11.3 DXI Metabolic and Crystalline Arthropathy

11.3 DXI Metabolic and Crystalline Arthropathy

A Third Radiographic Vocabulary: Deposition, Not Wear or Immune Attack

Metabolic and crystalline arthropathies form the third major DXI category, and they are recognizable because the pathology is driven by crystal or mineral deposition within and around the joint rather than mechanical wear (degenerative, Section 11.1) or immune-mediated synovitis (inflammatory, Section 11.2). The three conditions tested most on Part III are gout (monosodium urate crystals), calcium pyrophosphate deposition disease / CPPD ("pseudogout," calcium pyrophosphate dihydrate crystals), and hydroxyapatite deposition disease (HADD), also called calcific tendinitis (basic calcium phosphate/hydroxyapatite crystals). Each has a distinctive combination of crystal location, erosion morphology, and joint distribution that lets you separate it confidently from both degenerative and inflammatory disease.

Gout: The Overhanging Edge and the Tophus

Gout results from monosodium urate crystal deposition, most classically at the first metatarsophalangeal (MTP) joint (podagra), though any joint can be involved with chronicity, including the hands, wrists, elbows (olecranon bursa), and knees.

Key radiographic features of chronic tophaceous gout:

• Eccentric, well-marginated erosions with sclerotic borders — unlike the fuzzy, poorly defined erosions of RA, gouty erosions have a defined edge because the process is slow and allows time for a reactive bony rim to form.
• "Overhanging edge" (Martel sign) — the erosion's bony margin curls back over the soft-tissue tophus like a lip, a highly specific sign for gout when present.
• Relative preservation of joint space until late in the disease — because gout is primarily an extra-articular/juxta-articular process (tophus deposition adjacent to the joint) rather than a synovial-lining process that destroys cartilage early, unlike RA.
• Normal bone mineral density — gout does not cause the periarticular osteopenia seen in RA; bone density typically remains normal even adjacent to significant erosive change.
• Soft-tissue tophi — dense, sometimes eccentric soft-tissue masses adjacent to the joint, which may show internal calcification in long-standing disease and can markedly deform the digit.
• Asymmetric, "punched-out" appearance of erosions, often at a distance from the joint line rather than confined to the joint margin.

The four-part combination of preserved joint space + normal bone density + sclerotic "punched-out" eccentric erosions with overhanging edges + adjacent soft-tissue tophus is essentially pathognomonic and is the pattern DXI stems will build around when testing gout.

CPPD (Pseudogout): Chondrocalcinosis and the "Pseudo-" Patterns

Calcium pyrophosphate deposition disease (CPPD) deposits crystals within cartilage itself, producing a different visual signature than gout's peri-articular tophi:

• Chondrocalcinosis — linear or punctate calcification within fibrocartilage (the knee menisci, the triangular fibrocartilage complex/TFCC of the wrist, the pubic symphysis) and hyaline articular cartilage, running parallel to the subchondral bone. This is the single most important sign to recognize on a DXI image, because calcification within the substance of the cartilage — rather than in soft tissue — is what separates CPPD from both gout and simple degeneration.
• Atypical joint distribution for "osteoarthritis-like" change — CPPD can produce a degenerative-appearing arthropathy in joints that primary OA rarely affects, especially the radiocarpal joint of the wrist, the patellofemoral compartment in isolation, and the second and third MCP joints, sometimes with characteristic hook-like osteophytes at the MCP heads. Any "OA pattern" in one of these atypical locations should raise CPPD in your differential.
• CPPD can mimic multiple other categories depending on which tissue predominates clinically and radiographically: a pseudo-osteoarthritic pattern (most common), a pseudo-rheumatoid pattern (with symmetric involvement and some erosive change), and rarely a pseudo-neuropathic pattern with severe joint destruction resembling a Charcot joint.
• Acute CPPD crystal shedding produces the clinical syndrome of pseudogout — an acute, gout-mimicking monoarthritis, most often at the knee.

Hydroxyapatite Deposition Disease (HADD): Periarticular Cloud-Like Calcification

Hydroxyapatite deposition disease, also called calcific tendinitis, deposits basic calcium phosphate crystals within tendons rather than within cartilage or as juxta-articular tophi:

• Amorphous, cloud-like, homogeneous calcific deposits located within a tendon, most classically the supraspinatus tendon of the rotator cuff, but also seen at the gluteus medius/minimus (greater trochanter), the longus colli (anterior cervical soft tissue — "retropharyngeal calcific tendinitis," which can mimic a retropharyngeal abscess clinically and must be distinguished on imaging), and other tendon insertions.
• Deposits can be stable and asymptomatic for long periods, then acutely resorb, producing an intensely painful acute calcific periarthritis as the material extrudes into the surrounding bursa or soft tissue — a presentation that can mimic septic arthritis or gout clinically, making the deposit's characteristic cloud-like (rather than linear or punctate) morphology on imaging an important diagnostic anchor.
• No underlying joint erosion or cartilage destruction is typical of HADD in isolation, distinguishing it from the crystal arthropathies that directly damage the joint. The rare exception is Milwaukee shoulder syndrome, a severe, destructive form of HADD-associated arthropathy with joint destruction and instability, seen predominantly in older patients with rotator cuff deficiency.

Comparative Table: Deposition Disease Differentiators

Putting It Together for DXI Reasoning

When a DXI image shows calcification or an unusual erosive pattern, ask in sequence: Where is the calcium? (soft-tissue tophus vs. within cartilage vs. within a tendon), What does the erosion look like? (sclerotic with an overhanging edge vs. absent vs. atypical-OA-like), and What joint is it in? (first MTP favors gout; knee/wrist/pubic symphysis chondrocalcinosis favors CPPD; supraspinatus/greater trochanter/longus colli favors HADD). This three-question sequence resolves the large majority of metabolic/crystalline DXI stems without needing additional clinical history, though the vignette (acute podagra, chronic tophaceous deformity, acute shoulder pain with cloud-like calcification) will usually confirm the imaging impression.

Summary Reasoning Chain for the Full DXI Arthritic Category

Across all three sections of this chapter, the unifying DXI skill is the same describe → categorize → commit sequence: first decide whether the dominant process is mechanical (degenerative: osteophytes, sclerosis, preserved-until-late density, asymmetric wear pattern), immune-mediated (inflammatory: erosion, osteopenia or mixed proliferation, characteristic symmetry and joint predominance), or deposition-driven (metabolic/crystalline: calcification location and erosion morphology tied to a specific crystal). Practicing this sequence on image sets, rather than memorizing isolated facts, is what converts DXI arthritic items from guesswork into a reliably scorable category.