Infection in Artificial Joints
Artificial joints are one of the most successful medical interventions, with the highest quality of life improvements in the medical field.
Infection is the most feared and important of the complications which can occur after joint replacement. It is responsible for a significant amount of pain, disability and health care costs.
The risk of infection from the operation itself has dropped to 1% after total hip and shoulder replacement, and 2% after total knee replacement.
This very low level of infection after operation is due to the use of antibiotics before, during and after the operation and the technical improvements in airflow systems which reduce bacterial contamination from the people in the operating theatre.
The management of joint infections is not standardised, in other words there is no clear and obvious way to manage this problem. Doctors look at each complex problem as it appears and manage it according to their understanding and advice from microbiologists.
How Infection Is Diagnosed
Pain or inability to weight bear properly can be signs of infection but the scientific work diagnoses infection using specific criteria. For example:
What Bugs Are Usually Found?
Staphylococcus of various kinds are common, there are mixed pictures (with various kinds of bacteria), Streptococcus, so-called “gram negative bacteria” (which don’t stain under gram testing), enterococci and anaerobic bacteria (bacteria which don’t need oxygen to live and grow). In 11% of what seem to be infections, no microorganism can be found.
The Development of Infection
So how do bacteria grow in joints? Not a question which gets asked much by most of us including me. However, bacteria in these cases usually grow in what are called biofilms.
In a biofilm they grow in a matrix and develop into complex structures with considerable variation. When enough bacteria are present they start to produce biofilm, and within this structure they obtain protection from antibiotics and the immune response of the body.
In the biofilm, bacteria are much more resistant to death from drugs than bacteria loose in the body. As bacteria in biofilms have a reduced growth rate, perhaps due to poor nutrition getting through the biofilm, they are less sensitive to antibiotics.
Interactions Between Bugs, Body and Artificial Joints
Various body chemicals cover a metal implant immediately it is placed in the body. Staphylococcus aureus sticks to these chemicals using its own complex adhesive molecules.
Foreign implants obviously do not have any circulation close to their surfaces. In normal tissues this circulation is important for immune defenses and delivering antibiotics. You need 100,000 fewer Staphylococcus bacteria to cause an infection in the presence of a foreign body in the tissues. Infection can even occur in the presence of antibiotics used to insert artificial joints.
Less than 100 units of microorganisms can be enough to cause infection in these cases, and small numbers of bacteria in the blood can cause the same problem in artificial implants. Due to all these reasons, infection does not only occur during an operation but during the whole lifetime of the joint. This means anyone with an artificial joint is vulnerable to the risk of infection and will need to take care to prevent this.
The Way Infections Present In Practice
Artificial joint infections can be classified as to the time they occur:
How Joint Infections Are Diagnosed
White blood cell counts may be raised but do not by themselves indicate joint infection. Taking successive measurements over time can be helpful, with high levels making joint infection a strong suspicion.
Microscopic studies of the tissues around the joint can be useful but the technique of doing them and the interpretation is complex.
Bacterial culture of tissues around the joint is the most reliable way of identifying the bug responsible. It can be difficult to distinguish the bug causing the problem from contamination, ie bugs which get in accidentally while the sample is taken or for other reasons. If someone is taking antibiotics this will tend to complicate the picture as it may then be difficult to get a successful culture.
X-rays and Scans
X-rays can be useful if they are taken over a period of time (serial x-rays). The implant may gradually migrate in the bone over time and the bone weaken through reabsorption. This can occur, however, also with joint loosening without infection. CT scanning and MRI scanning are limited by the presence of metal in the tissues which affects the images. Various kinds of radioactive bone scans can be useful.
Treatment of Joint Infections
Artificial joints are supposed to give the recipients a painless, fully functioning, virtually normal joint, at least in older people. Infection gets in the way of this and threatens the exact opposite, with significant pain and disability.
There are two main treatments, often used in combination. Drug therapy with antibiotics is routinely given, and the joint may be removed and replaced within certain time scales.
In normal infections the bacteria are identified and tested to find out if they are sensitive to particular antibiotics. This allows the treatment to correctly target the bug responsible. However, in artificial joint infections this process is less reliable.
The antibiotics used need to be effective against the types of bacteria which adhere to foreign surfaces, grow slowly and form the biofilms mentioned above. Combinations of powerful antibiotics are often used.
Surgery for Joint Infection