Knee Infection

Knee Infection

Normal Anatomy

The bones of the knee are the femur (thigh bone), tibia (shin bone) and patella (kneecap). The femur and tibia meet to form the knee joint which is a hinge with the knee cap(patella) in front of these two bones acting as a mobile shield to protect the joint. The patella slides up and down in a groove in the femur called the femoral groove as the knee is bent and straightened.


Ligaments hold the knee together and give it stability. The medial (inner) collateral ligament (MCL) and outer (lateral) collateral ligament (LCL) limit sideways motion of the knee. The posterior and anterior cruciate ligaments (PCL and ACL) limit forward and backward translational motion of the knee bones, keeping them stable and allowing the knee to function as a unit.


Two structures known as menisci sit between the femur and the tibia and act as cushions or shock absorbers for the knee. Menisci are one of two types of cartilage in the knee. The other type, articular cartilage, is a smooth and very slick material that covers the end of the femur, the femoral groove, the top of the tibia and the underside of the patella. This articular cartilage allows the bones to move smoothly without pain. If this layer wears away, it is called arthritis and then the motion becomes painful.


Tendons connect muscles to the bones of the knee. The quadriceps muscles on the front of the thigh are connected to the top of the patella by the quadriceps tendon, which covers the patella and becomes the patellar tendon. The patellar tendon then attaches to the front of the tibia. The hamstring muscles in the back of the leg attach to the leg bone (tibia) at the back of the knee. The quadriceps muscles straighten the knee and the hamstring muscles bend the knee both of which are required for walking, running or simply standing.

Pathology

The knee is one of the most well-perfused (good blood supply) areas of the human body and because of this ample blood supply to the knee, circulating white blood cells offer excellent protection against infection. Consequently a knee infection is a rare condition. In spite of this, there are certain conditions that predispose individuals to having a knee infection:

1. A penetrating knee injury that is not cleaned and repaired properly. All penetrating injuries should seek medical advice for prophylactic antibiotics, a tetanus booster if one has not been received within a 10 year period for clean injuries and within a five year period for dirty wounds.
2. Postoperative wound infection is another cause of a knee infection. During the post-operative period, the fresh wound has not yet sealed and because of this, the organisms that are normal skin flora can penetrate a wound and cause an infection. This is why patients after surgery are advised to keep the dressing on and the wound dry for at least one week after surgery.
3. After a joint replacement (arthroplasty), there is a risk of infection if similar care is not provided to the post-operative wound. The risk of infection is higher after a joint replacement since the implant is a foreign object in the human body and organisms (bacteria) can hide and avoid antibiotics that are given. Individuals can also seed an implant in the body from infections elsewhere, such as a tooth abscess. This is why prophylactic antibiotics are recommended when visiting a dentist for a dental procedure after a patient has had a joint replacement.
4. A patient with a weak immune system, because of a chronic medical condition, is at an increased risk of developing a knee infection. Patients with diabetes, kidney failure, and/or inflammatory conditions, on immunosuppressive medication that suppress the protective effects of the immune system, are all at an increased infection risk. Similarly, a patient with HIV or AIDS that has had a penetrating injury, surgery or any blood-borne infection is also at an increased risk of developing a hip infection.

Treatment

The treatment of a knee infection depends on several factors which include:

1. The initial cause of the infection
2. Whether the infection is superficial or deep
3. The length of time the infection has been present
4. The type of organism causing the infection
5. The general medical condition and nutrition of the patient

When an infection is superficial, oral antibiotics, warm soaks and resting the extremity may be adequate treatment. If the infection has spread and lymph node involvement, fever, chills or other systemic symptoms accompany the infection, hospital admission for intravenous antibiotics/fluids and systemic support is preferred. If the infection is loculated, fluctuant, or an abscess has formed, it is a surgical condition requiring incision and drainage.


When a joint replacement has been done, it is prudent to admit the patient to the hospital to possibly wash out the wound. If the infection of the joint replacement occurs within 3 weeks of the initial surgery and the infection is deemed “acute”, surgery usually involves washing out the wound, replacing the plastic liner between the metal implants, and retaining the knee replacement components that were originally implanted. If the infection of the joint replacement has occurred more than 3 weeks after the initial surgery and the infection is deemed “chronic”, surgery often involves washing out the wound, removing all of the knee replacement components, and placing an antibiotic cement spacer. After the organism that caused the infection has been identified, the patient is given at least 6 weeks of intravenous antibiotics to eradicate the infection. Once the infection has been completely cleared, a revision knee replacement surgery may be scheduled to remove the cement spacer and reimplant the knee replacement components. If the patient’s medical health or immune status is compromised, fully treating the infection and leaving the cement spacer in place or leaving the joint without an implant may be the best option.