Periodontal Research: Basics and beyond - Part I (Defining the research problem, study design and levels of evidence).

Research in the field of periodontology has witnessed a tremendous upsurge in the last two decades unveiling newer innovations in techniques, methodologies, and material science. The recent focus in periodontal research is an evidence-based approach which offers a bridge from science to clinical practice. This three part review series intends to take a reader through a maze of periodontal research, unraveling and simplifying various issues in the design, conduct and interpretation of various study designs routinely used in the field of periodontal research. This understanding would facilitate a researcher with a focused and an enhanced vision toward formulating studies which can more efficiently translate sound scientific phenomena into clinically meaningful results.

INTRODUCTION

“All progress is born of inquiry. Doubt is often better than over-confidence, for it leads to inquiry, and inquiry leads to invention.”

–Hudson Maxim (1853-1927)

Research is a voyage of discovery from the known to the unknown. Research can also be defined as a scientific and systematic search for pertinent information on a specific topic. Research comprises defining and redefining problems, formulating hypothesis or solutions; collecting, organizing, and evaluating data; making deductions and reaching conclusions; and at last carefully testing the conclusions to determine whether they fit the formulating hypothesis. Over the past few decades, the branch of Periodontology witnessed a tremendous leap in various aspects of clinical, epidemiological and molecular research and many mysteries pertaining to the etiopathogenesis and treatment aspects of periodontal disease have since been unveiled.

Various steps in the research process involve formulating the research problem, an extensive literature survey, developing the hypothesis, preparing the research design, sample size estimation, collecting the data, execution of the project, data analysis, hypothesis testing, generalizations and interpretation and preparation of the report. For the research to be more meaningful and clinically relevant, research does not stop with the preparation of the report but is disseminated and promulgated through presentations and publication of the findings.

The purpose of this review series is to give the reader an insight into the various aspects of the research process and provide a better understanding to execution of efficient, ethical and meaningful research and apply the results of the appraisal to good clinical practice.

The research problem

The formulation of a general topic into a specific research problem, thus, constitutes the first step in a scientific enquiry. There are two types of problem statements generally used. The first is a “research question” and the other is a “hypothesis”. A research question is the question that you are trying to answer when you do research on a topic or write a research report. It is narrowly constructed and addresses the important factors of the problem.

Three types of questions are identified in research, questions of description; questions about relationships between phenomena; and questions of comparison.[1]

Who? What? Where? and When? form the essential parts of questions of description, where usually certain descriptions of phenomenon are dealt with.[1] Examples of the types of studies which use descriptive questions would include factors responsible for dental practitioners’ preference of antibiotics and inclusion of metronidazole in the treatment of periodontal abscess or attitudes of pregnant women toward their oral hygiene during pregnancy or the prevalence of periodontitis among smokers in a given population.

Questions of relationships ask how phenomena are related.[1] Examples of such studies would include, studies pertaining to periodontal medicine (stress and periodontitis, adverse pregnancy outcomes and periodontitis, etc), studies pertaining to various risk factors and periodontal disease, e.g., smoking and periodontitis.

Questions of comparison attempt to demonstrate cause and effect between different therapies or interventions. For example, if we give one individual (or group of individuals) a particular treatment, does that improve their outcome in comparison to an individual (or group of individuals) who did not receive that treatment?[1] The PICO question is a gold standard in all studies related to clinical comparative studies. The parts of a well-built research question (PICO question) includes Patient/population/problem (P), Intervention (I), Comparison (C), and Outcome (O).[2] For example in the PICO question “In angular defects, does DFDBA compared to FDBA, have better bone fill?”, “angular defects” refer to the patient or problem, “DFDBA” the intervention, “FDBA” the comparison and “bone fill” the outcome.

Hypothesis is defined as a “supposition arrived at from an observation or reflection. A hypothesis is a statement that can be proved or disproved. If a typically framed research question would be: “Is chlorhexidine (CHX) chip in conjunction with scaling and root planing (SRP) more effective in periodontal therapy than SRP alone?” then, a hypothesis would state: “The use of SRP and CHX chip is efficacious in the reduction of probing depth when compared to SRP alone.”

There are two types of hypothesis:

Null hypothesis (H0): It states that there is no difference in the population or the sample regarding any research question and the difference seen is purely accidental or may arise due to bias in sampling. The investigators seek to accept or reject the null hypothesis as the outcome of research. For example, the adjunctive use of a CHX chip along with SRP does not have a beneficial effect in the reduction of probing depth compared to SRP alone

Alternative hypothesis (HA): It states that there exists a difference among the groups. An alternative hypothesis is sometimes also referred to as research hypothesis. Example of an alternative hypothesis would be: There would be a difference in probing depth reduction among the two groups (CHX chip along with SRP vs. SRP alone). An alternative hypothesis can be directional or non-directional in nature.

Directional hypothesis confers magnitude on the effect of the independent variable. A directional hypothesis would state: “Adjunctive use of CHX chip along with SRP is effective in reduction of probing depth when compared to SRP alone, atleast by 1 mm.” A non-directional version of the same hypothesis might state: “Adjunctive use of CHX chip along with SRP has an effect on probing depth when compared to SRP alone. Both hypotheses convey that there is an effect on probing depth but the directional hypothesis makes it clear that the effect is to decrease the probing depth.”

After the research question and hypothesis is framed, it is important to design the type of study to be undertaken to answer the research question with minimal bias.

There are two basic approaches to research, i.e., quantitative approach and the qualitative approach. Quantitative research is based on the measurement of quantity or amount. It is applicable to phenomena that can be expressed in terms of quantity. Quantitative studies aim to test a hypothesis and the results are given in numbers or proportions. An example of this kind of research would be to estimate the prevalence of gingivitis in a group of pregnant patients or the reduction of probing depths after the use of a regenerative surgical procedure. Qualitative approach to research is concerned with subjective assessment of attitudes, opinions, and behavior. The results are described in words rather than numbers. Evaluating the awareness and perceptions of pregnant women about proper oral hygiene during pregnancy would be an example of qualitative research.

Types of study designs

Clinical research can be either experimental or observational [Figure 1]. In observational studies, the researcher observes patients at a point in time (cross-sectional studies) or over a period of time (longitudinal studies) whereas in experimental studies, the intervention is under the control of the researcher [Figure 2]. Prospective studies are those which go forward in time (following up and gathering new data) whereas retrospective are those that go back in time (e.g. based on data/records that already exist [Figure 2].

Figure 1

Types of study designs

Figure 2

Basic concepts of research design

Prospective studies are carried out forward in time in order to examine the etiology of disease (observational study) or to assess the effectiveness of an intervention (experimental study). Retrospective studies collect past exposure information on participants through interview or recorded information. Observational studies may be either prospective or retrospective. Experimental studies are always prospective.

Cross-sectional studies give a snapshot of the outcome of interest in a population at any given time and are often used to look at prevalence. Risk factor or exposure of interest and the outcome of interest are measured at the same point in time. E.g: Salivary cytokine levels in subjects with chronic periodontitis and in periodontally healthy individuals: A cross-sectional study.[3]

Longitudinal studies are used to examine changes in health status over time, including incidence rates of disease outcomes. Risk factor or exposure of interest and the outcome of interest are measured at different points in time. If it is to be observed how many pregnant women with poor periodontal health develop adverse pregnancy outcomes, then a longitudinal study would be designed where pregnant patients with periodontitis are recruited and then followed up to their period of delivery and the adverse events noted. In such a situation, the exposure or risk factor is the poor periodontal health of the pregnant women and the adverse pregnancy events constitute the outcomes. These studies often give an incidence of the disease or outcome. e.g.: Periodontal infection and preterm birth: Results of a prospective study.[4]

Study designs can be broadly categorized as [Figure 1]:

Observational: Observational studies are used to monitor or describe the health status of a population. They are referred to as observational as the investigators merely observe what is happening rather than attempting to intervene in any way.[5] The design does not involve a specific intervention other than normal standard care. Various observational studies include:

Correlational study: These studies intend to explore a correlation between two variables. A correlation would not necessarily mean causation. Correlational studies in periodontology would constitute those which explore on the correlation between coronary heart disease and chronic periodontitis or obesity and periodontal disease etc. It would also include studies investigating relationships between socio economic status, smoking, various periodontopathogens, inflammatory markers, and periodontal disease.

Case reports and case series: Study of a single individual (case report) or small group (case series) is descriptive studies in which the possibility of an association between an observed effect and a specific environmental exposure is based on detailed clinical evaluations and histories of the individual(s). Because they are reports of cases and use no control groups with which to compare outcomes, they have little statistical validity. E.g.: Periodontal findings in a patient with Mauriac syndrome: A case report.[6] Labial piercing resulting in gingival recession – A case series.[7]

Cross sectional study [Figures 1 and 2]: These studies provide a snapshot picture of a community at a point in time, and do not involve following a group of individuals over time, e.g. prevalence and risk of gingival overgrowth in patients treated with diltiazem or verapamil.[8] The limitation of a cross sectional study is that the temporal sequence between the exposure and disease cannot be inferred. It is very difficult to predict which came first – the exposure or the disease.

Case-control study: (Case-referent, retrospective or trohoc (cohort spelt backwards) study). Typically examines multiple exposures in relation to a disease; subjects are defined as cases and controls, and exposure histories are compared [Figure 1 and 2]. A case control study starts with patients who already have the outcome and looks backwards to possible exposures. For example in such a study, women with adverse pregnancy outcomes are recruited as cases and a group of women without any adverse outcomes constitute the controls. Both these groups are retrospectively compared for their exposure histories in an attempt to identify possible exposures which predispose the group of interest to adverse pregnancy outcomes. E.g.: Association between periodontitis and low birth weight: A case–control study.[9] Limitations of a case control study are bias (because of the retrospective nature) and the difficulty to establish the correct temporal relationship between the exposure and disease.

Cohort study [Figures 1 and 2]: These studies take a large population who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare them for outcomes with a similar group that has not been affected by the treatment or exposure being studied. A cohort study starts with the exposure and follows patients forward to an outcome. E.g.: Periodontal disease as a risk factor for adverse pregnancy outcomes: A prospective cohort study.[4] Cohort studies can be either a prospective cohort where participants are grouped on the basis of past or current exposure and are followed into the future in order to observe the outcomes of interest or a retrospective cohort study, where both the exposures and outcomes have already occurred when the study begins. The latter, examines only prior outcomes and not future ones.

Ecological study: It examines the rates of disease in relation to a factor described on a population level. The units of analysis are populations or groups of people rather than individuals.

Systematic Reviews: Usually focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify studies with sound methodology. The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question. E.g: A systematic review of guided tissue regeneration for periodontal infrabony defects.[10]

Whenever possible, the results of a systematic review are statistically pooled together to generate a Meta-analysis. It is not necessary that every systematic review should culminate in a meta-analysis. Sometimes it may not be possible. A meta-analysis can also be done independent of a systematic review. In such a case, a number of valid studies on a topic are identified and examined after which the results are combined using accepted statistical methodology to report the results as if it were one large study. E.g. Periodontal diseases and cardiovascular events: Meta-analysis of observational studies.[11]

Experimental: An experimental (or interventional) study is where something specific is done in the study – i.e., using a treatment, strategy, or other intervention, that is recorded and analyzed. Experimental studies are commonly classified by their objective – that is, by whether they investigate a measure that prevents disease occurrence or a measure that treats an existing condition. The former is known as a preventive or prophylactic trial, and the latter is known as a therapeutic or clinical trial. In this type of study, the investigator assigns individuals to two or more groups that either receive or do not receive the preventive or therapeutic agent.

Randomized, controlled clinical trials (RCTs) are carefully planned projects that introduce a treatment or exposure to study its effect on real patients. They include methodologies that reduce the potential for bias (randomization and blinding) and that allow for comparison between intervention groups and control groups (no intervention). A randomized controlled trial is an experiment and can provide sound evidence of cause and effect. A RCT randomly assigns the exposures and then follows patients forward to an outcome.

Randomization, also called random allocation, implies that the subjects are randomly (i.e., using a method based on chance) assigned the treatment groups. The advantage of randomization is that potential confounding factors will be approximately evenly distributed in the different intervention groups. For example, in a study of the effects of a therapeutic mouthwash in the treatment of patients with periodontitis in a large multiracial society, random allocation of the subjects to the mouthwash or control “treatments” would ensure that each ethnic group is approximately equally represented in both the groups.

Controls: An essential feature of a clinical trial is that it is comparative in nature. A clinical trial which includes a comparative group is called a controlled clinical trial. The reason for making the trial controlled is to ensure that, provided the composition of the treatment groups is similar, any conclusions drawn from the trial as to the effectiveness of the new treatment under consideration can be attributed solely to the administration of that treatment and not to any other factors.[12] In periodontal research, a number of controls may be chosen alone or in combination, namely:[13]

No treatment control

Minus active (Placebo) control: One that is identical to the test product or formulation without the active agent

Benchmark control: A product already in use by the public or a previously evaluated agent

Positive control: An accepted effective or most effective formulation available to date

Historical control: Individuals who have received the standard treatment in a previous trial so that the results of the current patients on the new treatment under investigation can be compared to these historical controls.[12]

Blinding

Blinding or masking is intended to limit the occurrence of conscious and unconscious bias in the conduct and interpretation of a clinical trial. The “double-blind” trial is one in which both the researcher and the patient do not know whether the patient is in the experimental group or the control group. In a “single-blind” trial the investigator and/or his staff are aware of the treatment but the subject is not, or vice versa. In an “open-label trial,” the identity of treatment is known to all. Blinding is maintained throughout the conduct of the trial, and unblinding is done usually prior to statistical analysis.

Experimental designs in periodontal literature usually refer to four kinds of trials:

Parallel-groups – each participant is randomly assigned to a group, and all the participants in the group receive (or do not receive) an intervention. For example, systemic lycopene administration in one group and placebo in the other group.

Crossover trials – the subject acts as his/her own control with the same subject being allocated for both treatments, receiving them at different times. The possibility of a “carryover” of the effect of the intervention provided in the first period into the second intervention period is an important concern in this study design. To minimize this, a treatment-free or placebo “washout period” is built-in between the intervention periods. E.g: The additional effect of a dentifrice on the instant efficacy of toothbrushing: A crossover study.[14]

Split-mouth – separate parts of the mouth of each participant (i.e., the left and right) are randomized to receive (or not receive) an intervention. A split mouth study design can be used mostly when two treatment modalities have to be compared, in the same patient who acts as his own control. For example, to test the efficacy of bone graft with open flap debridement (OFD) vs. OFD alone.

Factorial – each participant is randomly assigned to a group that receives a particular combination of interventions or non-interventions (i.e., group 1 receives therapy X and therapy Y, group 2 receives therapy X and placebo Y, group 3 receives placebo X and therapy Y, and group 4 receives placebo X and placebo Y). E.g: A comparison of the effectiveness of the Charters’, scrub, and roll methods of toothbrushing in removing plaque.[15]

Single centre vs. multicenter trials

A clinical trial conducted according to a single protocol but at more than one site, and therefore, carried out by more than one investigator is a multi-center trial. Several investigators come together to work on a single objective. The protocol of the study is same and is carried out in various centers; the investigators are trained at one center before they start the trial to eliminate the observational bias. Multicenter studies offer the potential for increased recruitment and generalizability from conduct of the trial in several regions of the country (or world) and also take the contributions of multiple investigators with complementary expertise.[16]

Pilot study

A pilot study is another kind of study which is to be done before the starting of a fully-fledged research project to check the feasibility or to improve the design of the research. It is not appropriate for case studies and is frequently carried out before large-scale quantitative research in an attempt to avoid time and money being wasted on an inadequately designed project. A pilot study is usually carried out on members of the relevant population, but not on those who will form part of the final sample. A pilot study can reveal deficiencies in the design of a proposed experiment or procedure and these can then be addressed before time and resources are expended on large scale studies.

The hierarchy of evidence

Armed with a bagful of research designs, it is essential to understand where we are heading toward. To put the findings of research into a clinically meaningful perspective, the researcher at the inception, needs to thoroughly understand the various levels and strengths of research designs, filter and choose the most appropriate evidence-based design suitable for answering certain perplexing clinical dilemmas.

Evidence-based dentistry has been defined as “the conscientious, expedient, and judicious use of current best evidence in making decisions about the care of individual patients.”[17] The principles and methods of evidence-based dentistry have established their roots well in the present era, where various decisions regarding patient care are based on choosing the “best” evidence from the “available” evidence. Understanding “strength of evidence” and the idea of a “research design hierarchy” is at the heart of evidence-based dentistry.[18] Evidence is graded based on its strength into different levels where systematic reviews and randomized controlled trials represent the highest levels of evidence, whereas case reports and expert opinion are the lowest. This “ladder of evidence” [Figure 3] was developed to a large extent for questions related to interventions or therapy.[19] For questions related to diagnosis, prognosis or causation, other study designs such as cohort studies or case-control studies will often be more appropriate. For these types of studies, it may be more appropriate to view it not as a hierarchy, but as categories of evidence, where the strongest design which is possible, practical and ethical should be used.[19]

Figure 3

Levels of evidence

CONCLUSION

A well-tailored research proposal always begins with a thorough understanding of all the information related to the topic of interest, defining the objectives, framing a good research question and structuring an appropriate study design for effective implementation of the study. In this article we have introduced the hierarchy of evidence, framing of a research question and hypothesis and also various research designs employed. It is very pivotal that the researcher is well versed with all the intricacies of research methodology such that the outcomes of research are more meaningful, ethical and evidence based. The next part of this review series will address these issues related to the actual implementation of research.