Residency · Residency · Diagnostic Radiology

Arthritis Imaging: Inflammatory vs. Degenerative Patterns

Overview

Approach to Arthritis on Imaging

The key discriminating features when evaluating arthritis on imaging are distribution, the presence of erosions versus osteophytes, the pattern of joint space narrowing, bone density, and soft tissue findings. Inflammatory arthritis characteristically produces periarticular osteopenia, marginal erosions, uniform joint space narrowing, and soft tissue swelling. Degenerative arthritis produces subchondral sclerosis, osteophytes, non-uniform joint space narrowing, no osteopenia, and subchondral cysts.

Key Differentiating Features: Inflammatory vs. Degenerative

Distribution Patterns

The distribution of joint involvement is one of the most powerful diagnostic clues and should be assessed for symmetry versus asymmetry, small joint versus large joint predominance, upper extremity versus lower extremity involvement, and axial versus appendicular involvement.

Osteoarthritis (OA)

Hallmark Radiographic Findings

The hallmark findings of OA include joint space narrowing that is non-uniform and affects the weight-bearing or stress-bearing compartment, subchondral sclerosis with increased bone density along the articular surface, osteophytes as marginal bony projections, and subchondral cysts (geodes) appearing as well-defined lucencies with sclerotic margins.

Distribution

In the hands, OA affects the DIP joints (producing Heberden nodes), the PIP joints (producing Bouchard nodes), and the first CMC joint, while sparing the MCPs. In the knee, the medial compartment is predominantly affected, producing varus deformity, and the patellofemoral compartment is also commonly involved. In the hip, OA produces superior joint space narrowing with superolateral migration of the femoral head. In the spine, it manifests as facet joint arthropathy, disc space narrowing, osteophytes, and endplate sclerosis. In the foot, the first MTP joint is the most commonly affected.

Erosive OA

Erosive OA is an aggressive variant affecting the DIP and PIP joints that produces central erosions creating "gull-wing" or "saw-tooth" deformities. It may mimic inflammatory arthritis, but its distribution in the DIP and PIP joints is classic for OA rather than an inflammatory pattern.

Rheumatoid Arthritis (RA)

Hallmark Radiographic Findings

The hallmark findings of RA include periarticular osteopenia (the earliest finding), marginal erosions at the bare areas where synovium attaches to bone without cartilage coverage, uniform joint space narrowing with symmetric loss throughout the joint, and soft tissue swelling that is periarticular and fusiform in the hands. In late disease, subluxations and deformities develop, including ulnar deviation at the MCPs, swan-neck and boutonniere deformities, and hitchhiker thumb.

Distribution

RA involves joints symmetrically. In the hands, the MCP and PIP joints are affected while the DIPs are spared. In the wrists, there is pancompartmental involvement with scapholunate dissociation and ulnar translocation. In the feet, the MTP joints (especially the 5th) are affected. In the cervical spine, atlantoaxial subluxation (anterior atlantodental interval exceeding 3 mm in adults) and cranial settling (basilar invagination) may develop.

MRI and Ultrasound

MRI demonstrates synovitis (enhancing, thickened synovium), bone marrow edema (a pre-erosive change), erosions, and tenosynovitis. Ultrasound with power Doppler detects synovial hypertrophy and hyperemia and can identify erosions earlier than radiographs.

Psoriatic Arthritis (PsA)

Key Imaging Features

PsA has an asymmetric distribution, though it can also present symmetrically, mimicking RA. Unlike RA, PsA involves the DIP joints. It commonly shows a ray pattern where the entire digit is affected, producing dactylitis or a "sausage digit." The pencil-in-cup deformity describes whittling of the phalangeal tip within a widened base. Periostitis appears as fluffy periosteal new bone along the phalanges. Enthesitis, or inflammation at tendon and ligament insertions such as the Achilles tendon and plantar fascia, is characteristic. Bone proliferation alongside erosions distinguishes PsA from RA. In its most severe form, arthritis mutilans produces extensive bone resorption and telescoping digits (opera glass hand).

Axial Involvement

In the axial skeleton, PsA produces asymmetric sacroiliitis (unlike the symmetric pattern of ankylosing spondylitis), bulky and asymmetric paravertebral ossifications (syndesmophytes), and may show skip levels in the spine.

Ankylosing Spondylitis (AS)

Key Imaging Features

The earliest and most characteristic finding of AS is bilateral symmetric sacroiliitis, which progresses through erosions and sclerosis to eventual ankylosis of the SI joints. On MRI, active inflammation appears as bone marrow edema at the SI joints on STIR sequences, while structural changes (erosions and fat metaplasia) are visible on T1-weighted images. In the spine, thin, vertical, marginal bridging ossifications called syndesmophytes form and can progress to the "bamboo spine" with complete ankylosis. Romanus lesions are erosions at the anterior vertebral body corners that appear as "shiny corners" on MRI. Squaring of the vertebral bodies is another characteristic finding. Hip involvement produces symmetric joint space narrowing with axial migration (protrusio acetabuli). Enthesitis is common at the calcaneus and iliac crest.

Complications

The ankylosed spine is susceptible to chalk-stick fractures, which often occur at the cervicothoracic junction and carry a high risk for cord injury. These fractures may be occult on radiographs, requiring CT or MRI for detection. Cauda equina syndrome is a rare late complication. An Anderson lesion (discovertebral lesion) represents a pseudoarthrosis at the ankylosed disc space.

Crystal Arthropathies

Gout

Gout results from monosodium urate crystal deposition. The most classic location is the first MTP joint (podagra), with other common sites including the midfoot, ankle, knee, hand, wrist, and olecranon bursa. Radiographic findings include well-defined erosions with overhanging margins ("rat-bite" erosions with sclerotic borders), preserved joint space until late disease, preserved bone density, and tophi (soft tissue nodules that may calcify). Erosions may be both juxta-articular and intra-articular. Dual-energy CT (DECT) provides color-coded detection of monosodium urate deposits and is highly specific, identifying tophi in soft tissues, tendons, and bone. Ultrasound demonstrates the double-contour sign (urate crystals on the cartilage surface) and detects tophi.

Calcium Pyrophosphate Deposition Disease (CPPD)

CPPD results from calcium pyrophosphate crystal deposition and produces chondrocalcinosis, which is calcification of cartilage (both fibrocartilage and hyaline cartilage). Common sites of chondrocalcinosis include the triangular fibrocartilage of the wrist, knee menisci, symphysis pubis, and hyaline cartilage of the knee. Pyrophosphate arthropathy produces OA-like changes but in unusual joints: the knee shows patellofemoral and lateral compartment predominance (opposite to typical OA), the wrist shows radiocarpal joint involvement (SLAC wrist pattern), the MCPs (2nd and 3rd) develop hook-like osteophytes (unusual for OA), and the shoulder may develop a severe destructive form called Milwaukee shoulder. Crowned dens syndrome involves CPPD crystal deposition around the odontoid process causing acute neck pain, with calcification around the dens visible on CT.

Arthritis Distribution Pattern Summary

Hydroxyapatite Deposition Disease (HADD)

HADD most commonly presents as calcific tendinitis, usually of the supraspinatus tendon in the shoulder. It appears as amorphous cloud-like calcification within the tendon. During the acute resorptive phase, the calcification may appear ill-defined or migrating, accompanied by severe pain.

<image>A hand radiograph comparison panel showing three arthritis patterns. Panel 1: Osteoarthritis with joint space narrowing, osteophytes, and subchondral sclerosis at the DIP joints (Heberden nodes), PIP joints (Bouchard nodes), and first CMC joint, with sparing of the MCPs. Panel 2: Rheumatoid arthritis with marginal erosions and uniform joint space narrowing at the MCP and PIP joints with periarticular osteopenia, sparing the DIPs. Panel 3: Psoriatic arthritis with asymmetric DIP joint erosions, pencil-in-cup deformity, periostitis along the phalanges, and dactylitis (sausage digit). Each panel is labeled with arrows highlighting the key findings.</image>

<image>A pelvis radiograph and MRI comparison for sacroiliitis. Left panel: AP pelvis radiograph showing bilateral symmetric sacroiliitis with erosions, sclerosis, and joint space narrowing in a patient with ankylosing spondylitis. Right panel: Coronal STIR MRI of the sacroiliac joints showing bilateral bone marrow edema (bright signal) along both sides of the SI joints representing active sacroiliitis. An inset lateral spine radiograph shows the bamboo spine appearance with flowing syndesmophytes and squared vertebral bodies. Key findings are labeled.</image>

<image>A dual-energy CT (DECT) image of the foot in a patient with gout. The standard CT image shows erosions with overhanging margins at the first MTP joint and a soft tissue tophus. The DECT color-coded overlay highlights monosodium urate deposits in green (or user-specified color) within the tophi, along the articular surfaces, and in the soft tissues of the foot. An inset radiograph shows the classic rat-bite erosion with preserved joint space at the first MTP joint. Labels indicate tophi, erosions, and urate crystal deposits.</image>

Clinical Pearls

OA spares the MCPs and wrists, while RA spares the DIPs -- this simple distribution rule is the fastest way to differentiate them on hand radiographs. CPPD arthropathy mimics OA but in atypical locations (patellofemoral compartment, radiocarpal joint, MCPs), and when OA is found in unusual joints, CPPD should be considered. Gout preserves joint space and bone density until late stages, and the combination of well-defined erosions with overhanging margins and preserved joint space is characteristic. DECT has transformed gout diagnosis by directly identifying urate crystal deposits and is the most specific imaging tool for gout. In ankylosing spondylitis, the ankylosed spine is extremely vulnerable to fractures from even minor trauma, and the entire spine should be imaged with CT or MRI after trauma in these patients. Psoriatic arthritis is the great mimicker, capable of presenting as RA-like symmetric polyarthritis, OA-like DIP disease, axial spondyloarthropathy, or destructive arthritis mutilans; the combination of erosions and bone proliferation is the distinguishing feature.

References

• Resnick D. Diagnosis of Bone and Joint Disorders. 5th ed. Saunders, 2002
• Glazebrook KN, et al. "Identification of Intraarticular and Periarticular Uric Acid Crystals with Dual-Energy CT." Radiographics, 2011
• ACR Appropriateness Criteria: Chronic Joint Pain, 2017
• Rudwaleit M, et al. "The Assessment of SpondyloArthritis International Society (ASAS) Classification Criteria for Axial Spondyloarthritis." Annals of the Rheumatic Diseases, 2009

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