Moving Forward by Looking Back: ‘Retrospective’ Clinical Studies

I have been asked to comment on ‘how to do a retrospective study. ’ A truly authoritative response to this charge is perhaps best sought in one or more reputable epidemiology texts. The present communication, therefore, can be seen as a few comments on research design culled from the literature and a few decades of experience with ‘retrospective’ clinical research. These comments may be of use to the neophyte investigator; however, they are more likely to find an audience among those who wish to read the literature with an eye toward deciding ‘what works and what doesn’t. ’ Current clinical practice recognizes a variety of mutually exclusive therapeutic alternatives, among which are two-stage growth modification treatment in children (functional appliances followed by fixed appliances), one-stage fixed-appliance therapy (extraction and nonextraction, in either adolescents or adults), and combined orthodontic/surgical treatment in adults. It is unlikely that they are as interchangeable as we might hope; however, experience shows that everything works well enough and often enough to support a clinical practice. Perhaps as a result, there is surprisingly little demand for evidence. Instead, it is common to opt for a single treatment (early, often, and without extraction? ) for application to all patients. In effect, the problem of choosing an individualized strategy is avoided; however, this approach has a hidden cost that is passed through to the patient in the form of regret—the difference between what the patient gets and what he/she could have got from the most appropriate treatment. Unfortunately, the orthodontist who would provide evidence-based treatment to individual patients is faced with a literature largely bereft of interpretable content. Many, if not most, clinical papers are either ‘case reports’ or purely anecdotal ‘opinion pieces’. A few more are little more than extended case reports—carefully filtered ‘explanatory’ studies designed to extol the virtues of the author’s favourite technique. The typical orthodontic clinical report, therefore, is popular, inoffensive, but largely incapable of supporting valid inference about clinical orthodontics. Indeed, when the sample size goes beyond a handful of highly filtered ‘cases’, the paper apparently ceases to be seen as clinical and, instead, becomes some sort of despised exercise in theory. Given this popular mindset, the recent call for ‘evidence based dentistry’ (EBD) is welcome, rational, and not a moment too soon. Not all agree, however. Surprisingly, EBD has generated widespread controversy and contumely. Many feel threatened; others are insulted. Normally, one might expect only the more entrepreneurial to rail against a call for evidence; however, the EBD movement has raised the hackles of a surprisingly broad spectrum of the dental and orthodontic world. Part of the problem comes from a tendency by those who prosecute prospective randomized trials to publish divisive, self-serving ‘hierarchies of research’ that serve to denigrate all other forms of investigation, including the various types of ‘retrospective’ research, both good and bad. The goal of the present communication is to argue for a middle ground.