Epidemiology is the study of health and disease and associated factors in human populations. It also includes how the states of health and disease in the population are influenced by heredity, biology, physical environment, social environment, and personal behavior.1 Epidemiology measures disease in several ways:
Epidemiologic research differs from clinical research in that entire groups are the focus of study, not individuals, and that persons without the disease are included in studies to assess the risk of having the disease among the members of a population.1 It is important to understand that prevalence alone, simply counting the number of individuals in a given population with disease, is not sufficient to understand gingival and periodontal disease in the population.
Epidemiologic research uses relatively simple calculations to assess disease in a population. Prevalence is computed by dividing the number of persons with the disease by the number of persons in the population (P = number with disease/number in population).
These calculations are based on evaluations of lots of individuals who make up the population being studied. Common types of experimental designs used in epidemiologic research, cross-sectional studies, cohort studies, and case-control studies,2 are described in Box 3-1.
However, to make these observations about populations, researchers must define what constitutes gingival and periodontal disease. This is done by clinical assessment of a group within the population using scales or indices. All individuals within the population can be assessed or just a sample. Ideally, sample individuals are selected by identifying members of the group at random (there are computer programs called random number generators that can do this) so as not to select all individuals with some common characteristic. For example, if you chose to study everyone in the population whose last name began with “B,” you might be selecting many individuals who are related; thus your data could be affected by family characteristics. If you selected randomly from all members of the population, familial characteristics would be spread out over the entire group and much less likely to affect your results. Sample selection is a very important characteristic of epidemiologic research and every study describes the process used.
Once the sample is selected, the individuals must be examined for clinical characteristics of disease using indexes or indices. The data then describe a general picture of the disease in the population. For example, periodontal disease can be defined as having probe depths of greater than 4 mm in at least one site in the mouth, so you can determine how many people in the population have that characteristic. It can also be defined as having one or more probe depths of 7 mm or having the gingival tissue bleed when probed in one or more sites. Therefore, it is important to understand the definition of disease in the study. As you will see, a study that defines periodontal disease as bleeding at one site will provide a very different picture of disease than one that defines it as one or more 7-mm probe depths in the same population. In addition, you might want to know what percentage of the population with periodontal disease (as you have defined it) uses tobacco and compare that to the number of those in the population who use tobacco but do not fit your definition of periodontal disease.
The examiner or team of examiners is trained to apply the indices to members of a population. This means the examiners are calibrated, or standardized, so that what one examiner would interpret as a 4-mm probing depth, all would interpret the same way. It is impossible to have 100% agreement among examiners at all times, but with training they can agree most of the time. This agreement is essential for the information collected to be meaningful, and so that the data can be accurately interpreted. Imagine the difficulty if one examiner probed 7 mm and another probed 5 mm in the same pocket and the definition of periodontal disease is 6 mm. That patient might not be accurately classified as with or without the disease. The calibration process permits the epidemiologic data to provide meaningful information.
Another important point about epidemiologic research is related to the population of individuals to be examined. It is impossible to examine every person in large epidemiologic studies, so representative samples are selected. This can be done through random selection of subjects as mentioned earlier, or very sophisticated sampling techniques using subsets of the population. In either of these cases, the results found in the sample can be generalized to a much larger group of the population, but rarely to everyone in the world. This is referred to as generalization of results. As an example, in the past epidemiologic studies of heart disease were conducted solely on men, so although the findings could be generalized to men, there was real concern about applying the findings to women. These studies now include women because treatment effects or risk factors need to be assessed on the basis of differences, such as gender, in populations. Also, an epidemiologic study of an American population might not be at all representative of the population in Mexico, China, or any other country because there are many cultural differences that influence health.
Although it is complex, epidemiologic research gives us important insights into health problems. It is very important for dental hygienists to understand the number of people in the population with periodontal disease, how severe it is, and what behaviors or characteristics are associated with the disease.
Epidemiologic data do not provide absolute values for a population, and studies differ in their findings as a result of differences in population, examiners, or measuring scales. However, well-designed, well-run epidemiologic studies provide us with much information that is distinct from information gained from clinical studies. Epidemiology provides a general understanding of the disease occurring in patients. For example, we know that periodontal disease occurs in a relatively small but significant percentage of the U.S. population, so not everyone seeking dental hygiene treatment has periodontal disease. However, it is also important to remember that in clinical practice, when a patient with periodontal disease requires dental hygiene care, the disease affects 100% of that person. Dental hygiene care is dictated by the specific findings for that person, not general population findings.
The following section describes important epidemiologic indices to help the dental hygienist understand the common measurements used to define periodontal disease. Current data will be briefly reviewed to describe the overall periodontal health of U.S. populations. Several risk factors associated with more severe disease will be described, and other population study data will be presented.
Epidemiologic data are collected with well-defined measuring systems known as indices. These indices have defined scales, are easily applied to populations of individuals, and measure some specific aspect of the disease or condition of interest—in this case, signs of periodontal disease.
Several indices have been used to evaluate the periodontal status of populations. In each case, the units of measurement are defined with upper and lower limits. This section explains what it means to say that the population had a mean (average) plaque index of 2 or that 7% of the population had severe periodontal disease characterized by one or more deep pockets. It is important to consider the scoring system used when interpreting research results because they are all different. For a detailed discussion of all indices, see Wilkins.3
Plaque is an important quantity to define because it is the etiologic agent in periodontal disease. Screening populations for the presence of plaque determines whether all the people have plaque, how much, and how it relates to other signs of disease. Sometimes plaque is measured by a simple scale, presence (scored as 1) or absence (scored as 0). If all plaque caused periodontal disease, this approach might be sufficient. However, plaque is clearly associated with gingivitis.4,5 Thus, the quantity of plaque provides a picture of oral hygiene (e.g., where the accumulation is greatest and how good or poor oral hygiene practices are in a given population) and can indicate the presence of gingivitis. It does not correlate well with periodontal disease.
The Plaque Index of Silness and Löe (Pl I) places the most significance on the amount of plaque at the gingival margin because of the importance of the proximity and relationship of plaque in that location to gingival inflammation, which is measured clinically by bleeding. This index, which has been used in many studies, can be used to measure all teeth or selected teeth in a study, or it can be applied to selected surfaces of the teeth. The following criteria are used for scoring:
The tooth score is determined by adding the scores for the measured surfaces on each tooth and dividing this number by the number of surfaces. The whole mouth score is determined by adding the tooth scores and dividing this number by the number of teeth. These data can be evaluated by tooth, by groups of teeth, or over individuals in the population. In a population with an average plaque score of 2, a moderate level of plaque is present in most individuals. Alternatively, if a tooth has a plaque score of 1, little plaque is present and the plaque cannot be seen by the naked eye.
The Simplified Oral Hygiene Index of Greene and Vermillion (OHI-S) has both a debris index (DI-S) for plaque and a calculus index (CI-S). The scores can be used singly to provide a plaque index or a calculus index, or they may be combined to provide an oral hygiene index.
The DI-S, the plaque portion of the index, is a numeric assessment of plaque and other debris on the teeth. As representative of the entire dentition, six selected teeth are scored: the buccal surfaces of the maxillary first molars, the lingual surfaces of the mandibular first molars, and the labial aspects of the maxillary and mandibular left central incisors. Scoring criteria are as follows:
The debris score for the individual is obtained by adding the scores for all surfaces and dividing by the number of surfaces scored. An average score across the population can be determined by adding the debris scores for each individual and dividing by the number of individuals.
Calculus is a significant factor associated with periodontal disease, although it is not an etiologic agent (see Chapter 5). It has been measured in many epidemiologic studies, and the amount of calculus present is significant to the practice of the dental hygienist. Knowing the extent of calculus formation in the population helps to explain why so much practice time is spent removing it.
Selected teeth are scored using the calculus index, the CI-S. A score of 2 on the CI-S indicates a considerable quantity of supragingival calculus, some subgingival calculus, or both. The criteria are as follows:
To determine the OHI-S score (the combined score of DI-S and CI-S), the mean scores on both indices for all subjects are added and then divided by the total number of subjects. These OHI-S scores are higher, ranging from 0 to 6. A score of 6 is the highest score possible on the DI-S—3, the highest possible score on the DI-S, plus the highest score possible in the CI-S, 3. A combined OHI-S score does not indicate how much is due to debris or how much is due to calculus being present. An OHI-S score of 3 could indicate heavy calculus accumulation, heavy debris accumulation, or a combination. For this reason, it is often helpful to consider both components of the OHI-S score rather than combining them.
The Volpe-Manhold Index scoring system measures only supragingival calculus. It was designed to measure the mandibular incisors but has also been applied to other teeth. A periodontal probe with millimeter markings is used to bisect each of the three parts of the lingual surface of the incisor (mesial lingual, distal lingual, and direct lingual), and a measurement of the height of calculus is made for each of the three parts of the lingual surface. Scores for the direct lingual, mesial lingual, and distal lingual surfaces are averaged (added together and divided by three) to provide a tooth score. Tooth scores are averaged (added up and divided by the number of teeth measured) to provide a score for the individual. The score can be interpreted as indicating the approximate height of calculus, in millimeters, on the measured surfaces.
Indices of gingival disease are assessments of bleeding of the gingiva. Bleeding is important to monitor because it is associated with inflammation. Bleeding on probing is the most common sign used in clinical practice to monitor gingival health, and bleeding is associated with periodontal destruction.