Causes
There are many diseases and injuries that can cause back pain. If back pain comes on suddenly, it's usually a sign of a muscle tear, sprained ligament, or ruptured disc. The spine contains intervertebral discs, a kind of cushion made of cartilage that fits between two vertebrae (bone segments). A slipped disc occurs when pressure on a disc causes it to bulge or rupture, pushing cartilage sideways. The cartilage presses on the spinal cord nerves, causing intense pain.
Pregnancy can also cause back pain, because of the extra body weight, changes in muscle and ligament firmness, and because the baby's head can compress the mother's spinal nerves. Overweight people are also more prone to back pain due to poorer posture and the extra strain that increased body weight places on discs and muscle.
In older people, the vertebrae themselves may spontaneously fracture or compress in size. This can be the result of conditions that affect the bone, the most common being osteoporosis. That's why these conditions can make people shorter with age. Sometimes these fractures cause no pain.
Because of the wiring of the nervous system, conditions in other parts of the body can cause back pain. Pyelonephritis, the most common kidney disease, causes back pain as well as fever. Gallbladder disease may also cause back pain. When pain is felt at a place in the body different from the injured or diseased part where the pain is expected to be, it is called "referred pain."
Sciatica is not a disease but a pain that follows the path of the sciatic nerve into the lower back. It can be triggered by a slipped disc, muscle spasm in the buttocks or, more rarely, by a degenerative condition like osteoarthritis.
Young people, especially men, who start to develop recurring back pain in their 20s, 30s, or 40s may have ankylosing spondylitis or a similar disease of the connective tissue. Ankylosing spondylitis causes the vertebrae to slowly fuse together, causing pain and stiffness. Fortunately, ankylosing spondylitis is quite uncommon, responds well to modern treatments, and is less likely to cause disability than in the past.