The impact of epidemiological data on periodontal treatment strategies

Introduction

There are several definitions of the word epidemiology. According to ^{Hennekens and Buring (1987)}, epidemiology can be defined as « the study of the distribution and determinants of disease in human populations ». Another definition was proposed by ^{Burt and Eklund (1992)} : « The study of health and disease in populations and how these states are influenced by heredity, biology, physical environment, social environment and way of living. »

Epidemiology is a central issue within oral health care and must be considered a vital instrument for the analysis of a population's need for dental care, the planning of suitable measures, the evaluation of care received, and, furthermore, the development of quality.

In the 60s and 70s, prevalence of periodontal disease was expressed in terms of frequency, incidence, and severity or extent according to indices or scales that quantify levels of disease. However, with computer-based registration of clinical and radiographic periodontal variables such as plaque, gingivitis, probing pocket depth, loss of attachment, and periodontal bone loss, the same variables that clinicians use in their diagnostics and treatment of the individual patient can also be used in epidemiological surveys. It is then possible to « translate » results from epidemiological studies into the daily work of planning treatment strategies for both the population and the individual.

In epidemiological studies where the variables selected to describe oral health are expressed as mean values, a minority of the population with a higher disease prevalence than the rest will always be present but may remain undetected. This is important to consider when interpreting results from epidemiological studies for planning future dental care. Dental health care personnel should, therefore, have sufficient knowledge to recognize any deviation from population norms and the ability to investigate background factors.

Etiology and pathogenesis of gingivitis and periodontal disease

Current concepts regarding the etiology and pathogenesis of periodontal disease were originally and are largely derived from the results of epidemiological investigations. Some 10 years ago, findings from epidemiological studies were interpreted to demonstrate that destructive periodontal disease started as gingivitis in children and adolescents, affected more or less all subjects after the age of 40, and slowly progressed to also include the elderly population (^{Ainamo, 1983}). This concept of widespread, slowly progressing periodontal disease has now been abandoned. Still, the results from epidemiological studies performed during the 80s and 90s revealed that the prevalence and extent of periodontal disease increased with age and inadequate oral hygiene. The disease is thus regarded as an infectious disease, a plaque-associated condition, initiated and sustained by the supra - and subgingival microbiota that, when left undisturbed, colonize the teeth and root surfaces. It starts as an inflammation of the gingiva and may, if left untreated, in certain individuals and/or sites spread to involve deeper parts of the periodontium.

Severe periodontal disease

Recent cross-sectional epidemiological and longitudinal studies on periodontal disease progression have shown that severe periodontal disease only affects a small fraction of the population, although the prevalence of individuals with severe periodontal disease increases above the age of 50 ^{(Baelum et al., 1988} ; ^{Hugoson and Jordan 1982} ; ^{Hugoson et al., 1992} ; ^{Löe et al., 1986} ; ^{Okamoto et al., 1988} ; ^{Papapanou et al., 1988} ; ^{Papapanou et al., 1989} ; ^{Salonen et al., 1991).}

Long-term evaluation of changes in gingivitis and periodontal disease prevalence

In a series of epidemiological studies initiated in 1973, the distribution of periodontal disease in a Swedish adult population was evaluated over a period of 20 years (^{Hugoson et al., 1998a} and ^b). Cross-sectional studies were carried out in the city of Jönköping in 1973, 1983, and 1993. Individuals were randomly selected from the age groups 20, 30, 40, 50, 60, and 70 years. Based on clinical data and full-mouth intra-oral radiographs, all individuals were classified into one of five groups according to the severity of the periodontal disease experience : group 1, healthy periodontium ; group 2, gingivitis without signs of bone loss ; group 3, moderate alveolar bone loss not exceeding 1/3 of the normal bone height ; group 4, severe alveolar bone loss ranging between 1/2 and 2/3 of the normal alveolar bone height ; and group 5, alveolar bone loss exceeding 2/3 of normal bone height and angular bony defects and/or furcation defects. During the 20-year period, the number of individuals in groups 1 and 2 increased from 49 % in 1973 to 60 % in 1993. In addition, there was a decrease in the number of individuals in group 3, the group with moderate bone loss. Groups 4 and 5 comprised 13 % of the population and showed no change in general between 1983 and 1993 (^{fig. 1}). Corresponding changes in periodontal disease prevalence have also been demonstrated in cross-sectional studies among US adults (^{US Public Health Service, 1987}). Thus, despite a general shift towards improved periodontal health, an unchanged fraction of the population will still develop severe periodontal disease. These individuals can, today, not be predicted although some risk factors are identified.

Periodontal disease progression rate on an individual or site level

Longitudinal studies on the progression of natural periodontitis in man suggest that the rate of periodontal breakdown in most individuals and sites is very low ^{(Albander et al., 1986} ; ^{Albander, 1990} ; ^{Buckley and Crowley, 1984} ; ^{Lavstedt et al., 1986} ; ^{Lindhe et al., 1983} ; ^{Lindhe et al., 1989} ; ^{Papapanou et al., 1989} ; ^{Wennström et al., 1993)}, even in the absence of dental care ^{(Buckley and Crowley, 1984} ; ^{Löe et al., 1986).}

In the above-mentioned randomized cross-sectional survey of the population in Jönköping, Sweden, all dentate individuals 15-60 years at the 1973 examination were invited to participate in a new clinical and radiographic examination 17 years later. The examination included full-mouth plaque and gingivitis scores, probing pocket depth, and alveolar bone height measurements on full-mouth intra-oral radiographs. All age groups had very good oral hygiene with 50 % or more of the individuals having plaque and gingivitis scores below 20 %. From the age of 20 there was a general pattern of bone height reduction over time corresponding to an annual loss of around 0.1 mm (^{fig. 2}). Very few individuals, less than 5 %, exhibited an individual mean bone loss of 2-3 mm or more over the 17 years and the majority of sites remained fairly stable (^{fig. 3}). In fact, only 6.2 % of the total number of sites examined (15 641 sites) exhibited a bone loss of 2-3 mm or more (^{fig. 4}). However, from the age of 30 years, about 80 % of the population had at least 1-5 sites with bone loss corresponding to 2-3 mm or more. This is in agreement with findings by, e.g. ^Lavstedt , ^Wennström and ^{Ship and Beck (1996).}

Evidently, the current view is that periodontal disease is subject-related. Only a few individuals will experience advanced loss of periodontal tissue support and alveolar bone at several teeth. It is, at present, not understood why, in most individuals, the inflammatory process is demarcated by the gingival tissues while in others a progressive destructive periodontitis with loss of connective tissue attachment and alveolar bone is developed. This includes individuals with many or most of the teeth affected or just one or few individual sites. However, most subjects who maintain a normal standard of oral hygiene are not likely to develop (gingivitis or) destructive periodontal disease.

Risk factors for periodontal disease

The fact that some individuals are more susceptible to periodontitis than others has increased interest in identifying individuals at risk and risk factors that make them more prone to develop periodontal breakdown. Studies in individuals have been performed, both cross-sectional and longitudinal, using multivariable statistical models to seek associations between risk factors and the outcome variable severe periodontal disease/disease progression ^{(Beck et al., 1990} ; ^{Brown et al., 1994} ; ^{Grossi et al., 1995} ; ^{Haffajee et al., 1991} ; ^{Ismail et al., 1990} ; ^{Locker and Leake, 1993} ; ^{Norderyd and Hugoson, 1998} ; ^{Norderyd et al., 1999} ; ^{Oliver et al., 1991} ; ^{Papapanou et al., 1989} ; ^{Papapanou and Wennström, 1991} ; ^{Tervonen et al., 1991}). Certain factors, such as male sex, higher age, low socio-economic or educational status, diabetes mellitus, stress, smoking, and the presence of certain bacteria in subgingival plaque have been identified in more than one study as being associated with severe periodontal disease. Associations found in cross-sectional studies do not establish causal or temporal relationships but point to factors that can be considered risk indicators (^{Genco, 1996}). To fully evaluate the importance of and to determine whether they are true risk factors, researchers must conduct longitudinal studies that include random samples to assess the cause of the disease in individuals who have these risk indicators as compared to the cause of the disease in individuals in whom these indicators are absent ^{(Locker et al., 1998).}

The association between age and periodontitis has been questioned because different age groups have grown up under different circumstances and have experienced different levels of disease (^{Ainamo and Ainamo, 1996}). In the New England Elders Dental Study, the increased prevalence of periodontal disease among the elderly was explained by the fact that new cohorts of elderly have significantly more teeth than did previous cohorts (^{Fox et al., 1994}). This was also found in the Jönköping surveys in 1973, 1983, and 1993. The largest difference in number of remaining teeth was found among 70-year-olds who had an average of 5 more teeth per subject in 1993 than in 1973 (^{Hugoson et al., 1998a} and ^b). Periodontally diseased teeth therefore seem to be more frequently retained in the older age group now than before.

Numerous epidemiological and other studies have demonstrated that of all risk factors identified, cigarette smoking may be the one environmental risk most strongly associated with severe periodontitis (^{Bergström, 1989} ; ^{Grossi et al., 1994} ; ^{Magnusson and Walker, 1996} ; ^{Norderyd et al., 1999} ; ^{Page and Beck, 1997}). Elevated odds ratio for this association persisted even when adjusted for other factors such as oral hygiene, gender, ageing, education, and socio-economic status.

Several studies on adults have demonstrated more severe periodontal disease in long-duration insulin-dependent diabetics than in non-diabetics (^{Belting et al., 1964} ; ^{Campbell, 1972} ; ^{Genco and Löe, 1993} ; ^{Hugoson et al., 1989} ; ^{Sznajder et al., 1978} ; ^{Tervonen and Knuuttila, 1986} ; ^{Thorstensson, 1995}). The most substantiated evidence for modification of disease susceptibility and/or progression of periodontal disease arises from such studies. So, even if there is still no consensus about the reason for increased susceptibility, insulin-dependent diabetics are an important risk group to be considered.

Although supragingival dental plaque is essential to the initiation of adult periodontitis, the actual correlation between supragingival plaque levels and longitudinal disease progression has been found to be rather weak (^{Kornman and Löe, 1993} ; ^{Lindhe et al., 1989}). In the cross-sectional study on the city of Jönköping, however, plaque remained significantly associated with severe bone loss when tested multivariately (^{Norderyd and Hugoson, 1998}). Studies have not, so far, identified a critical maximum plaque level compatible with maintenance of periodontal health (^{Lang and Tonetti, 1996}). Deepened periodontal pockets ≥ 4 mm have also been correlated with severe periodontal disease progression (^{Badersten et al., 1990} ; ^{Claffey et al., 1990} ; ^{Locker et al., 1998} ; ^{Norderyd and Hugoson, 1998}). However, it is not the pocket per se or the depth itself that constitutes the risk factor but what it represents in terms of micro-organisms, subgingival calculus, inflammation, and/or previous disease experience and treatment ^{(Lang and Tonetti, 1996} ; ^{Locker et al., 1998).}

Conclusion

- Prevalence and extent of periodontal disease increase with age and inadequate oral hygiene.

- In contrast, most subjects who maintain a normal standard of oral hygiene are not likely to develop gingivitis or destructive periodontal disease.

- Periodontal disease is an infectious disease initiated and sustained by supra- and subgingival microbiota.

- Periodontal disease starts as an inflammation of the gingiva and may, if left untreated, spread in certain individuals and/or sites, to involve deeper parts of the periodontium.

- The progression rate is usually low (around 0.1 mm/year), and there is a gradual loss of bone height even in the absence of disease.

- Severe periodontal disease only affects a small fraction of the population. However, about 80 % of an adult population will have at least a few sites with bone loss of more than 2-3 mm.

- Certain risk factors have been proposed, such as local (plaque and deepened periodontal pockets), iatrogenic, and systemic (age, smoking, and diabetes melllitus) factors.

Ethics or inequality in periodontal care

In the introduction to the booklet from the WHO regional office for Europe, The Solid Facts (^{WHO Europe, 1999}), the following can be read : « Even in the richest countries, the better-off live several years longer, have fewer illnesses than the poor. These differences in health are an important social injustice and reflect some of the most powerful influences on health in the modern world. People's lifestyles and the conditions in which they live and work strongly influence their health and longevity ». It continues : « Each person is responsible for ensuring that he or she eats a healthy diet, gets enough exercise and avoids smoking and excessive drinking. Nevertheless, we now know the importance to health of social and economic circumstances that are often beyond individual control ». It should be the right of every human being to be healthy or at least receive all evidence-based information needed. In the Swedish dental care act, for example, it is stated that good dental health and dental care should be available on an equal basis to the whole population, regardless of age, gender, ethnicity, or other social attributes. Evaluation of the fulfilment of the human right requirements places special demands on epidemiological monitoring of oral conditions, and utilization of care among the population as a whole. The legal requirements should be interpreted as referring not only to the opinion of the dental profession but primarily to the patients' subjective opinion and satisfaction concerning oral health and availability of care in the population. The consequence of this is that the entire population or every patient who enters the practice should be offered a correct diagnosis, prevention, and treatment depending on the patient's choice and co-operation. The recall system should also be organized depending on the risk assessment of the individual.

Clinical implications of epidemiological studies on daily periodontal practice

The assessments of periodontal treatment need not only must include well-defined clinical diagnostic criteria but should preferably also be related to attainable goals for periodontal care and health in society, population (e.g. private practice cohorts), and the individual patient.

As mentioned above, cross-sectional and longitudinal epidemiological studies have shown that there is a gradual loss of bone height with age even in the absence of disease. Ninety-five per cent of the 70-year-old population will have a remaining alveolar bone support equal to or exceeding 40 % of the root length both on an individual mean and on a tooth site level ^{(Hugoson and Laurell, 2000} ; ^{Papapanou et al., 1988} ; ^{Papapanou et al., 1989} ; ^{Salonen et al., 1991). Figure2} illustrates mean values and 95 % confidence intervals of bone height expressed as per cent of tooth length in different age groups based on data from ^{Hugoson and Laurell (2000)}. Such data could be used as minimum attainable long-term goals for periodontal care and health.

While around 15 % of the population exhibited severe periodontal breakdown over a 17-year period, as much as 80 % of the population above 30 years had at least 1 yo 5 sites that lost 2-3 mm or more (^{Hugoson and Laurell, 2000}). As down-hill cases or loser sites cannot be predicted based on previous disease experience (^{Hugoson and Laurell, 2000} ; ^{Lindhe et al., 1983}), periodontal examination should be a mandatory part of all regular dental check-ups to detect early signs of periodontal breakdown and provide adequate treatment.

Routine periodontal examination should include local factors such as registration of visible plaque (Silness and Löe plaque indices 2 and 3), probing pocket depths above 4 mm, bleeding on probing, assessment of bone height on radiographs when indicated and bony defect morphology, iatrogenic factors such as restoration overhangs and poor restoration quality together with systemic risk factors as age, smoking, and diabetes mellitus, and behaviour risk factors such as compliance with recall programmes to determine treatment needs and regimes. The primary goal of periodontal treatment should be to regain periodontal health and, by individually designed maintenance programmes, based on risk assessments, to maintain health and/or prevent disease recurrence. Full-mouth plaque scores ≤ 20 %, achieved and maintained pocket reduction, and absence of bleeding on probing are considered compatible with periodontal health and should be the goals to strive for. However, with such goals there is always a risk of over treatment since bleeding on probing and residual pockets are quite weak predictors of further attachment loss (^{Badersten et al., 1990} ; ^{Claffey et al., 1990} ; ^{Lang and Tonetti, 1996}). ^Wennström presented an interesting model for decision-making regarding periodontal treatment needs in which the actual bone height of the tooth sites assessed from radiographs was considered in relation to the patient's age and the predetermined goal of bone height at the age of 75 years. Briefly, in a diagram, a cumulative bone loss regression line was drawn between the 100 % bone height at 25 years and the predetermined goal of 40 % at the age of 75 (which would be the lower level of the 95 % confidence interval of bone height at that age). By entering the bone height value of the actual site at the patient's age into the diagram, a bone loss rate factor can be assessed. The calculated bone height value for each critical tooth site at the age of 30, 35, 40... and 70 years will thus describe the level at which therapeutic intervention is necessary to secure the maintenance goal of 40 % of the bone height ( ^{fig. 5}). Apart from being used in decision-making about periodontal treatment at a particular site, this model, which is based on data from epidemiological studies, could also be used to evaluate periodontal treatment needs on a population level ^{(Wennström et al., 1990).}

Because of the low progression rate of periodontal disease in most individuals, maintenance care in low-risk patients may be fulfilled by a recall programme with recall intervals of more than a year while high-risk individuals would need to be monitored several times during a similar period.

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