There are 12 ribs in most people. The first seven are called "true ribs." They attach to the spine in the back and to the breast bone (sternum) in the front. There are actual joints where the first seven ribs attach to the spine.
The lower ribs attach to the back bone, but don't connect directly to the sternum. These are called "false ribs." The false ribs (numbers 8 - 12) don't form true joints and the ligaments are poorly developed. Ribs 8, 9, and 10 are connected to one another in the
front below the sternum by thick cartilage.
The bones in the spine are supported (in part) by the rib cage structure. However, this support is lacking in the false ribs, especially at the very bottom of the rib cage at T11, T12, and L1. The 11th and 12th ribs are called "floating ribs." These are very short
and don't attach in the front by cartilage to the other ribs.
A sudden, unexpected force in weak or osteoporotic bones can cause fractures. Injury is more likely in the smaller vertebrae without the supportive structure of the rib cage (T11, T12, and L1).