Probable bilateral bifid fifth ribs in a Joseon-period Korean skeleton: anatomical documentation using three-dimensional surface models.

BACKGROUND: A bifid rib is a congenital anatomical variation in which a rib divides into two branches near its distal or sternal end. Bilateral involvement at the same rib level is rare, and osteological documentation in archaeological skeletal remains is scarce because the distal ends of ribs are often damaged postmortem. CASE REPORT: We examined the skeleton of an adult male from the Joseon period recovered from Anseong-si, Gyeonggi Province, Republic of Korea. Serial arrangement of the preserved ribs and comparison with the adjacent fourth and sixth ribs supported the identification of the affected elements as the right and left fifth ribs. Both ribs displayed a single head and body, with a distal/sternal bifurcation forming deep, U-shaped clefts. Most terminal surfaces, where costal cartilage would have attached, were damaged, and radiographic or computed tomographic examination was not performed. Three-dimensional surface models were used to document the external morphology and illustrate the approximate anatomical positions of the affected ribs. CONCLUSIONS: These findings support a probable bilateral bifid morphology of the fifth ribs, providing a rare archaeological example of same-level bilateral costal variation. Although postmortem damage precluded the assessment of branch symmetry, the preserved features indicate a deep, U-shaped bifurcation. This case broadens the osteological record of bifid ribs in archaeological human remains and highlights the importance of explicitly addressing preservation-related limitations when evaluating congenital rib variations in such remains.