What is a total hip replacement (THR) implant made from?
1. The acetabular components
The acetabular component of a total hip replacement generally comprises a shell and a liner. The shell is made of titanium alloy and usually has a hydroxyapatite coating. The liner is either made from polyethylene (ceramic/cobalt chrome on polyethylene bearing) or ceramic (ceramic on ceramic bearing).
As you would expect, every surgeon has their own preference for implants and materials. Professor Walter always uses total hip replacement components that he considers to be the best for that individual patient. Great effort and research goes into understanding the performance of these devices to determine which is the most likely to give reliable and durable service.
Polyethylene is the oldest and most commonly used material in hip replacement bearings (it is the only widely used option in knee replacement bearings). A polyethyene hip bearing comprises a ball made of metal or ceramic and a polyethylene socket. Prior to 2000 polyethylene was sterilized by a method of gamma irradiation in air. This led to the creation of free radicals in the material, which oxidized during storage resulting in poorer wear properties in vivo.
Professor Walter’s father implanted thousands of hips in the 1980s 1990s with this material as it was the best available at the time. These patients were told that their hips would last 10 to 15 years. This prediction proved to be accurate with 50% of these hips being revised after 15 years for osteolysis (bone loss) caused by wear debris. Many of these patients had their femoral component retained, the bearing was upgraded to a modern polyethylene or ceramic on ceramic bearing and they achieved good function after the revision surgery.
Modern polyethylene components are gas sterilized or irradiated in an oxygen-depleted environment and reheated creating crosslinking in the material, reducing free radicals thus mitigating oxidization and improving wear performance. The modern cross-linked polyethylene has been available since the year 2000 meaning that 10 to 15 year wear performance data is available. This data shows that the wear has been reduced by at least a factor of 10 when compared to the old polyethylene. Professor Walter uses cross-linked polyethylene as a bearing material in older patients who are unlikely to wear out this material in their lifetime.
The ‘ball’ or head-of-femur and the lining of the acetabular ‘socket’ is made from ceramic. Ceramic is an excellent material because it is particularly inert (the body doesn’t react to it) and has a very long safety record.
Our own meticulous research and records since modern ceramic implants became available in 1997 show that ceramic implants are extremely durable and effective. We use 2 different types of ceramic: alumina ceramic (Al or biolox forte) and zirconia toughened alumina (ZTA or biolox delta). Breakage of a ceramic implant is very rare, occurring at a rate of 1 per 1000 or 0.1% in alumina ceramic and less in ZTA– in this event, revision surgery is required.
Ceramic bearings have the lowest wear of any bearing surface available to surgeons with wear rates , which are 100 times, (Al) and 1000 times (ZTA) less than old polyethelene (plastic) implants. The reason this is important is because reducing the wear, reduces the production of the particles which come away from the implant and float inside the joint or other parts of the body, dramatically reducing the incidence of osteolysis (weakening and disappearance of the natural bone).
Ceramic on ceramic bearings can occasionally make a squeaking noise. When this noise occurs it is usually infrequent and only occurs with specific activities such as bending. Very rarely squeaking is frequent and occurs with walking and requires revision surgery. The rate of squeaking depends in the techniques used over the years but the overall rate in the 5000 patients treated at Professor Bill Walter’s practice with ceramic on ceramic bearings in the last 14 years is between 2% and 3%.
The rate of problematic squeaking requiring revision is less than 0.05% (2 patients out of 5000). Improved accuracy using the computer navigation with the direct anterior approach seems to have significantly reduced the rate of squeaking observed in the last 3 years.
2. The femoral component
The femoral component of a total hip replacement generally comprises a stem, which is connected to a ball. In older patients the stem may be connected to the femur with polymethylmethacrylate bone cement in which case the stem is usually made of stainless steel. In younger patients Professor Walter uses cementless fixation and the stem is usually made from titanium alloy with hydroxyapatite coating. The ball is usually made of alumina ceramic although Professor Walter sometimes uses a cobalt chrome ball.
Titanium is the gold standard in cementless hip implants because it is strong, flexible and it is a particularly inert metal that does very well in the body. Two different titanium alloys are used TMZF – titanium, molybdenum , zirconium and iron and TiVAl –titanium, vanadium and aluminium. Titanium alloy is used for the stem, which is inserted down the middle of the femur and for the shell of the acetabular component.
The stems and shells Professor Walter uses also have coating surfaces, which promote the attachment or growth of the natural bone onto the component creating strength and stability in the bone. The coating surface is usually rougher than the substrate material surface for greater friction with the bone and may be pure titanium or titanium alloy applied by plasma spray or other technique. On top of this there is usually a fine layer of hydroxyapatite, which is similar to the mineral content of the patients bone. Hydroxyapatite is biologically active and facilitates a more rapid bond formation between the bone and the implant.
What is a resurfacing hip implant made from?
A resurfacing hip replacement has two components. The femoral component is a ball cemeted to the prepared femur with polymethylmethacrylate bone cement (fits over the natural head and neck of femur bone). The acetabular component – or the “socket” – is attached to the pelvis by cementless means. Both these parts made from cobalt chrome alloy and the acetabular component has an hydroxyapatitie coating. Unlike a total hip replacement, there is no stem put into the femoral canal with the resurfacing hip replacement.
Cobalt chrome has been used in implants in humans for over 100 years. Nearly all knee replacements used today in Australia contain cobalt chrome and cobalt chrome is still the most common material used around the world in total hip replacement balls. Cobalt chrome – like any bearing material – can cause problems if there is high wear.
In cases of high wear cobalt chrome debris (particles) can cause local soft tissue and bone destruction. There were some poorly designed implants implanted in Australia in the last decade that had high rates of wear resulting in higher rates of revision surgery with these implants. A well designed resurfacing hip implanted into the right patient with good surgical technique has a low risk of requiring revision surgery which is equivalent to or lower than the risk of revision of a total hip replacement done in the same patient (AOA NJRR).
How long do hip implants last?
Using the polyethylene from the 1980s and1990s as a standard. We can say that these hips lasted 15 years. That is to say roughly half of the hips were revised by 15 years most commonly due to polyethylene wear and osteolysis. Even though some hips were revised for wear as early as 5 years in very active patients and some hips have lasted 25 years.
Excluding wear as a failure mode the survival of the majority of these implants exceeds 2 decades and often lasts the life of the patient. Other modes of failure in these hips including periprosthetic bone fracture, infection, dislocation and implant breakage are far less common than wear related failures.
The bearing materials that we use today have between 10 times and 1000 times reduction in wear and one may predict that they would be capable of lasting 150 years. However, we only have 15 years experience with these materials. We do not know if the reduced wear performance will be sustained beyond 15 years.
Furthermore, there are other modes of failure besides wear that can lead to revision surgery. We can say that we have dramatically reduced the commonest cause of failure in the first 2 decades. Based on the most recent historical data a modern hip replacement has about a 6.5% chance of requiring revision surgery in a decade.
The risk for the second decade may be the same or may be greater or less than 6.5% but we don’t know. It is likely that half of the hips we perform today will last several decades. The goal of implant selection is to choose materials that will last the life of a patient while minimizing the risk of adverse outcome and maximizing the functional performance.