The most likely diagnosis is diffuse idiopathic skeletal hyperostosis (DISH), which is defined by the presence of flowing osteophytes involving the anterolateral aspect of the thoracic spine at four or more contiguous vertebrae with preservation of the intervertebral disk space and the absence of apophyseal joint or sacroiliac inflammatory changes such as erosions. DISH may occur with or without osteoarthritis or inflammatory arthritis and represents a separate finding of calcification and ossification of spinal ligaments and the regions where tendons and ligaments attach to bone (entheses). DISH is a noninflammatory condition of unknown cause that is common in the elderly population. Patients may be asymptomatic or may describe stiffness and reduced range of motion, particularly at the thoracic spine.

Ankylosing spondylitis is an inflammatory disorder that usually becomes symptomatic in adolescence and early adulthood. It is characterized by progressive morning stiffness and low back pain and typically becomes symptomatic in the lumbar spine rather than the thoracic spine early in the course. Radiographs of DISH and ankylosing spondylitis have similarities; however, ankylosing spondylitis demonstrates vertical bridging syndesmophytes rather than the flowing osteophytes that occur in DISH. Plain radiographs of ankylosing spondylitis also characteristically show changes of the sacroiliac joints that can include erosions, evidence of sclerosis, and widening, narrowing, or partial ankylosis.

Degenerative disk disease is thought to arise from age-related changes in proteoglycan content in the nucleus pulposus of the disk. Disks shrink as they become desiccated and more friable. Age-related changes also occur in the annulus fibrosus, which becomes more fibrotic, less elastic, and can shift its position. Vertebral body endplates adjacent to the disk develop sclerosis, and osteophyte formation occurs at the vertebral margins.

Psoriatic arthritis is an inflammatory disorder that can affect the spine as well as peripheral joints. When axial involvement is prominent, sacroiliitis and spondylitis can both be present; however, axial disease rarely presents in the absence of frank inflammatory arthritis of peripheral joints.