13.2 Infectious and Miscellaneous Skeletal Conditions

13.2 Infectious and Miscellaneous Skeletal Conditions

Quick Answer: Bone and joint infections are high-yield Part II content because imaging findings evolve predictably over time — normal plain films in the first week do not exclude osteomyelitis, MRI is positive within 24–48 hours, and the pattern of joint destruction distinguishes septic arthritis from degenerative disease. Knowing the route of spread, age-dependent patterns, and granulomatous versus pyogenic differences resolves most exam vignettes.

Why Infection Imaging Matters

Infectious skeletal conditions account for a significant share of Diagnostic Imaging items on Part II because chiropractors are portal-of-entry providers who must recognize red-flag patterns before adjusting. A missed vertebral osteomyelitis, discitis, or septic arthritis can lead to catastrophic neurologic compromise or joint destruction. Part II tests pattern recognition, imaging timeline knowledge, and the ability to distinguish pyogenic from granulomatous infection.

Osteomyelitis: Routes, Age Patterns, and Imaging Timeline

Osteomyelitis reaches bone through three routes:

The pediatric metaphyseal predilection reflects sluggish blood flow in metaphyseal sinusoids just short of the growth plate. The physis normally acts as a barrier to epiphyseal and joint spread — except at intracapsular metaphyses (hip, shoulder, elbow, ankle), where metaphyseal osteomyelitis can rupture directly into the joint and cause secondary septic arthritis. This is why a child with hip osteomyelitis may refuse to bear weight and hold the hip flexed, abducted, and externally rotated (the position of maximal joint volume).

Plain-Film Timeline of Acute Osteomyelitis

• Days 1–7: Radiographs typically normal — a critical exam point
• Days 7–10: Deep soft-tissue swelling, loss of fat-plane definition
• Days 10–14: Periosteal reaction (lamellated if subacute)
• Days 14–21: Permeative or moth-eaten lytic destruction

MRI is the gold standard — positive within 24–48 hours with T1-hypointense marrow, T2/STIR-hyperintense edema, and post-contrast enhancement. MRI also defines abscess cavities, sinus tracts, and soft-tissue extent.

Bone scan (technetium-99m three-phase) is sensitive early (24–48 hours) but less specific. A three-phase scan helps differentiate cellulitis (increased flow and blood-pool activity without focal delayed uptake) from osteomyelitis (focal increased uptake on all three phases).

Brodie abscess is a subacute, localized form — a well-defined metaphyseal lucency with a thick sclerotic rim and often a narrow tract extending toward the growth plate. It may be discovered incidentally and is a classic pediatric/young-adult finding.

Discitis and Spondylodiscitis

Pyogenic discitis/spondylodiscitis (most commonly Staphylococcus aureus in adults) produces:

• Early disc-space narrowing with adjacent endplate erosion (disc is avascular and infected first)
• Vertebral body destruction at both adjacent endplates
• Paraspinal soft-tissue swelling or abscess on MRI/CT
• Elevated ESR/CRP with fever and severe localized spinal pain

Tuberculous spondylitis (Pott disease) differs in several testable ways:

• Disc sparing early — infection begins in the anterior vertebral body (Batson's paravertebral venous plexus route), and disc destruction is a late finding because the disc is avascular and not the initial site
• Multiple contiguous vertebrae involved with large, calcified paraspinal abscesses (cold abscess) that track along fascial planes (psoas abscess, thoracic paravertebral)
• Anterior vertebral body collapse producing an angular gibbus deformity
• Insidious onset, low-grade fever, weight loss — unlike acute pyogenic infection

On imaging, Pott disease classically shows vertebral body destruction with relative disc preservation early, whereas pyogenic infection narrows the disc space early.

Septic Arthritis

Septic arthritis is a surgical emergency. Imaging findings reflect rapid enzymatic cartilage destruction:

• Rapid joint-space narrowing (days, not months)
• Periarticular osteopenia from hyperemia
• Marginal erosions and periosteal reaction
• Joint effusion (best seen on MRI or ultrasound)

The key differentiator from osteoarthritis is the speed of joint-space loss and the presence of acute periarticular osteopenia. OA narrows space slowly over years without acute osteopenia. In children, ultrasound can detect an effusion and guide aspiration before plain-film changes appear.

Miscellaneous Infectious and Skeletal Conditions

Syphilis (tertiary): Periostitis, saber shin (anterior tibial bowing with thickened cortex), and Clutton joints (painless synovial effusions, classically knees).

Fungal osteomyelitis (coccidioidomycosis, blastomycosis, histoplasmosis): Indolent, often mimics tumor with lytic lesions and minimal periosteal reaction; endemic geography is a clinical clue.

Chronic recurrent multifocal osteomyelitis (CRMO): A sterile inflammatory condition of children/adolescents mimicking infection — multiple metaphyseal lesions, often clavicle and tibia, with negative cultures. MRI shows marrow edema without abscess.

SAPHO syndrome (synovitis, acne, pustulosis, hyperostosis, osteitis): Anterior chest wall hyperostosis (manubriosternal and costochondral junctions), palmoplantar pustulosis, and sterile osteitis — can mimic infection but cultures are negative.

Neuropathic arthropathy (Charcot joint): Not infectious, but frequently tested alongside infection because diabetic foot osteomyelitis and Charcot arthropathy coexist. Charcot shows disorganization — joint destruction, debris, dislocation, and dense subchondral bone without proportional pain (loss of protective sensation).

Imaging Modality Selection Summary

Exam Strategy

When a Part II vignette describes a child with fever, localized bone pain, and normal radiographs at day 3, the correct next step is MRI — not "reassure and follow up." When the vignette describes vertebral destruction with a cold abscess and relative disc preservation, think Pott disease. When joint-space loss is rapid with periarticular osteopenia, think septic arthritis over OA. Always correlate imaging timeline with clinical urgency.