A total knee arthroplasty, commonly known as a knee replacement, is a frequent orthopedic procedure performed to restore mobility and relieve pain from severe arthritis. This surgery involves replacing the damaged joint surfaces with an artificial implant, typically composed of strong metal alloys and a durable plastic spacer. The metal components are often made from cobalt-chromium or titanium alloys, while the plastic part is usually highly specialized polyethylene. While the procedure is highly successful for most patients, the concern about the body “rejecting” the new joint is a common one.
Why Implants Are Not Immunologically Rejected
The concept of biological rejection, as seen in organ transplants like a heart or kidney, does not apply to orthopedic implants. True immunological rejection occurs because the recipient’s adaptive immune system recognizes foreign proteins, specifically Major Histocompatibility Complex (MHC) proteins, on the transplanted tissue. This recognition triggers a systemic attack, requiring the use of powerful immunosuppressant drugs to prevent the body from destroying the foreign biological material.
Knee replacement implants are made from non-biological, inert materials like titanium, cobalt-chromium, and polyethylene. These materials lack the complex protein structures needed to activate the body’s adaptive immune response. Therefore, the immune system does not recognize the implant as a foreign biological entity to be attacked. The materials are selected specifically for their biocompatibility, allowing them to exist within the body without causing a widespread immune reaction.
Non-Infectious Causes of Implant Failure
Although true rejection is not a risk, knee replacement failure can still occur due to mechanical and inflammatory issues that cause the implant to lose fixation. The most frequent non-infectious reason for long-term failure is aseptic loosening, where the bond between the implant and the bone weakens without bacteria present. This loosening is often triggered by osteolysis, the local breakdown of bone tissue.
Osteolysis begins when microscopic particles, primarily from the wear of the polyethylene spacer, are released into the joint space. Immune cells called macrophages attempt to clear this debris, but the particles are too small to be digested. This triggers a chronic inflammatory reaction that activates osteoclasts, the cells responsible for bone resorption. As the bone around the implant is dissolved, the stable fixation of the prosthetic component is compromised, leading to pain and instability.
Another potential, though rare, non-infectious complication is a localized metal hypersensitivity reaction. This is a delayed-type hypersensitivity reaction to trace metals like nickel, cobalt, or chromium found in the alloy components. The reaction is a local inflammatory response, causing symptoms like persistent pain, swelling, or an itchy rash around the joint. While metal allergies affect 10% to 15% of the general population, failure of a knee replacement specifically due to this reaction is uncommon.
Periprosthetic Joint Infection
The most serious biological reason for knee replacement failure is a Periprosthetic Joint Infection (PJI), which is bacterial contamination rather than immune rejection. PJI occurs when bacteria, often Staphylococcus aureus or Staphylococcus epidermidis, colonize the surface of the artificial joint. These bacteria rapidly form a protective layer known as a biofilm, a complex matrix that shields the bacteria from the body’s immune cells and systemic antibiotics.
This biofilm creates a persistent, low-grade infection that can occur shortly after surgery (acute) or months to years later (chronic). The biofilm makes it nearly impossible to eradicate the bacteria with antibiotics alone, as the drugs cannot penetrate the protective layer. The ongoing bacterial presence leads to massive inflammation and destruction of the surrounding bone and soft tissue. In cases of established, chronic PJI, the only reliable way to cure the infection is to surgically remove all components of the implant.
Recognizing Complications and Treatment
Recognizing the signs of a complication is important for preserving the lifespan of the implant and the patient’s health. Symptoms of a problem, whether mechanical loosening or infection, often include persistent or new pain that worsens with activity. Other warning signs are swelling, warmth around the joint, and increasing stiffness that limits the range of motion.
In the case of infection, specific signs like fever, chills, and drainage from the surgical wound are strong indicators of PJI. Diagnosing the exact cause typically involves imaging, such as X-rays to check for loosening and bone loss, and blood tests for elevated inflammatory markers. If infection is suspected, a joint fluid aspiration is performed to analyze the fluid and culture any present bacteria.
Treatment depends entirely on the cause and severity of the failure. For aseptic loosening or component wear, revision surgery is performed to replace the failed parts with new ones. Chronic joint infection often requires a complex, staged treatment plan where the infected implant is removed, a temporary antibiotic-loaded cement spacer is inserted, and a second surgery is performed months later to place a new implant once the infection is proven to be eradicated.