Pain sensation during cold stimulation of the teeth: differential reflection of A delta and C fibre activity?

Pulp sensitivity changes during orthodontic treatment at different time periods: a prospective study

Objective

The purpose of this investigation was to recognize pulp sensitivity changes in teeth receiving orthodontic treatment by means of an electric pulp tester (Vitality Scanner Model 2006; Kerr Corporation, Brea CA, USA).

Materials and methods

An electric stimulus response threshold of eight teeth in 22 patients was measured prior to positioning orthodontic attachments, immediately before ligation of a nickel titanium archwire, immediately after ligation of a stainless steel archwire and 9 to 15 months after having achieved the clinical purposes established with the nickel titanium archwires. The first measurement served as baseline.

Results

All teeth responded to an electrical stimulus at all times. No statistical differences were observed between the response thresholds obtained at different treatment times. The mean response threshold of the second measurement showed a decreasing response threshold tendency when compared with those of the baseline measurement. The mean response threshold of the third measurement showed an increasing tendency when compared with those of the baseline measurement. The first maxillary incisor and canine showed the lowest decreasing response threshold after the second measurement and the highest increasing response threshold after the third measurement. Less noticeable, but similar decreasing and increasing response threshold tendencies were observed in all other teeth after the second and third measurements, respectively.

Conclusions

The results obtained in this investigation suggest that pulp sensitivity can be monitored during orthodontic treatment by means of an electric pulp tester.

Clinical relevance

The importance of monitoring the pulp status during orthodontic treatment.

Thermal Pain in Teeth: Electrophysiology Governed by Thermomechanics

Thermal pain arising from the teeth is unlike that arising from anywhere else in the body. The source of this peculiarity is a long-standing mystery that has begun to unravel with recent experimental measurements and, somewhat surprisingly, new thermomechanical models. Pain from excessive heating and cooling is typically sensed throughout the body through the action of specific, heat sensitive ion channels that reside on sensory neurons known as nociceptors. These ion channels are found on tooth nociceptors, but only in teeth does the pain of heating differ starkly from the pain of cooling, with cold stimuli producing more rapid and sharper pain. Here, we review the range of hypotheses and models for these phenomena, and focus on what is emerging as the most promising hypothesis: pain transduced by fluid flowing through the hierarchical structure of teeth. We summarize experimental evidence, and critically review the range of heat transfer, solid mechanics, fluid dynamics, and electrophysiological models that have been combined to support this hypothesis. While the results reviewed here are specific to teeth, this class of coupled thermomechanical and neurophysiological models has potential for informing design of a broad range of thermal therapies and understanding of a range of biophysical phenomena.

Fluid mechanics in dentinal microtubules provides mechanistic insights into the difference between hot and cold dental pain

Dental thermal pain is a significant health problem in daily life and dentistry. There is a long-standing question regarding the phenomenon that cold stimulation evokes sharper and more shooting pain sensations than hot stimulation. This phenomenon, however, outlives the well-known hydrodynamic theory used to explain dental thermal pain mechanism. Here, we present a mathematical model based on the hypothesis that hot or cold stimulation-induced different directions of dentinal fluid flow and the corresponding odontoblast movements in dentinal microtubules contribute to different dental pain responses. We coupled a computational fluid dynamics model, describing the fluid mechanics in dentinal microtubules, with a modified Hodgkin-Huxley model, describing the discharge behavior of intradental neuron. The simulated results agreed well with existing experimental measurements. We thence demonstrated theoretically that intradental mechano-sensitive nociceptors are not “equally sensitive” to inward (into the pulp) and outward (away from the pulp) fluid flows, providing mechanistic insights into the difference between hot and cold dental pain. The model developed here could enable better diagnosis in endodontics which requires an understanding of pulpal histology, neurology and physiology, as well as their dynamic response to the thermal stimulation used in dental practices.

Analysis of thermal-induced dentinal fluid flow and its implications in dental thermal pain

OBJECTIVES

The initiation of the pain sensation experienced following the thermal stimulation of dentine has been correlated with fluid flow in the dentinal tubules. There may be other mechanisms.

METHODS

This study examines this possibility using a mathematical model to simulate the temperature and thermal stress distribution in a tooth undergoing thermal stimulation. The results obtained were then used to predict the fluid flow in a single dentinal tubule by considering the deformation of the dentinal tubules and dentinal fluid.

RESULTS

Deformation of the pulp chamber was observed before a noticeable temperature change was recorded at the dentine-enamel junction. Tubule deformation leads to changes in fluid flow more rapidly than fluid expansion or contraction. This finding agreed with previously reported experimental observations. An initially high rate of outward fluid flow under cooling was found to correspond to short latency neural responses whilst heating was associated with long latency neural responses.

CONCLUSION

Rapid fluid flow caused by thermal deformation of dentinal tubules may account for the short latency (<1s) activation of mechano-sensitive receptors after of cooling. Long latency (>10s) neural responses could be associated with the activation of thermo-sensitive receptors.

The development of peripheral cold neural circuits based on TRPM8 expression

Afferent nerve fibers of the somatosensory system are a molecularly diverse cell population that detects a varied range of environmental stimuli, converting these external cues ultimately into a sensory percept. Afferents mediating detection of thermal stimuli express a repertoire of temperature sensitive ion channels of the TRP family which endow these nerves with the ability to respond to the breadth of temperatures in the environment. The cold and menthol receptor TRPM8 is responsible for detection of cold and, unlike other thermosensors, detects both innocuous and noxious temperatures. How this single molecule can perform such diverse functions is currently unknown, but expression analyses in adult tissues shows that TRPM8 neurons are a molecularly diverse population and it is likely that this diversity underlies differential functionality. To determine how this phenotype is established, we examined the developmental time course of TRPM8 expression using a mouse transgenic line in which GFP expression is driven by the TRPM8 transcriptional promoter (Trpm8(GFP)). We find that Trpm8(GFP) expression begins prior to embryonic day 15.5 (E15.5) after which expression reaches levels observed in adult neurons. By E18.5, central axons of Trpm8(GFP) neurons reach the spinal cord dorsal horn, but anatomical localization and in vivo measurements of neural activity suggest that fully functional cold circuits are not established until after the first postnatal week. Additionally, Trpm8(GFP) neurons undergo a transition in neurochemical phenotype, ultimately reaching adult expression of markers such TRPV1, CGRP, peripherin, and NF200 by postnatal day 14. Thus, based on immunochemical, anatomical and functional criteria, active cold neural circuits are fully established by the second week postnatal, thereby suggesting that important extrinsic or intrinsic mechanisms are active prior to this developmental stage.

Functional expression of thermo-transient receptor potential channels in dental primary afferent neurons: implication for tooth pain

Temperature signaling can be initiated by members of transient receptor potential family (thermo-TRP) channels. Hot and cold substances applied to teeth usually elicit pain sensation. This study investigated the expression of thermo-TRP channels in dental primary afferent neurons of the rat identified by retrograde labeling with a fluorescent dye in maxillary molars. Single cell reverse transcription-PCR and immunohistochemistry revealed expression of TRPV1, TRPM8, and TRPA1 in subsets of such neurons. Capsaicin (a TRPV1 agonist), menthol (a TRPM8 agonist), and icilin (a TRPM8 and TRPA1 agonist) increased intracellular calcium and evoked cationic currents in subsets of neurons, as did the appropriate temperature changes (>43 degrees , <25 degrees , and <17 degrees C, respectively). Some neurons expressed more than one TRP channel and responded to two or three corresponding stimuli (ligands or thermal stimuli). Immunohistochemistry and single cell reverse transcription-PCR following whole cell recordings provided direct evidence for the association between the responsiveness to thermo-TRP ligands and expression of thermo-TRP channels. The results suggest that activation of thermo-TRP channels expressed by dental afferent neurons contributes to tooth pain evoked by temperature stimuli. Accordingly, blockade of thermo-TRP channels will provide a novel therapeutic intervention for the treatment of tooth pain.

Cold Testing Through Full-Coverage Restorations

Endodontic diagnosis often requires thermal testing through porcelain fused-to-metal (PFM) and all-ceramic restorations. The purpose of this study was to measure and compare the temperature change during thermal testing by three commonly used methods occurring at the pulp-dentin junction (PDJ) of nonrestored teeth and teeth restored with full coverage restorations made of PFM, all-porcelain, or gold. The methods used to produce a thermal change were (a) an ice stick, (b) 1,1,1,2-tetrafluoroethane (TFE), and (c) carbon dioxide snow. A thermocouple measured temperature changes occurring at the PDJ in 10 extracted premolars when thermal tested by each method over a period of 30 seconds. Temperature reduction was also measured for the same samples restored with full gold crowns, PFM, and Empress crowns. Results showed intact premolars and those restored with PFM or all-ceramic restorations to respond similarly to thermal testing. In these teeth, TFE produced a significantly greater temperature decrease than carbon dioxide snow between 10 and 25 seconds (p < 0.05). In conclusion, application of TFE on a saturated #2 cotton pellet was the most effective method for producing a temperature reduction at the PDJ of intact teeth and those restored with gold, PFM, and all-porcelain when testing for less than 15 seconds.

Response of feline intradental nerve fibers to tooth cutting by Er:YAG laser

BACKGROUND AND OBJECTIVE

The aim of this study was to investigate the response of intradental A- and C-fibers during tooth cutting by Er:YAG laser.

STUDY DESIGN/MATERIALS AND METHODS

Bipolar electrical stimulation was applied to the cat's canine to identify functional single nerve fibers of the inferior alveolar nerve. The tip of the canine tooth was cut in 0.5-mm steps until the pulp was exposed. Teeth were alternately cut by using Er:YAG laser (50 mJ, 5 pps) and micromotor under water cooling. The nerve response recorded from the single nerve fibers during laser cutting was compared with that during micromotor cutting.

RESULTS

All 26 A-fibers responded to laser cutting with high frequency of nerve firings. The nerve firing rate was significantly higher during laser cutting compared with that during micromotor cutting of superficial dentin (Chi(2) test, P < 0.05) but was not significantly different at deep dentin (P > or = 0. 05). Nine of 11 C-fibers responded to laser cutting when the deep dentin was cut. Among those nine nerve fibers, three also showed a low frequency response to laser cutting of the superficial dentin.

CONCLUSION

During the tooth cutting, Er:YAG laser was more effective in activating intradental A-fibers compared with micromotor and also caused the activation of intradental C-fibers.

Comparison of methods for measuring root and mucogingival sensitivity

OBJECTIVE

The aim of this study was to compare the variability of measurements of root and mucogingival sensitivity over a 24-hour period.

STUDY DESIGN

Sixteen individuals (46.8 +/- 3.2 years old) were randomly tested for pain thresholds with calibrated electrical stimulation of the root and adjacent mucosa (electric pulp tester), pressure on mucosa (pressure-sensitive probe), and cold on the root (experimental thermocoupler probe) at baseline and after 4, 8, and 24 hours. Variability between and within subjects was estimated by using analysis of variance for random effects.

RESULTS

Intrasubject variability was highest for electric testing of the root and lowest for cold testing of the root across time. Of all subjects, 93% fell within 5 degrees C at all periods for the cold stimulation/moderate pain threshold.

CONCLUSIONS

Calibrated cold stimulation of root areas appears to provide the most sensitive measure to assess therapeutic interventions to control cervical dental pain because of low intrasubject variability in untreated patients.

Pulpal ischemia in man: effects on detection threshold, A-delta neural response and sharp dental pain

Preferential blocks of peripheral nerves have shown that myelinated nerves are more susceptible to local compression and less resistant to asphyxia than unmyelinated fibers. Since two groups of functionally different nociceptors exist in the dental pulp, it is of theoretical and clinical interest to determine the influence of ischemia on the sensitivity of human dental pulp, using standard means for testing tooth vitality and at the same time investigating the intensity coding in one pathway of the afferent trigeminal system. Adrenaline was used to study the differential effect of adrenaline-induced ischemia on intradental A-delta nerve activity (INA) and the concomitant sharp pain, as well as on the detection threshold for monopolar electrical stimulation. Cold (ethyl chloride) and heat (heated gutta-percha) stimulation was applied to the tooth surface. In accordance with the hydrodynamic theory of dentin sensitivity the rapid fluid flow induced in the dentinal tubuli by these thermal stimuli is an adequate stimulus for selectively activating the A-delta nerves in healthy pulps. Consistency plots of the magnitude of the perceptual experience of sharp pain against the neural population response in linear coordinates yielded a high product-moment correlation, implying linearity for the intensity coding relationship. In contrast to the significant reduction of INA and its perceptual correlate of sharp pain after adrenaline administration, the electrical detection threshold remained constant during the full test period, suggesting that electrical threshold measurements have their limitations as a diagnostic tool or criterion for assessing the sensitivity of the dental pulp. The absence of A-delta activity was parallelled by no sensation of sharp pain. These findings suggested that the integrated neural A-delta activity constituted the underlying peripheral neurophysiological mechanism of the sensory intensity of sharp dental pain.

NMDA receptor mechanisms contribute to neuroplasticity induced in caudalis nociceptive neurons by tooth pulp stimulation

We recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant and prolonged increases in jaw muscle electromyographic (EMG) activity that are suggestive of central neuroplasticity. Because small-fiber afferents, including pulp afferents, access nociceptive neurons in trigeminal (V) subnucleus caudalis, this study examined whether pulpal application of MO induces neuroplastic changes in caudalis nociceptive neurons (wide dynamic range and nociceptive specific) and whether central N-methyl--aspartate (NMDA) receptor mechanisms are involved in these MO-induced neuroplastic changes. After pretreatment with vehicle (saline, 10 microliter i.t.) to the surface of the medulla, the pulpal application of MO to the maxillary molar tooth pulp produced a significant increase in neuronal spontaneous activity, a significant expansion of the pinch and/or tactile mechanoreceptive field (RF), a significant decrease in mechanical threshold, and significant increases in neuronal responses to graded pinch stimuli. Compared with vehicle-treated rats, pretreatment with the NMDA receptor antagonist MK-801 (10 microgram/10 microliter i.t.) followed by MO application to the pulp in another group of rats significantly reduced or abolished these MO-induced neuroplastic changes in nociceptive neurons. In another group of rats pretreated with saline (intrathecally), mineral oil application to the pulp did not show any significant changes in spontaneous activity or RF properties over the 40-min observation period. The pulpal application of MO in other rats (pretreated with saline, intrathecally) did not produce any significant neuroplastic changes in caudalis low-threshold mechanoreceptive neurons. These results indicate that the MO-induced activation of molar pulpal afferents can produce profound NMDA receptor-related neuroplastic changes in caudalis nociceptive neurons. Such neuroplastic changes may contribute to the hyperalgesia and spread of pain that can be associated with pulpal inflammation.

Cold-induced pain and prickle in the glabrous and hairy skin

Little is known concerning the mechanisms underlying the perception of cold pain in humans. An appreciation of these mechanisms is important to understand and possibly treat those disorders in which cold stimuli evoke unpleasant sensations. To study cold pain, I have conducted psychophysical experiments on 16 healthy subjects. A peltier-type stimulator (20 x 25 mm) was used to deliver stimuli to sites on the thenar eminence (glabrous skin) and volar forearm (hairy skin) of each arm. Each trial consisted of a 90 s, 2 degrees C stimulus that was preceded and followed by a 35 degrees C stimulus. A computer-based visual analog scale was used to collect continuous pain ratings throughout each trial. In experiment 1, nine subjects rated the overall evoked pain intensity (four trials/skin type) and the prickle component (four trials/skin type). Typically, subjects perceived the cold-evoked pain as prickly, cold/freezing and achy. The pain intensity and quality was similar for glabrous and hairy skin sites within individual subjects. Pain intensity gradually rose to a plateau by approximately 60 s into each trial. The prickle component differed amongst subjects due to its variable time course. Subjects consistently reported an intense, brief jab of prickle at both hairy and glabrous sites during the rewarming phase. In experiment 2, nine subjects rated the pain intensity during the cold stimulus before and during a compression-ischemic block of Abeta/Adelta fiber conduction. The dominant sensation evoked by the cold stimulus in the hairy and glabrous skin during the block was a sharp, hot/burning pain. The block did not consistently affect the total pain at the hairy sites. However, most subjects reported more pain during the block at the glabrous sites. These data suggest that noxious cold stimuli affect a mosaic of primary afferent input and central processing resulting in a complex pain experience which may differ in glabrous and hairy skin.

Neurogenic inflammation and tooth pulp innervation pattern in sympathectomized rats

This study investigated possible collateral C-fiber innervation between the pulps of rat molars by assessing neurogenic inflammation (NI) induced by the C-fiber excitant mustard oil (MO). MO was applied to the pulp of the left mandibular first molar in two groups of rats: group 1, guanethidine sympathectomized rats (to dismiss sympathetic activation by MO); and group 2, unsympathectomized rats. A third group of unsympathectomized rats (group 3) had saline applied to the pulp of the left mandibular molar and served as a MO control. The NI-related plasma extravasation was examined in these teeth and in the remaining left mandibular teeth by a spectrophotometric analysis of extravasated plasma protein bound to Evans' Blue (EB) dye. The collateral innervation pattern was inferred from the NI pattern. EB concentrations were measured in the left mandibular teeth and the corresponding contralateral teeth, and expressed as a ratio. Statistical analysis of the data revealed significant differences in EB ratios in the first, second, and third molars between groups 1 and 3. This result suggests collateral C-fiber innervation exists within the pulps of molar teeth in the same dental quadrant. No difference in EB ratios was noted in the first and second molars between groups 1 and 2. Therefore, sympathetic efferents have no apparent effect on the degree of MO-induced NI.