Case consideration of replacement joints
12.2.3 Case consideration of replacement joints
12.2.3.1 Hip joints
The basic design of an artificial hip joint includes an alloy femoral stem, with a metallic or ceramic
femoral head moving in an ultra-high-molecular-weight polyethylene (UHMWPE) acetabular cup, as shown in Figures 12.2 and 12.3. The average life of the joint is better than 10 years, but implants tend to loosen as a result of bone resorption due to modulus mismatch. Friction and wear also cause wear debris between the cancellous, i.e. more porous, bone and the cup, and also between the femoral head and the softer UHMWPE. Overall the failure rate is now about 1–2% per year, as shown in Figure 12.4.
Friction and wear problems are being improved by using ultra-hard materials as the bearing material.
A good resistance to frictional wear together with biocompatibility makes alumina well suited for the femoral head. Unfortunately, Al 2 O 3 has a low impact strength and is liable to failure when subjected to stresses introduced by extra activity, e.g. jumping. Research has also shown that ion implantation of the UHMWPE in a nitrogen atmosphere at 10 −5 mbar at 80 keV to a dose of 1
× 10 17 ions cm −2 reduces the wear behavior to virtually zero. The enchanced surface properties, together with higher
hardness and elastic moduli, would very much improve the interaction problems. A compromise solution is to have a titanium stem with stainless steel head coated with a thin layer of ceramic.
The problem of loosening of the joint due to bone resorption is being tackled by second generation biomaterials and implants which mimic the body’s tissues. Thus, while continuing with improved bioinert materials, development has focused on bioactive materials which influence the biological
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Figure 12.3 (a) Conventional hip joint made of stainless steel and UHMWPE acetabular cup (courtesy R. Grimer, Royal Orthopaedic Hospital, Birmingham). (b) Modular artificial hip made of titanium stem, alumina head and UHMWPE acetabular cup (courtesy P. Marquis, Dental School, Birmingham).
response in a positive way, e.g. encourage bonding to surrounding tissue with stimulation of new bone growth. With this bioactive approach the interface between the body cells and the implant is critical and the materials science of the biomaterials surface extremely important. One such material is a composite of hydroxyapatite (HA) in high-density polyethylene. The HA provides the strengthening and stiffness reinforcement to the tough polyethylene matrix. HAPEX, as it is called, is biocompatible and bioactive, encouraging bone growth onto the implant material which, of course, contains HA.
The biological response has been demonstrated by cell culture studies with human oesteoblasts in which the cells grew and spread over the composite initiated at the HA particles. In vivo testing has shown a stable interface between the implant and bone.
12.2.3.2 Shoulder joints
These joints, like hip joints, are increasingly being replaced by metal prostheses (Figure 12.5). Initially, similar principles were applied to shoulder joints as to hip joints, using a stainless steel ball and socket with polyethylene cup. However, because it is not a load-bearing joint, the trend in shoulder replacement is simply to resurface the worn part on the humerus side with a stainless steel cap rather than replace both sides of the joint.
Case examination of biomaterials, sports materials and nanomaterials 589 100
THR success 90
TKR success 80 70 60 50 40 30 20
Success of hip and knee replacements 10
Time (years)
Figure 12.4 The failure rate for a total hip replacement (THR) and total knee replacement (TKR) (courtesy R. Grimer, Royal Orthopaedic Hospital, Birmingham).
12.2.3.3 Knee joints
Total knee replacements (TKR) are increasingly being made to give pain-free improved leg function to many thousands of patients suffering from osteoarthritis or rheumatoid arthritis. As with hip replacements, TKR involves removing the articulating surfaces of the affected knee joint and replacing them with artificial components made from biomaterials. Such operations have a successful history and a failure rate of less than 2% per year (Figure 12.4). A typical replacement joint consists of
a tibial base plate or tray, usually made of stainless steel, Co–Cr or titanium alloy, with a tibial insert (UHMWPE) that acts as the bearing surface (see Figure 12.6). The femoral component largely takes the shape of a natural femoral condyle made from the above alloys, and articulates with the bearing surface, together with the kneecap or patella. The patella may be all polyethylene or metal backed. The components are either fixed by cement or uncemented. Cemented components use acrylic cement (polymethyl methacrylate). Uncemented components rely on bone ingrowth into the implant.
Titanium used on its own shows evidence of bone ingrowth, but more recently this has been improved by use of hydroxyapatite (HA) coating. Problems include wear debris from the PTFE and possible fracture of the component.
One of the continuing challenges is improved design against failure, currently about 2%, as a result of overloading or increased physical activity, and possibly bone resorption.
12.2.3.4 Finger joints
Replacement finger and hand joints are far more complex than other joints because of the degree of flexibility required through large angles while maintaining overall stability. Early designs were based on the movement of a metal ‘hinge’, but had a problem of fatigue and/or corrosion. Nowadays silicone rubber is more commonly used, showing little tendency to fatigue failure while being biocompatible with an ability to absorb lipids and fatty acids.
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Figure 12.5 Shoulder joint prosthesis (courtesy P. Marquis, Dental School, Birmingham).
Polysiloxane is used in hand surgery to form a ‘tunnel’ to allow transplanted tendons to slide back and forward. It is also used to replace carpal bones. It is, however, not suitable for longer joints or bones unless reinforced.