Visual Assessment of Rib Cartilage Mineralization in Thoracic Radiographs as an Indicator of Age in Juvenile Dogs of Various Breeds

Anecdotally, the onset of mineralization of rib cartilage, as visualized by radiographic studies, is assumed to occur at 3 months of age. Determining radiographically the exact day when rib cartilage mineralization begins in juvenile dogs could aid in age estimation of young dogs with unknown histories. This retrospective study aimed to establish the radiographic onset and progression of rib cartilage mineralization. A number of 1310 dogs across 132 different breeds were included, and 2463 laterolateral or orthogonal radiographs of newborn to 400-day-old dogs were evaluated. Rib cartilage mineralization was divided into three grades (I: no radiographic evidence of mineralization; II: partial mineralization [speckled or fine linear mineralization]; and III: complete rib cartilage mineralization). Dogs were divided into five breed groups (toy, small, medium, large, and giant) depending on the estimated final body weight. The earliest radiographically visible rib cartilage mineralization in all dogs was 71 days for Grade II and 132 days for Grade III. Between 132 and 168 days, all three grades of rib cartilage mineralization were present. After 223 days, only complete rib cartilage mineralization was observed. Statistical analysis showed that small-breed dogs had significantly earlier onset and complete rib cartilage mineralization than medium, large, and giant breeds (p < 0.02). Chondrodystrophic dogs of the toy, small, and medium-breed groups showed significantly earlier onset of costal cartilage mineralization than non-chondrodystrophic dogs of the same breed group (p < 0.001). The predominance of chondrodystrophic breeds among small dogs may explain their earlier rib cartilage mineralization.