The hip joint is a ‘ball and socket’ joint. It allows movement to occur between the thigh bone (femur), and the hip bone (pelvis). The pelvis contains the ‘socket’ called the acetabulum. The ball shaped head of the femur fits into the acetabulum, forming a ball and socket joint that enables the leg to have a wide range of movement.
The outer surface of the femoral head and the inside surface of the acetabulum are covered with cartilage. The cartilage surface is a tough and very smooth material that allows the two surfaces to slide with ease against one another during movement.
Envelopes of tough ligaments connect the pelvis and femur, covering the joint and stabilising it. The hip joints’ movements are initiated and controlled by the thick muscles of the buttock at the back and the thick muscles of the thigh at the front. A healthy hip joint will allow the leg to move freely within its range of motion, while supporting the upper body and absorbing the impact that results from activities such as walking and running. Refer to figure 1.
Hip Joint Replacement
Approximately 30,000 hip joint replacements are performed each year.¹ The replacement can involve the femur only, or the femur and acetabulum. The procedure option selected will be based on your requirements and surgeon preference.
Components of a Hip Replacement
Each hip prosthesis is made up of several components: Refer to figure 2.
- The acetabular component replaces the acetabulum (socket). The acetabular component can either be made of a metal alloy outer shell with a fitted plastic, metal or ceramic liner or it can be made of one plastic component.
- The femoral component replaces the femoral head. The femoral component can be a single or two-piece design. The single piece is made of metal alloy. The two-piece design consists of the femoral stem made of metal alloy, while the femoral head that attaches to the stem can be made of either ceramic or metal alloy. This is known as a modular prosthesis.
Procedure Option
Total Hip Replacement
Hip joint replacement surgery involves replacing the head of the femur (ball) and the acetabulum (socket) with manmade components, called prostheses. The hip prostheses are designed to simulate the human anatomy as closely as possible.
There are many different designs of hip prostheses available and your surgeon will choose the one considered most suitable for you. However, the final decision may need to be made during the time of your operation. Refer to figure 3.
Hip Resurfacing
In a resurfacing procedure the head of the femur (ball) is not removed, but is trimmed and covered with a round metal cap. As with total hip replacement, resurfacing of the socket is achieved by firmly fitting a metal shell into the acetabular bone. The bone will then grow into the shell to hold it in place. Refer to figure 4.
Revision Hip Replacement
Revision surgery involves re-operation on a previously performed total hip replacement and resurfacing that has worn out, failed, or become loose. Part or all of the previous components are removed and replaced with new ones.
It is important to be aware that even the most successful procedures are not guaranteed to last forever. Artificial joints can become loose and unstable requiring a revision surgery (repeat). The life expectancy of joint replacement implants is difficult to estimate even though improvements in surgical techniques and instrumentation should help the joint last longer.
Bilateral Total Hip Replacements
This is a procedure where both hips are replaced at the same time.
For more information on hip replacement surgery, visit the Orthopaedic Research Institute’s All About Hip Replacement at www.ori.org.au/bonejoint/hip/contents.htm.
¹ Australian Orthopaedic Association Joint Replacement Registry, see www.aoa.org.au/docs/njrrrep06.pdf
