Linda G. Levin, DDS, PhD
Is the dental pulp a vestigial organ whose sole raison d’etre is dentin formation? Or is the pulp a highly specialized connective tissue necessary for immune competence and sensation within the tooth proper, long after dentinogenesis is complete? While modern research and clinical observation have supported the latter view, the ability of clinicians to preserve and revive pulpal tissue subsequent to carious exposure is poor, and their ability to replace the necrotic tissue in pulpless teeth is nonexistent.
Attempts to effect pulpal repair and dentin regeneration represent one of the oldest therapeutic goals in dentistry. Despite the wealth of knowledge that has been generated on the biology and pathophysiology of the dental pulp, very little progress has been made in the treatment of the cariously exposed pulp since Pfaff first performed pulpal cautery over 242 years ago.
This is not to indicate that contemporary aseptic techniques have not been applied to pulpal wound healing. In fact, the research of Kakehashi et al in 1965 effectively removed the mystique of pulp capping. This classic treatise demonstrated that exposed and inflamed pulps will heal and form reparative dentin in the absence of bacteria. As a result of this work, pulp biologists began to realize that the variable prognosis of pulp capping is predominantly a restorative issue. Until materials that consistently disinfect and hermetically seal the underlying pulp are developed, pulp capping of mature teeth exposed by caries will not be a reliable or predictable therapy. Consequently, conventional root canal therapy must be performed for the cariously exposed mature tooth, and temporary pulpotomies must be used for apexigenesis of immature teeth, both of which are followed by endodontic therapy (Figures 1-2-3).
Despite the paucity of restorative materials available to accomplish a long-term seal, several provisional means have demonstrated the ability to establish a short-term hermetic seal. Zinc oxide eugenol-based provisional cements have been shown to provide a bacteria-proof barrier for the healing pulp, provided that they are placed under aseptic conditions and with a minimum thickness of 3mm. Attempts have been made to hasten pulpal wound healing with a variety of medicaments and trophic factors to obtain a biological seal prior to the deterioration of the filling material. Chemical and biological pulp capping materials (eg, calcium hydroxide, collagen, fibronectin, transforming growth factors, and dentin fragments) have been utilized with variable degrees of success. Of these materials, calcium hydroxide is used most commonly in clinical practice.
Regardless of the stimulant used, the feasibility of pulp capping is dependent on the presence of endogenous pulpal tissue and is, therefore, by definition, a treatment for vital pulp only. When successful, pulp capping results in pulpal wound healing and partial dentin regeneration superficial to the wound; in mature permanent teeth, however, it has a variable and unpredictable prognosis. This has been attributed not only to the lack of materials that disinfect and hermetically seal the underlying pulp, but also to the inability of clinicians to determine the degree and extent of pulpal inflammation (Figure 4). Until these restorative and diagnostic concerns are resolved, the therapeutic options remain conventional root canal therapy for the cariously exposed mature tooth and temporary pulpotomies for apexification of immature teeth, both again followed by endodontic therapy.
At present, treatment options for the pulpless tooth are even more limited. Endodontic therapy for these teeth is the only alternative to extraction. In teeth with incompletely formed apices, long-term calcium hydroxide therapy can provide rooted closure but not an increase in root length or dentinal wall thickness—two factors that impact negatively on the eventual prognosis for the tooth. For these teeth in particular, regeneration of a pulplike tissue wit the capacity for continued tooth development would be a superior alternative to the conventional treatments currently available.
Tissue engineering is the field of biomedical science that has focused on the production of synthetic extracellular matrices, which, when placed in situ, can promote regeneration of a targeted tissue. It has been used effectively in animal models for regenerations of the peripheral nerves, spinal cord, bone, bladder urothelium, smooth muscle, and dermis. While the some of these techniques are still in experimental stages, a few have made their way into clinical practice. Keratinocytes seeded onto synthetic membranes have been used to positively manipulate the healing environment of skin wounds. Using certain collagen-GAG copolymer matrices known to inhibit wound contraction during healing, practitioners have been able to control and minimize scar formation. This has had enormous implications in the treatment of patients prone to keloid formation.
The characteristics of the extracellular matrix (ECM) analogue for a given tissue appear to be pivotal to the success of regeneration. Numerous investigators have analyzed various matrix parameters and indicated that the speed of degradation, pore size, and chemical composition of the matrix are important and variable for each tissue. Custom cell scaffolds have been fabricated by in situ transfer of native extracellular matrix from cells in culture. Matrix has also been extracted from resident tissues and subsequent reconstituted with specific growth mediators and used to repair tissue defects. Some of the most promising matrix analogues are the polyesters, which demonstrate superior biocompatibility and easily manipulated degradability. These matrices have proven to be optimally suited for bone and offer promise for dentin regeneration.
Preliminary studies indicate that the dental pulp is amenable to tissue engineering. Utilizing primary human pulpal fibroblasts, investigators seeded cells on the nonwoven polyglycolic acid (PGA) matrices and cultured them over a 60-day period. Fibroblasts readily adhered to the PGA scaffold and proliferated. As the matrix degraded, the cells condensed until they histologically resembled native pulpal tissue. By the end of the observation period, the artificial pulps had achieved the same cellular density as that of native adult pulpal tissue.
Numerous points must be addressed before a successful in vivo model of pulpal tissue regeneration can be devised. The pulpal ECM analogue should be able to stimulate neovascularization of the newly engineered tissue. The seeded pulpal cells must be able to survive long enough for vascularization to occur. In avulsed teeth with incompletely formed apices, revasculariztion is known to occur, albeit over a span of several months. This has been confirmed by laser Doppler. Studies on tumorigenesis have established that no cell in a rapidly growing tumor is greater than 100 µm from the nearest capillary. One possible way to circumvent the lag between placement of a seeded matrix and vascularization would be to utilize an acellular matrix analogue over vital pulpal tissue. A rat model designed to evaluate the regeneration of bladder urothelium, smooth muscle, blood vessels, and nerves into an acellular tissue matrix exhibited neovascularization within 2 weeks and the formation of neural elements within 4 weeks., provided that these structures were present in the native tissue bed. In order to augment cellular ingrowth in the artificial matrix, chemotactic factors for endothelium and neuronal elements could be incorporated into the three dimensional matrix. This approach could not be utilized in the necrotic pulp, which would require a progenitor cell source (such as the aforementioned seeded matrix) for regeneration. In this situation, autogeneous pulpal cell digests or gingival fibroblasts could be used to seed the matrix. Additional research on the characterization of cell population within the dental pulp as well as the potential of other mesenchymally derived cell lines to differentiate into “pulpal cells” will be required before satisfactory “surrogate” cells can be identified.
Contamination is also a significant complication to pulpal regeneration in the pulpless tooth. All of the studies on tissue regeneration with cell-seeded matrix analogues were conducted in a sterile environment. The majority of pulpless teeth treated clinically have an infected root canal space. It has been demonstrated that, by the use of a combination of instrumentation, debridement with an antibacterial irrigant, and a 7-day regimen of calcium hydroxide, the microorganism titer can be reduced below culturable levels; this criterion, however, does not ensure sterility. In addition, while calcium hydroxide has demonstrated the greatest efficacy as an intracanal medicament, its pH is high, and residual amounts could alter the physiological pH of the artificial matrix, thereby exerting a toxic effect on the regenerating cells. Thus, it appears that a new standard of canal disinfection will be required before the necrotic root canal can be utilized as a “test tube” for regenerating tissue.
The clinical presentation of a tooth with an irreversible pulpitis secondary to exposure is most ideally suited for pulpal regeneration. Removal of the coronal pulp and placement of a three-dimensional soluble matrix—either acellular with mediators (ie, integrins and chemotactic factors) or seeded with the patient’s fibroblasts—may offer the best prognosis, provided the coronal access of the pulp chamber can be completely sealed. Once again, microleakage appears as a significant variable in pulpal regeneration. Dentistry is in great need of materials that will reliably ensure a tight seal to protect the root canal space during pulpal regeneration. Once this has been achieved, a variety of new pulpal therapies will be available to endodontic patients—including regeneration of lost dental structures.
*Department of Endodontics, University of North Carolina School of Dentistry, Chaple Hill, North Carolina.