Accessory bones of the elbow: Prevalence, localization and modified classification

Accessory bones in the region of the elbow are rare variants with high clinical significance as they may be confused with avulsion fractures. We investigated their prevalence and performed a statistical analysis to support their congenital origin. Their localization was mapped to show their exact site of occurrence. We evaluated anteroposterior and lateral X-ray images of 2413 elbows in a Central European population from which a group of accessory bony structures was selected. Their character was evaluated, and accessory bones were identified. We used logistic regression to evaluate the potential relationship between the occurrence of accessory bones, the age of patients, and the occurrence of calcar olecrani. The prevalence of accessory bones of the elbow in the sample was 0.77%. Our results did not show a statistically significant relationship with the occurrence of calcar olecrani or with the age of patients. The most common type was os subepicondylare mediale (type V) in 0.46%, which was located distal to the medial epicondyle of the humerus, followed by os subepicondylare laterale (type III; 0.21%), situated laterally to the lateral epicondyle of the humerus. In comparison with previous reports, os sesamoideum mediale (type IV) was located more distally. Our data suggest that congenital accessory bones are a rare entity. Knowledge of their exact localization should be considered during diagnosis of avulsion fractures and other unclear diagnoses including accessory bony structures in the elbow region.