and to the gonial angle (P < .01).

CONCLUSIONS

A difference in masseter muscle metabolism between long- and short-faced subjects was confirmed at rest position.

Deep and superficial masseter muscle blood flow in women

PURPOSE

Although changes in blood perfusion have been described as being associated with temporomandibular disorder (TMD) myofascial pain, very little is known about blood flow levels in the deep and superficial masseter muscle. This study investigated blood flow in deep and superficial sites of six healthy female participants at baseline and during intermittent and continuous biting exercises and recovery.

MATERIALS AND METHODS

Blood flow was monitored unilaterally using a single-fiber probe laser Doppler flowmeter. The blood flow was continuously monitored at baseline and during two biting exercises: (a) intermittent at 25%, 50%, and 100% maximum voluntary bite force for 30 seconds each followed by 90 seconds rest between each biting level and (b) continuous biting at similar maximum voluntary bite force levels followed by 90 seconds rest.

RESULTS

There was significantly higher blood flow in the deep sites compared to the superficial sites (p < 0.001) and a significant increase in blood flow during biting compared to baseline (p < 0.001). There were no significant changes in blood flow among the three levels of biting, between the intermittent and continuous exercises, or from baseline blood flow compared to recovery.

CONCLUSIONS

This study showed regional differences in masseter muscle blood flow, perhaps related to differences in muscle fiber type and pattern of muscle fiber recruitment.

Involvement of ATP and its receptors on nociception in rat model of masseter muscle pain

The exact mechanism of the masseter muscle pain recognized as a prominent symptom in temporomandibular disorders remains unclear, although it is clinically known that excessive muscular contraction causes tenderness in masseter muscles. It has been demonstrated that P2X3 receptors (P2X3Rs) in sensory neurons play a role in pain signaling from the periphery. We determined the role of P2X(3)R on pressure pain and mechanical hyperalgesia in a newly developed rat model of masseter muscle pain. The pain in the masseter muscle was assessed by the pressure pain threshold (PPT), which was defined as the amount of pressure required to induce head flinching. In naive animals, systemic treatment with morphine was associated with increase of PPTs. Changes in PPTs were examined after administration of P2XR agonists or antagonists into the masseter muscle. The masseter muscle injection of alpha,beta-meATP (P2X(1,3,2/3)R-specific agonist) induced a significantly greater behavioral response than its vehicle. This enhanced response was completely blocked by the co-application of alpha,beta-meATP with PPADS (P2X(1,2,3,5,1/5,2/3)R-specific antagonist). Excessive contraction in masseter muscle was produced by electrical stimulation. The exerted masseter muscles showed a significant reduction in PPTs indicating the induction of mechanical hyperalgesia of the muscle. Moreover, administration of PPADS to the exerted masseter muscles produced a complete recovery of reducing PPT. Immunohistochemically, the number of P2X3R-positive neurons innervating the masseter muscles increased in the excessively contracted condition in trigeminal ganglia. Our results suggested that P2X3R plays an important role in pressure pain and mechanical hyperalgesia in masseter muscle caused by excessive muscular contraction.

The influence of tissue blood flow volume on energy metabolism in masseter muscles

This study investigated the energy metabolism of masseter muscles by 31P-Magnetic Resonance Spectroscopy (MRS) during increased blood flow induced by hot pack application to clarify the influence of changes in blood flow on muscle fatigue. Twelve healthy subjects with no history of muscle pain in the masticatory system participated in this study. The 31P-MRS measurements were performed before and after hot pack application and the ratio of phosphocreatine (PCr) acting as the energy source to reproduce ATP to beta-ATP, the PCr/beta-ATP ratio, was analyzed. Results showed that PCr/beta-ATP ratios increased significantly by an average of 22.4% after the hot pack application. The results suggest that changes in blood flow volume influence the energy metabolism in masseter muscles and that blood flow increases due to the hot pack cause higher energy levels in masseter muscles and offer an advantageous condition for preventing and relieving muscle fatigue.

Effect of sustained contraction on motor unit action potentials and EMG power spectrum of human masticatory muscles

The sequelae of a sustained clench are incompletely understood. Our experimental questions were to compare the responses of men and women, to compare masseter and anterior temporalis muscles, and to test hypotheses for the reduction of the median frequency of power spectra. We recorded duration, amplitude, number of phases, and area of the motor unit action potential before and after a sustained clench and the median frequency of the electromyographic power spectrum in 41 subjects. After the clench, the median frequency was lower, the action potential duration longer, the number of phases increased, and the area larger, but the amplitude was not different. Males and females failed to differ. Compared with the temporalis, the masseter had a lower median frequency, longer duration, larger number of phases, and increased area. Our results are consistent with a decrease in the conduction velocity of the muscle action potential as an explanation of the spectral shift.

Antero-posterior activity changes in the superficial masseter muscle after exposure to experimental pain

The aim of this randomized, controlled, double-blind study was to examine how the activation pattern of the masseter muscle changes during natural function when experimental pain is induced in a discrete anterior area of the muscle. In 20 subjects, three bipolar surface electrodes and three intramuscular fine-wire electrodes (antero-posterior mapping) were simultaneously attached above and in the right masseter muscle to record the electromyographic (EMG) activity during unilateral chewing before and after infusion of a 0.9% isotonic and 5% hypertonic saline bolus in the anterior area of the muscle. The activity of the contralateral masseter muscle was registered by surface electrodes. In addition, the development of pain intensity was quantitatively measured with a numerical rating scale (NRS). While both saline concentrations caused pain, the hypertonic solution evoked stronger pain. The experiments also provided evidence of a significant although differential activity reduction of the ipsilateral masseter muscle in the antero-posterior direction. The activity reduction decreased with increasing distance from the location of the infusion. The results support the idea that the strategy of differential activation protects the injured muscle while simultaneously maintaining optimal function.

Analyses of muscular activity, energy metabolism, and muscle fiber type composition in a patient with bilateral masseteric hypertrophy

Hyperwork of the masseter muscles due to habitual parafunction is thought to induce masseteric hypertrophy (so called work hypertrophy). However, the causes underlying this disease are not yet fully understood. Recently, we had a patient with bilateral masseteric hypertrophy, and we performed a partial excision of the masseter muscles. In this patient's case, we examined muscular activity, energy metabolism, and fiber type composition of the masseter muscles using electromyograms (EMG), 31P-magnetic resonance spectroscopy (MRS), and enzyme-histochemistry. The EMG showed no hyperactivity, and the 31P-MRS showed normal energy spectral patterns and PCr contents of the masseter muscles. The fiber type composition, however, in the muscles in this case was very different from that in muscles with "work hypertrophy" and also that in normal masseter muscles: 1. Loss of type IIB fibers; 2. Increases in type IIA and in type IM & IIC fibers; and 3. Decrease in type I fibers. The findings suggest that this is not a case of work hypertrophy but a case of compensatory hypertrophy possibly due to a lack of high-tetanus-tension type IIB fibers.

Magnetic resonance spectroscopy of the masseter muscle in different facial morphological patterns

The aims of this study were (1) to develop a reliable noninvasive method to evaluate the masseter muscle metabolism, by using (31)P-magnetic resonance spectroscopy, and (2) to evaluate the metabolic profile of the masseter muscle in subjects with various facial patterns. The maxillary-mandibular relationship, which varied from hypodivergent to hyperdivergent, was measured on lateral cephalograms of 20 adults, 22 to 35 years of age. (31)P-spectra were acquired from the masseter muscle at rest with a custom-made, single-turn, double-tuned, 3 x 5-cm oblong surface coil. The inorganic phosphate to phosphocreatine (Pi/PCr) ratios were measured and compared in relation to vertical and sagittal cephalometric measurements. A statistically significant (R(2) = 0.65, r = 0.81, P = .001) relationship was found between Pi/PCr ratio and the palatal-to-mandibular plane angle. As the maxillary-to-mandibular divergence increased, the Pi/PCr ratio decreased. This correlation suggests that muscles with a higher Pi/PCr ratio have a higher resting metabolic activity than those with a lower Pi/PCr ratio. Consequently, these muscles may keep bone under more tension and influence its growth in a more horizontal direction. Another possible explanation of the results is that the fiber type composition of the masseter muscle varies with facial morphology.

Effects of 28-day mechanical and chewing stress on content of bound and diffusible ions in muscles of mastication

Type I and type II muscle fibres have different ion concentrations. Muscles adapt to chronic stress by changing of fibre types and remodelling of the myosin heavy chains in the muscle fibres. The present investigation on ionic change during muscular contraction was carried out on 10-week-old pigs (6 treated animals, 6 controls) over a 28-day period. Six pigs received acrylic build-ups to induce mechanical advancement of the lower jaw and chronic chewing stress. Muscle tissue was taken from the masseter (M1, M2, M3), temporal (TP1, TP2), medial pterygoid (PM) and geniohyoid (GH) muscles by a standardized method. Eighty-four muscle samples were used for histological fibre differentiation with mATPase. Energy-dispersive X-ray microanalysis of muscles was carried out in an environmental scanning electron microscope. Endurance stress in the stressed muscles was seen as an increase of type I fibres (P < 0.001). This histological change and ionic alterations were measured in the anterior region of the masseter (M1 and M2) and in the posterior region of the temporal muscle (TP2). Smaller changes were found in the medial pterygoid muscle. We measured in this muscles increases in potassium, sulphur, chloride (P < 0.05) and even larger increases in phosphate (up to 1.5 mmol/g to 2.3 mmol/g, P < 0.001) and sodium (3-fold, P < 0.001). The results reveal the effects of chronic stress on muscle fibres and ion concentration in the muscle. Chronic stress resulted in an increase of type I fibres and increased ion concentration in the same muscle region. These are considered to be indicators of more efficient contraction. The changes in ion concentration are an important factor in muscle contraction.

Regional differences of metabolism in human masseter muscle by two-dimensional 31P-chemical shift imaging

Many reports have demonstrated significant region-dependent differences in the fiber-type composition of the human masseter muscle. Therefore, it is considered that there is intramuscular heterogeneity of metabolic activity in the muscle. The present study was carried out, with two-dimensional Chemical Shift Imaging, to detect differences between the deep and superficial parts of the human masseter muscle at rest. Masseter muscle from 11 volunteers, from 20 to 27 years old, was examined, and characteristic spectra of the inorganic phosphate (Pi), phosphocreatine (PCr), and alpha-, beta-, and gamma-adenosine triphosphate (ATP) from each part of the muscles were obtained. In this study, the deep and superficial parts of the masseter muscle were distinguished by the existence of aponeurosis. The Pi/PCr, PCr/beta-ATP, and Pi/beta-ATP ratios as well as the pH in the deep and superficial parts were calculated from the peak spectra. Compared with the deep part, the Pi/PCr of the superficial part was lower (p < 0.05) and the PCr/beta-ATP was higher (p < 0.01). The Pi/beta-ATP and pH showed no significant differences between the two parts. The results indicate that the superficial part of the masseter muscle contains more PCr than the deep part, and this may be related to functional differences between these two parts. In future examinations of the metabolic activity of the human masseter muscle, the deep and superficial parts must be measured separately.

Effect of maturation on 31P magnetic resonance spectroscopy of the rabbit masseter muscle

This work studies the dynamic metabolic changes of the rabbit masseter muscle during post-natal development. The composition and proportion of oxidative and glycolytic muscle fibers alter during maturation. The masseter muscle, as most muscles of the craniofacial region, exhibits unusual development in composition of isoforms of myosin. The effect of this unusual composition on the dynamic metabolic properties of the masseter muscle have not been assessed. The metabolism of the rabbit masseter muscle was studied by means of 31P-nuclear magnetic resonance (NMR) spectroscopy. Contraction was elicited by electrical stimulation of the muscle in the anesthetized animal. Five animals were studied at 8 weeks and 24 weeks so that both the juvenile and adult stages could be evaluated. The dynamic biochemical changes in the masseter muscle were studied by the analysis of NMR spectra. A single-turn surface coil (copper) was used, and the original signal was treated with Fourier transforms to obtain 31P spectra. The low signal-to-noise ratio required averaging 16 acquisitions (acquisition time = 400 msec, repetition rate = 1.8 sec) in 30 sec and then obtaining continuous spectra for 27 min. Each averaged spectrum demonstrated five peaks: inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to adenosine triphosphate (ATP). The protocol involved recording an initial three-minute rest period, stimulating the muscle at 5 Hz for 3 min twice, separated by three-minute rest periods, and stimulating the muscle at 50 Hz twice for 3 min separated by rest periods. The Pi/PCr ratio increased significantly in the adult masseter during both 5-Hz stimulations, evoking twitching, and the first 50-Hz stimulation, evoking tetany (repeated ANOVA, P < 0.05). The resting pH (6.96 +/- 0.13) was significantly lowered during both twitching (6.85 +/- 0.10; P < 0.0038) and tetany (6.55 +/- 0.13; P < 0.0001), but only in the adult masseter muscle. These finding suggest that the adult masseter muscle possesses more glycolytic fibers as it modifies its metabolism during postnatal development.