Partial Avulsion Patterns in the Pediatric Humeral Medial Epicondyle

The anterior oblique bundle of the medial ulnar collateral ligament (UCL) inserts on the anteroinferior aspect of the humeral medial epicondyle, while the flexor pronator mass (FPM) originates superficial and proximal to the UCL. With valgus stress, these distinct footprints may produce injury patterns that affect only focal areas of the medial epicondyle. The proximal UCL can act on the medial epicondyle either in isolation or in conjunction with the FPM to form partial avulsion fracture patterns within the pediatric medial epicondyle, and the predominant pattern involves only the proximal UCL footprint. Case series; Level of evidence, 4. A retrospective review was performed of medial epicondyle injuries from a single tertiary care institution documented from 2012 to 2022. Inclusion criteria consisted of (1) patients <18 years of age with a diagnosis of medial epicondyle fracture, (2) availability of radiographs within 3 weeks of injury date, and (3) no history of acute medial epicondyle fracture. Data collection included demographics, imaging, mechanisms of injury, associated injuries, and procedural details if operative notes were available. Of the 1951 patients screened, 709 were diagnosed with any form of medial epicondyle fracture. Of these patients, 15.9% (n = 113; mean ± SD age, 11.15 ± 2.56 years; 70% male; 40% throwing mechanism) had radiographic evidence of fragmentation of the inferior aspect of the medial epicondyle, suggesting a partial avulsion fracture/injury. Seventy-eight patients (69%) were noted to have a small, well-corticated fragment off the inferior pole of the medial epicondyle on radiographs, defined as a "proximal UCL avulsion fracture." Magnetic resonance imaging in 24 patients in this group confirmed attachment of the UCL to the avulsed fragment. This configuration was also confirmed by 2 operative reports. Thirty-five patients had a well-defined cortical shell or an indistinct radio-opaque body off the inferomedial aspect of the medial epicondyle on radiographs, defined as a "cortical sleeve avulsion." Magnetic resonance imaging in 8 patients in this group demonstrated the UCL in addition to the FPM origin still attached to the avulsed cortical sleeve. This was confirmed by 2 operative reports. The 2 injury patterns seen in this study-proximal UCL avulsion fracture and cortical sleeve avulsion-represented 15.9% of the patients with a medial epicondyle fracture. The proximal UCL avulsion fracture pattern was the more common of the 2 injury patterns. Accurate identification of these injury patterns and their underlying pathology is likely to be important in ultimately understanding the natural history of these injuries and the outcomes of different treatment strategies.