Total Knee Arthroplasty (TKA), commonly known as knee replacement surgery, is a highly effective and widely performed procedure intended to alleviate severe knee pain and restore function. This operation involves resurfacing the damaged bone and cartilage with artificial components made of metal alloys and specialized plastics. Over 90% of implants function well after 15 years, yielding impressive long-term success rates. Despite this success, the potential for complications remains a common source of patient concern, particularly the fear that the body might “reject” the new joint. Understanding the mechanics of implant failure requires a shift in terminology, as the causes of failure are complex and distinct from the immune-mediated processes seen in organ transplantation.
Immune Rejection Versus Implant Failure
The idea of the body “rejecting” a knee replacement misunderstands how the immune system interacts with inert materials versus living tissue. True immunological rejection, as occurs in organ transplants, involves the adaptive immune system attacking foreign cellular markers on living tissue. Knee implants are composed of materials like cobalt-chromium alloys, titanium, and polyethylene, which lack the cellular structures needed to trigger this systemic response. Implant failures are categorized as mechanical, infectious, or inflammatory reactions, not systemic immune rejection. Metal hypersensitivity, an allergy to components like nickel or cobalt, is the closest immune-related phenomenon, but it is a localized inflammatory sensitivity, not full-scale rejection.
The Major Causes of Non-Infectious Failure
Aseptic Loosening
The most common reason for long-term knee replacement failure, excluding infection, is aseptic loosening. This complication occurs when the bond between the bone and the implant breaks down without bacteria present. The primary mechanism involves tiny wear particles, mainly from the polyethylene plastic spacer, which are shed into the joint space during movement. Macrophages attempt to engulf these microscopic particles, releasing potent inflammatory chemicals called cytokines that activate osteoclasts, the cells responsible for breaking down bone tissue. The resulting bone loss, known as osteolysis, creates a gap around the implant components, allowing them to become loose and painful. Aseptic loosening can also occur due to fixation failure at the cement-implant interface, particularly if the surgical technique was compromised.
Instability and Malalignment
Instability or malalignment is another significant non-infectious cause of failure. This issue can stem from improper surgical technique, incorrect component sizing, or ligamentous imbalance. Malalignment causes the knee to feel unstable or “give way.” This leads to uneven loading and accelerated wear on the polyethylene insert. This increased stress can cause the plastic component to degrade, potentially leading to fracture or delamination.
Metal Hypersensitivity
Hypersensitivity reactions to the metal components, such as nickel, cobalt, or chromium, cause a localized inflammatory response. This reaction is a type IV delayed hypersensitivity. It can lead to persistent pain, swelling, and dermatitis around the joint. While relatively uncommon, this allergic response is distinct from true immunological rejection, instead representing an inflammatory reaction to the metal ions released from the implant surface.
Periprosthetic Joint Infection
Periprosthetic Joint Infection (PJI) is a distinct and serious complication of total knee arthroplasty, and it is a leading cause for revision surgery. PJI occurs when bacteria colonize the surface of the artificial joint components, forming a protective layer called a biofilm. This biofilm shields the bacteria from the body’s immune defenses and systemic antibiotics, making the infection exceptionally difficult to eradicate without surgery.
Infection can manifest in an acute form, typically within the first few weeks following surgery, often caused by highly virulent organisms like Staphylococcus aureus. In these cases, the infection is usually introduced during the surgical procedure itself or from an early wound complication. The diagnosis of PJI within the first year of surgery accounts for a significant portion of all infections.
Chronic or delayed infections occur months or even years later, often caused by less virulent organisms like Staphylococcus epidermidis. These result from hematogenous spread, where bacteria travel to the joint through the bloodstream from a distant site, such as a dental abscess or a urinary tract infection. Regardless of the timing, the presence of the biofilm requires aggressive treatment, which often involves surgical debridement and prolonged antibiotic therapy.
Recognizing Signs and Revision Options
Symptoms of Failure
Patients should be aware of specific warning signs indicating a problem with their knee replacement. The most common symptom of mechanical failure, such as aseptic loosening, is the return of persistent or worsening pain. This discomfort is often aggravated by weight-bearing activities. Signs of acute infection are generally more pronounced, while chronic infections may present with only subtle symptoms, making diagnosis challenging. Any new or sudden instability, such as a sensation of the knee “giving way,” also warrants prompt medical evaluation.
- Persistent or worsening pain in a previously pain-free knee.
- Recurrent swelling or a feeling of instability.
- Fever, chills, excessive warmth, redness, or drainage from the incision (acute infection).
- Generalized joint pain and fatigue (chronic infection).
- New or sudden instability, such as the knee “giving way.”
Revision Arthroplasty
When a knee replacement fails, the primary treatment is Revision Arthroplasty. This complex secondary surgery removes and replaces one or more original components with new, specialized implants. For non-infectious issues like loosening or instability, the goal of revision surgery is to restore alignment, stability, and proper fixation. Treating Periprosthetic Joint Infection is significantly more involved and frequently requires a two-stage revision procedure. The first stage involves removing all components, thoroughly cleaning the joint space, and inserting an antibiotic-loaded cement spacer to treat the infection. The second stage implants a new, permanent knee prosthesis once the infection is medically cleared after a period of weeks or months.