Infection Post-Hip Replacement
Total hip replacements are among the most successful surgeries in existence. The rate of complication is generally very low, and the surgery itself usually provides dramatic relief of symptoms. However, anytime a prosthetic joint is inserted into the body, there is a risk of infection due to the artificial joint surface’s susceptibility to bacterial growth. An infection involving a hip replacement may cause global effects such as a fever and chills, but more commonly causes symptoms localized to the hip. These symptoms include pain and swelling -and often the need to revise the hip replacement in an effort to eradicate the infection.
Diagnosis of a prosthetic joint infection involves a physical examination; obtaining x-rays of the joint; and blood tests such as a complete blood count, C reactive protein, and erythrocyte sedimentation rate. Synovial fluid is also collected by inserting a needle into the joint (needle aspiration) and sent for laboratory testing.
Treatment of an infected hip replacement usually involves surgery. If the infection comes on quickly (ex. a few weeks after the hip replacement surgery) it may be possible to treat the infection by extensively washing out the hip joint and placing the patient on antibiotics that are effective against the bacteria that caused the infection. More commonly, the infection will be chronic. In this situation, the entire prosthesis will need to be replaced. This is often done in two stages by first removing the prostheses and then putting a new prosthesis in at a later date. In some instances, it may be possible to remove the infected prostheses, clean out the infected joint, and replace it with a new prosthesis as a one-stage procedure (all in on surgery).
Total hip replacements are very common and successful procedures performed to treat osteoarthritis. The surgery involves removing the damaged bone making up the hip joint and replacing it with a prosthetic hip joint composed of various materials including metal, ceramic and/or plastic (polyethylene).
The vast majority of first time hip replacements have no complications. In a small number of cases (0.4-3%) however, an infection may develop following the procedure. Unfortunately, any time a foreign object such as an artificial hip is introduced into the body, an infection can occur because the artificial joint surfaces are a prime location for bacterial growth. Additionally, the prosthesis itself does not have a blood supply so antibiotics cannot reach all of the bacteria.
A prosthetic hip joint infection can occur at any time following a hip replacement. The time of infection onset can be determined using the surgery date as a starting point, and characterized as either early, delayed, or late onset. There are different types of infection-causing bacteria that are typically associated with each time point. Thus, proper characterization of the time of onset of infection can help determine which bacteria are responsible.
Early onset is defined as less than 3 months since the operation. Delayed onset is 3-12 months since the date of surgery and late onset is greater than 12 months post-surgery. For early and delayed onset infections, the infection was likely introduced during the surgery. This can occur despite strict adherence to sterile procedures such as maintaining a sterile field around the hip and limiting the amount of people in the operating room. Late onset infections typically occur as a result of bacteria spreading from another source of infection in the body and taking hold at the prosthetic joint. An example of this is a patient who has a very severe infection of their bladder or urinary tract resulting in spread of bacteria through the blood to the hip joint.
Risk factors for developing a prosthetic joint infection include having a compromised immune system such as is seen in HIV positive individuals or those receiving therapies that reduce immune function. These include chemotherapy and steroid-based treatments. This is because the body is unable to effectively defend itself against infection, leading to spread of bacteria that might otherwise have been cleared by the immune system. Individuals with diabetes may also be at increased risk of developing a prosthetic joint infection.
Signs and symptoms of an infected prosthetic hip joint include both local and global effects. The global symptoms may include fever, chills, night sweats and general tiredness. Local signs and symptoms of an infection may include redness (erythema), swelling, pain, or warmth around the prosthetic hip joint. If the infection occurs early on before the surgical incision has healed, pus and fluid can leak from the wound. For delayed or late onset infections, any of the above signs or symptoms can be present. However, there may also be pain, stiffness and reduced range of motion at the hip joint, even if the hip was working well before.
The physical exam performed in cases where a prosthetic joint infection is suspected includes not only an examination of the hip joint, but also a full body examination given that infections can cause whole body symptoms. If an infection is present, the individual’s vital signs may be higher than their normal values. This includes heart rate, breathing rate, blood pressure and temperature.
When examining the hip itself, there may be pus or other fluid leaking from the surgical incision site as well as redness, swelling and warmth around the joint. The presence of pain and loss of range of motion of the hip joint are also signs of a possible infection, and in many cases may be the only findings present on exam.
Imaging and Laboratory Tests
The main imaging used in the diagnosis of an infected prosthetic hip is an x-ray. A change in the position of the prosthesis in comparison to x-rays taken directly after the surgery, or motion of the prosthesis in special views known as stress views, means the prosthesis has subsided (changed position). Infection is a possible cause of subsidence of the prosthesis-although there are other common reasons as well. Unfortunately, many of the more advanced imaging techniques are not useful for visualizing joint replacements as the metal in the replacement decreases the quality of the image.
In addition to imaging, blood testing is commonly performed in order to determine the presence of infection. Basic blood tests such as a complete blood count will be ordered to determine any abnormalities in levels of different cells in the body. White blood cells, the immune cells of the body that fight infection, can be elevated with infection. Other common blood tests that are ordered include C reactive protein and erythrocyte sedimentation rate. An elevation in either of these blood tests is a sign of inflammation, a cause of which can be an infection. Blood samples may also be tested to look for the presence of infection-causing bacteria.
Hip joint Aspiration
In order to make a definitive diagnosis of a prosthetic joint infection, the bacteria that is causing the infection must be isolated. In order to do this, fluid is collected from the joint space through insertion of a needle into the joint and removing a sample of fluid (needle aspiration). The fluid is then analyzed for the presence of white blood cells and bacteria among other things. The presence of bacteria supports the diagnosis of an infection.
Treatment of an infected hip replacement depends on the severity of the infection and when the infection occurs. The first line of treatment for prosthetic joint infections is often surgical. Unfortunately, antibiotics alone are rarely curative of prosthetic joint infections. When orthopedic surgeons talk about an infected hip replacement they mean an infection that involves the actual prosthesis -an infection deep in the hip joint. This is different than a simple wound infection that does not extend deep to the hip joint. A simple wound infection can often be treated non-operatively with antibiotics. However, curing a deep infection that involves the hip prosthesis usually involves some form of surgery as antibiotics alone are not likely to be curative.
The reason that antibiotics alone are not effective when the infection is deep and involves the hip joint is because the hip prosthesis is a foreign material and therefore does not have a blood supply. Bacteria can grow on the artificial material of the joint and form a protective layer around themselves, known as a biofilm. This biofilm is very hard for the antibiotics to penetrate. As a result, the antibiotics cannot reach the prosthesis on a microscopic level. Therefore, the bacteria adhere to the prosthesis leading to a “chronic” infection.
The type of operation required to eradicate an infected hip replacement ranges from a simpler debridement procedure to remove the infected tissue to a more complex staged surgery. Cleaning out the wound in the operating room (irrigation and debridement) and then placing the patient on intravenous antibiotics for a period of time may be effective if the infection occurs in the first few weeks after the joint replacement was performed.
However, If the infected hip replacement is chronic, the hip prosthesis will need to be removed, the joint will have to be cleared of all infection, and a new joint replacement will be inserted. This has traditionally been done in two stages, although more recently some surgeons are trying to do this in a single “one-stage” surgery. In a “two-stage” surgery, the first stage involves removing the infected prosthesis and temporarily replacing it with an antibiotic spacer. The patient is then placed on intravenous antibiotics for a period of time (often 6 weeks). The antibiotics are then stopped for a period of time (often about 6 weeks) and then the hip joint is assessed for any residual infection via a review of blood work and primarily via a fluid aspiration and culture. If the fluid taken from the hip joint does not grow any bacteria and the blood work is normal, a “second-stage” revision surgery is performed where a new prosthetic will be inserted.
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