Morphological features of dentine and pulp related to dentine sensitivity

Trigeminal sensory nerve patterns in dentine and their responses to attrition in rat molars

OBJECTIVE

Our goal was to define trigeminal nerve ending quantities and patterns in rat molar dentine, their responses to attrition (tooth wear), and their associated odontoblasts and connections with pulpal plexuses.

DESIGN

Trigeminal ganglia were labeled for axonal transport of ³H-proteins to dentinal nerve endings in male rats (3-13 months old). Autoradiography detected radio-labeled dentinal tubules as indicators of nerve ending locations. Quantitative morphometry was done (ANOVA, t-tests), and littermates were compared for attrition and innervation.

RESULTS

There were six dentinal patterns, only two of which had an associated neural plexus of Raschkow and cell-free zone (Den-1, Den-2). Other nerves entered dentin from bush-like endings near elongated odontoblasts (Den-B), as single fibers (Den-X), as networks in predentine (PdN), or as single fibers in tertiary dentine at cusp tips (Den-S). There were at least 186,600 innervated dentinal tubules within the set of three right maxillary molars of the best-labeled rat, and similar densities were found in other rats. Attrition levels differed among cusps and in littermates (t-test p < 0.02-0.0001), but the matched right/left cusps per rat were similar. Innervations of tertiary and enamel-free dentine (Den-S, Den-X) were preserved in all rats. Den-B and Den-2 coronal patterns were unchanged unless displaced by dentinogenesis. Den-1 losses occurred in older cusps, while Den-2 patterns increased near cervical and intercuspal odontoblasts.

CONCLUSIONS

The extensive molar dentinal innervation had unique distributions per rat per cusp that depended on region (buccal, middle, palatal) and attrition, but only two of six patterns connected to a plexus of Raschkow.

The Effect of Calcium-Silicate Cements on Reparative Dentinogenesis Following Direct Pulp Capping on Animal Models

Dental pulp vitality is a desideratum for preserving the health and functionality of the tooth. In certain clinical situations that lead to pulp exposure, bioactive agents are used in direct pulp-capping procedures to stimulate the dentin-pulp complex and activate reparative dentinogenesis. Hydraulic calcium-silicate cements, derived from Portland cement, can induce the formation of a new dentin bridge at the interface between the biomaterial and the dental pulp. Odontoblasts are molecularly activated, and, if necessary, undifferentiated stem cells in the dental pulp can differentiate into odontoblasts. An extensive review of literature was conducted on MedLine/PubMed database to evaluate the histological outcomes of direct pulp capping with hydraulic calcium-silicate cements performed on animal models. Overall, irrespective of their physico-chemical properties and the molecular mechanisms involved in pulp healing, the effects of cements on tertiary dentin formation and pulp vitality preservation were positive. Histological examinations showed different degrees of dental pulp inflammatory response and complete/incomplete dentin bridge formation during the pulp healing process at different follow-up periods. Calcium silicate materials have the ability to induce reparative dentinogenesis when applied over exposed pulps, with different behaviors, as related to the animal model used, pulpal inflammatory responses, and quality of dentin bridges.

Chewing causes rapid changes in immunoreactive nerve patterns in rat molar teeth: Implications for dental proprioception and pain

OBJECTIVE

This study tests the hypothesis that normal use of teeth (chewing) causes changes in immunoreactive-(IR) patterns for endings of large Aβ and CGRP axons in rat molar cusps.

DESIGN

First, a new paradigm to test chewing in adult male rats was developed. Then IR patterns for large dental axons were analysed for a calcium-binding protein, parvalbumin (PV), heavy neurofilament protein-200 (NFP), and vesicle-release molecule synaptophysin (SYN) that all typify large dental axons and proprioceptors for comparison with endings of CGRP-IR neuropeptide axons. The behavior groups were: (1) daytime sleeping/fasting (Group:SF); (2) brief feeding after 8-11 h of daytime sleeping/fasting (Group:SF-C); (3) normal nocturnal feeding (Group:N); (4) nocturnal fasting (Group:NF); (5) brief feeding/chewing after nocturnal fasting (Group:NF-C).

RESULTS

Nerve endings with NFP-, PV-, or SYN-IR were lost or altered in pulp and dentin in all chewing groups. Other endings with CGRP-IR were near those with PV-, NFP- and SYN-IR at the pulp-dentin border and in dentin, and they also lost immunoreactivity in all chewing groups. The special beaded regions along the crown pulp/dentin borders lost neural labeling in all chewing groups. Nerves of molar roots and periodontal ligament were not changed.

CONCLUSIONS

Rapid neural reactions to chewing show extensive, reversible, non-nociceptive depletions of crown innervation. Those changes were rapid enough to occur during normal feeding followed by recovery during rest. The new dental paradigm related to chewing and fasting allows dissection of intradental proprioceptive-like mechanisms during normal tooth functions for comparison with nociceptive and mechanosensitive reactions after injury or inflammation.

Effect of Nd:YAG Laser Irradiation Pretreatment on the Long-Term Bond Strength of Etch-and-Rinse Adhesive to Dentin

PURPOSE

To investigate the effect of neodymium-doped yttrium aluminum garnet (Nd:YAG) laser irradiation pretreatment on the long-term bond strength of an etch-and-rinse adhesive to dentin.

METHODS

Fifty molars were sectioned parallel to the occlusal plane and randomly divided into two groups (n=25 per group): control group (no treatment) and laser group (dentin surfaces were treated with Nd:YAG laser at a setting of 100 mJ/10 Hz). Afterward, resin was bonded to the dentin surface using a two-step etch-and-rinse adhesive (Adper SingleBond 2), and then 150 beams of each group were produced. Each group was divided into three subgroups (n=50 each group): 24 hours of water storage, thermocycling, and NaOCl storage. The microtensile bond strength (MTBS), failure modes, nanoleakage expression, and Masson's trichrome staining were evaluated. An additional 20 molars were sectioned to obtain 2-mm-thick flat dentin slices. These slices were randomly divided into control and laser-treated groups as mentioned previously. Then slices of each group were examined by scanning electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), and the Knoop hardness test.

RESULTS

The results of ATR-FTIR and Masson's trichrome verified that laser irradiation partly removed collagen fibers from the dentin surface; however, no significant difference was found in the Knoop hardness (p>0.05). The XRD result showed similar crystalline structure regardless of laser pretreatment. There is no significant difference in short-term MTBS between control and laser-treated groups (p>0.05); however, long-term MTBS differed between the groups (p>0.05). Furthermore, the laser-treated group showed less silver deposition than the control group after aging (p<0.05).

CONCLUSION

Pretreatment by Nd:YAG laser irradiation appeared to have a positive effect on the adhesive-dentin bonding in vitro test, and the bonding effectiveness could be preserved after aging.

Dentin Morphology of Root Canal Surface: A Quantitative Evaluation Based on a Scanning Electronic Microscopy Study

Dentin is a vital, hydrated composite tissue with structural components and properties that vary in the different topographic portions of the teeth. These variations have a significant implication for biomechanical teeth properties and for the adhesive systems utilized in conservative dentistry. The aim of this study is to analyse the root canal dentin going from coronal to apical zone to find the ratio between the intertubular dentin area and the surface occupied by dentin tubules varies. Observations were conducted on 30 healthy premolar teeth extracted for orthodontic reasons in patients aged between 10 and 14. A SEM analysis of the data obtained in different canal portions showed that, in the coronal zone, dentinal tubules had a greater diameter (4.32 μm) than the middle zone (3.74 μm) and the apical zone (1.73 μm). The average number of dentinal tubules (in an area of 1 mm²) was similar in coronal zone (46,798 ± 10,644) and apical zone (45,192 ± 10,888), while in the middle zone they were lower in number (30,940 ± 7,651). However, intertubular dentin area was bigger going from apical to coronal portion. The differences between the analysed areas must be considered for the choice of the adhesive system.

Sensory ability in the narwhal tooth organ system

The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high-salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence.

Dental pulp stem cells and their potential roles in central nervous system regeneration and repair

Functional recovery from injuries to the brain or spinal cord represents a major clinical challenge. The transplantation of stem cells, traditionally isolated from embryonic tissue, may help to reduce damage following such events and promote regeneration and repair through both direct cell replacement and neurotrophic mechanisms. However, the therapeutic potential of using embryonic stem/progenitor cells is significantly restricted by the availability of embryonic tissues and associated ethical issues. Populations of stem cells reside within the dental pulp, representing an alternative source of cells that can be isolated with minimal invasiveness, and thus should illicit fewer moral objections, as a replacement for embryonic/fetal-derived stem cells. Here we discuss the similarities between dental pulp stem cells (DPSCs) and the endogenous stem cells of the central nervous system (CNS) and their ability to differentiate into neuronal cell types. We also consider in vitro and in vivo studies demonstrating the ability of DPSCs to help protect against and repair neuronal damage, suggesting that dental pulp may provide a viable alternative source of stem cells for replacement therapy following CNS damage.

Treatment of dentin hypersensitivity

Dentinal hypersensitivity is exemplified by brief, sharp, well-localized pain in response to thermal, evaporative, tactile, osmotic, or chemical stimuli that cannot be ascribed to any other form of dental defect or pathology. Pulpal pain is usually more prolonged, dull, aching, and poorly localized and lasts longer than the applied stimulus. Up to 30% of adults have dentinal hypersensitivity at some time. Current techniques for treatment may be only transient in nature and results are not always predictable. Two methods of treatment of dentin hypersensitivity are tubular occlusion and blockage of nerve activity. A differential diagnosis needs to be accomplished before any treatment.

Mechanotransducers in rat pulpal afferents

The hydrodynamic theory suggests that pain associated with stimulation of a sensitive tooth ultimately involves mechanotransduction as a consequence of fluid movement within exposed dentinal tubules. To determine whether putative mechanotransducers could underlie mechanotransduction in pulpal afferents, we used a single-cell PCR approach to screen retrogradely labeled pulpal afferents. The presence of mRNA encoding BNC-1, ASIC3, TRPV4, TRPA1, the alpha, beta, and gamma subunits of ENaC, and the two pore K+ channels (TREK1, TREK2) and TRAAK were screened in pulpal neurons from rats with and without pulpal inflammation. ASIC3, TRPA1, TREK1, and TREK2 were present in approximately 67%, 64%, 14%, and 10% of pulpal neurons, respectively. There was no detectable influence of inflammation on the proportion of neurons expressing these mechanotransducers. Given that the majority of pulpal afferents express ASIC3 and TRPA1, our results raise the possibility that these channels may be novel targets for the treatment of dentin sensitivity.

An overview of the dental pulp: its functions and responses to injury

The dental pulp is a unique tissue and its importance in the long-term prognosis of the tooth is often ignored by clinicians. It is unique in that it resides in a rigid chamber which provides strong mechanical support and protection from the microbial rich oral environment. If this rigid shell loses its structural integrity, the pulp is under the threat of the adverse stimuli from the mouth, such as caries, cracks, fractures and open restoration margins, all of which provide pathways for micro-organisms and their toxins to enter the pulp. The pulp initially responds to irritation by becoming inflamed and, if left untreated, this will progress to pulp necrosis and infection. The inflammation will also spread to the surrounding alveolar bone and cause periapical pathosis. The magnitude of pulp-related problems should not be underestimated since their most serious consequence is oral sepsis, which can be life threatening, and hence correct diagnosis and management are essential. Clinicians must have a thorough understanding of the physiological and pathological features of the dental pulp as well as the biological consequences of treatment interventions.

Ultrastructural patterns of human dentinal tubules, odontoblasts processes and nerve fibres

The structure of the dentin, consists of the following elements: the odontoblastic processes, dentinal tubules and their periodontoblastic spaces. The odontoblasts are aligned in a single layer in the periphery of the dental pulp and secrete the organic components of dentin. The vitality of dentin is mediated too by the nerve fibres. The ultrastructure of the trigeminal sensory nerves in dentin, especially in relation to odontoblasts remains to be clarified. We studied the third molars and young premolars. The specimens were fixed in glutaraldehyde immediately after extraction. Our investigations give evidence to prove that the distribution of the dentinary tubules is homogeneous, containing a principal odontoblastic prolongation in the regions of the inner dentine, and only in special cases more than one. The area of the dentinary tubules and the odontoblastic prolongations' area were studied. The nervous fibres appeared accompanying 30-70% of the odontoblastic prolongations and their synapsis-like relation with the odontotoblastic processes was demonstrated. The existence of very few periodontoblastic spaces, and intradentinal sensory axons, as well as the intercellular connections will allow us to discover more about the mechanisms of the dentinary permeability, and its significance in maintenance and repair of the human pulpodentinal complex.

The effect of three types of rasps on the occlusal surface of equine cheek teeth: a scanning electron microscopic study

Two hand rasps (tungsten chip blade, solid carbide blade) and an electrically-driven solid carbide axial bur were used to treat the cheek teeth of 2 horses immediately postmortem. All teeth were normal and were rasped to a standard considered satisfactory in practice. Six teeth from each horse served as untreated controls. Following treatment, the teeth were extracted and the clinical crown removed and prepared for scanning electron microscopy. Teeth were also extracted and examined from a horse that had excessive dental treatment previously. Dental debris created by the procedures was collected and examined. All three rasp techniques resulted in amputation of odontoblast processes. The solid carbide blade cut deep gouges and grooves into the surface of the dentin, chipping the enamel and peripheral cement. No smear layer was created. Rasping with a tungsten chip blade created a partial smear layer and a smoother surface than the solid carbide blade. The electrically-driven bur produced a complete smear layer and removed all dental tissues to a smooth layer. The enamel had also been damaged by the electric bur. Crown particles collected after the procedures were larger following hand rasping compared with particles produced by the electric bur. The extent of damage to sensitive and vital dentin tissue was of concern. Further studies are required to establish the optimum technique for rasping equine cheek teeth.

The distribution of intratubular dentine in equine incisors: a scanning electron microscopic study

The distribution of intratubular (peritubular) dentine was studied by scanning electron microscopy in 12 equine incisor teeth. High levels of intratubular dentine were found in the peripheral regions of the dentine. In these areas, a marked asymmetry occurred, as intratubular dentine was predominantly deposited onto the side of the dentinal tubular walls nearest to the dentino-enamel junction. The quantity and asymmetry of intratubular dentine were reduced towards the centre of the tooth. The significance of these variations in the amount and distribution of intratubular dentine between the different dentinal regions is discussed.

The Dentin Disc surface: a plausible model for dentin physiology and dentin sensitivity evaluation

Dentin sensitivity (DS) is a painful clinical condition which may affect 8-35% of the population. Various treatment modalities have claimed success in relieving DS, although at present there does not appear to be a universally accepted desensitizing agent. Current opinion based on Brännström's Hydrodynamic Theory would suggest that following exposure of the dentin surface (through attrition, abrasion, or erosion), the presence of open dentinal tubules, patent to the pulp, may be a prerequisite for DS. The concept of tubule occlusion as a method of dentin desensitization, therefore, is a logical conclusion from the hydrodynamic theory. The fact that many of the agents used clinically to desensitize dentin are also effective in reducing dentin permeability tends to support the hydrodynamic theory. This paper reviews the in vitro evaluation of desensitizing agents, the techniques used to characterize their effects on the prepared dentin surface, and the ability of these agents to reduce permeability through tubule occlusion, and presents recent findings from ongoing research based on the Pashley Dentin Disc model. It can be concluded that the use of this model to determine surface characteristics, and reductions in dentin permeability through tubule narrowing or occlusion, provides a useful screening method for evaluating potential desensitizing agents. Interpreting changes observed in vitro is difficult, and extrapolation to the clinical situation must be tempered with caution.