# Orthopedic Approaches to Infection: Osteomyelitis and Septic Arthritis

## Pathophysiology of Musculoskeletal Infection

### Routes of Infection

Musculoskeletal infections reach bone and joints through three principal routes. Hematogenous spread involves blood-borne bacterial seeding and is most common in children (who have susceptible metaphyseal blood vessels) and the elderly (who are prone to vertebral body seeding). Contiguous spread occurs from adjacent soft tissue infections or through open fracture wounds. Direct inoculation introduces organisms through trauma, surgical procedures, or intra-articular injections.

### Key Organisms

| Organism | Associated Clinical Setting |
|----------|---------------------------|
| Staphylococcus aureus (MSSA/MRSA) | Most common overall; all ages |
| Coagulase-negative staphylococci | Prosthetic joint infections (indolent) |
| Streptococcus species | Second most common in adults |
| Neisseria gonorrhoeae | Sexually active young adults |
| Kingella kingae | Children 6 months - 4 years |
| Salmonella | Sickle cell disease |
| Pseudomonas | Puncture wounds through shoes; IV drug users |
| Gram-negative organisms | GU/GI source; immunocompromised; IVDU |
| Mycobacterium tuberculosis | Endemic areas; Pott disease of spine |
| Fungi | Immunocompromised patients |

Staphylococcus aureus is the most common causative organism across all ages and clinical settings, and the rising prevalence of MRSA must be considered in empiric antibiotic selection. Streptococcus species are the second most common in adults. Coagulase-negative staphylococci cause many prosthetic joint infections, typically with indolent presentations. Gram-negative organisms are associated with genitourinary or gastrointestinal sources, immunocompromised states, and intravenous drug use. Neisseria gonorrhoeae causes septic arthritis in sexually active young adults. Kingella kingae is the predominant pathogen in children aged 6 months to 4 years. Salmonella has a particular association with sickle cell disease. Pseudomonas is classically linked to puncture wounds through shoes (particularly of the foot) and IV drug users. Mycobacterium tuberculosis causes skeletal infection (including Pott disease of the spine) in endemic areas and immigrant populations. Fungi are encountered primarily in immunocompromised patients.

### Biofilm Formation

Bacteria adhere to implant surfaces and begin forming biofilm within hours of contamination. The glycocalyx matrix that encases the bacterial colony protects organisms from both antibiotics and the host immune system. As biofilm matures, eradication becomes progressively more difficult, which is why implant removal is often necessary in chronic periprosthetic infections. Early biofilm (less than 3-4 weeks old) may still be disrupted by aggressive debridement combined with antibiotics and implant retention (the DAIR protocol: debridement, antibiotics, and implant retention).

## Osteomyelitis

### Classification: Cierny-Mader System

The Cierny-Mader classification combines the anatomic extent of infection with the physiologic status of the host to guide treatment decisions.

#### Anatomic Type

| Type | Description | Bone Stability After Debridement |
|------|-------------|----------------------------------|
| I (Medullary) | Confined to endosteal canal | Stable |
| II (Superficial) | Cortical surface involvement | Stable |
| III (Localized) | Full-thickness cortical involvement | Stable |
| IV (Diffuse) | Full-thickness involvement | Unstable — requires reconstruction |

Type I (medullary) infection is confined to the endosteal canal, typically from hematogenous seeding. Type II (superficial) involves the cortical surface from contiguous infection. Type III (localized) indicates full-thickness cortical involvement but with bone remaining mechanically stable after debridement. Type IV (diffuse) describes full-thickness involvement with bone that will be mechanically unstable after debridement, requiring reconstruction.

#### Physiologic Host Class

Class A hosts have normal immune function. Class B hosts are compromised either locally (Bl) from factors such as radiation, scarring, lymphedema, or vascular disease; systemically (Bs) from diabetes, malnutrition, renal failure, immunosuppression, or smoking; or both (Bls). Class C hosts are those in whom the morbidity of treatment would exceed the morbidity of the disease itself, representing prohibitive surgical risk.

### Acute Hematogenous Osteomyelitis

Acute hematogenous osteomyelitis is most common in children and has a metaphyseal predilection due to the sluggish blood flow in metaphyseal venous sinusoids, which allows bacterial seeding. The age of the patient determines the pattern of spread. In neonates (younger than 18 months), transphyseal vessels permit infection to spread from the metaphysis across the growth plate into the epiphysis and adjacent joint. In children between 18 months and 16 years, the physis acts as a barrier to spread, and infection instead tends to rupture through the thin metaphyseal cortex to form a subperiosteal abscess. In adults, the vascular anatomy still permits metaphyseal involvement, but hematogenous osteomyelitis is less common overall.

The clinical presentation includes fever, localized pain, and refusal to bear weight (in children). Laboratory studies reveal elevated ESR and CRP. Radiographs may appear normal for the first 10-14 days; MRI is the most sensitive early imaging study. Treatment consists of IV antibiotics (historically 4-6 weeks, though the OVIVA trial now supports transition to oral antibiotics with good bioavailability) and surgical drainage if an abscess has formed.

### Chronic Osteomyelitis

Chronic osteomyelitis results from failed treatment of acute infection or from indolent organisms. Three characteristic pathologic findings define chronic disease: the sequestrum (necrotic, devascularized bone that harbors bacteria), the involucrum (new periosteal bone forming around the sequestrum), and the cloaca (a sinus tract through the involucrum draining to the skin surface). Treatment principles include radical debridement of all necrotic bone and soft tissue (Papineau technique), dead space management using antibiotic-loaded cement spacers, antibiotic beads, or the Masquelet induced-membrane technique, soft tissue coverage with local or free flaps, pathogen-directed antibiotics for a minimum of 6 weeks (often longer), and skeletal stabilization when structural integrity is compromised.

### Workup

Laboratory evaluation includes WBC (which may be normal in chronic infection), ESR, and CRP (the most sensitive marker for monitoring treatment response). Blood cultures should be drawn before initiating antibiotics. MRI with gadolinium is the gold standard imaging modality. Deep tissue and bone cultures obtained at the time of surgical debridement are the gold standard for organism identification. Swab cultures of sinus tracts are unreliable due to polymicrobial surface contamination. Specimens should be sent for aerobic, anaerobic, fungal, and mycobacterial culture. On histopathology, greater than 5 polymorphonuclear leukocytes per high-power field suggests acute infection.

## Septic Arthritis

### Epidemiology and Risk Factors

The knee is the most commonly affected joint (approximately 50% of cases), followed by the hip, ankle, and shoulder. Risk factors include age greater than 80, diabetes, rheumatoid arthritis, skin infection, recent joint surgery or injection, immunosuppression, and intravenous drug use.

### Diagnosis

Septic arthritis presents as acute-onset monoarticular joint pain with swelling, warmth, restricted range of motion, and often fever. The differential diagnosis includes gout, pseudogout, reactive arthritis, and rheumatoid arthritis flare. In children with an acute painful hip, the Kocher criteria help differentiate septic arthritis from transient synovitis: fever greater than 38.5 degrees Celsius, non-weight-bearing status, ESR greater than 40, and WBC greater than 12,000. When all four criteria are met, the probability of septic arthritis is 99%. The modification by Caird added CRP greater than 2.0 mg/dL as a fifth criterion to increase diagnostic accuracy.

### Synovial Fluid Analysis

A synovial fluid white blood cell count exceeding 50,000/mm3 with more than 75% polymorphonuclear cells is highly suggestive of septic arthritis, though lower counts can be seen early in the disease course, in immunosuppressed patients, or in prosthetic joints. Gram stain is positive in only 30-50% of cases. Culture is positive in 70-90% with proper technique. Glucose is typically low (less than 40 mg/dL or less than 50% of serum glucose). Crystal analysis should always be performed to evaluate for gout or pseudogout, keeping in mind that crystal arthropathy can coexist with infection.

### Management

Septic arthritis is a surgical emergency because delay in joint washout leads to progressive cartilage destruction and worsened outcomes. Treatment requires emergent irrigation and debridement, which can be performed arthroscopically (effective for the knee and shoulder) or through open arthrotomy (preferred for the hip, especially in children). Serial washouts may be necessary if clinical improvement is inadequate. Empiric antibiotics should be started after cultures are obtained: vancomycin plus ceftriaxone for adults (covering MRSA and gram-negative organisms), cefazolin or nafcillin for children older than 5 years (targeting MSSA as the most common pathogen), and cefazolin or ceftriaxone for children younger than 5 years (providing Kingella coverage). Antibiotic selection is refined based on culture and sensitivity results. Duration is typically 4-6 weeks, with many centers now stepping down to oral antibiotics after an initial 2-week IV course based on the OVIVA trial evidence.

### OVIVA Trial (2019)

The OVIVA trial was a landmark randomized controlled trial comparing IV to oral antibiotics for bone and joint infection. It demonstrated that oral antibiotics with good bioavailability were non-inferior to IV therapy when used after an initial short IV course. Oral agents with excellent bioavailability include fluoroquinolones, rifampin, linezolid, trimethoprim-sulfamethoxazole, and clindamycin. This trial has changed practice at many centers, permitting earlier step-down to oral antibiotics and reducing the need for prolonged IV access with its associated complications (line infections, thrombosis). Important exclusions were spine infections and prosthetic joint infections, which were not well represented in the study population.

## Special Considerations

### Pediatric Osteomyelitis

In children, hematogenous spread is the predominant mechanism. Adjacent septic arthritis can develop when the metaphysis is intracapsular, as occurs at the hip, shoulder, elbow, and ankle. Brodie abscess is a subacute form of metaphyseal infection that appears as a well-circumscribed lucency on radiographs. Most pediatric osteomyelitis without abscess formation can be treated with antibiotics alone; surgical drainage is indicated for subperiosteal or intraosseous abscesses.

### Vertebral Osteomyelitis (Spondylodiscitis)

Vertebral osteomyelitis is hematogenous in most cases, with L2-L4 being the most commonly affected levels. S. aureus is the most common organism, though tuberculosis must be considered in endemic regions. MRI is the imaging study of choice. When blood cultures are negative, CT-guided biopsy provides tissue for culture. Most cases respond to antibiotics alone (6-8 weeks). Surgical indications include epidural abscess causing neurologic deficit, spinal instability, and failure of medical management.

### Infection Following Open Fracture

Infection risk correlates directly with the Gustilo-Anderson grade: approximately 2% for Type I, 5% for Type II, 5-10% for Type IIIA, 10-25% for Type IIIB, and 25-50% for Type IIIC. Antibiotic prophylaxis includes a first-generation cephalosporin for all open fractures, with the addition of an aminoglycoside for Type III injuries. Penicillin is added for farm or barnyard injuries to provide Clostridium coverage. Prompt debridement and appropriate soft tissue coverage remain the most important measures for reducing infection risk.

<image>A medical illustration of the pathophysiology of hematogenous osteomyelitis in a child's long bone. Show the metaphyseal blood supply with sluggish venous sinusoids where bacteria seed. Depict the physis as a barrier to epiphyseal spread. Show the formation of a subperiosteal abscess from infection penetrating through the thin metaphyseal cortex. Label the periosteum, physis, metaphysis, and abscess cavity. Include an inset showing biofilm formation on bone surface.</image>

<image>An illustration showing the Cierny-Mader classification of osteomyelitis. Display four cross-sectional views of a long bone: Type I (medullary — infection confined to endosteal canal), Type II (superficial — outer cortical surface involvement), Type III (localized — full-thickness but bone stable after debridement), Type IV (diffuse — full-thickness with structural instability). Label each type clearly with clinical examples.</image>

<image>A clinical algorithm flowchart for the evaluation of an acutely swollen, painful joint. Start with history and physical exam, then branch to synovial fluid aspiration. Show the analysis pathway: cell count (WBC > 50,000 with >75% PMNs = septic until proven otherwise), crystal analysis (rule out gout/pseudogout), Gram stain, and culture. Branch to treatment: emergent I&D for septic arthritis with empiric antibiotics, vs. medical management for crystal arthropathy. Include the Kocher criteria sidebar for pediatric hip evaluation.</image>

## Clinical Pearls

Septic arthritis is a surgical emergency, and delay in washout leads to irreversible cartilage destruction within 24-48 hours. The Kocher criteria guide management of the acute pediatric hip, but adding CRP (the Caird modification) increases diagnostic accuracy. Sinus tract swab cultures are unreliable; deep tissue and bone cultures obtained at surgical debridement are essential for guiding antibiotic therapy. In chronic osteomyelitis, the sequestrum must be completely removed because it harbors bacteria within devascularized bone that antibiotics cannot penetrate. The OVIVA trial supports early transition to oral antibiotics with good bioavailability, reducing the complications associated with prolonged IV access. Intracapsular metaphyses (proximal femur, proximal humerus, proximal radius, and distal lateral tibia) allow osteomyelitis to spread directly into the joint in children. MRSA must always be considered in empiric antibiotic coverage given its increasing community prevalence. CRP is the most useful laboratory marker for monitoring treatment response because it rises and falls fastest with disease activity.

## References

- Li HK, et al. Oral versus intravenous antibiotics for bone and joint infection (OVIVA): a multicentre, randomised, non-inferiority trial. *Lancet*. 2019;393(10183):2175-2186.
- Cierny G, Mader JT, Penninck JJ. A clinical staging system for adult osteomyelitis. *Clin Orthop Relat Res*. 2003;(414):7-24.
- Kocher MS, et al. Differentiating between septic arthritis and transient synovitis of the hip in children. *J Bone Joint Surg Am*. 1999;81(12):1662-1670.
- Parvizi J, et al. New definition for periprosthetic joint infection: from the Workgroup of the MSIS. *Clin Orthop Relat Res*. 2011;469(11):2992-2994.
- Lew DP, Waldvogel FA. Osteomyelitis. *Lancet*. 2004;364(9431):369-379.