The hip prosthesis has come a long way since the early metal and polythene model developed by John Charnley in the 1960s. These original artificial hip joints were made from stainless steel with a basic plastic shell, which were heavier than a natural hip joint. Today, surgeons have a range of materials to choose from and a modern hip prosthesis can weigh in at less than 400 grams. These are the most commonly used materials in modern artificial hip joints:
Metal alloys – the part of the hip prosthesis that acts as the ball of the joint is usually made of a metal alloy. Common combinations are based on titanium or cobalt and chromium. Different shapes are available and the long stem of the hip prosthesis can be either porous (for uncemented hip replacement) or non-porous (for cemented hip replacement).
Ceramics – this is a recent development that aims to ensure that the artificial hip joint does not fail over time. A metal/plastic hip prosthesis tends to have a life of around 15-20 years (Arthritis Research UK), so someone who has a hip replacement in their 50s or 60s could need a hip revision later in life. Although a ceramic hip prosthesis can be longer lasting, it is more prone to breaking compared to a metal or metal and plastic artificial hip joint.
Ultra-high molecular weight polyethylene used as the socket part of the hip prosthesis has high strength and is smooth and long lasting.
Polyethylene plus metal – the socket of the artificial hip joint can be made with a central metal component surrounded by a ‘skin’ of plastic, as well as from metal alone or plastic only.
For cemented hip replacements, polymethylmethacrylate, which is an acrylic polymer, is most often employed as the cement to hold the hip prosthesis in place.
What qualities does a hip prosthesis need?
Developing an artificial hip joint has not been an easy task; continually improving the design and the materials used still takes a lot of research time. Metals, alloys and plastics need to mimic the properties of living tissue very closely. A hip prosthesis has to take the weight of the body without buckling or snapping but it must be flexible enough not to restrict movement. The materials used need to be accepted by the body and to allow the artificial hip joint to heal and then function well. Developing a hip prosthesis that can stand up to the massive wear and tear of constant movement over many years is still a challenge.
Hundreds of thousands of people worldwide have a hip replacement every year (OECD Health at a Glance 2013), so there is a good incentive for manufacturers of artificial hip joints to produce very high quality products at a reasonable cost.
Which type of hip prosthesis is right for me?
This is a difficult question to answer and will ultimately fall to your surgeon to determine, with your input. He or she will probably advise that you have a type of hip prosthesis that has been tried and tested and is known to be long-lasting. The new ceramic hip joints might seem appealing but they are not right for everyone. We still don’t know how these joints will behave in the long term (National Joint Registry for England and Wales: Report 2013).
Your surgeon will probably advise a type of hip prosthesis that they have used before, and that have provided good, long-lasting results. Whether you have a cemented or uncemented hip replacement depends mainly on your age and activity level but most of the common types of hip prosthesis are available for each of these surgical techniques.