The proximal 5 to 6 caudal vertebrae have complete arches and spinous processes. The remaining caudal vertebrae consist simply of vertebral body. The bone unit definition indicates which set of caudal elements is referred to. Only the anterior and posterior epiphyses on the body of the element are considered when coding whether the element is fused or un-fused. Loose un-fused anterior or posterior epiphyses unassociated with the rest of the element are coded as being of weathered condition and are assigned their appropriate BU. Epiphyses on the spinous and transverse processes are ignored in considering element fusion. When isolated un-fused epiphyses from the spinous or transverse processes are identified, they are coded as “undifferentiated” BU.

For the Fincastle Project, Bone Units 12-19 were added.