Essential endodontology: prevention and treatment of apical periodontitis. 3rd ed

Chapter 11. Endodontic Treatment of Apical Periodontitis

Dag Orstavik

11.1 Introduction

11.1.1 Rationale for Treatment

The diagnosis of any form of apical periodontitis is generally considered a need to treat the condition. Acute/symptomatic or chronic/asymptomatic apical periodontitis, periapical abscesses with or without a sinus tract, as well as radicular cysts, start from an infected root canal system, which is the common target for therapeutic measures in these situations.

A chief complaint of pain or discomfort brings its own indication for intervention, and if apical periodontitis is the source of the pain, then curing the disease is the obvious treatment plan. Asymptomatic, chronic apical periodontitis, on the other hand, needs a rationale for treatment by itself. Patients do not necessarily feel any need for a costly and complicated treatment without having any pain or discomfort. There are three good reasons for attempting to cure also chronic, asymptomatic apical periodontitis.

11.1.1.1 Pain Control

A chronic inflammation caused by bacteria has the potential to exacerbate. Chronic apical periodontitis may develop insidiously to form large cysts without any symptoms, or it may remain stable and unchanged in size. The risk of such a lesion changing into an acute inflammation with toothache, swelling and abscess formation is a primary reason why one advocates treatment of the disease. It must be recognized, however, that predicting the incidence and severity of such exacerbations is difficult [7, 97, 198, 207], and is influenced by many variables that may be outside the control of patients and dentists.

11.1.1.2 Local Spread of Infection

A second rationale for treatment is the pos- sibility of the infection spreading to tissues and organs in the immediate and regional vicinity. The medulla of the jawbones may be infected to cause osteomyelitis, which in many cases is not easily treated. The maxillary sinus is frequently involved, and many cases of sinusitis have apical periodontitis as a contributing or causative factor [249, 250]. Brain abscesses have been reported with tooth infection as the source in over 30% of cases, but the type of tooth infection is often not known [109]. Acute infections follow t issue spaces or fasciae and may reach the mediastinum with life-threatening complications, particularly in immunocompromised individuals [100, 156].

11.1.1.3 Association with Systemic Diseases

Finally, the association of local dental infections or inflammation with heart and circulatory disease is an issue of concern (see also Chapter 4). While the risk posed by dental infection is low, there is no longer doubts about its existence [20, 112]. Patients with heart valve problems may be susceptible to infections from bacteremia following endodontic procedures. Here elimination of apical periodontitis traditionally is considered mandatory [42].

11.1.2 Purpose and Challenges of Treatment

The microbial etiology of the disease itself and of its sequelae defines the purpose of treatment: the elimination of microbes infecting the root canal system and occasionally the periapical tissues. All other efforts made during treatment are subsidiary to this primary goal, and one cannot compensate for failure to control the infection by other technical or clinical aspects of the procedures, such as pain control, use of antibiotics, or a visually pleasing root filling.

There are multiple protocols for treatment of apical periodontitis. This chapter does not aim to set out in detail the practicalities of treatment, but to highlight the principles and biological underpinnings of the elements of treatment.

11.1.2.1 Elimination of Infecting Microbes

The complexity of the root canal system makes treatment difficult: biofilms formed on surfaces in isthmuses and canal ramifications are poorly accessible by physical and chemical means; and antibiotics hardly penetrate dental tissues in sufficient concentrations for clinical effect. In addition to the treatment principles for root filling after vital pulp extirpation (Chapter 10), treatment of the infected tooth needs to ensure maximal antibacterial effect by mechanical instrumentation and application of disinfectants.

The classical works of Sundqvist and associates from the 1970s through the 1990s remain the backbone for our concepts and principles for reducing or eliminating endodontic infections. They and others established that apical periodontitis in humans does not develop in the absence of root canal infection [18, 225]. They went on to document how a systematic approach with instrumentation [26], irrigation [27, 28], and dressing [24, 212] had the potential to render the canal bacteriafree, and that subsequent treatment with root canal obturation was followed by success rates similar to those found after vital pulp extirpation [25, 213].

11.1.2.2 Outcome of Endodontic Treatment

These clinical bacteriological studies suggested that a treatment protocol with predictable curing of apical periodontitis could be estab- lished. Moreover, it lent support to a concept of a qualitative approach to disinfection: it should be possible to eliminate the infection and thereby re-establish a healthy apical periodontium completely free of microbes. However, subsequent clinical experiments did not reproduce the same level of disinfection, and follow-up studies of treatments performed according to the same principles did not always show the same good clinical-radiological results. It is also recognized that culturing of samples from root canals may give both false positive and false negative results, especially when sampling is performed with suboptimal techniques [195]. Thus, it remains a challenge to get the same high success rate for treatment of established apical periodontitis as for root fillings after pulpectomies.

11.2 Anatomic Location of the Microbes

11.2.1 Infection of the Root Canal System, Dentin and Cementum

The anatomical location of the infecting microbes (Figure 11.1) is a major reason why disinfection of root canals and the periapical area is so difficult. The main bulk of microor- ganisms are located inside the root canal itself, and even in long-standing and post-treatment apical periodontitis it is the bacteria inside the root canal that are mainly responsible for the inflammatory response. These pulpally located bacteria should be readily susceptible to mechanical and chemical debridement, but the complexity of the pulp ramifications limits the accessibility of instruments and medicaments even here. Moreover, the microbes are organized in biofilms colonizing the least accessible areas on the internal root surface [259].

Figure 11.1 Anatomical location of infecting microbes in apical periodontitis. (a) Overview. The bulk of bacteria are located inside the pulp lumen. (b) Microbes in pulp canal ramification (reproduced with permission from [176]). (c) Bacteria in dentin tubules (DT) infected through to the periodontal membrane (PM) (reproduced with permission from [244]). (d) Biofilm on external root surface (reproduced with permission from [174]). (e) Filamentous and coccoid bacteria and red blood cells from the center of a granuloma (courtesy of Dr Pia T. Sunde). (f) Specific fluorescent staining of bacteria extraradicularly in the connective tissues of an endodontic lesion (reproduced with permission from [223]).

With long-standing infection of the pulp space, microorganisms also occupy lateral and furcal side canals. This will further com- plicate efforts to disinfect the space.

Infection of dentinal tubules also occur. When the overlying cementum is intact and vital, bacteria penetrate the tubules to a limited depth. With longer-standing infections where the destructive processes have reached the periodontal membrane causing necrosis of the cementum, a “through and through” infection of the dentinal tubules may occur [244]. In such cases, there is often an irregular resorption of the dentin surface on the periodontal side of the apical foramen, caused by clast cells attempting to get at the microbes infecting the tubules and the pulp space. Fortunately, successful elimination of the infection leads to remodeling of the periodontal tissue even after extensive apical resorption [179].

11.2.2 Extraradicular Infection

Modern concepts of bacterial infections emphasize the importance of a foothold or surface for bacteria to attach to and multiply on [63, 105]. Skin, mucosal surfaces, and the interior of blood and lymphatic vessels offer such surfaces for adhesion. At the root apex there is limited opportunity in the surrounding tissues for attachment and growth of bacteria with limited pathogenicity and viru- lence. In acute phases, the balance of bacterial virulence and tissue resistance is shifted in favor of the bacteria, and their interaction with tissue and immune cells results in lique- faction and abscess formation. When a sinus tract develops, the infecting organisms are kept at bay by constant drainage through the tract; but the tracts interior surface may be colonized by the bacteria.

A bacterial biofilm may cover the cementum itself in chronic apical periodontitis [118, 134, 175]. This biofilm is similar to bacterial plaque with a tendency for calcification [165]. It is reasonable to speculate that conservative measures to eliminate the infection are much less likely to succeed when such a plaque is established.

The extraradicular bacteria in abscesses, sinus tracts, and expanding cysts may not be easily susceptible to the body's immune defenses. Moreover, in other situations, particular types of microorganisms manage to survive within the soft tissues of the apical granuloma without the presence of an external, solid surface [180]. Actinomyces and Propionibacterium colonies are the classical example. The cells cluster and grow from the inside, and when the aggregate is large enough, even massive attacks from phagocytic cells on its outside fail to limit growth in the central area. Moreover, also other cells may join forces to create clusters within the soft tissues, which help them survive the host defenses [50, 68, 180, 222, 223, 238-240].

While systemic antibiotics may limit growth and expansion of the infection, long- term success is questionable at best, and surgical removal by apicectomy is the preferred treatment modality [177]. It should be noted, however, that a clinical diagnosis of an extraradicular infection is always a speculative one until verified by a biopsy.

11.2.3 Infection in Endodontically Treated Teeth

The bacterial flora of infected, previously root-filled teeth may be different from that of primary infections (see Chapter 4). The microenvironment at the apex was disturbed during the first treatment, and the presence of root filling materials at or beyond the fora- men provides niches and physical surfaces different from the original. Thus bacteria may also grow on surfaces of the root filling material, core material, and sealer [87].

This may induce a selection of a more facultative flora, with streptococci and particularly Enterococcus faecalis increasing in relative numbers [130, 278]. Yeasts also have been associated with persistent apical periodontitis [256], but initial findings of relative dominance in post-treatment disease have not been confirmed [160]. On the other hand, if the post-treatment inflammation is associated with a root filling ending at a distance from the apical constriction, the residual space provides an environment similar to the pretreatment state, and the flora is most likely similar to that of a primary infection.

The differences in bacterial composition may be one reason why the success rate of retreatments is generally poorer than pri- mary treatment of apical periodontitis [130, 226], and attempts at elucidating mechanisms for the persistence of these infections are still being made [89, 231]. However, molecular studies of the microbial flora in different endodontic infections do not show any clear-cut differences among them [182], and it appears that generally, the microbial profile of post-treatment endodontic infections [274, 275] is similar to that of primary infections [191]. While there is still the pos- sibility that the relative proportions of species or taxa may differ systematically, such differences have not yet been translated into differential treatment strategies with documented efficacy.

11.3 Bacteriological Status During Treatment

Given the microbial etiology of apical periodontitis, it makes sense to monitor the presence or extent of infection during treatment, from entry into the pulp chamber to the time of root canal obturation. The infection fol- lows a course as outlined in Figure 11.2 (modified from [148]). Given the proven concept of “no bacteria, no lesion", a bacteria- free canal ensures treatment success, and a bacterial sample taken at the time of filling should not show any growth.

Figure 11.2 Development and healing of apical periodontitis related to the level of infection. Microbes in the root canal system (a, b) initiates and sustains the periapical inflammation (c); mechanical instrumentation (d), irrigation and medication (e), and root filling (f) reduce the level of infection sufficiently for repair and regeneration of the apical periodontal structures (g) [148].

11.3.1 History and Status of Microbiological Sampling from the Root Canal

The bacteriological sample may be seen as a surrogate measure of treatment success, and was advocated as a routine procedure to be applied before a root canal is considered ready to be filled [69]. For many reasons, however, this procedure is no longer applied in everyday practice. It is cumbersome; avail- able techniques are often error-prone; and evaluating results does not give an accurate bacteriological picture at the filling session. However, bacteriological sampling remains an essential tool for experimental and comparative studies on the efficacy of treatment procedures, and bacterial growth from the canal is the only parameter that has an etiology-based, direct correlation to treatment outcome. Chairside, real-time methods for assessing the bacteriological status have been developed, and some techniques hold a potential for use in clinical practice [74].

There may be good reasons to assess techniques and materials for their ability to adhere to dentin walls, to eliminate necrotic or vital tissue, to penetrate tubules; to limit extrusion of debris etc., but these parameters are all subordinate to the effect of microbial infection on the healing or development of apical periodontitis. Therefore, over the dec- ades, a standardized way of checking for infection during treatment has been developed and applied for assessments of the performance of endodontic treatment methods [26, 51, 153, 181, 225]. An initial sample from the root canal is taken on first entry into the canal space. This is to ascertain that the root canal is indeed infected, and provides a reference for subsequent samples. A second sample is taken at the end of the appointment, and the reduction or elimination of the bacterial content is a measure of the efficacy of the mechanical and/or chemical/biochemical methods used. If a second session is scheduled, another sample is taken on re- entry, reflecting the effect of the temporary medication; and a final sample is again taken just before filling. These samples are usually labeled S1, S2, S3, and S4.

Methods of sampling vary with the clinical state of the tooth, but the general principle is to collect as much of the infecting flora as possible. Multiple paper points usually collect initial samples and samples of the instrumented canal. Various modifications may apply to, e.g., root-filled teeth and for sampling of dentin filings. A sterile file retrieving root-filling material or with dentin chips in its flutes may be cut off and collected in transport medium for growth [153, 275]. False positive samples can be avoided by strict adherence to asepsis during sampling, but false negatives remain a significant prob- lem for research as well as clinical applications: many organisms cannot be cultivated with current techniques, and biofilms may be located distant from the site of sampling.

The technique initially recorded growth/no growth of the microbes. Sensitivity increased with improved conditions for growth of, especially, anaerobic bacteria. For research purposes, sampling also included characterization of the genera and species that could be determined by cultural and biochemical means. Molecular methods for detection of genetic material from microbes have expanded the sensitivity even further. But the association of treatment outcome with bacteriological status rests so far with bacteria detected by culture, i.e., live microbes. Moreover, the association is qualitative. While there may be a relation- ship between the quantity of organisms in the root canal and treatment prognosis, the established, significant association is with cultivable bacteria at the time of filling [55, 131, 214, 246, 254]. Whether and to what extent genetic material from dead bacteria and/or non-cultivable organisms play a role in the pathogenesis of primary and post-treatment apical periodontitis remains to be established, but it is generally recognized that for maintenance of disease, live bacteria in sufficient numbers and organization are necessary [209].

The concept of root canal infection as a surrogate measure of treatment efficacy has been applied to ex vivo models: extracted teeth or root segments are infested with microbes, typically E. faecalis, and surviving organisms are monitored after various mechanical and chemical disinfection procedures. There is a long way from such ex vivo studies to proven clinical efficacy, but the methodology may have value in finding dif- ferences in efficacy among products and techniques and thereby provide a rationale for clinical studies with these variables.

11.4 Infection Control During Treatment

The basic principles and practices of asepsis outlined in Chapter 10 for vital, uninfected teeth fully apply also to infected teeth with apical periodontitis, as do the principles of instrumentation and root canal filling. Endodontic treatment of the infected root canal has the added purpose to maximize the possibility of completely disinfecting the root canal system and to maintain asepsis for the lifetime of the tooth.

11.4.1 Field Isolation and Disinfection

Practitioners with limited skills and insight repeatedly question the need for tooth isolation by rubber dam. Given the infectious nature of pulpitis and apical periodontitis, any means of reducing the microbial burden, including application of the rubber dam, are of course helpful. A research protocol omitting such procedures would be ethically inappropriate, and in comparisons of treatments done with or without rubber dam the rubber dam cases have shown better prognosis [116]. Furthermore, patients run a greater risk of aspirating or swallowing sharp objects or harmful chemicals applied during the procedure [2]. Judicious, often extensive, removal of existing restorations that may harbor bacteria and of residual carious dentin is obligatory. This may create a need for complex reassembly of structure to facilitate placement of the rubber dam (see Figure 11.3) Traditionally, chlorhexidine or iodine solutions have been used for field disinfection of the isolated tooth and its surroundings. Five percent iodine tincture alternating with full- strength hydrogen peroxide was shown by Moller to be effective [133]; and satisfactory disinfection by swabbing with 3% hydrogen peroxide followed by 2.5% NaOCl has also been documented [275].

Figure 11.3 Pre-endodontic build-up: (a) on admission; (b) after complete removal of caries with protection of the exposed pulp and hemorrhage control; and (c) bonded build-up in situ with rubber dam fixed with clamps on neighboring teeth to limit stress on the interim restoration. (Courtesy of Dr Nikola Petronijevic.)

11.4.2 Mechanical Instrumentation

Quantitatively, mechanical instrumentation by any means, handor machine-operated, stainless steel or nickel-titanium instruments, removes the vast majority of cultivable microbes from the infected root canal in vivo [26, 51, 153]. The greater the initial load of root canal infection, the greater the likeli- hood that more efforts are needed for optimal reduction or total elimination of the microbes [26, 275]. Moreover, even if the quantitative reduction is impressive, residual bacteria are usually detectable in all or almost all teeth subjected to mechanical instrumen- tation only [26]. Therefore, supplemental, antimicrobial efforts need to be applied for effective disinfection.

11.4.3 Antimicrobial Irrigation

Irrigation solutions are used for various pur- poses during instrumentation or preparation of root canals. Solubilization of soft/necrotic tissue, solubilization of dentin, lubrication/ wetting for ease of instrumentation, and opening of dentinal tubules are functions considered important for treatment. Keeping the primary treatment goal in mind, all of these functions serves the purpose of minimizing the microbial presence in the root canal.

Irrigating solutions range from sterile saline to strong disinfectants. Sodium hypochlorite, in concentrations ranging from 0.5 to 5.25%, remains the standard for root canal disinfection. It kills a broad spectrum of bacteria; it dissolves necrotic tissues; it disrupts and remove biofilms; and it contributes to the cleansing of the canal system [72]. Chlorhexidine in concentrations from 0.5 to 2% also has strong antibacterial activity and its substantivity to hard tissues gives it prolonged activity in dental tissues, but it is less effective on biofilms [67, 72]. Ethylene diamine tetra-acetate (EDTA) is used not for its antibacterial activity, which is is low, but for dissolving and eliminating the smear layer and dentin debris in the instrumented canal [251]. Old and new formulations are regularly proposed and tested for suitability in endodontics. Formaldehyde and glutaraldehyde may match the antibacterial effect of NaOCl, but their use has been abandoned due to their toxicity and carcinogenicity. Other compounds tested are less antibacterial than NaOCl and are at best adjuncts in the disinfection of the root canal.

11.4.4 Principles of Chemical Disinfection and Mechanisms of Action

Disinfection is the process of killing patho- genic organisms or rendering them inert. Most commonly, this is achieved by thermal or chemical means. There are general principles for the process of disinfection, principles that apply to root canal disinfection as well. Primarily, the disinfectant must be effective against the pathogenic organism or organisms. Apical periodontitis is a non-specific disease, so the disinfectant must reach a broad range of potential pathogens.

Second, the agent must penetrate relevant tissues and physically touch the microbes. The solution or suspension must have surface and chemical properties that allow penetration through vital and necrotic tissues, biofilms, and dentin structures.

Third, disinfection takes time. Time is required for the agent to reach the target organisms, to break down extracellular matrix that protect them, and for the direct destructive action of individual cells. Classical studies on the reference disinfectant, phenol, showed that full strength phenol required 30 minutes to kill Escherichia coli even in planktonic suspensions [82]. The time-to-kill issue may have bearing on the single-visit controversy (see below).

Fourth, disinfectants generally show increased activity against microorganisms at higher concentrations. However, tissue toxicity of the disinfectant may limit the con- centration that may be safely applied. The high relative toxicity of the most potent agents, the aldehydes, has made them poorly suitable for endodontic use [216]. Because there may be consumption of the active ingredient by adsorption to tissues and debris and by chemical reactions, adequate concentration is maintained as much by replenishment during use as by having an initially high, but potentially toxic, concentration of the agent.

Fifth, the disinfectant may be energized through physical/thermal means. Ultrasound activation is generally acknowledged to increase the antimicrobial effect of irrigants in the root canal [139], particularly as so-called passive ultrasonic irrigation (PUI). Instruments for and methods of applying ultrasound to endodontic irrigation fluids have been developed and extensively studied [71, 136, 253, 271, 273]; however, the effect may be marginal and have limited clinical significance [155], particularly as the most apical part may not be much affected [252].

11.4.5 Principles of Biomechanical Instrumentation

Biomechanical instrumentation in endodontics means the mechanical preparation of the root canal with due respect for the biological and anatomical features of the canal system. Mechanical instrumentation serves three purposes: it should shape the canal to accom- modate and fit a root filling; it should remove residual pulpal tissue, debris, and microbes from the canal lumen; and it should facilitate and support the antimicrobial action of the irrigating solution. The latter two purposes link directly to the biological purpose of eliminating pathogenic microbes as the over- all goal of treatment.

Standardization of instruments: Harmonizing instruments with root filling core materials has a long history in endodontics. Championed by Ingle in the 1950s [81] and formalized with the basic 2% taper in international standards, the concept was revitalized in the 1970s with supporting clinical data by Kerekes and Tronstad [91-94]. Many, but not all, root canals may be instrumented to standard apical sizes with elimination of pulpal tissue and infected debris in a circular preparation in the apical 3 to 5 mm region. The standardized technique also showed good clinical results, as good as or bet- ter than the step-back technique used traditionally [90, 94]. Two per cent taper is optimal for hand instrumentation: it limits extensive con- tact with canal walls coronally, thereby preventing fastening of the instrument and risk of fracture, while allowing effective dentin removal in the apical part.

Greater taper has become the norm with machine-operated instruments. One perceived drawback with the standardized, hand instrumentation was the danger of enlarging the apical part too much, creating ledges and causing perforations. Step-back hand instrumentation [257] reduced this risk, but produced a non-standardized apical preparation that did not correspond to prefabricated cores. Rotary and reciprocating instruments with greater or varying taper produce, at least in theory, a canal that can be filled completely with cores of corresponding sizes, thus combining the theoretical advantages of the two classical hand instrumentation techniques.

Mechanical efficacy: The tissue-removing ability of instruments is dependent on their size and ability to touch canal walls. This is traditionally tested by scanning electron microscopy [29, 77, 135, 159, 168, 190, 200, 248], more recently also by micro-computed tomography (micro-CT) [17, 22, 35, 49, 157, 158, 162-164, 247, 279]. While results vary and some instrumentation systems may perform better than others, it is a universal finding that instruments do not touch a significant part of the canal surface area (some 30 to 50%). Therefore, mechanical instrumentation alone cannot ensure disinfection during treatment of apical periodontitis. In bacteriologically controlled clinical studies, most teeth remain infected after instrumentation in the absence of a disinfectant [26, 153].

Facilitation ofdisinfection: Instrumentation supports the activity of disinfectants by facilitating contact of the agents with biofilms, and in the case of sonic and ultrasonic instrumentation by energizing the antibacterial agent. Intentional dentin erosion by ultrasound is currently not a preferred method, and energizing the disinfectant by ultrasound is mostly done by passive ultrasonic activation, where the instrument is not supposed to act on the dentin walls. Any handor machine-driven instrumentation technique will drive the accompanying irrigation liquid towards the dentin walls and the apical area. As more debris and infected material are removed by mechanical action, the distance between the disinfectant and residual microbes becomes shorter and the volume of cells to be killed becomes smaller. These con- siderations formed the basis for maintaining that the apical preparation should be made as large as possible (without compromising the integrity of the root end). This would more likely eliminate bacteria in a greater part of the apical delta and facilitate diffusion of the irrigation liquid into spaces inaccessible to the instrument [153], possibly improving the incidence and rate of healing [127]. The counter-argument is that extensive instrumentation weakens the root and renders it more prone to cracks that may lead to fracture, and carries a high risk of perforation.

11.4.6 Intracanal Medication

An extraordinary range of substances has been placed in the root canal with the pur- pose of curing apical periodontitis: creosote, formaldehyde, glutaraldehyde, iodine formulations, chloramine, cresols, even radioactive substances [137] are among them. Concerns of toxicity and carcinogenicity and simple lack of demonstrable effectiveness have eliminated most of these.

The infectious nature of apical periodontitis was unequivocally demonstrated in the 1960s and early 1970s [85, 225, 235, 236], but a concept of medication as a means of supporting the host's healing ability remained. The pioneering work of Cvek [43-48] indicated that a thick slurry of calcium hydroxide was effective on both fronts: he demonstrated a reduction or elimination of bacterial growth in teeth thus treated, and after long periods of dressing (months to more than a year), healing was seen with formation of a hard tissue barrier (apexification) in immature teeth.

Buffered formaldehyde and camphorated chloro-phenol compounds were routinely used as dressing at that time, based in part on their favorable antibacterial-to-tissue toxicity ratio [216]. Iodine potassium iodide is more effective than chloro-phenols, but its action is short-lived and it was used for short intervals of 2 to 4 days only. When comparative tests favored calcium hydroxide as medication over both camphorated chloro-phenol [24] and iodine potassium iodide [189], Ca(OH)became established as the medicament of choice for inter-appointment dressing.

Figure 11.4 Microbial control during endodontic treatment. Percentage of cases positive for bacteria. SI, sample on entry; S2, after instrumentation and irrigation in first session; S3, after an inter-appointment dressing with calcium hydroxide. Data from [123, 153, 161, 201, 212, 275]. Zandi and Rodriguez data are with molecular techniques and from retreatment cases.

From the onset, calcium hydroxide was supposed to support the healing processes as much as being an effective disinfectant. It was applied for months while watching for healing by sinus tract closure, elimination of pain or discomfort, and radiographic signs of lesion reduction. As healing is difficult to assess before three months [95], this became the standard duration for this treatment modality. However, focus shifted to an emphasis on the antimicrobial effects. Using microbiological monitoring, the time for calcium hydroxide to complete its antibacterial effect came down from many months [47] to 4 weeks [24] and, finally, 7 days [212]. When applied for 10 minutes at the end of the first session no benefit over irrigation was observed [212].

11.4.7 Infection Control in One and Multiple Visits

The rationale for a dressing period is based on first, the surrogate measure of root canal infection as a predictor of success, and sec- ond, the improved incidence of healing when canals are bacteria-free at the time of filling. Several subsequent studies have used a similar methodology to optimize the disinfection process [1, 31, 37, 51, 84, 123, 124, 126, 140, 155, 161, 173, 181, 184, 185, 201, 206, 254, 258, 264, 270] (Figure 11.4), but it has proven difficult to establish a protocol that will predictably eliminate bacteria from the root canal. Rather, the importance of an added antibacterial effect of calcium hydrox- ide dressing has been questioned [107, 161]. If a dressing does not improve infection control to a clinically significant level, then an interim period with a temporary filling carries a risk of new contamination that may outweigh any small benefit [275].

Treatment of apical periodontitis in one appointment saves time and cost, and many well-designed, randomized clinical experiments have been done comparing the short and long-term clinical outcomes after treatments in one or two visits. These include symptomatic as well as asymptomatic cases. As would be expected, there are differences in results among individual clinical experiments, some favoring two-visit [186, 241], others one-visit treatments [61], and many finding no difference of importance [21, 64, 262, 266, 268]. It seems safe to draw the conclusion that in terms of subjective (pain experience, anal- gesic usage) and conventional objective (radiographic) signs of healing after treatment, there is virtually no difference between the two treatment modalities. This has been repeatedly documented for symptomatic as well as asymptomatic teeth in almost all of several systematic reviews [59, 193, 194, 197, 221, 267].

While clinical and conventional radiographic criteria are the practical yardstick for success, the histopathological appearance and CBCT may be more sensitive in detecting differences. Data from animals [76, 88] and humans [245] certainly suggests that fewer bacteria may be seen and healing appears better after a dressing with calcium hydroxide, and a randomized study using CBCT for assessment showed a somewhat greater lesion volume reduction for teeth treated in two sessions [52].

It is in the interest of all parties to limit the time and effort necessary for treatment success. There is no arguing against prolonged disinfection, by any means, for the purpose of maximal reduction of canal bacteria. But achieving complete asepsis may require sev- eral visits or longer intervals [24], and the protocol to confirm it is complicated [138]. Given the results of systematic reviews and meta-analyses, it is becoming increasingly difficult to maintain two or more visits as the standard protocol for treatment leading to success by conventional clinical-radiographic criteria. On the other hand, a dressing with calcium hydroxide remains the effective procedure when dictated by time constraints or particular clinical considerations.

11.4.8 Special Techniques

Alternative approaches to mechanical instrumentation with rigid files are sought and developed (see Chapter 10). A “non-instrument” cleaning of root canals was developed where an airtight seal was established over the tooth after access to the pulp, whereby a vacuum or reduced and variable pressure could be established and activate irrigating solutions [119, 120]. Recent developments of this con- cept has been tested ex vivo and clinically in cases of chronic apical periodontitis with favorable results in terms of both bacterial reduction and clinical-radiographic outcome [32, 73, 132, 202, 203, 261, 265], but rand- omized comparisons with other techniques in clinical studies are as yet lacking.

11.5 Root Filling Phase

11.5.1 Purpose and Functions of the Root Filling

The primary function of the root filling is the prevention of new infection of the root canal system. There are two ways of fulfilling this task: either to prevent microbes from entering or to kill them when they try to penetrate the barrier created by the material. The first depends on the physical integrity and adap- tation to tooth substance by the materials; the second requires some form of biological activity, which in turn is dependent on a degree of solubility of the material. Any anti- microbial properties of the materials may also support disinfection of the root surface and canal system by the biomechanical procedures [149].

In mature teeth with closed apices, the area of contact of the filling material with soft tissue is so small that biocompatibility is of limited importance. However, in other clinical situations, e.g., pulp exposures, perforations, and surgical approaches, the area of contact between the material and soft tissue is much larger. Here the material must possess properties that permit, at best promote, healing and regeneration of the relevant tissues [60], in addition to preventing bacterial activity.

There are thus three properties necessary for the clinical performance of endodontic materials: sealing ability, antimicrobial activity, and biocompatibility. In addition, the materials must have properties that allow their proper placement and which ensures durability [70].

These properties of the root filling reside mainly with the sealer cement, and there is a large body of literature characterizing the chemistry and physical properties of the various types of sealer. Some comparative clinical studies have compared the long-term performance of sealers, and almost all have shown very small if any differences [56, 78, 150, 152, 255]. However, the finding of long- term failure of some new products [14, 263] is a reminder of the need for vigilance in the process of accepting and promoting new methods and materials.

Table 11.1 Sealer types, examples and clinical tests.

Material

Subgroup

Examples

Clinical testing on cases with apical periodontitis

ZnO-eugenol

 

ProcoSol, Roth 811

Eriksen et al., 1988 [56] Trope et al., 1999 [241]

Resins

epoxy

AH26, AH plus

Conner et al., 2007 [14]

 

methacrylate

EndoRez, RealSeal

Barborka et al., 2017 [14]

Ca(OH)2

 

Apexit, Sealapex

Waltimo et al., 2001 [255]

Silicone

 

RoekoSeal, GuttaFlow

Huumonen et al., 2003 [78]

Ceramic

Ca-Si

BioRoot

 
 

Ca-Si-P

Endosequence

 

Gutta-percha

beta

generic

 
 

alpha

GuttaFusion, GuttaCore, Herofill

 

Resin cores

complete

Resilon

Barborka et al., 2017 [14]

 

resin coated

EndoRez, Endosequence

 

Core materials are developed and designed to be inert and stable, yet must possess some plasticity to adapt to the instrumented canal. They act as pistons for the sealer, which determines the functional properties of the root filling. Table 11.1 lists some sealer material types used for root filling.

11.5.2 Antibacterial Properties

Antibacterial properties are usually associated with some degree of tissue toxicity [217, 218], and there is limited incentive to develop sealer materials with strong antibacterial activity. However, most products are anti- bacterial before setting; many have remaining activity towards bacteria also after setting and into dentinal tubules [39, 86, 167, 169]; and additions of antibacterial components to sealers are commonplace [11, 16, 38, 187].

Bioceramic sealers create a strong alkaline environment locally, which acts as a longlasting inhibitor of microbial growth [4, 260].

11.5.3 Biological Properties

Root-filling sealers need to be tolerated by the tissues, and even if they may be strongly cytotoxic before setting, most are quite bland when set [117, 146, 147, 280]. Bioceramic sealers have properties that may stimulate repair [34, 269]; this may be valuable in situ- ations where there is a large contact area with the periodontal tissues; e.g., root-end fillings and perforation repair [149].

11.5.4 Basic Clinical Guidelines for Root Filling

Apical level. While the apical extent of the root filling has little bearing on the the prog- nosis in pulpectomy cases [213], it is clear from follow-up studies that it is of crucial importance for treatment of apical periodontitis [142, 145, 213] (Figure 11.5). Radiologically, this means that the root filling should end 0 to 2 mm short of the radiographic apex in infected cases, probably reflecting that the bulk of the infection is coronal to the apical constriction.

Figure 11.5 Critical influence of root-filling-to-apex distance on outcome of treatment of apical periodontitis. Root fillings ending within 2 mm from the radiographic apex may give results comparable to root fillings of vital pulps. Too short fillings or overfills strongly reduces the success rate. Modified from [213].

Width of apical preparation. One treatment philosophy argues for extensive apical instrumentation to eliminate as much as pos- sible of infected dentin [31, 94]; other investigators stress the importance of selecting the final file size after gauging the actual diameter of the apical root canal [143, 257] and avoid the risk of ledging and transportation (Figure 11.6). There is little data on the clinical importance of apical preparation size. Large case series show successful results with the extensive approach [94, 152], others have reported good, maybe better, outcome with the step-back, apical conservative technique [143]. Irrespective of method chosen, clinicians should be aware that infected teeth with lesions generally have larger apical foramen diameters than teeth with vital pulps [62].

Patency. The concept of patency is contro- versial [79]. Maintaining apical patency by repeatedly securing access beyond the apical constriction with very small files would seem to favor access of the irrigating solution to the most apical areas. It may also prevent accumulation of infected debris and be beneficial in producing a good apical seal of the filling. There are data to suggest that patency may have a positive effect on prognosis [108, 128, 142, 230]. On the other hand, the procedure carries the risk of transporting infected material into the periapical area with potentially negative consequences for development of exacerbation and for the long-term prognosis [80, 121, 128]. It seems safe to conclude that if maintenance of patency is desirable in a given case, then very small files (ISO 008 or 010) should be used, and mainly in cases where the apical constriction is sufficiently large to allow unimpeded passage of the patency file [79].

Figure 11.6 Apical box and tapered preparations. (a) the apical box philosophy emphasizes wide instrumentation for maximal removal of infected dentin and access of disinfectant; (b) a tapered preparation is designed for better conformity with the original canal shape.

Compaction of filling. The quality of the root filling as judged by radiographic homo- geneity appears to affect the prognosis after root filling of teeth with apical periodontitis [94], but results vary among studies and may be of particular significance for retreatment procedures [142, 213].

Coronal seal. The long-term success of treatment also depends on the quality of the coronal restoration [101, 103, 104, 171, 215, 237]. More specifically, a tight coronal seal extending below the marginal bone level is associated with less apical periodontitis [219]. This may also prevent bacteria from entering the filled root canal from lateral canals and dentinal tubules exposed to a periodontal pocket.

11.6 Clinical Issues During Diagnosis and Treatment of Primary Apical Periodontitis

Successful disinfection and effective filling of an infected pulp canal ensures success to the level achieved for vital pulp extirpations [94, 152, 213]. However, there are situations where the disinfection becomes especially difficult or impossible, and the infection may pose special problems not encountered in pulpectomies.

11.6.1 Fracture

If a fracture or crack involving the pulp and extending to or from the gingival margin is present, one cannot predictably treat the root canal. Disinfection and filling procedures may mask symptoms for a time, but eventually the crack will allow ingress of microbes to renew the infection and inflammation.

11.6.2 Marginal Periodontitis

The so-called endo-perio-problem (Figure 11.7) has been extensively discussed and reviewed [3, 196, 277]. The pulp status governs the endodontic treatment options: pulpal sensitivity strongly suggests that there is no apical periodontitis as part of the diagnostic issue. However, if a periodontal pocket is established with communication to the apical area and the root filling, the continuous seeding of bacteria from the plaque in the pocket will maintain the apical inflammation. While there are case reports of pulpal infection from a periodontal pocket [276], these situations are rare and usually do not interfere with successful endodontic treatment of the ensuing apical periodontitis. However, secondary infection from deep periodontal pockets may contribute to post-treatment apical periodontitis in some cases [188].

Figure 11.7 Histological analysis of tissues generated in the root canal of a human tooth after revitalization treatment. Detail from the middle third portion of the canal. The canal is partly occupied by a newly formed calcified tissue (between arrows; original magnification 950). Reproduced with permission from [115].

11.6.3 Exacerbations

Subjective symptoms, if present, normally subside quickly after instrumentation with disinfection, but the procedures carry some risk of initiating an acute exacerbation of the infection. The environmental change in the root canal may favor growth of more virulent organisms, and instrumentation beyond the apex may transport microbes to that area, giving rise to a transient, but some- times painful, even dramatic, clinical condition. Treatment procedures follow general recommendations for acute oral infections. Significant inter-appointment pain hardly occurs in extirpation procedures [9], testifying to infection, not physical effects of over instrumentation or chemical irritation by irrigation solutions, as the sole cause of socalled flare-ups [208, 210].

11.6.4 Anatomical Variations and Anomalies

Isthmuses between root canals in the same root is a source of persistence of infected material [98], and particular efforts are made to clean and disinfect these areas [5, 54, 205]. Simple forms of dens invaginatus can be fairly easily treated by conventional methods (Figure 11.8), while more complex types require combinations of surgical and operative adjunctive treatments.

Figure 11.8 Apical periodontitis in a tooth with dens invaginatus successfully treated despite highly irregular apical anatomy: (a) on admission; (b) immediate postoperative; (c) 5 months follow-up. (Courtesy of Dr Line Hardersen.)

11.6.5 Influence of Systemic Disease

Patients with certain chronic diseases may have a healing pattern that varies from the healthy individual. They may also be more prone to exacerbations. Diabetes is the typical case. Apical periodontitis seems to occur with higher frequency in diabetic patients and healing after treatment may be slower [75, 113, 192, 199, 228].

11.7 Treatment of Persistent or Recurrent Apical Periodontitis

All treatment procedures in retreatment cases have the same aim as for treatment of primary apical periodontitis, but the presence of a root filling complicates the issue. The root filling itself, sealer or core material, may be the substrate for biofilm formation, which typically occurs in the interface between filling material and the dentin wall [178]. Residual bacteria in association with remnants of root filling material are difficult to reach and eliminate, which makes it crucial that one removes all root filling material.

11.7.1 Instrumentation and Removal of Filling Material

Any of several types of rotary or hand instruments are used for removal of the root filling. The Hedstrom file is useful for manual approaches, and many commercial file systems include rotary or reciprocating instruments designed for root filling removal [30, 83, 96, 125, 204, 272].

Chemical solvents may assist in the removal of gutta-percha. Chloroform is highly effective, but toxic and potentially carcinogenic [229]. Xylene is quite effective, but also toxic. Limonene or orange oil is a substance derived from citrus fruits and is available commercially [170, 242, 243], which while less effective than chloroform or xylene is also less volatile and toxic.

Given the range of compounds and of the chemistry that go into the sealers setting around the gutta-percha cone, there is no general solvent for these materials. Essentially, one must rely on the mechanical procedures for maximal removal of the sealer, and by current methodologies, complete removal may be virtually impossible [10].

11.7.2 Biofilms in Recurrent Apical Periodontitis

The bacteria associated with filling material and the difficulty in physically eliminating the residual material offer one explanation for the reduced prognosis of retreatments [19, 141, 144, 145]. It also helps explain the experience that retreatment of root fillings of poor quality has a better prognosis [53], as material removal is easier and the infecting organisms reside in the previously unfilled regions of the pulp canal space.

It has been proposed that the microbial composition in persistent lesions differs from that of primary apical periodontitis (see Chapter 4). While there may be more facultative organisms, particularly streptococci and enterococci, attempts to augment disinfection procedures in retreatments by targeting this group of organisms [8, 129, 211, 227] have so far not resulted in proven clinical protocols.

Bacteria may establish a foothold in the tissues beyond the apical foramen and outside the reach of medicaments in extraradicular infections [68, 165, 174, 175, 180, 183, 224, 238, 240]. Attempts to treat such cases with antibiotics [15] have not been shown to reli- ably lead to healing, and root-end surgery is routinely applied [180].

11.8 Treatment of Immature Permanent Teeth with Apical Periodontitis

Apical periodontitis in immature teeth with open apices could be successfully treated with conventional methods, and formed part of the basis for calcium hydroxide dressing as a modality [48]. However, teeth thus treated have thin walls and may be prone to fracture. The risk of a new trauma is increased in children with a previous history of oral trauma [166]. Therefore, the finding of hard tissue formation in immature teeth with apical periodontitis treated with an antibiotic paste (Figure 11.9) opened for an improvement of the fracture resistance of such teeth [13] and stimulated extensive research in regenerative endodontics [99] (See Chapter 8).

11.8.1 Definition of Terms

As with other regenerative or tissue engineering methods in dentistry, procedures are not yet standardized to a point where the outcome is predictable. Moreover, there are many biological processes involved that are poorly understood [99], and the clinical aim of treatment is often only vaguely formulated. The cells involved and mechanisms for their activation are described in Chapter 8. In cases of apical periodontitis in immature teeth, the primary goal is to stimulate com- pletion of apexogenesis [66] for root length and strength. Other methods also predictably achieve this: a dressing with calcium hydroxide was the standard procedure, today, bioceramic materials applied in a single visit give similar results [41].

Figure 11.9 A gutta-percha point is inserted in a periodontal pocket buccal to a lower left second molar (a) with diffuse radiolucency (shown in b). The point traces to the apex of the distal root (c). The lesion has healed 12 months after endodontic treatment (d). (Courtesy of Dr Dyveke Knudsen.)

Figure 11.10 Preoperative radiograph of tooth #35 with open apex (a), and 12-month postoperative radiograph (b). Continuous root extension and canal wall thickening, relative to the preoperative image. Reproduced with permission from [110].

Revascularization. This is the simplest and least biologically demanding process applied to pulpless teeth. First attempted in 1961 [154], it aims basically to achieve ingrowth of a vascularized tissue into the vacated pulp space, supporting and promoting apexification or apexogenesis [6]. Revascularization is the first step towards repair or regeneration.

True regeneration may be said to occur when vital pulp tissue with functioning odontoblasts is established in the canal. This is still experimental at best, and the practical aim in clinical practice is biological repair by induction of some form of hard tissue in the root canal [115]. If integrated with the preexisting dentin, the root may be strengthened by the osteoid tissue, but new dentin integrated with pre-existing dentin is currently not achieved (see Figure 11.10).

11.8.2 Case Selection

The first cases with impressive hard tissue formation were in premolars with dens evaginatus and no history of trauma [13]. Teeth with apical periodontitis caused by caries affecting the convolutions in the occlusal enamel may respond better to regenerative treatments than traumatized teeth [114], which often have concomitant damage to other dental tissues. If apexification (Figure 11.11) or apexog- enesis is the main clinical purpose of treatment, then classical protocols with applications of calcium hydroxide or bioceramic materials followed by conventional filling of the coronal pulp space have predictable results [41].

Figure 11.11 Apexification. The infected canal (a) was cleansed with light instrumentation and copious irrigation, followed by a dressing of calcium hydroxide. Four mm of MTA was packed in the apical part and the tooth root-filled with a resin restoration coronally. The 15-year follow-up radiograph shows dental hard tissue formation with generation of a complete periodontal ligament (b). (Reproduced with permission from [172].)

11.9 Monitoring Healing, Prognostication

Chapter 6 gives an overview of prognostic factors and the fate of endodontically treated teeth. Many follow-up studies do not separate the outcomes for initially healthy and diseased periapices. While one may assume that most factors are common for all diagnoses, it is known that some are exclusively linked with the presence of a preoperative lesion, e.g., the apical end-point of the filling [213].

There are essentially three aspects of healing apical periodontitis: relief from pain, resolution of clinical signs (swelling, sinus tract), and radiographic assessment of replacement of the lesion's soft tissue with bone. In addition, tooth survival is often used for recording the clinical success of treatment.

11.9.1 Signs and Symptoms

Signs and symptoms, if present at start, are generally eliminated or reduced by most treatment protocols, but may persist or develop in up to some 10% of cases [143]. While some changes in the nervous system due to the inflammation and/or manual severance of pulpal nerves may account for some altered sensitivity, some of these minimally or symptomatic teeth probably harbor a residual infection that causes inflammation.

11.9.2 Radiographic Monitoring

Most studies of treatment outcome in teeth with apical periodontitis focusses specifically on radiographic healing. Classical success/ failure-analysis utilizes a comparison of preoperative, periapical radiographs with radiographs taken at control, but stresses the importance of a re-established lamina dura and normal periodontal structures [143]. These so-called Strindberg criteria [220], with modifications, have been and are widely used. Interpretation of radiographs is very prone to personal bias, and the success/failure approach is no exception [65]. The periapical index (PAI) [151] is an attempt to reduce bias by relating experimental, test images to histologically verified references [23] and has been used extensively in recent years.

Irrespective of the scoring system used, it is essential that observers are trained and calibrated for results to be comparable over time and geographical area. Such calibrations should be standardized and related to a common set of images with recognized status of healthy or diseased. Few studies document global calibration and caution must be exercised in comparing results from one study with others.

11.9.3 Cone Beam Computer Tomography

CBCT offers a more sensitive and probably more specific detection of apical disease (see Chapter 5). Comparison of PAI scores with the lesion volume show a clear and signifi- cant correlation, but the same study high- lights the limitations of a 2-dimensional radiograph compared to the 3-dimensional CBCT images [122]. Indices have been proposed also for CBCT recording of apical lesions [57, 233], which may become important for comparisons of results across studies.

Follow-up data on treatment suggest that many teeth considered healed by conventional radiography may show residual lesions and other factors may prove relevant for prognostication when monitored with CBCT [111]. The associations of prognostic factors observed by conventional radiographs remain, but they cannot automatically be transferred to similar studies with CBCT. Moreover, some lesions detected by CBCT may reflect fibrous, not infected and inflamed tissue, at least in surgical cases [106].

11.9.4 Tooth Survival

Tooth survival is a crude measure of follow- up of endodontically treated teeth. It is frequently used in studies comparing endodontic treatment with other treatment options, particularly implants [33, 234]. Tooth survival after endodontic treatment is, however, not only a reflection of the endodontic treatment itself. It is as much, or more, dependent on the periodontal conditions and on the degree of loss of coronal tooth substance [12]. Assessing the survivability of teeth is of course essential for treatment planning, especially for complex clinical cases, but treatment of apical periodontitis is generally so successful that it rarely is a decisive factor in such considerations.

11.10 Concluding Remarks

11.10.1 Principles of Case Selection and Treatment Decisions

Prevention is better than cure. This is true for endodontology as for other diseases. Successful treatment of many teeth with large lesions [143] does not alter the fact that in all prospective outcome studies the prognosis is significantly better for root filling after vital pulp extirpation [36]. Moreover, the larger the lesion, the poorer the prognosis [58, 102]. A traditional, wait-and-see approach to endodontic treatment runs the strong risk of reducing the prognosis when treatment eventually is initiated. Moreover, investigations with CBCT detect approximately twice as many lesions as periapical radiographs [111], magnifying the difference in prognosis between teeth with and without preoperative apical periodontitis.

Furthermore, the generally poorer prognosis of retreatments [213, 232] points to the need for high-quality endodontic treatment of pulpitis and primary apical periodontitis. As dental students [94, 152] as well as specialists [143] routinely obtain more than 90% success by conventional radiography for teeth without a lesion, early intervention with optimal aseptic, disinfection, and obturation techniques is the key to a general improvement in periapical health.

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