What causes mid back pain?

Many possibilities.: Common are: disk changes, smoking, sprains of ligaments, muscular strains and spasms, vitamin deficiencies, joint inflammation, altered walking or sitting patterns, lack of sleep, poor circulation in the spinal blood vessels, tumors, aortic aneurysm or dissection, trauma, and spinal deformities.

Sometimes a Concern: Mid back pain/thoracic pain can be due simply to musculoskeletal causes such as strain/sprain or could be a sign of more serious disease such as a thoracic/abdominal aortic aneurism or perhaps a kidney issue amongst other causes. It is best to be evaluated for any pain that does not go away over the course of a few days or becomes progressively worse.