Knee infection explained – methods to diagnose it

Knee and hip replacements are two of the most commonly performed elective surgeries. Such an operation is of critical importance in the patient’s life, not only does it reduce their pain, but also gives them back the possibility to move their body without limitations. In the shadow of successful surgeries, there is, however, a massive challenge that today’s medicine is struggling with: a relatively high number of patients who are undergoing hip or knee replacement may develop an infection after the operation. According to the latest studies periprosthetic joint infection (PJI) represents 25% of failed total knee arthroplasties (TKA).  

This article provides a deep insight into the topic of knee infection:

  • What kind of knee infections exist?
  • What causes PJI and how does it develop in the body?
  • What are the methods of knee infection diagnosis?
  • How does Lyfstone® Calprotectin work and what makes it a highly effective method for diagnosing PJI?

Keep reading to learn the answers.

What kind of knee infections exist?

Infections occurring after hip replacement surgery are divided into two groups: superficial infection and deep infection. Their occurrence, severity, and treatment are different.

Acute knee infection

In the skin around the incision after knee replacement surgery, an infection can develop. These infections are referred to by doctors as acute, superficial, or early-onset infections. Acute infections typically emerge shortly (up to 90 days) after surgery. The treatment is not complicated, but if left untreated, an acute knee infection might advance.

Chronic knee infection

An infection can also develop deeper, around the patients’ prosthetic knee. These infections are classified as chronic, deep, delayed-onset, or late-onset by doctors. Chronic infections are dangerous, and they can arise months or even years after knee replacement surgery. The treatment often consists of numerous phases and in many cases, the infected prosthetic knee must be removed.

Inflammation is one of the most common symptoms of periprosthetic knee infection

Knee infection after knee replacement surgery

Generally, 60 to 70% of prosthetic joint infections arise within the first two years of surgery, but they can occur at any time after the replacement. Bacteria are the main cause of knee infections. 

The most typical ways bacteria enter the body:

  • Infections of the airways
  • During extensive dental surgeries, through skin breaches or cuts (such as a tooth extraction or root canal)
  • As a result of wounds from prior surgical operations

Although bacteria are prevalent in our gastrointestinal tract and on our skin, our immune system normally keeps them under control. If germs enter our circulation, for example, the immune system reacts quickly and eliminates the invading bacteria. Knee replacements, however, are made of metal (cobalt-chromium), plastic (polyethylene), or ceramic and a prosthetic knee does not react to the immune system in the same way that the patient’s own knee would. Therefore the immune system has difficulties attacking microorganisms that get inside these implants. 

Patients with infected joint replacements frequently require surgery to cure the infection, despite medicines and preventative therapies.

What else can cause a knee infection after surgery?

Some patients are more likely to acquire infections after a knee replacement surgery.

The following risk factors enhance the chance of infection:

  • Vascular disease of the periphery
  • Immunosuppressive medications (such as chemotherapy or corticosteroids)
  • Immune dysfunction (such as HIV or lymphoma)
  • Type 2 diabetes
  • Obesity

Diagnosing knee infection

Signs and symptoms of knee infection after replacement surgery may vary from patient to patient, but the warmth of the joint, swelling, fevers, chills and night sweats, increased pain or stiffness in a previously well-functioning joint, and fatigue are the most common ones. 

Traditional diagnosing methods

There are several methods for detecting a potential knee infection:

  • Imaging test (MRI, X-ray, CT scan, or bone scan)
  • Blood test (CRP/ESR)
  • Joint aspiration, in which the doctor draws the synovial fluid out of the knee joint and gets it tested in a laboratory.

Although these methods are the most common, their efficiency, accuracy, and speed are not necessarily the best. The result of the blood test can take a long time, and the result only shows whether there is an infection in the patient’s body, but not where. Imaging tests are not suitable for detecting or ruling out an infection. 

A sample taken from the joint fluid can provide an accurate result, but it can take hours or even days for the result to arrive which is a waste of precious time. When a doctor decides to puncture the knee, it means that there is already an urgency and suspicion about an infection. Therefore in a situation like this, a quick result is crucial, as the patient is probably already in the hospital or at the emergency department. 

So what is the solution?

Innovation in prosthetic joint infection diagnosis: Lyfstone® Calprotectin

The presence of neutrophils is a hallmark of prosthetic joint infection (PJI), and infections result in a significant release (from the neutrophils) of calprotectin. Calprotectin levels have been found to be higher in both acute and chronic prosthetic joint infections. We started the development of our innovative product based on this fact. During the development process of the product, our experts constantly consulted with experienced international orthopedic specialists and surgeons, to make sure that it meets their needs in every aspect.

Lyfstone® Calprotectin rapid test and app for precise PJI diagnosis

The result is Lyfstone® Calprotectin, a unique bedside orthopedic test that can rule out prosthetic joint infection from a patient’s synovial fluid in about 15 minutes. The Lyfstone® test requires only a 20 μl sample and thanks to its simple work-up, it can be used at any stage of care, from consultation to pre-op and post-op. The test used together with the Lyfstone® mobile app provides a precise quantitative evaluation of calprotectin levels in synovial fluid.

Lyfstone® provides a professional solution for surgeons to deliver an accurate diagnosis and make the right decision sooner and more efficiently.

Possible treatments for knee infection – DAIR procedure

If the infection is confirmed, the patient needs immediate treatment. Debridement, antibiotics, and implant retention (DAIR) is a surgical treatment used to treat PJI following total joint arthroplasty (TJA). 

Antibiotics: If a surface hip or knee infection is detected early, antibiotics might be enough to successfully heal it.

Debridement: Deeper infections that are detected within a few days or weeks might be treated with a process known as debridement. During this procedure, the diseased tissues are surgically removed by the surgeon. The knee implant gets thoroughly cleaned, and some components are changed. Intravenous antibiotics are given for the patient roughly for the following 6 weeks?

Staged surgery: When a knee infection is detected a two stage procedure is required. If the infection is not detected major re-infection will occur. 

  • During the first stage, the orthopedic surgeon removes the implant and cleans the infected area. The doctor then places an antibiotic spacer where the implant was and closes the wound. This is followed by intravenous antibiotic treatment. It is not recommended that the patient put weight on the treated leg until the next surgery.
  • During the second, so-called revision surgery, the spacer gets removed and the doctor inserts a new implant into the knee.

The Lyfstone® rapid test is the most effective for detecting 90+ days post-surgery infections. It can be used perfectly in all steps of staged surgery so that the orthopedic surgeon can get quick and reliable results about the patient’s condition.

How to prevent knee infection?

Periprosthetic joint infection is definitely a dangerous complication that has a high morbidity and mortality rate. Therefore it is always recommended to prioritize prevention over treatment.

Numerous methods have been implemented by the Consensus in order to avoid infection and successfully treat it when it occurs. 

Identifying host risk factors, optimizing the patient’s overall health, providing correct wound care, and improving the operating room environment are some of the essential actions that can help reduce the overall incidence of infection.

And last but not least, patients undergoing TJA are always at risk of infection; consequently, prophylactic antibiotics must be prescribed accordingly.


Total joint arthroplasty is one of the most successful operations in medicine, improving the quality of life and function level of patients with degenerative joint problems. Despite all the developments in this field, fast and precise diagnosis of periprosthetic joint infection is still a significant issue for traumatologists and orthopedic surgeons. 

The Lyfstone® Calprotectin rapid bedside test provides a comprehensive solution for this situation. It highly increases the surgeon’s ability to rule out hip and knee infections quickly and accurately while helping to save a significant amount of healthcare costs. Thanks to its innovative technology surgeons can reliably determine the presence and extent of PJI from the patient’s synovial fluid in only 15 minutes using the Lyfstone® orthopedic test and mobile app. This allows surgeons to make decisions at the “point-of-care” which is a clear game changer in the quality of care for patients with implants.

Learn more about Lyfstone® and the possibilities that this unique, innovative solution can provide. Contact one of our professional distributors who are available throughout Europe.

Get in touch with

our distributors

in europe

Our professional distributors
are available throughout Europe. Choose the one closest to you and contact your sales representative today.

Get in touch with

our distributors

in europe

Our professional distributors
are available throughout Europe. Choose the one closest to you and contact your sales representative today.