In a recent commentary in the Journal of Periodontology, Merchant and Josey (2016) had suggested directed acyclic graphs to better comprehend the partly conflicting results from randomized controlled trials (RCT) on diabetic control after periodontal treatment in diabetic patients. In particular the influence of obesity caught their attention.
As a matter of fact, a remarkable number of systematic reviews (whose varying quality have recently been reviewed in at least two further SRs of SRs) have shown that numerous small-scale, single-center, often poorly designed RCTs had shown that the marker for diabetic control, HbA1c, might be reduced by, say 0.4% 3 months after in essence non-surgical periodontal therapy. The only large-scale, multi-center trial (DPTT) by Engebretson et al. (2013) couldn’t confirm that, though, which sparked harsh criticism of a large number of our thought leaders. A professor in the Department of Epidemiology and Biostatistics at the University of South Carolina, Columbia, Dr. Anwar Merchant himself had written a letter to the editors of JAMA pointing first to the fact that most participants in the paper by Engebretson et al. were utterly obese. He had further noticed that, “[i]n RCTs conducted among mostly nonobese individuals, periodontal treatment has been shown to reduce systemic inflammation2,4 and improve glycemic control among those with type 2 diabetes.2 However, periodontal treatment has not been shown to affect glycemic control in RCTs conducted among predominantly obese individuals with type 2 diabetes.1,3”
Obesity is positively correlated with inflammatory markers in the blood and strongly related to insulin resistance and metabolic dysregulation mediated by chronic systemic inflammation.5 These findings, taken together with results from RCTs evaluating the effects of periodontal treatment, suggest that the lack of effect of periodontal treatment on glycemic control observed in the study by Engebretson et al may be attributed to the high level of obesity in the study population. Therefore, the findings may be generalizable only to predominantly obese populations with type 2 diabetes.
What do statistical significance and clinical relevance actually mean? To be clear, that the American Journal of Periodontology has decided to accept another educational paper for clarifying common misconceptions is of course a good idea. The journal is mainly read by practicing periodontists who may not be so much familiar with statistics in general. What Chambrone and Armitage actually deliver (the accepted paper has just gone online in JOP) is, however, disappointing. Most scientists will probably stop reading after the first sentence of the second paragraph. Kannste vergessen.
It has been unmistakably demonstrated that statistically differences (e.g., P-value < 0.05) are more likely to be detected with large sample sizes compared to small ones.1-3,11