The possible connections between oral ill health and general health have long been present. Indeed in the fourth century BC, Hippocrates, the Greek physician known as ‘the father of medicine’ attributed the cure of a case of arthritis to the extraction of a tooth. Arguably, this was the earliest recorded observation of a possible link between oral infection and a systemic condition. Then, as now, the difficulty arises in being able to directly link cause and effect in a biological system as complex as the human body. Similarly, however, there is a distinct human trait in making an observation of a change and wishing to attribute it to a specific reason. It provides us with the basis of scientific method and it is from this discipline that we can begin to establish a hierarchy of evidence to attempt to better answer the questions of possible causative links between periodontal disease and systemic conditions.
Early in the twentieth century William Hunter, a British doctor, developed the theory of focal infection after noting links between oral sepsis and diseases of other body organs which he attributed to the dissemination of organisms or toxic products from the mouth.1 Extending his theory, which had a major effect on dentistry, Hunter connected a large range of conditions with oral sepsis including gastro-intestinal problems, cirrhosis, rheumatoid arthritis and also rather more vague symptoms such as debility and cardiac irregularity. Many of these associations were based on clinical reports of improvement after removal of the septic focus, usually the teeth. The theory gained much support from leaders of the dental profession on both sides of the Atlantic. One of its attractions was that it improved the standing of dentists who could be considered to be working in concert with the medical profession to improve the health of their patients and coincided with a campaign in Britain to limit dentistry to those with qualifications.2
The acceptance of the theory of focal infection resulted in wholesale removal of teeth in an attempt to treat a variety of conditions. The oral health impact of this practice was demonstrated, for example, in the data from the first Adult Dental Health Survey of the UK in 1968 which found that 71% of those aged 55 or older were edentulous, almost certainly representing the consequences of this approach in the first half of the century.3 A growing recognition that no controlled clinical studies supported this theory meant that it went into abeyance, especially as the studies into dental plaque improved our understanding of the pathogenesis of dental diseases.4 Additionally, the emphasis on disease prevention and on maintaining a functional dentition throughout life hastened the end of the era of adherence to the belief in focal infection.
Not long after the theory had been laid to rest, reports started to emerge in the late 1980s and early 1990s from well-designed scientific studies of possible linkages between periodontal disease and other diseases.5-8 Prospective cohort studies then began to report that periodontal disease was associated with an increased risk of premature death from any cause, suggesting the hypothesis that periodontitis could be a risk factor for other diseases.9,10
There was a subsequent explosion in clinical research in this area and periodontal disease has been linked to a number of diseases and conditions that have a major impact on public health, including cardiovascular disease, adverse pregnancy outcomes, diabetes, respiratory disease, chronic kidney disease, rheumatoid arthritis, cognitive impairment, obesity, metabolic syndrome and cancer, which have been variously reviewed.11,12