(-)-α-Bisabolol reduces nociception and trigeminal central sensitisation in acute orofacial neuropathic pain induced by infraorbital nerve injury.

Neuropathic orofacial pain conditions represent a challenge to diagnose and treat. Natural substances are promising therapeutic options for the control of pain. AIMS: This study aimed to examine whether (-)-α-bisabolol (BISA), a natural terpene, can attenuate nociceptive behaviour and central sensitisation in a rodent model of trigeminal neuropathic pain. MATERIALS AND METHODS: Infraorbital nerve transection (IONX) or sham operation was performed in adult male rats. Head withdrawal thresholds as a measure of facial mechanical sensitivity were tested with von Frey monofilaments applied bilaterally to the facial vibrissal pad pre-operatively (baseline) and then post-operatively before and at 60, 120, 240 and 360 min after administration of vehicle control per oris (p.o.) or BISA (200 mg/kg p.o.) (n = 8/group). Effects of BISA or vehicle on the activity of nociceptive neurons recorded in the medullary dorsal horn (MDH) were tested on post - operative day 8-10. ANOVA followed by post-hoc Bonferroni tested for statistically significant differences (p < 0.05) across study groups and time points. KEY FINDINGS: IONX animals (but not sham or naïve animals) showed post-operative facial mechanical hypersensitivity that was unaffected by vehicle. However, administration of BISA at post-operative day 7 significantly reversed the mechanical hypersensitivity in IONX rats; this effect lasted for at least 6 h. BISA also attenuated IONX-induced central sensitisation of MDH nociceptive neurons, as reflected in reversal of their reduced activation thresholds, increased responses to graded mechanical stimuli and enhanced spontaneous activity. SIGNIFICANCE: BISA may attenuate nociceptive behaviour and central sensitisation in a rat model of acute trigeminal neuropathic pain.

Jaw sensorimotor control in healthy adults and effects of ageing.

The oro-facial sensorimotor system is a unique system significantly distinguished from the spinal sensorimotor system. The jaw muscles are involved in mastication, swallowing and articulatory speech movements and their integration with respiration. These sensorimotor functions are vital for sustaining life and necessitate complex neuromuscular processing to provide for exquisite sensorimotor control of numerous oro-facial muscles. The function of the jaw muscles in relation to sensorimotor control of these movements may be subject to ageing-related declines. This review will focus on peripheral, brainstem and higher brain centre mechanisms involved in reflex regulation and sensorimotor coordination and control of jaw muscles in healthy adults. It will outline the limited literature bearing on age-related declines in jaw sensorimotor functions and control including reduced biting forces and increased risk of impaired chewing, speaking and swallowing. The mechanisms underlying these alterations include age-related degenerative changes within the peripheral neuromuscular system and in brain regions involved in the generation and control of jaw movements. In the light of the vital role of jaw sensorimotor functions in sustaining life, normal ageing involves compensatory mechanisms that utilise the neuroplastic capacity of the brain and the recruitment of additional brain regions involved in sensorimotor performance and closely associated functions (e.g. cognition and memory). However, these regions are themselves susceptible to detrimental age-related changes. Thus, better understanding of the peripheral and central mechanisms underlying age-related sensorimotor impairment is crucial for developing improved treatment approaches to prevent or cure impaired jaw sensorimotor functions and to thereby improve health and quality of life.

Decreased face primary motor cortex (face-M1) excitability induced by noxious stimulation of the rat molar tooth pulp is dependent on the functional integrity of medullary astrocytes.

We have recently shown that application of the small-fiber excitant and inflammatory irritant mustard oil (MO) to the rat molar tooth pulp can decrease face-M1 excitability, but increase the excitability of trigeminal medullary dorsal horn (MDH) nociceptive neurons and that application of the astrocytic inhibitor methionine sulfoximine (MSO) to the face-M1 or MDH can attenuate the MO-induced changes. The present study aimed to determine whether medullary MSO application could modulate the MO-induced decreased face-M1 excitability. Under ketamine general anesthesia, electromyographic (EMG) electrodes were implanted into the right anterior digastric (RAD, jaw-opening muscle) of adult male Sprague-Dawley rats. A microelectrode was positioned at a low-threshold (≤30 µA) site in the left face-M1. Then MO (n = 16) or control-solution (n = 16) was applied to the previously exposed molar tooth pulp, and intracortical microstimulation threshold intensities for evoking RAD EMG activities were monitored for 15 min. MSO (0.1 mM, n = 8) or phosphate-buffered saline (PBS, n = 8) was then applied to the MDH and RAD thresholds monitored every 15 min for 120 min. Statistics used ANOVA followed by post hoc Bonferroni as appropriate (p < 0.05). As compared to baseline, RAD thresholds significantly increased (i.e., decreased excitability) within 1 min (26.3 ± 7.9%, p = 0.007) and peaked at 15 min following pulpal MO application (49.9 ± 5.7%, p < 0.001) but not following control-solution. Following MSO (but not PBS) application to the medulla, RAD thresholds significantly decreased within 15 min (26.5 ± 3.0%, p = 0.05) and at 60 min approached 6.3 ± 2.4%, of baseline values (p = 0.1). These novel findings suggest that clinically related motor disturbances arising from dental pain may involve decreased face-M1 excitability that is modulated by medullary astrocytes.

Dental Occlusal Changes Induce Motor Cortex Neuroplasticity.

Modification to the dental occlusion may alter oral sensorimotor functions. Restorative treatments aim to restore sensorimotor functions; however, it is unclear why some patients fail to adapt to the restoration and remain with sensorimotor complaints. The face primary motor cortex (face-M1) is involved in the generation and control of orofacial movements. Altered sensory inputs or motor function can induce face-M1 neuroplasticity. We took advantage of the continuous eruption of the incisors in Sprague-Dawley rats and used intracortical microstimulation (ICMS) to map the jaw and tongue motor representations in face-M1. Specifically, we tested the hypothesis that multiple trimming of the right mandibular incisor, to keep it out of occlusal contacts for 7 d, and subsequent incisor eruption and restoration of occlusal contacts, can alter the ICMS-defined features of jaw and tongue motor representations (i.e., neuroplasticity). On days 1, 3, 5, and 7, the trim and trim-recovered groups had 1 to 2 mm of incisal trimming of the incisor; a sham trim group had buccal surface trimming with no occlusal changes; and a naive group had no treatment. Systematic mapping was performed on day 8 in the naive, trim, and sham trim groups and on day 14 in the trim-recovered group. In the trim group, the tongue onset latency was shorter in the left face-M1 than in the right face-M1 (P < .001). In the trim-recovered group, the number of tongue sites and jaw/tongue overlapping sites was greater in the left face-M1 than in the right face-M1 (P = 0.0032, 0.0016, respectively), and the center of gravity was deeper in the left than in the right face-M1 (P = 0.026). Therefore, incisor trimming and subsequent restoration of occlusal contacts induced face-M1 neuroplasticity, reflected in significant disparities between the left and right face-M1 in some ICMS-defined features of the tongue motor representations. Such neuroplasticity may reflect or contribute to subjects' ability to adapt their oral sensorimotor functions to an altered dental occlusion.

Decreased face primary motor cortex (face-M1) excitability induced by noxious stimulation of the rat molar tooth pulp is dependent on the functional integrity of face-M1 astrocytes.

Acute inflammatory dental pain is a prevalent condition often associated with limited jaw movements. Mustard oil (MO, a small-fiber excitant/inflammatory irritant) application to the rat molar tooth pulp induces increased excitability (i.e., central sensitization) of trigeminal medullary dorsal horn (MDH) nociceptive neurons that can be modulated by MDH application of the astrocytic inhibitor methionine sulfoximine (MSO). The objectives of the study were to determine whether MO application to the rat right maxillary first molar tooth pulp affects left face-M1 excitability manifested as altered intracortical microstimulation thresholds for evoking electromyographic activity in the right anterior digastric (RAD, jaw-opening muscle), and whether MSO application to face-M1 can modulate this MO effect. Under Ketamine general anesthesia, Sprague-Dawley male rats had a microelectrode positioned at a low-threshold (≤30 µA) face-M1 site. Then MO (n = 16) or control solution (n = 16) was applied to the previously exposed tooth pulp, and RAD threshold was monitored for 15 min. MSO (0.1 mM, n = 8) or saline (n = 8) was then applied to the face-M1, and RAD thresholds were monitored every 15 min for 120 min. ANOVA followed by post hoc Bonferroni was used to analyze data (p < 0.05). Within 15 min of MO (but not control) pulp application, RAD thresholds increased significantly (p < 0.001) as compared to baseline. One hour following MSO (but not saline) application to the face-M1, RAD thresholds decreased significantly (p = 0.005) toward baseline. These novel findings suggest that acute inflammatory dental pain is associated with decreased face-M1 excitability that may be dependent on the functional integrity of face-M1 astrocytes and related to mechanisms underlying limited jaw movements in acute orofacial pain conditions.

Cortical orofacial motor representation: effect of diet consistency.

Jaw and tongue motor alterations may occur following changes in food consistency, but whether such changes are associated with re-organization of motor representations within the facial sensorimotor cortex is unclear. We used intracortical microstimulation (ICMS) and recordings of evoked electromyographic responses to determine jaw (anterior digastric) and tongue (genioglossus) motor representations within the histologically defined face primary motor cortex (face-M1) and adjacent somatosensory cortex (face-S1) of rats fed hard (N = 6) or soft (N = 6) diet for 2 to 3 weeks. ICMS evoked jaw and tongue responses from an extensive area within the face-M1 and a smaller area within the face-S1. A significant contralateral predominance was reflected in the number and latency of ICMS-evoked jaw responses (p < 0.05). There were no significant differences between the hard- and soft-diet groups in jaw and tongue motor representations, suggesting that the rat's ability to adapt to changes in diet consistency may not be associated with significant neuroplasticity of sensorimotor cortex motor outputs.

Neuroplasticity of face primary motor cortex control of orofacial movements.

We have carried out a series of studies to address the role of the face primary motor area (MI) in the cerebral cortex in trained or semi-automatic orofacial motor behaviours and in behavioural adaptations to an altered oral environment. These studies have utilized intracortical microstimulation (ICMS), reversible cold block or single neurone recordings in face MI. Our studies in monkeys have revealed that face MI plays a strategic role in elemental and learned motor behaviours and in certain aspects of chewing and swallowing. Furthermore, successful training of awake monkeys in a novel tongue-protrusion task is associated with significant neuroplastic changes in face MI. These findings in monkeys are supported by correlated findings in humans which have revealed significantly enhanced corticomotoneuronal excitability when humans learn the novel tongue-protrusion task. Our related ICMS studies in rats reveal that trimming or extraction of the rat's lower incisors or damage to the rat's lingual nerve can result in significant changes in the MI representations of the tongue or jaw muscles. These various findings suggest that the face MI is important in orofacial motor skill acquisition and adaptation to an altered occlusion or loss of teeth or lingual sensory function, and that it reflects dynamic and modifiable constructs that are modelled by behaviourally significant experiences and that are critical to learning and adaptive processes.

A computer-assisted measurement technique to assess bone proximal to oral implants on intraoral radiographs.

A computer-assisted measurement technique for measuring bone levels proximal (mesial and distal) to oral implants imaged on standardized intraoral radiographs offers promise for accuracy and reliability. There were no differences between bone measurements made directly from bone proximal to implants placed in a dry mandible and those resulting from the computer-assisted measurement technique. In addition, there were no differences between measurements made of bone proximal to oral implants in vivo using the microscope and the computer techniques. The computer technique had a low intra- and inter-operator variability, and operators found fewer "unreadable" sites compared to the microscope technique. The computer-assisted measurement of bone levels proximal to oral implants on standardized intraoral radiographs offers accuracy and reliability.

Amalgam alternatives--micro-leakage evaluation of clinical procedures. Part I: direct composite/composite inlay/ceramic inlay.

This study investigated the degree of dye penetration with three different types of tooth-coloured restorations. Twenty-four intact extracted molars were collected. The teeth were immediately stored in water at room temperature. Class II cavity preparations were prepared and restored with three different types of tooth-coloured restorations: A, composite resin in the incremental technique; B, composite inlay technique; and C, ceramic inlay. Specimens were subjected to 700 cycles of thermal stress. They were than immersed in 2% basic fuchsin dye. The teeth were sectioned in three planes before being ranked as to the amount of dye penetration. The highest score obtained on three plano-parallel sections was adopted as the representative value. The three groups were compared using the Kruskal Wallis non-parametric test. Dye penetration was significantly lower at the enamel margins when using the composite inlay system and the incremental technique compared to the ceramic inlay technique. The restorations placed using the composite inlay technique showed less dye penetration than the incremental technique at the dentine margins (P < 0.017).

Clinical effectiveness of implant-supported single-tooth replacement: the Toronto Study.

Forty-one patients received 49 single-tooth implants placed in different jaw locations. One implant was not osseointegrated at stage 2 surgery. Three successfully osseointegrated implants were not available for follow-up. Forty-five implants were monitored for 1 to 8 years after loading. Each one of the implants met all of the traditionally accepted success criteria, except for nine individual implants that did not meet the proposed bone level criterion. No success criteria exist for natural teeth adjacent to implants. Although preliminary results are favorable, extensive long-term studies are needed to determine which specific criteria comprise optimal functional and esthetic results with minimal risk of morbidity.