It may seem strange (unless you are an engineer or an orthopaedic surgeon) but every time you get up from a chair or walk you are exerting a huge amount of force on your new joint. If the implant moves, even just a little bit, it will cause you pain. It is therefore essential to ensure that the new joint is very solidly fixed in place.
There are two methods of hip replacement fixation generally used; cemented and uncemented fixation.
The cement used during your surgery can be likened to that used for building a brick house. It is paste like to start with but soon sets. It is not a glue - although it can be sticky when it is first mixed up.
Bone has small holes called interstices. The cement works by filling these spaces.
Over the years three different methods for introducing the cement have been used. These are referred to by the medical profession as the three generations. The second generation is the most widely used and will be described here.
In this process a plug is placed in the femoral canal, which prevents the cement from going down further than is needed. It also means that when pressure is applied to the cement the only direction it can go in is sideways into the honeycombed surface of the bone. This makes for a pretty solid fixture.
One disadvantage of this technique is that with time the cement will crack and the prosthesis will become loose
This is often referred to as a press-fit fixation.
When the surgeon prepares the femoral bone to receive the implant he needs to make sufficient space in the femoral canal to receive it. With a press-fit fixation the idea is to make the space slightly smaller than the implant. The implant is then jammed into this smaller space making for a solid fixture.
The implant used in this procedure is manufactured with small holes - like pores on your skin. It is expected that bone will grow into these holes and keep the implant firmly secured. A coating of hydroxyapatite can be applied before insertion. This encourages bone growth.
Surgeons worked with engineers to find the right texture for the surface and the best metal to use (cobalt-chrome and titanium alloys).
Patients who have undergone the uncemented technique will take longer to recover as they need to take precautions until the new bone growth has attached to the implant - normally for three months post-op
Press-fit cups are also available. These have outer shells peppered with holes and they may have small spikes which help with fixation. In some cases screws may be used.
Uncemented hip replacement fixations tend to last longer than the cemented versions but they are not suitable for all patients. As the technique relies on the growth of new bone to hold the prosthesis in place it is essential that the patient has strong and healthy bones.
Your surgeon should talk through your options and you must be given time to ask as many questions as you need. As a rule of thumb: -
A hybrid hip replacement fixation is also possible. Here one part is inserted using cement (normally the femoral stem) and the other part (normally the acetabulum) uses a uncemented fixation.
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