Metaphyseal Fracture Displacement is Predictive of Intra-Articular Displacement in Adolescent Triplane Ankle Fractures

CATEGORY: Trauma; Ankle INTRODUCTION/PURPOSE: Triplane ankle fractures are a commonly encountered pediatric injury with potential long-term sequelae if not appropriately managed. Computed tomography (CT) imaging is frequently obtained in the workup of a triplane fracture, however the radiation and cost associated with CT imaging is not benign. We aim to determine the utility of a novel and simple plain radiographic measurement as a predictor of intra-articular displacement in adolescent triplane ankle fractures. METHODS: An IRB approved, retrospective review of all adolescent triplane ankle fractures from 2003-2013 was performed at a single level 1 pediatric trauma center. Plain radiographs (XR) and CT imaging obtained at initial presentation were independently reviewed by three orthopaedic surgeons. Maximum articular displacement and step off were measured using digital calipers on AP, mortise and lateral XRs as well as on axial, coronal, and sagittal CT imaging. In addition, maximal metaphyseal fracture displacement was recorded on lateral XR. Articular displacement and step off were compared between the two imaging modalities using student's t-test. Pearson and Spearman coefficients were used to identify correlations between XR and CT measurements. RESULTS: 87 triplane ankle fractures were identified to meet inclusion criteria with a mean age of 12.9 years old (10-16 years old). XR was found to underestimate fracture displacement by 233% in the sagittal plane (1mm vs.3.3mm on sagittal CT; p<0.05). XR underestimated displacement by 24% in the coronal plane (2.3mm on AP XR vs. 2.9mm on coronal CT; p<0.05), and articular step off by 184% in the coronal plane and 177% in the sagittal plane (p<0.05). Axial CT demonstrated a significantly greater amount of articular displacement (4.2mm, range 0-19mm) when compared to coronal (2.9mm, range 0-18.7mm) and sagittal (3.3mm, range 0- 11mm) cuts (p<0.05). 65 patients demonstrated metaphyseal fracture displacement >=1mm on lateral XR, of whom 61 demonstrated articular displacement of >2.5mm on CT (positive predictive value of 94%). CONCLUSION: Plain radiographs appear to underestimate the articular displacement of triplane fractures. Maximal metaphyseal fracture displacement is easily measured on XR and appears to correlate with articular displacement. Maximal metaphyseal fracture displacement >= 1mm is strongly predictive of articular displacement above the commonly accepted threshold for operative intervention. This finding should raise suspicion for significant articular displacement that may not be appreciated on XR and should prompt consideration of a CT before a decision is made on definitive treatment.