Total Knee Arthroplasty (TKA), or total knee replacement, is a highly successful procedure that restores function and significantly reduces pain. While most patients experience excellent, long-term results, a small percentage continue to experience chronic pain or develop new discomfort months or years after recovery. This persistent pain can be frustrating and indicates underlying issues requiring medical investigation. Understanding why an artificial knee might hurt is the first step toward finding a solution.
Mechanical Failures of the Implant
Pain arising from the hardware often centers on the interface between the artificial components and the bone. Aseptic loosening is a common mechanical failure where the bond between the bone and the cement or implant material breaks down without infection. This process begins due to microscopic wear particles from the polyethylene plastic rubbing off the implant surface. The body reacts to these fragments, triggering inflammation that gradually dissolves the surrounding bone tissue, a process called osteolysis.
This bone loss creates a gap, allowing the implant to shift microscopically. This leads to pain, instability, and a sensation of the knee buckling, especially during weight-bearing activities.
Another issue is the degradation of the polyethylene liner, the durable plastic component acting as cartilage between the metal parts. Over years, this plastic wears down, losing thickness and integrity. This wear destabilizes the joint, causing abnormal motion and increased stress on surrounding tissues, resulting in chronic discomfort.
Component malalignment during the original surgery can also cause chronic pain. Rotational malalignment of the femoral or tibial components causes abnormal forces and tracking issues for the kneecap. Patients often describe this chronic pain as an awkward or “not right” feeling.
Infection and Adverse Biological Reactions
Pain originating from the body’s reaction to the artificial joint, separate from mechanical stress, demands immediate attention. Periprosthetic Joint Infection (PJI) is a serious complication where bacteria contaminate the joint. PJI is classified by timing: acute infections appear within weeks of surgery with clear symptoms like fever, warmth, and drainage. Chronic or delayed infections surface years later with subtle signs.
In chronic PJI, bacteria often form a protective biofilm on the implant surface, making them difficult for the immune system and antibiotics to reach. Symptoms may include only persistent, deep joint pain and stiffness, which can be mistaken for mechanical loosening.
A less common, non-infectious biological reaction is metal hypersensitivity, an adverse response to metal alloys used in the implant (e.g., nickel, cobalt, or chromium).
This hypersensitivity is considered a diagnosis of exclusion, meaning other causes must be ruled out first. It manifests as chronic, painful synovitis—inflammation and swelling of the joint lining. Patients may experience persistent pain, stiffness, and sometimes a skin rash around the knee, often beginning two months to two years after TKA. While a history of skin reactions to metal jewelry suggests a risk, the definitive link between a metal allergy and a painful implant remains complex and challenging to confirm.
Pain Stemming from Nearby Structures
Not all post-TKA pain originates from the implant or infection; often, surrounding soft tissues or distant joints are the source. Arthrofibrosis, or excessive scar tissue formation, is a common issue where dense, fibrous tissue builds up around the knee joint. This scar tissue limits the knee’s normal range of motion and causes pain, leading to stiffness that prevents full bending or straightening of the leg. Restrictive stiffness may require a procedure to gently manipulate the joint under anesthesia to break up the adhesions.
Soft tissue inflammation, such as tendinitis or bursitis, can also cause localized pain around the knee. Surgery can irritate structures like the quadriceps or patellar tendons, or the bursae sacs that cushion the joint. This pain is manageable with physical therapy, anti-inflammatory measures, and rest, and does not indicate artificial joint failure.
Another common source of knee pain is referred pain, which originates from a separate location but is felt in the knee. Conditions like hip arthritis or nerve compression in the lumbar spine can manifest as discomfort in the knee because the nerves supplying these regions are interconnected.
Neuropathic pain arises from irritation or damage to the nerves around the knee, often occurring during the surgical incision or tissue manipulation. This pain is characterized by distinct sensations like burning, tingling, or shooting pain, and can persist for months after the procedure. Although most nerve irritation resolves as the nerves heal, up to half of those experiencing neuropathic symptoms at three months post-surgery may still report this pain 15 months later.
Guidance on Symptoms and Next Steps
If you are experiencing persistent or new pain in your artificial knee, contact your orthopedic surgeon for an evaluation. Before the appointment, track your symptoms, noting when the pain started, its severity, and what activities make it better or worse. Specific warning signs necessitate an urgent visit.
These urgent signs include:
- A sudden onset of intense pain
- A feeling of heat or warmth around the knee
- An inability to bear weight
- Fever, chills, or any new drainage from the surgical incision
These symptoms are particularly concerning and require immediate medical assessment, as they can signal an active infection. Providing your surgeon with detailed information assists them in determining the cause of your discomfort.