Infection After Knee Replacement

What Is a Knee Replacement Infection?

A periprosthetic joint infection occurs when microorganisms infect tissues around a joint replacement.

Because bacteria can attach to prosthetic surfaces and form biofilm, infection involving an artificial joint behaves differently from many routine superficial infections.

When Can Infection Occur?

Infection can occur:

• Soon after surgery
• Weeks or months later
• Years after knee replacement

Later infection may occasionally occur after bacteria enter the bloodstream from another source and reach the prosthetic joint.

What Are the Symptoms?

Possible symptoms include:

• Increasing knee pain
• Swelling
• Warmth
• Redness
• Persistent wound drainage
• Wound breakdown
• Stiffness
• Reduced function
• Fever in some cases
• Recurrent joint swelling

Chronic low-grade infection may produce more subtle symptoms.

Can I Have an Infection Without Fever?

Yes.

The absence of fever does not exclude prosthetic joint infection.

Some chronic infections present mainly with pain, stiffness or progressive implant problems.

Can an Infected Knee Look Normal?

Sometimes the external appearance is relatively unremarkable.

This is one reason that diagnosis should not rely solely on skin redness or wound appearance.

How Is Periprosthetic Joint Infection Diagnosed?

There is no single test that diagnoses every periprosthetic joint infection.

Diagnosis uses a combination of clinical findings, blood tests, synovial-fluid findings, microbiology and operative findings where relevant.

Some findings can strongly establish infection, whereas others only increase or decrease suspicion.

What Blood Tests Are Used?

ESR and CRP are commonly used as screening or supportive inflammatory markers.

Elevated results can increase suspicion, but they are not specific to prosthetic joint infection.

Normal results do not absolutely exclude every prosthetic joint infection, particularly in some chronic or low-grade infections.

What Is Knee Aspiration?

Joint aspiration involves obtaining fluid from the knee using a sterile needle.

The fluid may be assessed for:

• Cell count
• Differential
• Microbiology and culture
• Other validated synovial tests when appropriate

The result should be interpreted in the context of the timing from surgery, clinical presentation and other diagnostic findings.

Aspiration can provide important information when infection is suspected, but it is not required in every single painful prosthesis.

Can Antibiotics Affect Culture Results?

Yes.

Previous or ongoing antibiotics can reduce microbiological culture yield in some circumstances.

When diagnostic sampling is planned, antibiotic timing is coordinated by the treating team according to the clinical situation.

Patients must not stop, delay or alter prescribed antibiotics without specific instructions from their treating team.

If a patient is septic or systemically ill, necessary antimicrobial treatment should not be delayed merely to improve culture yield.

Why Are Cultures Important?

Identifying the causative organism can help guide antibiotic treatment, but microbiological findings must be interpreted with the complete clinical, laboratory and operative picture.

Culture-negative prosthetic joint infection can occur.

When surgery is undertaken for suspected prosthetic joint infection, multiple separately collected periprosthetic tissue samples are commonly obtained for microbiological assessment.

What Is Biofilm?

Biofilm is a structured community of microorganisms that can adhere to an implant surface.

This makes established prosthetic infection more difficult to eradicate with antibiotics alone.

Biofilm is one reason surgical treatment may be required.

Can Antibiotics Alone Treat an Infected Knee Replacement?

In many established prosthetic joint infections, antibiotics alone do not provide definitive eradication.

Treatment depends on:

• Timing
• Organism
• Implant stability
• Soft tissues
• Patient health
• Previous treatment

Some patients require surgical treatment combined with appropriate antimicrobial therapy.

When Can DAIR Be Considered?

DAIR — debridement, antibiotics and implant retention — may be considered for appropriately selected acute postoperative or acute hematogenous prosthetic joint infections.

Factors that may favor DAIR include:

• Well-fixed components
• Acceptable implant position
• A short duration of acute symptoms
• Manageable soft tissues
• Ability to perform thorough surgical debridement
• Exchange of accessible modular components when appropriate
• Microbiological and patient factors compatible with the strategy

DAIR is not appropriate for every infection. There is no single absolute number of days that determines suitability, and the decision remains individualized.

What Is One-Stage Revision?

In selected patients, all infected components can be removed, appropriate debridement performed and definitive revision components implanted during the same operation.

Suitability depends on factors such as:

• Patient condition
• Soft-tissue quality
• Bone loss
• Organism or microbiological information where available
• Ability to perform adequate debridement
• Reconstructive feasibility
• Local multidisciplinary experience and treatment strategy

One-stage revision is not universally superior, and culture positivity is not necessarily mandatory in every possible strategy.

What Is Two-Stage Revision?

Two-stage revision remains an important strategy for selected chronic, complex or difficult-to-treat infections.

Two-stage revision is not mandatory or a universal gold standard for every infection.

What Is an Antibiotic Spacer?

A spacer can:

• Occupy the joint space temporarily
• Deliver local antibiotics
• Assist management between stages
• Provide varying degrees of temporary stability and mobility depending on design

Spacers can be static or articulating depending on the reconstruction strategy.

How Do You Know When the Infection Is Gone?

There is no single test that proves eradication with perfect certainty in every patient.

Assessment may consider:

• Clinical progress
• Wound condition
• Inflammatory-marker trends when relevant
• Microbiology
• Aspiration in selected cases
• Operative findings

The decision to proceed with reimplantation should consider the complete clinical picture. Normalized ESR or CRP alone does not prove infection eradication. An antibiotic holiday is not a mandatory patient instruction.

What Happens If There Is Bone Loss?

Bone defects are reconstructed according to their location, size, remaining metaphyseal or diaphyseal bone and stability requirements.

Potential tools may include:

• Stems
• Augments
• Metaphyseal cones
• Sleeves
• Bone graft in selected circumstances
• Greater constraint where ligament function requires it

Not every listed option is needed together, and cones and sleeves are not interchangeable in every case.

What Happens If the Ligaments Are Damaged?

Severe infection, previous surgery and debridement can compromise knee stability.

More constrained revision implants may be necessary when native ligament function is insufficient.

What Are the Goals of Treatment?

The objectives are to treat and control the infection, obtain a durable stable reconstruction when feasible, relieve symptoms and restore useful function.

Permanent eradication and a particular outcome cannot be guaranteed; the exact strategy must be individualized.