Chronic Osteomyelitis

Chronic osteomyelitis is persistent or recurrent bone infection that lasts longer than 6 weeks, distinguished from acute osteomyelitis by the presence of necrotic bone (sequestrum), reactive new bone formation around dead bone (involucrum), chronic inflammatory tissue, and frequent sinus tract formation. Chronic osteomyelitis represents one of the most challenging conditions in orthopedic infection management, requiring complex multidisciplinary treatment including surgical debridement, prolonged antibiotic therapy, and management of underlying conditions and host factors.

Pathophysiology and characteristics: 1) Persistent infection — bacteria harbored in necrotic bone (sequestrum) and biofilms providing protected sanctuary from immune system and antibiotics; 2) Necrotic bone (sequestrum) — devitalized bone fragments that lose blood supply and harbor bacteria; can be small or extensive; 3) Reactive new bone (involucrum) — new periosteal bone formation around devitalized sequestrum, creates characteristic radiographic appearance; 4) Sinus tract — chronic draining tract from infected bone to skin surface; presence indicates communication with infected bone; can be source of new infection; 5) Biofilm formation — bacteria within biofilms have decreased susceptibility to antibiotics and immune mechanisms (10-1000 fold less susceptible); biofilm formation common with S. aureus, S. epidermidis, P. aeruginosa; 6) Chronic inflammation — ongoing inflammatory response with possible amyloidosis after long-standing infection; 7) Compromised local environment — fibrosis, scar tissue, decreased blood supply, decreased antibiotic penetration; 8) Pathologic fracture risk — weakened bone from extensive necrosis; 9) Amyloidosis — rare but recognized complication of long-standing chronic infection (5+ years); systemic AA amyloidosis affecting kidneys, heart, GI tract.

Etiologies and risk factors: 1) Acute hematogenous spread progressing to chronic — failed acute treatment, undiagnosed, undertreatment; 2) Open fracture — most common cause of chronic osteomyelitis after trauma in adults; high-energy injuries, severe contamination, inadequate initial debridement, prolonged time to surgery; 3) Surgical site infection following orthopedic surgery — especially with implanted hardware, prolonged operative time, immunocompromised patients; 4) Diabetic foot infection — extensive soft tissue ulceration extending to underlying bone, particularly in long-standing diabetics with peripheral neuropathy and vascular insufficiency; 5) Pressure sore (decubitus ulcer) extending to bone — sacral, ischial, heel locations particularly; 6) Vertebral osteomyelitis — typically hematogenous spread from urinary tract, IV drug use, post-procedural; 7) Post-puncture wound foot infection — Pseudomonas particularly common (especially with rubber sole footwear); 8) IV drug use — hematogenous spread to spine particularly; 9) Sickle cell disease — Salmonella osteomyelitis classic, also S. aureus; 10) Post-radiation osteonecrosis with secondary infection; 11) Periodontal disease — extension to mandible (osteomyelitis of jaw); 12) Specific predisposing conditions — diabetes mellitus, peripheral vascular disease, immunosuppression, alcoholism, malnutrition, chronic kidney disease, IV drug use, sickle cell disease, HIV.

Microbiology: 1) Staphylococcus aureus — most common pathogen overall (30-60 percent of cases); MRSA increasingly common (community-acquired and hospital-acquired); particularly common in hematogenous and post-traumatic infections; biofilm-producing strains pose treatment challenges; 2) Coagulase-negative Staphylococci (S. epidermidis) — particularly with implanted hardware (orthopedic implants, prosthetic joints), biofilm formation typical; 3) Pseudomonas aeruginosa — diabetic foot infection, water exposure, post-puncture wound through rubber sole, immunocompromised patients; 4) Streptococcus species — Group A streptococcus (S. pyogenes), Group B streptococcus (more in adults); 5) Enterobacteriaceae (E. coli, Klebsiella, Proteus) — diabetic foot, pressure sores, hematogenous from urinary tract; 6) Anaerobes — Bacteroides fragilis, Peptostreptococcus, Clostridium; common in diabetic foot, pressure sores, post-traumatic; 7) Mixed polymicrobial — particularly diabetic foot ulcers, pressure sores, post-traumatic infections; often 3-5 different organisms; 8) Mycobacterium tuberculosis — Pott disease (spinal tuberculosis), peripheral skeletal tuberculosis; particularly important in endemic regions, immunocompromised patients (HIV); 9) Atypical mycobacteria (M. fortuitum, M. marinum) — water exposure, immunocompromised; 10) Fungal — Candida in immunocompromised, IV drug users; dimorphic fungi (Coccidioides, Blastomyces, Histoplasma) in endemic regions; 11) Brucella — endemic regions, contact with cattle and dairy products.