Reviewing the Literature

Traditional, Systematic, Evidence-based

The literature review has been around for a long time, but now new terms (or are they just buzz words) abound. Good news! The goal today is not very different from the past, but the focus is profoundly different. Today's literature review holds real promise of continued improvement in our ability to use information and even improvements in the articles themselves. This can only lead to greater evidence, which will build our scientific base. In addition, the new rules precisely describe the basis for any conclusions, and sometimes even allow grouping of data from different studies.

Almost everyone carries the notion that they should read more professional journals. The obvious answer is a literature review—a synopsis or summary that will allow you to survey the topic. In the past this meant an author would identify a topic, read the available literature and write a review article. On the surface this appeared to be useful, but the deficiencies of the traditional review soon are apparent.

The traditional system has no constraints on the articles selected by the author. This coupled with the heterogeneity or failure to describe the methods used, often gave conflicting and contradictory reviews. The result was that a review of the literature often failed to achieve its goals and failed to give conclusions you could rely upon and use. Worst case scenario, the review potentially could be highly biased. There was no way the reader could know this.

Now an improved version of the literature review, the systematic review, is becoming the accepted norm. How is it different and what does this really mean? The traditional review is subjective and may use a carefully selected group of articles to develop a conclusion. The systematic review tells you the exact references and literature sources that were searched and the exact articles identified needed to qualify for the review. As with a research article, the systematic review should be replicable. This was virtually impossible with the traditional literature review.

You may have noticed that the Angle Orthodontist publishes very few traditional literature reviews because the traditional review does not produce data comparable to that of a systematic review. We do welcome good systematic reviews and here is an example of how the abstract of one looked in a recent paper in the Angle Orthodontist.1

OBJECTIVE: To assess the orthodontic treatment effects on unilateral posterior crossbite in the primary and early mixed dentition by systematically reviewing the literature.

MATERIALS AND METHODS: A literature search was performed by applying the Medline database (Entrez PubMed) and covering the period from January 1966 to October 2002. The inclusion criteria were primary and early mixed dentition with unilateral posterior crossbite, randomized controlled trials (RCT), prospective and retrospective studies with concurrent untreated as well as normal controls, and clinical trials comparing at least two treatment strategies without any untreated or normal group involved. Two reviewers extracted the data independently and also assessed the quality of the studies. The search strategy resulted in 1001 articles, and 12 met the inclusion criteria.

RESULTS: Two RCTs of early treatment of crossbite have been performed, and these two studies support grinding as treatment in the primary dentition. There is no scientific evidence available to show which of the treatment modalities, grinding, Quad-helix, expansion plates, or rapid maxillary expansion, is the most effective. Most of the studies have serious problems of lack of power because of small sample size, bias and confounding variables, lack of method error analysis, blinding in measurements and deficient or lack of statistical methods.

CONCLUSIONS: To obtain reliable scientific evidence, better-controlled RCTs with sufficient sample sizes are needed to determine which treatment is the most effective for early correction of unilateral posterior crossbite. Future studies should also include assessments of long-term stability as well as analysis of costs and side effects of the interventions.

What are some of the advantages of the systematic review over the traditional approach? A clear Objective is stated. No wandering through free associations.

The Materials and Methods state exactly what literature was searched and the parameters necessary for inclusion. Now we are beginning to compare apples to apples. The traditional approach gave the author free reign to compare selected studies and ignore the fact that the studies either did not state all the methods used or even used different approaches.

The Results of a rigorous systematic review may be disappointing because so few studies will qualify for inclusion. This is not the fault of the systematic review—it is the problem with our literature and the incompletely described or ambiguous way studies often are reported. The more you loosen the inclusion parameters of the systematic review, the more the systematic review will look like the traditional review and the less useful it will be in providing good evidence.

Better Conclusions will be forthcoming as our studies are better structured and better described. As we amass better studies, we will produce systematic reviews that will contribute to better evidence based treatment. To learn more about systematic reviews, enter the term, systematic review, in Google to get a glimpse of the future. Enter the same term in the search engine of your favorite orthodontic journal and see what has already happened.