The science and art of endodontics has come a long way since the early days. A brief review of the history of endodontics is helpful in understanding its influence on current practice. The scope of modern endodontics is now wider and encompasses a variety of procedures. Patients are no longer willing to accept tooth loss and have higher treatment and care expectations. Microorganisms have an essential role in the pathogenesis of pulpal and periapical diseases. The host defence response against root canal infection includes numerous inflammatory mediators and a range of cells. Continuing research is increasing our knowledge of the root canal microbiota, which will hopefully result in dedicated strategies to manage the different types of root canal infection. Advances in endodontics are continuing and many recent developments have been successfully translated into everyday clinical practice.

Endodontology is the branch of dental sciences concerned with the form, function, health of, injuries to and the diseases of the dental pulp and periradicular region, and their relationship with systemic well-being and health. Endodontic treatment can be defined as the prevention or treatment of apical periodontitis, the principal disease. The concept of treating the pulp of the tooth to preserve the tooth itself is a relatively modern development in the history of dentistry and it may be useful to review, very briefly, the history of pulp treatment in order to appreciate better modern views on endodontic treatment. Toothache has been a scourge of mankind from the earliest times. Both the Chinese and the Egyptians left records describing caries and alveolar abscesses. The Chinese believed that these abscesses were caused by a white worm with a black head which lived within the tooth. The ‘worm theory’ was current until the middle of the eighteenth century when doubts were raised, ¹ but they could not be expressed forcibly because those in authority still believed in the worm theory. ² The Chinese treatment for an abscessed tooth was aimed at killing the worm with a preparation that contained arsenic. The use of this drug was taught in most dental schools as recently as the 1950s in spite of the realization that it was self-limiting and that extensive tissue destruction occurred if even minute amounts of the drug leaked into the soft tissues. Pulp treatment during Greek and Roman times was aimed at destroying the pulp by cauterization with a hot needle or boiling oil, or with a preparation containing opium and hyoscyamus. About the end of the first century AD, it was realized that pain could be relieved by drilling into the pulp chamber to obtain drainage. In spite of modern antibiotics, there is still no better method of relieving the pain of an abscessed tooth than drainage.

Endodontic knowledge remained static until the sixteenth century when pulpal anatomy was described. Until the latter part of the nineteenth century, root canal therapy consisted of alleviating pulpal pain and the main function of the opened root canal was to provide retention for a dowel crown. ^3,4 At the same time, bridgework became popular and many dental schools taught that no tooth should be used as an abutment unless it was first devitalized. ⁵ Root canal therapy became commonplace partly for these reasons and also because the discovery of cocaine led to painless pulp extirpation. The injection of 4% cocaine as a mandibular nerve block was first reported in 1884; ⁴ and 20 years later, the first synthetic local anaesthetic, procaine was produced. Around this time, reports of endodontic surgery appeared. ⁶ The first radiograph of teeth was taken in 1896, ^3,7 shortly after the discovery of X-rays by Roentgen in 1895. This further popularized root canal therapy and gave it some respectability. About the same time dental manufacturers began to produce special instruments, which were used primarily to remove pulp tissue or clean debris from the canal. There was no concept of filling the root canals since the object of the procedure was to provide retention for a post crown.

By 1910 ‘root canal therapy’ had reached its zenith and no self-respecting dentist would extract a tooth. Every root stump was retained and a crown constructed. Sinus tracts often appeared and were treated by various ineffective methods for many years. The connection between the sinus tract and the pulpless tooth was known but not acted upon. In 1911, William Hunter^8,9 attacked ‘American dentistry’ and blamed bridgework for several diseases of unknown aetiology. He reported recovery from these conditions in a few patients following extraction of their teeth. It is interesting to note that he did not condemn root canal therapy itself but rather the ill-fitting bridgework and the sepsis that surrounded it. About this time microbiology became established and the findings of microbiologists added fuel to the fire of Hunter’s condemnations. Radiography, which at first helped the dentist, now provided irrefutable evidence of apical periodontitis surrounding the roots of pulpless teeth.

Whilst the theory of ‘focal infection’ was not enunciated by Billings¹⁰ until 1918, Hunter’s condemnations started a reaction to root canal therapy, and there began the wholesale removal of both non-vital and perfectly healthy teeth. The blame for obscure diseases was placed on the dentition, and as dentists could not refute this theory, countless mouths were mutilated. Naturally, not all dentists accepted this wholesale dental destruction. Some, particularly in continental Europe, continued to save teeth in spite of the focal sepsis theory. It is difficult to know why dentists in continental Europe disregarded this theory and one explanation may be that their patients equated the loss of teeth with a loss of virility, and therefore, did not allow their dentists to mutilate their dentitions. Alternatively, it could be that these practitioners were not so readily swayed by fashion as were their British colleagues.

The re-emergence of endodontics as a respectable branch of dental science began in the 1930s. ^11,12 The occurrence and degree of bacteraemia during tooth extraction was shown to depend on the severity of periodontal disease and the amount of tissue damage at operation. The incongruity between microbiological findings in the treatment of chronic oral infection and the histological picture was demonstrated. When the gingival sulcus was disinfected by cauterization before extraction, microorganisms could not be demonstrated in the bloodstream immediately postoperatively.

Gradually, the concept that a ‘dead’ tooth, one without a pulp, was not necessarily infected began to be accepted. Further, it was realized that the function and usefulness of the tooth depended on the integrity of the periodontal tissues and not on the vitality of the pulp. ¹³ Another important advance was clarification of the ‘hollow tube’ theory¹⁴ by research using sterile polyethylene tube implants in rats. ^15,16 The tissue surrounding the lumina of clean, disinfected tubes, which were closed at one end, was relatively free of inflammation and displayed a normal capacity for repair. When such tubes were filled with muscle, the inflammatory reaction was only severe around the openings of the tubes containing tissue contaminated with Gram-negative cocci. These findings place stress on the microbial contents of the tube; if the tube contains microorganisms then the potential for repair is far less favourable than when the lumen of the tube is clean and sterile. ¹⁷ This infected situation is likely to be found in most root canals requiring treatment. The concept that ‘apical seal’ was important led to the search for filling and sealing materials that were stable, non-irritant and provided a perfect seal at the apical foramen. With the more recent realization of the importance of coronal leakage^18,19 and the biodegradation of root canal fillings, total filling of the root canal space, including lateral and accessory canals, has assumed much greater importance.

Until relatively recently, practitioners were preoccupied with a mechanistic approach to root canal treatment and to the perceived effects of various potent drugs on the microorganisms within the root canal rather than a total antimicrobial approach of effective cleaning, adequate shaping and complete filling of the root canal space. ²⁰ This preoccupation diverted attention from the effects of such drugs on the periapical tissues. Medicaments that kill microbes may also be toxic to living tissue. ²¹ The consequences of such materials passing out of the tooth into the surrounding vital tissues can be localized tissue necrosis. These avoidable problems cause distress to patients and can lead to litigation. Effective elimination of microorganisms from root canal system is best achieved by instrumentation combined with irrigation.

The extent of the subject has altered considerably in the last 50 years. Formerly, endodontic treatment confined itself to root canal filling techniques by conventional methods; even endodontic surgery, which is an extension of these methods, was considered to be in the field of oral surgery. Modern endodontics has a much wider field^22,23 and includes the following:

The Chinese belief that dental abscesses were caused by small organisms, worms, persisted until the eighteenth century. At the end of the nineteenth century Miller²⁴ demonstrated the role of bacteria in root canal infection, and noted that different microorganisms were found in the root canal compared with the open pulp chamber. Shortly afterwards, systematic culturing of root canals was undertaken. ²⁵ Unfortunately, these methods, which were potentially so valuable for improving the outcome of root canal treatment, were used to condemn much of the dentistry carried out at the time. ⁹ During the 1930s, microbiological techniques were used to re-establish the scientific basis of root canal treatment. However, techniques at that time only readily identified aerobic bacteria, and led to confusing results in later clinical studies. ^26,27 This resulted in clinicians being complacent about the role of microorganisms, and performing treatment simply as a technical exercise.

The development of anaerobic culturing allowed the identification of many previously unknown microorganisms present in root canals. ²⁸ This led rapidly to the demonstration that the majority of these microorganisms were anaerobes, ^29,30 and the realization that canals previously considered sterile contained anaerobes alone. Furthermore, when traumatized teeth were examined, there was a close correlation between the presence of anaerobic bacteria in the root canal and a periapical radiolucency; ³⁰ in the absence of infection, the necrotic pulp and stagnant tissue fluids cannot induce or perpetuate a periapical lesion. This was later demonstrated experimentally in teeth where the pulp tissue had been removed; only in those where the pulp was infected did periapical inflammation occur. ³¹ Although anaerobic culturing of root canals is not a technique applicable to everyday clinical practice, the results of research have provided rational explanations for pulp and periapical diseases and its treatment. ³² Microorganisms, which previously could not be cultured, and so were mistakenly considered absent, can now be identified. Over 50% of the oral microbiota is still uncultivable. ^33,34 However, the use of molecular biology techniques has enabled the identification of microbes that would be undetectable by conventional culturing techniques.^{33.35. and 36.} As knowledge in this area expands, our understanding of the root canal microbiota is changing. The presence of specific or combination of microorganisms and their implications are yet to be fully appreciated. ³⁷

Most root canal infections contain a mixture of microorganisms with bacteria being the main candidate pathogen.^{30.38. and 39.} It has also been shown that the relative proportions of different microorganism are determined by the local environmental conditions. ³⁹ Endodontic pathogens do not occur at random but are found in specific combinations. ⁴⁰ If a selection of microorganisms is inoculated into root canals in fixed proportions, their relative numbers will change over time, with a decline in aerobes and an increase in anaerobes. ³⁹ It has also been established that combinations of microorganisms are more likely to survive than inocula of single species. ³⁸ It is clear that one species can produce substances that others can metabolize in order to survive, forming complex ecological and nutritional relationships. ³²

Microorganisms are normally confined to the root canal system in pulpless teeth and exist in two forms:

The intraradicular infection may be divided into three categories:

It is unusual for microorganisms to be present in periapical lesions; the host defences prevent them from invading the periapical tissues. ⁴¹ However, in certain circumstances and with some species, microorganisms may establish an extraradicular infection, which can be dependent or independent of the intraradicular infection; thankfully, the incidence of independent extraradicular infection in untreated teeth is low. ⁴²

The role of infection in the demise of damaged pulps was demonstrated in a classic study by Kakehashi and co-workers in the 1960s, ⁴³ and eventually led to a biological approach to operative dentistry. ⁴⁴ The presence of microorganisms, their byproducts or damaged tissue in the root canal can cause apical periodontitis, typically at the apical foramen but also around the foramina of any lateral or accessory canals or at a fracture. The periapical inflammation prevents the spread of infection from the tooth into the alveolar bone, otherwise osteomyelitis would occur. The inflammatory lesion contains inflammatory mediators and numerous inflammatory cells, e.g. polymorphonuclear leucocytes, macrophages, B- and T-lymphocytes and plasma cells. ⁴⁵ The interaction between these cells and the antigenic substances from the root canal results in the release of a large number of inflammatory mediators. The inflammatory mediators include neuropeptides, the complement system, lysozymes and metabolites of arachidonic acid. ⁴⁶ Prostaglandins, leucotrienes and cytokines play an important role in the development of periapical lesions. ^45,46

As long as antigens emerge from the canal foramina, there will be a continuing inflammatory response, mediated in a number of different ways. This is a very dynamic response to rapidly multiplying microorganisms in the root canal, and may not be readily apparent to the clinician observing a radiograph or a histologist examining a slide of fixed cells. Endodontic treatment is primarily directed at effective elimination of the microorganisms, allowing inflammation to subside and healing to occur.

The general public across the world now expect professional people to deliver a high standard of service; dentistry, and endodontic treatment in particular, is no exception. In the UK, guidance has been published by the regulatory body on the standards expected of, ⁴⁷ and the scope of practice for, ⁴⁸ the whole dental team. The European Society of Endodontology has issued quality guidelines for endodontic treatment. ⁴⁹ It is essential that dental practices have a quality control system to ensure that each step in history, diagnosis and treatment is carried out in a logical and consistent manner. This is to ensure a high standard of care and treatment, known as clinical governance. Patients are increasingly well informed and will not tolerate poor standards, e.g. in sterilization procedures or out-of-date views. In the UK and in line with many other countries, one of the largest source of dental negligence claims relate to endodontics. ⁵⁰ The dramatic rise in litigation is a reflection that patients are increasingly prepared to seek redress for any failures regarding their care or treatment.

In a recent survey in England of newly qualified dentists in vocational training to join the National Health Service, most expressed a lack of preparedness with regards to complex/molar endodontics, with 66% rating their preparedness as ‘poor’ and 3% ‘very poor’. ⁵¹ Those dentists who have undertaken further training to become specialists are expected to achieve consistently high standards in diagnosis and treatment. However, general practitioners cannot continue to practise in the way that they were taught at dental school many years ago; they must keep up to date and offer referral to an appropriate specialist when the treatment required is beyond their skill. This change has already occurred in the USA and is spreading to other countries. In the UK, continuing professional development is now mandatory for recertification with the regulatory body and practitioners are also required to refer patients for further advice or treatment when it is necessary, or if requested by the patient. ⁴⁷

Almost all endodontic procedures can be carried out with a predictable outcome. Root canal treatment has a reported success rate of over 90%, ^52,53 even though closer analysis reveals that retreatment of teeth with apical periodontitis is less successful than initial treatment in teeth without apical periodontitis. ^52,/>