Chronic nonbacterial osteomyelitis, or CNO, is a sterile autoinflammatory disorder of bone. Chronic recurrent multifocal osteomyelitis, or CRMO, describes the recurrent multifocal end of the same spectrum. The name is confusing because it contains the word osteomyelitis, but the disease is not primarily bacterial.

The practical clinical problem is a child or adolescent with recurrent bone pain, imaging evidence of osteitis and no microbiological proof of infection. Some children have a single symptomatic lesion. Others have multiple symptomatic and silent lesions in the long-bone metaphyses, clavicle, spine, pelvis, sternum, mandible or foot.

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CNO is commonly delayed because the first X-ray can be normal, the symptoms can resemble injury or infection, and the child may look well between flares. Delay matters because untreated disease can cause chronic pain, functional limitation, pathological fracture, vertebral collapse, angular deformity, limb-length problems or unnecessary surgery.

The topic is important for orthopaedics because the child often enters the system through fracture clinic, tumour clinic, emergency department or infection pathway. The orthopaedic surgeon must know when to biopsy, when not to debride, when to protect a painful bone, and when to involve paediatric rheumatology early.

CNO is best understood as disordered innate immune regulation causing sterile bone inflammation. The exact pathway is not one single gene or one single cytokine for most children. Contemporary reviews describe imbalance between pro-inflammatory and anti-inflammatory signalling, inflammasome activation and osteoclast-mediated bone injury as important themes.

The pathology is osteitis without a proven bacterial driver. Bone biopsy, when performed, may show inflammatory infiltrate, chronic sterile osteomyelitis-like change, fibrosis or sclerosis, but it does not provide a pathognomonic signature. This is why the diagnosis is made from the whole clinical, laboratory, imaging and sometimes histological pattern.

CNO is best classified clinically by distribution, risk and diagnostic certainty. This is more useful than trying to force the condition into a fracture-style grading system.

Typical History

Children usually present with recurrent or persistent bone pain. The pain may flare, settle and recur at the same or different sites. Limp, activity limitation, focal swelling and local tenderness are common. Fever is absent or low-grade in many children; marked toxicity is a warning against uncomplicated CNO.

Important history points are not generic. They are chosen to separate CNO from infection, malignancy, stress injury and inflammatory disease:

Examination

Examination should localise symptomatic lesions and look beyond the painful bone. Inspect gait, posture, spine, limb alignment, local swelling, warmth, tenderness, joint motion and neurovascular status. Examine skin and nails for psoriasis or pustulosis. Check jaw opening, clavicles, sternoclavicular joints and spine when symptoms suggest those regions.

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Plain Radiographs

Radiographs are often the first test because they are accessible and help exclude fracture, aggressive tumour, chronic infection and structural damage. Early radiographs may be normal. Established lesions may be lytic, sclerotic, mixed, expansile or show periosteal reaction. Radiographs are useful for structural complications, but they do not map disease burden.

Blood Tests

Order full blood count, ESR and CRP. Results can be normal, mildly raised or inflammatory. The absence of a marked inflammatory response does not exclude CNO. However, cytopenia, very high inflammatory markers, bacteremia, toxic appearance or persistent fever should push the assessment back toward infection, malignancy or systemic disease.

MRI

MRI is the central imaging test. Targeted MRI defines a symptomatic site, marrow oedema, periosteal reaction, soft-tissue inflammation, abscess-like appearances and structural damage. Whole-body MRI is preferred when CNO is suspected because it detects silent lesions and typical distribution patterns.

Biopsy

Biopsy is not required for every child, but it is mandatory when the diagnosis is not secure. The biopsy should be planned like a tumour or infection biopsy: correct compartment, safe tract, cultures and histology, and avoidance of contaminating future surgical planes.

CNO is not classified by one universally used orthopaedic fracture-style system. It is more useful to describe disease by burden, risk and certainty of diagnosis.

Management has two parallel aims: keep the diagnosis safe and control inflammation enough to preserve function and prevent structural damage. The child should usually be managed with paediatric rheumatology, paediatrics, radiology and orthopaedics rather than orthopaedics alone.

Treatment pathwayEscalationOrthopaedic role

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First-line Treatment

NSAIDs are commonly used as first-line anti-inflammatory treatment when safe and appropriate. They are not just analgesics in this setting; they may control inflammatory bone pain in a proportion of children. The child also needs activity modification, physiotherapy for gait and function, school and sport planning, and monitoring for flare recurrence.

First-line treatment is reasonable when the diagnosis is secure, the child is clinically well, there is no high-risk spinal collapse, no aggressive lesion, no fracture risk and symptoms are tolerable.

Escalation

Escalation is considered when pain persists, function is limited, MRI shows progression, NSAIDs fail or are not tolerated, or high-risk sites are involved. Options used in paediatric rheumatology practice include bisphosphonates, conventional disease-modifying antirheumatic drugs, TNF inhibitor therapy and short steroid bridging in selected cases. There is no single universally accepted best second-line drug for every child.

Treatment Choice

Situation	Reasonable direction	Rationale
Mild typical multifocal CNO	NSAID-based treatment and monitoring.	Many children improve, and overtreatment can be avoided when risk is low.
Persistent pain or MRI progression	Escalate anti-inflammatory treatment.	Ongoing active osteitis can damage bone and function.
Spinal disease	Early escalation is often favoured.	The endpoint is prevention of vertebral collapse and deformity, not just pain relief.
Mandibular or destructive lesions	Specialist escalation and site-specific monitoring.	Structural and functional consequences may be significant.
Uncertain diagnosis	Do not escalate immunosuppression until mimics are excluded.	Tumour and bacterial infection must not be masked.

Orthopaedic Role

Orthopaedics should not be passive in CNO. The orthopaedic role includes:

• recognising the pattern and stopping repeated infection-only thinking;
• arranging tumour-safe or infection-safe biopsy when required;
• protecting painful or structurally weak lesions from fracture;
• monitoring spine, alignment, limb length and deformity;
• avoiding unnecessary debridement when there is no pus, sequestrum or bacterial infection;
• coordinating care when lesions threaten joint function, growth or mechanical stability.

Antibiotics are a diagnostic clue, not a default treatment

If cultures are negative, imaging is multifocal and the child repeatedly relapses after antibiotics, the next step is not automatically a longer antibiotic course. Reassess for CNO, mimics and disease burden.

When biopsy is required, plan it properly. A poorly planned biopsy can contaminate compartments, miss the lesion or create misleading results.

Surgery is not a standard treatment for CNO inflammation. Operative intervention is reserved for diagnostic biopsy, fracture management, deformity correction, treatment of true infection if present, or rare structural complications. Repeated washouts without pus or culture-positive infection usually represent a management error.

Follow-up should track symptoms, function, inflammatory markers when useful, medication response, adverse effects and imaging activity. Whole-body MRI is used for disease burden and monitoring, but imaging should be interpreted with the clinical picture; persistent signal change does not always mean the child needs the same escalation if symptoms and structure are stable.

CNO is sterile inflammatory osteitis, and CRMO is its recurrent multifocal form. The safe approach is to recognise the pattern, exclude infection and malignancy, map disease with whole-body MRI, biopsy atypical or unifocal lesions, and treat according to pain, function, disease burden and high-risk sites. Orthopaedics contributes most by making the diagnosis safe, protecting structure, monitoring deformity and avoiding unnecessary infection surgery when cultures and imaging support sterile inflammatory disease.