The mechanism of pulpal wound healing

The favourable response of exposed pulp tissue against a variety of materials used for pulp capping in experimental conditions, as observed by hard tissue (reparative dentine) formation, demonstrates an intrinsic capacity of pulp tissue for healing. However, in the clinical situation, in which a pulpal exposure is usually accompanied by a long-term external irritation with the subsequent long-term inflammatory response to that irritation, the outcome of pulp capping procedures is not as predictable. While some of the factors related to the defensive reactions and healing after pulp exposure and capping procedures are well understood, the mechanisms and importance of others remain less well-known. Understanding the mechanisms regulating the spread of inflammation and necrosis in pulp tissue, and the factors regulating healing after closure of the wound, would facilitate the development of new and better treatment procedures with more predictable outcomes. In this review, some of the aspects considered to be important in pulpal wound healing are discussed.     

Effects of injury and inflammation on pulpal and periapical nerves

Several studies dealing with the reactions of dental nerve fibers to injury and inflammation are reviewed in this article. The subgroup of dental nerve fibers that contains calcitonin gene-related peptide (CGRP) was examined by immunocytochemistry at various times (1 to 35 days) after one of three degrees of injury: (a) Mild: Four days after making shallow cavities into cervical dentin of first molars of anesthetized adult rats, we found that CGRP fibers had sprouted into the subjacent odontoblast layer and dentin, and then returned to normal by 3 wk. (b) Intermediate: If the cervical cavities were acid etched, we found damage to the odontoblast layer, microabscess formation, and sprouting of CGRP fibers near the abscess, with subsequent formation of reparative dentin and healing. (c) Severe: If the pulp was exposed, a variety of reactions could occur, the most prevalent of which was a severe necrosis leading to development of periapical lesions. Analysis of the progressive stages of pulpal abscess and necrosis showed sprouting CGRP nerve fibers (a) at the retreating interface between abscess and vital pulp; (b) in periapical areas during onset of lesions; and (c) around chronic abscesses in granulomatous periodontal tissues. These studies are discussed in relation to various dental clinical problems such as hypersensitive teeth, episodic toothache, early onset of periapical lesions, dental anesthesia, and possible roles for sensory fibers and neuropeptides in tissue defense and healing.     

牙本质非胶原蛋白诱导牙髓-牙本质复合体反应的动物实验观察

目的：观察提取的猪牙本质非胶原蛋白对修复性牙本质形成的诱导活性。方法：用提取的猪EDTA可溶性牙本质非胶原蛋白作盖髓剂,对Wistar大鼠的健康第一磨牙进行直接盖髓实验,采用氢氧化钙（Dycal）和空白组为对照,通过组织学实验观察牙髓-牙本质复合体的反应。结果：盖髓术后7 d,各组之间软、硬组织反应的差异无统计学意义。术后14 d,NCPs组和Dycal组的修复反应均明显优于空白对照组（P〈0.01）,且NCPs组对牙髓的刺激小于Dycal组（P〈0.05）。结论：NCPs对牙髓刺激性小,可以诱导牙髓细胞分化为成牙本质细胞样细胞,形成修复性牙本质,其诱导活性至少与常规盖髓剂相当。     

The induction of reparative dentine by enamel proteins

AIM: This study was designed to examine whether enamel matrix derivative (EMD) could induce reparative dentine formation without eliciting adverse side-effects in pulpotomized teeth in the miniature swine. METHODOLOGY: Pulpotomy was performed in 36 mandibular incisor teeth from 11 adult miniature swine. Following the surgical procedure, the exposed pulp tissue was treated with EMD or covered with a calcium hydroxide preparation (Dycal). Following an observation period of 3, 4 and 8 weeks, the experimental teeth were extracted and examined using light microscopy and histometric analysis. The total amount of reparative dentine formed in the EMD-treated teeth was calculated as total area using digital histomorphometry analysis of the five central-most sections from each experimental tooth. RESULTS: In the EMD-treated teeth, substantial amounts of dentine-like tissue formation consistently led to a complete hard-tissue bridging of the defects. The onset of hard tissue formation could be observed after 2 weeks and was located only on the pulpal wound. More limited dentine formation was also observed in Dycal-treated teeth. However, in these teeth the new hard tissue formed at the expense of pulp chamber width, causing narrowing of root canals. The total amount of reparative dentine formed in the EMD-treated teeth was significantly higher (P<0.005) than in the Dycal-treated specimens. CONCLUSION: These results demonstrate the potential of EMD as a biologically active pulp-dressing agent that specifically induces pulpal wound healing and dentine formation in the pulpotomized teeth without affecting the normal function of the remaining pulp.     

矿物三氧化物凝聚体用于犬牙直接盖髓的实验研究

目的：观察矿物三氧化物凝聚体(MTA)直接盖髓后牙髓炎症反应和修复性牙本质的形成。方法：人工机械暴露犬牙髓，用MTA或氢氧化钙直接盖髓。结果：实验2周和8周，MTA诱导修复性牙本质形成的效果优于氢氧化钙，炎症反应较轻。结论：MTA是一种效果较好的盖髓剂。     

Effect of ageing on responses of nerve fibres to pulpal inflammation in rat molars analysed by quantitative immunocytochemistry.

The response of sensory nerve fibres to inflammation in young adult rat molars has recently been shown to include increases in nerve sprouting and neuropeptide content. The objective was to evaluate neural responses to class V dental preparations in molars of old (1-2 yr) as compared with young adult rats (3-4 months). Tissues were investigated immunocytochemically 4 days post-injury for the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P. Quantitative image analysis of the material demonstrated that more immunoreactivity was present for CGRP than for substance P in intact control teeth for each age group. Four days after injury, both immunoreactivities were increased in pulp adjacent to the injury in both young and old teeth. The increase depended on at least three factors: (1) enhanced immunoreactivity of the nerve fibres; (2) increased terminal nerve sprouts near the injury and (3) elevated peptide content of the pulp tissue. Although the incidence of CGRP- and substance P-immunoreactive nerve fibres had decreased in older teeth, the proportional increases in both neuropeptides near the injury were greater in old than in young teeth, owing to a reduction in pulpal volume during ageing. Pulpal tissue was also immunostained for the low-affinity nerve growth factor receptor (p75-NGFR) as an index of pulpal ageing; and an extensive decrease was found in the old adult as compared to young adult rats. These results indicate that old rats maintain the capacity for nerve sprouting despite the decreases in p75-NGFR labelling of pulp cells, pulp volume and nerve fibre numbers that occur as part of dental ageing.     

The dentinogenic effect of mineral trioxide aggregate (MTA) in short-term capping experiments

AIM: The objective of the present experiment was to study the early pulpal cell response and the onset of reparative dentine formation after capping application of MTA in mechanically exposed pulps. METHODOLOGY: Thirty-three teeth from three dogs, 12-18 months of age were mechanically exposed via class V cavities. Light pressure was applied to control haemorrhage. ProRoot MTA (Dentsply Simfra, Paris) was placed at the exposure site and light pressure was applied with a wet cotton pellet. The cavities were restored with amalgam and the pulpal tissue reactions were assessed by light and electron microscopy (transmission and scanning) after healing intervals of 1, 2 or 3 weeks. RESULTS: A homogenous zone of crystalline structures was initially found along the pulp-MTA interface, whilst pulpal cells showing changes in their cytological and functional state were arranged in close proximity to the crystals. Deposition of hard tissue of osteotypic form was found in all teeth in direct contact with the capping material and the associated crystalline structures. Formation of reparative dentine (tubular matrix formation in a polar predentine-like pattern by elongated polarized cells) was consistently related to a firm osteodentinal zone. CONCLUSIONS: The present experiments indicate that MTA is an effective pulp-capping material, able to stimulate reparative dentine formation by the stereotypic defensive mechanism of early pulpal wound healing.     

Caries-induced changes in the expression of pulpal neuropeptide Y

Recent evidence suggests that the sympathetic nervous system may have a role in modulating neurogenic inflammation and bone remodelling. Neuropeptide Y (NPY) is a well-characterized neuropeptide transmitter in the peripheral sympathetic nervous system. NPY is known to be present in human dental pulp; however, quantitative data on NPY levels in pulpal health and disease in an adult population remain to be determined. The aims of the current study were to assess, quantitatively, NPY levels by radioimmunoassay and confirm the distribution of NPY fibres by immunocytochemistry in carious and non-carious adult human pulp tissue. Our results suggest changes in the levels and distribution of NPY in human dental pulp during the caries process, with significantly higher levels of NPY in carious compared with non-carious adult human teeth. Within the carious samples studied, our finding, that NPY levels were significantly elevated in mild/moderate caries, concurs with the hypothesis that NPY could have a modulatory role in pulpal inflammation and in reparative dentine formation.     

Is hard tissue formation in the dental pulp after the death of the primary odontoblasts a regenerative or a reparative process?

Objectives

Conceptually, two types of tertiary dentine may be produced in response to caries and environmental irritations: “reactionary dentine” that is secreted by existing primary odontoblasts and “reparative dentine”, formed after the death of the odontoblasts by proliferation and differentiation of progenitor cells into odontoblast-like cells. Because histologic evidence for tubular dentine generated by newly differentiated odontoblast-like cells is lacking in human teeth, the present study examined pulpal cellular changes associated with caries/restorations, in the presence or absence of pulpal exposures.

Methods

Ninety-six extracted human teeth were histologically processed and serial sectioned for light microscopy: 65 contained untreated enamel/dentine caries; 20 were heavily restored and 11 had carious exposures managed by direct pulp-capping.

Results

Sparsely distributed, irregularly arranged dentinal tubules were identified from the tertiary dentine formed in teeth with unexposed medium/deep caries and in restored teeth; those tubules were continuous with the tubules of secondary dentine; in some cases, tubules were absent. The palisade odontoblast layer was reduced to a single layer of flattened cells. In direct pulp-capping of pulp exposures, the defects were repaired by the deposition of an amorphous dystrophic calcified tissue that resembled pulp stones more than dentine, sometimes entrapping pulpal remnants. This atubular hard tissue was lined by fibroblasts and collagen fibrils.

Conclusions

Histological evidence from the present study indicates that reparative dentinogenesis cannot be considered as a regenerative process since the so-formed hard tissue lacks tubular features characteristic of genuine dentine. Rather, this process represents a repair response that produces calcified scar tissues by pulpal fibroblasts.

Clinical significance

Formation of hard tissue in the dental pulp after the death of the primary odontoblasts has often been regarded by clinicians as regeneration of dentine. If the objective of the clinical procedures involved is to induce healing, reduce dentine hypersensitivity, or minimise future bacteria exposure, such procedures may be regarded as clinical success. However, current clinical treatment procedures are not adept at regenerating physiological dentne because the tissues formed in the dental pulp are more likely the result of repair responses via the formation of calcified scar tissues.     

Effects of a new antibacterial adhesive on the repair capacity of the pulp-dentine complex in infected teeth

AIM: To evaluate the effects of a self-etching/priming adhesive system, containing the antibacterial monomer 12-methacryloyloxy-dodecylpyridinium bromide (MDPB), on the repair capacity of the pulp-dentine complex in infected cavities in dog's teeth. METHODOLOGY: Class V cavities with a residual dentine thickness ranging from 0.3-0.8 mm were prepared on the buccal surface of permanent teeth in four dogs. Pulpal exposures were performed in half of the cavities. Millipore filters that had been incubated for 3 h in a 10(5) milky suspension of a-streptococci were placed in the cavities, which were then filled temporarily. After 24 h, the filters were removed and both the exposed and non-exposed cavities were washed with sterile saline and assigned to four groups which were treated with either the experimental antibacterial adhesive system, or Clearfil SE bond, Dycal and Teflon discs. Stereotype connective tissue reactions (inflammatory cell response and/or tissue necrosis) and pulp-specific reparative tissue responses (reduction of odontoblasts and tertiary dentine formation) were assessed at post-operative periods of 4 and 8 weeks. RESULTS: Neither severe inflammation nor tissue necrosis was observed, either in the dentinal cavities or pulpal exposures treated with the self-etch adhesive containing MDPB. Rates of tertiary dentine formation in infected dentinal cavities treated with this system were comparable with those observed after dentine treatment with the Ca(OH)2-based material. Dentinal bridging was not seen in pulpal exposures treated with the experimental adhesive. CONCLUSIONS: The new antibacterial adhesive system maintained pulp vitality and primary odontoblastic function in infected nonexposed and exposed cavities but interfered with reparative dentine formation in infected pulpal exposures.     

Histologic effects of antigenically altered collagen as a heterograft for mammalian pulp exposures

Mammalian tooth pulp can heal an exposure by synthesis of a layer of reparative dentine at the site of injury. Since reconstituted antigenically altered collagen gels have been shown to assist in the repair of surgically created defects in bone, this compound has been investigated to determine if it could stimulate healing and reparative dentine formation over a pulp exposure. The results demonstrated that the classical treatment with calcium hydroxide has a better prognosis than does the antigenically altered collagen gel.     

Regeneration of calcitonin gene-related peptide immunoreactive nerves in replanted rat molars and their supporting tissues.

First maxillary right molars in 66 rats were elevated and replanted and the pulps allowed to regenerate for 1-90 days. The contralateral tooth served as control. Regeneration of nerves in the pulp and periodontium was studied by CGRP-immunohistochemistry and the avidin-biotin-peroxidase method. The pulp and periodontium of the controls were richly supplied with CGRP-labelled nerves. One day after replantation the pulp was completely devoid of CGRP-immunoreactive nerves. After 2 days, axon sprouts were present in the apical, regenerated pulp and in the periodontium. From 3-7 days CGRP-immunoreactive axons were regularly seen to have regenerated in front of the cellular inflammation in the pulp. After 10 days, the pulps were reinnervated up to the horns, although more sparsely than in the controls. From day 20-90 there was a marked divergence in pulpal healing: 17 pulps formed irregular postoperative dentine with a gradual increase in nerve density; 16 pulps remained sparsely innervated and were gradually replaced by bone. Root resorption was most extensive in the teeth with bone replacement of pulp. The soft tissue adjacent to extensive resorbing areas had many more CGRP-labelled axons than in the controls. The reinnervation of the regenerating pulp occurred at the same time as pulpal wound healing, but did not achieve the innervation density of the controls.     

Pulpal Injury: Pathology,Diagnosis And Periodontal Reactions

Diagnosis of pulpal disease can be difficult due to the lack of diagnostic signs and symptoms available to the practitioner. An understanding of the possible underlying pathological processes, combined with an exact assessment of the pain history, and appropriate clinical tests, should aid the practitioner in determining the nature of pulpal inflammation, and differentiating it from dentine sensitivity and cracked teeth. The responses of the pulp to traumatic injury to the periodontal membrane (PDM) require special consideration, particularly with respect to the assessment of pulp vitality, and the determination of cases requiring pulp extirpation in order to avoid inflammatory root resorption. Although the pulp is relatively isolated from the rest of the dentoalveolar complex by a dentine/cementum barrier, it is important to remember that it can communicate with the PDM through apical and lateral foramina, and areas of damaged cementum. Hence, it is a priority to both preserve the integrity of the cemental layer in cases of traumatic injury and periodontal disease, and to prevent the inflammation and resorption associated with periapical lesions by accurate diagnosis of irreversible pulpitis and pulp necrosis, followed by appropriate endodontic debridement procedures.     

Delayed direct pulp capping in permanent incisors of monkeys

Summary. The effect of preoperative oral contamination for either 4 or 48 hours on pulpal response to direct pulp capping was studied. Four young monkeys provided a contralateral experimental series of 16 pairs of incompletely developed permaent incisors. Radiographic examination, labelling in vivo , and conventional histological observations at 6 months postoperatively indicated no difference between the two series. All incisors in either series showed continued apical development, and all but one demonstrated increase in cervical dentine. All but two teeth in each series showed apparently fully bridged pulpal wounds. Full bridging generally correlated with normal pulpal conditions; however, in three teeth of the 48-hour and one of the 4-hour series, full bridging occurred along with moderate or dense coronal infiltrates of chronic inflammatory cells. All these teeth were mandibular and showed a large denticle formed around dentine chips. Failed bridging seen in two teeth of each series correlated with severe inflammation in the adjacent pulpal tissue. No consistent adverse effect of coronal pulpal inflammation on cervical increase in dentine was recorded.     

Dentine hypersensitivity - A review

The exposure of cervical dentine has a multifactorial aetiology and pain may frequently be elicited by a number of stimuli. Management of the condition, dentine hypersensitivity, tends to be empirical because of the lack of knowledge concerning the mechanism of pain transmission through dentine. The pulpal changes associated with the condition and any modulating effect on symptoms are by no means clearly established. Evidence suggests that exposed dentine which is sensitive exhibits patent tubules. The question of how pain is initiated across the dentine has received considerable attention but still remains somewhat debatable. The literature reviewed indicates that, at most, nerve fibres only penetrate a limited distance along some dentinal tubules. The theories that either the odontoblasts and their processes act as dentinal receptors or the nerves in the pulp are the pain receptors, are discussed. Evidence for the stimulation of pulp nerve fibres by a hydrodynamic mechanism would appear the most likely mechanism. Nevertheless, whichever theory proves to be correct, occlusion of dentinal tubules would appear an essential prerequisite for an effective desensitising agent.     

Dissecting dentine–pulp injury and wound healing responses: consequences for regenerative endodontics

A thorough understanding of the biology of the dentine–pulp complex is essential to underpin new treatment approaches and maximize clinical impact for regenerative endodontics and minimally invasive vital pulp treatment (VPT) strategies. Following traumatic and carious injury to dentine–pulp, a complex interplay between infection, inflammation and the host defence responses will occur, which is critical to tissue outcomes. Diagnostic procedures aim to inform treatment planning; however, these remain clinically subjective and have considerable limitations. As a consequence, significant effort has focussed on identification of diagnostic biomarkers, although these are also problematic due to difficulties in identifying appropriate diagnostic fluid sources and selecting reproducible biomarkers. This is further compounded by the link between inflammation and repair as many of the molecules involved exhibit significant multifunctionality. The tertiary dentine formed in response to dental injury has been purposefully termed reactionary and reparative dentine to enable focus on associated biological processes. Whilst reactionary dentine produced in response to milder injury is generated from surviving primary odontoblasts, reparative dentine, in response to more intense injury, requires the differentiation of new odontoblast‐like cells derived from progenitor/stem cells recruited to the injury site. These two diverse processes result in very different outcomes in terms of the tertiary dentine produced and reflect the intensity rather than specific nature (nonexposure versus exposure) of the injury. The subsequent identification of the odontoblast‐like cell phenotype remains challenging due to lack of unique molecular or morphological markers. Furthermore, the cells ultimately lining the newly deposited dentine provide only a snapshot of events. The specific source and plasticity of the progenitor cells giving rise to the odontoblast‐like cell phenotype are also of significant debate. It is likely that improved characterization of tertiary dentine may better clarify the influence of cell derivation for odontoblast‐like cells and their diversity. The field of regenerative endodontics offers exciting new treatment opportunities, and to maximize outcomes, we propose that the term regenerative endodontics should embrace the repair, replacement and regeneration of dentine–pulp.     

The influence of haemostatic agents on healing of healthy human dental pulp tissue capped with calcium hydroxide

AIM: To investigate the hypothesis that different haemostatic agents could impair the histological response of human pulps capped with calcium hydroxide. METHODOLOGY: Forty-five third molars scheduled for extraction were selected. Class I cavities with pulp exposures were prepared. Three agents were used to control bleeding: 0.9% saline solution (control, n = 14), 5.25% sodium hypochlorite (n = 16) and 2% chlorhexidine digluconate (n = 15). The pulps were dressed with hard-setting calcium hydroxide cement. After 7, 30 or 90 days, teeth were extracted, formalin-fixed and prepared for histochemical techniques. The biological response was categorized using the following criteria: inflammatory response, soft tissue organization, reactionary dentine and reparative dentine. Data were submitted to statistical analysis, using nonparametric Kruskal-Wallis one-way analysis of variance on ranks. Differences amongst groups were detected using Dunn's method. RESULTS: The statistical analysis disclosed that whilst inflammatory response decreased over time, reactionary dentine deposition and reparative dentine formation increased in the latter periods of evaluation (P < 0.05). The three agents had similar performances for all criteria evaluated. The conventional pulp response to calcium hydroxide was observed over time, and complete pulp healing was observed in 88% of the specimens after 90 days. CONCLUSION: The three haemostatic agents did not impair the healing process following pulp exposure and capping with calcium hydroxide at different time intervals investigated.     

The response of four calcium hydroxides on monkey pulps

Abstract Dentinal bridge formation and pulpal responses of four calcium hydroxide materials, pulp capping medicaments, MFC®, Experimental MFC-12, Dycal® and Pulpdent®, were evaluated in primary and permanent monkey teeth. A total of 60 primary and 60 permanent teeth were used with each material placed in a Class V cavity exposure in Rhesus monkey teeth. The materials were placed on the exposed pulp tissue and were histologically evaluated at 3 days, 5 weeks and 8 weeks. After perfusion the teeth were processed using routine histological procedures. The 3-day pulpal responses in both primary and permanent teeth were moderate, characterized by disruption of the pulpal tissue directly beneath the exposure site and a zone of acute inflammation and hemorrhage in the underlying pulp. The 5-week response showed histological differences between the four medicaments, with Dycal producing the least amount of pulpal irritation with reparative dentin bridges occurring in 50% of the permanent teeth. Experimental MFC-12 stimulated one reparative dentin bridge, while Pulpdent and MFC showed no evidence of bridge formation. Pulpal responses to Dycal were moderate and moderate to severe for the other calcium hydroxide compounds. No reparative dentin bridges were seen in the primary teeth at 5 weeks with any of the materials, and the pulpal responses were of a moderate degree at that time. Eight-week responses were similar to the 5-week responses. Dycal provoked a slight to moderate pulpal response with 50% success at bridging. Experimental MFC-12 initiated pulpal responses in the moderate to severe range with some bridging evident. Pulpdent incited moderate to severe histological responses with three teeth demonstrating bridge formation, and MFC provoked severe pulpal responses with no bridging. Primary teeth showed some bridging for all compounds except those treated with MFC, in which no evidence of bridging occurred, and moderate to severe pulpal responses were present.     

光敏黏接剂盖髓后牙髓的组织病理反应

目的观察应用3种光敏黏接剂直接盖髓后,兔牙髓的反应。方法兔前臼齿及臼齿72颗,分为4组,制备Ⅰ类洞机械穿髓,实验组3组(Excite,Prime & Bond NT,XenoⅢ),Dycal为对照,光敏树脂(SureFil)充填。于第7、14、21天每组拔除6颗牙齿,常规脱钙切片,镜下观察充血反应、炎细胞浸润、纤维化程度、牙本质桥形成、修复性牙本质形成、细菌污染等情况。结果所有实验牙齿都有牙本质桥形成,修复性牙本质封闭穿髓孔。同一时期内,兔牙髓对不同盖髓剂的反应无显著性差异。除对照组外,同种黏接剂在不同时期内对牙髓的影响无显著性差异。结论此3种黏接剂对兔牙髓具有良好的生物相容性,并能促进修复性牙本质生成。