What parts of the hip are affected?
The hip joint is a true ball-and-socket joint. The bones of the hip are the femur (the thighbone) and the pelvis. The top end of the femur is shaped like a ball. This ball is called the femoral head. The femoral head fits into a round socket on the side of the pelvis. This socket is called the acetabulum.
Transient synovitis affects the inner lining of the hip joint including the synovium and joint capsule. The synovium is a membrane that seals the synovial fluid in the joint to lubricate the joint surface. It is between the articular cartilage and the joint capsule. The joint capsule is a watertight sac that surrounds a joint. In the hip, the joint capsule is formed by a group of three strong ligaments that connect the femoral head to the acetabulum. These ligaments are the main source of stability for the hip. They help hold the hip in place.
Articular cartilage is the material that covers the ends of the bones of any joint. Articular cartilage is about one-quarter of an inch thick in the large, weight-bearing joints like the hip. In the hip, articular cartilage covers the end of the femur and the socket portion of the acetabulum in the pelvis. The cartilage is especially thick in the back part of the socket, as this is where most of the force occurs during walking and running.
Articular cartilage is white and shiny and has a rubbery consistency. It is slippery, which allows the joint surfaces to slide against one another without causing any damage. The function of articular cartilage is to absorb shock and provide an extremely smooth surface to make motion easier. We have articular cartilage essentially everywhere that two bony surfaces move against one another, or articulate.