Basics of Arthritis Imaging

How are joints classified anatomically/histologically?

There are two major types of joint classification systems. One is based on type of motion, the other on histology. Based on motion type, the joints are classified into synarthroses, amphiarthroses and diarthroses. Based on histology, the joints are classified into:

  • Fibrous joints – sutures, syndesmosis, gomphoses
    • Examples: skull sutures, interosseous membranes
  • Cartilagenous – symphysis, synchondrosis
    • Examples: symphysis pubis, spinal disks
  • Synovial – diarthroses
    • Examples: extremity joints, facet joints, sternoclavicular and inferior portions of sacroiliac joints.

How can arthritis be classified?

There are numerous ways to classify arthritis, and each individual entity can be classified even further. In an urgent care situation, the most important task is to differentiate infectious arthritis from all the other possibilities. Once infection is ruled out, the other arthritides can be further evaluated. Among non-infectious etiologies, one major division is degenerative arthritis (osteoarthritis) and “all others.” “All others” include rheumatoid-type and other inflammatory cytokines associated arthritides (lupus, psoriasis, reactive arthritis, inflammatory bowel disease associated arthritis, ankylosing spondylitis, etc).


How to diagnose septic arthritis on imaging?

Without knowing clinical history, it is next to impossible to definitively rule out infectious arthritis based on imaging findings alone in the majority of real life situations. Obviously, a completely normal joint MRI with no joint effusion, no erosions, no bone marrow edema, no abnormal enhancement, and no periosteal reaction confidently demonstrates “no evidence of septic joint.” But, in the majority of real life situations, there are at least some findings that can overlap with findings in septic joints. For example, how much of synovial enhancement is normal and how much is too much? More on this later. To complicate things further, it is possible for low grade indolent infections to present without obvious signs of flagrant septic arthritis both clinically and on imaging.

There are some signs that are helpful in narrowing differential diagnosis when dealing with unknown type of arthritis. A radiograph that demonstrates an isolated joint affected by joint space narrowing (cartilage destruction), or joint space widening (effusion), ill defined erosions, adjacent periosteal reaction and soft tissue swelling is very suspicious for an infectious process. Similar findings can be seen on MRI, which may also reveal irregular synovial enhancement, patchy bone marrow edema (on both T1 and T2 weighted images), restricted diffusion on DWI/ADC images, and adjacent peripherally enhancing abscesses. Once in a while, a soft tissue tract that courses from skin defect into the joint is revealed.

Septic Arthritis
Septic Arthritis

What are classic imaging signs of degenerative arthritis?

  • Joint space narrowing
  • Osteophytosis
  • Subcortical cysts
  • Subcortical sclerosis

What are different types of inflammatory (synovial, autoimmune) arthritis?

  • Rheumatoid Arthritis
  • Juvenile Inflammatory arthritis
  • Ankylosing Spondylitis
  • Lupus Arthritis
  • Inflammatory Bowel Disease Associated Arthritis
  • Reactive Arthritis
  • Psoriatic Arthritis

What are some crystal deposition arthropathies?

  • Calcium Pyrophosphate Deposition Disease
  • Hydroxyapatite Deposition Disease
  • Gout (uric acid)