The most serious complication is infection of the joint, which occurs in <1% of patients. Deep vein thrombosis occurs in up to 15% of patients, and is symptomatic in 2-3%. Nerve injuries occur in 1-2% of patients. Persistent pain or stiffness occurs in 8-23% of patients. Prosthesis failure occurs in approximately 2% of patients at 5 years.

Deep Vein thrombosis
According to the American Academy of Orthopedic Surgeons (AAOS), "blood clots in the leg veins are the most common complication of knee replacement surgery. Your orthopedic surgeon will outline a prevention program, which may include periodic elevation of your legs, lower leg exercises to increase circulation, support stockings and medication to thin your blood."

Fractures
Periprosthetic fractures are becoming more frequent with the aging patient population and can occur intraoperatively or postoperatively.

Loss of Motion
The knee at times may not recover its normal range of motion (0 - 135 degrees usually) after total knee replacement. Much of this is dependent on pre-operative function. Most patients can achieve 0 - 110 degrees, but stiffness of the joint can occur. In some situations, manipulation of the knee under anesthetic is used to improve post operative stiffness. There are also many implants from manufacturers that are designed to be "high-flex" knees, offering a greater range of motion.

Instability
In some patients, the kneecap is unstable post-surgery and dislocates to the outer side of the knee. This is painful and usually needs to be treated by surgery to realign the kneecap. However this is quite rare.

In the past, there was a considerable risk of the implant components loosening over time as a result of wear. As medical technology has improved however, this risk has fallen considerably. One implant manufacturer claims to have reduced this risk of wear by 79% in fixed-bearing knees. Another implant manufacturer claims to have reduced the risk of wear by 94% in mobile-bearing, also known as rotating platform, knees. Knee replacement implants can last up to 20 years in many patients; whether or not they actually survive that long depends largely in part upon how active the patient is after surgery.

Infection
The current classification of AAOS divides prosthetic infections into four types.

• Type 1 (Positive intraoperative culture): 2 positive intraoperative cultures
  
• Type 2 (early postoperative infection): Infection occurring within first month after surgery
  
• Type 3 (acute hematogenous infection): Hematogenous seeding of site of previously well-functioning prosthesis
  
• Type 4 (late chronic infection): Chronic indolent clinical course; infection present for >1 month

While it is relatively rare, periprosthetic infection remains one of the most challenging complications of joint arthroplasty. A detailed clinical history and physical remain the most reliable tool to recognize a potential periprosthetic infection. In some cases the classic signs of fever, chills, painful joint, and a draining sinus may be present, and diagnostic studies are simply done to confirm the diagnosis. In reality though, most patients do not present with those clinical signs, and in fact the clinical presentation may overlap with other complications such as aseptic loosening. In those cases diagnostic tests can be useful in confirming or excluding infection.