Introduction

Septic arthritis is an acute infection of the synovial membrane and joint space characterized by purulent inflammation within a synovial joint. It is most commonly caused by bacterial pathogens, although fungi and mycobacteria may also be responsible, particularly in immunocompromised individuals. Septic arthritis is an orthopaedic emergency because irreversible cartilage destruction can occur within 24–48 hours of symptom onset, potentially resulting in permanent joint dysfunction, septicemia, or death if treatment is delayed.

The condition most commonly affects large joints such as the knee and hip but may involve any synovial joint. Early diagnosis, prompt antimicrobial therapy, and urgent joint drainage are essential to preserve joint function and prevent systemic complications.

Epidemiology

• Occurs in all age groups but is most common in young children and older adults.

• Annual incidence ranges from 2–10 cases per 100,000 population in the general population.

• Higher incidence among patients with rheumatoid arthritis, diabetes mellitus, immunosuppression, prosthetic joints, and intravenous drug use.

• The knee is the most frequently affected joint, followed by the hip, shoulder, ankle, elbow, and wrist.

• Polyarticular septic arthritis occurs in approximately 10–20% of cases and carries a higher mortality.

Anatomy and Commonly Affected Joints

Commonly involved joints include:

• Knee (most common)

• Hip

• Shoulder

• Ankle

• Elbow

• Wrist

Less commonly:

• Sternoclavicular joint

• Sacroiliac joint

• Small joints of the hands and feet

Etiology

Septic arthritis results from microbial invasion of the synovial membrane and joint space through hematogenous spread, direct inoculation, or contiguous spread from adjacent infection.

Common causative organisms

Adults

• Staphylococcus aureus (most common)

• Streptococcus species

• Gram-negative bacilli

Children

• Staphylococcus aureus

• Kingella kingae

• Streptococcus species

Sexually active young adults

• Neisseria gonorrhoeae

Sickle Cell Disease

• Salmonella species

• Staphylococcus aureus

Immunocompromised patients

• Gram-negative organisms

• Mycobacteria

• Fungal organisms

Classification

According to duration

• Acute septic arthritis

• Chronic septic arthritis

According to microbiology

• Pyogenic bacterial arthritis

• Gonococcal arthritis

• Tuberculous arthritis

• Fungal arthritis

According to joint involvement

• Monoarticular (most common)

• Polyarticular

Pathophysiology

1. Microorganisms enter the bloodstream or joint directly.

2. Organisms colonize the synovial membrane.

3. Rapid bacterial multiplication triggers intense inflammation.

4. Neutrophils release proteolytic enzymes.

5. Synovial effusion and increased intra-articular pressure develop.

6. Articular cartilage is rapidly destroyed.

7. Untreated infection leads to bone erosion, osteomyelitis, ankylosis, septicemia, and permanent disability.

Risk Factors

• Extremes of age

• Diabetes mellitus

• Rheumatoid arthritis

• Osteoarthritis

• Prosthetic joints

• Previous joint surgery

• Recent intra-articular injection

• Trauma

• Open joint injuries

• Immunosuppression

• HIV infection

• Chronic kidney disease

• Intravenous drug use

• Sickle cell disease

• Skin or soft tissue infection

• Bacteremia

Clinical Presentation

Symptoms usually develop rapidly over hours to a few days.

Common features include:

• Fever

• Malaise

• Severe joint pain

• Swollen joint

• Warm joint

• Erythema over the affected joint

• Marked tenderness

• Reduced range of motion

• Inability to bear weight (lower limb involvement)

• Refusal to use the affected limb in children

• Irritability in infants

• Joint held in a position of comfort

The clinical features are similar to those of acute osteomyelitis but are localized primarily to the affected joint.

Diagnostic Criteria

Diagnosis is based on a combination of clinical findings, laboratory investigations, imaging, and synovial fluid analysis.

Major Criteria

• Purulent synovial fluid obtained by joint aspiration

• Positive synovial fluid culture

• Positive synovial tissue culture

• Organism identified by Gram stain or molecular testing from synovial fluid

Minor Criteria

• Acute painful swollen joint

• Fever

• Elevated CRP

• Elevated ESR

• Leukocytosis

• Positive blood culture

• Imaging consistent with septic arthritis

• Clinical improvement after antimicrobial therapy

Kocher Criteria (Primarily for Septic Arthritis of the Hip in Children)

The likelihood of septic arthritis increases with the number of criteria present:

• Fever >38.5°C

• Non-weight bearing on the affected limb

• ESR >40 mm/hour (or CRP >20 mg/L in modified criteria)

• White blood cell count >12,000/mm³

Four positive criteria indicate a very high probability of septic arthritis and warrant urgent orthopaedic assessment.

Investigations

Laboratory Investigations

• Complete blood count

• Differential white blood cell count

• Erythrocyte sedimentation rate (ESR)

• C-reactive protein (CRP)

• Blood culture and sensitivity before antibiotics

• Renal and liver function tests (baseline)

Synovial Fluid Analysis (Gold Standard)

Joint aspiration should be performed before antibiotics whenever possible.

Assess for:

• Appearance

• White blood cell count

• Differential cell count

• Gram stain

• Culture and sensitivity

• Crystal analysis

• Polymerase chain reaction (PCR) where available

Imaging

Plain X-ray

May demonstrate:

• Joint effusion

• Soft tissue swelling

• Joint space narrowing (late)

• Bone erosion (late)

Ultrasound

• Detects joint effusion

• Guides aspiration

• Particularly useful for the hip

MRI

Most sensitive modality for:

• Early septic arthritis

• Adjacent osteomyelitis

• Soft tissue abscess

CT Scan

Useful for:

• Deep joints

• Complex anatomy

• Surgical planning

Differential Diagnosis

• Acute osteomyelitis

• Crystal arthritis (gout, pseudogout)

• Reactive arthritis

• Rheumatoid arthritis

• Juvenile idiopathic arthritis

• Transient synovitis

• Lyme arthritis

• Hemarthrosis

• Bone tumour

Management

Management is an orthopaedic emergency and should begin immediately once septic arthritis is suspected.

Treatment includes:

• Prompt antibiotic therapy

• Urgent joint drainage

• Joint immobilization initially

• Early physiotherapy after infection control

• Monitoring inflammatory markers

Pharmacological Treatment

First-line Treatment

Cloxacillin

• 1–2 g IV every 6 hours

Then continue with:

Ampicillin + Cloxacillin (fixed-dose combination)

• 500 mg orally every 8 hours

Continue treatment for 3–6 weeks or until CRP normalizes and radiological improvement is demonstrated.

OR

Ampicillin + Sulbactam (fixed-dose combination)

• 3 g IV every 6 hours for two weeks

Then:

Amoxicillin + Clavulanate (fixed-dose combination)

• 625 mg orally every 12 hours for four weeks.

Patients with Penicillin Allergy

Clindamycin

• IV every 6 hours for two weeks

Then continue orally to complete 4–6 weeks

PLUS

Ciprofloxacin

• 400 mg IV every 12 hours for two weeks

Then continue orally to complete 4–6 weeks

Sickle Cell Disease (Suspected Salmonella Infection)

Ciprofloxacin

• 400 mg IV every 12 hours for four weeks

May be changed to oral therapy after two weeks if clinically improving.

Surgical Management

Urgent surgical drainage is recommended for all patients, particularly those presenting more than 24 hours after symptom onset.

Surgical options include:

• Open arthrotomy with incision and drainage followed by meticulous irrigation

• Arthroscopic drainage and irrigation (where expertise and facilities are available)

• Repeat irrigation if infection persists

• Synovectomy when indicated

• Debridement of necrotic tissue where necessary

Following surgery:

• Limb elevation

• Temporary immobilization

• Early supervised joint mobilization once infection is controlled

Monitoring and Follow-up

Patients should be monitored for:

• Clinical improvement

• Reduction in pain and swelling

• Restoration of joint movement

• Serial CRP and ESR

• White blood cell count

• Wound healing

• Repeat aspiration if symptoms persist

• Follow-up radiographs when indicated

• Antibiotic adverse effects

Complications

• Irreversible cartilage destruction

• Osteomyelitis

• Joint instability

• Joint stiffness

• Ankylosis

• Growth plate injury in children

• Pathological dislocation

• Chronic pain

• Recurrent infection

• Septicemia

• Septic shock

• Permanent disability

• Death

Prevention

• Prompt treatment of bacteremia

• Early treatment of skin and soft tissue infections

• Strict aseptic technique during joint injections and surgery

• Appropriate management of open injuries

• Good glycaemic control in patients with diabetes

• Early recognition of septic arthritis symptoms

• Appropriate perioperative antibiotic prophylaxis

Patient Education

Patients should be advised to:

• Complete the full course of prescribed antibiotics.

• Attend all scheduled follow-up appointments.

• Report persistent fever, worsening pain, increasing swelling, or wound discharge immediately.

• Participate in physiotherapy to restore joint function.

• Avoid excessive weight-bearing until advised by the treating clinician.

• Maintain good control of chronic medical conditions such as diabetes.

• Seek immediate medical attention if symptoms recur.

Prognosis

The prognosis depends on the speed of diagnosis, prompt initiation of antimicrobial therapy, adequacy of joint drainage, the causative organism, and patient comorbidities. When treated within the first 24–48 hours, most patients achieve good functional recovery with preservation of joint integrity. Delayed diagnosis or inadequate treatment significantly increases the risk of irreversible cartilage destruction, chronic joint dysfunction, osteomyelitis, recurrent infection, and permanent disability. Mortality is highest among elderly patients, immunocompromised individuals, and those with polyarticular disease or septic shock.