The Birmingham Mandible and Mid-face (BruMM) rules: interim data analysis.

Introduction: Clinical predictor rules are useful heuristics that can inspire confidence in clinicians on the front line to make decisions that are safe and reproducible. Rules such as the Ottawa Ankle Rules can also reduce the number of unnecessary radiographs taken, reducing radiation exposure and cost, as well as improving quality of care.

Methods: A previous Delphi study delineated 11 variables associated with an increased likelihood of finding a mandibular fracture and 14 variables associated with an increased likelihood of finding a zygomatic fracture on plain film radiographs. In the current study, clinicians suspecting a mandibular and/or zygomatic fracture were invited to complete a proforma identifying any of these variables in advance of requesting plain film radiograph(s). An interim analysis was conducted with predictors being cross-tabulated against relevant outcomes using: sensitivity, specificity, Jaccard index, odds ratio (OR) and Fisher's exact probability.

Results: During the period January to October 2022 inclusive, 69 records were inputted into REDCap, of which 20/69 demonstrated a fracture. Fisher's exact test produced several significant results including malocclusion (p<0.0001, OR 31.99), presence of a new open bite (p<0.001, OR undefined) and subconjunctival haemorrhage (p<0.05, OR undefined).

Conclusions: Data collection is continuing and initial interim analysis would suggest a sample size of n=252 will be required, assuming a negative scan rate of 55%, to achieve a specificity of 0.90 within ±0.05. We aim to present finalised data in 2025.