In this, the fourth in a series of posts on hip replacement surgery, we address one of the commonest questions that we get asked. It’s important to know what is being placed inside your body so our expert Consultant Hip and Knee Surgeon, Mr Chethan Jayadev, has created this short guide – including what to know when you go to the airport!
Your own (native) hip
Your own hip is a ball-and-socket joint. In a normal hip, the smooth and spherical head of your femur (thigh bone) fits perfectly into the natural seat of the acetabulum, a cup-shaped cavity on the outside of your pelvic bone.
A total hip replacement is typically made of 4 individual components:
- Cup – fits into the pelvic bone
- Liner – inserts into the cup and can be considered as your “new cartilage”
- Stem – inserts into the top of your femur (thigh bone)
- Ball – attached to the stem and sits in the liner
The Cup
The acetabular component (cup) fits into your outer pelvis and serves to hold the liner. Together with the liner, it forms the socket of the ball-and-socket joint.
Most modern cups are made of alloys of the metal Titanium. There are small amounts of the metals such as Aluminium and Vanadium. This material was originally developed for the aerospace industry. It is very well tolerated by the human body (biocompatible) and its mechanical properties make it well suited for use in hip replacements. The outer curved surface of the cup is porous (has lots of small voids). This provides an excellent surface for fixation to the pelvis due to bone growing into the tiny pores (biological fixation).
On occasion, the cup and liner are a single component made of a specialised form of the plastic, polyethylene. This component is fixed to the pelvic bone using an acrylic bone cement that acts as a firm grout.
The Liner
The liner fits into the cup and acts as your “new cartilage”. The femoral head (ball) sits and rotates within the liner allowing hip movement. This movement will very slowly cause wear of the liner. However, modern liners are much more wear resistant. The 2 most commonly used liner materials are polyethene and ceramic.
- Polyethylene – This plastic polymer undergoes highly specialised preparation and treatment to make it hard wearing. It is by far the most commonly used liner material and has excellent results and longevity when combined with metal or ceramic balls.
- Ceramic – This is an extremely hard-wearing material resistant to scratching and other damage. It is also exceptionally well tolerated by the body. It is paired with ceramic balls. One of few drawbacks of ceramic is that it is so stiff that it can fracture (crack) like your porcelain kitchenwear at home. However, modern surgical grade ceramics have an extremely low risk of fracture.
Cobalt-chromium (see below) metal liners are no longer used routinely. They were previously used in combination with Cobalt-chromium balls, but this combination can produce adverse reactions through the production of metal debris.
The Stem
The stem fits into the top end of your femur (thigh bone). It can be fixed into the femur with or without bone cement; cemented and uncemented stems respectively. Both have excellent long-term results.
Cemented stems are most often made of cobalt-chromium metal alloys or surgical grade stainless steel. Cobalt-chromium alloys also contain small amounts the metals molybdenum and nickel, and trace amounts of carbon. Stainless steel is made of iron, carbon and chromium. Orthopaedic stainless steel used in hip replacements also contains the metals nickel and molybdenum. Both cobalt-chromium and stainless-steel stems are strong and biocompatible.
Uncemented stems are routinely made of titanium metal alloys. As mentioned previously, it is very biocompatible with a low risk of adverse reactions. The stems are designed to allow bone to grow onto and into the implant (biological fixation).
The very top of the femoral stem (trunnion), regardless of material or fixation, is specially shaped to securely hold the new ball of the artificial joint.
The Ball
The femoral head (ball) fits securely to the end of the stem (trunnion). The ball is usually made of either cobalt-chromium alloy or ceramic.
- Cobalt-chromium – this is the most common material used for the ball. It is relatively scratch resistant and hard wearing. It usually coupled to polyethylene liners and this combination has excellent long-term results.
- Ceramic – As mentioned previously, ceramic is a very strong material with low wear. It can be combined with either polyethylene or ceramic liners. Ceramic-on-ceramic combinations can very rarely give rise to a squeaking noise which could prove annoying.
Where can I read more?
There are many high quality websites that you can visit to learn more about Total Hip Replacement including;
The Royal College of Surgeons of England
The American Academy of Orthopaedic Surgeons
Summary
Every person is unique, and no one size fits all. All hip replacement components come in numerous different sizes or diameters. Your surgeon will plan and choose the components to best restore your anatomy and function.
And before you ask, the answer is yes! Over 90% of implanted total hip replacements will set off airport metal detectors.
