Cochrane systematic reviews in orthodontics: trends across updates

Systematic reviews (SR) are regularly updated to reflect new evidence. However, updates are time-consuming and costly, and therefore should ideally be informed by new high-quality research. The purpose of this study is to assess trends in the quantity, quality, and recency of evidence intervening updates of orthodontic SR.

SR relevant to orthodontics with at least two versions were identified from the Cochrane Database. The number, risk of bias, and year of publication of included trials were recorded for each update. Multivariate regression was conducted to assess factors affecting the risk of bias in trials, and the proportions within SR.

Forty-five SR inclusive of updates were included. The median number of trials was three per review and this increased across subsequent versions. Seven reviews (15.6%) included no evidence, and 40.74% of updates included no new evidence. Most of the primary research was considered high risk of bias (57.3%), although this was reduced marginally across updates. The proportion of studies considered low risk did not improve significantly between updates. There was no impact of publication year of clinical trials on the risk of bias (P = 0.349). However, average age of trials included in a systematic review significantly affected the proportion of low risk-of-bias studies (P = 0.039).

SR are frequently updated without including new evidence. New evidence that is included is commonly deemed to be at high risk of bias. Targeted strategies to improve the efficient use of resources and improve research quality should be considered.