15.2 DXI Bone and Joint Infections

Why Infection Imaging Matters on DXI

Bone and joint infection is one of the few DXI topics where imaging timing is itself part of the diagnosis: a normal radiograph in the first days of infection does not rule out osteomyelitis or septic arthritis, and knowing which modality is reliable at which time point is a recurring exam theme. This section works through osteomyelitis, discitis/spondylodiscitis, and septic arthritis in the order their imaging findings actually evolve.

Osteomyelitis: Routes of Spread and Age-Dependent Patterns

The pediatric metaphyseal predilection is a mechanistic fact worth memorizing: the metaphyseal nutrient artery loops back on itself into sluggish sinusoidal venous channels just short of the growth plate, and this slow flow combined with relatively less efficient local phagocytic activity favors bacterial seeding at that exact location. The growth plate itself normally acts as a barrier to spread into the epiphysis and joint in children, except in joints where the metaphysis is intracapsular (hip, shoulder, elbow, ankle), where metaphyseal osteomyelitis can rupture directly into the joint and cause secondary septic arthritis.

Imaging Timeline of Acute Osteomyelitis

• Plain film lag: radiographs are typically normal for the first 7-10 days of infection. The earliest plain-film sign is deep soft-tissue swelling with loss of normal fat plane definition, followed by periosteal reaction, then permeative/moth-eaten lytic bone destruction by roughly 10-21 days.
• MRI is the most sensitive and specific modality, often positive within 24-48 hours: T1-hypointense marrow signal, T2/STIR-hyperintense marrow edema, and post-contrast enhancement; MRI also defines abscess extent, sinus tracts, and soft-tissue involvement better than any other modality.
• Bone scan (technetium-99m) is sensitive early (positive within 24-48 hours) but less specific than MRI; a three-phase bone scan helps differentiate cellulitis (increased flow/blood-pool activity without a focal delayed-phase abnormality) from osteomyelitis (focal increased uptake on all three phases, especially delayed).

Chronic Osteomyelitis Terminology

A Brodie abscess is subacute pyogenic osteomyelitis (most often Staphylococcus aureus), presenting as an oval lucent metaphyseal lesion surrounded by a sclerotic rim. On MRI, the penumbra sign — a smooth, mildly T1-hyperintense rim around the abscess cavity relative to the surrounding marrow edema, representing vascularized granulation tissue — is a fairly specific marker for a subacute abscess versus other lytic lesions, including tumor.

Discitis and Spondylodiscitis

In children, isolated "discitis" is a recognized primary entity because the pediatric intervertebral disc retains its own blood supply; it presents with irritability or refusal to bear weight/sit, disc space narrowing with only minimal endplate irregularity, and is often self-limited with antibiotics.

In adults, the disc is avascular, so infection almost always begins in the vertebral body endplate (via hematogenous spread, often through Batson's venous plexus, or direct arterial seeding) and secondarily crosses into the disc — hence the preferred adult term vertebral osteomyelitis/spondylodiscitis rather than isolated discitis.

Imaging progression: disc space narrowing → endplate erosion and destruction (irregularity, loss of the normal cortical line) → vertebral body height loss → paraspinal or epidural abscess formation.

MRI is the gold standard: T1-hypointense vertebral bodies and disc, T2/STIR-hyperintense disc and adjacent marrow edema, and disc/endplate enhancement after contrast, frequently accompanied by a rim-enhancing paraspinal or epidural abscess. This pattern is distinguished from degenerative Modic type 1 endplate change, which preserves normal disc morphology and lacks a discrete enhancing abscess collection.

Tuberculous Spondylitis (Pott Disease) — A Classic Distractor

• Typically begins in the anterior vertebral body, with relative preservation of the disc space early — unlike pyogenic infection, which destroys the disc early — because Mycobacterium tuberculosis lacks the proteolytic enzymes pyogenic organisms use to digest disc material.
• Produces large, often calcified paraspinal "cold" abscesses disproportionate to the degree of bone destruction visible.
• Skip lesions (noncontiguous vertebral involvement) are more common than in pyogenic infection.
• Progressive anterior vertebral body collapse produces the gibbus deformity (sharp, angular kyphosis).
• Predilection for the thoracolumbar spine.

Septic Arthritis: The Imaging Progression

Septic arthritis is an orthopedic emergency, especially in the pediatric hip, because cartilage-digesting enzymes from bacteria and the host inflammatory response can destroy articular cartilage within days.

• Earliest finding: joint effusion/capsular distension — in the hip, ultrasound is used to confirm an effusion, since distinguishing septic arthritis from transient synovitis by imaging alone is unreliable and often requires aspiration.
• Early plain-film finding: juxta-articular osteopenia and periarticular soft-tissue swelling.
• Rapid, uniform loss of joint space (over days to weeks) from cartilage destruction — this speed is the key differentiator from osteoarthritis (years) and even rheumatoid arthritis (months), and is a frequently tested discriminator.
• Subchondral erosions and marginal bone destruction follow.
• Untreated disease progresses to osseous ankylosis, gross joint destruction, or, in children, growth disturbance from physeal damage.

Common Pathogens by Host