Osteomyelitis detailed explanation

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Osteomyelitis - Detailed Explanation

Definition

Osteomyelitis is inflammation of bone and marrow, virtually always secondary to infection. It may complicate any systemic infection but frequently presents as a solitary primary focus. All types of organisms - bacteria, viruses, parasites, and fungi - can cause it, but pyogenic bacteria and mycobacteria are by far the most common.

Robbins, Cotran & Kumar Pathologic Basis of Disease

Routes of Infection

Three pathways allow organisms to reach bone:

Route	Description	Typical Setting
Hematogenous	Bacteremia seeds bone via the bloodstream	Children (long bone metaphyses), adults (vertebrae)
Contiguous spread	Extension from adjacent soft tissues/joints	Diabetic foot, pressure sores
Direct implantation	Penetrating trauma, surgery, open fractures	Post-operative, compound fractures

In children, hematogenous spread is most common. The metaphyses of long bones are preferentially involved because the slow blood flow through looped capillaries, combined with microtrauma, encourages bacterial seeding during a bacteremia. In adults, osteomyelitis is most often a complication of open fractures, surgical procedures, or diabetes (feet).

Bailey and Love's Short Practice of Surgery, 28th Ed.

Microbiology / Causative Organisms

Setting	Typical Organism(s)
General (all ages)	Staphylococcus aureus (80-90% of culture-positive cases)
Neonates	Group B Streptococcus, E. coli
Older children	S. aureus, Streptococcus pyogenes, Haemophilus influenzae
Sickle cell disease	Salmonella spp., gram-negative organisms
IV drug users / elderly	Gram-negatives, polymicrobial
Immunocompromised	Fungi (Coccidioides, Blastomyces, Cryptococcus), Mycobacteria
Post-surgical / trauma	Mixed bacterial infections

Why S. aureus? Staphylococcal cell-wall proteins bind bone matrix components such as collagen, facilitating bacterial adherence to bone. MRSA is now the dominant organism in acute hematogenous osteomyelitis (AHO) in many regions.

Robbins, Cotran & Kumar

Specific organisms are only identified in ~50% of patients on culture; PCR-based assays are expected to improve this rate.

Age-Related Vascular Anatomy (Key Concept)

The site of infection is determined by the osseous vascular anatomy, which changes with age:

Neonates: Metaphyseal vessels penetrate the growth plate, so infection can spread to both the epiphysis and joint - septic arthritis frequently co-exists.
Older children (>18 months): The growth plate acts as a barrier; metaphyseal involvement is typical, epiphysis is spared.
Adults: After growth plate closure, anastomoses form between metaphyseal and epiphyseal vessels - infection tends to involve the epiphyses and subchondral regions. Vertebral osteomyelitis becomes more common.
Grainger & Allison's Diagnostic Radiology

Pathology: Stages of Disease

Acute Phase (within 48 hours)

Bacteria proliferate → neutrophilic infiltration
Bone cell and marrow necrosis within 48 hours
Bacteria + inflammatory cells spread via Haversian canals to reach the periosteum
Periosteum (loosely attached in children) is elevated → subperiosteal abscess forms
Periosteal lifting further impairs cortical blood supply → more necrosis

Subacute / Progressive Phase

Increased pressure from abscess → pus perforates cortical bone
Pus elevates the strong periosteum, rendering cortical bone avascular
Periosteal elevation is a potent stimulus for new bone formation (involucrum)
Soft tissue abscesses form; may channel to skin as draining sinuses

Chronic Phase

Dead avascular bone = sequestrum
New reactive bone forming a shell around the sequestrum = involucrum
This creates the classic "bone-within-a-bone" appearance on imaging
Marrow shows fibrosis and a chronic infiltrate of lymphocytes and plasma cells
Pus may track through gaps (cloacae) in the involucrum to the surface

Brodie Abscess: A walled-off chronic abscess, appearing as a well-defined lytic lesion with a sclerotic rim - characteristic of subacute osteomyelitis.

Below is a gross pathology specimen showing chronic osteomyelitis - the outer involucrum (new viable bone shell) has been opened to reveal the inner white sequestrum (necrotic cortex):

Resected femur with chronic osteomyelitis showing involucrum and sequestrum

Fig. - Resected femur with draining osteomyelitis. The outer periosteal shell of viable new bone (involucrum) reveals the inner necrotic cortex (sequestrum). - Robbins, Cotran & Kumar

Clinical Features

Symptoms

Fever, rigors, may appear toxic
Systemic: headache, fatigue, malaise, anorexia (less consistent in chronic disease)
Children: sudden limp or inability to bear weight, localized warmth, swelling, erythema

Signs

Point tenderness over the infected segment (most consistent finding)
Palpable warmth and soft-tissue swelling with erythema
In chronic disease: palpable involucrum/sequestrum, sinus tracts to skin
A sympathetic effusion in the adjacent joint may develop even without joint infection

Acute Hematogenous Osteomyelitis (AHO) in Children

Male preponderance (2:1 to 3:1)
Involves long bones ~80% of cases
Distal metaphysis most common site (increased vascularity)
Blood cultures positive in ~40% of cases
Rosen's Emergency Medicine

Vertebral Osteomyelitis (Adults)

Increasingly common as population ages
Risk factors: IV access devices, indwelling lines, urinary infections, IV drug use
Presents with back pain, low-grade fever; neurological signs if epidural extension

Investigations

Laboratory

CBC: Leukocytosis (variable)
ESR and CRP: Elevated - useful for monitoring response to treatment
Blood cultures: Positive in ~40% of pediatric AHO; lower in adults
Bone biopsy + culture: Gold standard for organism identification
PCR assays: Emerging to improve yield above culture

Imaging

Modality	Key Points
Plain X-ray	Lags behind disease by 10-14 days; shows soft tissue swelling, cortical irregularity, periosteal reaction, lytic destruction
Ultrasound	Detects subperiosteal fluid/abscess early; guides drainage; especially useful in children
Bone scan (scintigraphy)	Sensitive early; useful for multifocal disease screening; less specific
CT	Best for defining extent of cortical destruction, detecting sequestra; higher radiation
MRI	Highest sensitivity and specificity; best for early disease, soft tissue involvement, spinal complications, epidural abscess

MRI features: Bone marrow edema on T1 (low signal) and T2 (high signal); periosteal reaction; soft tissue edema. The penumbra sign in subacute infection = a peripheral high-signal ring of granulation tissue surrounding a low-signal abscess cavity.

Even on appropriate therapy, radiographic signs of improvement lag behind clinical recovery.

Grainger & Allison's Diagnostic Radiology

Classification

By Duration

Acute: < 2-4 weeks symptoms; hematogenous; responds to antibiotics alone (often)
Subacute: Weeks to months; Brodie abscess typical; culture often negative
Chronic: >4-6 weeks; sequestrum/involucrum/sinus tracts; requires surgery + prolonged antibiotics

Cierny-Mader Classification (Chronic Osteomyelitis)

Used clinically to guide surgical planning:

Stage 1 (Medullary): Endosteal infection only
Stage 2 (Superficial): Cortical surface involvement from contiguous source
Stage 3 (Localized): Full-thickness cortical involvement, stable bone
Stage 4 (Diffuse): Through-and-through infection, mechanically unstable bone

Host class: A (normal), B (compromised local or systemic), C (treatment worse than disease)

Special Subsets

Diabetic Foot Osteomyelitis

Contiguous spread from soft tissue ulcers
Polymicrobial; often includes gram-negatives and anaerobes
Probe-to-bone test: positive = high specificity for osteomyelitis
MRI is imaging modality of choice

Vertebral Osteomyelitis (Spondylodiscitis)

Usually involves two adjacent vertebral bodies + intervening disc
Most common site: lumbar spine
Complications: epidural abscess, spinal cord compression, vertebral collapse
MRI is mandatory to assess intraspinal involvement

Neonatal Osteomyelitis

Multiple sites in ~50% of cases
Septic arthritis frequently co-exists (shared vasculature)
Flat bones (facial bones) more commonly involved than at other ages
Whole-body MRI or skeletal scintigraphy for multifocal screening

CRMO (Chronic Recurrent Multifocal Osteomyelitis)

Autoinflammatory (non-infectious) form
Multiple foci, culture-negative
Associated with psoriasis subtypes
Diagnosed by imaging pattern; treated with NSAIDs/bisphosphonates

Treatment

General Principles

Identify the organism (blood cultures, bone biopsy)
Start empirical antibiotics, then narrow based on culture
Drain any pus surgically
Splint/rest the affected limb
Treat underlying conditions (sickle cell, nutritional deficiency, diabetes)

Antibiotic Therapy

Setting	Empirical Choice
Children (MSSA suspected)	Anti-staphylococcal penicillin (flucloxacillin) or 1st-gen cephalosporin
MRSA suspected (most AHO)	Vancomycin IV; or clindamycin if susceptible
Neonates	Vancomycin + cefotaxime (cover GBS + gram-negatives)
Adults (vertebral/diabetic)	Vancomycin + broad gram-negative cover until cultures return

IV-to-oral switch: Once clinical and laboratory improvement is documented, oral step-down is appropriate in uncomplicated cases. Debate continues on exact duration.
Total duration typically 4-6 weeks for acute; 3-6 months for vertebral or chronic.
A 2025 meta-analysis on oritavancin (PMID 41224240) found it effective for osteomyelitis, offering the advantage of once-weekly dosing - relevant for long-course outpatient treatment.

Surgical Treatment

Indication: Abscess formation, failure of medical treatment, chronic osteomyelitis with sequestrum, sinus tracts, unstable bone
Procedures: Debridement, sequestrectomy, saucerization, dead space management (antibiotic-impregnated beads, bone grafting, muscle flaps), stabilization if unstable
Shortened IV and oral regimens apply only to uncomplicated cases in patients who are clinically and hematologically improving.
Bailey and Love's Short Practice of Surgery

Complications

Complication	Notes
Chronic osteomyelitis	Occurs in 5-25% of acute cases; delayed diagnosis, extensive necrosis are risk factors
Septic arthritis	Especially neonates and when metaphysis is intracapsular
Bacteremia / Sepsis	Systemic spread of infection
Growth disturbance	Physeal damage in children → limb shortening or deformity
Pathologic fracture	Through sites of bone destruction
Secondary amyloidosis	Complication of long-standing chronic infection
Squamous cell carcinoma	Malignant transformation in chronic draining sinus tracts (rare, late)
Sarcoma	Rare malignant transformation in infected bone
Brain abscess / meningitis	Hematogenous spread from vertebral/skull osteomyelitis
Spinal cord compression	Epidural extension in vertebral osteomyelitis

Mycobacterial Osteomyelitis (Skeletal TB)

More destructive and resistant than pyogenic osteomyelitis
Occurs in ~1-3% of individuals with pulmonary or extrapulmonary TB
Typically solitary, insidious onset; years may pass before diagnosis
Classic site: spine (Pott's disease) - involves two adjacent vertebrae + disc
Spina ventosa: TB dactylitis - cyst-like cavities with diaphyseal expansion, predominantly in hand/foot bones
Treatment: standard anti-TB regimen (minimum 9-12 months)
Robbins, Cotran & Kumar

Key Terms Summary

Term	Meaning
Sequestrum	Fragment of dead, avascular necrotic bone
Involucrum	Shell of new periosteal bone surrounding the sequestrum
Cloaca	Gap/opening in the involucrum through which pus drains
Brodie abscess	Walled-off subacute abscess with sclerotic rim on imaging
Sinus tract	Channel from the infected bone to the skin surface
Penumbra sign	MRI finding in subacute osteomyelitis - high-signal granulation tissue rim around abscess

Sources:

Robbins, Cotran & Kumar - Pathologic Basis of Disease, Ch. 26
Bailey and Love's Short Practice of Surgery, 28th Ed., Ch. 44
Rosen's Emergency Medicine, Ch. 125
Grainger & Allison's Diagnostic Radiology, Ch. 73
Recent evidence: Systematic review on chronic pediatric osteomyelitis (PMID 41329650, 2025); Meta-analysis on oritavancin for osteomyelitis (PMID 41224240, 2025)

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