The trochlear physeal line angle: a novel method to assess coronal plane alignment of the paediatric distal humerus

Supracondylar humeral fractures account for 60% of elbow fractures in childhood, with cubitus varus being its most common complication. It can be avoided by accurate assessment of distal humeral alignment after fracture reduction, and the Baumann angle is the usual measurement for this purpose. However, several reports demonstrate an inconsistency of this angle, and our hypothesis is that reliability could be improved with angles drawn between the trochlear physeal line and the humeral longitudinal axis. The aim of the study was to compare intra- and interobserver reliability between the Baumann angle and angles drawn between the trochlear physeal line and (1) humeral longitudinal axis (X-angle), (2) humeral lateral cortex line (Y-angle), (3) humeral medial cortex line (Z-angle). Angle measurements were performed on 141 distal humerus' radiographs of children aged 3-10 years by five observers in a tertiary hospital. The measurements were compared for their intra- and interobserver agreement. All measurements showed good or excellent intraobserver agreement levels, with the highest for Z-angle [intraclass correlation coefficient (ICC) = 0.906] and the lowest for Baumann angle. There were also excellent interobserver agreement levels for all angles, the highest for Z-angle (ICC = 0.885). There are indications that Z-angle had higher agreement levels when compared to the others (ICC = 0.954; 0.936-0.969) in the group aged 7-10 years. The angle drawn between the humeral medial cortex and trochlear physeal line showed greater agreement than the other angles analysed, becoming a potentially useful tool for evaluation of children with supracondylar humeral fractures in daily practice.