Septic arthritis is an infection in the joint tissues and the joint's fluid (synovial fluid). The infection usually reaches the joints though the bloodstream, although some joints may become infected due to an injection, surgery, or injury. Different bacteria and viruses can infect a joint and usually are associated with a person's age.
What is a joint?
Joints are the areas where two bones meet. Most joints are mobile, allowing the bones to move. Joints consist of the following:
- cartilage: at the joint, the bones are covered with cartilage (a connective tissue), which is made up of cells and fibers and is wear-resistant; cartilage helps reduce the friction of movement
- synovial membrane: a tissue called the synovial membrane lines the joint and secretes synovial fluid (a clear, sticky fluid) around the joint to lubricate it
- ligaments: strong ligaments (tough, elastic bands of connective tissue) surround the joint to give support and limit the joint's movement
- tendons: tendons (another type of tough connective tissue) on each side of a joint attach to muscles that control movement of the joint
- bursas: fluid-filled sacs, called bursas, between bones, ligaments, or other adjacent structures help cushion the friction in a joint
- synovial fluid: a clear, sticky fluid secreted by the synovial membrane
- femur: the thigh bone
- tibia: the shin bone
- patella: the knee-cap
- meniscus: a curved part of cartilage in the knees and other joints
The different types of joints
There are many different types of joints, and they are classified according to structure and how they move. Joints that do not move are called "fixed." Other joints may move a little, such as the vertebrae. Examples of mobile joints include:
- ball-and-socket joints, such as the shoulder and hip joints: allow backward, forward, sideways, and rotating movements
- hinge joints, such as in the fingers, knees, elbows, and toes: allow only bending and straightening movements
- pivot joints, such as the neck joint: allow limited rotating movements
- ellipsoidal joints, such as the wrist join: allow all types of movement except pivotal movements
What joints are commonly affected by septic arthritis?
- knee
- hip
- ankle
- elbow
- wrist
- shoulder
- pelvis
Most infections only affect one joint.
Swift diagnosis and treatment can help clear the infection within a matter of days. But if treatment doesn't begin soon enough, the joints may become permanently damaged because of pressure and reduced blood flow.
What causes septic arthritis?
Many different types of bacteria can cause septic arthritis, but the most common type of bacteria is called Staphylococcus aureus, also referred to as S. aureus.
The bacteria can enter the body in a variety of ways. The most common routes of entry include the following:
- infection that spreads from another source inside the body, such as ear infections (this is the most common source of septic arthritis)
- infected wounds
- open fractures, or bones that penetrate through the skin
- foreign object penetrating the skin
- trauma
In some rarer cases, septic arthritis can stem from a viral or fungal infection.
What are the symptoms of septic arthritis?
The most common joints affected by septic arthritis are the knee, hip, shoulder, elbow, and wrist. The following are the most common symptoms of septic arthritis. However, each child may experience symptoms differently. Symptoms may include:
- fever
- joint pain, usually severe
- joint swelling
- redness in the affected area
- warmth around the infected area
- limited use of the affected extremity
- guarding or protecting the affected area to prevent it from being touched or looked at
- other symptoms of illness, such as vomiting, sore throat, and/or headache.
Symptoms of septic arthritis may resemble other medical conditions or problems. Always consult your child's physician for a diagnosis.
How Boston Children's Hospital approaches septic arthritis
To ensure your child gets the quickest treatment possible, doctors at Boston Children's have different tests to make sure your child is diagnosed properly, including tests of blood, urine, and joint fluid. The exact nature of your child's infection, whether it's viral or bacterial, will determine how doctors treat your child's septic arthritis.
