Hip replacement surgery (hip arthroplasty) has been touted by many experts as one of the most significant medical device innovations of the last 40 years. It has helped millions of people overcome painful arthritis, recover from hip fractures and improve quality of life.

However, hip implants do not come without risk. Some implant recipients experience implant failure and other severe hip replacement complications. These complications are more serious than common side effects.

Common side effects of hip replacements include minor pain or discomfort during the first three months after surgery. Blood clots are a common risk after any surgery that involves anesthesia, but physical therapy decreases the risk.

Major complications do not subside with medication or physical therapy. They often require revision surgery or intensive treatment. Some risks are avoidable. For example, patients can remove the risk of developing metal poisoning if they choose not to receive a metal-on-metal implant. However, some complications can develop even if surgeons and patients follow proper procedures.

Many patients experience relief from hip pain or increased mobility following a recovery period after hip replacement surgery. But many people also feel minor side effects, and a small number of people experience major complications.

Common side effects of hip replacement surgery include dislocation and inflammation. Doctors recommend avoiding certain movements, such as pulling the knees to the chest, in the first weeks after surgery to avoid dislocation. They prescribe anti-inflammatory drugs to reduce inflammation.

In some patients, a small amount of pain or instability can occur after surgery. There may also be stiffness, bleeding or nerve injury.

The type of implant may increase the risk for certain complications. For example, studies show metal-on-metal implants have greater risk of loosening or early failure because metal particles released from the device may weaken nearby bone and tissue. However, most manufacturers stopped making metal-on-metal implants after they faced lawsuits seeking compensation for unexpected hip complications.

The natural hip is held in place by a ligament that connects the ball to the socket. It’s also held in place by thick, dense tissue that surrounds the joint. During hip replacement surgery, this tissue is removed and dislocation can occur. Proper placement of the implant during surgery is vital to it remaining in place for the long term.

Although dislocation is not very common, it occurs in 1 to 5 percent of first-time hip implant recipients. The risk for dislocation rises as high as 20 percent during revision surgery. Dislocation leading to implant failure is most likely to occur within the first few months after surgery.

Heterotopic ossification is the process of bone forming outside of the skeleton. It happens when soft tissue is calcified. It usually occurs in areas of the body where severe trauma has occurred. In the case of hip arthroplasty, the muscles around the hip joint calcify and become stiff.

Heterotopic ossification is considered one of the most common hip replacement complications, occurring in nearly 50 percent of patients. However, only 10 percent of those suffer any side effects from the condition, such as tenderness, swelling or decreased range of motion. The condition can be treated with low-dose radiation and anti-inflammatory drugs. In severe cases, surgery is required to remove the calcified tissue.

With an incidence rate of about 1 percent, infections following hip surgery are not common.  If the infection is caught within four weeks of surgery, doctors may re-open the wound to clean it and inject antibiotics.

If that does not work or if the infection is not discovered until later, the implant may need to be removed and re-implanted after the infection has been eradicated. To avoid scar tissue and bone degeneration, re-implantation needs to occur within three months.

Infection is one of the most feared postoperative complications. In the most extreme cases, when the infection cannot be controlled, the entire hip and leg may have to be amputated. It is exceptionally rare for a patient to have to undergo a transpelvectomy, or hip-disarticulation. That occurs when the entire leg, including the hip joint, is removed and replaced with a prosthetic limb.

After total hip replacement or hip resurfacing, particle debris from the cup, ball or ball covering can lead to complications and implant failure.

As the implant recipient moves, the surfaces of the components rub against each other. The friction and abrasive wear between the components produces debris. It is estimated that every step taken with a hip implant produces between 100,000 and 1 million particulates of debris.

The type of debris depends on the type of implant. Different materials can cause different long-term complications. For example, metal-on-metal hip implants create metal debris called ions that can cause a condition known as metallosis. Metal-on-plastic implants create polyethylene particles that can lead to a condition known as osteolysis.

Metallosis is a rare condition that is caused by the buildup of metallic debris in the soft tissue of the body.  Metal debris from metal-on-metal hip implants can cause grey discoloration of the tissue surrounding the implant, usually indicating that the tissue is oxygen-starved and dying. The effects of metallosis can be particularly acute in implant recipients who have a condition known as metal sensitivity. In patients who have metal sensitivity, metallosis can lead to metal poisoning.

Osteolysis, or periprosthetic osteolysis, is the loss of bone around the hip replacement. It is the body’s attempt to remove particle debris, plastic or metal that’s produced by the implant. Osteolysis is the most common long-term hip replacement complication. It’s also the cause of nearly 75 percent of all hip implant failures.

Osteolysis occurs when the body recognizes implant debris as foreign and reacts to remove the particles. The body releases enzymes, cytokines and other cellular reactions into the area in an attempt to either expel or dissolve the particulates. The process causes a chronic inflammatory condition.

A July 2012 study in Orthopedics medical journal found that more than three-fourths of all metal-on-metal hip devices, such as the DePuy ASR and DePuy Pinnacle models , fail within two years of being implanted, and 98 percent of the patients in the study needed revision surgery within three years of the original operation.

Data from 80 patients who had revision surgery was analyzed for the study, and researchers attributed the early failure rates to aseptic acetabular loosening, metal hypersensitivity reactions, pseudotumor formation and component seizing.

Acetabular cup loosening was the most common culprit of early failure, with rates at 56.25 percent, and researchers described which brands were most often at fault:

“Of the 45 cups that loosened, 35 (78%) were Durom acetabular cups (Zimmer). The second most common cup that experienced aseptic loosening was the ASR cup (DePuy), with 4 (9%) failed cups. The remainder of the cups that loosened were 2 (4.4%) Magnum cups (Biomet), 2 (4.4%) Pinnacle cups (DePuy), 1 (2.2%) Conserve cup (Wright Medical), and 1 (2.2%) unknown cup.”

Partial hip replacements (hemiarthroplasty) usually treat injury, and total hip replacements usually treat bone decay caused by disease. The surgical procedure for a partial hip replacement surgery is less complicated than a total hip replacement.

In a partial hip replacement, the femoral head (the ball) of the hip is replaced and the socket is left alone. The artificial metal ball that’s implanted touches natural cartilage in the hip socket. This can cause the cartilage and surrounding tissues to become weak and less healthy, causing problems over time.

That’s why recovery results from partial hip replacements are less consistent than total hip replacements. During a total hip replacement, the socket is replaced so the artificial ball doesn’t touch natural cartilage.

There is also a risk of femur fracture during the hip replacement procedure because the surgeon purposefully dislocates the hip and forces the femur back into the socket. These movements can cause bones to fracture. However, fractures occur in only 5 percent of partial hip replacement operations.

Although hip arthroplasty is performed more than 450,000 times a year in the United States, and it’s considered by many to be routine surgery, complications can be dangerous and deadly.

Implant components can loosen over time. Loosening is considered the most serious long-term complication because it is the strongest indicator that a revision surgery will be required. A component could loosen because it’s worn out or because cement fails to hold it in place.

However, loosening of a hip implant can also be a warning sign of a more serious condition. If loosening is suspected, doctors will use a series of X-rays to detect movement in the components and to determine if any corrective actions are necessary.

Avascular necrosis is bone death caused by lack of blood. It can lead to implant failure. Avascular necrosis is more often associated with hip resurfacing than total hip replacement. After hip resurfacing, the metal cap that covers the femoral ball can reduce the amount of blood reaching the ball. If the bone is deprived of blood for an extended period of time, it will collapse, destroy the bone and disrupt use of the joint. In extreme cases, the implant has to be replaced.

Periprosthetic fractures are bone breaks that occur around the implant. The fractures can cause the implant to fail. They typically occur because the bone around the implant was weakened by osteoporosis, medications, pressure from the implant or stress placed on the implant. Periprosthetic fractures occur around hip implants following a little more than 1 percent of first-time surgeries and 4 percent of revision surgeries.

A 2003 study in the Iowa Orthopaedic Journal found that the mortality rate for patients having hip surgery for the first time was nearly 1 percent. For patients undergoing a revision surgery, the mortality rate skyrocketed to 2.5 percent. However, more recent studies have found that mortality risk has decreased as hip replacement techniques have become more sophisticated.

In England and Wales, hip replacement mortality decreased from 0.56 percent in 2003 to 0.29 percent in 2011, according to a 2013 study in The Lancet. In the United States, mortality decreased from 1.2 percent in 1991 to 0.8 percent in 2008, according to a 2011 study in the Journal of the American Medical Association. Studies indicate that old age, male gender and cardiovascular disease increase the risk of death during the first three months after a hip replacement.

When surgeons implant hip replacements properly and patients follow doctor’s advice during recovery from surgery, major complications are rare. But complications do affect thousands of people every year, and the adverse events can disrupt a person’s life.