Fracture morphology and ossification process of the keel bone in modern laying hens based on radiographic imaging

Keel bone fractures (KBF) are one of the most important welfare problems in commercial laying hens. Despite extensive research on the matter, its etiology remains unclear. Studying fracture characteristics in radiographic images can aid in the understanding of the disorder. The aim of the current study was to provide detailed description of fracture characteristics and explore ossification in the keel bone. In this descriptive study, repeated cross-sectional sampling was performed in one commercial laying hen flock. The flock was visited at 11 time points from 17-57 weeks of age (WOA), radiographing 30 laying hens at each visit resulting in altogether 330 unique radiographs. Fracture characteristics and the keel bone's level of ossification were assessed in each radiograph. In total, 344 fractures were recorded, of which 71.5% were complete and 28.5% were incomplete. Of the complete fractures, 82.9% were recorded as transverse, and 15.9% as oblique. One comminuted and two butterfly fractures were recorded. The caudal third of the keel was the most common area for fractures. Fracture characteristics differed between the different regions of the keel bone; all incomplete fractures in the cranial third appeared on the ventral surface of the keel, whilst the majority of incomplete fractures on the caudal third appeared on the dorsal surface. This indicates that the underlying etiology might differ between the cranial and caudal part. Folding fractures were observed in 18.6% of all the fractures, and occurred in both cranial-, and caudal third of the keel, indicating possible underlying disorders of calcium metabolism. All hens at 32 WOA and older had a fully ossified keel, based on radiographic evaluation. Displacement and soft tissue swelling are common characteristics in fractures of traumatic origin. We found a high frequency of simple fractures, without these characteristics, indicating that non-traumatic causes may be of higher importance than conventional beliefs.