Imaging

1. Why Imaging Matters

The diagnostic problem in Charcot can be reduced to three questions:

1. Is this Charcot at all? — or cellulitis, DVT, gout, fracture, septic arthritis?
2. If Charcot, what is the Eichenholtz stage? — this drives offloading vs surgery.
3. Is there superimposed osteomyelitis? — especially in patients with overlying ulcer.

Imaging is the dominant tool for all three. The clinical exam supports but does not settle them. Plain radiographs are the screening test. MRI is the workhorse for early disease and for the Charcot-vs-osteomyelitis question. CT is for surgical planning. Nuclear medicine is the second-line discriminator when MRI is contraindicated or equivocal.

2. Plain Radiographs — The Screening Tool

Weight-bearing AP, lateral, and oblique foot radiographs (and AP / mortise ankle if hindfoot/ankle is suspected) are the universal first study. The critical caveat: Stage 0 (acute, pre-radiographic) CN has normal plain radiographs. Up to ~50% of plain films are normal in early CN. A normal radiograph in a warm, swollen, neuropathic foot does not exclude CN — it is an indication for MRI.

In the acute developmental phase (Eichenholtz I), the radiographic features are:

• Periarticular fragmentation — small avulsed fragments at joint margins.
• Osteopenia / juxta-articular bone loss — hyperaemic osteolysis.
• Joint effusion — soft-tissue density, sometimes with displacement of fat planes.
• Subluxation / dislocation — especially at Lisfranc.
• Soft-tissue swelling.

In coalescence (Eichenholtz II): absorption of fine debris, periosteal new bone, sclerosis. In consolidation (III): fusion, residual deformity (rocker- bottom), and remodelled architecture — the “burnt-out” foot.

The Meary’s line (long axis of talus through 1st MT on lateral) and the calcaneal pitch angle are the standard measurements for arch collapse. Loss of Meary’s alignment with apex-plantar break at the midfoot is the classic rocker-bottom deformity.

3. The Five Ds — Classic Mnemonic

The classical radiographic features of established Charcot are summarised by the “5 Ds” (or 6 Ds, depending on author):

• Distension — joint effusion, soft-tissue swelling.
• Density increase — sclerosis of fragments and the joint margin.
• Debris — loose intra-articular bony fragments.
• Disorganisation — gross loss of joint architecture.
• Dislocation — subluxation/dislocation with abnormal alignment.
• Destruction — bone erosion / loss (sometimes considered the 6th D).

When all five Ds are present in a non-infected setting, CN is essentially certain. The 5 Ds are most evident in Eichenholtz I–II; consolidation (III) sees recoalescence so the foot can look more organised again, just deformed.

4. MRI — The Gold Standard for Early Charcot

MRI is uniquely suited to CN because it visualises bone marrow oedema, subchondral microfracture, soft-tissue inflammation, joint effusion, and ligamentous integrity in a single study. Standard protocol:

• T1 — anatomy; marrow fat replacement appears low signal.
• T2 fat-saturated / STIR — bone marrow oedema appears high signal; the most sensitive sequence.
• Post-contrast T1 fat-saturated — abscess delineation, sinus tracts, cellulitis vs Charcot.
• PD / proton-density — ligamentous integrity, especially Lisfranc complex.

Stage-by-stage MRI features:

The MRI signature of acute Charcot: periarticular bone-marrow oedema centred on joints, with capsular distention, subchondral microfracture, and ligamentous oedema, especially in the midfoot at the tarsometatarsal and naviculocuneiform joints. Multiple bones & multiple joints are typically affected (multifocal).

5. Charcot vs Osteomyelitis — The Hard Question

The single most important MRI question is differentiating CN from osteomyelitis (OM). The decision typically arises in a patient with an overlying ulcer or healing wound. The wrong answer means either months of unnecessary IV antibiotics (CN treated as OM) or a missed amputation- threatening infection (OM treated as CN).

The discriminating MRI features (Ahmadi et al., AJR 2006; Mautone and Naidoo, Insights Imaging 2015; Donovan and Schweitzer, RadioGraphics 2010):

The probe-to-bone test (Lavery et al., Diabetes Care 2007) — touching bone with a sterile probe through an ulcer — has a positive predictive value of ~90% for osteomyelitis in high-risk feet, and is the simplest bedside test.

Bone biopsy and culture remains the gold standard for OM. CT- or fluoroscopy-guided percutaneous biopsy through uninvolved skin (avoiding the ulcer) is preferred to swabs (which sample colonising flora, not pathogens). Histology of acute Charcot bone shows microfracture, callus, and bony fragments without polymorphonuclear infiltrate; OM shows neutrophil-rich infiltrate and necrotic bone.

6. CT & Surgical Planning

CT is not a primary diagnostic tool but is invaluable for:

• Surgical planning — high-resolution bony detail, 3D reconstruction for arthrodesis hardware planning, computer-assisted templating.
• Sequestrum identification — better than MRI for visible cortical detachment / dead bone.
• Stress fracture vs Charcot — in early or atypical cases.
• Hardware-related complications — broken screws, loose fixation, peri-implant lucencies (where MRI is artefacted).
• CT angiography — for the vascular workup that often accompanies surgical planning.

Modern weight-bearing cone-beam CT (PedCAT, Curvebeam) acquired in the standing position is increasingly used in foot & ankle for true biomechanical assessment of deformity — the gold standard for pre-operative planning of complex midfoot arthrodesis or ankle reconstruction in Charcot.

7. Nuclear Medicine

When MRI is contraindicated (severe pacemakers, implants causing artefact) or equivocal, nuclear-medicine techniques offer a complementary view:

• Three-phase Tc-99m methylene diphosphonate (MDP) bone scan — hot in both CN and OM. Sensitive but not specific. A negative scan effectively rules out either; a positive scan does not distinguish.
• In-111 labelled white blood cell (WBC) scan — WBCs accumulate at sites of pyogenic infection; not at sites of Charcot. Combined Tc-MDP + In-WBC with image co-registration distinguishes CN (MDP+ / WBC−) from OM (MDP+ / WBC+) with sensitivity ~85%, specificity ~85%.
• Tc-99m HMPAO labelled WBC — a similar In-WBC alternative with shorter half-life, lower radiation, often more available.
• Sulphur-colloid bone-marrow scan — combined with WBC scan to subtract physiologic bone-marrow uptake; this is the so-called “dual-isotope” technique — the most accurate nuclear discriminator (Palestro et al., Semin Nucl Med 2009).
• F-18 FDG-PET / PET-CT — uptake in metabolically active inflammation. CN tends to have low-to-moderate FDG uptake; OM has intense focal uptake. Promising data (Kagna et al., Eur J Nucl Med Mol Imaging 2012) but limited availability and cost.
• Anti-granulocyte antibody scintigraphy — alternatives such as Tc-99m sulesomab.

In current practice, MRI remains first-line. Nuclear medicine is reserved for the patient with metallic implants, MRI contraindication, or genuinely equivocal MRI. Where available, dual-isotope WBC + sulphur colloid or FDG-PET-CT are the modern complements.

8. Ultrasound & Bone-Health Tools

• Ultrasound — useful for soft-tissue collections, abscess, foreign body, and Achilles morphology in equinus contracture; not for bone or joint architecture.
• Skin-temperature scanning — infrared dermal thermometry as covered in Part I; \(\Delta T \ge 2\,^\circ\text{C}\) drives suspicion and follows treatment response.
• DEXA — useful for systemic bone health in patients being considered for anti-resorptive therapy; not directly for CN diagnosis.
• Bone turnover markers — serum CTX, P1NP, urinary NTX; reflect global resorption / formation balance, not local CN activity, but track response to bisphosphonates in the small CN trials.
• High-resolution peripheral QCT (HR-pQCT) — research tool for trabecular microarchitecture in the diabetic foot.

For the broader bone-imaging context (DEXA, QCT, T-scores) and the pharmacology of anti-resorptive agents see Cell Physiology and Pharmacology.

9. Imaging by Eichenholtz Stage — A Synthesis

Imaging is therefore not a one-shot diagnostic; it is the longitudinal monitoring tool that maps the patient through the natural history. Repeat radiographs at 6–8 week intervals during cast immobilisation track the transition from Stage I (fragmentation) to Stage II (coalescence) to Stage III (consolidation) — and when combined with the temperature criterion (\(\Delta T < 2\,^\circ\text{C}\)) they form the basis of the “ready for protected weight-bearing” decision discussed in Part VI.

The next part formalises the Eichenholtz staging system in detail, along with the modern Sella & Barrette modification (adding Stage 0) and the Brodsky and Sanders anatomic classifications.