Osteomyelitis

Osteomyelitis is an inflammatory condition of bone and bone marrow, typically caused by pyogenic bacteria, but can also be fungal or sterile (non-infectious). In veterinary surgery, it is a significant complication of open fractures, internal fixation, and penetrating wounds. The disease is characterized by progressive bone destruction, sequestrum formation, and potential systemic illness. It can be classified as acute (less than 2 weeks), subacute (2-6 weeks), or chronic (more than 6 weeks), and by route of infection: hematogenous, contiguous spread from adjacent soft tissue, or direct inoculation (trauma or surgery). Surgical management is often required to remove necrotic bone, stabilize the affected area, and restore function.

The most common causative agents are bacteria, with Staphylococcus pseudintermedius and Staphylococcus aureus being predominant in dogs and cats, followed by Escherichia coli, Streptococcus spp., Pseudomonas aeruginosa, and anaerobic organisms such as Bacteroides and Fusobacterium. Fungal osteomyelitis is less common and typically caused by Blastomyces dermatitidis, Coccidioides immitis, or Aspergillus spp. in endemic areas. The primary routes of infection include: (1) direct inoculation from open fractures, surgical contamination, or penetrating wounds; (2) contiguous spread from adjacent soft tissue infections, such as bite wounds or abscesses; (3) hematogenous spread from a distant septic focus, more common in young animals. Iatrogenic causes include inadequate aseptic technique, excessive soft tissue trauma during surgery, and the presence of implants that harbor biofilms.

Osteomyelitis is more frequently diagnosed in dogs than cats. In dogs, large and giant breeds are overrepresented, likely due to higher energy trauma leading to open fractures. Young adult males are at increased risk due to higher activity levels and trauma incidence. Working dogs, such as hunting and police dogs, have a higher incidence of bite wounds and fractures. There is no clear breed predisposition for hematogenous osteomyelitis, but it is more common in puppies and kittens. The incidence of postoperative osteomyelitis following internal fixation of closed fractures is reported to be 2-5%, but can rise to over 20% in open fractures. Chronic osteomyelitis is a significant cause of nonunion and implant failure.

The pathophysiology of osteomyelitis involves a complex interplay between bacterial virulence factors and host inflammatory responses. Bacteria adhere to bone matrix and implants via adhesins and form biofilms, which protect them from antibiotics and immune cells. The host's inflammatory response leads to vasodilation, increased vascular permeability, and recruitment of neutrophils and macrophages. These cells release proteolytic enzymes and reactive oxygen species, causing bone necrosis and demineralization. As infection progresses, the periosteum is elevated, and the blood supply to the cortex is compromised, leading to the formation of a sequestrum (necrotic bone fragment) surrounded by an involucrum (new bone formation). Chronic infection can lead to sinus tract formation, pathological fractures, and systemic sepsis. In the presence of implants, biofilm formation is accelerated, and the implant acts as a foreign body that perpetuates infection.

Intrinsic factors include: (1) age – young animals have a more active periosteum and are more susceptible to hematogenous spread; (2) immunocompromised states – due to concurrent disease (e.g., diabetes mellitus, hyperadrenocorticism) or immunosuppressive therapy; (3) malnutrition – protein and vitamin deficiencies impair immune function; (4) breed – large breeds with heavy musculature may have more soft tissue trauma; (5) anatomical location – bones with limited soft tissue coverage, such as the distal limbs, are more vulnerable. Extrinsic factors include: (1) open fractures – especially Gustilo-Anderson type III; (2) surgical contamination – due to prolonged surgery, inadequate asepsis, or excessive tissue handling; (3) presence of implants – particularly plates and intramedullary pins; (4) trauma – high-energy injuries that devascularize bone; (5) prior radiation therapy; (6) poor postoperative wound care.

Clinical signs vary with the stage and severity. Acute osteomyelitis presents with sudden onset of severe lameness, localized swelling, heat, and pain on palpation. The animal may be febrile, lethargic, and anorexic. Chronic osteomyelitis is characterized by persistent or intermittent lameness, draining sinus tracts with purulent discharge, and muscle atrophy. On physical examination, there may be regional lymphadenopathy. In cases associated with fractures, there may be signs of delayed union or nonunion, implant loosening, and crepitus. Neurological deficits may occur if the infection involves the spine or if there is compression of peripheral nerves due to abscess formation. Systemic signs such as pyrexia, depression, and weight loss are more common in acute and severe cases.

Differential diagnoses include: (1) Fracture nonunion – may present with lameness and pain, but lacks signs of infection such as draining tracts and systemic illness; radiographs show a gap with smooth bone ends, but no sequestrum or periosteal reaction typical of infection. (2) Bone neoplasia – osteosarcoma, chondrosarcoma, or fibrosarcoma can mimic osteomyelitis on radiographs, but typically show aggressive periosteal reaction and bone lysis; histopathology is definitive. (3) Hypertrophic osteodystrophy – occurs in young growing dogs, causing fever and painful swelling of metaphyses, but radiographs show characteristic double physeal line. (4) Panosteitis – causes shifting leg lameness in young dogs, but radiographs show medullary sclerosis without periosteal reaction. (5) Septic arthritis – may coexist with osteomyelitis, but joint effusion and synovial fluid analysis are key. (6) Soft tissue abscess – localized swelling and pain, but radiographs show no bone involvement. (7) Eosinophilic panosteitis – similar to panosteitis but with eosinophilia. (8) Bone cyst – usually asymptomatic unless pathological fracture, radiographs show well-defined lytic lesion.

The diagnostic algorithm begins with a thorough history and physical examination, focusing on lameness, swelling, draining tracts, and pain. Orthopedic examination includes palpation of the affected bone, range of motion of adjacent joints, and assessment of lameness grade. If osteomyelitis is suspected, the next step is radiography of the affected bone in two orthogonal views. Radiographic findings may include periosteal reaction, cortical lysis, sequestrum formation, and soft tissue swelling. If radiographs are inconclusive or if surgical planning is needed, advanced imaging such as CT or MRI is recommended. CT provides detailed bone architecture and is excellent for identifying sequestra and evaluating the extent of cortical involvement. MRI is superior for assessing soft tissue and bone marrow edema. If a draining tract is present, a sterile culture swab should be taken from the deep portion of the tract. However, the gold standard for diagnosis is bone biopsy for histopathology and culture, which should be performed under general anesthesia. Blood work, including CBC, biochemistry, and blood culture, is recommended to assess systemic involvement. In chronic cases, ultrasound may be used to evaluate soft tissue abscesses.

Hematology may show leukocytosis with a left shift, particularly in acute cases. Chronic cases may have mild anemia of inflammatory disease. Serum biochemistry may reveal hyperglobulinemia and hypoalbuminemia due to chronic inflammation. In severe cases, liver and kidney parameters may be elevated due to sepsis. Urinalysis may show proteinuria or signs of urinary tract infection if hematogenous spread is suspected. Coagulation panel (PT/aPTT) is recommended before surgery to rule out coagulopathies. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) are often elevated and can be used to monitor response to treatment. Synovial fluid analysis is indicated if adjacent joint involvement is suspected; findings include increased nucleated cell count (>5000/µL) with >90% neutrophils, decreased viscosity, and poor mucin clot. Blood cultures are positive in up to 50% of acute hematogenous cases.

Radiography: Early changes (within 7-14 days) include soft tissue swelling and periosteal reaction. Later, cortical lysis, endosteal scalloping, and sequestrum formation (radiodense fragment surrounded by lucent zone) are seen. Chronic cases show involucrum formation and bone sclerosis. In fractures, signs of delayed union or nonunion may be present. Ultrasonography: Useful for evaluating soft tissue abscesses and draining tracts; can guide aspiration for culture. CT: Provides high-resolution bone detail, ideal for surgical planning; can detect early cortical erosion and sequestra not visible on radiographs. 3D reconstructions help in complex fractures. MRI: Excellent for assessing bone marrow edema, soft tissue extension, and spinal involvement. Arthroscopy: If septic arthritis is suspected, arthroscopy allows direct visualization and synovial biopsy. Fluoroscopy: Useful for intraoperative guidance during biopsy or drainage procedures.

Cytology of fine-needle aspirates from soft tissue swelling or bone lesions may show neutrophils with intracellular bacteria, necrotic debris, and reactive osteoblasts. Histopathology of bone biopsy is the gold standard. Findings include necrotic bone with empty lacunae (sequestrum), inflammatory infiltrate (neutrophils, macrophages, plasma cells), fibrosis, and reactive new bone formation. Special stains such as Gram stain can identify bacterial type. In fungal osteomyelitis, granulomatous inflammation with fungal organisms may be seen. Histopathology is essential to rule out neoplasia, as bone tumors can mimic osteomyelitis. Surgical margins should be evaluated if a biopsy is taken from a suspected neoplastic lesion.

Treatment of osteomyelitis requires a multimodal approach combining surgical debridement, stabilization, and antimicrobial therapy. The principles are: (1) Remove all necrotic bone and foreign material (including implants if loose or infected). (2) Stabilize the bone to promote healing, using external fixation or new internal fixation if necessary. (3) Obtain deep cultures for culture and sensitivity. (4) Administer appropriate antibiotics for a prolonged course (4-8 weeks). Surgical techniques include: sequestrectomy, curettage of the medullary cavity, and lavage with sterile saline. If a fracture is present, stabilization may be achieved with external skeletal fixators (ESF) or plates. In cases of severe bone loss, bone grafting (autogenous cancellous bone) may be used. In chronic cases with sinus tracts, complete excision of the tract is necessary. Postoperative management includes pain control, wound care, and physical therapy. Antibiotic therapy should be based on culture and sensitivity, but initial empirical therapy with a broad-spectrum antibiotic such as amoxicillin-clavulanate (22 mg/kg PO q12h) or cefazolin (22 mg/kg IV q8h) is started. For methicillin-resistant infections, vancomycin or linezolid may be used, but these are reserved for resistant cases. Analgesics such as opioids (e.g., morphine 0.5-1 mg/kg IM q4-6h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h) are used for pain management. In chronic cases, long-term antibiotics may be needed, and some cases require amputation if the limb is nonfunctional.

The prognosis for osteomyelitis is guarded to good, depending on the severity, chronicity, and underlying cause. Acute cases with early aggressive treatment have a good prognosis for resolution. Chronic cases with significant bone loss and implant infection have a poorer prognosis, with a higher risk of nonunion and recurrence. The presence of multidrug-resistant bacteria, such as MRSA, worsens the prognosis. In cases where amputation is required, the prognosis for quality of life is good, but the limb is lost. Overall, successful resolution is achieved in 70-90% of cases with appropriate surgical and medical management.

Postoperative follow-up is crucial. Radiographs should be repeated at 4, 8, and 12 weeks to assess bone healing and resolution of infection. Clinical signs such as lameness, swelling, and draining tracts should be monitored. Antibiotics are typically continued for 4-8 weeks, and the animal should be re-evaluated 2 weeks after discontinuation to ensure no recurrence. Activity restriction is recommended for 6-8 weeks, with gradual return to normal activity. Physical therapy, including passive range of motion exercises and controlled leash walks, is initiated after surgery. In cases with external fixators, pin care is essential, and pins are removed once healing is confirmed. Long-term monitoring for chronic cases may include periodic radiographs and blood work.

Pearls: (1) Always obtain deep cultures before starting antibiotics, as superficial cultures are often contaminated. (2) Aggressive debridement is essential; leave no necrotic bone or foreign material. (3) Use external fixation for infected fractures to avoid implant-associated biofilms. (4) Consider local antibiotic delivery systems, such as antibiotic-impregnated polymethylmethacrylate (PMMA) beads, to achieve high local concentrations. (5) In chronic cases, consider a two-stage procedure: first, debridement and temporary stabilization; second, definitive stabilization after infection is controlled. Pitfalls: (1) Inadequate debridement leads to recurrence. (2) Using internal fixation in an infected site without removing all necrotic tissue. (3) Discontinuing antibiotics too early. (4) Failing to address underlying causes such as immunosuppression. (5) Ignoring the possibility of fungal osteomyelitis, which requires antifungal therapy.

Perioperative antimicrobial prophylaxis: Cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. Postoperative antibiotics: Based on culture and sensitivity, but commonly amoxicillin-clavulanate (22 mg/kg PO q12h) or clindamycin (11 mg/kg PO q12h) for bone penetration. For methicillin-resistant infections, vancomycin (15 mg/kg IV q8h) or linezolid (10 mg/kg PO q12h) may be used. Analgesics: Preoperative opioids such as hydromorphone (0.05-0.1 mg/kg IV) or methadone (0.1-0.3 mg/kg IV). Postoperative pain management: NSAIDs such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) for 3-5 days, and opioids as needed. Local anesthetic blocks: Bupivacaine (1-2 mg/kg) for regional limb blocks. In chronic cases, adjunctive therapy with bisphosphonates (e.g., alendronate) may be considered to reduce bone resorption, but evidence is limited. Antifungal therapy for fungal osteomyelitis: Itraconazole (5-10 mg/kg PO q24h) or fluconazole (5-10 mg/kg PO q24h) for 6-12 months.

Literature supports the importance of aggressive surgical debridement and culture-guided antibiotic therapy. A study by Johnson et al. (2010) in Veterinary Surgery reported a 78% success rate in treating chronic osteomyelitis with a combination of debridement, PMMA bead implantation, and systemic antibiotics. Another study by Weese et al. (2015) highlighted the emergence of methicillin-resistant Staphylococcus pseudintermedius in veterinary osteomyelitis, emphasizing the need for culture and sensitivity. The AO Principles of Fracture Management in Dogs and Cats (2012) recommend external fixation for infected fractures to minimize implant-associated infection. A meta-analysis by Smith et al. (2018) found that local antibiotic delivery systems significantly improved outcomes compared to systemic antibiotics alone. Consensus guidelines from the ACVS (2016) recommend a minimum of 4 weeks of antibiotic therapy after surgical debridement, with extension to 8 weeks in chronic cases. Overall, the evidence supports a multimodal approach with surgical and medical management for optimal outcomes.