ORIGINAL ARTICLE MR imaging of capitellar ossification: a study in children of different ages Lauren M. Fader & Tal Laor & Emily A. Eismann & Roger Cornwall & Kevin J. Little Received: 20 November 2013 /Revised: 17 January 2014 /Accepted: 5 February 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Background The capitellar ossification center is used routine- ly to evaluate elbow alignment on radiography. However, whether capitellar ossification is central and concentric to support this practice is unknown. Objective To define the pattern of capitellar ossification at different ages of childhood. Materials and methods This HIPAA-compliant study was IRB approved. MR imaging examinations from 81 children (ages 1–13 years, at least 3 boys and 3 girls in each age group) were included. We determined the center points of the ossified capitellum and the cartilaginous capitellum on the sagittal and coronal sequences that best showed differentiation between cartilage and bone. Percentage offset of the center of the ossified capitellum from the center of the cartilaginous capitellum was calculated in anterior–posterior, proximal–dis- tal and medial–lateral dimensions, and compared across age groups and between genders. Linear regressions were used to ascertain the effect of age on percentage offset for all patients and for each gender. Results Capitellar ossification begins eccentrically with sagittal anterior proximal offset and coronal medial offset. With age, ossification proceeds posteriorly, distally and laterally. Percent- age offset gradually diminishes with age. The ossified capitellum centralizes in the sagittal plane by 12–13 years. In the coronal plane, the capitellum ossifies medially beyond the proximal radioulnar joint and remains eccentric at 12–13 years. Centralization in boys lags in the anterior–posterior dimension. Conclusion Capitellar ossification is an eccentric process, with lag in anterior–posterior centralization in boys. Medial offset persists at 12–13 years. Recognition of this eccentric ossification may allow for more accurate assessment of elbow alignment on radiographs, especially in younger children. Keywords Capitellum . Elbow . Ossification . Magnetic resonance imaging . Children . Skeletal maturation Introduction The structural anatomy of the elbow joint is complex, with three distinct articulations between the distal humerus, prox- imal radius and proximal ulna. These joints, coupled with the large amount of unossified cartilage in a growing child, and the unique pattern of skeletal epiphyseal maturation can make injury assessment challenging. There are six secondary cen- ters of ossification in the elbow of a child, which appear on radiographs in relatively predictable patterns but at different ages during skeletal development [1, 2], and may be asym- metrical within the same child [3]. These secondary centers generally appear earlier in girls [4]. The capitellum is the first center to begin to ossify, often by 6 months of age [1, 2, 5], and is commonly used as a landmark for normal articulation of the pediatric elbow [6]. Traditionally, it is considered that a line drawn along the anterior humeral cortex on a lateral radio- graph, when extended distally, should travel through the middle third of the ossified capitellum [7]. As such, since most supracondylar fractures of the distal humerus result in posterior displacement of the distal fracture fragment, this line would then pass through or be entirely anterior to the anterior- third of the capitellum. Similarly, it is described that a line L. M. Fader University of Cincinnati College of Medicine, Cincinnati, OH, USA T. Laor (*) Department of Radiology, Cincinnati Children’ s Hospital Medical Center, 3333 Burnet Ave., Cincinnati, OH 45229, USA e-mail: tal.laor@cchmc.org E. A. Eismann : R. Cornwall : K. J. Little Division of Orthopaedic Surgery, Cincinnati Children’ s Hospital Medical Center, Cincinnati, OH, USA Pediatr Radiol DOI 10.1007/s00247-014-2921-4