11.2 DXI Inflammatory Arthritic Disorders

11.2 DXI Inflammatory Arthritic Disorders

The Inflammatory Signature: What Changes From Section 11.1

Where degenerative disease is a mechanical, "add bone, narrow unevenly" process, inflammatory arthropathies are immune-mediated processes that destroy bone and cartilage from within the joint, producing a fundamentally different radiographic vocabulary: erosions instead of osteophytes, osteopenia instead of sclerosis, and — depending on the specific disease — either symmetric or asymmetric distribution with characteristic soft-tissue and periosteal reactions. DXI stems in this category test whether you can (1) recognize that a pattern is inflammatory at all, then (2) split it correctly among rheumatoid arthritis, the spondyloarthropathies (ankylosing spondylitis/axial spondyloarthritis, psoriatic arthritis, reactive arthritis), because each has a distinct fingerprint that changes the differential diagnosis and case-management pathway.

Rheumatoid Arthritis: The Prototype Erosive Arthropathy

Rheumatoid arthritis (RA) is a symmetric, polyarticular, erosive synovitis. Its radiographic hallmarks:

• Symmetric, bilateral joint involvement, favoring the small joints of the hands and feet, wrists, and cervical spine (particularly C1-C2).
• Periarticular osteopenia — juxta-articular bone loss from hyperemia and disuse, visible before frank erosions appear.
• Marginal erosions at the "bare area" of the joint (the unprotected margin not covered by articular cartilage), first seen at the second and third MCP and PIP joints and the ulnar styloid.
• Uniform (concentric) joint space narrowing — the entire joint narrows together, unlike the compartment-specific narrowing of OA.
• Soft-tissue swelling around affected joints from synovial proliferation (pannus) and effusion.
• No osteophyte formation in early/moderate disease — a key negative finding that separates RA from OA even when both narrow the joint space.
• Late-stage deformities: ulnar deviation of the fingers, boutonnière and swan-neck deformities, subluxations, and in severe disease, bony ankylosis (especially the wrist and cervical spine).
• Cervical spine: atlantoaxial subluxation from erosion of the transverse ligament and dens, a finding with direct clinical importance for chiropractic case management because it changes manipulation safety decisions.

The MCP/PIP-predominant, DIP-sparing distribution is the direct mirror image of primary OA's DIP/PIP-predominant, MCP-sparing pattern — DXI writers pair these two conditions constantly because the "opposite distribution" test is easy to write and easy to miss under time pressure.

Ankylosing Spondylitis and Axial Spondyloarthritis: The Spine-Centered Pattern

Ankylosing spondylitis (AS), the prototype of axial spondyloarthritis, is defined radiographically by its assault on the axial skeleton, beginning at the sacroiliac (SI) joints and ascending the spine.

• Sacroiliitis is nearly always present and is usually bilateral and symmetric — this symmetry is the single most useful discriminator from the asymmetric sacroiliitis of psoriatic and reactive arthritis (see below).
• Vertebral body squaring — loss of the normal anterior concavity of the vertebral body on lateral view, from erosion of the anterior corners (Romanus lesions, seen acutely as the "shiny corner sign").
• Syndesmophytes — thin, vertical, marginal ossifications that bridge adjacent vertebral bodies by following the outer annulus fibrosus. Contrast this explicitly with the thick, horizontal, non-marginal osteophytes of spondylosis deformans and DISH.
• Progressive ascending ankylosis produces the "bamboo spine" appearance on AP radiograph in advanced disease.
• Ossification of spinal ligaments produces the "dagger sign" (ossified supraspinous ligament, seen on AP view as a single vertical radiodense line) and the "trolley-track sign" (ossification of both the supraspinous ligament and the facet joint capsules, producing three parallel vertical lines on AP view).
• Facet and costovertebral joint ankylosis and enthesitis at the pelvis (ischial tuberosities, iliac crests) are common associated findings.

Grading Sacroiliitis (Modified New York Criteria Concept)

DXI stems test whether you know that sacroiliitis is graded on a spectrum, not called present-or-absent:

A stem that describes "bilateral grade 3 sacroiliitis with vertebral squaring and thin marginal syndesmophytes" is unambiguously pointing to AS/axial spondyloarthritis, not psoriatic or reactive arthritis, because of the bilateral symmetry.

Psoriatic Arthritis: Asymmetric and Destructive with a Signature Deformity

Psoriatic arthritis (PsA) shares the spondyloarthropathy family with AS but diverges sharply in the peripheral joints:

• DIP joint predominance (in contrast to RA's MCP/PIP predominance), often with a "ray" pattern — all joints of a single digit affected together, correlating clinically with dactylitis ("sausage digit").
• Asymmetric distribution, both peripherally and at the sacroiliac joints (unilateral or asymmetric bilateral sacroiliitis, versus AS's symmetric pattern).
• "Pencil-in-cup" deformity — erosion tapers the proximal phalanx to a pencil point while the distal bone widens into a cup, from combined bone resorption and adjacent new bone formation.
• Fluffy, ill-defined periostitis and bony proliferation adjacent to erosions — bone destruction and bone formation occurring side by side, a mixed pattern not seen in RA.
• Spine involvement, when present, favors bulky, asymmetric, non-marginal paravertebral ossification, distinct from the thin marginal syndesmophytes of AS.

Reactive Arthritis: Asymmetric Lower-Extremity Oligoarthritis With Enthesitis

Reactive arthritis (ReA), classically triggered by GI or genitourinary infection, produces a pattern that overlaps with PsA and is often distinguished by history and distribution rather than a single pathognomonic sign:

• Asymmetric oligoarthritis of the lower extremities, especially the knees and ankles/feet.
• Enthesitis — inflammation at tendon/ligament insertions, producing fluffy periostitis at the calcaneus (Achilles tendon and plantar fascia insertions), sometimes called "lover's heel."
• Asymmetric sacroiliitis, similar in pattern to PsA and distinct from AS's symmetric involvement.
• Can produce erosive changes in the feet resembling PsA; the two are differentiated mainly by clinical context (psoriasis history and skin/nail findings favor PsA; preceding urethritis, conjunctivitis, or dysentery favor ReA).

Comparative Summary Table

DXI Test Strategy for Inflammatory Sets

When a DXI item presents an erosive-appearing joint, immediately check three things in order: (1) symmetry of peripheral joints, (2) symmetry of sacroiliitis if the SI joints are shown, and (3) whether new bone formation (periostitis, syndesmophytes) accompanies the erosion or not. Symmetric peripheral disease with pure erosion and no new bone points to RA. Symmetric sacroiliitis with thin vertical syndesmophytes points to AS. Asymmetric disease, DIP involvement, or mixed erosion-plus-fluffy-periostitis points to PsA or ReA, with the clinical vignette (psoriasis vs. preceding infection) making the final split.