The arthritides can be divided into Osteoarthritis, Inflammatory Arthritis, Connective Tissue Arthritis, Metabolic/Endocrine Arthritis and Infectious Arthritis.
Under the larger category of 'Inflammatory Arthitis' is Rheumatoid Arthritis and Rheumatoid Variants aka Seronegative Spondylarthropathies. The Seronegative Spondylarthropathies include Ankylosing Spondylitis, Reiter Syndrome, Psoriatic Arthritis, and Enteropathic Arthritis (UC, Crohn, and Whipple disease).
The prototype and most common of the seronegative spondylarthropathies is ankylosing spondylitis. Rheumatoid factor is negative (thus the name seronegative) but a high percentage of patients (up to 95%) possess histocompatibility antigen HLA-B27 (HLA-B27 in 25%-60% Psoriatic patients - usually those with spondylarthropathy - and 80% Reiter's patients).
Ankylosing spondylitis is a chronic, progressive inflammatory arthritis principally affecting the synovial joints of the spine and adjacent soft tissues as well as the sacroiliac joints; however, peripheral joints (ie hips, shoulders, and knees) may be involved. Extraarticular features include iritis, cardiac conduction defects, pulmonary fibrosis, and aortic incompetence. (Psoriatic extraarticular manifestations include skin disease which often predates arthropathy, soft tissue swelling in the digits, and nail changes including thickening/pitting/discoloration, a barometer of severity of the arthropathy. Reiter's extraarticular manifestations include urethritis/cervicitis and conjunctivitis).
AS is seen predominantly in men (7:1) at a young age (15-35 yo). (PA males = females, Reiter's males > females).
In AS, the sacroiliac joints are classically the site of initial involvement. First there is cortical unsharpness/loss of definition followed by erosions and joint widening. (These findings may be more prominent initially on the iliac side of the joint where the cartilage is thinner though eventually both sides are involved). Later in the process the synovial portions of the SI joints fuse. The hallmark of late disease is bilaterally symmetric fusion.
In AS, spine involvement classically follows SI abnormalities. It begins in the spine at the thoracolumbar and lumbosacral junctions and extends contiguously (RARELY are there skip areas as can be seen more with Reiter's and Psoriatic arthropathies. A caveat to this is that AS in females can skip more often). First there is osteitis at the peripheral corners of the vertebral bodies (if + reactive sclerosis, then 'shiny corners' appearance). The osteitis leads to a loss of the normal vertebral concavity giving it a characteristic squaring. Eventually syndesmophytes form IN the annulus fibrosis at the discovertebral junction (these AS syndesmophytes are thin/delicate vertical ossifications in contradistinction to the bulky nonmarginal osteophytes that arise from the vertebral body itself in Reiter's and Psoriatic arthropathies). End stage is ankylosis, a 'bamboo spine'. In a bamboo spine there is undulating ankylosis of vertebral bodies and fusion of apophyseal joints. At this stage, osteoporosis ensues secondary to limited mobility and disuse. This makes the fused osteoporotic spine vulnerable to fracture even from mild trauma. Fractures often extend through the disk space and the posterior elements. If undetected, a fracture could result in motion across this segment, the consequences of which could prove devastating.
In up to 50% of patients, the hip is involved in AS. Radiographically the appearance is of concentric narrowing, erosive and productive change, and protrusio acetabuli. As with the SI joints, involvement of the hips is often bilateral.
The images provided delineate a classic case of the SI joint involvement in ankylosing spondylitis. There are no spine findings... yet.