Many forms of arthritis and related conditions that affect the joints, muscles and/or bones can cause problems like pain, stiffness and swelling in the back. While any part of the back can be affected, the lower back is the most common site of arthritis back pain, most likely because it bears more of the body's weight.

Several forms of arthritis – collectively referred to as the spondylarthropathies (meaning spinal arthritis)– primarily affect the spine. These include:

• Ankylosing spondylitis. Ankylosing spondylitis is a chronic inflammatory disease that primarily affects the spine, particularly the sacroiliac joints near the pelvis, and the hip joints. Ankylosing is a term meaning stiff or rigid and spondylitis means inflammation of the spine.

Enthesitis (inflammation of the place where ligaments and muscles attach to bones) accounts for much of the pain and stiffness of ankylosing spondylitis. This inflammation eventually can lead to bony fusion of the joints (where the fibrous ligaments transform to bone, and the joint permanently grows together).

Other joints can also develop synovitis (inflammation of the lining of the joint), with lower limb joints more commonly involved than upper-limb joints.

Read more about anklyosing spondylitis from the Arthritis Foundation.

Learn more about ankylosing spondylitis from the National Library of Medicine.

• Reactive arthritis. Reactive arthritis is a chronic form of arthritis that often occurs following an infection of the genital, urinary or gastrointestinal system. Features of reactive arthritis include inflammation of the joints, eyes and structures within the gastrointestinal or genitourinary tracts, such as intestines, kidneys or bladder.

Read about the cause of reactive arthritis

The National Institute of Arthritis and Musculoskeletal and Skin Diseases answers your questions about reactive arthritis.