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European Journal Of Oral Sciences
Volume 111 Issue 4 Page 285  - August 2003
Key words: oral health, complexity, oral ecology

More about the Ecology of Health, see:
Dimitrov, V: A New Kind of Social Science, 2003, Morrisville: Lulu press

Ecology of oral health: a complexity perspective

Harald M. Eriksen and Vladimir Dimitrov


Health and disease are determined by many interwoven factors. Complexity theory may facilitate an understanding of oral health and disease by studying their complex interplay, and not focusing only on the importance of each individual factor involved. This introduces a theoretical basis for a dynamic, holistic approach to oral health with possible far-reaching consequences for dental education and diagnosis, prevention and treatment of oral diseases.

Epidemiological research has convincingly documented that health and disease and the balance between the two conditions are determined by many interwoven factors, which may reinforce, coact, mask or inhibit each other in a dynamic web of interactions (1, 2). However, in a reductionist scientific tradition one tries to understand and explain the outcome of such interactions by isolating and studying separately the effect of each factor involved. This is achieved either through the research design (i.e. controlled clinical trials and experiments) or through multivariate statistical analyses disclosing the impact of each health- or disease-promoting factor by controlling for the effect of all other factors included.

This approach, which emphasizes single causes and bivariate associations, has dominated dental research and contributed with an overwhelming amount of information regarding specific factors associated with oral diseases (3-6). However, the major oral diseases (dental caries and periodontal diseases) are usually slow-progressing and chronic in nature and cannot be fully understood in terms of the reductionist concept of well-defined disease entities (2, 4, 7).

Another limitation related to the reductionist approach to studying health and disease is the lack of differentiation between the disease process (pathogenesis) and the origin (initiation and progression) of a disease. Factors involved in a disease process (e.g. acidogenic bacteria, sugar, and fluorides in dental caries) are more easily subjected to experimental investigations and controlled clinical trials than factors related to initiation of the disease, such as social conditions, attitudes and habits. Consequently, the importance of factors involved in the disease process may be over-emphasized. This relates directly to a strict vs. a liberal definition of causality. A liberal definition, commonly used in epidemiological research, includes all factors that comply with the concept of association, time order and direction, while a strict version, most commonly found in experimental studies, is assuming that causal factors have to be directly and actively involved in the disease process (3, 8).

Based on the prevailing research activities, clinical interventions are still to a large extent based on the mechanisms through which oral diseases operate, the disease processes, in order to interfere with them. This is reinforced by the recent focus on evidence-based medicine and dentistry emphasizing controlled clinical trials as the 'gold standard' for intervention (9). An alternative strategy would be to try to understand the complex interplay between the web of component causes responsible for maintenance of health or initiation and progression of disease, both in general (8) and related to oral conditions in particular (10). Complex systems research incorporating concepts described as dynamic complexity, chaos theory, fuzzy logic, neural network, evolutionary programming, and computational methods (11) emerges as a promising alternative in order to explore complex, dynamic biological interactions (12).

Complexity theory has been widely applied in many branches of biology (12). Although the term 'complex' is frequently used to describe the multifactorial etiology of oral diseases, only a few attempts have been made to really use the theory of complexity in dental research. Landini applied a fractal model for periodontal breakdown (13) and Manji and coworkers studied dental caries and periodontal breakdown and argued for the relevance of applying the concept of random motion (14, 15). There are also examples of complex modeling in oral sciences (16). In the present paper, it is our aim to argue for the benefit of applying the concepts of complexity theory and complexity thinking in order to broaden our understanding of oral health/disease conditions.

The way of thinking in complexity may differ from what is commonly encountered in natural sciences. While a reductionist approach focuses on separate aspects of reality, the complexity approach encounters reality as a totality. Interactions of dynamics at various levels of manifestation are ubiquitous throughout the universe. A brief introduction to the basic concepts of this theory is therefore included before we continue with a more specific application of the theory related to dentistry.

Bridging complexity and ecology                           

The paradigm of complexity is centered in the rich conceptual basis of the non-linear science - the science of turbulence and chaos, emergence and fractals, self-organizations and criticality (17-19). The word 'complexity' originates from the Latin word 'complexus' which means 'totality'. The science of complexity explores the wholeness of dynamics (i.e. forces, energies, substances, and forms that permeate the whole universe and connect everything that exists in a web of dynamic interrelationships and interactions). The scales of manifestation of this web are manifold (i.e. micro and macro, organic and inorganic, natural and simulated, individual and social, animal and human). But even if the scales of the web are different, the dynamics at each scale exhibit similar characteristics and regularities. The study of these characteristics and regularities forms the conceptual basis of the paradigm of complexity.

Ecology studies the web of dynamic interactions of living species, including humans and their natural and artificial (human-made) environment. While complexity science tries to reveal the inner secrets of the dynamic interactions in nature and society, ecology explores the variety of their 'outward' manifestations. The unified framework of the emergent dynamic ecologies serves as a bridge between the two holistic branches of human inquiry, ecology and complexity (17).

According to the principle of fractality (19), which represents an integral part of complexity thinking, similar structures repeat themselves at different scales of generalization. If one considers the health of an organ (heart, lungs or the oral cavity), this is not only similar to, but also nested in the health dynamics of the individual that again is nested in the health of the family. Going from a higher level to a lower level of organization, family health envelops the health dynamics of the individual, which envelops the health of an organ (20). What is 'outward' (ecological) manifestation of health at one level of generalization will therefore (based on the principle of fractality) become part of the 'inner' (complexity) interactions at a higher level and thus bridge complexity and ecology (17).

Ecology of oral health                           

There have been many attempts to define health. The most general (holistic) and also one of the most debated is the definition given by the World Health Organization (WHO) stating that: 'Health is a state of complete physical, mental and social well-being and not merely the absence of illness or infirmity' (21). Other definitions either based on a normative or a naturalist philosophy, all go beyond the reductionist level of definition (22). Health ecology is defined as the unique web of life- and health-supporting interactions at all levels of their fractal, self-organizing emergence (i.e. molecular, cellular, organ-related as well as intra- and interindividual) between the individual and the environment and the individual and society (17). Based on this concept, ecology of oral health deals with interactions at the level of organization confined to the oral cavity, but with consequences beyond this level.

At the molecular level, oral health is dependent on immunology, which involves complex interactions between antigens, both humoral and cellular (23). The microbiology of the oral cavity is in itself an ecological system of great complexity, with interactions creating properties beyond what can be deduced from the sum of individual microbial characteristics (24, 25). Health of the structures constituting the oral cavity (i.e. teeth, including their attachment to the jawbones), the oral mucosa and the tongue, constitute an entity of great complexity. The health of these structures is dependent both on the internal factors mentioned earlier (immunological and microbiological), saliva, with both digestive, lubricating and other protective properties, and external factors such as nutrition, habits and lifestyle, including stress and coping (26), and various addictions, with smoking being the best documented (27). In addition, both social and cultural factors may have both a direct (sports activities) and an indirect (socio-economic status, occupation) impact on oral health (5, 6), emphasizing the complex structure of oral health.

Oral diseases: a complexity perspective                        

There is a variety of oral diseases and ailments, both genuinely oral with dental caries and periodontal diseases predominant, and oral manifestations of general diseases, where acquired immune-deficiency syndrome (AIDS) is a relevant example both regarding early diagnosis and proper treatment (28). Primary oral diseases may also have general health implications (29). However, in the present paper only the two major oral diseases mentioned will be considered even if the viewpoints presented may be equally relevant for many other oral conditions.

Dental caries has been explained as the result of an interaction between oral microbes (acidogenic bacteria colonizing the tooth surface), diet (primarily sucrose) and host factors, including saliva buffering capacity visualized by the Keyes' triad (30). The response is modified by the use of fluorides. This concept was based on controlled animal studies and represented a breakthrough in the understanding the pathogenesis of dental caries. However, an acknowledgment of the additional importance of more 'distant' factors including behavioral, psychological, and social factors involved in the disease process has gradually emerged (31-33). This has initiated a steadily increasing number of multifactorial, analytical epidemiological studies, applying advanced statistical methods. Such studies are frequently referred to as complex. However, with a few exceptions (13-16, 34), they do not apply aspects of complexity theory. Most of them are sophisticated extensions of the prevailing paradigm of disclosing the most important disease determinant(s) made possible through powerful computers.

The prevailing concept of the etiology of periodontal diseases was established by Waerhaug based on experimental studies in man and monkeys (35). These studies clearly showed that accumulation of microbial plaque at and below the gingival margin was the cause of periodontitis. Experimental and epidemiological studies have expanded and refined this concept (36). A growing appreciation of the importance of environmental factors together with strong focus on host resistance through immune responses has emerged. However, the prevailing paradigm based on a reductionist approach has, with a few exceptions (11, 12, 14), also dominated periodontal research during the last decades.

Relevance of complexity theory                        

In order to see how complexity theory might supplement our present comprehension of oral health and disease, the characteristics of dental caries and periodontal disease briefly described above will be related to some principles of complexity. These principles, explicitly stated by Dimitrov (20) and Reason& Goodwin (37), can be summarized as follows (Table 1):
Principle of emergence (the development from simple to complex structures);

Principle of self-organization or self-healing (38);
Principle of attraction (strange dynamic attractors usually driven by human desires) (20);
Principle of fractality or iteration (similar structures that repeat itself at different scales of generalization) (19);
Principle of rich interconnections and interdependence (multivariation).

Complex systems are most likely to change (bifurcate) under 'far from equilibrium' conditions, which is frequently described as 'the edge of chaos' in complexity theory (38, 39). In this context, instability has the potential of both creativity and destruction (restoring health or developing disease) depending on the direction of the bifurcation.

Principle of emergence

This principle is primarily of relevance to evolution (38), but it has also relevance to oral conditions when combining the principle of fractality with the position expressed by Gorowitz& MacIntyre (40) that: 'The characteristics of one level of organization can not be understood solely as the sum of characteristics of their component parts'. This statement represents a serious criticism of the reductionist approach and is in favor of complexity theory. An aspect of this principle has recently been elaborated on in a paper dealing with caries in populations (41).

Principle of self-organization

The principle of self-organization or self-healing goes very much together with conditions at 'the edge of chaos'. Because of the dynamic interconnectedness of all biological systems, they possess a stable and robust region comparable to the buffering zone of an inorganic buffering solution. In this zone, impacts are 'absorbed' or 'buffered' without creating much change (38). However, 'at the edge' very little additional impact can dramatically change the conditions that comply with chaos theory (14, 39). For dental caries and periodontal disease, self-organization or self-healing is of relevance as long as the conditions are reversible (i.e. to 'the edge of chaos'). Beyond, self-reorganization may occur, but a return to the original condition is impossible: a cavity in a tooth or attachment lost do not heal by self-organization. Dental investigations have addressed this topic (14, 15) and the pattern of punctuated equilibrium related to self-organization described by Bak& Sneppen (42) shows obvious similarities to the burst-theory explaining periodontal breakdown (43).

Principle of attraction

This principle relates directly to behavioral aspects where attraction to frequent consumption of sweets and beverages are caries-promoting, and addiction to smoking relates to periodontal problems and oral cancer. These examples illustrate that there is not necessarily something innovative in the principle of attraction. However, the concept of attraction shifts the focus from the destructive habit to the urge for something that has deleterious effect, a substantial difference regarding, for example, preventive strategies.

Principle of fractality

Both dental caries and periodontal disease may be characterized at different levels of organization, from micro to macro, to a global level. This points to the relevance of applying a fractal approach. An understanding of the interconnectedness between these levels through fractality may bridge knowledge from experimental research (microlevel), clinical investigations (individual or macrolevel) and epidemiology (population or global level) into a holistic unit of knowledge based on the principle of fractality that similar structures repeat themselves at different scales of organization. In this sense, communication within biofilms on the tooth surface may show some similarities with communication in societies.

Principle of rich interconnections (multivariation)

As documented earlier, medical and dental research, including research on dental caries and periodontal diseases (44, 45), have used multifactorial analytical designs in order to control inherent interconnectedness and disclose health- and disease-promoting factors. This indicates the relevance of searching for methods in order to elucidate complex dynamic structures, and in this context complexity theory may facilitate a deeper understanding of disease entities.

Possible consequences                            

The consequences of applying theory of complexity in oral research may, as indicated above, be far-reaching with a variety of aspects. In order to frame possible consequences, the following aspects related to oral health and disease will be discussed: causality/explanation, prediction, diagnosis, prevention, treatment/cure, rehabilitation and prognosis. In addition, consequences for dental education will be mentioned.


Complexity theory complies with a liberal definition of causality based on an ecological understanding (3, 7). The concept of causation is probabilistic and inclusive rather than deterministic and exclusive and implements uncertainty as a basic assumption for causality and explanation. As complexity theory is of relevance in areas as different as organization management, economics, weather, ecology, biology, and learning (37-39). The insight gained through a complexity perspective may therefore act as a key to interdisciplinary understanding that is relevant for dentistry.


Many attempts have been made to develop sets of rational criteria for prediction of oral diseases, none of them really successful (4, 5, 10, 44-46). The multifactorial and complex nature of oral diseases is often used as an explanation why these attempts have never succeeded. In addition, many of the sets of criteria introduced assume that the actual exposure status of the individual remains unchanged during the period in question (46) in order to secure predictability. This also explains why previous disease experience frequently turns up as the strongest predictor of future disease. Instead of trying to isolate determinants of importance, definition of attractors of relevance, 'edge of chaos' analyses and interconnectedness evaluations may add valuable information regarding prediction.


Prevention is closely linked to causality/explanation, using preventive methods of relevance both for the disease(s) in question and the level of generalization. The ecological perspective and complexity theory as basic assumptions may place prevention in a wider than usual context. This is in accord with thoughts presented by Sheiham& Watt in their 'common risk factor' approach (47).

Diagnosis and treatment/cure/rehabilitation

The commonly recommended procedure for diagnosis and treatment planning includes collection of anamnestic information, recording of the present status of the patient, including clinical registration, a diagnosis based on evaluation of information collected and, consequently, a treatment plan. This procedure appears objective and well defined; however, dentists end up with vastly different suggestions for treatment based on such principles (48, 49), indicating a need for a more comprehensive conceptual basis for treatment planning (50).


A complexity perspective to prognosis will shift the focus from the usual life-table estimates of possible years of function for dental restorations to expected years of maintained oral health and functions, biologically, socially and mentally. This is relevant for clinical dentistry (51) and might in addition contribute to a highly needed theoretical basis for esthetic dentistry.

Complexity theory, with its holistic perspective, may also be of relevance in order to understand the more obscure ailments and conditions linked to the oral cavity such as 'oral galvanism' (52), 'burning mouth syndrome' (53) and some temporomandibular dysfunction conditions (54).

The application of complexity theory has a direct impact on dental education. This includes both the content and priorities within the curriculum and didactic aspects. Theory of complexity has been used in pedagogic programs (55) and the concept of problem-based learning indicates that such ideas are already implemented in some dental curricula (51).

Application of complexity theory will support the concept of the oral cavity as an integral part of the human body rather than as an isolated entity, which has been the prevailing tendency reflected in many dental curricula. This is also in accord with recent developments in dental curriculum planning (51).

Complexity theory will not substitute existing knowledge about the oral cavity. Much relevant information of biological and pathological processes has to be derived from experiments, controlled clinical trials and epidemiology. The concept of oral ecology based on complexity theory has, however, the potential to go beyond the fragmented knowledge derived through the reductionist approach and facilitate a broader understanding and appreciation of the dynamic nature of oral health and diseases.

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Accepted for publication April 2003

To cite this article
Eriksen, Harald M. & Dimitrov, Vladimir (2003)
Ecology of oral health: a complexity perspective.
European Journal Of Oral Sciences  111 (4), 285-290.
doi: 10.1034/j.1600-0722.2003.00053.x