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

Triplane fractures are rare enough that large homogeneous series to support management decisions are lacking. During initial evaluation, the addition of computed tomography (CT) to conventional X-rays (XR) does not always alter the patient's clinical course. Therefore, routine use of CT is controversial. This study aims to: (1) clarify quantitative relationships between articular displacement measured on XR versus CT and (2) identify whether metaphyseal displacement on the lateral XR predicts clinically relevant articular displacement on a CT scan.

A 10-year retrospective review of consecutive triplane fractures was performed at a level 1 pediatric trauma center. Maximum articular and metaphyseal displacement were recorded from XR and CT. Quantitative relationships between XR and CT measurements were compared among imaging modalities and radiographically operative versus nonoperative fractures.

Eighty-seven patients met the inclusion criteria. XR underestimated articular displacement by 229% in the sagittal plane (1 mm on XR vs 3.3 mm on CT; P < 0.05) and 17% in the coronal plane (2.3 mm on XR vs 2.7 mm on CT; P < 0.05). XR underestimated articular step-off by 184% in the coronal plane and 177% in the sagittal plane ( P < 0.05). CT measurements more often differentiated patients who did or did not undergo surgery at our institution. Metaphyseal displacement was significantly higher in patients with traditionally operative articular displacement (≥2.5 mm on CT) versus those with articular displacement below traditionally operative thresholds (2.4 vs 0.9 mm, P = 0.001). Sixty patients had metaphyseal displacement >1 mm on the lateral XR, of whom 56 had surgical-magnitude articular displacement (≥2.5 mm) on CT (positive predictive value = 94%).

Conventional radiographs underestimate the true articular displacement of triplane fractures. Surgical-magnitude articular step-off is rare, and the largest articular gap is usually visualized on the axial CT image. Metaphyseal displacement >1 mm, which is easily measured on a lateral XR, is strongly predictive of clinically relevant articular displacement on CT. This radiographic finding should prompt advanced imaging before proceeding with nonoperative management.

Level III.