Osteomyelitis. Paget's disease

1. Osteomyelitis

Definition

• Inflammation of bone + marrow cavity.
• Can be acute or chronic.
• Most common etiologies: pyogenic bacteria + M. tuberculosis.

2. Pyogenic Osteomyelitis

Pathogens

• Staphylococcus aureus = #1 overall (↑ binding via adhesion proteins to bone matrix).
• E. coli, S. agalactiae (group B Strep), N. gonorrhoeae = neonates.
• Salmonella = sickle cell disease patients.
• Pseudomonas = IV drug users, diabetic foot.
• Pasteurella multocida = cat/dog bites.

Routes of Bacterial Entry

• Hematogenous dissemination (#1 in children, especially metaphysis of long bones).
• Extension from adjacent joint / soft tissue infection.
• Direct inoculation (trauma, surgery, open fracture).

Morphology

• Bacterial proliferation → cell death.
• Subperiosteal abscess may form → can spread along bone surface; periosteal rupture → abscess + draining sinus.
• Sequestrum: dead bone fragment.
• Involucrum: reactive new bone shell around sequestrum.
• Infants: epiphyseal infection may spread to joint → suppurative arthritis.
• Inflammatory cells appear after a week → cytokine release → bone remodeling.
• Brodie abscess: walled-off intraosseous abscess (subacute/chronic).

Clinical Features

• Spectrum: mild (fever, local pain) to acute systemic illness (malaise, fever, throbbing pain).
• Acute can progress to chronic → pathologic fracture, sepsis, endocarditis, malignant transformation (SCC of draining sinus tract).
• Labs: ↑ WBC, ↑ ESR, ↑ CRP.
• Diagnosis: radiologic findings (MRI most sensitive), blood culture, bone biopsy.

3. Tuberculous Osteomyelitis

Overview

• Complication of pulmonary TB.
• Reaches bone via hematogenous route or direct spread from foci.
• More destructive + resistant to treatment than pyogenic.

Sites

• Long bones + vertebrae.
• Pott disease = TB of the vertebrae:
  • Vertebral deformity + collapse.
  • Extension to adjacent soft tissues (psoas abscess).
  • Gibbus deformity (sharp angular kyphosis).
• Also affects synovium → typical granulomatous inflammation with caseous necrosis.

4. Paget Disease (Osteitis Deformans)

Definition

• Excessive breakdown + quick disorganized regrowth of bone.
• Net effect: gain in bone mass, but disordered + weak.
• Usually occurs in late adulthood (> 55 yr).

Stages

1. Osteolytic stage: hyperactive osteoclastic bone resorption.
2. Mixed osteoclastic-osteoblastic stage: hectic bone formation.
3. Osteosclerotic (burnt-out) stage: cellular activity exhausted; sclerotic bone remains.

Pathogenesis

• ↑ Osteoclastic + osteoblastic activity.
• Suggested association with paramyxovirus infection (possible viral etiology) + genetic predisposition (SQSTM1 mutations).

Morphology

• Solitary lesion (femur, vertebra, tibia, skull) or multiple sites (pelvis, spine).
• Osteoclasts numerous + abnormally large (multinucleated).
• Bone surfaces lined by osteoblasts → marrow replaced by loose CT → osteoblast activity burns out → fibrovascular tissue recedes → marrow returns.
• Hallmark: mosaic pattern of lamellar bone with prominent cement lines.

Clinical Features

• Often mild; many asymptomatic.
• Localized bone pain (microfractures + nerve compression).
• Hearing loss (CN VIII compression / ossicle involvement).
• “Lion-like” facies (leontiasis ossea) — enlarged skull.
• Enlarged skull → hat size ↑.
• High-output heart failure (↑ vascularity of pagetic bone → AV shunting).
• Risk of osteosarcoma ↑ (mutated osteoblasts; uncommon but serious).
• Labs: ↑↑ ALP, normal Ca²⁺ + PO₄.

Treatment

• Calcitonin (↓ osteoclast resorption).
• Bisphosphonates (zoledronic acid).

5. Summary Table

Condition	Cause	Key features
Pyogenic osteomyelitis	S. aureus (#1); neonates: E. coli/GBS/gonococcus; sickle cell: Salmonella; IVDU: Pseudomonas	Hematogenous (metaphysis in kids), sequestrum + involucrum, Brodie abscess
TB osteomyelitis	M. tuberculosis post-pulmonary	Pott disease of vertebrae → gibbus + psoas abscess; granulomatous inflammation
Paget disease	Idiopathic / paramyxovirus / SQSTM1	3 stages; mosaic pattern; ↑↑ ALP; lion facies, hearing loss, high-output HF, ↑ osteosarcoma

💡 High-yield: Osteomyelitis = S. aureus #1; neonates = E. coli/GBS/gonococcus; sickle cell = Salmonella; IVDU = Pseudomonas; cat/dog bites = Pasteurella. Routes: hematogenous (kids, metaphysis), contiguous, direct. Morph: sequestrum (dead bone) + involucrum (reactive new bone shell); chronic → draining sinus + SCC of sinus tract. TB = Pott disease of spine (gibbus deformity + psoas abscess); granulomatous. Paget disease: > 55 yr; 3 stages (osteolytic → mixed → osteosclerotic); mosaic pattern; possible paramyxovirus; ↑↑ ALP (Ca + PO₄ normal); lion facies + hearing loss (CN VIII) + high-output HF + ↑ osteosarcoma risk; Tx bisphosphonates + calcitonin.