Orthopaedic Surgery - Septic Knee

Basics

A septic knee is:

An infection of the synovial lining and joint space of the knee.

It is most commonly caused by:

Bacteria

and represents an:

Orthopaedic emergency

because untreated infection can rapidly destroy:

Articular cartilage

and lead to permanent joint dysfunction.

Predisposing Factors

Important predisposing conditions include:

Pre-existing arthritis

Intravenous drug use

Alcohol misuse

Corticosteroid therapy

and other causes of:

Immunosuppression.

Epidemiology

Septic arthritis of the knee is:

Common among native-joint infections.

It may occur in:

Infants

Children

Adults

and

Older adults.

In adults, the knee is one of the:

Most frequently affected joints.

Risk Factors

Important risk factors include:

Bacteremia

Intravenous drug use

Alcohol misuse

Recent trauma

Previous knee surgery

Recent joint injection or aspiration

HIV infection

Diabetes mellitus

Corticosteroid use

Other immunocompromised states

Pre-existing inflammatory or degenerative joint disease

Pathogenesis

Infection may reach the knee through:

Hematogenous spread

Direct inoculation

or

Contiguous extension from nearby infection.

Hematogenous Spread

Because the synovium is:

Highly vascular

bacteria circulating in the bloodstream can seed the:

Knee joint.

This is a common mechanism in:

Native-joint septic arthritis.

Direct Inoculation

Direct introduction of organisms may occur after:

Trauma

Surgery

Arthrocentesis

or

Intra-articular injection.

Etiology

The most common causative organism is:

Staphylococcus aureus.

Other Organisms

Additional organisms include:

Streptococcus species

Streptococcus pneumoniae

Neisseria gonorrhoeae

Neisseria meningitidis

Salmonella species

Brucella species

and, historically in young children,

Haemophilus influenzae.

Haemophilus influenzae

Haemophilus influenzae type b was historically an important cause of septic arthritis in:

Infants and young children.

Its incidence has fallen substantially following widespread:

Hib vaccination.

Diagnosis

Diagnosis is based on:

Clinical examination

Synovial fluid aspiration

Laboratory testing

and, when necessary,

Imaging.

The most important diagnostic procedure is:

Knee aspiration.

Signs and Symptoms

Common symptoms include:

Knee swelling

Pain

Pain with movement

Difficulty bearing weight

and occasionally:

Fever.

Joint Swelling

The joint capsule may become:

Distended

and

Fluctuant

because of a large:

Effusion.

Pain

Pain is typically aggravated by:

Active movement

Passive range of motion

and

Weight bearing.

Systemic Features

Patients may have:

Fever

Malaise

or

Leukocytosis.

However, systemic signs may be absent, particularly in:

Older

Immunocompromised

or partially treated patients.

Physical Examination

The key findings are:

Joint effusion

Painful range of motion

and

Restricted motion.

Effusion

A significant:

Intra-articular effusion

is commonly present.

Pain With Short-Arc Motion

Marked pain with even:

Small arcs of passive motion

strongly suggests an intra-articular inflammatory process such as:

Septic arthritis.

Erythema

Overlying erythema may be:

Absent

because the knee joint lies beneath several layers of:

Soft tissue.

Its absence does not exclude infection.

Warmth and Tenderness

The knee may demonstrate:

Warmth

Diffuse tenderness

and

Protective muscle spasm.

Laboratory Tests

Peripheral White Blood Cell Count

CBC may demonstrate:

Leukocytosis

with a:

Left shift.

However, a normal peripheral WBC count does not exclude:

Septic arthritis.

ESR

The:

Erythrocyte sedimentation rate

is commonly elevated.

It is useful as a supportive marker and may help monitor:

Treatment response.

C-Reactive Protein

CRP is also commonly elevated and is particularly useful because it:

Rises and falls more rapidly than ESR.

Serial measurements can assist in assessing:

Clinical improvement.

Synovial Fluid Aspiration

Joint aspiration is the:

Primary diagnostic test.

The aspirate should be sent for:

Cell count

Differential

Gram stain

Culture

and

Crystal analysis.

Synovial White Blood Cell Count

A very high synovial WBC count strongly supports:

Septic arthritis.

Historical descriptions emphasized counts above:

100,000 cells/µL

with greater than:

90% polymorphonuclear leukocytes.

However, infection may occur with substantially lower counts.

Therefore:

No single synovial WBC threshold reliably rules septic arthritis in or out.

Neutrophil Predominance

A high percentage of:

Polymorphonuclear leukocytes

supports the diagnosis, particularly in the appropriate clinical setting.

Synovial Glucose and Protein

In bacterial infection, synovial fluid may demonstrate:

Reduced glucose

and

Elevated protein.

These findings are nonspecific and are not relied upon as primary diagnostic criteria.

Gram Stain

The aspirate should be sent for:

Gram stain.

A positive result may guide immediate antibiotic selection, but sensitivity is limited.

A negative Gram stain does not exclude:

Septic arthritis.

Culture

Synovial fluid culture is essential to identify:

The causative organism

and its:

Antimicrobial sensitivities.

Crystal Analysis

The aspirate should also be examined for:

Monosodium urate crystals

and

Calcium pyrophosphate crystals

to evaluate for:

Gout

or

Pseudogout.

The presence of crystals does not completely exclude:

Concomitant infection.

Blood Cultures

Patients with suspected septic arthritis should have:

Blood cultures

obtained before antibiotic administration whenever possible.

Blood cultures may identify the organism even when:

Synovial fluid cultures are negative.

Imaging

Plain Radiographs

Early radiographs may show:

Joint effusion

Soft-tissue swelling

or pre-existing:

Degenerative changes.

They may otherwise be normal.

Chronic Infection

Long-standing infection may eventually produce:

Joint-space narrowing

Subchondral erosion

Bone destruction

and secondary:

Degenerative change.

MRI

MRI is particularly useful when the diagnosis is:

Uncertain

or when there is concern for:

Adjacent osteomyelitis

Soft-tissue abscess

Popliteal cyst infection

or extensive:

Synovitis.

Baker Cyst

A large or infected:

Popliteal or Baker cyst

may coexist with septic arthritis.

If infected, it may serve as a persistent reservoir and potentially:

Reinoculate the knee joint.

Pathological Findings

Untreated infection causes progressive:

Synovial inflammation

followed by:

Articular cartilage destruction.

Cartilage Damage

Bacterial toxins and inflammatory enzymes can damage cartilage within:

A few days.

This is the major reason prompt:

Drainage

and

Antibiotic treatment

are required.

Bone Destruction

The severity of bone involvement depends on:

Organism virulence

and

Duration of untreated infection.

Long-Standing Infection

Advanced disease may progress to:

Fibrous ankylosis

Bony ankylosis

Osteomyelitis

or

Septicemia.

Differential Diagnosis

Important alternative diagnoses include:

Acute osteomyelitis

Periarticular cellulitis

Prepatellar bursitis

Gout

Pseudogout

Acute rheumatoid arthritis

Juvenile idiopathic arthritis

Hemarthrosis from hemophilia

Lyme arthritis

Prepatellar Bursitis

Prepatellar bursitis usually causes swelling:

Anterior to the patella

without the profound pain on passive short-arc knee motion typical of:

Intra-articular septic arthritis.

Gout and Pseudogout

Crystal arthritis can closely mimic infection with:

Acute pain

Effusion

Warmth

and

Erythema.

Definitive distinction often requires:

Joint aspiration.

Lyme Arthritis

Lyme arthritis may produce a:

Large knee effusion

but often causes less pain with:

Short-arc passive motion

than typical acute bacterial septic arthritis.

Treatment

General Principles

Treatment requires:

Early diagnosis

Prompt antibiotics

and

Adequate joint drainage.

The knee usually requires:

Arthroscopic or open irrigation and débridement.

Early Nonoperative Management

In carefully selected cases diagnosed very early, an initial trial of:

Intravenous antibiotics

with

Serial joint aspiration

may be considered.

This requires:

Very close monitoring.

Failure of Aspiration

Persistent or recurrent:

Effusion

Purulence

Fever

or clinical deterioration should prompt:

Surgical drainage.

Urgent Irrigation and Débridement

The infected knee should generally be:

Irrigated and débrided urgently

to reduce bacterial burden and protect:

Articular cartilage.

Repeat Débridement

Some infections require:

Multiple surgical washouts

before infection is controlled.

Popliteal Cyst Drainage

An infected or communicating:

Popliteal cyst

may need drainage if it serves as a persistent source of:

Reinfection.

Immobilization

A:

Knee immobilizer

may be used temporarily during the acute painful phase.

Duration of Immobilization

Prolonged immobilization should be avoided.

Once infection and pain improve, the patient should begin:

Gentle active and passive range-of-motion exercises.

Physical Therapy

After control of the acute infection, rehabilitation should focus on:

Restoring knee motion

Quadriceps strength

Gait

and overall:

Lower-extremity function.

Medication

Empiric Antibiotics

Antibiotic treatment should begin promptly after:

Synovial fluid

and

Blood cultures

are obtained whenever clinically feasible.

Initial Coverage

Empiric therapy should provide coverage for:

Gram-positive organisms

particularly:

Staphylococcus aureus.

Coverage for:

MRSA

should be considered according to patient risk factors and local resistance patterns.

Additional Coverage

Broader antibiotic coverage may be required in patients with:

Immunocompromise

Intravenous drug use

Recent surgery

Gram-negative risk factors

or other specific exposures.

Definitive Antibiotics

Once Gram stain, culture, and susceptibility results are available, antibiotics should be narrowed to:

Organism-specific therapy.

Surgery

Arthroscopic Irrigation and Débridement

Arthroscopy is commonly used to:

Drain the joint

Break up loculations

Remove purulent material

and perform:

Synovectomy when necessary.

Open Irrigation and Débridement

An open approach may be used when:

Arthroscopic drainage is inadequate

Infection is advanced

or extensive tissue destruction requires:

Direct exposure.

Irrigation

The joint is washed with large volumes of:

Sterile saline

until gross contamination and purulence are removed.

Loculations

All accessible:

Loculations

should be disrupted to permit:

Complete drainage.

Drains

A temporary:

Intra-articular drain

may occasionally be used until:

Inflammation and drainage decrease.

Follow-Up

Patients require close monitoring after treatment.

Important parameters include:

Pain

Fever

Knee swelling

Range of motion

CRP

ESR

and clinical evidence of:

Recurrent effusion.

Prognosis

When diagnosed and treated:

Early

the prognosis is generally:

Good.

Delayed Treatment

Outcomes deteriorate substantially when diagnosis is delayed.

Historical reports suggested that delays beyond approximately:

2–4 days

increase the risk of:

Permanent cartilage injury

and poor joint function.

Complications

Articular Cartilage Destruction

Persistent infection can cause irreversible:

Cartilage erosion

leading to:

Post-infectious osteoarthritis.

Fibrous Ankylosis

Severe inflammation may cause scar formation and eventual:

Fibrous ankylosis.

Bony Ankylosis

Advanced destructive infection may rarely lead to:

Bony fusion of the knee.

Osteomyelitis

Infection may extend into the:

Femur

Tibia

or

Patella

causing:

Osteomyelitis.

Septicemia

Bacterial dissemination can result in:

Sepsis

or

Septicemia.

Degenerative Joint Disease

Even after eradication of infection, damaged cartilage may result in:

Chronic pain

Stiffness

and progressive:

Degenerative joint disease.

Recurrent Infection

Incomplete drainage or inadequate antimicrobial treatment may lead to:

Persistent or recurrent septic arthritis.

Patient Monitoring

Patients should be monitored closely for:

Clinical improvement

Resolution of fever

Reduction in joint swelling

Improved motion

and declining:

Inflammatory markers.

Reaccumulating effusion or failure to improve should prompt consideration of:

Repeat aspiration

Repeat imaging

or

Repeat surgical débridement.

Key Principle

Septic knee is a serious infection of the knee joint, most commonly caused by Staphylococcus aureus, that can rapidly destroy articular cartilage if treatment is delayed.

The key diagnostic test is:

Joint aspiration with synovial fluid cell count, differential, Gram stain, culture, and crystal analysis.

Treatment generally requires:

Prompt empiric antibiotics after cultures and urgent drainage of the knee, most commonly by arthroscopic irrigation and débridement.

Early treatment usually results in a good outcome, whereas delay may lead to:

Cartilage destruction, osteomyelitis, ankylosis, septicemia, and secondary degenerative joint disease.