There are several classic signs on this xray of ankylosing spondylitis. This is a seronegative spondyloarthropathy which results in fusion of the spine and sacroiliac joints. On this xray, the SI joints are fused and cannot be clearly delineated.
Ossification of the supraspinous and interspinous ligaments leads to a continuous vertical dense line known as the "dagger spine" appearance. This line is broken in the mid lumbar region, and this is very suspicious for a fracture. This is the lateral view, which confirms a fracture and also shows other classic signs of ankylosing spondylitis:
A fracture across the L3 vertebral body is shown, with a marked acute kyphosis at this level. The continuous ossification of the anterior longitudinal ligament running down the front of the vertebral column is broken. The rest of the vertebral column shows the "bamboo spine" sign due to flowing syndesmophyte formation (ossification of the edges of the intervertebral discs), and squared vertebral bodies.
This is a sagittal CT image which shows a 3-column fracture and the other changes of ankylosing spondylitis:
This fracture is known as a chalkstick or carrot stick fracture because of how brittle the spine becomes in ankylosing spondylitis. Fractures can occur with minimal trauma, and are usually at the cervicothoracic or thoracolumbar junctions.