Hip and Knee Replacements

x ray hip replacenment

Hip replacement

A hip replacement is usually made up of a number of different components being formed from different materials. The articulation between the femoral head and the acetabular component is sometimes termed as the “bearing surface”.

Femoral stem

The femoral stem is the part of the joint replacement which is seated in the thigh bone on top of which sits the femoral head. The femoral stem may be either cemented or uncemented. The cemented stems are held in place with bone cement whereas the uncemented stems are held in place with a tight fit between the femoral stem and the femoral bone. Some femoral stems have special coatings applied to them to encourage bone ingrowth into the femoral stem (hydroxyapatite).

Femoral head

This is the ball that sits on top of the femoral stem and articulates with the acetabular component. Femoral heads can be made of metal or ceramics. Femoral heads come in different sizes that are matched to the size of the acetabular component. In addition the length of the femoral head can be changed at the time of surgery by the surgeon to adjust leg lengths and the tension that the hip is held into place with.

Acetabular components

Acetabular components are again of two different types:

Cementless components

Cementless components rely on a tight fit of the bone of the pelvis to keep in place. Again they may be sprayed with a coating to enhance this fixation. A liner is placed inside the cementless cup, this may be either of a plastic material or of a ceramic type material.

Cemented cups

Cemented cups are held into place by bone cement and are made up of a special plastic called ultra high molecular weight polyethylene.

Bearing surfaces

As can be gathered above, the bearing surfaces between the head of the femur and the acetabulum can be of numerous different types. The commonest types are metal on plastic (this is a metal head and an ultra high molecular weight polyethylene acetabular component or lining), ceramic on plastic (this is a bearing surface with a ceramic femoral head and a plastic acetabular component or liner), ceramic on ceramic (this bearing surface is where both the femoral head and the acetabular lining are both made up of ceramic materials.

Footnote

Research is ongoing as to which is the best bearing surface. It may vary from patient to patient and is always worthwhile discussing with your surgeon.

IMG_0177[1]-01 IMG_0181[1]-01

Knee replacements

Knee replacement surgery involves the removal of the surfaces of the bone of the bottom of the femur and top of the tibia to create a new articulation between these bones. The most common type of knee replacement is the total condylar knee replacement discussed below.

Total condylar knee replacement

Total condylar knee replacements are made up of a femoral and tibial component with a plastic insert between. In addition it is usual to place a plastic button on the undersurface of the patella.

Femoral component knee replacement

The femoral component of a knee replacement sits like a cap on the end of the femur. It is held in place with bone cement. During surgery the surgeon will prepare the end of the femur with various cutting jigs. These jigs are of different sizes and allow a close match of the femoral component to the size of the individual’s femur. The femoral component is made up of a sophisticated metal alloy that has a highly polished surface.

Tibial component knee replacement

The tibial component of a knee replacement is again made up of a sophisticated metal alloy. It is a plate that sits on the top of the tibial surface. In addition this plate had a keel that extends into the tibial medullary cavity to enhance its fixation. Again the tibial component is usually fixed to bone with bone cement. The femoral and tibial components are separated by a plastic spacer made up of ultra high molecular weight polyethylene. This is fixed to the tibial component. The articulation between this and the femur provides with a very low friction bearing surface.

Patellar component

It is usual that the undersurface of the patella is shaved during surgery and a small button is placed onto the surface of the patella (kneecap). It is held with bone cement and between one and three small pegs that fit into the patellar surface.

Introduction into anaesthesia for hip and knee replacements

Different types of anaesthesia are used for hip and knee replacement patients and patients may be more suitable to one type than the other. The two major different types are of a general anaesthesia and spinal anaesthesia.

General anaesthesia

General anaesthesia will mean that you are unconscious throughout the operation. The anaesthetist will give you various drugs to enable you to relax and that you do not feel pain and are not aware throughout the procedure. At the end of the procedure the drugs are withdrawn and the patient wakes up. Drugs given during the procedure will lessen the discomfort from surgery and also local infiltration of local anaesthetic will help ease any discomfort.

Spinal anaesthetics

It is possible to do both hip and knee replacements with a spinal anaesthetic. This will mean that during the procedure you are aware of what is going on, although various drugs can be given to lessen the anxiety that this may cause. The spinal anaesthesia will mean that you will not have to go to sleep during the surgery and it may be that you would want to listen to some music during the procedure. Spinal anaesthetics wear off some hours after surgery and following this the patients can start to use their new joints.