Septic (Infectious) Arthritis Symptoms & Causes

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 thighbone.
•   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
•   pivot joints, such as the neck joint - allows limited rotating movements
•   ellipsoidal joints, such as the wrist joint - allow all types of movement except pivotal movements

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.