Design

The Charnley hip or low frictional torque arthroplasty - LFA - has quite specific design features.  Charnley relied on the femoral stem to get good fixation to the bone using acrylic bone cement and also used a high density polyethylene socket which was also cemented into the acetabulum.  The unique features of this design is the small head compared to the normal femoral head.  The diameter of femoral head is quite commonly 50 to 54mm however Charnley used a diameter of 7/8inch or 22.225mm.  The reasons for this are mainly due to wear, friction and lubrication.  The articulation is mainly dry and is not reliant on joint fluid and a bearing which is small has a low or short sliding distance, ie the amount of plastic the femoral head runs over as the hip moves backwards and forwards from side to side and it is this low sliding distance which causes relatively low wear of the hip.  The second reason for using a small femoral head was that this imparts a relatively low torque on the interface between the socket and the bone due to the relatively low ratio of the head diameter to the external diameter of the socket. It was for these reasons that Charnley used a small femoral head.  Subsequently some surgeons were concerned about the dislocation potential of this design and therefore subsequent attempts have been made to increase the diameter of the femoral head.

Other types of design principles relate to the geometry of the stem whether it be a flat stem, a tapered stem, a textured stem.  The stem is to be fixed to the bone either with bone cement or with cementless fixation.  It is now generally accepted for cementless stems that of a tapered polished design produces reliable long term results.  This was mainly the philosophy of the Exeter group of surgeons and is the design of the Exeter stem from Robin Ling and Clive Lee.  However, other cemented designs have quite a rough surface.  They have a different design principle in that the stems are essentially held up and fixed rigidly whereas the tapered designs do allow some movement or engagement of the femoral stem and load the cement under compression.  There are therefore two quite different but both acceptable design principles of design and fixation of the stem to the bone.

In terms of the acetabular component, high density polyethylene is still used although there are newer generations of the polyethylene to make it more wear resistant and this can be achieved by cross linking the polyethylene by using radiation during the manufacturing process.  The newer cross linked polyethylenes show very low wear properties, maybe an 80 per cent reduction in wear although there is slight concern regarding a reduced mechanical strength of the material although these risks are probably very low indeed.  Certainly in the United States there is an increase usage of these cross link polyethylenes.

By increasing the wear resistance of the polyethylene it is therefore possible to increase the diameter of the femoral head without introducing grossly adverse wear environments and this has led to a reintroduction of larger femoral head designs.  If the femoral head is larger, the potential advantages are greater stability of the joint, potentially a greater functional range of movement before the hip impinges on the acetabular bone and therefore potentially better function.  There is some evidence that the larger femoral head has a lower dislocation potential.